WO2017215772A1

WO2017215772A1 - Microwave radio link chain employing constructive interference

Info

Publication number: WO2017215772A1
Application number: PCT/EP2016/080973
Authority: WO
Inventors: Bengt-Erik Olsson; Per-Arne Thorsén
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2016-06-13
Filing date: 2016-12-14
Publication date: 2017-12-21

Abstract

A wireless communication system comprising first, second, and third serially connected communication devices, the second communication device arranged to receive a first communication signal, and to transmit a second communication signal comprising at least part of the first communication signal to the third communication device. The third communication device is arranged to generate and transmit a feedback 5 signal to the second communications device indicating a phase difference between the first and the second communication signal at the third communication device. The second communication device is arranged to receive the feedback signal and to, based on the feedback signal, apply a phase shift to the second communication signal to improve a reception condition of the second communication signal at the third communication device.

Description

M ICROWAVE RADIO LINK CHAIN EMPLOYING CONSTRUCTIVE INTERFERENCE

TECHNICAL FIELD

The present disclosure relates to wireless communication systems, and in particular to a system of microwave transceivers arranged in series to form a chain of microwave radio links.

BACKGROUND

A microwave radio link or radio link system is a communication system that transmits data between two fixed locations over a point-to-point radio link, often via highly directive antennas and in line-of-sight transmission conditions. A microwave radio link transmitter and receiver are often incorporated into one unit, herein denoted as microwave transceiver. Microwave transceivers operate at high carrier frequency, i.e., on the order of tens of GHz.

To provide connectivity over longer distances, or to provide connectivity to multiple locations along some path, several radio links may be arranged in series. The microwave transceivers are then said to act as repeaters and to implement a method for repeating a wireless transmission.

Microwave repeater transceivers are known, see, e.g., US 3,018,370.

Systems of serially connected microwave radio links are also known, see, e.g., WO 2014/194945, where a microwave radio link chain comprising microwave repeaters is disclosed.

Interference and signal distortion degrade reception conditions during wireless communication. Interference between different transmissions along a chain of microwave radio links can be reduced by using different frequency bands for re-transmissions along the chain and applying filtering techniques to recover specific signal transmissions by extracting signals received in a given frequency band. However, this requires occupying multiple frequency bands, which is a drawback since frequency is often a limited resource. WO 2014/194945 discloses a system where re-transmissions along the chain occupy the same frequency band thus avoiding the use of different frequency bands. In WO 2014/194945, interference due to hardware impairments in microwave repeaters is reduced by each repeater inverting the spectrum of received signals prior to re-transmission, thus improving reception conditions along the chain. SUMMARY

An object of the present disclosure is to provide wireless communication devices, systems, and methods which provide improved reception conditions of wireless communication signals in serially connected microwave radio link communication systems. This object is obtained by a wireless communication system comprising first, second, and third serially connected communication devices. The first communication device is arranged to transmit a first communication signal to the second communications device. The second communication device is arranged to receive the first communication signal, and to transmit a second communication signal comprising at least part of the first communication signal to the third communication device. The third communication device is arranged to receive the second communication signal, and to also receive an interference signal from the first communication device comprising the first communication signal. The third communication device is further arranged to generate and to transmit a feedback signal to the second communications device indicating a phase difference between the first and the second communication signal received at the third communication device. The second communication device is arranged to receive the feedback signal and to, based on the feedback signal, apply a phase shift to the second communication signal to obtain constructive interference between the first and second communication signal at the third communication device.

Since constructive interference is obtained due to the applied phase shift, the received signal strength at the third communications device increases. Thus, improved reception conditions in the serially connected microwave radio link communication system are obtained, and more reliable communication links are obtained over longer distances and/or at reduced output powers.

According to aspects, the second communication device is arranged to down-convert the received first communication signal in frequency by mixing with a signal from a local oscillator, and to up-convert the second communication signal in frequency by mixing with a second signal from said local oscillator. Hereby, since the same local oscillator is used for both down- and up-conversion, phase distortion at the second communication device will not affect the interference state at the third communications device, i.e., phase distortion from, e.g., phase noise at the local oscillator, will not affect whether constructive or destructive interference occurs at the third communication device.

According to aspects, the second communication device is attached to an aerial vehicle, or to an autonomous aerial vehicle, or to an aerial drone. Hereby improved communication conditions are obtained in a system comprising an aerial vehicle, or an autonomous aerial vehicle, or an aerial drone

According to aspects, the second communication device is attached to a vehicle for road transport.

Hereby improved communication conditions are obtained in a system comprising a vehicle for road transport.

According to aspects, the second communication device comprises a remote radio head, or a small-cell radio base station arranged for communication with further wireless devices.

Hereby improved communication conditions are obtained in a system comprising a remote radio head, or a small-cell radio base station arranged for communication with further wireless devices. The object is also obtained by a wireless communication device for connection in series between a first and a third communication device. The wireless communication device is arranged to receive a first communication signal via a first antenna port from the first communication device, and to transmit a second communication signal comprising at least part of the first communication signal to the third communication device via a second antenna port. The wireless communication device comprises a control module arranged to receive feedback data from the third communication device and to generate a control signal for adjusting a phase of the second communication signal to improve a reception condition of the second communication signal at the third communication device. The wireless communication device comprises a phase shift module arranged to receive the control signal, and to apply a phase shift to the second communication signal based on the control signal. By applying said phase shift, communication conditions are improved at the third communication device since constructive interference between the first and the second communication signal is obtained. This way more reliable communication is obtained over longer distances and/or at reduced output powers.

According to aspects, the wireless communication device is arranged to down-convert the received first communication signal in frequency by mixing with a signal from a local oscillator, and to up-convert the second communication signal in frequency by mixing with a signal from said local oscillator.

Hereby phase distortion introduced by the wireless communication device when processing the received first communication signal is compensated for, and will not affect the interference state at the third communications device, i.e., phase distortion from, e.g., phase noise at the local oscillator, will not affect whether constructive or destructive interference occurs at the third communication device. This way less costly components, e.g., local oscillators, can be used in designing the wireless communication device. According to aspects, the first and the second communication signal each comprises first and second pilot symbols. The first pilot symbol in the second communication signal is adjusted in phase in a first direction relative to the first pilot symbol in the first communication signal, and the second pilot symbol in the second communication signal is adjusted in phase in a direction opposite to the first direction relative to the second pilot symbol in the first communication signal.

