WO2018082768A1

WO2018082768A1 - Device and method for wireless communication network synchronization

Info

Publication number: WO2018082768A1
Application number: PCT/EP2016/076430
Authority: WO
Inventors: Fan Wang; Branislav M POPOVIC; Peng Wang; Fredrik Berggren
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2016-11-02
Filing date: 2016-11-02
Publication date: 2018-05-11
Also published as: CN109906591B; CN109906591A

Abstract

A system and method for wireless communication network synchronization is described. A wireless communication system transmits SS blocks that include at least a primary synchronization signal, a secondary synchronization signal and an index of SS blocks. Transmitted bocks are received by a user equipment and the user equipment may extract signals and the index from any of the received blocks. In addition to the SS block index it is possible to encode a beam index into each SS blocks. The user equipment may respond to the received information appropriately.

Description

DEVICE AND METHOD FOR WIRELESS COMMUNICATION NETWORK

SYNCHRONIZATION

FIELD

[0001 ] The present disclosure relates to the field of wireless communications, and more particularly to transmission of synchronization signals.

BACKGROUND

[0002] Cellular wireless systems are dependent on correct synchronization.

Typically the synchronization is performed by using synchronization signals. Base stations transmit synchronization signals that are detected by a mobile device, which may be a mobile phone, any type of user equipment, a further device comprising wireless communication unit or similar.

[0003] Long Term Evolution (LTE) is currently widely used technology used for wireless communication. In LTE synchronization signals are transmitted periodically by the base station (BS), for example, in subframe 0 and subframe 5 of every frame for Frequency Division Duplex (FDD). In LTE, the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) are transmitted in different Orthogonal Frequency Division Multiplexing (OFDM) symbols. Within one periodicity, i.e., 5ms, PSS and SSS each is transmitted in one synchronization block, i.e., one OFDM symbol. The UE acquires time and frequency synchronization as well as cell identity (ID) by detecting PSS and SSS.

[0004] Currently a New Radio (NR) access technologies which may support beamformed synchronization signals is being developed. For example, the NR base station applies a number of beams to a number of synchronization intervals of a synchronization occasion within one periodicity. On one hand, the network node, such as a base station, access point, or transmission reception point, may support three implementations of beamforming, which are analog beamforming, digital beamforming and hybrid beamforming. Especially for analog beamforming and hybrid beamforming, the maximum number of concurrent beams may be limited by the number of Radio Frequency (RF) chains. On the other hand, according to detailed deployment scenarios and required coverage areas, the numbers of beams adopted at different network nodes may also be different from each other. It is therefore an issue to provide a synchronization signal which can be transmitted using beamforming, while enabling a mobile device to acquire and track a flexible number of beamformed synchronization signals adopted by the network node, and providing efficient network node operation.

[0005] In one proposed solution a number of different spatial beams are applied to the same number of synchronization blocks of one synchronization occasion within one periodicity. The mapping between the beams (also interpreted as beam directions or beam indexes) and the synchronization blocks is one-to-one and pre-defined, e.g., Beam 1, Beam 2, and Beam N are adopted in Synchronization block 1, Synchronization block 2, and Synchronization block N, respectively. The UE derives the beam index by detecting the synchronization signals, and may feedback this beam information, e.g., through random access procedure or uplink feedback, to the network node.

[0006] In another proposed solution the synchronization signal comprises

PSS containing part of the cell ID, SSS containing the remaining part of the cell ID, and Extended Synchronization Signal (ESS) containing the synchronization block index where each synchronization block is an OFDM symbol. Each PSS, SSS and ESS triple corresponds uniquely to one beam and is frequency division multiplexed in one of a pre-defined number (i.e., 14) of OFDM symbols. For each OFDM symbol, PSS and SSS follow the same design as those of LTE, and ESS is generated using a product of a Zadoff Chu (ZC) sequence and a pseudo-random sequence, where the symbol index is encoded in the ZC sequence via different cyclic shifts. The UE derives the cell ID from detecting PSS/SSS and the synchronization interval index from detecting ESS.

[0007] In the proposal discussed above the network node implementation has restrictions. It is required the network node to generate a fixed number of beams, i.e., equal to the number of synchronization intervals. In some implementations the network node may produce a different number of beams, e.g., smaller or larger than the number of synchronization blocks, however, then it is also required the network node to apply the beamforming in a pre-defined manner, which unnecessarily increases the implementation restriction, as well as the specification effort.

SUMMARY

[0008] In the following means to transmit synchronization signal with a flexible number of beams in one or multiple synchronization signal blocks, SS blocks of a pre-defined period will be described, where a pre-defined period may be an SS burst or SS burst set. A labeling method encoding the cell identity, the intervals block index, the Transmit Receive Point (TRP) identity and/or the beam index, is provided in the synchronization signal to allow UE to perform continuous searching for suitable transmission points by acquiring the synchronization and/or the beams, even when the UE is not aware the number of beams used at the network node. Specifically, it allows the network node to transmit multiple beams in one SS block concurrently, while it also allows the networks to transmit a flexible number of SS blocks in one SS burst.

