WO2024186248A1 - A method of continuous phase m-fsk modulation compatible with ofdm systems - Google Patents

Abstract

A method, system and apparatus are disclosed. According to some embodiments, a transmitter node configured to communicate with a receiver node is provided. The transmitter node configured to generate a multiple-frequency-shift keying, MFSK, signal having a continuous phase in a time domain by, at least in part, by mapping a signal on a plurality of subcarriers per OFDM symbol, perform OFDM modulations based on the generated MFSK signal, and cause transmission of an OFDM transmission based on the OFDM modulations.

Description

A METHOD OF CONTINUOUS PHASE M-FSK MODULATION COMPATIBLE WITH OFDM SYSTEMS TECHNICAL FIELD The present disclosure relates to wireless communications, and in particular, to methods, systems, and apparatuses for continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. BACKGROUND The Third Generation Partnership Project (3GPP) has developed and is developing standards for Fourth Generation (4G) (also referred to as Long Term Evolution (LTE)) and Fifth Generation (5G) (also referred to as New Radio (NR)) wireless communication systems. Such systems provide, among other features, broadband communication between network nodes, such as base stations, and mobile wireless devices (WD), as well as communication between network nodes and between WDs. The 3GPP is also developing standards for Sixth Generation (6G) wireless communication networks. Wake-up receiver (WUR), sometimes also referred to as ‘wake-up radio’, relates to enabling a low power receiver in wireless devices, which, in case of the detection of a wake-up signal (WUS), wakes up the main (baseband/RF/less power efficient) receiver to detect an incoming message, typically paging (e.g., physical downlink control channel (PDCCH) in paging occasions (PO), scheduling the paging message on physical downlink shared channel (PDSCH)). One benefit of employing WUR is lowering energy consumption and longer device battery life, or at a fixed energy consumption the downlink latency can be reduced (shorter Discontinuous Reception (DRX)/duty-cycles and more frequent checks for incoming transmissions). FIG.1 is a diagram of an example of the location of WUS and a paging occasion to which it is associated. Two approaches for detecting WUS are described below: • Using the main receiver: o No need for additional dedicated hardware/receiver for monitoring WUS o Coverage of the main receiver is not typically impacted o Limited power saving gain as the main receiver monitors WUS • Having a dedicated receiver (WUR) different from the main receiver: o Extremely low power, low complexity and low-cost receiver architecture, relaxed requirements, noisier (i.e., less accurate) clock or oscillator o Significant power saving gain can be achieved by maximizing the time in which the main receiver can be in the sleep mode o Enablers for zero energy/battery-less devices, and energy harvesting operations. o There are coverage considerations given the tradeoff between WUR power consumption and sensitivity. As an example, FIG.2 is a diagram of an example of a dedicated wake up radio (WUR) that is used for monitoring a wake-up signal (WUS). Once WUR detects the intended WUS, the WUR wakes up the main (baseband/RF/less power efficient) receiver to detect further incoming messages. Therefore, the main receiver can go to sleep mode and save power until it is triggered by WUR. Here, the WUR is an ultra- low power and low-complexity receiver which can support simple modulation schemes such as On-Off Keying (OOK), frequency-shift keying (FSK), or phase-shift keying (PSK). However, the WUS is transmitted using an OFDM-based transmitter. WUS for Narrowband IoT (NB-IoT) and LTE-M 3GPP Release 15 (Rel-15) In 3GPP Rel-15, 3GPP TS 36.304, v.15.4.0 “User Equipment (UE) procedures in idle mode”, in WUS was specified for NB-IoT and LTE-M. One motivation was wireless device energy consumption reduction since with the coverage enhancement Physical Downlink Control Channel (PDCCH) could be repeated many times and the WUS is relatively much shorter and hence requires less reception time for the wireless device. The logic is that a wireless device would check for a WUS a certain time before its PO, and only if a WUS is detected the wireless device would continue to check for PDCCH in the PO, and if not, which is most of the time, the wireless device can go back to a sleep state to conserve energy. Due to the coverage enhancements, the WUS can be of variable length depending on the wireless device 22’s coverage. FIG.3 is a diagram of an example WUS for NB-Iot and LTE-M. A ‘Wake-up signal’ (WUS) is based on the transmission of a short signal that indicates to the wireless device that it should continue to decode the DL control channel, e.g., full NPDCCH for NB-IoT. If such signal is absent (DTX, i.e., wireless device does not detect it) then the wireless device can go back to sleep without decoding the DL control channel. The decoding time for a WUS is considerably shorter than that of the full NPDCCH since it essentially only needs to contain one bit of information whereas the NPDCCH may contain up to 35 bits of information. This, in turn, reduces wireless device power consumption and leads to longer wireless device battery life. The WUS would be transmitted only when there is a paging for the wireless device. But if there is no paging for the UE then the WUS will not be transmitted (i.e., implying a discontinuous transmission, DTX) and the wireless device would go back to deep sleep, e.g., upon detecting DTX instead of WUS. This is illustrated in FIG.1, where blocks with no pattern indicate possible WUS and PO positions whereas the boxes with a pattern indicate actual WUS and PO positions. The specification of Rel-15 WUS is discussed in several parts of the LTE 36- series standard, e.g., 3GPP Technical Specifications (TS) 36.211, 36.213, 36.304 and 36.331. WUS UE grouping objective in 3GPP Rel-16 In the 3GPP Rel-16, GPP TS 38.213, v.16.11.0 “NR; Physical layer procedures for control,” work item description (WID), it was agreed that WUS should be further developed to also include wireless device grouping, such that the number of wireless devices that are triggered by a WUS is further narrowed down to a smaller subset of the wireless devices that are associated with a specific paging occasion (PO). The purpose is to reduce the false paging rate, i.e., avoid a wireless device from being unnecessarily woken up by a WUS transmission intended for another wireless device. This feature is referred to as Rel-16 group WUS, or GWUS. However, this is not directly related to WUR and is not further described herein. 3GPP Rel-17 NR PEI In 3GPP Rel-17, 3GPP TS 38.331, v.17.3.0, “Evolved Universal Terrestrial Radio Access (EUTRA) and Evolved Universal Terrestrial Radio Access Network (EUTRAN),” discussions started on introducing a WUS for NR, which was then referred to as ‘Paging Early Indication’ (PEI). However, since at the time no coverage enhancement was specified for NR, the only gain for 3GPP Rel-17 PEI was for scenarios where the small fraction of wireless devices are in bad coverage and with large synchronization error due to the use of longer DRX cycles. The gain for such wireless devices were that with the use of PEI they would typically only have to acquire one SSB before decoding PEI, instead of up to 3 SSBs if PEI is not used (value according to wireless device vendors). So, for most wireless devices, 3GPP Rel-17 PEI may result in gains or increased performance. 3GPP Rel-17 PEI will also support wireless device grouping for false paging reduction, similar to the Rel-16 GWUS, which will have some gains at higher paging load. In RAN#93e it was agreed that PEI will be PDCCH-based, making it less interesting for and/or applicable to WUR (i.e., the main baseband receiver is required for decoding PEI). 3GPP Rel-18 NR WUR In 3GPP Rel-18, 3GPP TR 38.869, v.1.0.0 , "Study on low-power Wake-up Signal and Receiver for NR", there has been interest to introduce WUR for NR, with a purpose for achieving more significant energy efficiency improvement compared to solutions already specified in earlier 3GPP releases. As explained above, the only specification support needed to be able to use a WUR in the wireless device is the specification of a WUS and a long enough time gap between the WUS and the PDCCH in the PO (to allow the wireless device 22 to start up the main receiver). Therefore, one difference to 3GPP Rel-17 PEI is the WUS in 3GPP Rel-18 that should not be PDCCH- based and allow for a less complex and low power receiver, i.e., WUR with low complexity modulation and detection techniques (e.g., using on-off keying,(OOK) modulation and non-coherent detection). In 3GPP Rel-18, a study item on “low-power wake-up signal and receiver for NR” was approved. The relevant justification and objective sections are described in RP-213645, some of which is described below. 5G systems are designed and developed targeting for both mobile telephony and vertical use cases. Besides latency, reliability, and availability, wireless device energy efficiency is also critical to 5G. Currently, 5G devices may have to be recharged per week or day, depending on individual’s usage time. In general, 5G devices consume tens of milliwatts in RRC idle/inactive state and hundreds of milliwatts in RRC connected state. Designs to prolong battery life is a necessity for improving energy efficiency as well as for better user experience. Energy efficiency is even more critical for wireless devices without a continuous energy source, e.g., wireless devices using small rechargeable and single coin cell batteries. Among vertical use cases, sensors and actuators are deployed extensively for monitoring, measuring, charging, etc. Generally, their batteries are not rechargeable and expected to last at least few years as described in 3GPP Technical Reference (TR) 38.875, v.17.0.0 , "Study on support of reduced capability NR devices" . Wearables include smart watches, rings, eHealth related devices, and medical monitoring devices. With typical battery capacity, it is challenging to sustain up to 1-2 weeks as required. The power consumption depends on the configured length of wake-up periods, e.g., paging cycle. To meet the battery life requirements above, Extended Discontinuous Reception (eDRX) cycle with large value is expected to be used, resulting in high latency, which is not suitable for such services with requirements of both long battery life and low latency. For example, in fire detection and extinguishment use case, fire shutters shall be closed and fire sprinklers shall be turned on by the actuators within 1 to 2 seconds from the time the fire is detected by sensors, long eDRX cycle cannot meet the delay requirements. eDRX is apparently not suitable for latency-critical use cases. Thus, the intention is to study ultra-low power mechanism that can support low latency in Rel-18, e.g., lower than eDRX latency. Currently, wireless devices need to periodically wake up once per DRX cycle, which dominates the power consumption in periods with no signalling or data traffic. If wireless devices are able to wake up only when they are triggered, e.g., paging, power consumption could be dramatically reduced. This can be achieved by using a wake-up signal to trigger the main radio and a separate receiver that has the ability to monitor wake-up signal with ultra-low power consumption. Main radio works for data transmission and reception, which can be turned off or set to deep sleep unless it is turned on. The power consumption for monitoring wake-up signal depends on the wake-up signal design and the hardware module of the wake-up receiver used for signal detecting and processing. The study should primarily target low-power WUS/WUR for power-sensitive, small form-factor devices including IoT use cases (such as industrial sensors, controllers) and wearables. Other use cases are not precluded, e.g., XR/smart glasses, smart phones. Different from the work on wireless device power savings in previous releases, this study item will not require existing signals to be used as WUS. All WUS solutions identified shall be able to operate in a cell supporting legacy wireless devices. Solutions should target substantial gains compared to the existing Rel-15/16/17 wireless device power saving mechanisms. Other aspects such as detection performance, coverage, wireless device complexity, should be covered by the evaluation. • Identify evaluation methodology (including the use cases) & KPIs [RAN1] o Primarily target low-power WUS/WUR for power-sensitive, small form-factor devices including IoT use cases (such as industrial sensors, controllers) and wearables ^ Other use cases are not precluded. • Study and evaluate low-power wake-up receiver architectures [RAN1, RAN4] • Study and evaluate wake-up signal designs to support wake-up receivers [RAN1, RAN4] • Study and evaluate L1 procedures and higher layer protocol changes needed to support the wake-up signals [RAN2, RAN1] • Study potential wireless device power saving gains compared to the existing Rel-15/16/17 wireless device power saving mechanisms and their coverage availability, as well as latency impact. System impact, such as network power consumption, coexistence with non-low- power-WUR wireless devices, network coverage/capacity/resource overhead should be included in the study [RAN1] Note: The need for RAN2 evaluation will be triggered by RAN1 when necessary. A benefit of WUR is to reduce the energy consumption of the receiver, such that unless there is any paging and data for the wireless device, it can remain in a power saving state. This will extent the battery life of the device, or alternatively enable shorter downlink latency (shorter DRX) at a fixed battery life. For short-range communication, the WUR power can be low enough (~10 uW) that this can even, in combination with energy harvesting, enable that the WUR is continuously on (i.e., DRX or duty-cycling is not used) without the need for a battery. This can be considered as an enabler of battery-less devices towards 6G. Ambient IoT and zero energy devices The 3GPP Rel-18 RAN is discussing the concept of Zero-Energy (ZE) IoT, also known as Ambient IoT. These devices are designed to operate without the need for manual battery replacement or recharging by harvesting energy from the surrounding environment, resulting in low maintenance and long-lasting functionality. However, the small size, ultra-low cost, and battery-less nature of ZE IoT devices present design challenges. Supporting ZE IoT devices requires significant reduction of power consumption and complexity by simplifying the RF chain and baseband architecture, reducing memory size, and eliminating unnecessary components. To achieve ultra-low power consumption, communication procedures between ZE IoT devices and access points (AP) must be designed as low complex as possible. While OFDM may not be suitable for ZE IoT devices due to its high-power consumption requirements, lower complexity waveforms such as OOK/FSK modulation offer a more promising option for enabling ultra-low complexity data transmission and reception. However, one of the key challenges in adopting these simpler waveforms is ensuring compatibility with existing OFDM-based architecture. The 3GPP study on Ambient IoT (e.g., 3GPP TR 22.840, v.2.1.0 "Study on Ambient power-enabled Internet of Things ") investigates the feasibility of a new IoT technology to open new markets within 3GPP systems, whose number of connections and/or device density can be orders of magnitude higher than existing 3GPP IoT technologies, and which can provide complexity and power consumption orders-of- magnitude lower than existing 3GPP Low-Power, Wide-Area (LPWA) technologies such as NB-IoT and LTE-MTC. IEEE WUR Institute of Electrical and Electronics Engineers (IEEE) 802.11 standardized the support for WUR in the task group (TG) “ba” (TGba). Similar to the 3GPP solution, the use of WUR is only enabled in stations and not in access points (APs), that is for downlink communication only. The AP advertises that it has WUR operation capability, along with WUR configuration parameters (among other info, in which band/channel WUR is operational, which can be different from the band/channel used for data transmission using the main receiver, e.g., WUR in the 2.4 GHz band but data communication is in 5 GHz band. Also note that the WUR operating channel is advertised in the beacon, and that the WUR discovery operating channel may be different from the WUR operating channel.). Stations can then request to be configured with WUR mode of operation. This request has to be granted by the AP, and in case it is granted, the station is further configured/setup for WUR mode of operation (the configuration is only valid for the connection to the associated AP, and further the configuration must be torn down/de-configured if WUR is not used anymore). Both continuous WUR (receiver open all the time) and duty-cycled WUR (receiver only open during preconfigured time slots) mode of operations are supported. For the latter, the length of the duty-cycles and on-time during wake up is part of the WUR configuration. Unlike the 3GPP solution, the WUR operation mode is a “sub-state” of the regular operation and upon the detection of a WUS transmission from the AP, the station will resume the power saving mechanism it was configured with before entering the WUR operation mode. That is, IEEE has specified a number of different power saving mechanisms, and for example, if duty-cycled monitoring of the downlink has been configured for the station, it will switch to that upon detection of the WUS (i.e., unlike the specified 3GPP mechanism which only covers paging, and the wireless device will continue to monitor PDCCH if WUS is detected). In this way the IEEE WUR functionality is more general, and still allows for the station to, upon detection of WUS, “monitor paging” by checking in the beacon from the AP for which stations there is data, or for the station to directly respond with an uplink transmission. The physical wake-up signal (WUS) in IEEE contains complete frames which must be processed by the station. The drawback with this design is that it requires more handling and processing in the station, i.e., compared to a low complexity WUR design which trigger one pre-defined activity in case WUS is detected. The benefit is that it contains more information and the solution is more general. The IEEE WUS contains information to indicate if the WUS is a WUR sync beacon, a WUR discovery beacon, or a regular WUS (intended to wake the station up). The WUS can also contain proprietary frames, which could, e.g., be used to directly turn actuators on/off. The transmission uses on/off keying (OOK) modulation, using Manchester coding, but is using multi-carrier OOK which can be generated by an OFDM transmitter (i.e., WUR can be enabled as a software upgrade in APs). The WUS is 4 MHz wide, but a whole 20 MHz channel is reserved. The WUS starts with a 20 MHz legacy preamble (to allows other stations to perform carrier sense) followed by 4 MHz Manchester coded OOK. Two data rates are supported: 62.5 kbps and 250 kbps, and link adaptation is up to the AP (each packet is self-contained and includes the data rate, i.e., in the WUR there are two possible sync words used to signal the data rate). OFDM transmission Orthogonal frequency-division multiplexing (OFDM) is a multi-carrier modulation system where data is transmitted as a combination of orthogonal narrowband signals known as subcarriers. OFDM is more robust to frequency selective fading and simplifies equalization at the receiver. OFDM is a foundational scheme found in many common wireless communications standards such as WIFI, LTE, and 5G. The OFDM transmission scheme can be described in several components. The data is first coded and modulated, usually into QAM symbols. These symbols are loaded into equally spaced frequency bins and an inverse fast Fourier transform (IFFT) is applied to transform the signal into orthogonal overlapping sinusoids in the time domain. The N samples at the output of the IFFT make up one OFDM symbol. A cyclic prefix is then appended to each OFDM symbol, which allows for computation of circular convolution through linear convolution if the cyclic prefix is at least as long as the channel impulse response. This allows equalization at the receiver to remove inter- symbol interference through a straightforward complex scalar multiplication applied to each OFDM symbol independently. A block diagram of an OFDM transmitter is illustrated in FIG.4. The ^^^^-point IFFT expression for input ^^^^^[ ^^^^^], ^^^^ ∈ {0, ... ^^^^ − 1} is given by: 1 ^{^^^^−1} Here, ^^^^[ ^^^^] represents the