Hereby a way of reliably determining the control signal is provided.

According to aspects, the wireless communication device is arranged to receive a feedback signal comprising the feedback data. The feedback data comprises information about which pilot symbol out of the first and the second pilot symbol that was received under best reception conditions. Hereby a way of reliably communicating the feedback data is provided.

According to aspects, the first and the second communication signal comprises different identification symbols.

Hereby an alternative or complementary way of reliably determining the control signal is provided.

According to aspects, the wireless communication device is arranged to receive a feedback signal comprising the feedback data. The feedback data comprises information about the relative phase difference with which the different identification symbols was received at the further wireless device.

Hereby an alternative or complementary way of reliably communicating the feedback data is provided.

According to aspects, the wireless communication device is arranged to receive a reset signal. The control signal is arranged to control a phase shift that varies periodically, at a first frequency, around an operating point. The periodical variation is arranged to be reset in response to receiving the reset signal.

According to aspects, the wireless communication is arranged to receive a feedback signal comprising the feedback data. The feedback data comprises information about a phase of an envelope variation of the second communication signal relative to the reset signal. Hereby an alternative or complementary way of reliably determining the control signal is provided.

The object is also obtained by a wireless communication device for connection in series with a first and a second communication device. The wireless communication device is arranged to receive a second communication signal from the second communication device comprising a first communication signal, and to also receive an interference signal from the first communication device comprising the first communication signal. The wireless communication device is arranged to generate and to transmit a feedback signal to the second communications device indicating a phase difference between the first and the second communication signal received at the third communication device.

By generating and transmitting said feedback signal, the wireless communication device enables the second communication device to apply a suitable phase shift to the second communication signal in order to improve communication conditions at the wireless communication device. This way more reliable communication is obtained over longer distances and/or at reduced output powers.

The object is also obtained by a method in a wireless communication system comprising first, second, and third serially connected communication devices. The method comprises transmitting a first communication signal from the first communication device to the second communications device, receiving the first communication signal by the second communication device, and transmitting a second communication signal comprising at least part of the first communication signal to the third communication device. The method also comprises receiving the second communication signal and also an interference signal comprising the first communication signal by the third communication device, generating and transmitting a feedback signal from the third communication device to the second communications device, the feedback signal indicating a phase difference between the first and the second communication signal at the third communication device. The method further comprises receiving the feedback signal by the second communication device and, based on the feedback signal, applying a phase shift to the second communication signal to improve a reception condition of the second communication signal at the third communication device.

The object is also obtained by a method in a wireless communication device for serial connection with further communication devices. The method comprises receiving a first communication signal via a first antenna port from a first communication device, transmitting a second communication signal comprising at least part of the first communication signal to a third communication device via a second antenna port, receiving, by a control module, feedback data from the third communication device and generating a control signal for adjusting a phase of the second communication signal to improve a reception condition of the second communication signal at the third communication device, and also applying, by a phase shift module, a phase shift to the second communication signal based on the control signal.

The object is also obtained by a method in a wireless communication device for connection in series with a first and a second communication device. The method comprises receiving a second communication signal from the second communication device comprising a first communication signal, receiving an interference signal from the first communication device comprising the first communication signal, generating and transmitting a feedback signal to the second communications device indicating a phase difference between the first and the second communication signal received at the third communication device for applying a phase shift to the second communication signal prior to transmission. There is also provided herein computer programs comprising computer program code which, when executed in a microwave radio transceiver, causes the microwave radio transceiver, to execute methods according to the present disclosure.

The computer programs and the methods provide advantages corresponding to the advantages already described in relation to the various wireless communication devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features, and advantages of the present disclosure will appear from the following detailed description, wherein some aspects of the disclosure will be described in more detail with reference to the accompanying drawings, in which: Figure 1 is a block diagram schematically illustrating a wireless communication system.

Figure 2 is a block diagram schematically illustrating a wireless communication device.

Figures 3 and 4 are block diagrams schematically illustrating wireless communication systems.

Figures 5a-5c are block diagrams schematically illustrating wireless communication devices.

Figures 6a and 6b illustrate wireless communication signals in a wireless communication system. Figure 7 is a signaling diagram illustrating phase control in a wireless communication system.

Figures 8a-8b illustrate wireless communication signals in a wireless communication system.

Figure 9 is a signaling diagram illustrating phase control in a wireless communication system.

Figure 10a is a block diagram schematically illustrating a wireless communication system.

Figure 10b is a signaling diagram illustrating phase control in a wireless communication system. Figures 11 -13 are block diagrams schematically illustrating applications of wireless communication systems.

Figures 14-16 are flowcharts illustrating methods according to aspects of the disclosure. DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments and aspects of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional. The present teaching is based on the understanding that two identical waveforms arriving at a receiving antenna with the same phase or with similar phases are subject to constructive interference, i.e., the two waveforms combine into a single waveform with increased power, while the same two waveforms arriving at the receiving antenna with opposite phases are subject to destructive interference, i.e., the two waveforms combine into a single waveform with reduced power. Various levels of constructive and destructive interference ensue as relative phase difference between waveforms is varied from zero to 180 degrees.

Figure 1 is a block diagram schematically illustrating a wireless communication system 100. The communication system comprises microwave transceivers 110, 120a, 120b, 130 deployed at different locations A, B, C, D along some path 160. Thus, the system of serially connected microwave radio links provide connectivity along the path 160 between locations A, B, C, and D. The chain is terminated at end points A and D by the leftmost 110 and rightmost 130 transceivers.

The type of serial connection, or chain, of microwave radio links illustrated in Figure 1 can be used in a wide variety of different applications. For instance, microwave radio links can be used to link remote radio heads, or small cell access points, to a central radio base station. This example and other example applications of the present teaching will be discussed below in connection to Figures 11 -13.