[0009] In the first aspect a network node for wireless communication comprising a processor is disclosed. The processor is configured to provide an at least one SS block, wherein said at least one SS block comprises at least a primary synchronization signal and a secondary synchronization signal. The processor is further configured to encode an SS block index into each of said synchronization signal (SS) blocks. The network node further comprises a transceiver configured to transmit said SS block. In some terminology, an SS block may be an SS burst, and the SS block index may be the SS burst index accordingly.

[001 0] The network node transmits multiple synchronization signals each comprising at least PSS and SSS and potentially ESS in multiple SS blocks of the same SS burst or SS burst set, where each synchronization signal comprises information of the SS block index. The network node may transmit with beamforming in each SS block in a proprietary way, i.e., there is no restriction of how the network node applies the beams in an SS block. Specifically, the SS block index can be encoded in various ways, e.g., encoded in PSS by modulating a ZC sequence, or in SSS/ Extended Synchronization Signal (ESS) by reusing the SSS sequences, or split into more than one part, each corresponding to one of PSS, SSS and potentially a further part which is transmitted in the Physical Broadcast CHannel (PBCH). In addition, a different network node identity other than cell identity, e.g., TRP identity, may be encoded in the synchronization signals, which allows the UE to synchronize to multiple TRPs within a same cell. The UE derives the SS block index from the detected synchronization signal sequence resource(s). The UE may utilize the SS block index for feedback to help the eNodeB to derive the associated beam information, e.g., implicitly.

[001 1 ] In the first implementation of the first aspect the transceiver is configured to transmit said SS block using multiple beams. Using multiple beams provides beamforming gain to extend the coverage of the synchronization signals in the SS block.

[001 2] In the second implementation of the first aspect said SS block further comprises at least one of an additional secondary synchronization signal and a physical broadcast channel information. It is advantageous to allow SS block to include also other signals than the primary synchronization signal and secondary synchronization signal. It allows the network node to transmit additional information other than those in PSS and SSS, e.g. System Frame Number (SFN).

[001 3] In the third implementation of the first aspect said processor is configured to encode said synchronization block index into at least two of said primary synchronization signal, secondary synchronization signal, additional synchronization signal and physical broadcast channel information. When the SS block index is split into multiple parts and separately included in different types of signals it is efficient that the network node may freely select to which signal said a part of SS block index is encoded. This provides further flexibility. Furthermore, the network node is free to select into how parts it will split SS block index is divided.

[001 4] In the fourth implementation of the first aspect the number of said SS blocks is from 1 to N, wherein N is a pre-defined maximum value. It is advantageous to allow using any number of synchronization blocks between one and the pre-defined maximum. This allows the network node to select a flexible number of used SS blocks which further result in a flexible overhead of the synchronization signals. In the fifth implementation of the first aspect said processor is configured to encode said SS block index into any of: said primary synchronization signal, wherein said processor is further configured to derive primary synchronization signal sequence from multiplying a Zadoff Chu sequence by an additional sequence, wherein said additional sequence is used in encoding the SS block index, or said additional synchronization signal, wherein said processor is configured to derive said additional synchronization signal from the same set of sequences as said secondary synchronization signal, or said secondary synchronization signal, wherein the periodicity of said secondary synchronization signal is an integer multiple of one system frame. By encoding the SS block index in PSS, it is advantageous as the UE may derive the SS block index from PSS sequence resource, while the signaling overhead is not increased in SSS. By encoding the SS block index in additional synchronization signal reusing the same set of sequences as SSS, this is advantageous as the existing synchronization signal design/detection can be inherited to the largest extent while the additional function is fulfilled. By encoding the SS block index in SSS, it is advantageous as the UE may derive the SS block index from SSS sequence, while not increasing the signaling overhead in PSS.

[001 5] In the sixth implementation of the first aspect of a device and method for wireless communication network synchronization said processor is further configured to encode a beam index into each of said at least one SS block. In the sixth implementation of the first aspect, the beam information (e.g., beam index) is encoded in the synchronization signal sequence, in addition to the SS block index. The total number of beams associated to the total synchronization signal sequences is pre-defined. The network may utilize a flexible number of beams which is no greater than the pre-defined total number of beams. The UE derives the SS block index as well as the beam information from the detected synchronization signal sequence(s). The UE may utilize the beam information for feedback to the network node. [001 6] In the seventh implementation of the first aspect the number of said beam indexes is encoded into said at least one SS block is from 1 to M, wherein M is a pre-defined maximum value. It is advantageous to allow using any number of beam between one and the pre-defined maximum. This allows the network node to apply beamforming using different implementations, e.g. analog/hybrid/digital beamforming. Especially this allows the network node to apply concurrent transmission of multiple beams to further provide reduced control overhead or finer beam resolution.

