WO2018088951A1

WO2018088951A1 - Method and apparatus for synchronization

Info

Publication number: WO2018088951A1
Application number: PCT/SE2017/051110
Authority: WO
Inventors: Yunxi LI; Qianxi Lu; Yanli Zheng
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2016-11-11
Filing date: 2017-11-08
Publication date: 2018-05-17

Abstract

The disclosure relates to a method for synchronizing to a wireless communication system performed by a wireless device configured for sidelink-based V2X communication is provided. The method comprises determining (510) a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a GNSS and a DFN offset. The method also comprises estimating (520) a timing of a radio frame boundary of the wireless communication system to correspond to the determined timing of the radio frame boundary for sidelink-based V2X communication, and initiating (530) a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system. The disclosure also relates to the wireless device configured to perform the method.

Description

METHOD AND APPARATUS FOR SYNCHRONIZATION

TECHNICAL FIELD

The disclosure generally relates to synchronization, and particularly relates to a method for synchronizing to a wireless communication system performed by a wireless device configured for sidelink-based Vehicle-to-X communication, and apparatus therefore.

BACKGROUND

The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of Universal Mobile Telecommunication System (UMTS), and Long Term Evolution (LTE). LTE is also sometimes referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is a next generation wireless communication system relative to UMTS. LTE brings significant improvements in capacity and performance over previous radio access technologies.

The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and E-UTRAN is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a wireless device, also called a User Equipment (UE), is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to the UE and receiving signals transmitted by the UE. The area served by one or sometimes several RBSs may be referred to as a cell.

Wireless devices, which are referred to as UE in 3GPP terminology, may comprise, for example, cellular telephones, personal digital assistants, smart phones, laptop computers, handheld computers, machine-type communication/machine-to-machine (MTC/M2M) devices or other devices with wireless communication capabilities. Wireless devices may refer to terminals that are installed in fixed configurations, such as in certain machine-to-machine applications, as well as to portable devices, or devices installed in motor vehicles. Hereinafter, a wireless device may sometimes be referred to as a UE or simply as a device.

During Release 12, the LTE standard has been extended with support of Device-to- device (D2D) communication, which has been specified as sidelink in the standard. There are D2D features targeting both commercial and Public Safety applications. One feature enabled by Rel-12 LTE D2D is device discovery, allowing wireless devices to sense the proximity of another wireless device by the broadcasting and detection of discovery messages that carry device and application identities. Another D2D feature is direct communication based on physical channels terminated directly between the devices. This new direct D2D interface is sometime designated as PC5, also known as sidelink at the physical layer.

In Rel-14, the extensions for D2D consist of support of Vehicle-to-Everything or Vehicle-to-X (V2X) communication, which includes any combination of direct communication between vehicles, pedestrians and infrastructure equipped with D2D/V2X enabled wireless devices. V2X communication may take advantage of a NW infrastructure, when available, but at least basic V2X connectivity should be possible even in case of lack of coverage. Providing an LTE-based V2X interface may be economically advantageous because of the LTE economies of scale and it may enable tighter integration between communications with the NW infrastructure (Vehicle to Infrastructure (V2I)) and Vehicle to Pedestrian (V2P) and Vehicle to Vehicle (V2V) communications, as compared to using a dedicated V2X technology. Some V2X scenarios for an LTE based network are schematically illustrated in Figure 1.

V2X communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirement sets, e.g., in terms of latency, reliability, and capacity.

LTE Synchronization procedure

After a UE has been switched on, a Public Land Mobile Network (PLMN) selection is done, which in LTE begins with a synchronization procedure, also referred to as a cell search procedure. The cell search procedure is used by the UE to acquire time and frequency synchronization with a cell, and to obtain a physical-layer Cell Identity (PCI) of that cell. There are mainly two cell search procedures in LTE: initial synchronization or cell search at power on of the UE, and cell search for detecting neighbor cells e.g. in preparation for a handover. Figure 2A schematically illustrates the main steps of the LTE cell search procedure, and indicates the information acquired at each step of the cell search.