Communication links 140, 141 , 142 may according to prior art occupy different frequency bands, in which case interference from neighboring transmissions can be filtered out using, e.g., passive filters having a pass-band at a given frequency band of interest. However, if transmissions 140, 141 , 142 are in the same frequency band, then interference may arise, shown here by dashed lines 150. The different receivers will then not just receive the transmission from the neighboring transceivers, but also from other transceivers in the wireless communication system 100. If the different transmissions are identical, or comprise frequency band portions which are identical, destructive or constructive interference may occur in some frequency bands depending on the phase relationship between the different signals at the receiving antennas. As discussed above, in case two identical transmissions arrive in phase at a receiving antenna, the received combination of transmissions will have increased amplitude, i.e., will be received with higher power. In case the two identical transmissions arrive with opposite phases at a receiving antenna, the received combination will have decreased amplitude, i.e., will be received with reduced power. Thus, best reception conditions are obtained when identical waveforms arrive in phase at a receiving antenna, generating constructive interference as opposed to the destructive interference which occurs when signals with phases that differ more than 90 degrees arrive at a receiver. The relative phase difference between waveforms arriving at a receiving antenna is determined by their respective phases at transmission, by the communication medium, and also by the path distance from transmitting antenna to receiving antenna. Phase is also affected by reflection and diffraction effects, but such effects will not be discussed in detail here.

The present teaching is based on the realization that one can adjust the transmitted phase of one or both waveforms prior to transmission in a chain of microwave radio links, based on feedback data reported back from a receiver of the waveforms to one or both transmitters, in order to generate constructive interference at the receiving antenna. Constructive interference occurs when signals arrive at a receiving antenna with relative phase difference magnitude below 90 degrees, i.e., as long as the relative phase difference is between -90 and +90 degrees. According to some aspects, it is an object of the present technique to maintain the relative phase difference of received waveforms at a third receiver between -90 and +90 degrees.

According to some aspects, it is an object of the present technique to minimize the relative phase difference of received waveforms at a third receiver.

Advantageously, processing in order to re-transmit a received communication signal along the chain involves down-conversion and up-conversion in frequency using a shared local oscillator. Such frequency conversion will not introduce differential phase noise into the re-transmission since phase noise incurred during down-conversion is effectively compensated for during up-conversion.

Figure 3 is a block diagram schematically illustrating a wireless communication system 300 according to some aspects of the present disclosure. The wireless communication system 300 comprises first 310, second 320, and third 330 serially connected communication devices, i.e., wireless devices forming a chain of communication devices, meaning that a communication path is formed in a first direction from the first communication device 310 on the left, via the second communication device 320 in the middle to the third communication device 330 on the right. Communication links between the different devices can be two-way, meaning that each device can both transmit to and receive signals from a neighboring device. This way a communication path is also formed in a second direction from the third communication device 330 on the right, via the second communication device 320 in the middle to the first communication device 310 on the left.

According to aspects, the communication chain comprises more than three serially connected communication devices. In this case interference from additional devices can be experienced. However, interference is often dominated by the interference from the closest transmitters. The first communication device 310 is arranged to transmit a first communication signal 311 to the second communications device 320. The second communication device 320 is arranged to receive the first communication signal, and to transmit a second communication signal 321 comprising at least part of the first communication signal to the third communication device 330. Thus, the second communication device re-transmits or repeats at least part of the transmission of the first communication device. This way the second communication device acts as repeater, and provides a repeater function, and so extends the reach of the transmission of the first communication device 310. Information carried by the first communication signal can be tapped and processed by the second communication device.

A communication signal is, according to aspects, a wirelessly transmitted signal arranged to carry information. An example of a communication signal is a payload signal for a cellular communication system, such as a Long Term Evolution (LTE) communication system. Another example is a quadrature amplitude modulated (QAM) signal. A further example is an orthogonal frequency division multiplexed (OFDM) signal. Another example of a communication signal is a control signal used to control machinery in, e.g., a factory. This way control signaling for machinery deployed along a production line can be provided for.

The communication links 311 , 321 , 331 between communication devices share a frequency band, or share part of a frequency band, so if a signal transmission from the left-most first device 310 reaches the rightmost third device 330, interference will occur in this shared frequency band or shared part of a frequency band. This interfering transmission, or interference signal, is shown as a dashed line 312 in Figure 3. The third communication device 330 is arranged to receive the second communication signal 321 , and to also receive this interference signal 312 from the first communication device comprising the first communication signal. The third communication device 330 is arranged to generate and to transmit a feedback signal 331 to the second communications device indicating a phase difference between the first and the second communication signal received at the third communication device. There are many different ways in which this feedback signal may be generated, some of which will be discussed in detail below. There also exist different ways in which the third communications device 330 may indicate said phase difference, i.e., by indicating the sign of a determined phase difference, by indicating the actual value of the phase difference, or by indicating the reception condition resulting from the phase difference. It is appreciated that indicating the reception condition by, e.g., communicating a received signal strength or mean-squared error (MSE) value derived from symbol detection serves to indicate phase difference since the second communications device may try different applied phase shifts and see which applied phase shift resulted in the best reception condition. Having indicated the phase difference between the first and the second communication signal, it becomes possible for a receiver of the feedback signal to adjust a phase of either the first or the second communication signal in order to obtain constructive interference at the receiving antenna of the third communications device. Hereby reception conditions, or communication conditions are improved at the third communication device. Alternatively, or additionally, the third communication device 330 is arranged to generate and to transmit a feedback signal 331 to the second communications device indicating a state of interference between the first and the second communication signal received at the third communication device. I.e., the indicated state of interference can be indicated as constructive or as destructive, or indicate the level of constructive interference on a scale from maximum destructive interference to maximum constructive interference. Having indicated the state of interference between the first and the second communication signal, it becomes possible for a receiver of the feedback signal to adjust a phase of either the first or the second communication signal in order to obtain constructive interference, or to increase the level of constructive interference, at the receiving antenna of the third communications device.

According to some aspects, the second communication device is arranged to receive the feedback signal and to, based on the feedback signal, apply a phase shift 322 to the second communication signal to obtain constructive interference between the first and second communication signal at the third communication device.

