WO2019201438A1

WO2019201438A1 - Techniques for network-based time synchronization for ue sidelink and/or uplink communication

Info

Publication number: WO2019201438A1
Application number: PCT/EP2018/059916
Authority: WO
Inventors: Konstantinos MANOLAKIS; Markus Martin DILLINGER; Wen Xu
Original assignee: Huawei Technologies Duesseldorf Gmbh
Priority date: 2018-04-18
Filing date: 2018-04-18
Publication date: 2019-10-24
Also published as: CN111989960B; CN111989960A

Abstract

The disclosure relates to techniques for network-based time synchronization for UE sidelink and/or uplink communication, in particular for inter-operator sidelink and/or uplink communication. The disclosure particularly relates to a base station, in particular an eNodeB or a gNodeB, for synchronizing at least one user equipment, UE, for its uplink and/or sidelink communication, the base station comprising a processor configured to: forward at least one time synchronization message of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, between a time reference server and at least one UE receive synchronization information of the at least one UE about the synchronization between the at least one UE and the time reference server, in particular an end-to-end delay between the time reference server and the at least one UE; and transmit a synchronization instruction to the at least one UE. The disclosure further relates to a corresponding UE and a corresponding time reference server.

Description

Techniques for network-based time synchronization for UE sidelink and/or uplink communication

TECHNICAL FIELD

The disclosure relates to techniques for network-based time synchronization for UE sidelink and/or uplink communication, in particular for inter-operator sidelink and/or uplink communication. The disclosure particularly relates to a base station, in particular an eNodeB or a gNodeB, for synchronizing at least one user equipment, UE, for its uplink and/or sidelink communication. The disclosure further relates to corresponding UEs, time reference server and synchronizing methods.

BACKGROUND

Device-to-device (D2D) communication is considered as a key component for future 5G networks, mainly in the context of vehicle-to-anything (V2X) communications. In order to guarantee reliable communication and fast link establishment, quick and accurate time synchronization is required in the cellular sidelink, including different types of single- or multi-link D2D/V2V communication (unicast, broadcast etc.). The nature of V2X communication requires communication between users assigned to different base stations, i.e. multi-cellular V2V, which users may even belong to different mobile network operators (MNOs). In order to enable this, a global time reference serving as a common beckon of time is required for all mobile users. Based on this reference, sidelink time synchronization, as for example provided by the cellular network or mutually achieved between the users can be performed and refined. Since time references provided by the base station to mobile users for cellular (uplink/downlink) transmission are in general different, these cannot directly serve as a reference for the sidelink, which is why a global reference is needed at first place. Moreover, it is noted that external sources as GNSS references are not always available and cannot be used as a main global reference for V2V/D2D communication. SUMMARY

It is the object of the invention to provide a concept for improving communication, in particular UE sidelink and/or uplink communication in mobile communication networks, in particular 5G networks, to guarantee reliable communication and fast link establishment.

In particular it is an object of the invention to provide a common perception of time, in particular a common time reference, to all mobile users involved in mobile communication.

This object is achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

In this disclosure, a new procedure for distributing a global time reference from a remote network entity (e.g. cloud server) through the MNO core and access network until the mobile user is presented. The procedure uses basic parts of the IEEE 1588 protocol to estimate and compensate the time offset between a mobile user and the cloud server and measure the delay. As part of the presented extension, attached mobile users report certain measurements back to their base station and receive instructions in the form of control information regarding the time reference to use for the sidelink and/or the uplink. As an overall result, all users, including the ones assigned to different MNOs, reach a common time perception, which they can follow for the V2V sidelink or uplink, and based on which they can receive further instructions or perform mutual synchronization to align their sidelink or uplink transmissions in time. While the focus lies on UE’s sidelink communication, the concepts can be used as well for the UE’s uplink communication.

The scope of the disclosure is the definition of a procedure for remote network-based time synchronization, used for sidelink communication between UEs of same or different MNOs. To this end, a basic idea of this disclosure is the introduction of new signaling and information/measurement exchange between a UE and its eNB, while in parallel a PTP is executed between the UE and a remote server used for sidelink coordination. Moreover, signaling is extended to include out-of-coverage UEs in order to synchronize them and allow for synchronized sidelink transmission, e.g. in partial cellular coverage scenarios. Different implementations regarding the protocol stack implementation and aspects of the UE internal architecture are also discussed and solutions are presented. In order to describe the invention in detail, the following terms, abbreviations and notations will be used:

DL: Downlink, i.e. link from network to UE

UL: Uplink, i.e. link from UE to network

SL: Sidelink, i.e. link between UEs

UE: User Equipment

BS: Base Station, eNodeB

PTP: Precise Time Protocol

NTP: Network Timing Protocol

C-server: Cloud server or central server

D2D: Device-to-device

V2X: Vehicle-to-anything

MNO: Mobile Network Operator

According to a first aspect, the invention relates to a base station, in particular an eNodeB or a gNodeB, for synchronizing at least one user equipment, UE , for its uplink and/or sidelink communication, the base station comprising a processor configured to: forward at least one time synchronization message of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, between a time reference server and at least one UE; receive synchronization information of the at least one UE about the synchronization between the at least one UE and the time reference server, in particular a time offset and an end-to-end delay between the time reference server and the at least one UE; and transmit a synchronization instruction to the at least one UE.

Such a base station improves communication, in particular UE sidelink and/or uplink communication in mobile communication networks, in particular 5G networks. The base station thus guarantees reliable communication and fast link establishment. By applying such base station, a common perception of time can be achieved, in particular a common time reference can be provided to all mobile users involved in mobile communication.

The base station is enabled to acquire synchronization information about the time reference server and/or an end-to-end delay. Further advantages are that the base station enables the at least one UE synchronizing with the time reference server; and UEs’ uplink and/or sidelink communication follow a time reference based on the time reference server and are time-aligned/synchronized with each other.

Note that the BS does not need to synchronize with the time reference server as well, but this can be an optional feature. The time reference finally used by the UEs does not need to be the server reference, but is based/depending on it.

In an exemplary implementation form of the base station, the processor is configured to use a second time reference, which is not based on a time reference of the time reference server, for synchronization of downlink communication with the at least one UE.

This provides the advantage that synchronization of UE’s downlink communication is independent from synchronization of UEs’ uplink and/or sidelink communication.

In an exemplary implementation form, the base station is configured to use alternatively a time reference depending on the time reference server for synchronizing of downlink communication with the at least one UE.

This provides the advantage that synchronization of UE’s downlink communication is dependent on synchronization of UEs’ uplink and/or sidelink communication.

In an exemplary implementation form of the base station, the synchronization information, in particular the time offset and end-to-end delay, between the time reference server and the at least one UE is based on an offset between the time reference server and the at least one UE and a delay from the time reference server to the at least one UE and/or a delay from the at least one UE to the time reference server.

Note that the term“based on” can be in particular an average, e.g. a weighted average, between delays in different directions and/or from more than one UEs.

This provides the advantage that the end-to-end delay can be precisely determined.

In an exemplary implementation form of the base station, the processor is configured to determine an access delay between the base station and the at least one UE. This provides the advantage that synchronization can be improved when determining the access delay between BS and UE.

In an exemplary implementation form of the base station, the processor is configured to determine the access delay between the base station and the at least one UE based on UE-specific information provided by the at least one UE, in particular depending on radio propagation delay, known contribution of the base station to the access delay and timing advance, TA.

This provides the advantage that the access delay can be precisely determined.

In an exemplary implementation form of the base station, the processor is configured to determine a network delay between the time reference server and the base station based on the synchronization information, in particular the end-to-end delay, and the access delay.

This provides the advantage that the network delay can be precisely determined.

In an exemplary implementation form of the base station, the base station is configured to use the network delay for synchronization of the at least one UE.

This provides the advantage that synchronization can be improved when utilizing the network delay for synchronization of the UE.

In an exemplary implementation form of the base station, the processor is configured to determine the time reference of the time reference server based on the network delay.

This provides the advantage that determining the time reference of the time reference server can be improved when determining it based on the network delay.

In an exemplary implementation form of the base station, the processor is configured to transmit synchronization instructions to a plurality of UEs, and in particular wherein the synchronization instructions are UE-specific or specific to a group of UEs. This provides the advantage that specific synchronization can be transmitted to all UEs. Hence synchronization can be optimized for each UE.

In an exemplary implementation form of the base station, the synchronization instructions are based on the network delay and UE-specific time measurements and parameters, in particular access delay, radio propagation delay and known contribution of the base station to the access delay and UE-specific timing advance, TA.

