WO2014162200A2

WO2014162200A2 - Neighbor discovery method and apparatus for user mobile terminal device to user mobile terminal device communication system

Info

Publication number: WO2014162200A2
Application number: PCT/IB2014/000565
Authority: WO
Inventors: Dong Li; Yong Liu; Kai Yang; Fang-Chen Cheng
Original assignee: Alcatel Lucent
Priority date: 2013-04-03
Filing date: 2014-03-21
Publication date: 2014-10-09
Also published as: CN104105083A; CN104105083B; TW201503729A; WO2014162200A3

Abstract

There is provided a neighbor discovery method and apparatus for a user mobile terminal device to user mobile terminal device communication system comprising multiple eNode Bs and a plurality of user mobile terminal equipments. The method comprises receiving a system message at each of the plurality of user mobile terminal equipments from a respective eNode B of the multiple eNode Bs, wherein the system message comprises a configuration information indicating how a plurality of neighbor discovery channels are configured in radio resources; selecting, at each of the plurality of user mobile terminal equipments, a respective neighbor discovery channel from the plurality of neighbor discovery channels based on the configuration information in the system message; transmitting, at each of the plurality of user mobile terminal equipments, respective beacon signals on the selected neighbor discovery channel; and receiving, on unselected neighbor discovery channels, at each of the plurality of user mobile terminal equipments, beacon signals from neighbor mobile terminal devices of a respective user mobile terminal equipment. The neighbor discovery method and apparatus of the present invention can achieve good performance with neighbor discovery latency constraint.

Description

NEIGHBOR DISCOVERY METHOD AND APPARATUS FOR USER MOBILE TERMINAL DEVICE TO USER MOBILE TERMINAL DEVICE COMMUNICATION

SYSTEM TECHNICAL FIELD

[0001] Embodiments of the present invention relate to the field of user mobile terminal device to user mobile terminal device communications, and more particularly, relate to a neighbor discovery method and apparatus for a user mobile terminal device to user mobile terminal device communication system.

BACKGROUND ART

[0002] In recent years, there is a social-technogical trend for the proximity-based applications and services. The proximity based device to device (D2D) communications can greatly improve the spatial spectral efficiency (from small communication range) and can also obtain the hop gain (from direct communication, instead of two-hop via base station). Motivated by these benefits, 3 GPP recently created a study item for the D2D communications. The D2D communication is expected to be an important component of the solutions to the incredible mobile data tunami in the next decades.

[0003] In D2D communications, each D2D node (i.e., user mobile terminal equipment, UE) has direct radio link to only a small number of other nodes, called its neighbors. A fundamental precondition for the D2D communications is the awareness of respective neighbors, that is, each D2D node shall discover and identify respective neighbors which are within its communication range, and shall be discovered and identified by the respective neighbors as well. This is called neighbor discover (ND).

[0004] Guo Dongning et al. proposed a neighbor discovery scheme based on compressive sensing in a document entitled "neighbor discovery for wireless networks via compressive sensing" (hereinafter abbreviated as "document 1 ", which is a available from the Internet at http://arxiv.org/pdf/1012.1007.pdf). The compressive sensing based neighbor discovery proposed in the document 1 has the following disadvantages: 1) each node shall have prior information on the discovery sequence matrix (includes sequences of all nodes), which may be a large signaling overhead; 2) the neighbor discovery complexity increases with the number of nodes, which will be a large burden, especially when the number of nodes are very high; and 3) the pseudorandom on-off signals requires frequent TX/RX switching, which complicates the implementation.

SUMMARY OF INVENTION

[0005] In view of the foregoing, one or more of the objectives of embodiments of the present invention is to propose a new neighbor discovery solution so as to solve or at least partially mitigate at least part of problems in the prior art.

[0006] According to a first aspect of embodiments of the present invention, there is provided a neighbor discovery method for a user mobile terminal device to user mobile terminal device communication system. The user mobile terminal device to user mobile terminal device communication system comprises multiple eNode Bs and a plurality of user mobile terminal equipments. The method comprises receiving a system message at each of the plurality of user mobile terminal equipments from a respective eNode B of the multiple eNode Bs, wherein the system message comprises a configuration information indicating how a plurality of neighbor discovery channels are configured in radio resources; selecting, at each of the plurality of user mobile terminal equipments, a respective neighbor discovery channel from the plurality of neighbor discovery channels based on the configuration information in the system message; transmitting, at each of the plurality of user mobile terminal equipments, respective beacon signals on the selected neighbor discovery channel; and receiving, on unselected neighbor discovery channels, at each of the plurality of user mobile terminal equipments, beacon signals from neighbor mobile terminal devices of a respective user mobile terminal equipment.

[0007] According to an exemplary embodiment, the plurality of neighbor discovery channels are repeated in neighbor discovery periods configurable by the multiple eNode Bs.

[0008] According to an exemplary embodiment, each of the neighbor discovery periods comprises a plurality of subperiods, and each of the neighbor discovery channels spans all the subperiods of one neighbor discovery period.

[0009] According to an exemplary embodiment, each of the neighbor discovery channels comprises a plurality of segments, and a number of the segments is equal to a number of subperiods within one neighbor discovery period.

[0010] According to an exemplary embodiment, the system message further comprises a number of user groups determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; wherein the plurality of user mobile terminal equipments are grouped based on the number of user groups such that each of the user mobile terminal equipments in each of the user groups can select a respective neighbor discovery channel and transmit respective beacon signals only in neighbor discovery periods corresponding to a respective user group.

[0011] According to an exemplary embodiment, the method further comprises: transmitting the respective beacon signals on a predetermined number of segments of the selected neighbor discovery channel, and receiving, on rest segments of the selected neighbor discovery channel, beacon signals from the neighbor mobile terminal devices.

[0012] According to an exemplary embodiment, the system message further comprises a reference waiting value M_BO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels. Selecting the respective neighbor discovery channel comprises: comparing a first configuration information currently broadcast by the respective eNode B with a second configuration information previously broadcast by the respective eNode B . If the first configuration information is the same as the second configuration information, there is detected, at each of the plurality of user mobile terminal equipments, average channel energies for all the neighbor discovery channels, and there is selected a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies. If the first configuration information is different from the second configuration information, there is generated, at each of the plurality of user mobile terminal equipments, a waiting value R indicating that a respective user mobile terminal equipment should select a neighbor discovery channel in the R^th subperiod, wherein R is an integer and 0<R<M_Bo-l - [0013] According to an exemplary embodiment, the system message further comprises a reference waiting value N_BO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels. Selecting the respective neighbor discovery channel comprises comparing a first configuration information currently broadcast by the respective eNode B with a second configuration information previously broadcast by the respective eNode B . If the first configuration information is the same as the second configuration information, there is detected, at each of the plurality of user mobile terminal equipments, average channel energies for all the neighbor discovery channels, and there is selected a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies. If the first configuration information is different from the second configuration information, a neighbor discovery channel is selected, and an average channel energy of the selected neighbor discovery channel is detected so as to determine whether there is a selection collision on the selected neighbor discovery channel. If there is the selection collision, a waiting value S indicating that a respective user mobile terminal equipment should re-select a neighbor discovery channel in the S^th subperiod is generated, wherein S is an integer and 0<S<NBO- 1

[0014] According to an exemplary embodiment, the method further comprises re-selecting a neighbor discovery channel in the S^th subperiod.