According to another aspect, the first communication device is arranged to receive the feedback signal and to, based on the feedback signal, generate the second communication signal by applying a phase shift to the first communication signal to obtain constructive interference between the first and second communication signal at the third communication device. The phase shift is applied at the second communications device 320 by means of a phase shift module 322 arranged between a receiving antenna that receives the first communication signal and a transmitting antenna that transmits the second communication signal. The phase shift module is controlled by a control module 323. I.e., the control module determines which value of phase shift to apply at any given point in time based on a preconfigured initial value followed by phase shift values determined based on the received feedback signal. The control module may either output discrete control values for the phase shift module or may output a continuous control signal. The control signal is, according to aspects an analog signal, and according to other aspects a digital signal.

Regarding the rightmost or third communication device 330, this device is a wireless communication device 330 for connection in series with a first 310 and with a second 320 communication device. The wireless communication device 330 is arranged to receive a second communication signal 321 from the second communication device comprising a first communication signal, and to also receive an interference signal 312 from the first communication device comprising the first communication signal. The wireless communication device 330 is arranged to generate and to transmit a feedback signal 331 to the second communications device indicating a phase difference between the first and the second communication signal received at the third communication device.

The first and the second communication signals may, according to some aspects, be identical, i.e., comprise identical payload data parts. In this case the second communications device acts as a repeater which retransmits the received first communication signal as is. However, according to some other aspects, the first communication signal comprises a payload data part, and wherein the second communication signal comprises the payload data part and also a further payload data part. This means that the second communications device re-transmits the signal received from the first communication device, and so acts as a repeater, but also adds some content of its own in the transmission of the second communication signal . According to aspects this additional content is added in a frequency band different from that occupied by the first communication signal.

It is appreciated that, in order to avoid interference in the case where only parts of the payload signals are identical, it is possible to either add additional signal content to the transmission along the chain, or to subtract signal, but not to both remove and add signal content in the same frequency band.

Herein, the term payload data refers generally to any data or information quantity carried by the communication signal between communication devices. Payload data may comprise, e.g., LTE data channels, or a QAM modulated information bearing signal, or a control signal for controlling, e.g., machinery of some sort.

According to some aspects the received feedback signal 331 comprises a request to increase or to decrease the applied phase shift. The receiver of the feedback signal, which in Figure 3 is the second communication device 320, may then apply a phase shift accordingly, i.e., increase or decrease the applied phase shifts according to the last received instruction and with some pre-determined or adaptable step-size.

According to some other aspects the received feedback signal comprises an indication of a phase difference. The receiver of the feedback signal, which in Figure 3 is the second communication device 320, may then apply a phase shift accordingly, i.e., change the currently applied phase shift to compensate for the indicated phase difference. Thus constructive interference between the first and second communication signals may be obtained at the third communications device. For example, the feedback data may comprise information that the relative phase difference as measured is now 20 degrees. The control module 323 can then change or adapt the control signal to decrease the applied phase shift by 20 degrees, which would improve reception conditions at the third communication device. In some cases, the indication of phase difference comprised in the feedback signal 331 comprises noise or is otherwise distorted. Such noise or distortion may be reduced by filtering. Thus, according to some aspects, the second communication device 320 comprises a control module arranged to filter feedback data comprised in the feedback signal prior to generating a control signal for applying a phase shift. The applied filter is, according to different aspects, a Kalman filter, a Wiener filter, or a low-pass filter arranged to suppress high frequency noise components in the feedback data.

Figure 4 is a block diagram schematically illustrating a wireless communication system according to aspects of the present disclosure. If Figure 4, the second communication device 420 is arranged to down-convert the received first communication signal in frequency by mixing 422a with a signal from a local oscillator 421 , and to up-convert the second communication signal in frequency by mixing 422b with a second signal from said local oscillator 421.

The fact that the two mixers 422a and 422b use signals from the same local oscillator 421 , or signals at least partly derived from the same local oscillator signal, means that the two signals will exhibit the same or at least correlated phase noise. Thus, the relative phase difference between the interference signal 312 comprising the first communication signal and the second communication signal 321 at the third communication device 330 will not be affected by the frequency conversion at the second communication device, since mixing is done using signals from, or derived from, the same local oscillator. Aspects of the present disclosure relates in particular to the second communication device. Figure 2 is a block diagram that schematically illustrates this wireless communication device 220 which can be used in the wireless communication system 300 discussed in connection to Figure 3, as the second communication device 320, 420. Figure 2 shows a wireless communication device 220 for connection in series between a first and a third communication device. The wireless communication device is arranged to receive a first communication signal 211 via a first antenna port 221 from the first communication device, and to transmit a second communication signal 223 comprising at least part of the first communication signal to the third communication device via a second antenna port 222. It is appreciated that the wireless communication device 220 according to some aspects, does not comprise antenna elements per se, but comprises antenna ports for connecting the communication device to external antenna elements, such as illustrated in, e.g., Figure 3.

The wireless communication device 220 comprises a control module 225 arranged to receive feedback data from the third communication device and to generate a control signal 226 for adjusting a phase of the second communication signal to improve a reception condition of the second communication signal at the third communication device.

As discussed above, the reception conditions, or equivalently communication conditions, can be measured in many different ways. Some examples of reception condition measurements, include received signal power, received signal strength, error vector magnitude (EVM) of signal detection, or a mean-squared error (MSE) measure of signal detection.

Also, as discussed above, the feedback data may take different forms, some of which will be discussed in detail below. There are also many different ways in which the third communications device 330 may indicate said phase difference in its feedback to the wireless communications device 220, i.e., by indicating the sign of the phase difference, by indicating the actual value of the phase difference, or by indicating the reception condition as discussed above. Indicating the reception condition by, e.g., communicating a received signal strength or mean-squared error value from symbol detection serves to indicate phase difference since the second communications device may try different applied phase shifts and see which applied phase shift results in the best reception condition.

The wireless communication device also comprises a phase shift module 227 arranged to receive the control signal 226, and to apply a phase shift to the second communication signal based on the control signal. Thus, having received the feedback data and processed it by the control module 225, the wireless communication device 220 can adjust the phase of the second communication signal by the phase shift module 227 in order to obtain constructive interference at the receiving antenna of the third communications device. Figures 5a-5c are block diagrams schematically illustrating wireless communication devices, and in particular illustrates further details related to the phase shift module 227. The phase shift module, according to some aspects, implements a digital rotation of the signal. The phase shift module, according to some other aspects, implements an analog phase shift operation, such as a variable delay operation, resulting in a phase shift. Figure 5a shows a wireless communication device 510 arranged to receive a signal on a first antenna port 221 , and to apply a phase shift to the received signal by a phase shift module 227, and to transmit the phase shifted signal on a second antenna port 222.