This provides the advantage that synchronization can be improved when using such specific synchronization instructions.

In an exemplary implementation form of the base station, the processor is configured to forward the at least one time synchronization message between the time reference server and the at least one UE without participating in the time synchronization protocol.

Note that the term“without participating in the time synchronization protocol” in the sense of the invention comprises that the BS does not implement the synchronization protocol itself and takes a role within the communication according to this protocol, and/or that the BS does not read the time synchronization message.

This provides the advantage that quick and accurate time synchronization can be achieved guaranteeing reliable communication and fast link establishment.

In an exemplary implementation form of the base station, the processor is configured to prioritize forwarding the at least one time synchronization message between the time reference server and the at least one UE.

This provides the advantage that no extra delay is induced during the synchronization due to queuing in particular in case of network traffic jam.

In an exemplary implementation form of the base station, the processor is configured to request the at least one UE providing the synchronization information, in particular the end-to-end delay, between the time reference server and the at least one UE. This provides the advantage that if the base station realizes that the synchronization should be updated it can request so.

In an exemplary implementation form of the base station, the synchronization information, in particular the end-to-end delay, between the time reference server and the at least one UE is periodically received from the at least one UE.

This provides the advantage that the synchronization process is very robust and can tolerate lost synchronization information.

In an exemplary implementation form of the base station, the processor is configured to request the at least one UE changing a period for reporting the synchronization information, in particular the end-to-end delay, in particular if the base station detects changes in network delay between the time reference server and the base station.

This provides the advantage that the synchronization process can be optimally adapted to changing network conditions.

According to a second aspect, the invention relates to a user equipment, UE, for assisting a base station for synchronizing at least one user equipment, UE, for its uplink and/or sidelink communication, the UE comprising a processor configured to: receive a time synchronization message of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, from a time reference server;

determine synchronization information about a synchronization between the UE and the time reference server, in particular a time offset and an end-to-end delay between the time reference server and the UE, based on the time synchronization message; report the synchronization information to the base station; and receive a synchronization instruction from the base station for synchronizing the UE’s uplink and/or sidelink communication.

Such a UE improves communication, in particular UE sidelink and/or uplink

communication in mobile communication networks, in particular 5G networks. The UE thus can guarantee reliable communication and fast link establishment. By applying such UE, a common perception of time can be achieved, in particular a common time reference can be provided to all mobile users involved in mobile communication. Further advantages are that the UE is enabled synchronizing with the time reference server; and UEs’ uplink and/or sidelink communication follow a time reference based on the time reference server and are time-aligned/synchronized with each other.

In one embodiment the UE is configured to receive and/or report the time synchronization message, the synchronization information, and/or synchronization instruction through another UE.

This provides the advantage that the UE can be out-of-coverage and communicates with the time reference server and/or the base station via second UE that is in-coverage. The second UE operates as relay node for the respective functionality then.

In an exemplary implementation form of the UE, the processor is configured to report the synchronization information to the base station via an Uplink feedback channel.

This provides the advantage that a standard channel that is already available can be used for reporting synchronization parameters such as the end-to-end delay.

In an exemplary implementation form of the UE, the processor is configured to receive the UE-specific synchronization instructions from the base station via a Downlink control channel.

This provides the advantage that a standard channel such as the DL control channel that is already available can be used for receiving the synchronization instructions.

In an exemplary implementation form of the UE, the processor can be configured to align a clock offset with the time reference server based on a first synchronization message received from the time reference server and particularly based on a first follow-up message to the first synchronization message according to the PTP/NTP protocol.

This provides the advantage that an available implementation of the standard PTP or NTP protocol can be (re)used.

In an exemplary implementation form of the UE, the processor can be configured to determine a master-to-slave delay indicating a delay between the time reference server and the UE based on the aligned clock offset and a second synchronization message received from the time reference server and particularly based on a second follow-up message to the second synchronization message according to the PTP/NTP protocol.

In an exemplary implementation form of the UE, the processor can be configured to determine a slave-to-master delay indicating a delay between the UE and the time reference server based on a delay response message received from the time reference server and in particular based on a follow-up message to the delay response message according to the PTP/NTP protocol.

In an exemplary implementation form of the UE, the first synchronization message, the second synchronization message and the delay response message can be received from the time reference server without modification by the base station message according to the PTP/NTP protocol.

In an exemplary implementation form, the UE comprises: a first modem comprising a first protocol stack, PC5, for processing the UE’s sidelink communication; and a second modem comprising a second protocol stack, Uu for processing Uplink/Downlink communication with the base station, wherein the first protocol stack and the second protocol stack comprise a shared IP layer, a shared Radio Resource Connection, RRC, layer and separate MAC layers.

This provides the advantage that implementation of the UE’s sidelink communication link and the UE’s Uplink/Downlink communication link are independent with respect to each other. In an exemplary implementation form of the UE, the processor is configured to process the time synchronization protocol based on the shared IP layer and to synchronize the UE’s uplink and/or sidelink communication based on the shared RRC layer or based on the separate MAC layers.

This provides the advantage that by using shared layers such as shared IP or shared RRC, implementation costs can be reduced and synchronization efficiency can be increased.

In an exemplary implementation form of the UE, the processor is configured to

compensate an internal delay between the first modem and the second modem and to synchronize the UE with the time reference server based on the compensated internal delay.

This provides the advantage that by compensating the internal delay, synchronization accuracy can be increased.

In an exemplary implementation form of the UE, the processor is configured to report the synchronization information to the base station, wherein the synchronization information includes an internal delay between the first modem and the second modem.

This provides the advantage that by reporting the internal delay to the base station, the base station can increase synchronization accuracy.

In an exemplary implementation form of the UE, the processor is configured to provide a synchronization instruction to another UE that is out of coverage from the base station.

This provides the advantage that out-of-coverage UEs can be efficiently synchronized.

In an exemplary implementation form of the UE, the processor is configured to provide the synchronization instruction to the other UE via a sidelink control channel between the UE and the other UE.

The synchronization information provided to the other UE can be specific for the other UE. This provides the advantage that the out-of-coverage UE can be efficiently synchronized via the sidelink control channel to the (in-coverage) UE.

In an exemplary implementation form of the UE, the UE is configured to measure a delay, in particular a round-trip delay, between the UE and the other UE and to base the synchronization instruction on this delay.

This provides the advantage that synchronization of the out-of-coverage UE can be improved when synchronization is based on the round-trip delay between the UE and the other UE.

This provides the further advantage that an overall delay is complemented by the delay between the UE and the other UE.

In an exemplary implementation form of the UE, the UE is configured to receive a request from the other UE to measure the delay.

This provides the advantage that UE can measure the delay upon request. There is no need for a permanent monitoring for other non-coverage UEs.

According to a third aspect, the invention relates to a time reference server for

synchronizing at least one user equipment, UE, for its uplink and/or sidelink

communication, the time reference server comprising a processor configured to: transmit at least one time synchronization message of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, to the at least one UE, wherein the at least one synchronization message comprises information to enable the at least one UE to report synchronization information about a synchronization between the at least one UE and the time reference server, in particular an end-to-end delay between the time reference server and the at least one UE.

Such a time reference server provides the advantage that the UE can then be configured to report the synchronization information to a base station to enable the base station transmitting a synchronization instruction to the at least one UE for synchronizing the UE’s uplink and/or sidelink communication. Such a time reference server, also referred to as C-server, improves communication, in particular UE sidelink and/or uplink communication in mobile communication networks, in particular 5G networks. The time reference server can provide reliable communication and fast link establishment. By applying such time reference server, a common perception of time can be achieved, in particular a common time reference can be provided to all mobile users involved in mobile communication.

Further advantages are that the time reference server enables the at least one UE synchronizing with the time reference server; and UEs’ uplink and/or sidelink communication follow a time reference based on the time reference server and are time- aligned/synchronized with each other.

In an exemplary implementation form of the time reference server, the processor is configured to do at least one of the following: transmit a first synchronization message and particularly a first follow-up message to the first synchronization message via bypassing the base station to the UE; transmit a second synchronization message and particularly a second follow-up message to the second synchronization message via bypassing the base station to the UE; receive a delay request message via bypassing the base station from the UE; and transmit a delay response message and particularly a follow-up message to the delay response message via bypassing the base station to the UE.

In an exemplary implementation form, the time reference server is configured to transmit the time synchronization message to a plurality of operator networks; and/or to be operated extern of an operator network.

This provides the advantage that the time reference server can be used operator- independent.