[ ^^^^] is the time domain representation of the signal. The existing 3GPP and IEEE systems are based on orthogonal frequency division multiplexing (OFDM) modulation. For WUS transmissions, it is desired to reuse the OFDM-based transmitter with minimum hardware/software upgrade. However, on the receiver side, employing an OFDM-based WUR may not be feasible/desired due its significant power consumption. Hence, one or more issues remain unsolved with respect to WUS and WUR in 3GPP and/or the IEEE. SUMMARY Some embodiments advantageously provide methods, systems, and apparatuses for continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. One or more embodiments described herein provide low-complexity and high data rate solutions for generating low complexity modulations such as FSK using an OFDM-based transmitter. The methods for generating continuous phase M-FSK modulation based on OFDM system are presented which prevent spectrum spread and significantly reduces receiver complexity/power consumption for timing synchronization. According to one aspect of the present disclosure, a transmitter node configured to communicate with a receiver node is provided. The transmitter node is configured to generate a multiple-frequency-shift keying, MFSK, signal having a continuous phase in a time domain by, at least in part, mapping a signal on a plurality of subcarriers per OFDM symbol, perform OFDM modulations based on the generated MFSK signal, and cause transmission of an OFDM transmission based on the OFDM modulations. According to one or more embodiments of this aspect, the mapping of the signal on the plurality of subcarriers per OFDM symbol occurs before converting the signal from the frequency domain to the time domain. According to one or more embodiments of this aspect, the transmitter node is further configured to perform phase shift compensation before the converting of the signal for each OFDM modulation where the phase shift compensation is configured to, at least in part, configure the continuous phase in the time domain of the MFSK signal. According to one or more embodiments of this aspect, the transmitter node is further configured to add a cyclic prefix, CP, to the OFDM transmission where the MFSK signal has a continuous phase in a time domain after the CP is added. According to one or more embodiments of this aspect, the signal corresponds to an initial sequence of bits, ^^^^ _^^^^, and when an information binary of MFSK is 0, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� of the plurality of subcarriers where ^^^^ the subcarrier index vector. According to one or more embodiments of this aspect, when the information binary of MFSK is 1, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}� of the plurality of subcarriers. According to one or more embodiments of this aspect, a subset of carriers are used for a guard band and have no power. According to one or more embodiments of this aspect, the signal is a wake-up signal, WUS, and/or provides a wake-up indication. According to another aspect of the present disclosure, a receiver node configured to communicate with a transmitter node is provided. The receiver node is configured to receive an orthogonal frequency-division multiplexing, OFDM, transmission that is based on OFDM modulations, where the OFDM modulations are based on a multiple-frequency-shift keying, MFSK, signal that has a continuous phase in a time domain based, at least in part, on a mapping of a signal on a plurality of subcarriers per OFDM symbol, and perform at least one action based on the received OFDM transmission. According to one or more embodiments of this aspect, the mapping of the signal on the plurality of subcarriers per OFDM symbol occurs before converting the signal from the frequency domain to the time domain. According to one or more embodiments of this aspect, the OFDM transmission is based on phase shift compensation performed before the converting of the signal for each OFDM modulation where the phase shift compensation is configured to, at least in part, configure the continuous phase in the time domain of the MFSK signal. According to one or more embodiments of this aspect, the OFDM transmission includes a cyclic prefix, CP. According to one or more embodiments of this aspect, the signal corresponds to an initial sequence of bits, ^^^^ _^^^^, and when an information binary of MFSK is 0, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� of the plurality of subcarriers where ^^^^ is the subcarrier index vector. According to one or more embodiments of this aspect, when the information binary of MFSK is 1, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}� of the plurality of subcarriers.