[0015] According to an exemplary embodiment, re-selecting a neighbor discovery channel in the S^th subperiod further comprises: detecting average channel energies for all the neighbor discovery channels, and selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies.

[0016] According to an exemplary embodiment, the system message further comprises a reference waiting value P_Bo determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and the method further comprising: generating a list of neighbors based on the received beacon signals, the list containing identifiers of the neighbor mobile terminal devices for the respective user mobile terminal equipment; comparing the list currently generated with a list previously generated so as to determine a difference between current neighbors and previous neighbors; comparing the difference with a threshold predetermined by the respective eNode B; if the difference exceeds the threshold, detecting an average channel energy of the selected neighbor discovery channel so as to determine whether there is a selection collision on the selected neighbor discovery channel, and if there is the selection collision, generating a waiting value T indicating that a respective user mobile terminal equipment should re-select a neighbor discovery channel in the T^th subperiod, wherein T is an integer and 0<T<PBO- 1 ·

[0017] According to an exemplary embodiment, the method further comprises re-selecting a neighbor discovery channel in the T^th subperiod.

[0018] According to an exemplary embodiment, re-selecting a neighbor discovery channel in the T^th subperiod further comprises: detecting average channel energies for all the neighbor discovery channels, and selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies.

[0019] In a second aspect of embodiments of the present invention, there is provided a neighbor discovery apparatus for a user mobile terminal device to user mobile terminal device communication system comprising multiple eNode Bs and a plurality of user mobile terminal equipments. The apparatus comprises means for receiving a system message at each of the plurality of user mobile terminal equipments from a respective eNode B of the multiple eNode Bs, wherein the system message comprises a configuration information indicating how a plurality of neighbor discovery channels are configured in radio resources; means for selecting, at each of the plurality of user mobile terminal equipments, a respective neighbor discovery channel from the plurality of neighbor discovery channels based on the configuration information in the system message; means for transmitting, at each of the plurality of user mobile terminal equipments, respective beacon signals on the selected neighbor discovery channel; and means for receiving, on unselected neighbor discovery channels, at each of the plurality of user mobile terminal equipments, beacon signals from neighbor mobile terminal devices of a respective user mobile terminal equipment.

[0020] According to an exemplary embodiment, the plurality of neighbor discovery channels are repeated in neighbor discovery periods configurable by the multiple eNode Bs.

[0021] According to an exemplary embodiment, each of the neighbor discovery periods comprises a plurality of subperiods, and each of the neighbor discovery channels spans all the subperiods of one neighbor discovery period.

[0022] According to an exemplary embodiment, each of the neighbor discovery channels comprises a plurality of segments, and a number of the segments is equal to a number of subperiods within one neighbor discovery period.

[0023] According to an exemplary embodiment, the system message further comprises a number of user groups determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and wherein the apparatus further comprises means for grouping the plurality of user mobile terminal equipments based on the number of user groups such that each of the user mobile terminal equipments in each of the user groups can select a respective neighbor discovery channel and transmit respective beacon signals only in neighbor discovery periods corresponding to a respective user group.

[0024] According to an exemplary embodiment, the apparatus further comprises: means for transmitting the respective beacon signals on a predetermined number of segments of the selected neighbor discovery channel, and means for receiving, on rest segments of the selected neighbor discovery channel, beacon signals from the neighbor mobile terminal devices.

[0025] According to an exemplary embodiment, the system message further comprises a reference waiting value M_BO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and wherein means for selecting the respective neighbor discovery channel comprises: means for comparing a first configuration information currently broadcast by the respective eNode B with a second configuration information previously broadcast by the respective eNode B; if the first configuration information is the same as the second configuration information, means for detecting, at each of the plurality of user mobile terminal equipments, average channel energies for all the neighbor discovery channels, and means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies; or if the first configuration information is different from the second configuration information, means for generating, at each of the plurality of user mobile terminal equipments, a waiting value R indicating that a respective user mobile terminal equipment should select a neighbor discovery channel in the R^th subperiod, wherein R is an integer and 0<R<M_Bo- l -

[0026] According to an exemplary embodiment, the system message further comprises a reference waiting value NBO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and wherein means for selecting the respective neighbor discovery channel comprises: means for comparing a first configuration information currently broadcast by the respective eNode B with a second configuration information previously broadcast by the respective eNode B ; if the first configuration information is the same as the second configuration information, means for detecting, at each of the plurality of user mobile terminal equipments, average channel energies for all the neighbor discovery channels, and means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies; or if the first configuration information is different from the second configuration information, means for selecting a neighbor discovery channel, means for detecting an average channel energy of the selected neighbor discovery channel so as to determine whether there is a selection collision on the selected neighbor discovery channel, and means for generating a waiting value S indicating that a respective user mobile terminal equipment should re-select a neighbor discovery channel in the S^th subperiod if there is the selection collision, wherein S is an integer and 0<S≤N_Bo- l -

[0027] According to an exemplary embodiment, the apparatus further comprises means for re-selecting a neighbor discovery channel in the S^th subperiod.

[0028] According to an exemplary embodiment, means for re- selecting a neighbor discovery channel in the S"¹ subperiod further comprises: means for detecting average channel energies for all the neighbor discovery channels, and means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies.

[0029] According to an exemplary embodiment, the system message further comprises a reference waiting value P_Bo determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and the apparatus further comprises: means for generating a list of neighbors based on the received beacon signals, the list containing identifiers of the neighbor mobile terminal devices for the respective user mobile terminal equipment; means for comparing the list currently generated with a list previously generated so as to determine a difference between current neighbors and previous neighbors; means for comparing the difference with a threshold predetermined by the respective eNode B; if the difference exceeds the threshold, means for detecting an average channel energy of the selected neighbor discovery channel so as to determine whether there is a selection collision on the selected neighbor discovery channel, and means for generating a waiting value T indicating that a respective user mobile terminal equipment should re-select a neighbor discovery channel in the T^th subperiod if there is the selection collision,, wherein T is an integer and 0<Τ<Ρ_Βο- 1 ·

[0030] According to an exemplary embodiment, the apparatus further comprises means for re-selecting a neighbor discovery channel in the T^th subperiod.

[0031] According to an exemplary embodiment, means for re-selecting a neighbor discovery channel in the T^th subperiod further comprises: means for detecting average channel energies for all the neighbor discovery channels, and means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies.

[0032] According to the solutions of the embodiments of the present invention, a neighbor discovery (ND) resource configuration is proposed that has hierarchical structure of ND subperiod and ND period. A ND channel contains multiple ND channel segments that not only improve the detection performance with time diversity, but also the structure design greatly facilitates the sensing and ND channel selection operations.

[0033] Several ND channel selection scheme are proposed, including sensing-based selection based on random initialization, backoff and sensing based method (BSS), and grouping based random selection method (GRS). The GRS method can achieve aggregation gains and is suitable for the neighbor discovery over multi-cells with much different user densities. The BSS method can achieve good performance with neighbor discovery latency constraint.