In the procedure of cell search at power on of the UE, the UE attempts to measure the wideband received power for frequencies over a set of supported frequency bands one after another after power on, and ranks those frequencies/cells based on the signal strength. The UE then uses downlink synchronization signals to correlate with the received signal. The UE blindly detects the Primary Synchronization Signal (PSS) as there is no a priori information. The PSS is transmitted in the last Orthogonal frequency-division multiplexing (OFDM) symbol of a first time slot of the first and 5th sub-frames. The detection of the PSS in 210 thus provides symbol timing detection and physical-layer identity detection, as illustrated in Figure 2A. A Secondary Synchronization Signal (SSS) is also located in the same subframe as PSS but in the symbol before the PSS carrying symbol. From the detection of SSS, 220, the UE is able to obtain the Radio Frame Timing, PCI, Cyclic Prefix (CP) length, and duplex mode (Frequency division duplex, FDD, or Time division duplex, TDD). Once the UE knows the PCI for a given cell, it also knows the location of cell reference signals which are used for channel estimation, cell selection/reselection and handover procedures.

There are 504 unique physical-layer cell identities (PCI). The PCIs are grouped into

168 unique PCI groups, each group containing three unique identities. The grouping is such that each PCI is part of one and only one PCI group. A PCI is thus uniquely defined by a number in the range of 0 to 167, representing the PCI group, and a number in the range of 0 to 2, representing the physical-layer identity within the PCI group.

As described above, after detection of PSS, the symbol timing, physical-layer identity and carrier frequency are acquired. The UE will then search for the SSS at four possible positions based on the acquired position of PSS. After SSS detection, the radio frame timing, PCI, duplex mode and CP length is acquired. After the radio frame timing is determined, the UE will decode the master information block (MIB) that is broadcasted by the eNodeB. Some critical system information, such as System Frame Number (SFN) and downlink bandwidth will be obtained based on the MIB.

In the LTE synchronization procedure (schematically illustrated in Figure 2A), the UE needs to do the PSS detection blindly as there is no prior knowledge. Although the PSS can be detected with a frequency offset up to ±7.5 kHz due to the flat frequency domain auto correlation property and the low frequency offset sensitivity, the crystal oscillator tolerance may be as large as ±17 ppm. Therefore, for the frequency synchronization a worst-case initial frequency offset is about ±45 kHz in the 2.6 GHz band (E-UTRA band 38). The frequency offsets hypothesis will thus be [-45:7.5:45] kHz for each physical-layer identity.

For the time synchronization of PSS, matched filtering is used to fetch the correlation peak to determine the possible PSS position. The received signal will be down-sampled to 1.92Msps, as PSS only occupy the central six resource blocks. For each frequency offset hypothesis and each physical-layer identity, there will be 9600 correlation results in one time slot data segment, i.e. in 5 ms, where 9600 corresponds to the received signal length. There is one PSS and one SSS in every 5ms. This means that there will be 3*13*9600 candidates of PSS timing and frequency offset, as there are three PSS candidates, and for each PSS candidate there are 13 frequency hypothesis and 9600 timing possibilities in 5ms. As there is noise, interference, imperfections, and spurious correlation peaks with timing offset and frequency offset, it will take a certain time to get a successful blind detection for the PSS, thus implying a certain latency for the cell search. Furthermore, for the detection of SSS, four possible positions needs to be checked based on the detected PSS timing, and the PCI group candidates may be checked using either coherent or non-coherent techniques.

Although both the PSS and SSS are transmitted twice per radio frame, it may take up to 200 ms - with a corresponding power consumption - to find a suitable cell (excluding the broadcast reading of the MIB). When a V2X UE moves across a cell border of e.g. an LTE cell, or from out of coverage to LTE coverage, the V2X UE has to obtain synchronization information from the eNodeB to secure service continuity of the V2X service. An LTE synchronization procedure that may take up to 200 ms, may be a challenge for the V2X UE which may have a high requirement on e.g. latency. There is thus a need for improved synchronization procedures for a UE configured for sidelink-based V2X communication.

SUMMARY

An object of embodiments is to alleviate or at least reduce one or more of the above mentioned problems, and to provide an improved synchronization procedure. This object, and others, is achieved by a method and apparatus according to the independent claims, and by the embodiments according to the dependent claims.

According to a first aspect, a method for synchronizing to a wireless communication system performed by a wireless device configured for sidelink-based V2X communication is provided. The method comprises determining a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a GNSS and a DFN offset. The method also comprises estimating a timing of a radio frame boundary of the wireless communication system to correspond to the determined timing of the radio frame boundary for sidelink-based V2X communication, and initiating a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system.

According to a second aspect, a wireless device configured for sidelink-based Vehicle-to-X, V2X, communication and configured to synchronize to a wireless communication system is provided. The wireless device is further configured to determine a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a Global Navigation Satellite System, GNSS, and a Device-to-Device Frame Number, DFN, offset. The wireless device is also configured to estimate a timing of a radio frame boundary of the wireless communication system to correspond to the determined timing of the radio frame boundary for sidelink-based V2X communication, and initiate a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system.