According to one or more embodiments of this aspect, a subset of carriers are used for a guard band and have no power. According to one or more embodiments of this aspect, the receiver node includes a first receiver, and a second receiver that consumes less operational power than the first receiver where the second receiver is configured to wake-up the first receiver when a wake-up signal, WUS, is received, and the second receiver is configured to receive OFDM transmission, the signal being a WUS and/or provides a wake-up indication, and where the at least one action includes causing the first receiver to wake up based on the received OFDM transmission. According to another aspect of the present disclosure, a method implemented by a transmitter node that is configured to communicate with a receiver node is provided. A multiple-frequency-shift keying, MFSK, signal having a continuous phase in a time domain is generated by, at least in part, mapping a signal on a plurality of subcarriers per OFDM symbol. OFDM modulations are preformed based on the generated MFSK signal. Transmission is caused of an OFDM transmission based on the OFDM modulations. According to one or more embodiments of this aspect, the mapping of the signal on the plurality of subcarriers per OFDM symbol occurs before converting the signal from the frequency domain to the time domain. According to one or more embodiments of this aspect, phase shift compensation is performed before the converting of the signal for each OFDM modulation, where the phase shift compensation is configured to, at least in part, configure the continuous phase in the time domain of the MFSK signal. According to one or more embodiments of this aspect, a cyclic prefix, CP, is added to the OFDM transmission, the MFSK signal having a continuous phase in a time domain after the CP is added. According to one or more embodiments of this aspect, the signal corresponds to an initial sequence of bits, ^^^^ _^^^^, and when an information binary of MFSK is 0, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� of the plurality of subcarriers where ^^^^ is the subcarrier index vector. According to one or more embodiments of this aspect, where the information binary of MFSK is 1, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}� of the plurality of subcarriers.

According to one or more embodiments of this aspect, a subset of carriers are used for a guard band and have no power. According to one or more embodiments of this aspect, the signal is a wake-up signal, WUS, and/or provides a wake-up indication. According to another aspect of the present disclosure, a method implemented in a receiver node that is configured to communicate with a transmitter node is provided. An orthogonal frequency-division multiplexing, OFDM, transmission that is based on OFDM modulations is received, and the OFDM modulations are based on a multiple- frequency-shift keying, MFSK, signal that has a continuous phase in a time domain based, at least in part, on a mapping of a signal on a plurality of subcarriers per OFDM symbol. At least one action is performed based on the signal. According to one or more embodiments of this aspect, the mapping of the signal on the plurality of subcarriers per OFDM symbol occurs before converting the signal from the frequency domain to the time domain. According to one or more embodiments of this aspect, the OFDM transmission is based on phase shift compensation performed before the converting of the signal for each OFDM modulation where the phase shift compensation is configured to, at least in part, configure the continuous phase in the time domain of the MFSK signal. According to one or more embodiments of this aspect, the OFDM transmission includes a cyclic prefix, CP. According to one or more embodiments of this aspect, the signal corresponds to an initial sequence of bits, ^^^^ _^^^^, and when an information binary of MFSK is 0, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� of the plurality of subcarriers where ^^^^ is the subcarrier index vector.

According to one or more embodiments of this aspect, where the information binary of MFSK is 1, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}� of the plurality of subcarriers.