[0034] A mechanism is proposed for sensing-based neighbor discovery channel reselection for support of mobility in the D2D networks.

[0035] Other features and advantages of the embodiments of the present invention will also be apparent from the following description of specific embodiments when read in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of embodiments of the invention.

BRIEF DESCRIPTION OF DRAWINGS

[0036] Embodiments of the invention are presented in the sense of examples and their advantages are explained in greater detail below, with reference to the accompanying drawings, wherein

[0037] Fig. 1 is a flowchart of a neighbor discovery method for a D2D communication system according to an embodiment of the present invention;

[0038] Fig. 2 is a diagram schematically illustrating the ND resource configuration;

[0039] Fig. 3a shows an example of a ND channel segment structure in time-frequency grids, and Fig. 3b shows another example of a ND channel segment structure in time-frequency grids;

[0040] Fig 4a schematically illustrates a ND channel segment structure for the first slot in a LTE frame in time domain;

[0041] Fig 4b schematically illustrates a ND channel segment structure for the second slot in a LTE frame in time domain;

[0042] Fig 5a schematically illustrates a TX/RX switching in ND wherein the ND resource is in UL subframe;

[0043] Fig 5b schematically illustrates a TX/RX switching in ND wherein the ND resource is in DL subframe;

[0044] Fig. 6 schematically illustrates a procedure of the GRS method according to an example of the embodiment of the present invention;

[0045] Fig. 7 is a flowchart of the GRS scheme according to an example of the present invention;

[0046] Fig. 8 schematically illustrates a procedure of the BSS scheme according to an example of the present invention;

[0047] Fig. 9 is a flowchart of the BSS scheme according to an example of the present invention;

[0048] Fig. 10 is a flowchart of the RSSU scheme according to an example of the present invention;

[0049] Fig. 11 is a flowchart of a ND channel reselection procedure for supporting user mobility according to an example of the present invention.

[0050] Fig. 12 schematically illustrates a beacon signal generation procedure according to an example of the embodiment of the present invention;

[0051] Fig. 13 shows CM comparison of the ND signal

[0052] Fig. 14 shows ND beacon signal detection performance in AWGN

[0053] Figs. 15a-15c shows the simulation results for User density 200 users/km , for User density 500 users/km 2 and for User density 1000 users/km 2 in the GRS scheme, respectively; and

[0054] Figs. 16a-16c shows the simulation results for User density 200 users/km , for User density 500 users/km 2 and for User density 1000 users/km 2 in the BSS scheme, respectively.

DETAILED DESCRIPTION OF EMBODIMENTS

[0055] It should be appreciated that, while this specification contains many specific implementation details, they should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

[0056] In a first aspect of embodiments of the present invention, there is provided a neighbor discovery method for a device to device communication system.

[0057] Fig. 1 is a flowchart of a neighbor discovery method for a D2D communication system according to an embodiment of the present invention. The D2D communication system may comprise multiple eNode Bs (eNBs) and a plurality of UEs. As shown in Fig.l, at step 101, receiving a system message at each of the plurality of user mobile terminal equipments from a respective eNode B of the multiple eNode Bs. It should be noted that the "respective eNode B" comprises one or more eNBs. When a UE is located at boundaries of multiple cells, the UE may receive more system messages from more eNBs. The system message comprises a ND resource configuration information indicating how a plurality of ND channels are configured in radio resources. In general, the period with which the eNB broadcasts the system message containing the ND resource configuration information can be relatively large, e.g., several minutes.

[0058] Hereinafter, reference will be made to Figs. 2 to 5 to describe the ND resource configuration according to the embodiments of the present invention.

[0059] Fig. 2 is a diagram schematically illustrating the ND resource configuration. The ND resource comprises a plurality of ND channels and may be divided into a plurality of ND periods, such as ND period K-l, ND period K, ND period K+l as shown in Fig. 2. The plurality of ND channels are repeated in the ND periods. Each of the ND periods comprises a plurality of subperiods. For example, Fig. 2 shows that a ND period comprises N_ND ND subperiods, such as ND subperiod 0, ND subperiod 1, ND subperiod N_ND-1- Each of the ND channels comprises a plurality of segments. One segment of a ND channel is illustrated with a dark rectangle in Fig. 2. It will be understood that, in Fig. 2, within each ND subperiod, P_NDXQ_ND ND channels are formed over the time-frequency grids. Each ND channel spans all the ND subperiods of one ND period, that is, within one ND period, there are totally P_NDXQ_ND ND channels, each of which contains N _D channel segments. The numbers of the ND periods and ND subperiods are configurable by the eNB. It will be understood that the number of the ND subperiods will be typically not greater than 4.

[0060] The ND resource may be configured in radio resource, such as a LTE frame. The mapping of the ND resource to a LTE frame structure is also illustrated in Fig. 2. As shown in Fig.2, the P_ND frequency ND channels are mapped to a contiguous carrier segment within a LTE carrier (e.g., 1.4MHz for P_ND=6). The ND resource can be configured at the UL subframes or DL subframes in TDD LTE or FDD LTE. Generally, UL subframe configuration for ND resource is preferred, but the configuration of the ND resource at the DL subframe in TDD LTE or FDD LTE is not excluded.

[0061] Fig. 3a shows an example of a ND channel segment structure in time-frequency grids, and Fig. 3b shows another example of a ND channel segment structure in time-frequency grids.

[0062] As shown in Fig. 3a and Fig. 3b, in frequency domain, each ND channel segment occupies 12 contiguous subcarriers (i.e., one RB in LTE terminology) (Fig. 3a) or 6 contiguous subcarriers (i.e., half RB in LTE terminology) (Fig. 3b) and multiple ND channels are frequency multiplexed in the ND carrier segments as shown in Fig. 2. As can be seen from Fig. 3a and Fig. 3b, there are totally 12*5=60 or 6*5=30 data resource elements for one ND channel segment and within one ND period, totally 60*NN_D or 30*NN_D (e-g-, 240 or 120 for N_ND=4) resource elements can be used to carry beacon signals of one UE in one ND channel.

[0063] In the ND channel segment structures shown in Fig. 3a and Fig. 3b, the pilots are multiplexed with the beacon signals in TDM. The motivation of this design is to facilitate DFT-precoding for the beacon signals, as done in the LTE UL, so as to achieve a lower PAPR (which means higher power efficiency and larger D2D operation range). Note that in the descriptions of the ND channel structures, five data symbols are only given as an example. Other ND channel structures (e.g., with 6 data symbols) are also possible, which depends on the numerology design for the CP length and zero-padding length.

[0064] Fig 4a schematically illustrates a ND channel segment structure for the first slot in a LTE frame in time domain and Fig 4b schematically illustrates a ND channel segment structure for the second slot in a LTE frame in time domain.

[0065] In the framework of ND resource configuration shown in Fig. 2, in order to support both discovering neighbors and being discovered by neighbors in the same ND period, a flexible switching between transmitting and receiving shall be supported. In the present invention, an improved ND channel structure design is provided to better support switching between transmitting and receiving, as shown in Figs. 4a and 4b. In particular, for each ND channel segment which spans one slot in a LTE frame, a full zero-padding and a half zero-padding are inserted at the beginning and the end of the useful waveform. Note that the half zero-padding is located at the end of the first slot, and at the beginning of the second slot, as shown in Figs. 4a and 4b.