[001 7] In the eighth implementation of the first aspect said processor is further encode a TRP identity and cell identity into each of said at least one SS block. It is advantageous to include additional network node identity related information in the SS block as it provides for UE the possibility to identify the network when the UE is connected to multiple TRP in a cell.

[001 8] In the second aspect of the device and method for wireless communication network synchronization a user device for wireless communication is provided. The user device comprises a transceiver configured to receive at least one SS block comprising at least primary synchronization signal and secondary synchronization signal and a processor configured to extract said SS block index from a received SS block.

[001 9] In the first implementation of the second aspect said user device is configured to extract said SS block index from said at least one received SS block by decoding any of said primary synchronization signal derived from multiplying a Zadoff Chu sequence by an additional sequence, wherein said additional sequence encodes the SS block index, or an additional secondary synchronization signal derived from the same set of sequences as said secondary synchronization signal, or said secondary synchronization signal, wherein the periodicity of said secondary synchronization signal is an integer multiple of one system frame. By encoding the SS block index in PSS, it is advantageous as the UE may derive the SS block index from PSS sequence resource, while the signaling overhead is not increased in SSS. By encoding the SS block index in additional synchronization signal reusing the same set of sequences as SSS, this is advantageous as the existing synchronization signal design/detection can be inherited to the largest extent while the additional function is fulfilled. By encoding the SS block index in SSS, it is advantageous as the UE may derive the SS block index from SSS sequence, while not increasing the signaling overhead in PSS.

[0020] In the second implementation of the second aspect said processor is further configured to derive any of beam index and TRP identity. It is advantageous to include additional network node identity related information in the SS block as it provides for UE the possibility to identify the network when the UE is connected to multiple TRP in a cell In the third implementation of the second aspect said user device is configured to derive an uplink transmission from any of said SS block index, said beam index, and said TRP identity. It is advantageous to derive an uplink transmission from the received information so that the best possible conditions can be chosen for communication between the network and the UE.

[0021 ] In the third aspect of the device and method for wireless communication network synchronization a method for wireless communication is disclosed. The method comprises providing at least one SS block, wherein said at least one SS block comprises at least a primary synchronization signal and a secondary synchronization signal; encoding the SS block index into each of said SS block; and transmitting said at least one SS block. It is beneficial to encode an SS block index into each of transmitted SS blocks. This will allow UE to derive the SS block index even if only one SS block will be received at the UE.

[0022] In the first implementation of the third aspect the method further comprises transmitting said at least one SS block using multiple beams. Using multiple beams provides beamforming gain to extend the coverage of the synchronization signals in the SS block. In the second implementation of the third aspect said SS block further comprises at least one of additional secondary synchronization signal and physical broadcast channel. It is advantageous to allow SS block to include also other signals than the primary synchronization signal and secondary synchronization signal. It allows the network node to transmit additional information other than those in PSS and SSS, e.g. System Frame Number (SFN). [0023] In the third implementation of the third aspect the method further comprises encoding said synchronization block index into at least two of said primary synchronization signal, secondary synchronization signal, additional synchronization signal and physical broadcast channel information. When the SS block index is split into multiple parts and separately included in different types of signals it is efficient that the network node may freely select to which signal said a part of SS block index is encoded. This provides further flexibility.

[0024] In the fourth implementation of the third aspect the number of said at least one SS blocks is from 1 to N, wherein N is a pre-defined maximum value. It is advantageous to allow using any number of synchronization blocks between one and the pre-defined maximum. This allows the network node to select a flexible number of used SS blocks which further result in a flexible overhead of the synchronization signals.

[0025] In the fifth implementation of the third aspect the method further comprises encoding said SS block index into any of said primary synchronization signal, wherein the method further comprises deriving primary synchronization signal sequence from multiplying a Zadoff Chu sequence by an additional sequence, wherein said additional sequence is used in encoding the SS block index, or said additional synchronization signal, wherein the method further comprises deriving said additional synchronization signal from the same set of sequences as said secondary synchronization signal or said secondary synchronization signal, wherein the periodicity of said secondary synchronization signal is an integer multiple of one system frame. By encoding the SS block index in PSS, it is advantageous as the UE may derive the SS block index from PSS sequence resource, while the signaling overhead is not increased in SSS. By encoding the SS block index in additional synchronization signal reusing the same set of sequences as SSS, this is advantageous as the existing synchronization signal design/detection can be inherited to the largest extent while the additional function is fulfilled. By encoding the SS block index in SSS, it is advantageous as the UE may derive the SS block index from SSS sequence, while not increasing the signaling overhead in PSS. [0026] In the sixth implementation of the third aspect the method further comprises encoding a beam index into each of said at least one SS block. It is beneficial for UE to receive and use all information received from the network. It provides more efficient use of resources and increases the usability of the device. In the seventh implementation of the third aspect the number of said beam indexes encoded into said at least one SS block is from 1 to M, wherein M is a pre-defined maximum value. It is advantageous to allow using any number of beam between one and the pre-defined maximum. This allows the network node to apply beamforming using different implementations, e.g. analog/hybrid/digital beamforming. Especially this allows the network node to apply concurrent transmission of multiple beams to further provide reduced control overhead or finer beam resolution.