According to further aspects, a computer program comprising computer readable code which when executed by at least one processor of a wireless device causes the wireless device to carry out the method according to the first aspect is provided, as well as a carrier containing the computer program.

One advantage of embodiments is that the time and corresponding power consumption of the synchronization procedure will decreases substantially as the complexity of the synchronization procedure is decreased.

Another advantage of embodiments is that not only the complexity will be reduced for the PSS and SSS detection, but also the accuracy will be increased as there will be no spurious correlation peaks as the radio frame boundary timing is known.

Other objects, advantages, and features of embodiments will be explained in the following detailed description when considered in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically illustrates example scenarios for V2X based on LTE.

Figure 2A schematically illustrates steps of an LTE synchronization or cell search procedure according to prior art.

Figure 2B schematically illustrates steps of an LTE synchronization or cell search procedure according to embodiments of the invention.

Figure 3 schematically illustrates an example of misalignment of DFN and SFN.

Figure 4 schematically illustrates an example where the DFN is totally aligned with the SFN when the DFN timing has been derived from GNSS and DFN offset.

Figures 5A-5B are flow charts schematically illustrating embodiments of the method performed by a wireless device according to various embodiments.

Figure 6 is a block diagram schematically illustrating embodiments of the wireless device. DETAILED DESCRIPTION

In the following, different aspects will be described in more detail with references to certain embodiments and to accompanying drawings. For purposes of explanation and not limitation, details are set forth, such as particular scenarios and techniques, in order to provide a thorough understanding of the different embodiments. However, other embodiments that depart from these details may also exist.

Furthermore, in some instances detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as to not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or in several nodes. Some or all of the functions described may be implemented using hardware circuitry, such as analog and/or discrete logic gates interconnected to perform a specialized function, or ASICs. Likewise, some or all of the functions may be implemented using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Where nodes that communicate using the air interface are described, it will be appreciated that those nodes also have suitable radio communications circuitry. Moreover, the technology may be embodied entirely within any form of computer-readable memory, including non-transitory embodiments such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

Hardware implementations of the present invention may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analog) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and where appropriate, state machines capable of performing such functions.

In terms of computer implementation, a computer is generally understood to comprise one or more processors or one or more controllers, and the terms computer, processor, and controller may be employed interchangeably. When provided by a computer, processor, or controller, the functions may be provided by a single dedicated computer or processor or controller, by a single shared computer or processor or controller, or by a plurality of individual computers or processors or controllers, some of which may be shared or distributed. Moreover, the term "processor" or "controller" also refers to other hardware capable of performing such functions and/or executing software, such as the example hardware recited above. Embodiments are described in a non-limiting general context in relation to a UE configured for sidelink-based V2X communication in a 3GPP LTE system. However, embodiments of the invention may be applied to any other wireless device configured for D2D communication with another wireless device over a direct D2D interface in a wireless communication system supporting a synchronization or cell search procedure.

Sidelink-based Communication and Operation

Sidelink transmission (sometimes also referred to as D2D or ProSe transmission) over the so-called PC5 interface between devices in cellular spectrum has been standardized in 3GPP since Rel-12, as already described above. In 3GPP Rel-12, two different operation modes have been specified. In a first operation mode (referred to as mode-1 or mode-3), a UE in Radio Resource Control Connected (RRC_CONNECTED) mode requests D2D resources and the eNodeB grants them via a Physical Downlink Control Channel (PDCCH) or via dedicated signaling. In a second operation mode (referred to as mode-2 or mode-4), a UE autonomously selects resources for transmission from a pool of available resources. The pool of available resources is provided by the eNodeB in broadcast, via SIB signaling for transmissions on carriers other than the PCell, or via dedicated signaling for transmission on the PCell. Therefore, unlike the first operation mode, the second operation mode can be performed also by UEs in Radio Resource Control Idle (RRCJDLE).

In Rel.14, the usage of sidelink is extended to the V2X domain. The design of the sidelink physical layer in Rel.12 has been made under the assumption that there would be a small amount of UEs competing for the same physical resources in the spectrum and low- mobility of the UEs. This is due to that the sidelink was initially designed to carry voice packets for Mission Critical Push To Talk (MCPTT) traffic. However, with V2X the sidelink should be able to cope with higher load scenarios. One such scenario may be a scenario with hundreds of cars equipped with wireless devices configured for V2X. These cars have high mobility, need physical resources, and send time/event triggered V2X messages. For such reasons, 3GPP has discussed possible enhancements to the sidelink physical layer for Rel. 14.