According to one or more embodiments of this aspect, a subset of carriers are used for a guard band and have no power. According to one or more embodiments of this aspect, the receiver node includes: a first receiver and a second receiver that consumes less operational power than the first receiver, where the second receiver is configured to wake-up the first receiver when a wake-up signal, WUS, is received, and where the second receiver is configured to receive OFDM transmission, and where the signal is a WUS and/or provides a wake-up indication. The at least one action includes causing the first receiver to wake up based on the received OFDM transmission. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: FIG.1 is a diagram of the location of WUS and the paging occasion to which it is associated; FIG.2 is a diagram of a dedicated wake-up radio accompanying the main receiver; FIG.3 is a diagram of WUS for NB-IoT and LTE-M; FIG.4 is diagram of an OFDM transmitter; FIG.5 is a schematic diagram of an example network architecture illustrating a communication system connected via an intermediate network to a host computer according to the principles in the present disclosure; FIG.6 is a block diagram of a host computer communicating via a network node with a wireless device over an at least partially wireless connection according to some embodiments of the present disclosure; FIG.7 is a flowchart illustrating example methods implemented in a communication system including a host computer, a network node and a wireless device for executing a client application at a wireless device according to some embodiments of the present disclosure; FIG.8 is a flowchart illustrating example methods implemented in a communication system including a host computer, a network node and a wireless device for receiving user data at a wireless device according to some embodiments of the present disclosure; FIG.9 is a flowchart illustrating example methods implemented in a communication system including a host computer, a network node and a wireless device for receiving user data from the wireless device at a host computer according to some embodiments of the present disclosure; FIG.10 is a flowchart illustrating example methods implemented in a communication system including a host computer, a network node and a wireless device for receiving user data at a host computer according to some embodiments of the present disclosure; FIG.11 is a flowchart of an example process in a network node according to some embodiments of the present disclosure; FIG.12 is a flowchart of an example process in a wireless device according to some embodiments of the present disclosure; FIG.13 is a diagram of an example WUS generation based on FSK modulation using an OFDM system according to some embodiments of the present disclosure; FIG.14 is a diagram of an example MFSK modulation in the frequency domain based on an OFDM system according to some embodiments of the present disclosure; FIG.15 is a diagram of an example time domain signal with discontinuous phase 2-FSK modulation based on OFDM system according to some embodiments of the present disclosure; FIG.16 is diagram of an example subcarrier mapping before IFFT on the i-th OFDM symbol according to some embodiments of the present disclosure; FIG.17 is a diagram of an example time domain signal with continuous phase 2-FSK modulation based on an OFDM system according to some embodiments of the present disclosure; and FIG.18 is a flowchart of an example process for continuous phase M-FSK modulation based on OFDM system according to some embodiments of the present disclosure. DETAILED DESCRIPTION As described above, on the receiver side, employing an OFDM-based WUR may not be feasible/desired due its significant power consumption. In fact, a low power WUR may need to work with a low complexity modulation scheme such as FSK which requires an ultra-low-complexity receiver architecture. One challenge is to generate a WUS using an OFDM-based transmitter which can be received by a low complexity WUR receiver while maintaining orthogonality of OFDM-based transmissions (i.e., minimizing inter-subcarrier interference). In addition, the complexity of generating WUS at the transmitter needs to be minimized. Another challenge is to reduce the complexity of receiver when the generation FSK signal is based on OFDM modulation, CP should be added after OFDM modulation. It is likely to cause a sharp jump in the amplitude or phase of the FSK signal between two adjacent OFDM symbols. For the FSK signal, the sharp jump causes not only spectrum spread but also requires more complexity to make timing synchronization for receiver. One or more embodiments described herein solves at least one problem with existing systems by, for example, providing low-complexity and high data rate solutions for generating low complexity modulations such as FSK using an OFDM- based transmitter. The methods for generating continuous phase M-FSK modulation based on OFDM system are presented which prevent spectrum spread and significantly reduces receiver complexity/power consumption for timing synchronization. Before describing in detail example embodiments, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. Accordingly, components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Like numbers refer to like elements throughout the description. As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the concepts described herein. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. In embodiments described herein, the joining term, “in communication with” and the like, may be used to indicate electrical or data communication, which may be accomplished by physical contact, induction, electromagnetic radiation, radio signaling, infrared signaling or optical signaling, for example. One having ordinary skill in the art will appreciate that multiple components may interoperate and modifications and variations are possible of achieving the electrical and data communication. In some embodiments described herein, the term “coupled,” “connected,” and the like, may be used herein to indicate a connection, although not necessarily directly, and may include wired and/or wireless connections. The term “network node” used herein can be any kind of network node comprised in a radio network which may further comprise any of base station (BS), radio base station, base transceiver station (BTS), base station controller (BSC), radio network controller (RNC), g Node B (gNB), evolved Node B (eNB or eNodeB), Node B, multi-standard radio (MSR) radio node such as MSR BS, multi-cell/multicast coordination entity (MCE), integrated access and backhaul (IAB) node, relay node, donor node controlling relay, radio access point (AP), transmission points, transmission nodes, Remote Radio Unit (RRU) Remote Radio Head (RRH), a core network node (e.g., mobile management entity (MME), self-organizing network (SON) node, a coordinating node, positioning node, MDT node, etc.), an external node (e.g., 3rd party node, a node external to the current network), nodes in distributed antenna system (DAS), a spectrum access system (SAS) node, an element management system (EMS), IEEE based node (e.g., access point, transmission reception point (TRP)), etc. The network node may also comprise test equipment. The term “radio node” used herein may be used to also denote a wireless device (WD) such as a wireless device (WD) or a radio network node. In some embodiments, the non-limiting terms wireless device (WD) or a user equipment (UE) are used interchangeably. The WD herein can be any type of wireless device capable of communicating with a network node or another WD over radio signals, such as wireless device (WD). The WD may also be a radio communication device, target device, device to device (D2D) WD, machine type WD or WD capable of machine to machine communication (M2M), low-cost and/or low-complexity WD, a sensor equipped with WD, Tablet, mobile terminals, smart phone, laptop embedded equipped (LEE), laptop mounted equipment (LME), USB dongles, Customer Premises Equipment (CPE), an Internet of Things (IoT) device, or a Narrowband IoT (NB-IOT) device, etc. Also, in some embodiments the generic term “radio network node” is used. It can be any kind of a radio network node which may comprise any of base station, radio base station, base transceiver station, base station controller, network controller, RNC, evolved Node B (eNB), Node B, gNB, Multi-cell/multicast Coordination Entity (MCE), IAB node, relay node, access point, radio access point, Remote Radio Unit (RRU) Remote Radio Head (RRH). In some embodiments, the term "transmitter node" may refer to one or more of a network node, wireless device, radio network node, IEEE based device/node, non- 3GPP based device/node, etc., among other entities that are configured with an OFDM based transmitter. In some embodiments, the term "receiver node" may refer to one or more of a network node, wireless device, radio network node, IEEE based device/node, non- 3GPP based device/node, etc., among other entities that are configured with a FSK- based receiver. Note that although terminology from one particular wireless system, such as, for example, 3GPP LTE and/or New Radio (NR), may be used in this disclosure, this should not be seen as limiting the scope of the disclosure to only the aforementioned system. Other wireless systems, including without limitation Wide Band Code Division Multiple Access (WCDMA), Worldwide Interoperability for Microwave Access (WiMax), Ultra Mobile Broadband (UMB) and Global System for Mobile Communications (GSM), may also benefit from exploiting the ideas covered within this disclosure. Note further, that functions described herein as being performed by a wireless device or a network node may be distributed over a plurality of wireless devices and/or network nodes. In other words, it is contemplated that the functions of the network node and wireless device described herein are not limited to performance by a single physical device and, in fact, can be distributed among several physical devices. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Some embodiments provide continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. Referring again to the drawing figures, in which like elements are referred to by like reference numerals, there is shown in FIG.5 a schematic diagram of a communication system 10, according to an embodiment, such as a 3GPP-type cellular network that may support standards such as LTE and/or NR (5G), which comprises an access network 12, such as a radio access network, and a core network 14. The access network 12 comprises a plurality of network nodes 16a, 16b, 16c (referred to collectively as network nodes 16), such as NBs, eNBs, gNBs or other types of wireless access points, each defining a corresponding coverage area 18a, 18b, 18c (referred to collectively as coverage areas 18). In one or more embodiments, network node 16 is an example of a transmitter node. Each network node 16a, 16b, 16c is connectable to the core network 14 over a wired or wireless connection 20. A first wireless device (WD) 22a located in coverage area 18a is configured to wirelessly connect to, or be paged by, the corresponding network node 16a. A second WD 22b in coverage area 18b is wirelessly connectable to the corresponding network node 16b. While a plurality of WDs 22a, 22b (collectively referred to as wireless devices 22) are illustrated in this example, the disclosed embodiments are equally applicable to a situation where a sole WD is in the coverage area or where a sole WD is connecting to the corresponding network node 16. Note that although only two WDs 22 and three network nodes 16 are shown for convenience, the communication system may include many more WDs 22 and network nodes 16. In one or more embodiments, wireless device 22 is an example of a receiver node. Also, it is contemplated that a WD 22 can be in simultaneous communication and/or configured to separately communicate with more than one network node 16 and more than one type of network node 16. For example, a WD 22 can have dual connectivity with a network node 16 that supports LTE and the same or a different network node 16 that supports NR. As an example, WD 22 can be in communication with an eNB for LTE/E-UTRAN and a gNB for NR/NG-RAN. The communication system 10 may itself be connected to a host computer 24, which may be embodied in the hardware and/or software of a standalone server, a cloud-implemented server, a distributed server or as processing resources in a server farm. The host computer 24 may be under the ownership or control of a service provider, or may be operated by the service provider or on behalf of the service provider. The connections 26, 28 between the communication system 10 and the host computer 24 may extend directly from the core network 14 to the host computer 24 or may extend via an optional intermediate network 30. The intermediate network 30 may be one of, or a combination of more than one of, a public, private or hosted network. The intermediate network 30, if any, may be a backbone network or the Internet. In some embodiments, the intermediate network 30 may comprise two or more sub- networks (not shown). The communication system of FIG.5 as a whole enables connectivity between one of the connected WDs 22a, 22b and the host computer 24. The connectivity may be described as an over-the-top (OTT) connection. The host computer 24 and the connected WDs 22a, 22b are configured to communicate data and/or signaling via the OTT connection, using the access network 12, the core network 14, any intermediate network 30 and possible further infrastructure (not shown) as intermediaries. The OTT connection may be transparent in the sense that at least some of the participating communication devices through which the OTT connection passes are unaware of routing of uplink and downlink communications. For example, a network node 16 may not or need not be informed about the past routing of an incoming downlink communication with data originating from a host computer 24 to be forwarded (e.g., handed over) to a connected WD 22a. Similarly, the network node 16 need not be aware of the future routing of an outgoing uplink communication originating from the WD 22a towards the host computer 24. A network node 16 is configured to include an MFSK unit 32 which is configured to perform one or more network node 16 functions as described herein such as with respect to continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. A wireless device 22 is configured to include a detection unit 34 which is configured to perform one or more wireless device 22 functions as described herein such as with respect to continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. Example implementations, in accordance with an embodiment, of the WD 22, network node 16 and host computer 24 discussed in the preceding paragraphs will now be described with reference to FIG.6. In a communication system 10, a host computer 24 comprises hardware (HW) 38 including a communication interface 40 configured to set up and maintain a wired or wireless connection with an interface of a different communication device of the communication system 10. The host computer 24 further comprises processing circuitry 42, which may have storage and/or processing capabilities. The processing circuitry 42 may include a processor 44 and memory 46. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 42 may comprise integrated circuitry for processing and/or control, e.g., one or more processors and/or processor cores and/or FPGAs (Field Programmable Gate Array) and/or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 44 may be configured to access (e.g., write to and/or read from) memory 46, which may comprise any kind of volatile and/or nonvolatile memory, e.g., cache and/or buffer memory and/or RAM (Random Access Memory) and/or ROM (Read-Only Memory) and/or optical memory and/or EPROM (Erasable Programmable Read-Only Memory). Processing circuitry 42 may be configured to control any of the methods and/or processes described herein and/or to cause such methods, and/or processes to be performed, e.g., by host computer 24. Processor 44 corresponds to one or more processors 44 for performing host computer 24 functions described herein. The host computer 24 includes memory 46 that is configured to store data, programmatic software code and/or other information described herein. In some embodiments, the software 48 and/or the host application 50 may include instructions that, when executed by the processor 44 and/or processing circuitry 42, causes the processor 44 and/or processing circuitry 42 to perform the processes described herein with respect to host computer 24. The instructions may be software associated with the host computer 24. The software 48 may be executable by the processing circuitry 42. The software 48 includes a host application 50. The host application 50 may be operable to provide a service to a remote user, such as a WD 22 connecting via an OTT connection 52 terminating at the WD 22 and the host computer 24. In providing the service to the remote user, the host application 50 may provide user data which is transmitted using the OTT connection 52. The “user data” may be data and information described herein as implementing the described functionality. In one embodiment, the host computer 24 may be configured for providing control and functionality to a service provider and may be operated by the service provider or on behalf of the service provider. The processing circuitry 42 of the host computer 24 may enable the host computer 24 to observe, monitor, control, transmit to and/or receive from the network node 16 and or the wireless device 22. The processing circuitry 42 of the host computer 24 may include an information unit 54 configured to enable the service provider to one or more of process, analyze, determine, configuration, store, transmit, receive, communicate, forward, relay, etc., information related to continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. The communication system 10 further includes a network node 16 provided in a communication system 10 and including hardware 58 enabling it to communicate with the host computer 24 and with the WD 22. The hardware 58 may include a communication interface 60 for setting up and maintaining a wired or wireless connection with an interface of a different communication device of the communication system 10, as well as a radio interface 62 for setting up and maintaining at least a wireless connection 64 with a WD 22 located in a coverage area 18 served by the network node 16. The radio interface 62 may be formed as or may include, for example, one or more RF transmitters, one or more RF receivers, and/or one or more RF transceivers. The communication interface 60 may be configured to facilitate a connection 66 to the host computer 24. The connection 66 may be direct or it may pass through a core network 14 of the communication system 10 and/or through one or more intermediate networks 30 outside the communication system 10. In the embodiment shown, the hardware 58 of the network node 16 further includes processing circuitry 68. The processing circuitry 68 may include a processor 70 and a memory 72. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 68 may comprise integrated circuitry for processing and/or control, e.g., one or more processors and/or processor cores and/or FPGAs (Field Programmable Gate Array) and/or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 70 may be configured to access (e.g., write to and/or read from) the memory 72, which may comprise any kind of volatile and/or nonvolatile memory, e.g., cache and/or buffer memory and/or RAM (Random Access Memory) and/or ROM (Read- Only Memory) and/or optical memory and/or EPROM (Erasable Programmable Read- Only Memory). Thus, the network node 16 further has software 74 stored internally in, for example, memory 72, or stored in external memory (e.g., database, storage array, network storage device, etc.) accessible by the network node 16 via an external connection. The software 74 may be executable by the processing circuitry 68. The processing circuitry 68 may be configured to control any of the methods and/or processes described herein and/or to cause such methods, and/or processes to be performed, e.g., by network node 16. Processor 70 corresponds to one or more processors 70 for performing network node 16 functions described herein. The memory 72 is configured to store data, programmatic software code and/or other information described herein. In some embodiments, the software 74 may include instructions that, when executed by the processor 70 and/or processing circuitry 68, causes the processor 70 and/or processing circuitry 68 to perform the processes described herein with respect to network node 16. For example, processing circuitry 68 of the network node 16 may include MFSK unit 32 configured to perform one or more network node 16 functions as described herein such as with respect to continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. The communication system 10 further includes the WD 22 already referred to. The WD 22 may have hardware 80 that may include a radio interface 82 configured to set up and maintain a wireless connection 64 with a network node 16 serving a coverage area 18 in which the WD 22 is currently located. The radio interface 82 may be formed as or may include, for example, one or more RF transmitters, one or more RF receivers, and/or one or more RF transceivers. The hardware 80 of the WD 22 further includes processing circuitry 84. The processing circuitry 84 may include a processor 86 and memory 88. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 84 may comprise integrated circuitry for processing and/or control, e.g., one or more processors and/or processor cores and/or FPGAs (Field Programmable Gate Array) and/or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 86 may be configured to access (e.g., write to and/or read from) memory 88, which may comprise any kind of volatile and/or nonvolatile memory, e.g., cache and/or buffer memory and/or RAM (Random Access Memory) and/or ROM (Read-Only Memory) and/or optical memory and/or EPROM (Erasable Programmable Read-Only Memory). Thus, the WD 22 may further comprise software 90, which is stored in, for example, memory 88 at the WD 22, or stored in external memory (e.g., database, storage array, network storage device, etc.) accessible by the WD 22. The software 90 may be executable by the processing circuitry 84. The software 90 may include a client application 92. The client application 92 may be operable to provide a service to a human or non-human user via the WD 22, with the support of the host computer 24. In the host computer 24, an executing host application 50 may communicate with the executing client application 92 via the OTT connection 52 terminating at the WD 22 and the host computer 24. In providing the service to the user, the client application 92 may receive request data from the host application 50 and provide user data in response to the request data. The OTT connection 52 may transfer both the request data and the user data. The client application 92 may interact with the user to generate the user data that it provides. The processing circuitry 84 may be configured to control any of the methods and/or processes described herein and/or to cause such methods, and/or processes to be performed, e.g., by WD 22. The processor 86 corresponds to one or more processors 86 for performing WD 22 functions described herein. The WD 22 includes memory 88 that is configured to store data, programmatic software code and/or other information described herein. In some embodiments, the software 90 and/or the client application 92 may include instructions that, when executed by the processor 86 and/or processing circuitry 84, causes the processor 86 and/or processing circuitry 84 to perform the processes described herein with respect to WD 22. For example, the processing circuitry 84 of the wireless device 22 may include a detection unit 34 configured to perform one or more wireless device 22 functions as described herein such as with respect to continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. In some embodiments, the inner workings of the network node 16, WD 22, and host computer 24 may be as shown in FIG.6 and independently, the surrounding network topology may be that of FIG.5. In FIG.6, the OTT connection 52 has been drawn abstractly to illustrate the communication between the host computer 24 and the wireless device 22 via the network node 16, without explicit reference to any intermediary devices and the precise routing of messages via these devices. Network infrastructure may determine the routing, which it may be configured to hide from the WD 22 or from the service provider operating the host computer 24, or both. While the OTT connection 52 is active, the network infrastructure may further take decisions by which it dynamically changes the routing (e.g., on the basis of load balancing consideration or reconfiguration of the network). The wireless connection 64 between the WD 22 and the network node 16 is in accordance with the teachings of the embodiments described throughout this disclosure. One or more of the various embodiments improve the performance of OTT services provided to the WD 22 using the OTT connection 52, in which the wireless connection 64 may form the last segment. More precisely, the teachings of some of these embodiments may improve the data rate, latency, and/or power consumption and thereby provide benefits such as reduced user waiting time, relaxed restriction on file size, better responsiveness, extended battery lifetime, etc. In some embodiments, a measurement procedure may be provided for the purpose of monitoring data rate, latency and other factors on which the one or more embodiments improve. There may further be an optional network functionality for reconfiguring the OTT connection 52 between the host computer 24 and WD 22, in response to variations in the measurement results. The measurement procedure and/or the network functionality for reconfiguring the OTT connection 52 may be implemented in the software 48 of the host computer 24 or in the software 90 of the WD 22, or both. In embodiments, sensors (not shown) may be deployed in or in association with communication devices through which the OTT connection 52 passes; the sensors may participate in the measurement procedure by supplying values of the monitored quantities exemplified above, or supplying values of other physical quantities from which software 48, 90 may compute or estimate the monitored quantities. The reconfiguring of the OTT connection 52 may include message format, retransmission settings, preferred routing etc.; the reconfiguring need not affect the network node 16, and it may be unknown or imperceptible to the network node 16. Some such procedures and functionalities may be known and practiced in the art. In certain embodiments, measurements may involve proprietary WD signaling facilitating the host computer’s 24 measurements of throughput, propagation times, latency and the like. In some embodiments, the measurements may be implemented in that the software 48, 90 causes messages to be transmitted, in particular empty or ‘dummy’ messages, using the OTT connection 52 while it monitors propagation times, errors, etc. Thus, in some embodiments, the host computer 24 includes processing circuitry 42 configured to provide user data and a communication interface 40 that is configured to forward the user data to a cellular network for transmission to the WD 22. In some embodiments, the cellular network also includes the network node 16 with a radio interface 62. In some embodiments, the network node 16 is configured to, and/or the network node’s 16 processing circuitry 68 is configured to perform the functions and/or methods described herein for preparing/initiating/maintaining/supporting/ending a transmission to the WD 22, and/or preparing/terminating/maintaining/supporting/ending in receipt of a transmission from the WD 22. In some embodiments, the host computer 24 includes processing circuitry 42 and a communication interface 40 that is configured to a communication interface 40 configured to receive user data originating from a transmission from a WD 22 to a network node 16. In some embodiments, the WD 22 is configured to, and/or comprises a radio interface 82 and/or processing circuitry 84 configured to perform the functions and/or methods described herein for preparing/initiating/maintaining/supporting/ending a transmission to the network node 16, and/or preparing/terminating/maintaining/supporting/ending in receipt of a transmission from the network node 16. Although FIGS.5 and 6 show various “units” such as MFSK unit 32, and detection unit 34 as being within a respective processor, it is contemplated that these units may be implemented such that a portion of the unit is stored in a corresponding memory within the processing circuitry. In other words, the units may be implemented in hardware or in a combination of hardware and software within the processing circuitry. FIG.7 is a flowchart illustrating an example method implemented in a communication system, such as, for example, the communication system of FIGS.5 and 6, in accordance with one embodiment. The communication system may include a host computer 24, a network node 16 and a WD 22, which may be those described with reference to FIG.6. In a first step of the method, the host computer 24 provides user data (Block S100). In an optional substep of the first step, the host computer 24 provides the user data by executing a host application, such as, for example, the host application 50 (Block S102). In a second step, the host computer 24 initiates a transmission carrying the user data to the WD 22 (Block S104). In an optional third step, the network node 16 transmits to the WD 22 the user data which was carried in the transmission that the host computer 24 initiated, in accordance with the teachings of the embodiments described throughout this disclosure (Block S106). In an optional fourth step, the WD 22 executes a client application, such as, for example, the client application 92, associated with the host application 50 executed by the host computer 24 (Block S108). FIG.8 is a flowchart illustrating an example method implemented in a communication system, such as, for example, the communication system of FIG.5, in accordance with one embodiment. The communication system may include a host computer 24, a network node 16 and a WD 22, which may be those described with reference to FIGS.5 and 6. In a first step of the method, the host computer 24 provides user data (Block S110). In an optional substep (not shown) the host computer 24 provides the user data by executing a host application, such as, for example, the host application 50. In a second step, the host computer 24 initiates a transmission carrying the user data to the WD 22 (Block S112). The transmission may pass via the network node 16, in accordance with the teachings of the embodiments described throughout this disclosure. In an optional third step, the WD 22 receives the user data carried in the transmission (Block S114). FIG.9 is a flowchart illustrating an example method implemented in a communication system, such as, for example, the communication system of FIG.5, in accordance with one embodiment. The communication system may include a host computer 24, a network node 16 and a WD 22, which may be those described with reference to FIGS.5 and 6. In an optional first step of the method, the WD 22 receives input data provided by the host computer 24 (Block S116). In an optional substep of the first step, the WD 22 executes the client application 92, which provides the user data in reaction to the received input data provided by the host computer 24 (Block S118). Additionally or alternatively, in an optional second step, the WD 22 provides user data (Block S120). In an optional substep of the second step, the WD provides the user data by executing a client application, such as, for example, client application 92 (Block S122). In providing the user data, the executed client application 92 may further consider user input received from the user. Regardless of the specific manner in which the user data was provided, the WD 22 may initiate, in an optional third substep, transmission of the user data to the host computer 24 (Block S124). In a fourth step of the method, the host computer 24 receives the user data transmitted from the WD 22, in accordance with the teachings of the embodiments described throughout this disclosure (Block S126). FIG.10 is a flowchart illustrating an example method implemented in a communication system, such as, for example, the communication system of FIG.5, in accordance with one embodiment. The communication system may include a host computer 24, a network node 16 and a WD 22, which may be those described with reference to FIGS.5 and 6. In an optional first step of the method, in accordance with the teachings of the embodiments described throughout this disclosure, the network node 16 receives user data from the WD 22 (Block S128). In an optional second step, the network node 16 initiates transmission of the received user data to the host computer 24 (Block S130). In a third step, the host computer 24 receives the user data carried in the transmission initiated by the network node 16 (Block S132). FIG.11 is a flowchart of an example process in a transmitter node (e.g., network node 16, wireless device 22, etc.) according to some embodiments of the present disclosure. One or more blocks described herein may be performed by, for example, one or more elements of network node 16 such as by one or more of processing circuitry 68 (including the MFSK unit 32), processor 70, radio interface 62 and/or communication interface 60. The transmitter node is configured to generate (Block S134) a multiple-frequency-shift keying, MFSK, signal having a continuous phase in a time domain by, at least in part, mapping a signal on a plurality of subcarriers per OFDM symbol, as described herien. The transmitter node is configured to perform (Block S136) OFDM modulations based on the generated MFSK signal, as described herein. The transmitter node is configured to cause (Blocks S138) transmission of an OFDM transmission based on the OFDM modulations, as described herein. According to one or more embodiments, the mapping of the signal on the plurality of subcarriers per OFDM symbol occurs before converting the signal from the frequency domain to the time domain (e.g., IFFT or IDFT). According to one or more embodiments, the transmitter node is further configured to perform phase shift compensation before the converting of the signal for each OFDM modulation, the phase shift compensation configured to, at least in part, configure the continuous phase in the time domain of the MFSK signal. According to one or more embodiments, the transmitter node is further configured to add a cyclic prefix, CP, to the OFDM transmission, the MFSK signal having a continuous phase in a time domain after the CP is added. According to one or more embodiments, the signal corresponds to an initial sequence of bits, ^^^^ _^^^^, and when an information binary of MFSK is 0, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� of the plurality of subcarriers where ^^^^ is the subcarrier index vector. According to one or more embodiments, when the information binary of MFSK is 1, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}� of the plurality of subcarriers. According to one or more embodiments, a subset of carriers are used for a guard band and have no power. According to one or more embodiments, the signal is a wake-up signal, WUS, and/or provides a wake-up indication. FIG.12 is a flowchart of an example process in a receiver node (e.g., wireless device 22, network node 16, etc.) according to some embodiments of the present disclosure. One or more blocks described herein may be performed by, for example, one or more elements of wireless device 22 such as by one or more of processing circuitry 84 (including the detection unit 34), processor 86, radio interface 82 (including a first receiver 82a (main receiver) and a second receiver 82b (e.g., WUR)) and/or communication interface 60. The receiver node is configured to receive (Block S140) receive an orthogonal frequency-division multiplexing, OFDM, transmission that is based on OFDM modulations where the OFDM modulations are based on a multiple- frequency-shift keying, MFSK, signal that has a continuous phase in a time domain based, at least in part, on a mapping of a signal on a plurality of subcarriers per OFDM symbol, as described herein. The receiver node is configured to perform (Block S142) at least one action based on the received signal, as described herein. According to one or more embodiments, the mapping of the signal on the plurality of subcarriers per OFDM symbol occurs before converting the signal from the frequency domain to the time domain. According to one or more embodiments, the OFDM transmission is based on phase shift compensation performed before the converting of the signal for each OFDM modulation where the phase shift compensation is configured to, at least in part, configure the continuous phase in the time domain of the MFSK signal. According to one or more embodiments, the OFDM transmission includes a cyclic prefix, CP. According to one or more embodiments, the signal corresponds to an initial sequence of bits, ^^^^ _^^^^, and when an information binary of MFSK is 0, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� of the plurality of subcarriers where ^^^^ is the subcarrier index vector. According to one or more embodiments, when the information binary of MFSK is 1, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}� of the plurality of subcarriers. According to one or more embodiments, a subset of carriers are used for a guard band and have no power. According to one or more embodiments, the receiver node includes a first receiver, and a second receiver that consumes less operational power than the first receiver where the second receiver is configured to wake-up the first receiver when a wake-up signal, WUS, is received, and where the second receiver is configured to receive OFDM transmission, and where the signal is a WUS and/or provides a wake-up indication. The at least one action includes causing the first receiver to wake up based on the received OFDM transmission. Having described the general process flow of arrangements of the disclosure and having provided examples of hardware and software arrangements for implementing the processes and functions of the disclosure, the sections below provide details and examples of arrangements for continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. Some embodiments provide continuous-phase multiple frequency-shift keying (MFSK) modulation in orthogonal frequency-division multiplexing (OFDM) systems. One or more network node 16 functions described below may be performed by one or more of processing circuitry 68, processor 70, MFSK unit 32, radio interface 62, etc. One or more wireless device 22 functions described below may be performed by one or more of processing circuitry 84, processor 86, radio interface 82 (e.g., first receiver 82a (e.g., WUR), second receiver 82b (e.g., main receiver)), etc. That is, one or more functions described below may be performed by a receiver node or transmitter node where the receiver node or transmitter node may be a network node 16, wireless device 22, non-3GPP based device or node, among other entities in system 10. In one or more embodiments, an OFDM-transmitter can generate a desired (or configured) time-domain modulation scheme such as FSK which can be detected by a low complexity receiver such as a low power wake-up radio (WUR). The generated signal can be used for various purposes including a wake-up signal, data transmissions, or any other indications. In one or more embodiments, the sequence is mapped on the subcarriers on the WUS band before OFDM modulation, how to generate a time-domain signal using an OFDM transmitter which is close to FSK modulation with continuous phase is described, minimizing the impact of OFDM transmitter (e.g., gNB) and inter-subcarrier interference is described, as well as minimizing the cyclic prefix (CP) impact on the complexity in the receiver. Table 1 is a list of some notations used herein. Table 1: FSK signal generation based on OFDM parameters. Parameter description Notation The size of IFFT ^^^^ _{^^^^ ^^^^ ^^^^ ^^^^} The time domain WUS signal after IFFT on i-th ^^^^ _^ ^{^} ^^{^^^} ^_{^ ^^^^ ^^^^} ( ^^^^) OFDM symbol T^{he size of CP in one WUS transmission ^^^^ ^^^^ ^^^^} = _{{ ^^^^1 , ^^^^2 ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ , … , ^^^^ ^^^^ ^^^^ }} The number of OFDM symbols in one WUS ^^^^ transmission The FSK modulation tone ^^^^ The number of subcarriers used per FSK symbol ^^^^ The number of subcarriers shift between two adjacent ∆ ^^^^ tones of FSK symbol The subcarrier index of the m-th tone FSK symbol ^^^^ _{^^^^, ^^^^} and the ^^^^-th subcarrier T^{he initial sequence which is a complex vector} _{^^^^ =} ^[ _{^^^^1, ^^^^2, … , ^^^^ ^^^^} ^] The complex sequence mapped on the ^^^^-th OFDM _{^^^^ ^^^^ =} ^[ _{^^^^ ^} ¹ ^_{^^ , ^^^^ ^} ² ^_{^^ , … , ^^^^ ^} ^{^} ^_^ ^{^} ^^{^^]} symbol before IFFT. The subcarrier index that ^^^^ _^^^^ mapped on the ^^^^-th ^^^^ _^^^^ = [ ^^^^ _^ ¹ ^_^^, ^^^^ _^ ² ^_^^, … , ^^^^ _^ ^{^} ^_^ ^{^} ^^{^^}] OFDM symbol before IFFT. T^{he phase shift vector of ^^^^ ^^^^+1 based on ^^^^ ^^^^ ∆Θ ^^^^} = _{[∆ ^^^^1 2 ^^^^ ^^^^ ,∆ ^^^^ ^^^^ , … ,∆ ^^^^ ^^^^ ]} To minimize the impact on the OFDM transmitter (e.g., gNB, network node, device, node), the FSK modulation reuses the OFDM modulation function and fully utilizes the bandwidth. The FSK generation system with an OFDM transmitter is shown in FIG.13. In FIG.13, a subset of available subcarriers are used for WUS based on FSK modulation, and other subcarriers are used for non-WUS OFDM-based transmissions. Symbols of WUS transmission ^^^^ = [ ^^^^₁, ^^^^₂, … , ^^^^ _^^^^] are mapped onto subcarriers within the WUS bandwidth. ^^^^ is a complex vector with the length of ^^^^, where ^^^^ is the number of subcarriers used per FSK symbol. Each element in ^^^^ is mapped on the subcarriers respectively before IFFT. To perform multiple frequency-shift keying (MFSK) modulation in a communication system, where ^^^^ is the size of the alphabet for frequency shifts/tones, the transmitter selects one tone at a time from the alphabet to use for MFSK symbol transmission. That is, each FSK symbol carries ^^^^ ^^^^ ^^^^2( ^^^^) information bits. However, to fully utilize the bandwidth in an OFDM system, each tone of MFSK can be mapped on a certain bandwidth which includes more than one subcarrier. Let ^^^^ be the number of subcarriers used for each tone of FSK symbol and ^^^^ ≥ 1. To avoid inter-symbol interference, a guard band (i.e., frequency separation) is included between two adjacent tones of FSK symbol. Let ∆ ^^^^ = ^^^^_{1, ^^^^} − ^^^^_{2, ^^^^} be the number of subcarriers shift (or separation) between two adjacent tones of FSK symbol. FIG.14 shows M-FSK modulation in frequency domain based on an OFDM system, with ^^^^ _{^^^^, ^^^^} being the index of subcarriers, ^^^^ = 1,2, … ^^^^, ^^^^ = 1,2, … ^^^^ . One or more embodiments are described using IFFT as an example. However, the one or more embodiments also apply to OFDM modulation/OFDM baseband signal generation that is used in, for example, NR. To perform FSK modulation, a complex sequence of length ^^^^ is mapped onto the certain subcarriers as inputs to the OFDM modulation. If the sequence is fixed, and mapping on certain subcarriers is based on what the bit(s) should be carried, the phase would be discontinuous after CP insertion. Take 2FSK as an example, where FIG.15 is a diagram of an example that the phase is not continuous after CP insertion in case of bit 0 and bit 1 are modulated on single- carrier or multi-carrier. One or more embodiments described herein provide a design where the sequence is mapped on the subcarriers per OFDM symbol before IFFT to make a continuous phase MFSK signal in time domain, thereby minimizing the impact of OFDM transmitter and reducing the complexity of receiver. One or more embodiments described herein can be applied to M-FSK modulation based on OFDM system when ^^^^ ≥ 2. For simplicity, ^^^^ = 2 is used as an example in the following discussions. Initial sequence The WUS signal before mapping to subcarrier is indicated by complex vector ^^^^ _^^^^, where ^^^^ is the index of OFDM symbols in one WUS transmission and ^^^^ = 1,2,3 … , ^^^^ , where ^^^^ is the number of OFDM symbols per WUS transmission. In one or more embodiments, ^^^^ is also equal to the number of the information bits carried by WUS if using 2FSK (i.e., ^^^^ = 2). The initial sequence is ^^^^₀ = ^[ ^^^^₀ ¹, ^^^^₀ ², … , ^^^^₀ ^{^^^^]}, which can be generated as many kinds of sequences such as zadoff chu (ZC), m-sequence, etc.. For example, this sequence could be related to sequences used for cell ID based on PSS/SSS. When a new WUS transmission starts, the WUS signal, before mapping to subcarrier ^^^^₁ = ^^^^₀. ^^^^₁, is mapped onto the corresponding subcarriers based on the FSK source bit is 0 or 1. FSK modulation based on OFDM system Define the subcarrier index vector ^^^^ _^^^^ = ^[ ^^^^ _^ ¹ ^_^^, ^^^^ _^ ² ^_^^, … , ^^^^ _^ ^{^} ^^{^} ^_^ ^{^^]} for the sequence ^^^^ _^^^^ mapped on the ^^^^-th OFDM symbol before Take 2-FSK, for example, there are two cases for ^^^^ _^^^^ : • Case1: ^^^^ _^ ^{^} ^^{^} ^^{^} ^^{^} = ^^^^_{1, ^^^^} when the WUS bit is 1 and should be carried on the ^^^^-th OFDM symbol. • Case2: ^^^^ _^ ^{^} ^^{^} ^^{^} ^^{^} = ^^^^_{2, ^^^^} when the WUS bit is 0 and should be carried on the ^^^^-th OFDM symbol. FIG.16 is a diagram of the sequence ^^^^ _^^^^ mapped on the subcarriers before IFFT on the ^^^^-th OFDM symbol. There is no power on the subcarriers which is used for guard band. When the information binary of FSK is 0 which should be carried on i-th OFDM symbol, sequence ^^^^ _^^^^ are mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� , and there is no power on subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}�. In the same way, when the information binary of FSK is 1 which should be carried on i-th OFDM symbol, sequence ^^^^ _^^^^ are mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}�, and there is no power on � ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}�. The time domain signal of WUS on the i-th OFDM symbol without