[0027] In the eighth implementation of the third aspect the method further comprises encoding a TRP identity and network identity into each of said at least one SS block. It is advantageous to include additional network node identity related information in the SS block as it provides for UE the possibility to identify the network when the UE is connected to multiple TRP in a cell.

[0028] In the fourth aspect of the device and method for wireless communication network synchronization a method for a user device for wireless communication is provided. The method comprises receiving an SS block, wherein said SS block comprises at least a primary synchronization signal and a secondary synchronization signal and extracting said SS block index from a received SS block. It is advantageous for a user device to receive transmission wherein each SS block comprises SS block index. Thus, it is possible acquire all information required for synchronization by receiving one SS block only.

[0029] The device and method for wireless communication network synchronization a method for wireless communication disclosed above provides an efficient mechanism for providing required synchronization information from network side to user equipment. Particularly beneficial the arrangement disclosed above is in beamformed networks, wherein it is possible to remove a plurality of limitations by using aspects and implementations described above. These and other aspects of the invention will be apparent from and the embodiment(s) described below. DESCRIPTION OF THE DRAWINGS

[0030] The present description will be better understood from the following detailed description read in light of the accompanying drawings, wherein:

FIG. la illustrates a schematic representation of a network node and a UE interacting in a wireless communication system;

FIG. lb illustrates an example of a method used in a wireless

communication system;

FIG. lc illustrates an example of a method used in a wireless

communication system;

FIG. Id illustrates an example of the network node used in a wireless communication system;

FIG. 2 illustrates a signaling chart of a wireless communication system;

FIG. 3 illustrates an example of a synchronization signals

FIG. 4 illustrates example of SS blocks in relation to beams; and

FIG. 5 illustrates an example of an SS block.

DETAILED DESCRIPTION

[0031 ] The detailed description provided below in connection with the appended drawings is intended as a description of the embodiments and is not intended to represent the only forms in which the embodiment may be constructed or utilized. However, the same or equivalent functions and structures may be accomplished by different embodiments.

[0032] In the following description synchronization signal block, SS block, and synchronization block index, SS Block Index, are extensively discussed. The SS block is used to carry different signals and information. The SS block index is an index of SS blocks within a time period, such as a synchronization signal (SS) burst. For example, if an SS burst comprises three SS blocks the corresponding SS block index includes information of these three SS blocks. Other expressions used for the same purpose are, for example, synchronization signal interval, synchronization signal occasion, synchronization symbol or similar. However, the difference between, for example, a synchronization signal interval and a synchronization signal interval index corresponds with the difference described above with regard an SS block and an SS block index.

[0033] FIG. la illustrates a wireless communication system with one base station 100 and one user equipment 108. For the sake of clarity only one base station and user equipment is shown. Typically wireless communication systems involve very large number of base stations that are connected to serve very large number of users. The base station of figure la comprises a signal processing circuitry 101, a beamformer 102 and two antennas 103a, 103b. The network connection 105 is used to connect the base station 100 to the core network 105. The signal processing circuitry comprises at least one processor 106 and at least one memory 107 configured to process and produce signals that will be transmitted further. Processing includes several different tasks that are related to synchronization and synchronization signals. The beamformer 102 receives signals that include synchronization signals from signal processing circuitry 101 and provides the in beamformed form to a transceiver 104, which transmits beams using transmission antennas 103a, 103b. The number of antennas is not limited to two.

[0034] Referring to FIG. la, the base station 100 is configured to transmit data to at least one user equipment 108. The particular processing of the received data and an appropriate response to the received synchronization signals. The user equipment may be a mobile phone, smart phone or a similar device.

[0035] Referring to FIG. lb an aspect of a method used in a wireless communication system, such as the base station 100 of figure la or the network node 118 of FIG. Id, is disclosed. In the method first an SS block is provided, step 111. In the example an SS block index is then encoded in to the SS block, step 112. Then the encoded SS block is transmitted, step 113. The transmission may be in beamformed form, however, it is not necessary. [0036] Referring to FIG. lc another embodiment of a method used in a wireless communication system, such as the system of figure la, is disclosed. In the method first an SS block is provided, step 114. In the example an SS block index is then encoded in to the SS block, step 115. In addition to the SS block index also a beam index is encoded into the SS block, step 116. In the example of figure lc the transmission step 117 is in beamformed form, however, even if beamformed communications are used it is not necessary to transmit SS blocks in beamformed form.