V2X Synchronization aspects

One of the enhancements to sidelink is the introduction of a new synchronization framework. Unlike the Rel.12 sidelink synchronization, the UE can select as a synchronization source not only the eNodeB timing or the timing of a nearby UE via sidelink synchronization signals (SLSS), but also timing from a Global Navigation Satellite System (GNSS). Which synchronization source the UE should prioritize is indicated by the eNodeB or is pre-configured.

The UE can obtain a Coordinated Universal Time (UTC) from the GNSS. The UE can derive a timing of a radio frame boundary for a D2D Frame Number (DFN) from the current UTC time and a reference UTC time. Additionally, 3GPP has introduced a DFN offset. With the introduction of the DFN offset, the D2D frame boundary and corresponding system frame boundary for an SFN can be shifted with regards to each other. The main purpose of using this DFN offset is to mitigate the possible interference in uplink (UL) and downlink (DL) due to misalignment between eNodeB synchronization and GNSS synchronization. In the example illustrated in Figure 3, the eNodeB allocates resources located in SFN#1 to the UE. The UE transmits over the sidelink in UL resources during DFN#1. Because of the misalignment between eNodeB synchronization and GNSS synchronization, there might be an overlap in time referred to as T1 between V2V sidelink transmission during DFN#1 and eNodeB UL during SFN#2. The V2V traffic from UE during T1 will lead to interference to UL during SFN#2 for a FDD system, and interference to both UL and DL for a TDD system, especially when the cyclic prefix is not long enough to cope with GNSS and eNodeB timing misalignment. To overcome this problem, the eNodeB can e.g. configure the DFN offset such that the misalignment between the UTC timing (available at the eNodeB) and the eNodeB timing is smaller than the guard period.

The timing of the DFN is a function of the UTC time and the DFN offset, i.e. DFN = function (UTC time, DFN offset). The function of calculating DFN timing is predefined in 3GPP standards. Wth an accurate DFN offset value, the UE may thus determine or compute the timing for DFN such that the DFN is aligned with the SFN of the cellular network or the wireless communication system, as illustrated in Figure 4. It has been proposed that the DFN offset can be set per cell. However, in a real network deployment the benefit of the DFN offset is only there for a synchronized network. In a synchronized network, a DFN offset value can be shared among all cells of the network.

When a UE configured for sidelink-based V2X communication, also referred to as a V2X UE, powers on, moves from out-of-coverage to in coverage of the LTE system, or moves across a cell border, it needs to follow the LTE synchronization procedure to camp on a cell. The procedure starts with a PLMN selection where the first step is to search for a synchronization signal. As illustrated in Figure 2B, the problem related to latency in the LTE synchronization procedure for the V2X UE, is addressed by a solution where the UE determines or calculates a radio frame boundary timing of a D2D frame for V2X communication from the UTC time given by a GNSS and a DFN offset which is known to the UE. A D2D frame may be referred to as a direct frame, and the use of a direct frame for V2X communication corresponds to a sidelink-based V2X communication. As the direct frame for V2X communication and a system frame for the LTE wireless communication system are aligned in a synchronized system, the UE can consider or estimate the boundary of the direct frame to occur at the same time as a boundary of the system frame, 230. Based on the estimated boundary timing of the system frame, or the estimated information of system frame boundary timing, the UE starts the synchronization or cell search in the LTE system. With knowledge of the radio frame timing for the system already when starting the synchronization procedure, the following optimization of the procedure may be done:

1) For the PSS detection, 240, the UE only needs to check four possible positions of the PSS, corresponding to the two different duplex modes each combined with the two different CP lengths. When PSS is detected, also the CP length and the duplex mode can be determined.

2) The frequency synchronization of PSS will be less complex when the frame timing is known, as there will be no spurious correlation peak.

3) Wth determined radio frame timing, duplex mode, and CP length, the position of SSS may be determined. For the SSS detection, the UE only needs to check the different cell identity group candidates to determine the PCI, 250.

This optimization will result in a less complex procedure which enables a reduced latency and reduced power consumption.