According to a fourth aspect, the invention relates to a method for synchronizing user equipment, UE, for its uplink and/or sidelink communication, the method comprising:

forwarding at least one time synchronization message of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, between a time reference server and at least one UE; receiving synchronization information of the at least one UE about the synchronization between the at least one UE and the time reference server, in particular an end-to-end delay between the time reference server and the at least one UE; and transmitting a synchronization instruction to the at least one UE.

Such a method that can be implemented at BS site improves communication, in particular UE sidelink and/or uplink communication in mobile communication networks, in particular 5G networks. The method thus guarantees reliable communication and fast link establishment. By applying such a method, a common perception of time can be achieved, in particular a common time reference can be provided to all mobile users involved in mobile communication.

According to a fifth aspect, the invention relates to a method for synchronizing at least one user equipment, UE, for its uplink and/or sidelink communication, the method comprising: receiving a time synchronization message of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, from a time reference server; determining synchronization information about a synchronization between the UE and the time reference server, in particular an end-to-end delay between the time reference server and the UE, based on the time synchronization message; reporting the synchronization information to a base station; and receiving a synchronization instruction from the base station for synchronizing the UE’s uplink and/or sidelink communication.

Such a method that can be implemented at UE site improves communication, in particular UE sidelink and/or uplink communication in mobile communication networks, in particular 5G networks. The method thus guarantees reliable communication and fast link establishment. By applying such a method, a common perception of time can be achieved, in particular a common time reference can be provided to all mobile users involved in mobile communication.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the invention will be described with respect to the following figures, in which:

Fig. 1 shows a schematic diagram illustrating an exemplary mobile (vehicular) network 100 with in-cellular coverage users 101 , 102 and out-of-cellular coverage users 103, 104; Fig. 2 shows a schematic diagram illustrating a centralized C-Server architecture 200 according to the disclosure;

Fig. 3 shows a schematic diagram illustrating a C-Server architecture 300 according to the disclosure and sources of delay and time offset for the general scenario of multiple operators;

Fig. 4a shows a schematic diagram illustrating a delay measurement 400a in NTP/PTP based on one-step messaging;

Fig. 4b shows a schematic diagram illustrating a delay measurement 400a in NTP/PTP based on two-step messaging;

Fig. 5 shows a message sequence chart 500 illustrating the main steps of the IEEE 1588 time synchronization protocol;

Fig. 6 shows a schematic diagram illustrating an exemplary mobile (vehicular) network 600 with synchronized sidelink 601 according to the disclosure;

Fig. 7 shows a message sequence chart 700 illustrating signalling and operations between C-server, base station/eNB and mobile user/UE according to the disclosure;

Fig. 8 shows a message sequence chart 800 illustrating the principle of transparent clock according to the disclosure;

Fig. 9 shows a message sequence chart 900 illustrating synchronization of UEs without cellular coverage according to the disclosure;

Fig. 10 shows a schematic diagram illustrating an exemplary implementation of protocol stacks in a mobile (vehicular) network including C-server, base station/eNB and UE according to a first implementation form; Fig. 11 shows a schematic diagram illustrating an exemplary implementation of protocol stacks 1000 in a mobile (vehicular) network including C-server 232, base station/eNB 211 and UE 201 according to a second implementation form;

Fig. 12 shows a schematic diagram illustrating an exemplary implementation of clock distribution within the UE according to the disclosure;

Fig. 13 shows a schematic diagram illustrating a method 1300 for synchronizing a UE 201 for its uplink and/or sidelink communication, from base station 21 1 side, according to the disclosure; and

Fig. 14 shows a schematic diagram illustrating a method 1400 for synchronizing a UE 201 for its uplink and/or sidelink communication, from UE 201 side, according to the disclosure;

DETAILED DESCRIPTION OF EMBODIMENTS

In the following detailed description, reference is made to the accompanying drawings, which form a part thereof, and in which is shown by way of illustration specific aspects in which the disclosure may be practiced. It is understood that other aspects may be utilized and structural or logical changes may be made without departing from the scope of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims.

It is understood that comments made in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary aspects described herein may be combined with each other, unless specifically noted otherwise.

The methods and devices described herein may be implemented in wireless

communication networks based on mobile communication standards, e.g. LTE (Long Term Evolution), in particular 4.5G, 5G and beyond. The methods and devices described herein may also be implemented in wireless communication networks, in particular communication networks using WiFi communication standards according to IEEE 802.1 1 and higher. The described devices may include integrated circuits and/or passives and may be manufactured according to various technologies. For example, the circuits may be designed as logic integrated circuits, analog integrated circuits, mixed signal integrated circuits, optical circuits, memory circuits and/or integrated passives.

The devices described herein may be configured to transmit and/or receive radio signals. Radio signals may be or may include radio frequency signals radiated by a radio transmitting device (or radio transmitter or sender) with a radio frequency lying in a range of about 3 kHz to 300 GHz.

The devices and systems described herein may include processors, memories and transceivers, i.e. transmitters and/or receivers. In the following description, the term “processor” describes any device that can be utilized for processing specific tasks (or blocks or steps). A processor can be a single processor or a multi-core processor or can include a set of processors or can include means for processing. A processor can process software or firmware or applications etc.

The devices and systems described herein may be applied in base stations and User Equipments. Examples of a base station may include access nodes, evolved NodeBs (eNBs), gNBs, NodeBs, master eNBs (MeNBs), secondary eNBs (SeNBs), remote radio heads and access points.

Fig. 1 shows a schematic diagram illustrating an exemplary mobile (vehicular) network 100 with in-cellular coverage users 101 , 102 and out-of-cellular coverage users 103, 104. An exemplary number of five UEs 101 , 102, 103, 104, 105 is shown (there may be more or less UEs) from which two UEs 101 , 102 are in-cellular coverage, UE 101 is in-cellular coverage 1 11 of eNodeB 110 and UE 102 is in-cellular coverage 121 of eNodeB 120. In this exemplary network 100, first eNodeB 110 may be a base station of a first mobile network operator (MNO) while second eNodeB 120 may be a base station of a second MNO. Three UEs 103, 104, 105 are out-of-cellular coverage, where UEs 103, 104 (and additionally UEs 101 , 102) are within communication area and also within synchronization area but only one UE 105 is within synchronization area but outside communication area. Note that these numbers are only meant as exemplary numbers, any other numbers can be used as well. In this example, UEs 101 , 103 are equipped with a GNSS receiver for receiving timing information from a GNSS system 160 represented by a satellite. In this example, UE 101 can be connected to eNB 1 10 and additionally to remote small cell unit (RSU) 130.

A typical scenario of a mobile (vehicular) network in a cellular environment is shown in Figure 1 , including in- and out-of cellular coverage mobile users with User Equipments (UEs) 102, 103, 104, 105, some of which (e.g. 101 , 103) are also equipped with a Global Navigation Satellite System (GNSS) 160 receiver. Coexistence of multiple sidelink transmissions - also between users of different MNOs- within one frequency band requires time alignment of transmitted signals to avoid interference; in case a sidelink frequency band shall be hosted within a cellular band used for UL/DL, further time alignment with respect to these cellular transmissions will be required.

Main challenges and constraints of this global V2V/D2D time synchronization problem include:

• Time alignment of users attached to different and non-synchronized base stations e.g. using Frequency Division Duplex (FDD) and/or assigned to different MNOs’ network;

• Partial coverage scenarios with out-of-coverage users;

• Synchronization of the cellular UL/DL shall not be affected by further synchronizing the sidelink. Ideally, the UL/DL should not be aware or need to consider the requirements of the V2V sidelink;

• GNSS-like time references (GPS, Galileo etc.) are not always and everywhere provided and cannot serve as a main reference.

At this point it needs to be clarified that the current disclosure provides a solution for clock/time reference distribution through fixed/wireless network architectures. In addition to this level of first synchronization, additional information in the form of assignments may still be provided to the UEs by the base stations, similar to the timing advance (TA) for the cellular uplink. Moreover, on the receiver side, synchronization algorithms still need to be performed by UEs, similar to the downlink. Through detection of predefined synchronization signals- the so-called Beginning of Frame (BOF) and Beginning of Symbol (BOS) have to be still estimated in order to correctly process the received signal, e.g. remove the Cyclic Prefix (CP) in an Orthogonal Frequency Division Multiplexing (OFDM) waveform. Fig. 2 shows a schematic diagram illustrating a centralized C-Server architecture 200 according to the disclosure. The communication system includes a central server 232, also referred to as cloud server 232 that may be located in the cloud 230. The

communication system further includes a first mobile network operator (MN01 ) network 210 including a first MNO server 212 and a first eNodeB (eNB1 ) 21 1 , and a second mobile network operator (MN02) network 220 including a second MNO server 222 and a second eNodeB (eNB2) 221 . A first mobile user with first UE 201 may be connected to MN01 network 210, a second mobile user with second UE 202 may be connected to MN02 network 220 and a third mobile user with third UE 203 may be out-of-coverage but connected to first UE 201 via sidelink 204 connection. First UE 201 and second UE 202 may be connected via sidelink 204 connection.