The phase shift module 227 is, according to different aspects, implemented in digital domain as a complex multiplication, matrix operation, vector rotation or equivalent. The phase shift module 227 is, according to different aspects, implemented in analog domain as a variable delay or filter module which can be configured to provide a phase shift to an input signal in response to a control signal.

The phase shift operation can be implemented at baseband frequency, at an intermediate frequency (IF), or at a radio frequency (RF). Figure 5b shows a wireless communication device 520 arranged to down-convert the received first communication signal in frequency by mixing 422a with a signal from a local oscillator 421 , and to up- convert the second communication signal in frequency by mixing 422b with a signal from said local oscillator 421. Thus, the same local oscillator is used for both down-conversion and for up-conversion.

As discussed in connection to Figure 4, the fact that the two mixers 422a and 422b use signals from the same local oscillator, or signals derived from the same local oscillator signal, means that the two signals will exhibit the same or at least highly correlated phase noise. Thus, the relative phase difference between the interference signal 312 comprising the first communication signal and the second communication signal 321 at the third communication device 330 will not be affected by the frequency conversion at the second communication device, since mixing is done using signals from, or derived from, the same local oscillator. Figure 5b shows an example of the phase shift module implemented by a processing element PROC operating at baseband or at intermediate frequency, i.e., between down-conversion and up-conversion by mixers 422a, 422b.

Figure 5c illustrates examples of different alternative or complementary implementations, shown with dashed lines, of the phase shift module discussed above.

One such example relates to a wireless communication device 530 wherein the phase shift module 523 is arranged to apply the phase shift to the second communication signal at a baseband or at an intermediate frequency. In this case the phase shift module is comprised in a baseband processing module 423 located after down-conversion in frequency by mixer 422a and before up-conversion in frequency by mixer 422b. Another example comprises a phase shift module 531 , 532 which, according to some aspects, is arranged to apply the phase shift to the second communication signal by adjusting the phase of the signal from the local oscillator 421. In this case the phase shift module is located in connection to an output signal of the local oscillator 421. A phase shift of this oscillator output signal will translate into a phase shift of the communication signal when mixing to either down-convert or up-convert in frequency. Yet another example comprises a phase shift module 533, 534 which, according to some aspects, is arranged to apply the phase shift to the received first communication signal at radio frequency, or to the second communication signal at radio frequency prior to transmission. In this case the phase shift module 533 is located before down-conversion in frequency, or the phase shift module 534 is located after up- conversion in frequency. Figures 6a and 6b illustrate wireless communication signals in a wireless communication system, in particular the above-mentioned first and second communication signals.

The first and the second communication signal 610 each comprises first S1 and second S2 pilot symbols. The first pilot symbol in the second communication signal is adjusted in phase in a first direction relative to the first pilot symbol in the first communication signal, and the second pilot symbol in the second communication signal is adjusted in phase in a direction opposite to the first direction relative to the second pilot symbol in the first communication signal.

Alternatively, the first pilot symbol in the first communication signal is adjusted in phase in a first direction relative to the first pilot symbol in the second communication signal, and the second pilot symbol in the first communication signal is adjusted in phase in a direction opposite to the first direction relative to the second pilot symbol in the second communication signal. This way a receiver of the two waveforms is able to infer and compare communication condition, such as, e.g., received signal strength or detection error, when receiving the two pilot symbols. In case the first pilot symbol is received in better conditions than the second, a phase shift in the first direction is suitable in order to improve overall reception conditions. In case the second pilot symbol is received in better conditions than the first, a phase shift in the second direction is suitable in order to improve overall reception conditions.

Of course, noise and distortion may cause one pilot symbol to be received with better communication conditions than the other, despite having a phase shift in a direction which causes less constructive interference or even destructive interference. Filtering techniques are, according to some aspects, applied in order to alleviate such effects. Figure 6b illustrates the feedback signal 620 comprising feedback data. The wireless communication device discussed above is in this example arranged to receive a feedback signal 620 comprising feedback data, wherein the feedback data comprises information about which pilot symbol out of the first and the second pilot symbol that was received under best reception conditions.

In this particular example illustration, the feedback data comprises an indication S1/S2 of which pilot symbol that was received with best communication condition among the first and the second pilot symbol, i.e., was received with highest received signal strength (RSS), with largest amplitude, or with the smallest detection error in terms of mean-squared-error (MSE), error vector magnitude (EVM) or equivalent.

Alternatively, more information can be added to the feedback data. For instance, the feedback data may comprise data relating to the actual measured RSS, MSE, or EVM, of the two pilot symbols. The receiver of the feedback data is then able to leverage on more advanced signal processing techniques when determining a suitable phase shift to apply. For instance, a Kalman filter or a Wiener filter architecture.

Figure 7 is a signaling diagram illustrating phase control in a wireless communication system exemplifying that shown in Figure 3. The left-most communication device 710 generates and transmits the above- mentioned two pilot symbols S1 and S2 having different phase shifts. The rightmost communication device 720 receives the two pilot symbols in interference 150 and determines a communication condition of reception for each symbol. The rightmost communication device also compares communication conditions for the two pilot symbols, e.g., measures RSS for the two received pilot symbols, and then transmits the feedback signal 620 comprising feedback data back to the leftmost communication device 710.

The left-most communication device 710 optionally filters the feedback data to reduce effects from noise and distortion, and then controls the phase shift applied to its transmission in order to obtain constructive interference and improved communication conditions at the rightmost communication device. Figures 8a-8b illustrate wireless communication signals in a wireless communication system. In particular, Figure 8a illustrates an example wherein the first and the second communication signal 810 comprises different identification symbols ID.

A receiver of the first and second communication signal may then correlate known identification symbols with the received signal and thus determine a phase of the received first communication signal and a phase of the received second communication signal. By comparing these two determined phases a phase difference between the two received signals can be obtained.