Embodiments of method described with reference to Figures 5A-5B

Figure 5A is a flowchart illustrating one embodiment of a method for synchronizing to a wireless communication system. The method is performed by a wireless device configured for sidelink-based V2X communication, such as a V2X UE that comes into coverage of an LTE wireless communication system as in the scenario described previously. The method comprises:

- 510: Determining a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a GNSS and a DFN offset. As described previously, SFN timing may be acquired based on GNSS information. A V2X UE is supposed to obtain and keep GNSS information providing the UTC time. The DFN offset information is also know to the UE, e.g. obtained through pre-configuration or in signaling from the network. Wth the DFN offset value together with the UTC time, the UE can determine or calculate the DFN timing, where the important information is the time position or timing of the frame boundary. In embodiments, the timing of the radio frame boundary for sidelink-based V2X communication may be determined as a function of a UTC indicated by the GNSS and the DFN offset.

- 520: Estimating a timing of a radio frame boundary of the wireless communication system to correspond to the determined timing of the radio frame boundary for sidelink- based V2X communication.

- 530: Initiating a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system. The direct frame is expected to be totally aligned with the LTE system frame, and the frame boundary timing of the direct D2D frame will be used as an estimate of the system frame boundary timing in the steps of the synchronization procedure.

Figure 5B is a flowchart illustrating another embodiment of the method. The method comprises:

- 510: Determining a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a GNSS and a DFN offset.

- 530: Initiating a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system. Initiating the synchronization procedure comprises:

o 531 : Determining at least one candidate reception time of a PSS based on the estimated timing of the radio frame boundary of the wireless communication system and possible positions of the PSS in a radio frame of the wireless communication system.

o 532: Detecting the PSS at one of the at least one candidate reception time of the PSS.

The UE searches PSS based on the known radio frame timing. There will be four possible PSS positions or candidate reception times, considering the different duplex modes and CP lengths. This is due to that the PSS is mapped to different symbols for FDD and TDD, and the symbol length is different for normal CP and long CP. The four possible positions of PSS are for: FDD with normal CP, FDD with long CP, TDD with normal CP, and TDD with long CP. This implies that the UE will try to detect the PSS at up to four candidate reception times. At one of these four candidate reception times, the UE will determine that it detects the PSS. As mentioned before, there are three PSS sequence candidates, 13 frequency offset hypothesis for each PSS sequence candidate, and four possible positions for each candidate, so there are 3*13*4 correlation results of PSS timing candidates and frequency offset in one slot data segment of 5 ms. This should be compared to 3*13*9600 correlation results for the legacy case, which means that the complexity of computation is decreased by about 99% for this case.

In one example implementation, a solution to resist the high interference from Multicast Broadcast Single Frequency Network (MBSFN) and uplink in TDD, and to avoid the problem that correlation results cannot be accurately compared as the Automatic Gain Control (AGC) changes time to time, the ratio of correlation result to momentary energy is used as metric. There is no spurious correlation peak any more within the initial frequency offset range, as the radio frame timing has been determined before the PSS detection. The candidate with largest correlation peak could be selected after removing the fake candidates, such as noise candidates, redundant candidates (two candidates having the same timing, but a frequency hypothesis identity difference of 1 , makes it possible to remove the candidate with worse quality), and candidates with frequency offset bigger than the maximal offset computed by Voltage Controlled Crystal Oscillator (VCXO).

In embodiments of the method, the method may further comprise, as illustrated in Figure 5B:

- 540: Performing channel estimation based on the detected PSS.

- 550: Adjusting the estimated timing of the radio frame boundary of the wireless communication system based on the performed channel estimation.

To secure the timing accuracy, an adjustment of the timing estimation could be done using the channel estimation of the found PSS. First, an Inverse Fast Fourier Transform (IFFT) may be used to get the channel impulse response in the time domain, and to calculate the power of the channel profile. Then a search is done in the channel window with maximum power, and the gravity center of the channel window is calculated. A mean delay could be obtained and the timing could be adjusted to secure that the timing error is small enough. This may be done as the frame timing that was derived from GNSS and the DFN offset may have a timing offset of a couple of samples from the real frame timing. The adjustment is thus done to get a more accurate frame timing estimation, which may also increase the accuracy of the SSS detection.

In other embodiments of the method, also illustrated in Figure 5B, initiating the synchronization procedure, 530, may further comprise: - 533: Determining at least one of a duplex mode, and a CP length based on the detected PSS.

- 534: Determining a candidate reception time of a SSS, based on the determined at least one of the duplex mode and the CP length.

- 535: Detecting the SSS at the candidate reception time of the SSS.