[0066] The following reasons for this design are illustrated in Figs. 5a and 5b. It will be noted that, for the two ND channel segments within a ND subframe, only three states are possible for a ND node (UE): (state 1) transmitting its own beacon signal in the first slot and receiving beacon signals of other nodes in the second slot; (state 2) receiving beacon signals of other nodes in the first slot and transmitting its own beacon signal in the second slot; and (state 3) receiving beacon signals of other nodes in the first and second slots. It can seen that with the ND channel segment structures in Figs.4a and 4b, the ND nodes can work well in all the three states, regardless of the ND resource occupying the DL subframe or UL subframe.

[0067] The carrier segment that the ND resource is maped to generally has a small bandwidth (e.g., 1.4MHz), with the following motivations and benefits:

[0068] (1) under the maximum transmit power constrain, a relatively high transmitted power spectral density can be obtained and thus extends the ND range;

[0069] (2) with the small bandwidth configuration, the impact on the cellular LTE operations can be minimized; and

[0070] (3) under the half-duplex constraint in neighbor discovery (i.e., a D2D node cannot detect beacon signals of other D2D nodes during its deacon signal transmission), the missed potential neighbors can be minimized.

[0071] As mentioned above, one ND channel comprises multiple ND channel segments, each segment is within one ND subpeiod. The benefits and motivations from this design are as follows. [0072] Since a relatively small carrier segment is used, the frequency diversity is very limited. In this case, distributing one ND channel into multiple channel segments can obtain time diversity, thus improve the detection performance for ND beacon signals.

[0073] Based on the design of the ND channel, each ND channel segment is self-decodable and joint decoding of multiple channel segments may enhance the detection performance of the ND beacon signals. Thus, the ND channel segmentation facilitates flexible sensing operations. For example, within all the four channel segments, a D2D node can use three ND channel segments to transmit its beacon signal (so that its neighbors can find it) and mute one ND channel segment (i.e., receiving instead of transmitting on the ND channel segment) such that the D2D node can sense whether there are severe selection collisions on its selected ND channel. At the same time, the D2D node can also detect neighbors on the concurrent ND channels (i.e., the ND channels which are the same in time domain but are different in frequency domain). However, in the conventional neighbor discovery methods, due to the half-duplex constrains in neighbor discovery, the D2D nodes transmitting beacon signals on the concurrent ND channels can not be discovered. It will be understood that the so called "selection collision" refers to the case in which more D2D nodes select the same ND channel so that other D2D nodes can not discover the more D2D nodes. Sensing whether there is a selection collision on the selected neighbor discovery channel may comprises detecting an average channel energy of the selected neighbor discovery channel and comparing the average channel energy with a predetermined threshold. If the average channel energy exceeds the predetermined threshold, it is indicated that more D2D nodes are transmitting their beacon signals on the selected neighbor discovery channel. Thus, it can be determined there is the selection collision on the selected neighbor discovery channel.

[0074] Come back to Fig. 1. After receiving the system message from the eNB at step 101, selecting, at the respective UE, a respective neighbor discovery channel from the plurality of ND channels based on the configuration information in the system message at step 102. Hereinafter, reference will be made to Figs. 6 to 11 to describe the ND channel selection schemes according to embodiments of the present invention. Grouping-based random selection (GRS)

[0075] Fig. 6 schematically illustrates a procedure of the GRS method according to an example of the embodiment of the present invention.

[0076] In the GRS scheme, a plurality of D2D nodes are divided into a number of groups in a distributed manner such that the nodes in each of the groups can randomly select a respective ND channel and transmit the beacon signals only in ND periods corresponding to the respective group. The number of the groups is determined by the eNode B in accordance with a relative relationship between a user density and the number of the ND channels. The "relative relationship between a user density and the number of the ND channels" means that the user density is relatively large or small for a given number of the ND channels. For example, for ten ND channels, if there are 20 users in a certain area, it is considered that the user density is relatively large for this number of ND channels. The number of the groups is contained in the system message broadcast by the eNB.

[0077] An illustration of this scheme is shown in Figure 6 with users 1-8 (of 8 D2D nodes) as an example. In Fig. 6, it is assumed that each ND channel comprises four ND channel segments, and each segment is represented by a small rectangle. As shown in Fig. 6, it assumed for example that user 1 and user 3 are divided into a group, and the K^G_ND ¹¹¹ ND period corresponds to the group of users 1 and 3. Thus, the nodes of users 1 and 3 select respective ND channels and transmit their respective beacon signals only in the K^G _D ¹¹¹ ND period. The small rectangle denoted by 601 shows that the user 1 transmit its respective beacon signals in the K^G_ND ¹¹¹ ND period, and the small rectangle denoted by 603 shows that the user 3 transmit its respective beacon signals in the K^G_ND ¹¹¹ ND period. It should be understood that although both the user 1 and the user 3 transmit their respective beacon signals in the K^G _D ¹¹¹ ND period, the user 1 and user 3 transmit their beacon signals on their respective selected ND channels. Since the users select the ND channel independently, there is a high probability that the two users select different ND channels. This is indicated by different positions of the small rectangles 601 and 603.

[0078] Fig. 7 is a flowchart of the GRS scheme according to an example of the present invention.

[0079] As shown in Fig. 7, at step 701, ND function is activated. Next, at step 702, a

UE acquires the ND resource configuration information and the number of user groups G _D contained in the system message broadcast by the eNB. Then, at step 703, the UE randomly generates an integer number A in the range [0,1,...,G_ND-1]- Next, at step 704, the UE determines whether A is equal to the result of the number of the current ND period modular G_ND- If yes, the procedure proceeds to step 705. At step 705, the UE randomly select one ND channel to transmit beacon signals thereon and receive beacon signals on unselected ND channels. At step 704, if the result of determination is no, the procedure proceeds to step 706. At step 706, the UE receives beacon signals to find its neighbors.

[0080] The benefits of the GRS scheme are as below. [0081] (1) In distributed manner, very limited eNB signaling overhead involved (i.e., grouping number).

[0082] (2) The eNB can control ND channel selection quality by adjusting the group number parameter as per node density.

[0083] (3) Support different group number configuration in adjacent cells with the same beacon detection operation.

[0084] (4) Naturally support change of D2D user network topologies due to user motion.

Backoff and sensing based selection (BSS)

[0085] In the BSS scheme, after receiving the ND resource configuration information broadcast by eNB, a respective D2D node will compare a first ND resource configuration information currently broadcast by the eNB with a second ND resource configuration information previously broadcast by the eNB. Depending on the result of the comparison, there are two ND channel selection cases for the D2D node.