In the centralized C-server architecture 200, a central entity, e.g. a central server (c-server) 232 is located inside or outside the MNOs’ core network (CN) or in the cloud 230, controls the sidelink 204 transmission between mobile users 201 , 202 attached to base stations 212, 222 of the same of different operators. Multi-operator V2V is controlled by this c-server 232, which provides high-layer control of the UEs’ sidelink 204 in the form of control information 207, which is transmitted via each MNO’s core and access network 210, 220 to the UEs 201 , 202. The role of the base stations 21 1 , 212 is to forward this control information 207 to their attached UEs 201 , 202 and to receive and forward feedback to the cloud server 232. Control information 207 is transmitted from base stations 21 1 , 221 to UEs 201 , 202 in a dedicated channel within the downlink frequency band. This architecture allows for using a shared band for V2V between all MNOs’ UEs 201 , 202, which has several benefits, e.g. allows for centralized resource management and resource allocation. Based on this architecture, a subset of MNO functionalities can be shifted to the central server 232. Out- of-coverage UEs, e.g. 203, can further receive control information via sidelink 204 through in-coverage UEs, e.g. 201 .

Fig. 3 shows a schematic diagram illustrating a C-Server architecture 300 according to the disclosure and sources of delay and time offset for the general scenario of multiple operators. The C-server architecture 300 of Fig. 3 is a similar representation of the C- server architecture 200 illustrated in Figure 2, in which time delays DΪ_A 301 , Dί_b 302 between the C-server 232 and the base stations 21 1 , 221 of MN01 210 (denoted here as MNO_A) and MN02 220 (denoted here as MNOB) and time delays tm 304 and t_R2 305 between the base stations 21 1 , 221 and their associated UEs 201 , 202 are highlighted. From the synchronization point of view, the sidelink transmission will be subject to several time offset and delay contributing factors, which are shown in the overview of Fig. 3. In the modeling approach followed hereinafter, the C-server 232 is assumed to use a global time reference t_c, which can be provided by accurate GNSS, a highly-accurate local clock (rubidium or atomic clock) or other sources, and which is here considered as an ideal time reference.

Each base station, see here two base stations 21 1 , 221 belonging to MNO A 210 and MNO B 220, are driven by local time references t_A and t_B, which are in the general case different to global time reference t_c and have time offsets t_A,off and t_{B, off} with respect to the global reference t_c. In the present model, these offsets include systematic offsets between different timing references used by c-server 232 and the particular eNB 21 1 , 221 , as well as timing errors due to clock drift or clock distribution effects within each MNOs’ network 210, 220. In general, MNOs are not aware of these offsets. This error model also applies to base stations of the same MNO.

In addition to the time offsets (systematic and error-dependent), there is also a delay time At between c-server 232 and a particular base station 21 1 , 221 , which is in general unknown to the base station 21 1 , 221 and the MNO 210, 220. The value of At may change, e.g. if the path (routers, gateway etc.) between c-server 232 and base station 21 1 , 221 changes.

There are delays in the access network due to radio propagation between base station 21 1 and UE 201 , as well as due to local processing and queuing effects at the base station 21 1. These are all captured by access delay t_R. This delay is in general smaller than At but may change faster due to UEs’ mobility.

The problem of global synchronization in the sense of distribution of a time reference to the mobile users within cellular networks of multiple operators has not been faced in cellular mobile networks up to date. This challenging new requirement comes from the nature of future cellular systems (e.g. 3GPP NR Rel. 16 and beyond), where multi-operator sidelink shall be potentially carried out in one frequency band. Moreover, the c-server network architecture has been recently proposed for enabling multi-operator V2V, but has not been deployed yet. Of course, the above synchronization problem as described and shown in Fig. 3 will occur also within a single MNO. In this case, the server providing control and potentially serving as a synchronization reference can be some internal reference within the MNO’s network.

Existing State-of-the-Art (SoTA) systems as LTE and IEEE 802.1 1 p have to face very different and more relaxed synchronization requirements. For example, in cellular LTE, PHY-layer synchronization is performed by each UE through the detection of predefined synchronization sequences in DL with respect to the serving base station. In the uplink, time advance (TA) is assigned to each UE by the network for aligning signals from different UEs. This level of PHY layer synchronization is sufficient for connection establishment with the base station. The detection of synchronization signals provides time synchronization including BOF/BOS estimation as well as typically carrier frequency synchronization and user identification at the same time.

IEEE 802.1 1 p is by definition a non-synchronized system, in the sense that data is not transmitted in a predefined point in time and moreover spans the complete frequency band. Therefore, no mutual synchronization between users or global synchronization and time reference distribution is needed.

For fixed-point (wired or wireless) networks, the so-called network timing protocols (NTPs) and the more accurate Precision Time Protocols (PTPs) as the IEEE 1588 and 1588-2008 (also known as PTP Version 2) protocols as described below with respect to Figures 4a,

4b and 5 are often used. These are used to synchronize clocks within a network and achieve accuracies in the sub-microsecond range. These protocols have been mainly designed for fixed network topologies, but are not common for dynamic wireless networks, as e.g. the cellular radio access network.

Fig. 4a shows a schematic diagram illustrating a delay measurement 400a in NTP/PTP based on one-step messaging. The client 410 transmits a first message 401 at time ti to the server 420 which is received by the server 420 at time t₂. t₂ can be indicated by the server 420 as the timestamp of reception 403. The server 420 transmits a second message 402 at time h to the client 410 which is received by the client 410 at time t₄. h can be indicated by the server 420 as the timestamp of transmission 404. A round trip delay can be determined by the client 410 according to: round trip delay is equal to ((t₄-t-i)- (t₃-t₂)). One key point is the measurement of delay. This is achieved by exchange of packets including time stamps, i.e. the value the timer used when a particular event occurs. The client 410 can use the time value provided by the server 420 and estimate the roundtrip delay (if the delays are symmetric, one-way delay is half of the round trip), e.g. according to the above-indexed formula which is shown in Fig. 4a.

Fig. 4b shows a schematic diagram illustrating a delay measurement 400a in NTP/PTP based on two-step messaging. A master 430 transmits a first message 431 at time ti and a follow-up message 432 to the first message 431 to a slave 440. The slave 440 knows the value of ti precisely, can determine the delay 433 between ti and reception of the follow-up message 432 precisely and can adjust its clock correspondingly.

In the“two step messaging” shown in Fig 4b, more precise time stamps are inserted in a follow-up message 432, compared to“on the fly” messaging, which is used for“one step messaging”, as shown in Fig. 4a.

Fig. 5 shows a message sequence chart 500 illustrating the main steps of the IEEE 1588 time synchronization protocol.

The ultimate goal of the PTP is to estimate and compensate the time offset with respect to a master 430 clock and network delay between master 430 and slave 440. The main stages of the time synchronization protocol are the following.

• Stage A, 510: clock offset alignment

• The master 430 sends“sync” message 501 (at time ti) including timestamp and the slave 440 uses its local clock to timestamp the message arrival (at time t₂).

• The slave 440 compares it to the actual sync transmission timestamp (at time ti) in master’s 430 follow-up message 502. The difference between two timestamps (t₂-t-i) equals to the clocks’ offset plus the transmission delay as illustrated at the bottom of Fig. 5.

• Slave 440 adjusts its local clock by this difference. • Stage B, 51 1 : Master->Slave delay d_M->s

• Slave 440 receives a second sync/follow-up message set 503. Using its updated clock it calculates the master-to-slave delay d_M->s.

• Stage C, 512: Slave->Master delay ds->M

• The slave 440 timestamps a delay request message 505 (at time t3).

• The master 430 timestamps the arrival of the delay request message 505 (at time t₄) and sends back a delay response message 506.

• The timestamps’ difference t3- U gives the slave-to-master delay ds->M. The slave 440 averages the two directional delays and adjusts to the mean delay and offset, as shown at the bottom of Fig. 5.