[0037] FIG. Id illustrates an aspect of a wireless communication system. In the figure a network element 118 is illustrated. The network element 118 comprises a processor 119. The network element optionally comprises a beamformer 120. The network element 118 is configured to perform the method according to figure lb. Optionally the network element 118 is configured to perform the method of figure lc. When the method of figure lc is performed the beamformer 120 is included in the network node 118.

[0038] FIG. 2 illustrates a signaling chart of a wireless communication system. A network node 200, such as a base station, is configured to communicate with a user equipment (UE) 201. The network node 200 transmits signals 202 comprising at least one SS block to the user equipment 201. The SS block comprises a primary synchronization signal and a secondary synchronization signal. Synchronization signals blocks are sent in beamformed form. Each of SS blocks further comprise an SS block index. The SS block may also include a beam index.

[0039] From the received signals UE 201 derives the downlink synchronization and potentially the preferred beam information. Furthermore, the UE 202 is configured to indicate beam information and/or synchronization information to the network node in order to derive the preferred downlink beam information.

[0040] In the example of figure 2 the network 200 node transmits the synchronization signal using a flexible number of beams, and a fixed number of synchronization signal sequence resources each associated with a unique SS block in a fixed number of SS blocks. The UE 201 derives the SS block information from the detected synchronization signal sequence resource.

[0041 ] The network node transmits the synchronization signal 202 using a flexible number of beams (X) in a synchronization signal burst or a synchronization signal burst set comprising a flexible number (T) of SS blocks, where P beams are applied in each SS block, 1<=P <= M and 1<=T <= N, and M and N are pre-defined, e.g., in the standard. The number of beams P per SS block is not necessarily known at the UE prior to initial access.

[0042] Figure 3 provides an example of the transmission of primary synchronization signals (PSS) and secondary synchronization signals (SSSs) in a synchronization signal (SS) burst set comprising multiple SS bursts where each SS burst comprises multiple SS blocks. The same beam direction(s) will be applied in one SS block for PSS, SSS and potentially other signals such as public broad cast channel (PBCH). These signals in one SS block can either be time division multiplexed (TDM), i.e., as shown in figure 3, or frequency division multiplexed (FDM). One SS block may be one OFDM symbol, or any other pre-defined time interval. The transmit receive point (TRP) may implement the beamformed synchronization signals using P (P <= M) beam(s) per SS block and T (T <= N) SS block(s) per SS burst, where in Fig. 3 "nominal" means the pre-defined maximum value (M and N), and "actual" means the used value at TRP according to its beamforming capability. Note that this includes support of single beam transmission, e.g., when P = T = 1. The UE does not need to know the actual number of beams and the actual number of SS blocks for decoding synchronization signals as the same synchronization and cell search procedure (e.g., sliding window) can be adopted. Upon detecting the SS from any SS block, the cell search information can be derived.

[0043] In figure 4 selection of the number of beams per SS block and the number of SS blocks to achieve the same coverage is illustrated. The number of beams X may be equal to the number of SS blocks N, however, it does not necessary need to be equal. In one embodiment X is equal to N. The network node may apply sequential beamforming, i.e., each beam direction corresponds to one SS block. In another embodiment X is larger than N. This is especially beneficial in some implementations allowing concurrent transmission of a number of beams, for example, using hybrid beamforming or digital beamforming. The network node may apply concurrent transmissions of multiple beamformed synchronization signals in at least one SS block. Concurrent transmission of multiple beamformed synchronization signals may be labeled, for example, by different synchronization signal sequences. These synchronization signal sequences for the same SS block can have the same labels for the SS block index, while different labels for the beam index. Concurrent beam transmission can be used to achieve wider block angular width, resulting in a smaller number of SS blocks and therefore reduced overhead, as shown in Figure 4. Concurrent beam transmission can also be used to achieve better beam resolution over a given number of SS blocks, a wide beam to be transmitted in one SS block can be split into multiple concurrent narrow beams. In another embodiment X is smaller than N. The network node may apply one beam direction to multiple SS blocks. This is especially beneficial for some implementations producing broad beams, e.g., a network node with a limited number of antenna elements.

[0044] The arrangement described above is advantageous as there is no strict restriction on how many beams are used for transmitting synchronization signal, and it is not necessary to pre-define the mapping between the beams and the SS blocks within an SS burst or SS burst set.

[0045] Next encoding of SS block index in PSS or SSS is explained. The SS block index is encoded in the synchronization signal sequence resource, for example, using a processor of the base station. A synchronization signal resource is a container for SS block index, wherein one synchronization signal sequence corresponds to one index. A number of synchronization signal sequence resources are predefined in standards, where the total number is equal to the maximum number of SS blocks, i.e., N. The association between the N synchronization signal resources and the N SS blocks is one-to-one and predefined. A synchronization signal sequence resource can be a PSS sequence, a SSS sequence, or a combination of a PSS sequence and a SSS sequence. A synchronization signal sequence resource can also be a PSS sequence group, a SSS sequence group, a combination of a PSS sequence group and a SSS sequence group, a combination of a PSS sequence and a SSS sequence group, or a combination of a PSS sequence group and a SSS sequence.