[0086] (1) If the first ND resource configuration information is the same as the second ND resource configuration information, the D2D node will detect average channel energies for all the ND channels, and select a ND channel from multiple ND channels with the lowest average channel energies. That is, a D2D node (i) will select a ND channel with the index

l_ND(i) = arg in{E } where E_{(p q)} denotes the average energy of the ND channel (p,q), p e [0 : P_ND -l],qe [0 : Q_ND - 1] , which are contributed by all the D2D nodes that have already selected this ND channel and transmitted their respective beacon signals on it. Note that, due to the fact that geographically adjacent users may have similar sensing results, in order to reduce ND channel selection collisions, the D2D node can randomly select one out of multiple least congested ND channels, i.e., select a ND channel from multiple ND channels with the lowest average channel energies. It will be noted that the number of the ND channels with the lowest average channel energies is determined by the eNode B in accordance with a relative relationship between a user density and the number of the ND channels. The number is contained in the system message broadcast by the eNB.

[0087] (2) If the first ND resource configuration information is different from the second ND resource configuration information, the D2D node will generate a backoff value (waiting value) R indicating that the D2D node will should select a ND channel in the R^th subperiod, wherein R is an integer and 0<R<M_Bo-l, M_Bo is determined by the eNode B in accordance with a relative relationship between a user density and the number of the ND channels and is contained in the system message broadcast by the eNB.

[0088] Fig. 8 schematically illustrates a procedure of the BSS scheme according to an example of the present invention. It should be noted that the ND channel structure in Fig. 8 is similar to that of Fig. 6.

[0089] As shown in Fig. 8, where the first configuration information is different from the second configuration information, for example, a user 3 (of a D2D node) generates a backoff value 0, and selects a ND channel in the 0^th subperiod of the K^th ND period, a user 2 generates a backoff value 2, and selects a ND channel in the 2^th subperiod of the Κ^ώ ND period. Note that when the user 2 selects the ND channel, the user 3 has already selected its ND channel and transmitted its beacon signal, thus the user 2 can deliberately avoid collision with the user 3 in ND channel selection, as shown in Fig. 8. .

[0090] Fig. 9 is a flowchart of the BSS scheme according to an example of the present invention. At step 901, ND function is activated. Next, at step 902, a UE acquires ND resource configuration information and a reference backoff value (reference waiting value) M_BO- Then, at step 903, the UE will compare a first ND resource configuration (NDRC) information currently broadcast by the eNB with a second ND resource configuration information previously broadcast by the eNB so as to determine whether the ND resource configuration information is updated. If no, the procedure proceeds to step 904. At step 904, the UE senses all the ND channels and then select one out of the N_LC least congested ND channels at step 905. Note that, N_LC represents a number of the least congested ND channels and is configured by the eNB. At step 903, if it is determined that the NDRC is updated, the procedure proceeds to step 907. At step 907, the UE randomly generate an integer number R in the range [0,1,...,M_BO-1]- Then, at step 908, it is determined whether R is equal to 0. If yes, the UE in the next ND subperiod randomly selects one ND channel to transmit beacon signals and retain selection in subsequent ND periods. Then, at step 910, the UE receives beacon signals on unselected ND channels in each ND period. If it is determined in step 908 that R is not equal to 0, the procedure proceeds to step 911. At step 911, the UE in the next R-l ND subperiods receive beacon signals on all ND channels. Then, at step 912, the UE in the R^th ND subperiod select one ND channel out of the N_LC least congested ND channels. Thereafter, the UE transmits beacon signals on the selected ND channel and receive beacon signals on unselected ND channels (step 913).

[0091] The benefits of the BSS scheme are as below. [0092] (1) Through sensing, the D2D node can select a ND channel with least congestion.

[0093] (2) Operation is conducted only when the ND resource configuration update-generally infrequent (e.g., in order of seconds).

[0094] (3) The eNB can adapt the backoff value to user density to achieve good ND channel access quality with limited signaling.

[0095] (4) Utilize the smaller granularity of ND channel segments.

Random selection with sensing-based update (RSSU)

[0096] In the RSSU scheme, after receiving the ND resource configuration information broadcast by eNB, a respective D2D node will compare a first ND resource configuration information currently broadcast by the eNB with a second ND resource configuration information previously broadcast by the eNB. Depending on the result of the comparison, there are two ND channel selection cases for the D2D node.

[0097] (1) If the first ND resource configuration information is the same as the second ND resource configuration information, the D2D node will detect average channel energies for all the ND channels, and select a ND channel from multiple ND channels with the lowest average channel energies, as described in the BSS scheme.

[0098] (2) If the first ND resource configuration information is different from the second ND resource configuration information, the D2D node will perform the following operations.

A. Randomly selecting a ND channel.

B. Detecting an average channel energy of the selected ND channel so as to determine whether there is a severe selection collision on the selected ND channel.

C. If there is the severe selection collision, generating a backoff value (waiting value) S indicating that the UE should re-select a ND channel in the S"¹ subperiod, wherein S is an integer and 0<S<N_BO- 1, N_BO is determined by the eNB in accordance with a relative relationship between a user density and the number of the ND channels and is contained in the system message broadcast by the eNB.

D. If there is no severe collision in the random selection, the UE will retain its ND channel selection.

[0099] Fig. 10 is a flowchart of the RSSU scheme according to an example of the present invention. At step 1001, ND function is activated. Then, at step 1002, a UE acquires ND resource configuration information and a reference backoff value (reference waiting value) NBO- Then, at step 1003, the UE will compare a first ND resource configuration (NDRC) information currently broadcast by the eNB with a second ND resource configuration information previously broadcast by the eNB so as to determine whether the ND resource configuration information is updated. If no, the procedure proceeds to step 1004. At step 1004, the UE senses all the ND channels and then select one out of the NLC least congested ND channels at step 1005. Note that, NLC represents a number of the least congested ND channels and is configured by the eNB. At step 1003, if it is determined that the NDRC is updated, the procedure proceeds to step 1007. At step 1007, the UE makes a random ND channel selection. Then, at step 1008, the UE transmits beacon signals and senses all the ND channels. Note that, the UE senses its selected ND channel by muting one ND channel segment of the ND channel (i.e., transmitting no neighbor discovery signals on the one ND channel segment). Next at step 1009, it is determined whether there is a selection collision on the selected ND channel and whether it is needed to re- select a ND channel. If yes, the UE generates a waiting value S indicating that the UE should re-select a ND channel in the S^th subperiod (step 1010), wherein S is an integer and 0<S<NBO- 1 - Next, at step 1011 , the UE senses all the ND channels before the time indicated by the waiting value S is expired. Then, at step 1012, the UE selects one out of multiple least congested ND channels in the S^th subperiod. Next, at step 1013, the UE transmits beacon signals on the selected ND channel and receives beacon signals on unselected ND channels so as to make ND. If it is determined at step 1009 that there is no selection collision on the selected ND channel, the UE will retain its random ND channel selection (step 1014), transmit beacon signals on the selected ND channel and receive beacon signals on unselected ND channels so as to make ND (step 1015). ND channel reselection for support of Mobility

[00100] A challenge of the D2D network design is the mobility. The mobility of D2D nodes will lead to the change of D2D network topology with time. In case of ND, a good ND channel selection at some time that has no collision may suffer severe interference at a later time due to collisions after some mobility. In the present invention, a ND channel reselection scheme is designed to support user mobility. Hereinafter, reference will be made to Fig. 11 to describe a ND channel reselection scheme for supporting user mobility according to an example of the present invention.