After the position of PSS and physical layer identity are determined, the duplex mode and CP length is set or determined. The position of the SSS may therefore also be determined. The UE needs to check only the cell identity group candidates using e.g. a coherent or non-coherent method.

The coherent method comprises to equalize the SSS by channel estimation from the PSS, to combine the equalized SSS from different receiving Rx antennas, and to detect SSS identity by correlation. The non-coherent method comprises to perform the correlation of SSS directly and to combine the correlation result from different Rx antennas for the SSS identity.

As there are 168 SSS sequence candidates, 168 correlation results need to be checked. In the legacy case, as the duplex mode and CP length still is not determined at this stage, there are four possible SSS positions: FDD with normal CP, FDD with long CP, TDD with normal CP, and TDD with long CP. So there are 4*168 correlation results that need to be checked. Compared to the legacy situation, the computational complexity my thus be decreased by about 75% at this stage of the synchronization procedure as there is only one possible candidate reception time for the SSS when the duplex mode and the CP length is known.

Validity check of frame timing

Occasionally, the DFN frame with a timing determined based on the DFN offset is not aligned with the LTE SFN frame with regards to the frame boundary, e.g. due to an improper configuration. In this case, the estimated timing of the frame boundary determined based on GNSS and DFN offset may be considered to be invalid. A validity checking may be done during the synchronization process, either after the PSS detection or after the SSS detection. If the estimated timing of system frame is found not valid, the cell search procedure which has been based on a timing determined from GNSS will be ended, and a normal "legacy" cell search procedure may be initiated or started instead.

1) At the PSS detection stage, thresholds of both the correlation result and quality metric may be set for the validity checking. If the correlation value or the quality value is under the threshold, the DFN offset will be judged as incorrect and the estimated timing will be judged as invalid; otherwise, the procedure will continue. 2) At the SSS detection stage, thresholds of the correlation result may be set for validity checking. If the correlation value is under the threshold, the DFN offset will be judged as incorrect and the estimated timing will be judged as invalid; otherwise, after SSS is detected, the synchronization process is finished.

Thus, in one example embodiment, the method illustrated in Figures 5A and 5B may further comprise:

- Determining a validity of the estimated timing of the radio frame boundary based on at least one of a correlation result and a quality metric for the detected PSS.

And when the estimated timing of the radio frame boundary is determined to be invalid, the method may further comprise:

- Ending the synchronization procedure based on the estimated timing, and

- Initiating a synchronization procedure based on a blind detection of the PSS.

In another example embodiment which may be combined with the previous embodiment, the method may optionally comprise:

- Determining a validity of the estimated timing of the radio frame boundary based on at least a correlation result for the detected SSS.

- Ending the synchronization procedure based on the estimated timing, and

- Initiating a synchronization procedure based on a blind detection of the PSS.

Embodiments of apparatus described with reference to Figure 6

An embodiment of the wireless device 600 is schematically illustrated in the block diagram in Figure 6. The wireless device 600 is configured for sidelink-based V2X communication and is configured to synchronize to a wireless communication system. The wireless device is further configured to determine a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a GNSS and a DFN offset. The wireless device is also configured to estimate a timing of a radio frame boundary of the wireless communication system to correspond to the determined timing of the radio frame boundary for sidelink-based V2X communication, and to initiate a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system.

In embodiments, the wireless device may be configured to initiate the synchronization procedure by being configured to determine at least one candidate reception time of a PSS, based on the estimated timing of the radio frame boundary of the wireless communication system and possible positions of the PSS in a radio frame of the wireless communication system, and to detect the PSS at one of the at least one candidate reception time of the PSS. The wireless device may be further configured to perform channel estimation based on the detected PSS, and to adjust the estimated timing of the radio frame boundary of the wireless communication system based on the performed channel estimation. Furthermore, the wireless device may be configured to determine a validity of the estimated timing of the radio frame boundary based on at least one of a correlation result and a quality metric for the detected PSS. The wireless device may be configured to end the synchronization procedure based on the estimated timing, and initiate a synchronization procedure based on a blind detection of the PSS, when the estimated timing of the radio frame boundary is determined to be invalid.

In other embodiments, the wireless device may be configured to initiate the synchronization procedure by being further configured to determine at least one of a duplex mode, and a cyclic prefix length based on the detected PSS, determine a candidate reception time of a SSS, based on the determined at least one of the duplex mode and the cyclic prefix length, and detect the SSS at the candidate reception time of the SSS.