1 ^^^^ ^{2 ^^^^ ^^^^ ^^^^} ^^^^ ^_^^^ ^^^^ _^ ^{^} ^^{^^^} ^_{^ ^^^^ ^^^^ ( ^^^^) =} � ^^^^ ^^^^ ^^^^ ^^^^ ^{^^^^ ^^^^ ^^^^ ^^^^ ^^^^} , ^^^^ = 0,1, … , ^^ ^^^^ _^^^^ ^^ _{^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^} ^^^^=1 Where ^^^^ _{^^^^ ^^^^ ^^^^ ^^^^} is the size of IFFT. Complex sequence of each OFDM symbol Let the length of CP corresponding to WUS transmission spanning ^^^^ OFDM ^{symbols be denoted by a constant vector ^^^^ = { ^^^^1 , ^^^^2 , … , ^^^^ ^^^^ }, where ^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^ ^^^^ ^^^^ , ^^^^ ∈} _{{1,2, … , ^^^^} is the CP length of the ^^^^-th OFDM symbol in one WUS transmission.} One or more embodiments described herien proposes a new signal ^^^^ _^^^^+1 which is dependent on the previous value of ^^^^ _^^^^. Assume, ^^^^ _^^^^+1 = ^^^^ ^{^^^^∆Θ ^^^^+1}.∗ ^^^^ _^^^^ = ^^^^ ^{^^^^∆ ^^^^1 ^^^^+1} ^^^^ _^ ¹ ^_^^, ^^^^ ^{^^^^∆ ^^^^2 ^^^^+1} ^^^^ _^ ² ^_^^, … , ^^^^ ^{^^^^∆ ^^^^ ^^^^ ^^^^+1} ^^^^ _^ ^{^} ^_^ ^{^} ^^{^^} where ^^^^ _^^^^ = [ ^^^^