[00101] As shown in Fig. 11, at step 1101, a UE transmits beacon signals on the selected ND channel and receives beacon signals on unselected ND channels so as to make ND. Next, at step 1102, a list of neighbors is generated based on the received beacon signals, and the list contains identifiers of the neighbors for a respective UE. Then, at step 1103, the list currently generated is compared with the list previously generated so as to determine a difference between the current neighbors and the previous neighbors. Next, at step 1104, the difference is compared with a threshold predetermined by the eNB.

[00102] For example, a percentage, denoted by D, of the same neighbors over the total neighbors in the two lists may be computed. Thus, the difference may be determined as (1-D). Then, the value of (1-D) is compared with the threshold. If the value of (1-D) exceeds the threshold, it can be assumed that the surrounding network topology changes a lot, and the procedure proceeds to step 1105. Then, at step 1105, the UE mutes one or more ND channel segments of the selected ND channel and detects an average channel energy of the selected ND channel.

[00103] At step 1106, it is determined whether there is a selection collision on the selected ND channel and whether there is a need of re-selection. If yes, the UE generates a waiting value T indicating that the UE should re- select a ND channel in the T^th subperiod, wherein T is an integer and 0<T<P_BO- 1, P_BO is a reference backoff value (reference waiting value) determined by the eNB in accordance with a relative relationship between a user density and the number of the ND channels, and is contained in the system message broadcast by the eNB. Next, at step 1107, the UE detects average channel energies for all the ND channels, and selects a ND channel from multiple ND channels with the lowest average channel energies, i.e., reselects one out of multiple least congested ND channels. At step 1106, if it is determined there is no selection collision on the selected ND channel and there is no need of re- selection, the procedure will proceed to step 1108. At step 1108, the UE will retain its ND channel selection.

[00104] With reference back to Fig. 1, the UE transmits respective beacon signals on the selected ND channel at step 103 and receives, on the unselected ND channels, beacon signals from neighbors at step 104. Hereinafter, reference will be made to Figs. 12-14 to describe a beacon signal generation procedure according to an example of the present invention.

[00105] Assuming that the beacon information per ND period consists of M bits, for example, M=24. The processing for the beacon information bits are shown in Fig 12. The M beacon information bits are firstly CRC encoded and L parity bits are attached, wherein L =8 (step 1201). L=16 is also possible. The cyclic generator polynomials defined in section 5.1.1 of 3GPP TS 36.212, VIO.0.0, "Multiplexing and channel coding" (hereinafter abbreviated as "document 2", which is available from the 3GPP website at www.3gpp. org) can be used. Then, tail biting convolutional coding (TBCC) is applied for the K=M+L bits. The rate 1/3 tail biting convolutional coding defined in section 5.1.3.1 of the document 2 can be used.

[00106] The 3K output bits from TBCC are then processed by the rate matching module (step 1203), according to the specification in section 5.1.4.2 of the document 2, to match the resource elements in one ND channel. Taking NND=4 as an example, totally 240 resource elements will be used for ND beacon signal. Assuming QPSK modulation is applied, the overall effective coding rate for the ND information is 1/15 or 2/15 for 5-symbol ND channel segment structure as shown in Fig. 3, and the overall effective coding rate for the ND information is 1/18 or 1/9 for 6-symbol ND channel segment structure (not shown).

[00107] After rate matching, bit-wise scrambling (step 1204) is applied to randomize the potential colliding interference and achieve the processing gain and coding gain. Several scrambling patterns are predefined and the selected scrambling pattern is implicitly signaled by the pilot sequence index which is blindly detected by hypothesis tests. The scrambling sequences can be specified based on the definition of pseudo-random sequence generation in section 7.2 of 3 GPP TS 36.211, VIO.0.0, "Physical channels and modulation", which is available from the 3GPP website at www.3gpp. org..

[00108] After the scrambling at step 1204, the output bits are QPSK modulated (step 1205) and then the DFT-precoding is applied for each 12(or 6)-length symbol group. Finally the symbols after DFT-precoding (step 1206) are mapped (step 1207) to the selected ND channel. Note that as a special case, no DFT-precoding is not excluded (i.e., bypass the DFT-precoding in Fig. 12) which may be desired when the power efficiency isn't a major issue.

[00109] In Fig. 13, the cubic metric (CM, a more practical property than PAPR value) is compared for beacon signals with and without 12-length DFT-precoding. It can be seen from Fig. 13 that the DFT-precoding can reduce the CM by about 1.7dB at 50% CDF and 2.5dB at 90% CDF.

[00110] Fig. 14 shows the frame error rate (FER) of the beacon signal transmission through AWGN channel for different ND channel segment detections for 5-symbol option 1 ND channel segment structure. It can be seen that the performance of three-channel-segment detection is much better than single-channel- segment detection (by up to 4.8dB at 0.01 FER). Meanwhile, the three-channel- segment detection is worese than four- segment case by about 1.2dB. This comparison means that muting one ND channel segment in the ND channel sensing and reselection has limited impact on the detection performance.

System-level simulations

[00111] Some system level simulations are performed to evaluate the system performance of the neighbor discovery schemes. The main simulation conditions are listed as follows.

[00112] In the simulations, the numbers of found neighbors averaged over all UEs are evaluated. Figs. 15a- 15c shows the results for User density 200 users/km , for User density 500 users/km 2 and for User density 1000 users/km 2 in the GRS scheme, respectively. Note that, the lines 1501, 1502 and 1503 correspond to maximum number of neighbors without collisions, i.e., upper bounds for numbers of found neighbors for any ND scheme.

[00113] Below are some comments for the results of GRS scheme.

[00114] (1) Aggregation of multiple ND results approaches the maximum value and the larger the user density, the slower of the approaching.

[00115] (2) In the GRS scheme, more found neighbors are obtained at cost of larger delay.

[00116] Figs. 16a- 16c shows the results for User density 200 users/km , for User density 500 users/km 2 and for User density 1000 users/km 2 in the BSS scheme, respectively. Note that, the lines 1601, 1602 and 1603 correspond to the neighbor discovery results with ideal greedy ND channel selection, i.e., the users progressively access to the D2D network one by one and select the least congested ND channel based on sensing results.

[00117] Below are some comments for the results of BSS-based scheme,

[00118] There is a gap between the results and the ideal greedy method results. This gap comes from collisions in each ND subperiod.

[00119] (1) At the overloaded scenarios, the numbers of found neighbors remain similar, irrespective of user densities.

[00120] (2) After the backoff initialization stage, the number of found neighbors remains stable.

[00121] (3) After the backoff initialization stage, the number of found neighbors remains stable.

[00122] In a second aspect of embodiments of the present invention, there is provided a neighbor discovery apparatus for a user mobile terminal device to user mobile terminal device communication system. The system comprises multiple eNode Bs and a plurality of user mobile terminal equipments.