[0046] In one embodiment the SS block index is encoded in the PSS sequence. Each synchronization signal sequence resource comprises a distinct PSS sequence. The network transmits multiple PSS sequences in multiple SS blocks. Each PSS sequence is associated with one unique SS block. One example of the association is given in Table 1. A PSS sequence may be a Zadoff-Chu sequence as defined in LTE, or any other sequence which may or may not contain cell- specific information such as cell ID.

Table 1. Association between the synchronization signal sequence resource and the SS block

[0047] This is advantageous as the UE may derive the SS block index from

PSS sequence resource, while also not increasing the signaling overhead in SSS.

[0048] The PSS sequence can be generated by modulating a sequence with the SS block index, for example, by the following equation c_s (ri) = d_u (n)b_s (n mod m), n = 0,1,...,L - 1 [0049] In the equation L = tm , t and m are positive integers, d n) is the n-t element of a length-L ZC sequence as defined in 3 GPP LTE with root index u , b_s (n mod m) is the (n mod m)-th element of a length-m sequence restricted to consist of complex numbers having the same (unit) magnitude, and derived from the SS block index, i.e., each SS block corresponds to a unique sequence { b_s (n mod m) } .

[0050] This is advantageous as the UE derives from PSS the cell- specific information, i.e., part of cell ID from the used root index u , while also capable of deriving the SS block index. It shall be noted that in this case a group of PSS sequences are associated to one SS block, i.e., they constitute one synchronization signal resource.

[0051 ] In one embodiment, the SS block index is encoded in the SSS sequence. Each synchronization signal sequence resource comprises a distinct SSS sequence. The network transmits multiple SSS sequences in multiple SS blocks. Each SSS sequence is associated with one unique SS block. The SSS sequence may be a sequence as defined in LTE, or any other sequence which may contain cell- specific information such as cell ID, or any other information. This is advantageous as the UE may derive the SS block index from SSS sequence, while not increasing the signaling overhead in PSS.

[0052] In one embodiment, the SS block index s_m is jointly encoded in PSS and SSS. S_ID is a function of _¾> and S , where _¾> is encoded in the PSS sequence and is encoded in the SSS sequence. In one embodiment, there are four SS blocks indexed by {0, 1, 2, 3 }, is from {0, 1 }, S™ is from {0, 1 }, and S_ID = 2 x + . This is advantageous as the indication of the SS block index can be flexibly allocated to PSS and SSS, by setting and , to achieve better performance and complexity tradeoff.

[0053] In Figure 5 another embodiment is disclosed. In the embodiment, each PSS is associated with one SSS (namely SSS 1) and one additional synchronization signal (namely SSS2). SSS 1 is used to carry cell ID, as in LTE. SSS2 uses the same set or a subset of the SSS sequences, to indicate additional information. A detailed design can be found in Figure 5, where each SS block comprises PSS, SSSl, SSS2 and PBCH, corresponding to the same beam. In LTE, the SSS sequence used for the first half frame and the second half frame are different to encode the frame timing, e.g., 168 sequences for the first half and 168 sequences for the second half. The total available sequences for SSS2 is therefore 336. This is advantageous as the SSS sequences can be reused to carry additional information including SS block index and beam index. Additionally, the SSS detector may be reused for both SSSl and SSS 2 as they share the same set of sequences.

[0054] In a further embodiment, each PSS is associated with one SSS (namely SSSl) or one additional synchronization signal (namely SSS 2). SSS2 uses part of the SSS sequences for indicating information in addition to SSSl, e.g., the SS block index and/or beam index information. Specifically, SSSl uses the first 168 sequences to carry the cell ID and is only transmitted in the first half-frame of each frame. In this way, the frame timing information can be directly obtained by detecting SSSl. In addition, SSS2 uses the second 168 SSS sequences of all 336 SSS sequences and is only transmitted in the second half-frame of each frame. The selection of SSS2 sequence is independent of that of the SSSl sequence and can be used to carry additional information. For example, assuming there are 14 SS block, one can divide the second 168 SSS2 sequences into 14 groups each containing 12 SSS2 sequences. Each group is uniquely associated with one synchronization interval and each SSS2 sequence in one group is uniquely associated with one beam index. This is advantageous as the existing synchronization signal design/detection can be inherited to the largest extent while the additional function is fulfilled.