Fig. 6 shows a schematic diagram illustrating an exemplary mobile (vehicular) network 600 with synchronized sidelink 601 according to the disclosure. The mobile network 600 may correspond to the C-server architectures 200, 300 described above with respect to Figures 2 and 3. PTP protocol messages 602 are exchanged between C-server 232 and respective UEs 201 , 202. Control and feedback information 603 is exchanged between base stations 21 1 , 221 and corresponding UEs 201 , 202. Sidelink information is exchanged via a synchronized Sidelink 601 between the UEs 201 , 202.

The presented solution according to this disclosure for global synchronization and distribution of a common time reference within a network including internet, core and access network, can be explained based on Fig. 6, while a more detailed description of the procedure and the signaling between c-server 232, base station 21 1 , 221 and UE 201 , 202 is given by the sequence diagram shown in in Figure 7.

As shown in Figure 6, the PTP is implemented between the c-server 232 (that represents the master 430 according to Figures 4b and 5) and at least on UE (that represents the slave 440 according to Figures 4b and 5). In this way, the time offset can be compensated and the end-to-end (E2E) delay can be measured by the UE 201 , 202. It is noted that the PTP packets„bypass“ the base station 21 1 , 221 , meaning that it forwards PTP packets between c-server 232 and UE 201 , 202 without any need to read or modify them. Fig. 7 shows a message sequence chart 700 illustrating signalling and operations between C-server, base station/eNB and mobile user/UE according to the disclosure.

Already while running the PTP with the c-server 232, the UE 201 reports measurements 701 including time offset and -especially- E2E (end-to-end) delay to its serving base station 21 1 . Since the E2E delay can be different in the two directions, two measurements can be correspondingly reported to the base station 21 1 (first message Delay (dM->s) and offset report 701 , second message Delay (ds->M) report 702). In Fig. 8, dot-and-dash line arrows 501 , 502, 503, 504, 505, 506 and dashed line arrow 507 (optional) show the PTP signaling, while solid line arrows 701 , 702, 703 indicate the new disclosed signaling between UE 201 and the base station 21 1 .

The base station 21 1 is normally aware -or can measure- the access delay to the reporting UE 201 , which depends on parameters including timing advance (TA), delays due to queuing, processing etc. Considering all these, the base station 21 1 calculates the part of the delay corresponding to the path between c-server 232 and base station 21 1 (without access delay). Since this is the common delay part for all UEs 201 attached to this base station 21 1 , it is sufficient that one UE 201 or other node exchanging signals with the base station 21 1 , e.g. relay, road side unit etc. implements the PTP and reports measurements to the base station 21 1 . Of course, when measurements are reported from more than one UEs 201 , the accuracy can be naturally improved.

Finally, the base station 21 1 provides UE-specific sidelink synchronization instructions 703 to all attached UEs 201. These can have the form of a time offset with respect to a predefined known time reference, e.g. a„time shift" with respect to the UE-specific time reference instructed for UL, as shown in Fig. 8 by reference sign 703. All the synchronization-relevant information exchanged between UE 201 and eNB 21 1 can be e.g. transmitted in the control and feedback channels within the downlink frequency resources.

It is important that during the procedure, the UE 201 does not update its timing for DL/UL, neither does the base station 21 1. The UE 201 only estimates and tracks the c-server 232 timing, reports to the eNB 21 1 and receives instructions 703 for its own sidelink (SL) timing. According to the sequence diagram in Fig. 7, the overall procedure can be summarized as follows:

• The (at least one) UE 201 implements PTP and estimates E2E (C-server 232 to UE 201 ) delay and offsets, which are reported to its eNB 21 1 via UL.

• Based on E2E delay and own measurements and information, the eNB 21 1 calculates the c-server-to-eNB delay, which is common for all UEs 201 attached to the eNB 21 1.

• UE-specific synchronization instructions 703 are provided to all attached UEs 201.

These are given in a form which the UE 201 can identify, e.g. with respect to a time reference it already is aware of.

• In this way, UEs 201 can synchronize their time reference for sidelink.

• Time reference of UL as well as eNB synchronization is not affected by the above procedure.

• The procedure can be implemented by more than one UE 201 and with a variable frequency.

The three main entities shown in Fig. 7, i.e. C-server 232, base station/eNB 211 and mobile user/UE 201 can be implemented as described in the following:

The base station 21 1 may be for example an eNodeB or a gNodeB for synchronizing at least one user equipment, e.g. UE 201 , for its uplink 205 and/or sidelink 204

communication. The base station 211 comprises a processor which is configured to perform the following: forward at least one time synchronization message 501 , 503, 506 of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, between a time reference server (also denoted as C-server 232 in the figures) and at least one UE (201 ); receive synchronization information 701 of the at least one UE 201 about the synchronization between the at least one UE 201 and the time reference server 232, in particular an end-to-end delay between the time reference server 232 and the at least one UE 201 ; and transmit a synchronization instruction 703 to the at least one UE 201.

The base station 21 1 is enabled to acquire synchronization information about the time reference server 232 and/or an end-to-end delay. The base station 211 thus enables the at least one UE 201 synchronizing with the time reference server 232 as shown in Fig. 7. UEs’ 201 uplink and/or sidelink communication can follow a time reference based on the time reference server 232 and are time-aligned/synchronized with each other.

Note that the BS 21 1 does not need to synchronize with the time reference server 232 as well, but this can be an optional feature. The time reference finally used by the UEs 201 , 202, 203 does not need to be the server reference, but is based/depending on it.

The processor can use a second time reference, which is not based on a time reference of the time reference server 232, for synchronization of downlink communication with the at least one UE 201.

The BS 211 may be configured to use a time reference of the time reference server 232 for synchronizing of downlink communication with the at least one UE 201.

The synchronization information 701 , in particular the end-to-end delay, between the time reference server 232 and the at least one UE 201 may be based on a delay from the time reference server 232 to the at least one UE 201 and/or a delay from the at least one UE 201 to the time reference server 232.

The processor can determine an access delay between the base station 21 1 and the at least one UE 201.

The processor may be configured to determine the access delay between the base station 211 and the at least one UE 201 based on UE-specific information provided by the at least one UE 201 , in particular depending on radio propagation delay, known contribution of the base station 211 to the access delay and timing advance, TA.

The processor can determine a network delay between the time reference server 232 and the base station 21 1 based on the synchronization information 701 , in particular the end- to-end delay, and the access delay. The base station 21 1 may be configured to use the network delay for synchronization of the at least one UE 201.

The processor can determine the time reference of the time reference server based on the network delay.

The processor can transmit synchronization instructions 703 to a plurality of UEs. The synchronization instructions 703 can be UE-specific, for example, or specific to a group of UEs.

The synchronization instructions 703 may be based on the network delay and UE-specific time measurements and parameters, in particular radio propagation delay and known contribution of the base station 211 to the access delay and UE-specific timing advance, TA.

The processor can forward the at least one time synchronization message 501 , 503, 506 between the time reference server 232 and the at least one UE 201 without participating in the time synchronization protocol.“Without participating in the time synchronization protocol” in the sense of the invention comprises that the BS does not implement the synchronization protocol itself and takes a role within the communication according to this protocol, and/or that the BS does not read the time synchronization message.

The processor can be configured to prioritize forwarding the at least one time

synchronization message 501 , 503, 506 between the time reference server 232 and the at least one UE 201.

No extra delay is induced during the synchronization due to queuing in particular in case of network traffic jam.

The processor can request the at least one UE 201 providing the synchronization information 701 , in particular the end-to-end delay, between the time reference server 232 and the at least one UE 201. If the BS realizes that the synchronization should be updated it can request so. The synchronization information 701 , in particular the end-to-end delay, between the time reference server 232 and the at least one UE 201 can be periodically received from the at least one UE 201.

The processor can request the at least one UE 201 changing a period for reporting the synchronization information 701 , in particular the end-to-end delay, in particular if the base station 21 1 detects changes in network delay between the time reference server 232 and the base station 21 1.

The user equipment, UE 201 can be used for assisting the base station 21 1 (or another base station) for synchronizing at least one user equipment, UE 201 , e.g. UE 201 or another UE, for its uplink 205 and/or sidelink 204 communication. The UE 201 comprises a processor configured to perform the following: receive a time synchronization message 501 , 503, 506 of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, from a time reference server 232 (e.g. the C- server as depicted in the figures); determine synchronization information 701 about a synchronization between the UE 201 and the time reference server 232, in particular an end-to-end delay between the time reference server 232 and the UE 201 , based on the time synchronization message 501 , 503, 506; report the synchronization information 701 to the base station 21 1 ; and receive a synchronization instruction 703 from the base station 21 1 for synchronizing the UE’s uplink 205 and/or sidelink 204 communication.