[00123] The apparatus comprises means for receiving a system message at each of the plurality of user mobile terminal equipments from a respective eNode B of the multiple eNode Bs, wherein the system message comprises a configuration information indicating how a plurality of neighbor discovery channels are configured in radio resources; means for selecting, at each of the plurality of user mobile terminal equipments, a respective neighbor discovery channel from the plurality of neighbor discovery channels based on the configuration information in the system message; means for transmitting, at each of the plurality of user mobile terminal equipments, respective beacon signals on the selected neighbor discovery channel; and means for receiving, on unselected neighbor discovery channels, at each of the plurality of user mobile terminal equipments, beacon signals from neighbor mobile terminal devices of a respective user mobile terminal equipment.

[00124] According to an exemplary embodiment, the plurality of neighbor discovery channels are repeated in neighbor discovery periods configurable by the multiple eNode Bs.

[00125] According to an exemplary embodiment, each of the neighbor discovery periods comprises a plurality of subperiods, and each of the neighbor discovery channels spans all the subperiods of one neighbor discovery period.

[00126] According to an exemplary embodiment, each of the neighbor discovery channels comprises a plurality of segments, and a number of the segments is equal to a number of subperiods within one neighbor discovery period.

[00127] According to an exemplary embodiment, the system message further comprises a number of user groups determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and wherein the apparatus further comprises means for grouping the plurality of user mobile terminal equipments based on the number of user groups such that each of the user mobile terminal equipments in each of the user groups can select a respective neighbor discovery channel and transmit respective beacon signals only in neighbor discovery periods corresponding to a respective user group.

[00128] According to an exemplary embodiment, the apparatus further comprises: means for transmitting the respective beacon signals on a predetermined number of segments of the selected neighbor discovery channel, and means for receiving, on rest segments of the selected neighbor discovery channel, beacon signals from the neighbor mobile terminal devices.

[00129] According to an exemplary embodiment, the system message further comprises a reference waiting value M_BO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and wherein means for selecting the respective neighbor discovery channel comprises: means for comparing a first configuration information currently broadcast by the respective eNode B with a second configuration information previously broadcast by the respective eNode B; if the first configuration information is the same as the second configuration information, means for detecting, at each of the plurality of user mobile terminal equipments, average channel energies for all the neighbor discovery channels, and means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies; or if the first configuration information is different from the second configuration information, means for generating, at each of the plurality of user mobile terminal equipments, a waiting value R indicating that a respective user mobile terminal equipment should select a neighbor discovery channel in the R^th subperiod, wherein R is an integer and 0<R<MBO- 1 -

[00130] According to an exemplary embodiment, the system message further comprises a reference waiting value NBO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and wherein means for selecting the respective neighbor discovery channel comprises: means for comparing a first configuration information currently broadcast by the respective eNode B with a second configuration information previously broadcast by the respective eNode B ; if the first configuration information is the same as the second configuration information, means for detecting, at each of the plurality of user mobile terminal equipments, average channel energies for all the neighbor discovery channels, and means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies; or if the first configuration information is different from the second configuration information, means for selecting a neighbor discovery channel, means for detecting an average channel energy of the selected neighbor discovery channel so as to determine whether there is a selection collision on the selected neighbor discovery channel, and means for generating a waiting value S indicating that a respective user mobile terminal equipment should re-select a neighbor discovery channel in the S^th subperiod if there is the selection collision, wherein S is an integer and 0<S <N_Bo- 1 ·

[00131] According to an exemplary embodiment, the apparatus further comprises means for re-selecting a neighbor discovery channel in the S^th subperiod.

[00132] According to an exemplary embodiment, means for re- selecting a neighbor discovery channel in the S"¹ subperiod further comprises: means for detecting average channel energies for all the neighbor discovery channels, and means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies.

[00133] According to an exemplary embodiment, the system message further comprises a reference waiting value PBO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and the apparatus further comprises: means for generating a list of neighbors based on the received beacon signals, the list containing identifiers of the neighbor mobile terminal devices for the respective user mobile terminal equipment; means for comparing the list currently generated with a list previously generated so as to determine a difference between current neighbors and previous neighbors; means for comparing the difference with a threshold predetermined by the respective eNode B; if the difference exceeds the threshold, means for detecting an average channel energy of the selected neighbor discovery channel so as to determine whether there is a selection collision on the selected neighbor discovery channel, and means for generating a waiting value T indicating that a respective user mobile terminal equipment should re-select a neighbor discovery channel in the T^th subperiod if there is the selection collision,, wherein T is an integer and 0<Τ<Ρ_Βο- 1 ·

[00134] According to an exemplary embodiment, the apparatus further comprises means for re-selecting a neighbor discovery channel in the T^th subperiod.

[00135] According to an exemplary embodiment, means for re- selecting a neighbor discovery channel in the T^th subperiod further comprises: means for detecting average channel energies for all the neighbor discovery channels, and means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies.

[00136] It will be appreciated that aspects of the invention may be implemented in any suitable form including hardware, software, firmware or any combination of these. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit or IC, in a plurality of units or ICs or as part of other functional units.

Claims

WHAT IS CLAIMED IS:

1. A neighbor discovery method for a user mobile terminal device to user mobile terminal device communication system comprising multiple eNode Bs and a plurality of user mobile terminal equipments, the method comprising:

receiving a system message at each of the plurality of user mobile terminal equipments from a respective eNode B of the multiple eNode Bs, wherein the system message comprises a configuration information indicating how a plurality of neighbor discovery channels are configured in radio resources;

selecting, at each of the plurality of user mobile terminal equipments, a respective neighbor discovery channel from the plurality of neighbor discovery channels based on the configuration information in the system message;

transmitting, at each of the plurality of user mobile terminal equipments, respective beacon signals on the selected neighbor discovery channel; and

receiving, on unselected neighbor discovery channels, at each of the plurality of user mobile terminal equipments, beacon signals from neighbor mobile terminal devices of a respective user mobile terminal equipment.

2. The method according to claim 1, wherein the plurality of neighbor discovery channels are repeated in neighbor discovery periods configurable by the multiple eNode Bs .

3. The method according to claim 2, wherein each of the neighbor discovery periods comprises a plurality of subperiods, and each of the neighbor discovery channels spans all the subperiods of one neighbor discovery period.

4. The method according to claim 3, wherein each of the neighbor discovery channels comprises a plurality of segments, and a number of the segments is equal to a number of subperiods within one neighbor discovery period.

5. The method according to any one of claims 1 to 4, wherein the system message further comprises a number of user groups determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; wherein the plurality of user mobile terminal equipments are grouped based on the number of user groups such that each of the user mobile terminal equipments in each of the user groups can select a respective neighbor discovery channel and transmit respective beacon signals only in neighbor discovery periods corresponding to a respective user group.

6. The method according to claim 4, further comprises:

transmitting the respective beacon signals on a predetermined number of segments of the selected neighbor discovery channel, and

receiving, on rest segments of the selected neighbor discovery channel, beacon signals from the neighbor mobile terminal devices.