In one embodiment, the wireless device may be further configured to determine a validity of the estimated timing of the radio frame boundary based on a correlation result for the detected SSS, and when the estimated timing of the radio frame boundary is determined to be invalid, end the synchronization procedure based on the estimated timing, and initiate a synchronization procedure based on a blind detection of the PSS.

The wireless device may in any of the above described embodiments be configured to determine the timing of the radio frame boundary for sidelink-based V2X communication as a function of a UTC indicated by the GNSS and the DFN offset.

As illustrated in Figure 6, the wireless device 600 may comprise at least one processing circuitry 610 and optionally also a memory 630. In embodiments, the memory 630 may be placed in some other node or unit or at least separately from the wireless device 600. The wireless device 600 may also comprise one or more input/output (I/O) units 620 configured to communicate with a network node such as an eNodeB and with another wireless device over the sidelink. The input/output (I/O) unit 620 may in embodiments comprise a transceiver connected to one or more antennas over antenna ports for wireless communication with network nodes or wireless devices in the network. The memory 630 may contain instructions executable by said at least one processing circuitry 610, whereby the wireless device 600 may be configured to determine a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a GNSS and a DFN offset, estimate a timing of a radio frame boundary of the wireless communication system to correspond to the determined timing of the radio frame boundary for sidelink-based V2X communication, and initiate a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system.

In embodiments, the memory 630 contains instructions executable by the processing circuitry 610 whereby the wireless device is configured to perform any of the methods previously described herein with reference to Figures 5A-5B.

In an another embodiment also illustrated in Figure 6, the wireless device 600 may comprise a determining module 61 1 adapted to determine a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a GNSS and a DFN offset, an estimating module 612 adapted to estimate a timing of a radio frame boundary of the wireless communication system to correspond to the determined timing of the radio frame boundary for sidelink-based V2X communication, and an initiating module 613 adapted to initiate a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system.

The wireless device 600 may contain further modules adapted to perform any of the methods previously described herein.

The modules described above are functional units which may be implemented in hardware, software, firmware or any combination thereof. In one embodiment, the modules are implemented as a computer program running on the at least one processing circuitry 610.

In still another alternative way to describe the embodiment in Figure 6, the wireless device 600 may comprise a Central Processing Unit (CPU) which may be a single unit or a plurality of units. Furthermore, the wireless device 600 may comprise at least one computer program product (CPP) with a computer readable medium 641 , e.g. in the form of a nonvolatile memory, e.g. an EEPROM (Electrically Erasable Programmable Read-Only Memory), a flash memory or a disk drive. The CPP may comprise a computer program 640 stored on the computer readable medium 641 , which comprises code means which when run on the CPU of the wireless device 600 causes the wireless device 600 to perform the methods described earlier in conjunction with Figures 5A-B. In other words, when said code means are run on the CPU, they correspond to the at least one processing circuitry 610 of the wireless device 600 in Figure 6. The above mentioned and described embodiments are only given as examples and should not be limiting. Other solutions, uses, objectives, and functions within the scope of the accompanying patent claims may be possible.

Claims

WHAT IS CLAIMED IS:

1. A method for synchronizing to a wireless communication system performed by a wireless device (600) configured for sidelink-based Vehicle-to-X, V2X, communication, the method comprising:

- determining (510) a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a Global Navigation Satellite System, GNSS, and a Device-to-Device Frame Number, DFN, offset,

- estimating (520) a timing of a radio frame boundary of the wireless communication system to correspond to the determined timing of the radio frame boundary for sidelink-based V2X communication,

- initiating (530) a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system. 2. The method according to claim 1 , wherein initiating (530) the synchronization procedure comprises:

- determining (531 ) at least one candidate reception time of a primary synchronization signal, PSS, based on the estimated timing of the radio frame boundary of the wireless communication system and possible positions of the PSS in a radio frame of the wireless communication system,

- detecting (532) the PSS at one of the at least one candidate reception time of the PSS.

The method according to claim 2, further comprising:

- performing (540) channel estimation based on the detected PSS,

- adjusting (550) the estimated timing of the radio frame boundary of the

communication system based on the performed channel estimation.