This determined phase difference serves as an indication of phase difference, and is included in the feedback data comprised in the feedback signal 820. A receiver of this feedback data is then able to apply a suitable phase shift to its transmitted signal in order to reduce the phase difference at the receiving end and thus obtain constructive interference and improved communication conditions at the receiver of the first and second communication signal.

Thus, according to aspects, the wireless communication device discussed above is arranged to receive a feedback signal 820 comprising the feedback data from a further wireless device. The feedback data comprises information about the relative phase difference with which the different identification symbols was received at the further wireless device.

Figure 9 is a signaling diagram illustrating phase control in a wireless communication system exemplifying that shown in Figure 3.

A first communication device 910 transmits a signal comprising an identification symbol ID-A. This transmission is received by a second communication device 920, and also interferes 150 with reception at a third communication device 930.

The second communication device transmits 810 a different identification symbol ID-B to the third communication device 930.

The third communication device 930 receives the two identification symbols ID-A and ID-B, and determines a phase difference between them by, e.g., correlating against known identification symbols. The third communication device then generates and transmits a feedback signal back to the second communication device which comprises feedback data indicating said determined phase difference. It is appreciated that this difference can be indicated in many different ways, i.e., by its sign alone - positive or negative, or by its actual value. The pilot symbols S1 and S2, the identification symbols ID-A and ID-B, and the feedback data are according to some aspects part of a control channel that can be used to pass information, either in-band or out of band between the communication devices in the chain. This control channel can be located outside the band where the payload signals are carried in its own dedicated control band. Since the pilot symbols and the identification symbols will be different between transmissions along the chain, constructive interference cannot be guaranteed. There are a number of alternative solutions to this problem. For instance, according to some aspects this control channel can be located in different reserved frequency bands, where each link along the chain uses a different frequency band, or re-uses a frequency band that was used far away in order to avoid strong interference. According to other aspects, the control channel between different communication devices along the chain may use different orthogonal codes, i.e., apply code division multiple access, use different time slots, i.e., use time division multiple access, or operate according to an ALOHA scheme where the communication device first senses a frequency channel and only transmits in case no other transmission is ongoing.

Figure 10a is a block diagram schematically illustrating a wireless communication system comprising example implementations of the first 310, second 320 and third 330 communication devices discussed above. The second wireless communication device 1000 is here in this example arranged to receive a reset signal 1030. The control signal 226 is arranged to control a phase shift that varies periodically, at a first frequency f1 , around an operating point, wherein said periodical variation is arranged to be reset in response to receiving the reset signal 1030. Thus, the phase applied by the phase shift module 322 will vary around some operating point. This operating point corresponds to a phase shift controlled by the control signal 226 which is determined in order to obtain the type of constructive interference and improved communication conditions discussed above.

This variation in phase around the operating point will cause an effect similar to that of the two pilot symbols discussed in connection to Figure 6a-6b and Figure 7. The communication conditions will, on average or in the noise-free case vary periodically as illustrated in the graph 1040 of envelope versus time shown in Figure 10a. If the third communication device has access to the reset signal, it can determine or indicate the relative phase shift between the first and second communication signals received by comparing the instant communication conditions with the reset signal. The best communication conditions should be measured at the time instant of the reset signal. If the best communication conditions occur after the reset signal in time, then a phase shift in a first direction should be indicated. If the best communication conditions occur before the reset signal in time, then a phase shift in a second direction opposite the first direction should be indicated. According to aspects, an early-late gate detector is applied in order to determine and indicate the relative phase difference.

According to aspects, the first frequency f1 is configurable, or selectable from a pre-determined list of frequencies. Thus, different frequencies can be assigned to different communication devices. In this case filtering can be applied in order to separate out the effects from one phase shift module from another phase shift module.

The second communication device 1000 illustrated in Figure 10a is arranged to receive a feedback signal 1020 comprising the feedback data, wherein the feedback data comprises information about a phase of an envelope variation of the second communication signal relative to the reset signal. Figure 10b is a signaling diagram illustrating phase control in a wireless communication system exemplifying that shown in Figure 3. The second communication device, here shown as TRX-A receives the reset signal and resets the periodic variation by the oscillator 1010 at frequency f1. It transmits the second communication signal to the third communication device, here shown as TRX-B. The third communication device also receives the reset signal. The third communication device determines the envelope of the received signal, which corresponds to determining communication conditions over time relative to the time instant of receiving the reset signal. The third communications device then transmits the feedback signal to the second communications device, which signal comprises information related to said measurements made by the third communications device. The second communications device receives the feedback signal, optionally filters the feedback data in order to reduce effects from noise and distortion, and then proceeds to control the applied phase shift in order to improve on the communication conditions at the third communications device.

It is noted that the controlling of phase shift here refers to changing the operating point around which the phase variation occurs.

Several different applications of the above described communication system and communication devices are foreseen.

Figure 11 is a block diagram schematically illustrating an example application of a wireless communication system according to the present disclosure. Here, the second communication device 1120 is attached to an aerial vehicle, or to an autonomous aerial vehicle, or to an aerial drone. This way a connection from a chain of serially connected aerial vehicles to a central station, such as a radio base station, access point or eNodeb can be implemented. This serial connection then provides backhaul functionality that connects the aerial vehicles to a core network, the Internet or to some other communications resource. Figure 12 is a block diagram schematically illustrating another example application of a wireless communication system according to the present disclosure. Here, the second communication device is attached to a vehicle for road transport. Similar to the previous example discussed in connection to Figure 11 , a chain of vehicles travelling on a road, such as a caravan of trucks, can be connected to each other. Figure 13 is a block diagram schematically illustrating a further example of a wireless communication system according to the present disclosure. Here, the second communication device comprises a remote radio head, or a small-cell radio base station arranged for communication with further wireless devices 1322. This way a number of small cells deployed along some path can be connected to a core network, or to a centralized radio base station (RBS). The different communication devices and communication systems discussed above are arranged to perform methods.

Figure 14 is a flowchart illustrating such methods according to aspects of the disclosure.