7. The method according to claim 6, wherein the system message further comprises a reference waiting value M_BO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and

wherein selecting the respective neighbor discovery channel comprises:

comparing a first configuration information currently broadcast by the respective eNode B with a second configuration information previously broadcast by the respective eNode

B ;

if the first configuration information is the same as the second configuration information, detecting, at each of the plurality of user mobile terminal equipments, average channel energies for all the neighbor discovery channels, and selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies; or if the first configuration information is different from the second configuration information, generating, at each of the plurality of user mobile terminal equipments, a waiting value R indicating that a respective user mobile terminal equipment should select a neighbor discovery channel in the R^th subperiod, wherein R is an integer and 0<R<M_Bo-l ·

8. The method according to claim 6, wherein the system message further comprises a reference waiting value NBO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and

wherein selecting the respective neighbor discovery channel comprises:

B ;

if the first configuration information is the same as the second configuration information,

detecting, at each of the plurality of user mobile terminal equipments, average channel energies for all the neighbor discovery channels, and

selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies; or

if the first configuration information is different from the second configuration information,

selecting a neighbor discovery channel,

detecting an average channel energy of the selected neighbor discovery channel so as to determine whether there is a selection collision on the selected neighbor discovery channel, and

if there is the selection collision, generating a waiting value S indicating that a respective user mobile terminal equipment should re-select a neighbor discovery channel in the S^th subperiod, wherein S is an integer and 0<S≤NBO- 1 -

9. The method according to claim 8, further comprising re-selecting a neighbor discovery channel in the S^th subperiod.

10. The method according to claim 9, wherein re-selecting a neighbor discovery channel in the S^th subperiod further comprises:

detecting average channel energies for all the neighbor discovery channels, and selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies.

11. The method according to any one of claims 7 to 10, wherein the system message further comprises a reference waiting value P_BO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and

the method further comprising:

generating a list of neighbors based on the received beacon signals, the list containing identifiers of the neighbor mobile terminal devices for the respective user mobile terminal equipment;

comparing the list currently generated with a list previously generated so as to determine a difference between current neighbors and previous neighbors;

comparing the difference with a threshold predetermined by the respective eNode B ; if the difference exceeds the threshold,

if there is the selection collision, generating a waiting value T indicating that a respective user mobile terminal equipment should re-select a neighbor discovery channel in the T^th subperiod, wherein T is an integer and 0<T<P_Bcr 1 ·

12. The method according to claim 11, further comprising re-selecting a neighbor discovery channel in the T^th subperiod.

13. The method according to claim 12, wherein re-selecting a neighbor discovery channel in the T^th subperiod further comprises:

14. A neighbor discovery apparatus for a user mobile terminal device to user mobile terminal device communication system comprising multiple eNode Bs and a plurality of user mobile terminal equipments, the apparatus comprising:

means for receiving a system message at each of the plurality of user mobile terminal equipments from a respective eNode B of the multiple eNode Bs, wherein the system message comprises a configuration information indicating how a plurality of neighbor discovery channels are configured in radio resources;

means for selecting, at each of the plurality of user mobile terminal equipments, a respective neighbor discovery channel from the plurality of neighbor discovery channels based on the configuration information in the system message;

means for transmitting, at each of the plurality of user mobile terminal equipments, respective beacon signals on the selected neighbor discovery channel; and

means for receiving, on unselected neighbor discovery channels, at each of the plurality of user mobile terminal equipments, beacon signals from neighbor mobile terminal devices of a respective user mobile terminal equipment.

15. The apparatus according to claim 14, wherein the plurality of neighbor discovery channels are repeated in neighbor discovery periods configurable by the multiple eNode Bs .

16. The apparatus according to claim 15, wherein each of the neighbor discovery periods comprises a plurality of subperiods, and each of the neighbor discovery channels spans all the subperiods of one neighbor discovery period.

17. The apparatus according to claim 16, wherein each of the neighbor discovery channels comprises a plurality of segments, and a number of the segments is equal to a number of subperiods within one neighbor discovery period.

18. The apparatus according to any one of claims 14 to 17, wherein the system message further comprises a number of user groups determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and wherein the apparatus further comprises means for grouping the plurality of user mobile terminal equipments based on the number of user groups such that each of the user mobile terminal equipments in each of the user groups can select a respective neighbor discovery channel and transmit respective beacon signals only in neighbor discovery periods corresponding to a respective user group.

19. The apparatus according to claim 17, further comprises:

means for transmitting the respective beacon signals on a predetermined number of segments of the selected neighbor discovery channel, and

means for receiving, on rest segments of the selected neighbor discovery channel, beacon signals from the neighbor mobile terminal devices.

20. The apparatus according to claim 19, wherein the system message further comprises a reference waiting value M_BO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and

wherein means for selecting the respective neighbor discovery channel comprises: means for comparing a first configuration information currently broadcast by the respective eNode B with a second configuration information previously broadcast by the respective eNode B ;

if the first configuration information is the same as the second configuration information, means for detecting, at each of the plurality of user mobile terminal equipments, average channel energies for all the neighbor discovery channels, and means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies; or

if the first configuration information is different from the second configuration information, means for generating, at each of the plurality of user mobile terminal equipments, a waiting value R indicating that a respective user mobile terminal equipment should select a neighbor discovery channel in the R^th subperiod, wherein R is an integer and 0<R<M_Bo-l -

21. The apparatus according to claim 19, wherein the system message further comprises a reference waiting value NBO determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and

means for detecting, at each of the plurality of user mobile terminal equipments, average channel energies for all the neighbor discovery channels, and

means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies; or

means for selecting a neighbor discovery channel,

means for detecting an average channel energy of the selected neighbor discovery channel so as to determine whether there is a selection collision on the selected neighbor discovery channel, and

means for generating a waiting value S indicating that a respective user mobile terminal equipment should re-select a neighbor discovery channel in the S^th subperiod if there is the selection collision, wherein S is an integer and 0<S≤NBO- 1 -

22. The apparatus according to claim 21, further comprising means for re-selecting a neighbor discovery channel in the S^th subperiod.

23. The apparatus according to claim 22, wherein means for re-selecting a neighbor discovery channel in the S^th subperiod further comprises:

means for detecting average channel energies for all the neighbor discovery channels, and

means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies.

24. The apparatus according to any one of claims 22 to 23, wherein the system message further comprises a reference waiting value P_Bo determined by the respective eNode B in accordance with a relative relationship between a user density and a number of the neighbor discovery channels; and

the apparatus further comprising:

means for generating a list of neighbors based on the received beacon signals, the list containing identifiers of the neighbor mobile terminal devices for the respective user mobile terminal equipment;

means for comparing the list currently generated with a list previously generated so as to determine a difference between current neighbors and previous neighbors;

means for comparing the difference with a threshold predetermined by the respective eNode B ;

if the difference exceeds the threshold,

means for generating a waiting value T indicating that a respective user mobile terminal equipment should re-select a neighbor discovery channel in the T^th subperiod if there is the selection collision,, wherein T is an integer and 0<T≤P_BO-1 -

25. The apparatus according to claim 24, further comprising means for re-selecting a neighbor discovery channel in the T^th subperiod.

26. The apparatus according to claim 25, wherein means for re-selecting a neighbor discovery channel in the T^th subperiod further comprises:

means for detecting average channel energies for all the neighbor discovery channels, and means for selecting a neighbor discovery channel from multiple neighbor discovery channels with the lowest average channel energies.