[0055] In a further embodiment, the SS block index is jointly encoded in the synchronization signal sequence and PBCH. In LTE, different SSS sequences are used in the first half frame and the second half frame. In case the SS periodicity is increased from half system frame to a longer value, e.g., one system frame, the different SSS sequences for the second half frame can be reused to indicate two states of the SS block index. The other remaining SS block index can be indicated in the associated PBCH. This is advantageous as the existing synchronization signal sequence design can be reused, which simplifies the implementation and standardization

[0056] The SS block index can be conveyed in PBCH by reusing the existing information bits in LTE. This could be conditioned when the synchronization periodicity is further increased, e.g., to several system frames. In one embodiment, the SS periodicity is increased to 80ms, i.e., 8 frames. Then upon detecting the PSS/SSS and deriving the SS block index, the frame index within every 8 frames is also available, which means 3 bits saving in the field for system frame number indication in PBCH. This saved 3 bits can be used to indicate 2³ = 8 states of the SS block without increasing the PBCH overhead. Combined with ability of two states indication from the SSS sequence saving, there would be 16 states saving without increasing the number of synchronization signal sequences or the number of bits in PBCH. This applies the splitting of the SS block index into two parts, one in synchronization signal, and the other in PBCH. This is advantageous as both the existing synchronization signal sequence design and PBCH design can be reused, which simplifies the implementation and standardization.

[0057] In another embodiment the beam information can be associated with the SS block index. This association is known at the network node but unknown to the UE. The UE feeds back the SS block index explicitly (e.g., uplink control signaling) or implicitly (e.g., the selected Physical Random Access CHannel (PRACH) resource). The network node can then derive the beam information from the association between the used beam(s) and the SS blocks. This is because the network node has the information which beam(s) has been used in a specific SS block indicated by the UE. This is advantageous as the network node can acquire at least part of the beam information. In the case when there is only one beam adopted in each SS block, all the beam information can be obtained without any ambiguity.

[0058] In another embodiment, the network node transmits the synchronization signal using a flexible number of beams X, and a flexible number of synchronization signal sequence resources T each associated with a unique SS block. The total N synchronization signal sequence resources comprise a plurality of synchronization signal sequences each associated with one beam. The total N synchronization signal sequence resources are associated to a pre-defined maximum number of beam indexes M. The UE derives the explicit beam information from the detected synchronization signal sequence resource.

[0059] In one embodiment a pre-defined maximum beam number M is used.

The beam information is encoded in the synchronization signal sequence, in addition to the SS block index. The maximum number of beams M is pre-defined. The detailed radiation pattern of each beam is up to implementation of the network node and unknown at the UE. As already disclosed in the first embodiment, the number of beams X is not necessarily restricted by the number of SS blocks N. The UE has no information of X prior to initial access. However, the UE has the information of M, as well as the association of the N synchronization signal sequence resources to the M beam indexes. For a network node, the Ν synchronization signal sequence resources only comprises the sequences mapping to X beams, e.g., Beam ID {0, 1,..., X-l }. This is advantageous as the UE may derive the beam ID from the pre-defined association between the synchronization signals and the beams. The beam ID can then be fed back to the network node.

[0060] In one embodiment N synchronization signal sequence resources are associated to M beam. One synchronization signal sequence resource is uniquely associated with one SS block index, and also associated with beams, as in Table 2.

Table 2. Association between the synchronization signal sequence resources, the SS block, and beam

[0061 ] This is advantageous as reduced feedback signaling can be achieved. As the synchronization signal sequences typically comprise information for the cell ID and the SS block index, the total number of information states comprised in the synchronization signal sequences is typically greater than the number of beams. In one example, the number of information states is 5040 if there are 504 cell indexes and 10 SS blocks, while the maximum number of beams could be much smaller, e.g., M = 16. Hence the UE may feedback less information using the beam ID instead of using the synchronization signal sequence index.

[0062] In one embodiment a synchronization signal sequence resource comprises a group of SSS sequences comprising information for SS block index, cell ID and beam ID. The SSS sequences in the group associated with a synchronization signal sequence resource carry the same SS block index, but different beam ID. A group of sequences with the same SS block index form one synchronization signal sequence resource. The information for SS block index can be an index selected from N values, i.e., {0, 1, N-l }. The information for cell ID can be the cell index, or part of cell index as in LTE. The information for beam ID can be an index selected from M values, i.e., {0, 1, -l }. This is advantageous as there is a maximum flexibility of beams used in each SS block, i.e., up to M concurrent beams.

[0063] As a special case of Table 2, the beam ID associated to a synchronization signal sequence resource may only be a sub- set of {0, 1, M-l }. In one embodiment, the beam IDs are classified into Ν groups each associated to a unique synchronization signal sequence resource. For example, by assuming N = 4 and M = 8, the association is illustrated in Table 3 below.