The method according to any of claims 2-3, further comprising:

- determining a validity of the estimated timing of the radio frame boundary based on at least one of a correlation result and a quality metric for the detected PSS,

- when the estimated timing of the radio frame boundary is determined to be invalid, ending the synchronization procedure based on the estimated timing, and initiating a synchronization procedure based on a blind detection of the PSS. The method according to any of claims 2-4, wherein initiating (530) the synchronization procedure further comprises:

- determining (533) at least one of a duplex mode, and a cyclic prefix length based on the detected PSS,

- determining (534) a candidate reception time of a secondary synchronization signal, SSS, based on the determined at least one of the duplex mode and the cyclic prefix length,

- detecting (535) the SSS at the candidate reception time of the SSS. The method according to claim 5, further comprising:

- determining a validity of the estimated timing of the radio frame boundary based on a correlation result for the detected SSS,

- when the estimated timing of the radio frame boundary is determined to be invalid, ending the synchronization procedure based on the estimated timing, and initiating a synchronization procedure based on a blind detection of the PSS.

The method according to any of the preceding claims, wherein the timing of the radio frame boundary for sidelink-based V2X communication is determined as a function of a Coordinated Universal Time, UTC, indicated by the GNSS and the DFN offset.

A wireless device (600) configured for sidelink-based Vehicle-to-X, V2X, communication and configured to synchronize to a wireless communication system, the wireless device being further configured to:

- determine a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a Global Navigation Satellite System, GNSS, and a Device-to-Device Frame Number, DFN, offset,

- estimate a timing of a radio frame boundary of the wireless communication system to correspond to the determined timing of the radio frame boundary for sidelink- based V2X communication,

- initiate a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system. The wireless device according to claim 8, configured to initiate the synchronization procedure by being configured to:

- determine at least one candidate reception time of a primary synchronization signal, PSS, based on the estimated timing of the radio frame boundary of the wireless communication system and possible positions of the PSS in a radio frame of the wireless communication system,

- detect the PSS at one of the at least one candidate reception time of the PSS.

10. The wireless device according to claim 9, further configured to:

- perform channel estimation based on the detected PSS,

- adjust the estimated timing of the radio frame boundary of the wireless communication system based on the performed channel estimation.

11. The wireless device according to any of claims 9-10, further configured to:

- determine a validity of the estimated timing of the radio frame boundary based on at least one of a correlation result and a quality metric for the detected PSS,

- and when the estimated timing of the radio frame boundary is determined to be invalid, end the synchronization procedure based on the estimated timing, and initiate a synchronization procedure based on a blind detection of the PSS.

12. The wireless device according to any of claims 9-11 , configured to initiate the synchronization procedure by being further configured to:

- determine at least one of a duplex mode, and a cyclic prefix length based on the detected PSS,

- determine a candidate reception time of a secondary synchronization signal, SSS, based on the determined at least one of the duplex mode and the cyclic prefix length,

- detect the SSS at the candidate reception time of the SSS.

The wireless device according to claim 12, further configured to:

- determine a validity of the estimated timing of the radio frame boundary based on a correlation result for the detected SSS,

14. The wireless device according to any of the claims 8-13, configured to determine the timing of the radio frame boundary for sidelink-based V2X communication as a function of a Coordinated Universal Time, UTC, indicated by the GNSS and the DFN offset. 15. A computer program (640) comprising computer readable code which when executed by at least one processor of a wireless device (600) causes the wireless device to carry out the method of any of claims 1-7.

16. A carrier containing the computer program (640) of claim 15, wherein the carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium (641).

17. A wireless device (600) configured for sidelink-based Vehicle-to-X, V2X, communication and configured to synchronize to a wireless communication system, the wireless device comprising processing circuitry (610) and a memory (630), the memory containing instructions executable by the processing circuitry whereby the wireless device is configured to:

- initiate a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system.

18. The wireless device of claim 17, wherein the memory (630) contains instructions

executable by the processing circuitry (610) whereby the wireless device is configured to perform the method of any of claims 2-7.

19. A wireless device (600) configured for sidelink-based Vehicle-to-X, V2X, communication and configured to synchronize to a wireless communication system, the wireless device comprising:

- a determining module (611) adapted to determine a timing of a radio frame boundary for sidelink-based V2X communication based on a timing indicated by a Global Navigation Satellite System, GNSS, and a Device-to-Device Frame Number, DFN, offset,

- an estimating module (612) adapted to estimate a timing of a radio frame boundary of the wireless communication system to correspond to the determined timing of the

5 radio frame boundary for sidelink-based V2X communication,

- an initiating module (613) adapted to initiate a synchronization procedure for a cell search in the wireless communication system, based on the estimated timing of the radio frame boundary of the wireless communication system.

10 20. The wireless device of claim 19, further comprising modules adapted to perform the method of any of claims 2-7.