Figure 14 shows a method in a wireless communication system 300 comprising first 310, second 320, and third 330 serially connected communication devices. The method comprises transmitting Sa1 a first communication signal 311 from the first communication device 310 to the second communications device 320, receiving Sa3 the first communication signal by the second communication device 320, and transmitting a second communication signal 321 comprising at least part of the first communication signal to the third communication device 330. The method also comprises receiving Sa5 the second communication signal 321 and also an interference signal 312 comprising the first communication signal by the third communication device 330, generating and transmitting Sa7 a feedback signal 331 from the third communication device 330 to the second communications device, the feedback signal indicating a phase difference between the first and the second communication signal at the third communication device, and receiving Sa9 the feedback signal by the second communication device and, based on the feedback signal, applying a phase shift 322 to the second communication signal to improve a reception condition of the second communication signal at the third communication device.

According to aspects, the method further comprises down-converting Sa31 the received first communication signal in frequency by mixing 422a with a signal from a local oscillator 421 , and up-converting Sa33 the second communication signal in frequency by mixing 422b with a second signal from said local oscillator 421. Thus, the methods illustrated in Figure 14 corresponds to the actions performed in the communication system discussed in connection to, e.g., Figure 3 and Figure 4 above. Figure 15 is a flowchart illustrating other methods according to aspects of the disclosure.

Figure 15 shows a method in a wireless communication device 220 for serial connection with further communication devices. The method comprises receiving Sb1 a first communication signal 211 via a first antenna port 221 from a first communication device 210, transmitting Sb3 a second communication signal 223 comprising at least part of the first communication signal to a third communication device 230 via a second antenna port 222, and receiving Sb5, by a control module 225, feedback data from the third communication device and generating a control signal 226 for adjusting a phase of the second communication signal to improve a reception condition of the second communication signal at the third communication device. The method also comprises applying Sb7, by a phase shift module 227, a phase shift to the second communication signal based on the control signal.

According to aspects, the method comprises down-converting Sb11 the received first communication signal in frequency by mixing 422a with a signal from a local oscillator 421 , and up-converting Sb31 the second communication signal in frequency by mixing 422b with a second signal from said local oscillator 421.

Figure 16 is a flowchart illustrating further methods according to aspects of the disclosure. Figure 16 shows a method in a wireless communication device 330 for connection in series with a first and a second communication device. The method comprises receiving Sc1 a second communication signal 321 from the second communication device comprising a first communication signal, receiving Sc3 an interference signal 312 from the first communication device comprising the first communication signal, and generating and transmitting Sc5 a feedback signal 331 to the second communications device indicating a phase difference between the first and the second communication signal received at the third communication device for applying a phase shift to the second communication signal prior to transmission.

Thus, the methods illustrated in Figure 15 and Figure 16 corresponds to the actions performed in the communications devices discussed in connection to, e.g., Figures 2-5 above.

The embodiments disclosed herein, in particular in connection to Figures 14-16 thus relate to mechanisms for wireless communication and in particular to mechanisms for communication in a system of microwave transceivers arranged in series to form a chain of microwave radio links. In order to obtain such mechanisms there is provided one or more control nodes, and methods performed by the control nodes, a computer program comprising code, for example in the form of a computer program product, that when run on processing circuitry of a control node or nodes, causes the control node or nodes to perform the method. Processing circuitry is provided using any combination of one or more of a suitable central processing unit CPU, multiprocessor, microcontroller, digital signal processor DSP, application specific integrated circuit ASIC, field programmable gate arrays FPGA etc., capable of executing software instructions stored in a computer program product, e.g. in the form of a storage medium.

Particularly, the processing circuitry is configured to cause the control node to perform a set of operations, or steps, as discussed in connection to Figures 14-16. For example, the storage medium may store the set of operations, and the processing circuitry may be configured to retrieve the set of operations from the storage medium to cause the control node or nodes to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus the processing circuitry is thereby arranged to execute methods as herein disclosed.

The storage medium may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

Consequently, there is disclosed herein a computer program comprising computer program code which, when executed in a wireless communication system, causes the system to execute a method such as the methods discussed in connection to Figures 14-16.

The inventive concept has mainly been described above with reference to a few embodiments and example drawings. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the claims.

Claims

1. A wireless communication system (300) comprising first (310), second (320), and third (330) serially connected communication devices,

- the first communication device (310) arranged to transmit a first communication signal (311) to the second communications device (320),

- the second communication device (320) arranged to receive the first communication signal, and to transmit a second communication signal (321) comprising at least part of the first communication signal to the third communication device (330),

- the third communication device (330) arranged to receive the second communication signal (321), and to also receive an interference signal (312) from the first communication device comprising the first communication signal,

- the third communication device (330) arranged to generate and transmit a feedback signal (331) to the second communications device indicating a phase difference between the first and the second communication signal received at the third communication device,

- the second communication device arranged to receive the feedback signal and to, based on the feedback signal, apply a phase shift (322) to the second communication signal to obtain constructive interference between the first and second communication signal at the third communication device.

2. The wireless communication system (400) according to claim 1 , the second communication device (420) arranged to down-convert the received first communication signal in frequency by mixing (422a) with a signal from a local oscillator (421), and to up-convert the second communication signal in frequency by mixing (422b) with a second signal from said local oscillator (421).

3. The wireless communication system (1100) according to claim 1 or 2, wherein the second communication device is attached to an aerial vehicle, or to an autonomous aerial vehicle, or to an aerial drone.

4. The wireless communication system (1200) according to claim 1 or 2, wherein the second communication device is attached to a vehicle for road transport.

5. The wireless communication system (1300) according to claim 1 or 2, wherein the second communication device comprises a remote radio head, or a small-cell radio base station arranged for communication with further wireless devices (1322).

6. A wireless communication device (220) for connection in series between a first and a third communication device, the wireless communication device arranged to receive a first communication signal (211) via a first antenna port (221) from the first communication device, and to transmit a second communication signal (223) comprising at least part of the first communication signal to the third communication device via a second antenna port (222), the wireless communication device (220) comprising a control module (225) arranged to receive feedback data from the third communication device and to generate a control signal (226) for adjusting a phase of the second communication signal to improve a reception condition of the second communication signal at the third communication device, the wireless communication device comprising a phase shift module (227) arranged to receive the control signal (226), and to apply a phase shift to the second communication signal based on the control signal.