The processor can be configured to report the synchronization information to the base station via an Uplink feedback channel.

The processor can be configured to receive the UE-specific synchronization instructions from the base station via a Downlink control channel.

The processor can be configured to align a clock offset with the time reference server based on a first synchronization message received from the time reference server and particularly based on a first follow-up message to the first synchronization message according to the PTP/NTP protocol.

The processor can be configured to determine a master-to-slave delay indicating a delay between the time reference server and the UE based on the aligned clock offset and a second synchronization message received from the time reference server and particularly based on a second follow-up message to the second synchronization message according to the PTP/NTP protocol.

The processor can be configured to determine a slave-to-master delay indicating a delay between the UE and the time reference server based on a delay response message received from the time reference server and in particular based on a follow-up message to the delay response message according to the PTP/NTP protocol.

The first synchronization message, the second synchronization message and the delay response message can be received from the time reference server without modification by the base station message according to the PTP/NTP protocol.

The UE 201 may comprise: a first modem 1202 (e.g. as shown in Fig. 12) comprising a first protocol stack 1020, PC5, for processing the UE’s 201 sidelink 204 communication; and a second modem 1201 (e.g. as shown in Fig. 12) comprising a second protocol stack 1010, Uu, for processing Uplink/Downlink 205 communication with the base station 211 , wherein the first protocol stack 1020 and the second protocol stack 1010 comprise a shared IP layer 1001 , a shared Radio Resource Connection, RRC, layer 1002 and separate MAC layers 1005, e.g. as described below with respect to Figures 10 and 11.

The processor may be configured to process the time synchronization protocol based on the shared IP layer 1001 and to synchronize the UE’s 201 uplink 205 and/or sidelink 204 communication based on the shared RRC layer 1002 or based on the separate MAC layers 1005, e.g. as described below with respect to Figures 10 and 1 1.

The processor may be configured to compensate an internal delay between the first modem 1202 and the second modem 1201 and to synchronize the UE 201 with the time reference server 232 based on the compensated internal delay.

The processor may be configured to report the synchronization information, in particular the end-to-end delay to the base station 21 1 , wherein the synchronization information includes an internal delay between the first modem 1202 and the second modem 1201 , e.g. as described below with respect to Figure 12. The processor may be configured to provide a synchronization instruction 91 1 , 921 to another UE 203 that is out of coverage from the base station 211 , e.g. as described below with respect to Figure 9.

The processor may be configured to provide the synchronization instruction 91 1 , 921 to the other UE 203 via a sidelink 204 control channel between the UE 201 and the other UE.

The synchronization information provided to the other UE can be specific for the other UE.

The UE 201 can be configured to measure a delay, in particular a round-trip delay, between the UE 201 and the other UE 203 and to base the synchronization instruction 703 on this delay.

An overall delay is complemented by the delay between the UE and the other UE.

The UE 201 may be configured to receive a request from the other UE 203 to measure the delay.

The time reference server 232 can be used for synchronizing at least one user equipment, e.g. UE 201 , for its uplink and/or sidelink communication. The time reference server comprises a processor which is configured to: transmit at least one time synchronization message 501 , 503, 506 of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, to the at least one UE 201. The at least one synchronization message 501 , 503, 506 comprises information to enable the at least one UE 201 to report synchronization information 701 about a synchronization between the at least one UE 201 and the time reference server 232, in particular an end-to-end delay between the time reference server 232 and the at least one UE 201.

Such a time reference server 232 provides the advantage that the UE can then be configured to report the synchronization information to a base station to enable the base station transmitting a synchronization instruction to the at least one UE for synchronizing the UE’s uplink and/or sidelink communication. The processor can be configured to transmit a first synchronization message 501 and particularly a first follow-up message 502 to the first synchronization message 501 via bypassing the base station 211 to the UE 201.

The processor can be configured to transmit a second synchronization message 503 and particularly a second follow-up message 504 to the second synchronization message 503 via bypassing the base station 211 to the UE 201.

The processor can be configured to receive a delay request message 505 via bypassing the base station 21 1 from the UE 201.

The processor can be configured to transmit a delay response message 506 and particularly a follow-up message 507 to the delay response message 506 via bypassing the base station 211 to the UE 201.

The time reference server 232 can be configured to transmit the time synchronization message to a plurality of operator networks; and/or to be operated extern of an operator network. This provides the advantage that the time reference server can be used operator-independent.

Fig. 8 shows a message sequence chart 800 illustrating the principle of transparent clock according to the disclosure. If base stations should serve as bridges, the PTP would have to be implemented by all base stations and all attached UEs. This would result into a large signaling overhead compared to the disclosed solution.

A master 801 that may represent the C-server 232 described above exchanges synchronization messages 501 , 502, 505, 506 according to a PTP protocol with a bridge 802, e.g. a base station 211 as described above. Then, the bridge 802, e.g. the base station 21 1 exchanges synchronization messages 501 , 502 according to a PTP protocol with a slave 803, e.g. a UE 201 as described above. The master 801 can determine delay measurement 811 (only in P2P bridges) and the slave 803 can determine delay d and residence time t3-t2, where t₂ is the arrival time of synch message 501 at the bridge 802 and t₃ is arrival time of the delay response message 506 at the bridge 802. In order to avoid the disclosed solution as presented above with respect to Fig. 7, the most straightforward alternative would be that the base stations 21 1 , 221 are more actively involved in the synchronization procedure, e.g. by applying the PTP and serving as “transparent clock” for all attached UEs 201 , 202, which would also apply the PTP. In this way, delays between c-server 232 and base station 21 1 , 221 as well as“residence times” (delays introduced by a node) would be“invisible” to the UEs 201 , 202 as UEs 201 , 202 would see only the time stamps, i.e. time reference defined by the bridge 802.

However, the main drawback of such technical solution would the large overhead due to the signaling in both directions between base stations and all their attached UEs, instead of the at least one UE per base station, which the disclosed solution requires. Moreover, the UEs which would not be implementing the PTP would not be able to synchronize at all, meaning that the full PTP implementation would be required for all UEs.

For reference, the basic principle of“transparent clocks” or“bridges” 802 is shown in Fig. 8. The main drawback is that all UEs and base stations would have to implement the PTP, resulting in a large signaling overhead between c-server, base stations and mobile users.

It is underlined that the requirement of a fully synchronized cellular network (similar as in Time Division Duplex - TDD networks), further including base stations of multiple MNOs is a technically very hard and undesired requirement from the MNOs’ point of view. Moreover, assuming GNSS as a global reference for the sidelink is a feasible technical solution, but not recommended, as GNSS is not always available and seen as a non-reliable source.

Hence, advantages of the disclosed solution are that the disclosed solution

Does NOT require base stations to

o exchange data with c-server

o implement PTP

o synchronize to the c-server

o change the parameterization or time synchronization used for DL transmission.

Requires only ONE UE (or other node) to implement the PTP with the c-server. o Results into a low overhead for control/feedback information

o Allows synchronization of UEs which are not capable to run the PTP Allows UEs to synchronize in the sidelink without them or their base stations to exchanging information about cellular time references

Fig. 9 shows a message sequence chart 900 illustrating synchronization of UEs without cellular coverage according to the disclosure. In-coverage UEs 201 act as“transparent clocks”, allowing other UEs 203 to synchronize by implementing the PTP, without being aware of the delays and offsets of the C-server-to-UE path. In-coverage UE 201 provides direct synchronization information through a sidelink control channel.

Including out-of-coverage UEs, e.g. UE 203 shown in Fig. 9, is an essential component in the considered communication scenario. In order to achieve this, two possible solutions are hereby disclosed and shown in Fig. 9.

The first option 910 requires that in-coverage UEs 201 act as“transparent clocks” (firstly defined in IEEE 1588-2008), allowing attached out-of-coverage UEs 203 to synchronize by implementing the PTP. This architecture has the positive aspect that the attached UE 203 does not need to be aware or consider in any way the delays and offsets behind the in-coverage UE 201 , i.e. from C-server-to-UE path. Of course, this requires that all out-of- coverage UEs 203 indeed run the PTP.

The second option 920 goes into the direction of the in-coverage UE 201 taking over a role similar as the eNB 21 1. It provides directly synchronization information through a sidelink control channel to the out-of-coverage UE 203, including synchronization information 921 , 922, 923 similar to the one the eNB 21 1 would have provided. Optionally, a delay measurement can be included, in order to compensate differences in the access delay between different pairs or groups of UEs.