before IFFT on the i-th OFDM symbol. ∆_{Θ ^^^^+1 =} ^[ _{∆ ^^^^ ^} ¹ ^_{^^+1 ,∆ ^^^^ ^^} ² ^_{^+1 , … ,∆ ^^^^ ^^} ^{^} ^^{^} ^^{^} +^{^} 1 ^] _{is a vector with size of ^^^^. Each value} ∆ ^^^^ _^^ ^{^} ^^{^} ^^{^^} +₁ , ^^^^ ∈ {1,2, … ^^^^} is a phase shift based on the previous i-th OFDM symbol on the ^^^^- th element of the sequence. For the sequence on the first OFDM symbol ^^^^₁, there is an ^{initial phase shift based on the initial sequence ^^^^0, thus ∆Θ1 =} _{[∆ ^^^^1 2 ^^^^ ^^^^ 1,∆ ^^^^1, … ,∆ ^^^^1]where∆ ^^^^1 ∈ [0,2 ^^^^), ^^^^ ∈ {1,2, … ^^^^} and ∆ ^^^^ ^^^^ 1 can be any value.} ^^^^ _^^^^+1 = ^[ ^^^^ _^ ¹ ^_^^+1 , ^^^^ _^ ² ^_^^+1 , … , ^^^^ _^ ^{^} ^_^ ^{^} ^^{^} +^{^} 1 ^] is the result of element-wise multiplication between ^^^^ ^{^^^^∆Θ ^^^^+1} and ^^^^ _^^^^, that is ^^^^ _^ ^{^} ^^{^} ^^{^} ^^{^} +₁ = ^^^^ ^{^^^^∆ ^^^^ ^^^^ ^^^^+1} ∗ ^^^^ _^ ^{^} ^^{^} ^^{^} ^^{^}. To ensure the phase continuity between two adjacent OFDM symbols with added CP, the phase shift ∆Θ _^^^^+1 should meet the following condition, 2 ^{^^^^ ^^^^ ^^^^+1 ^ ^^^^} ^_{^^^ ^^^^ ^^^^} ^{^^^} _^^^^+1