7. The wireless communication device (520) according to claim 6, arranged to down-convert the received first communication signal in frequency by mixing (422a) with a signal from a local oscillator (421), and to up-convert the second communication signal in frequency by mixing (422b) with a signal from said local oscillator (421).

8. The wireless communication device (530) according to claim 7, wherein the phase shift module (523) is arranged to apply the phase shift to the second communication signal at a baseband or at an intermediate frequency.

9. The wireless communication device (530) according to claim 7, wherein the phase shift module (531 , 532) is arranged to apply the phase shift to the second communication signal by adjusting the phase of the signal from the local oscillator (421).

10. The wireless communication device (530) according to claim 7, wherein the phase shift module (533, 534) is arranged to apply the phase shift to the received first communication signal at radio frequency, or to the second communication signal at radio frequency prior to transmission.

11. The wireless communication device according to any of claims 6-10, wherein the first and the second communication signal (610) each comprises first (S1) and second (S2) pilot symbols, wherein the first pilot symbol in the second communication signal is adjusted in phase in a first direction relative to the first pilot symbol in the first communication signal, and wherein the second pilot symbol in the second communication signal is adjusted in phase in a direction opposite to the first direction relative to the second pilot symbol in the first communication signal.

12. The wireless communication device according to claim 11 , arranged to receive a feedback signal (620) comprising the feedback data, wherein the feedback data comprises information about which pilot symbol out of the first and the second pilot symbol that was received under best reception conditions.

13. The wireless communication device according to any of claims 6-10, wherein the first and the second communication signal (810) comprises different identification symbols (ID).

14. The wireless communication device according to claim 13, arranged to receive a feedback signal (820) comprising the feedback data, wherein the feedback data comprises information about the relative phase difference with which the different identification symbols was received at the further wireless device.

15. The wireless communication device (1000) according to any of claims 6-10, arranged to receive a reset signal (1030), wherein the control signal (226) is arranged to control a phase shift that varies periodically, at a first frequency (f1), around an operating point, wherein said periodical variation is arranged to be reset in response to receiving the reset signal (1030).

16. The wireless communication device according to claim 15, wherein the first frequency (f1) is configurable, or selectable from a pre-determined list of frequencies.

17. The wireless communication device according to claim 15 or 16, arranged to receive a feedback signal (1020) comprising the feedback data, wherein the feedback data comprises information about a phase of an envelope variation of the second communication signal relative to the reset signal.

18. The wireless communication device according to any of claims 6-17, wherein the first and second communication signals comprise identical payload data parts.

19. The wireless communication device according to any of claims 6-17, wherein the first communication signal comprises a payload data part, and wherein the second communication signal comprises the payload data part and also a further payload data part.

20. The wireless communication device according to any of claims 6-19, wherein the received feedback signal comprises a request to increase or to decrease the applied phase shift.

21. The wireless communication device according to claim 20, wherein the control module is arranged to filter the feedback data prior to generating the control signal.

22. A wireless communication device (330) for connection in series with a first and a second communication device, the wireless communication device (330) arranged to receive a second communication signal (321) from the second communication device comprising a first communication signal, and to also receive an interference signal (312) from the first communication device comprising the first communication signal, the wireless communication device (330) arranged to generate and transmit a feedback signal (331) to the second communications device indicating a phase difference between the first and the second communication signal received at the third communication device.

23. A method in a wireless communication system (300) comprising first (310), second (320), and third (330) serially connected communication devices, comprising

- transmitting (Sa1) a first communication signal (311) from the first communication device (310) to the second communications device (320),

- receiving (Sa3) the first communication signal by the second communication device (320), and transmitting a second communication signal (321) comprising at least part of the first communication signal to the third communication device (330),

- receiving (Sa5) the second communication signal (321) and also an interference signal (312) comprising the first communication signal by the third communication device (330),

- generating and transmitting (Sa7) a feedback signal (331) from the third communication device (330) to the second communications device, the feedback signal indicating a phase difference between the first and the second communication signal at the third communication device,

- receiving (Sa9) the feedback signal by the second communication device and, based on the feedback signal, applying a phase shift (322) to the second communication signal to improve a reception condition of the second communication signal at the third communication device.

24. The method according to claim 23, comprising

- down-converting (Sa31) the received first communication signal in frequency by mixing (422a) with a signal from a local oscillator (421), and - up-converting (Sa33) the second communication signal in frequency by mixing (422b) with a second signal from said local oscillator (421).

25. A method in a wireless communication device (220) for serial connection with further communication devices, comprising

- receiving (Sb1 ) a first communication signal (211 ) via a first antenna port (221 ) from a first communication device (210),

- transmitting (Sb3) a second communication signal (223) comprising at least part of the first communication signal to a third communication device (230) via a second antenna port (222),

- receiving (Sb5), by a control module (225), feedback data from the third communication device and generating a control signal (226) for adjusting a phase of the second communication signal to improve a reception condition of the second communication signal at the third communication device, and

- applying (Sb7), by a phase shift module (227), a phase shift to the second communication signal based on the control signal.

26. The method according to claim 25, comprising

- down-converting (Sb11) the received first communication signal in frequency by mixing (422a) with a signal from a local oscillator (421), and

- up-converting (Sb31 ) the second communication signal in frequency by mixing (422b) with a second signal from said local oscillator (421).

27. A method in a wireless communication device (330) for connection in series with a first and a second communication device, comprising

- receiving (Sc1) a second communication signal (321) from the second communication device comprising a first communication signal,

- receiving (Sc3) an interference signal (312) from the first communication device comprising the first communication signal,

- generating and transmitting (Sc5) a feedback signal (331) to the second communications device indicating a phase difference between the first and the second communication signal received at the third communication device for applying a phase shift to the second communication signal prior to transmission.

28. A computer program comprising computer program code which, when executed in a wireless communication system, causes the system to execute a method according to any of claims 23-24.

29. A computer program comprising computer program code which, when executed in a microwave radio receiver, causes the microwave radio receiver to execute a method according to any of claims 25-26.

30. A computer program comprising computer program code which, when executed in a microwave radio receiver, causes the microwave radio receiver to execute a method according to claim 27.