In the first option 910, synchronization and follow-up messages 91 1 are transmitted from in-coverage UE 201 to out-of-coverage UE 203. Out-of-coverage UE 203 answers with delay request (t_R) message 912 and in-coverage UE 201 transmits delay response (t_R) to out-of-coverage UE 203.

In the second option 920, synchronization message 921 is transmitted from in-coverage UE 201 to out-of-coverage UE 203. The synchronization message 921 includes timing offset Toff as a function of At, t_0ff and timing advance TA. Out-of-coverage UE 203 answers with delay request (t_R) message 922 and in-coverage UE 201 transmits delay response (t_R)/T_0ff update message 923 to out-of-coverage UE 203.

Figs. 10 and 11 show a schematic diagrams illustrating an exemplary implementation of protocol stacks 1000, 1 100 in a mobile (vehicular) network including C-server, base station/eNB and UE according to a first and a second implementation form.

In both implementations 1000, 1100, the c-server 232 includes an RRC/MAC controller and an IP layer; the eNB includes a physical (PHY) layer 1006, a MAC layer 1005, an RLC layer 1004, a PDCP layer 1003 and an RRC layer 1002; the UE 201 includes a protocol stack 1010 implementing the uplink/downlink communication link Uu and a protocol stack 1020 implementing the sidelink communication PC5. The uplink/downlink stack 1010 includes a physical (PHY) layer 1006, a MAC layer 1005, an RLC layer 1004, a PDCP layer 1003, a common RRC layer 1002 (common with the sidelink stack 1020) and a common IP layer 1001 (common with the sidelink stack 1020). The sidelink stack 1020 includes a physical (PHY) layer 1006, a MAC layer 1005, an RLC layer 1004, a PDCP layer 1003, a common RRC layer 1002 (common with the uplink/downlink stack 1010) and a common IP layer 1001 (common with the uplink/downlink stack 1010).

In the two exemplary implementations 1000, 1 100 shown in Figures 10 and 1 1 , arrows 101 1 (in Fig. 10) and 1 11 1 (in Fig. 11 ) indicate the PTP flow, while arrows 1012, 1013 (in Fig. 10) and 11 12, 1 113, 1 114 (in Fig. 1 1 ) indicate the new signaling required for sidelink synchronization. As the UE 201 has the capability of connecting to both UL/DL and SL, it will include two protocol stacks 1010, 1020, which may, however, share the upper Radio Resource Connection (RRC) 1002 and IP 1001 layers. It is noted that the PTP between UE 201 and C-server 232 has to be implemented on IP 1001 , in the general case where the C-server 232 is outside the MNO’s network. For the more special case where the C- server 232 is inside the MNO’s network, it can be also implemented on PDCP 1003. In the implementations shown in Figures 10 and 11 , synchronization information 1013 (in Fig.

10) and 1 1 13, 1 114 (in Fig. 11 ) between UE 201 and eNB 21 1 is exchanged on the RRC 1002 (in Fig. 10) or the MAC 1005 layer (in Fig. 1 1 ). A design solution should consider how delay-sensitive the procedure is, and whether resources on the MAC layer 1005 for regular control/feedback shall be used to provide lowest delay and highest reliability. Fig. 12 shows a schematic diagram illustrating an exemplary implementation of clock distribution within the UE according to the disclosure. Clock distribution within the UE 201 may introduce delays. The communication interface used for delay measurements needs to be carefully chosen. Internal delays can be either considered as part of the overall E2E delay or compensated internally by each UE separately.

The UE 201 includes a first component, e.g. a first modem 1201 to perform DL/UL communication and a second component, e.g. a second modem 1202 to perform sidelink communication. Internal clock distribution 1203 between the first modem 1201 and the second modem 1202 can result in different clock references.

A further implementation-related aspect on the UE side is the definition of the connection and measurement interface. In a real implementation, clock distribution within the UE 201 , e.g. between the UL/DL 1201 and SL 1202 units or modems, will introduce a delay.

Therefore, the communication interface used for delay measurements needs to be carefully chosen. Two possible solutions are hereby presented:

1. The DL/UL unit 1201 performs all measurements with c-server232 and receives instructions. Internally, instructions provided by eNB 21 1 need to be adjusted considering the measured/known internal delays before used for the SL unit 1202.

2. E2E delay is defined between c-server 232 and UE SL unit 1202. This means that the PTP is implemented on the SL unit 1202 and measurements/instructions are relayed over the DL/UL unit 1201 to the eNB 21 1.

Fig. 13 shows a schematic diagram illustrating a method 1300 for synchronizing a UE 201 for its uplink and/or sidelink communication, from base station 211 side, according to the disclosure.

The method 1300 includes forwarding 1301 at least one time synchronization message 501 , 503, 506, e.g. as described above with respect to Fig. 5, of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, between a time reference server 232 and at least one UE 201 , e.g. as described above with respect to Figs. 6 and 7. The method 1300 further includes receiving 1302 synchronization information 701 of the at least one UE 201 about the synchronization between the at least one UE 201 and the time reference server 232, in particular an end-to-end delay between the time reference server 232 and the at least one UE 201 , e.g. as described above with respect to Figs. 6 and 7.

The method 1300 further includes transmitting 1303 a synchronization instruction 703 to the at least one UE 201 , e.g. as described above with respect to Figs. 6 and 7.

Fig. 14 shows a schematic diagram illustrating a method 1400 for synchronizing a UE 201 for its uplink and/or sidelink communication, from UE 201 side, according to the disclosure.

The method 1400 includes receiving 1401 a time synchronization message 501 , 503, 506, e.g. as described above with respect to Fig. 5, of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, from a time reference server 232, e.g. as described above with respect to Figs. 6 and 7.

The method 1400 further includes determining 1402 synchronization information 701 about a synchronization between the UE 201 and the time reference server 232, in particular an end-to-end delay between the time reference server 232 and the UE 201 , based on the time synchronization message 501 , 503, 506, e.g. as described above with respect to Figs. 6 and 7.

The method 1400 further includes reporting 1403 the synchronization information 701 to a base station 211 , e.g. as described above with respect to Figs. 6 and 7.

The method 1400 further includes receiving 1404 a synchronization instruction 703 from the base station 21 1 for synchronizing the UE’s uplink 205 and/or sidelink 204

communication, e.g. as described above with respect to Figs. 6 and 7.

The present disclosure also supports a computer program product including computer executable code or computer executable instructions that, when executed, causes at least one computer to execute the performing and computing steps described herein, in particular the steps of the methods 1300, 1400 and the flowcharts 400a, 400b, 500, 700, 800, 900 described above with respect to Figs. 4-5, 7-9 and 13-14. Such a computer program product may include a readable non-transitory storage medium storing program code thereon for use by a computer. The program code may perform the processing and computing steps described herein, in particular the methods 1300, 1400 and the flowcharts 400a, 400b, 500, 700, 800, 900 described above with respect to Figs. 4-5, 7-9 and 13-14.

While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features or aspects of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms "include", "have", "with", or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term "comprise". Also, the terms "exemplary", "for example" and "e.g." are merely meant as an example, rather than the best or optimal. The terms“coupled” and “connected”, along with derivatives may have been used. It should be understood that these terms may have been used to indicate that two elements cooperate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other.

Although specific aspects have been illustrated and described herein, it will be

appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific aspects shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific aspects discussed herein.

Although the elements in the following claims are recited in a particular sequence with corresponding labelling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the invention beyond those described herein. While the present invention has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the present invention. It is therefore to be understood that within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described herein.

Claims

CLAIMS:

1. A base station (211 ), in particular an eNodeB or a gNodeB, for synchronizing at least one user equipment, UE (201 ), for its uplink (205) and/or sidelink (204)

communication, the base station (21 1 ) comprising a processor configured to: forward at least one time synchronization message (501 , 503, 506) of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, between a time reference server (232) and at least one UE (201 ); receive synchronization information (701 ) of the at least one UE (201 ) about the synchronization between the at least one UE (201 ) and the time reference server (232), in particular an end-to-end delay between the time reference server (232) and the at least one UE (201 ); and transmit a synchronization instruction (703) to the at least one UE (201 ).

2. The base station (21 1 ) of claim 1 , wherein the processor is configured to use a second time reference, which is not based on a time reference of the time reference server (232), for synchronization of downlink communication with the at least one UE (201 ).