for each OFDM modulation, continuous phase MFSK signal can be generated. Take 2FSK as an example where FIG.17 is a diagram of an example that shows that the phase is continuous after CP insertion in case of bit 0 and bit 1 are modulated on single-tone or multi-tone. Procedure of M-FSK modulation based on OFDM system One or more embodiments described herein proposes a method of phase- continuous M-FSK generation based on OFDM system. This method can be implemented with limited impact on OFDM transmitter and with no interference on other transmissions, and further provides the benefit of reducing the complexity and enhancing the M-FSK detection performance in the receiver. FIG.18 is a flowchart of steps for generating a M-FSK signal based on OFDM system where the M-FSK signal can be detected by a low complexity receiver. Step 1: Setup the subcarriers utilized ^^^^ _{^^^^, ^^^^}for each tone of FSK symbol, initial sequence ^^^^₀. Step 2: Calculate the phase shift Θ _^^^^ for the current i-th OFDM symbol. Step 3: Update the complex sequence ^^^^ _^^^^ for the current i-th OFDM symbol, ^^^^ _^^^^ = ^^^^ ^{^^^^Θ ^^^^} .∗ ^^^^ _^^^^−1

the complex sequence based on the FSK source bits that should be carried on the current i-th OFDM symbol. Step 5: Make OFDM modulation with other transmissions, add CP and transmit. Step 6: Repeat steps 2-5 on the next OFDM symbol until WUS transmission is finished. Hence, one or more embodiments, described herein provide one or more of the following advantages: 1) Low complexity for generating low complexity modulations such as FSK using an OFDM-based transmitter. 2) The methods for generating continuous phase M-FSK modulation based on OFDM system are presented which prevent spectrum spread and significantly reduce receiver complexity/power consumption for timing synchronization. 3) Efficient generation wake-up signals using an OFDM-based transmitter which can be received by a low complexity WUR receiver while maintaining orthogonality of OFDM-based transmissions. 4) Ensuring minimum impacts on transmitters (e.g., network node, gNB) for supporting wake-up radios. 5) Ensuring low complexity and efficient implementation. 6) Efficient use of WUR to maximize the power saving gain while maintaining the wireless device coverage in various deployment scenarios. 7) Network flexibility for properly employing WUR based on various requirements such coverage, energy efficiency, and latency. 8) The solutions can be considered as an enabler of battery-less (zero- energy) devices and energy harvesting operations towards 5G Advanced and 6G. As will be appreciated by one of skill in the art, the concepts described herein may be embodied as a method, data processing system, computer program product and/or computer storage media storing an executable computer program. Accordingly, the concepts described herein may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects all generally referred to herein as a “circuit” or “module.” Any process, step, action and/or functionality described herein may be performed by, and/or associated to, a corresponding module, which may be implemented in software and/or firmware and/or hardware. Furthermore, the disclosure may take the form of a computer program product on a tangible computer usable storage medium having computer program code embodied in the medium that can be executed by a computer. Any suitable tangible computer readable medium may be utilized including hard disks, CD- ROMs, electronic storage devices, optical storage devices, or magnetic storage devices. Some embodiments are described herein with reference to flowchart illustrations and/or block diagrams of methods, systems and computer program products. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer (to thereby create a special purpose computer), special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer program instructions may also be stored in a computer readable memory or storage medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. It is to be understood that the functions/acts noted in the blocks may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows. Computer program code for carrying out operations of the concepts described herein may be written in an object oriented programming language such as Python, Java® or C++. However, the computer program code for carrying out operations of the disclosure may also be written in conventional procedural programming languages, such as the "C" programming language. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer. In the latter scenario, the remote computer may be connected to the user's computer through a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). Many different embodiments have been disclosed herein, in connection with the above description and the drawings. It will be understood that it would be unduly repetitious and obfuscating to literally describe and illustrate every combination and subcombination of these embodiments. Accordingly, all embodiments can be combined in any way and/or combination, and the present specification, including the drawings, shall be construed to constitute a complete written description of all combinations and subcombinations of the embodiments described herein, and of the manner and process of making and using them, and shall support claims to any such combination or subcombination. It will be appreciated by persons skilled in the art that the embodiments described herein are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope of the following claims.

Claims

WHAT IS CLAIMED IS: 1. A transmitter node (16) configured to communicate with a receiver node (22), the transmitter node configured to: generate a multiple-frequency-shift keying, MFSK, signal having a continuous phase in a time domain by, at least in part, mapping a signal on a plurality of subcarriers per orthogonal frequency-division multiplexing, OFDM, symbol; perform OFDM modulations based on the generated MFSK signal; and transmit an OFDM transmission based on the OFDM modulations.

2. The transmitter node of Claim 1, wherein the mapping of the signal on the plurality of subcarriers per OFDM symbol occurs before converting the signal from the frequency domain to the time domain.

3. The transmitter node of Claim 2, wherein the transmitter node is further configured to: perform phase shift compensation before the converting of the signal for each OFDM modulation, the phase shift compensation configured to, at least in part, configure the continuous phase in the time domain of the MFSK signal.

4. The transmitter node of any one of Claims 1-3, wherein the transmitter node is further configured to add a cyclic prefix, CP, to the OFDM transmission, the MFSK signal having a continuous phase in a time domain after the CP is added.

5. The transmitter node of any one of Claims 1-4, wherein the signal corresponds to an initial sequence of bits, ^^^^ _^^^^; and when an information binary of

is 0, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� of the plurality of subcarriers where ^^^^ is the subcarrier index vector.

6. The transmitter node of Claim 5, wherein, when the information binary of MFSK is 1, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}� of the plurality of subcarriers.

7. The transmitter node of Claim 5 or 6, wherein the MFSK is a 2FSK.

8. The transmitter node of any one of Claims 5-7, wherein a subset of carriers are used for a guard band and have no power.

9. The transmitter node of any one of Claims 1-8, wherein the signal is a wake-up signal, WUS, and/or provides a wake-up indication.

10. A receiver node (22) configured to communicate with a transmitter node, the receiver node configured to: receive an orthogonal frequency-division multiplexing, OFDM, transmission that is based on OFDM modulations, the OFDM modulations being based on a multiple-frequency-shift keying, MFSK, signal that has a continuous phase in a time domain based, at least in part, on a mapping of a signal on a plurality of subcarriers per OFDM symbol; and perform at least one action based on the received OFDM transmission.

11. The receiver node of Claim 10, wherein the mapping of the signal on the plurality of subcarriers per OFDM symbol occurs before converting the signal from the frequency domain to the time domain.

12. The receiver node of Claim 11, wherein the OFDM transmission is based on phase shift compensation performed before the converting of the signal for each OFDM modulation, the phase shift compensation configured to, at least in part, configure the continuous phase in the time domain of the MFSK signal.

13. The receiver node of any one of Claims 10-12, wherein the OFDM transmission includes a cyclic prefix, CP.

14. The receiver node of any one of Claims 10-13, wherein the signal corresponds to an initial sequence of bits, ^^^^ _^^^^; and when an information binary of MFSK is 0, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� of the plurality of subcarriers where ^^^^ is the subcarrier index vector.

15. The receiver node of Claim 14, wherein, when the information binary of MFSK is 1, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}� of the plurality of subcarriers.

16. The receiver node of Claim 14 or 15, wherein the MFSK is a 2FSK. 17. The receiver node of any one of Claims 14-16, wherein a subset of carriers are used for a guard band and have no power. 18. The receiver node of any one of Claims 10-17, further comprising: a first receiver; a second receiver that consumes less operational power than the first receiver, the second receiver being configured to wake-up the first receiver when a wake-up signal, WUS, is received, the second receiver being configured to receive OFDM transmission, the signal being a WUS and/or provides a wake-up indication; and the at least one action including causing the first receiver to wake up based on the received OFDM transmission. 19. A method implemented by a transmitter node that is configured to communicate with a receiver node, the method comprising: generating (S134) a multiple-frequency-shift keying, MFSK, signal having a continuous phase in a time domain by, at least in part, mapping a signal on a plurality of subcarriers per OFDM symbol; performing (S136) OFDM modulations based on the generated MFSK signal; and transmitting of an OFDM transmission based on the OFDM modulations. 20. The method of Claim 19, wherein the mapping of the signal on the plurality of subcarriers per OFDM symbol occurs before converting the signal from the frequency domain to the time domain. 21. The method of Claim 20, further comprising: performing phase shift compensation before the converting of the signal for each OFDM modulation, the phase shift compensation configured to, at least in part, configure the continuous phase in the time domain of the MFSK signal. 22. The method of any one of Claims 19-21, further comprising adding a cyclic prefix, CP, to the OFDM transmission, the MFSK signal having a continuous phase in a time domain after the CP is added. 23. The method of any one of Claims 19-22, wherein the signal corresponds to an initial sequence of bits, ^^^^ _^^^^; and when an information

of MFSK is 0, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� of the plurality of subcarriers where ^^^^ is the subcarrier index vector. 24. The method of Claim 23, wherein, when the information binary of MFSK is 1, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}� of the plurality of subcarriers. 25. The method of any of Claims 23-24 wherein the MFSK is a 2FSK. 26. The method of any one of Claims 23-25, wherein a subset of carriers are used for a guard band and have no power. 27. The method of any one of Claims 19-26, wherein the signal is a wake-up signal, WUS, and/or provides a wake-up indication. 28. A method implemented in a receiver node that is configured to communicate with a transmitter node, the method comprising: receiving (S140) an orthogonal frequency-division multiplexing, OFDM, transmission that is based on OFDM modulations, the OFDM modulations being based on a multiple-frequency-shift keying, MFSK, signal that has a continuous phase in a time domain based, based at least in part, on a mapping of a signal on a plurality of subcarriers per OFDM symbol; and performing (S142) at least one action based on the received OFDM transmission. 29. The method of Claim 28, wherein the mapping of the signal on the plurality of subcarriers per OFDM symbol occurs before converting the signal from the frequency domain to the time domain. 30. The method of Claim 29, wherein the OFDM transmission is based on phase shift compensation performed before the converting of the signal for each OFDM modulation, the phase shift compensation configured to, at least in part, configure the continuous phase in the time domain of the MFSK signal. 31. The method of any one of Claims 28-30, wherein the OFDM transmission includes a cyclic prefix, CP. 32. The method of any one of Claims 28-31, wherein the signal corresponds to an initial sequence of bits, ^^^^ _^^^^; and when an information

of MFSK is 0, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_1,1, ^^^^_1,2, … , ^^^^_{1, ^^^^}� of the plurality of subcarriers where ^^^^ is the subcarrier index vector. 33. The method of Claim 32, wherein, when the information binary of MFSK is 1, sequence ^^^^ _^^^^ is mapped onto subcarriers� ^^^^_2,1, ^^^^_2,2, … , ^^^^_{2, ^^^^}� of the plurality of subcarriers.

34. The method of any of Claims 32-33 wherein the MFSK is a 2FSK. 35. The method of any one of Claims 32-34, wherein a subset of carriers are used for a guard band and have no power. 36. The method of any one of Claims 28-35, wherein the receiver node includes: a first receiver; a second receiver that consumes less operational power than the first receiver, the second receiver being configured to wake-up the first receiver when a wake-up signal, WUS, is received, the second receiver being configured to receive OFDM transmission, the signal being a WUS and/or provides a wake-up indication; and the at least one action including causing the first receiver to wake up based on the received OFDM transmission.