Table 3. Association between the synchronization signal sequence resource, the SS block, and beam. Ν = 4, M = 8, and each synchronization signal sequence resource is only associated to a sub-set of beam IDs.

index. The total sequence

resource index

0 0 {0, 1 }

1 1 {2, 3 }

2 2 {4, 5}

3 3 {6, 7}

[0064] In this case, the beam ID and the SS block index are jointly associated with the synchronization sequence resource. Part of the beam information number of synchronization signal sequences can be reduced. Please note that the UE need be aware of such an association to derive the SS block index and beam ID from the detected synchronization signal sequence. This is advantageous as a reduced number of synchronization signal sequences can be predefined in the standard. The total number of information states for each synchronization signal sequence resource is reduced from 8 to 2 in this embodiment. This allows up to 2 concurrent beams in each SS block. Similarly as in earlier embodiment with only SS block index, the beam information can be jointly encoded in the synchronization signal and PBCH, to reuse the LTE design.

[0065] In an all described embodiments the UE may work with multiple connections to the network, e.g., connected to multiple beams of the same TRP, or multiple beams of different TRPs in the same cell (i.e., with the same cell ID), or even multiple TRPs in different cells. Especially, for the UE connected to multiple TRPs of the same cell, it is desirable that the synchronization signal design may allow the UE to acquire the TRP information in order to maintain multiple TRP connections. The TRP information can be encoded similarly as in the previous embodiments, e.g., the TRP ID in PSS, SSS, ESS, PBCH, or a combination of them.

[0066] The system and method for wireless communication network synchronization has been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

1. A network node (100) for wireless communication comprising:

a processor (106) configured to

provide (111) an at least one synchronization signal block, SS block, wherein said SS block comprises at least a primary synchronization signal (PSS) and a secondary synchronization signal (SSS, SSSl);

encode (112) a SS block index into each of said SS blocks; and a transceiver (104) configured to transmit (113) said SS block.

2. The network node of claim 1, wherein said transceiver (104) is configured to transmit (115) said synchronization block using multiple beams. 3. The network node of claim 1 or 2, wherein said SS block further comprises at least one of an additional secondary synchronization signal (SSS2) and a physical broadcast channel information (PBCH).

4. The network node of any preceding claim 1 - 3, wherein said processor is configured to encode said synchronization block index into at least two of said primary synchronization signal (PSS), secondary synchronization signal (SSS, SSSl), additional synchronization signal (SSS2) and physical broadcast channel information (PBCH). 5. The network node of claim 1 - 4, wherein the number of said SS blocks is from 1 to N, wherein N is a pre-defined maximum value.

6. The network node of any preceding claim 1 - 5, wherein said processor (106) is configured to encode (112) said SS block index into any of:

- said primary synchronization signal (PSS), wherein said processor (106) is further configured to derive primary synchronization signal sequence from multiplying a Zadoff Chu sequence by an additional sequence, wherein said additional sequence is used in encoding the SS block index, or

- said additional synchronization signal (SSS2), wherein said processor (106) is configured to derive said additional synchronization signal (SSS2) from the same set of sequences as said secondary synchronization signal (SSSl), or

- said secondary synchronization signal (SSS, SSI), wherein the periodicity of said secondary synchronization signal (SSS, SSSl) is an integer multiple of one system frame. 7. The network node of any preceding claims 1 - 6, wherein said processor

(106) is further configured to encode (116) a beam index into each of said at least one SS block.

8. The network node of claim 7, wherein the number of said beam indexes encoded into said at least one SS block is from 1 to M, wherein M is a pre-defined maximum value.

9. The network node of claim 1-8, wherein said processor is further encode a TRP identity and cell identity into each of said at least one SS block .

10. A user device (108) for wireless communication comprising:

a transceiver (109) configured to receive an at least one SS block comprising at least primary synchronization signal (PSS) and secondary synchronization signal (SSS);

a processor (110) configured to extract said SS block index from a received

SS block.

11. A user device of claim 10, wherein said processor (110) of said user device (108) is configured to extract said SS block index from said at least one received SS block by decoding any of: - said primary synchronization signal (PSS) derived from multiplying a Zadoff Chu sequence by an additional sequence, wherein said additional sequence encodes the SS block index, or

- an additional secondary synchronization signal (SSS 2) derived from the same set of sequences as said secondary synchronization signal (SSSl), or

- said secondary synchronization signal (SSS, SSSl), wherein the periodicity of said secondary synchronization signal (SSS, SSSl) is an integer multiple of one system frame.

12. The user device of claim 10 or 11, wherein said processor is further configured to derive any of beam index and TRP identity.

13. The user device of claim 12, wherein said user device is configured to derive an uplink transmission from any of said SS block index, said beam index, and said TRP identity.

14. A method for wireless communication comprising:

providing (111, 114) an at least one SS block, wherein said at least one SS block comprises at least a primary synchronization signal (PSS) and a secondary synchronization signal (SSS, SSSl);

encoding (112, 115) the SS block index into each of said SS block; and transmitting (113, 117) said SS block.

15. A method for wireless communication comprising:

receiving a SS block, wherein said SS block comprises at least a primary synchronization signal (PSS) and a secondary synchronization signal (SSS, SSSl); and

extracting said SS block index from a received SS block.