3. The base station (21 1 ) of claim 1 , configured to use a time reference of the time reference server (232) for synchronizing of downlink communication with the at least one UE (201 ).

4. The base station (21 1 ) of claim 2 or 3, wherein the synchronization information (701 ), in particular the end-to-end delay, between the time reference server (232) and the at least one UE (201 ) is based on a delay from the time reference server (232) to the at least one UE (201 ) and/or a delay from the at least one UE (201 ) to the time reference server (232).

5. The base station (21 1 ) according to one of the preceding claims, wherein the processor is configured to determine an access delay between the base station (211 ) and the at least one UE (201 ).

6. The base station (21 1 ) of claim 5, wherein the processor is configured to determine the access delay between the base station (211 ) and the at least one UE (201 ) based on UE-specific information provided by the at least one UE (201 ), in particular depending on radio propagation delay, known contribution of the base station (211 ) to the access delay and timing advance, TA.

7. The base station (21 1 ) of claim 5 or 6, wherein the processor is configured to determine a network delay between the time reference server (232) and the base station (21 1 ) based on the synchronization information (701 ), in particular the end-to-end delay, and the access delay. 8. The base station (21 1 ) of claim 7, configured to use the network delay for synchronization of the at least one UE

(201 ).

9. The base station (21 1 ) of claim 7 or 8, wherein the processor is configured to determine the time reference of the time reference server based on the network delay.

10. The base station (21 1 ) of one of claims 7 to 9, wherein the processor is configured to transmit synchronization instructions (703) to a plurality of UEs and in particular wherein the synchronization instructions (703) are UE-specific or specific to a group of UEs. 1 1. The base station (21 1 ) of claim 10, wherein the synchronization instructions (703) are based on the network delay and UE-specific time measurements and parameters, in particular radio propagation delay and known contribution of the base station (211 ) to the access delay and UE-specific timing advance, TA.

12. The base station (21 1 ) of one of the preceding claims, wherein the processor is configured to forward the at least one time

synchronization message (501 , 503, 506) between the time reference server (232) and the at least one UE (201 ) without participating in the time synchronization protocol. 13. The base station (21 1 ) of one of the preceding claims, wherein the processor is configured to prioritize forwarding the at least one time synchronization message (501 , 503, 506) between the time reference server (232) and the at least one UE (201 ).

14. The base station (21 1 ) of one of the preceding claims, wherein the processor is configured to request the at least one UE (201 ) providing the synchronization information (701 ), in particular the end-to-end delay, between the time reference server (232) and the at least one UE (201 ).

15. The base station (21 1 ) of one of the preceding claims, wherein the synchronization information (701 ), in particular the end-to-end delay, between the time reference server (232) and the at least one UE (201 ) is periodically received from the at least one UE (201 ).

16. The base station (21 1 ) of claim 15, wherein the processor is configured to request the at least one UE (201 ) changing a period for reporting the synchronization information (701 ), in particular the end-to-end delay, in particular if the base station (21 1 ) detects changes in network delay between the time reference server (232) and the base station (21 1 ).

17. A user equipment, UE (201 ) for assisting a base station (211 ) for synchronizing at least one user equipment, UE (201 ), for its uplink (205) and/or sidelink (204)

communication, the UE (201 ) comprising a processor configured to: receive a time synchronization message (501 , 503, 506) of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, from a time reference server (232); determine synchronization information (701 ) about a synchronization between the UE (201 ) and the time reference server (232), in particular an end-to-end delay between the time reference server (232) and the UE (201 ), based on the time synchronization message (501 , 503, 506); report the synchronization information (701 ) to the base station (21 1 ); and receive a synchronization instruction (703) from the base station (211 ) for synchronizing the UE’s uplink (205) and/or sidelink (204) communication.

18. The UE (201 ) of claim 17, wherein the processor is configured to report the synchronization information to the base station via an Uplink feedback channel.

19. The UE (201 ) of claim 17 or 18, wherein the processor is configured to receive the UE-specific synchronization instructions from the base station via a Downlink control channel.

20. The UE (201 ) of one of claims 17 to 19, comprising: a first modem (1202) comprising a first protocol stack (1020, PC5) for processing the UE’s (201 ) sidelink (204) communication; and a second modem (1201 ) comprising a second protocol stack (1010, Uu) for processing Uplink/Downlink (205) communication with the base station (21 1 ), wherein the first protocol stack (1020) and the second protocol stack (1010) comprise a shared IP layer (1001 ), a shared Radio Resource Connection, RRC, layer (1002) and separate MAC layers (1005).

21. The UE (201 ) of claim 20, wherein the processor is configured to process the time synchronization protocol based on the shared IP layer (1001 ) and to synchronize the UE’s (201 ) uplink (205) and/or sidelink (204) communication based on the shared RRC layer (1002) or based on the separate MAC layers (1005).

22. The UE (201 ) of claim 20 or 21 , wherein the processor is configured to compensate an internal delay between the first modem (1202) and the second modem (1201 ) and to synchronize the UE (201 ) with the time reference server (232) based on the compensated internal delay. 23. The UE (201 ) of claim 20 or 21 , wherein the processor is configured to report the synchronization information to the base station (211 ), wherein the synchronization information includes an internal delay between the first modem (1202) and the second modem (1201 ).

24. The UE (201 ) of one of claims 17 to 23, wherein the processor is configured to provide a synchronization instruction (911 ,

921 ) to another UE (203) that is out of coverage from the base station (21 1 ).

25. The UE (201 ) of claim 24, wherein the processor is configured to provide the synchronization instruction (91 1 , 921 ) to the other UE (203) via a sidelink (204) control channel between the UE (201 ) and the other UE (203).

26. The UE (201 ) of one of claims 17 to 25, configured to measure a delay, in particular a round-trip delay, between the UE (201 ) and the other UE (203) and to base the synchronization instruction (703) on this delay. 27. The UE (201 ) of claim 26, wherein the UE (201 ) is configured to receive a request from the other UE (203) to measure the delay.

28. A time reference server (232) for synchronizing at least one user equipment, UE (201 ), for its uplink and/or sidelink communication, the time reference server comprising a processor configured to: transmit at least one time synchronization message (501 , 503, 506) of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, to the at least one UE (201 ), wherein the at least one synchronization message (501 , 503, 506) comprises information to enable the at least one UE (201 ) to report synchronization information (701 ) about a synchronization between the at least one UE (201 ) and the time reference server (232), in particular an end-to-end delay between the time reference server (232) and the at least one UE (201 ).

29. The time reference server (232) of claim 28, wherein the processor is configured to do at least one of the following: transmit a first synchronization message (501 ) and particularly a first follow-up message (502) to the first synchronization message (501 ) via bypassing the base station (21 1 ) to the UE (201 ); transmit a second synchronization message (503) and particularly a second follow- up message (504) to the second synchronization message (503) via bypassing the base station (21 1 ) to the UE (201 ); receive a delay request message (505) via bypassing the base station (211 ) from the UE (201 ); and transmit a delay response message (506) and particularly a follow-up message (507) to the delay response message (506) via bypassing the base station (211 ) to the UE (201 ).

30. The time reference server (232) of one of the preceding claims, configured to transmit the time synchronization message to a plurality of operator networks; and/or to be operated extern of an operator network.

31. A method (1300) for synchronizing user equipment, UE (201 ), for its uplink (205) and/or sidelink (204) communication, the method (1300) comprising: forwarding (1301 ) at least one time synchronization message (501 , 503, 506) of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, between a time reference server (232) and at least one UE (201 ); receiving (1302) synchronization information (701 ) of the at least one UE (201 ) about the synchronization between the at least one UE (201 ) and the time reference server (232), in particular an end-to-end delay between the time reference server (232) and the at least one UE (201 ); and transmitting (1303) a synchronization instruction (703) to the at least one UE (201 ).

32. A method (1400) for synchronizing at least one user equipment, UE (201 ), for its uplink (205) and/or sidelink (204) communication, the method comprising: receiving (1401 ) a time synchronization message (501 , 503, 506) of a time synchronization protocol, in particular a Precision Time Protocol, PTP or a Network Timing Protocol, NTP, from a time reference server (232); determining (1402) synchronization information (701 ) about a synchronization between the UE (201 ) and the time reference server (232), in particular an end-to-end delay between the time reference server (232) and the UE (201 ), based on the time synchronization message (501 , 503, 506); reporting (1403) the synchronization information (701 ) to a base station (21 1 ); and receiving (1404) a synchronization instruction (703) from the base station (21 1 ) for synchronizing the UE’s uplink (205) and/or sidelink (204) communication.