Random Backoff-Based Sidechain Communication Link Establishment Method

Abstract

The present invention discloses a method for establishing a side-chain communication link based on random backoff, which mainly solves the problem of link establishment failure caused by two nodes simultaneously initiating establishment requests to each other in the prior art. The implementation solution is as follows: conflict detection is performed on the link establishment process through the security capability value in the signaling; a minimum backoff interval is set, and when a link establishment conflict occurs, each of the two nodes selects a backoff time greater than the minimum backoff interval for backoff; the node that has completed backoff can initiate a link establishment request again without waiting for the timer to expire, and the node that has not completed backoff will no longer initiate a link establishment request to the other node after receiving this link establishment request, and will cooperate with the initiating node to complete the link establishment process in the identity of the target node. The present invention avoids link establishment conflicts, improves the link reconstruction speed, solves the problem that the existing D2D communication link establishment scheme in the 3GPP standard cannot solve the problem of link establishment failure caused by repeated link establishment between nodes, and can be used for the establishment of communication links in multi-user scenarios.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communication, and particularly relates to a side-chain communication link establishment process, which can be used for the establishment of communication links in a multi-user scenario. Technical Background

[0002] The 3rd Generation Partnership Project (3GPP) proposed to support direct communication between User Equipments (UEs) in Release 12 (Rel-12), that is, UEs directly exchange data and signaling services using the Sidelink protocol. D2D communication based on Sidelink realizes direct data transmission between users by multiplexing cellular network resources, which can effectively reduce the transmission power, improve the resource utilization rate, increase the system capacity, reduce the end-to-end transmission delay, and relieve the base station load. Therefore, the infrastructure-less D2D network based on Sidelink has the advantages of fast network formation and independence from infrastructure. In addition, compared with traditional short-distance wireless communication technologies, D2D communication under cellular network management can establish a more reliable transmission link in an environment with controllable interference.

[0003] The one-to-one Sidelink communication supported by the existing 3GPP standard is a connection-oriented transmission mechanism, and the link is established first before the user transmits data, making data transmission more reliable. Specifically, after the user sends a direct communication request message, the UE triggers a mutual authentication process to complete the establishment of the Layer 2 link on PC5. When two nodes initiate the establishment request simultaneously, the security capability matching fails due to the unclear dominance of the two nodes during the mutual link establishment process, resulting in the failure of link establishment. In an infrastructure-less D2D network, due to the lack of a centralized controller for coordination and scheduling, the communication link establishment process may be initiated to the peer UE after meeting the conditions for initiating the communication link establishment process, and the UE fails to establish the communication link due to repeated establishment of the communication link. Therefore, the possibility of the communication link establishment conflict problem occurring increases significantly.

[0004] Meanwhile, in a centerless D2D network, regardless of whether the D2D communication is in a direct communication, two-hop communication, multi-hop communication scenario, or a multi-device networking communication scenario, there is a possibility of conflicts in the establishment of communication links. Existing research in the D2D field mainly focuses on aspects such as D2D discovery, resource allocation, interference management, and power control. In most of these studies, the existence of D2D pairs is assumed, that is, the D2D communication link has been successfully established. Therefore, designing a reasonable and reliable communication link establishment scheme is crucial for D2D communication. The 3GPP standard provides solutions for common link establishment failure situations, such as reasons like IP version mismatch and lack of resources in the link resulting in the rejection of the communication link establishment process. However, these solutions proposed by the standard cannot effectively solve the problem of link establishment failure caused by UEs establishing communication links with each other. And in the current version of the standard, in order to prevent frequent link establishment conflicts, the initiation interval of link establishment is restricted. After a link establishment failure, it is necessary to wait for the timer to end before starting the link establishment process again, which will cause a relatively large delay. Summary of the Invention

[0005] The object of the present invention is to propose a side-chain communication link establishment method based on random backoff to avoid the problem of link establishment failure caused by UEs establishing communication links with each other and reduce the link establishment delay in view of the problems of the 3GPP standard.

[0006] To achieve the above object, the technical solution of the present invention includes the following:

[0007] (1) Two nodes UE1 and UE2 in side-chain communication each perform initialization, start the discovery process, and broadcast discovery messages containing their own Layer2 IDs. The first node UE1 discovers the neighbor, that is, the second node UE2, through the discovery message, obtains its Layer2 ID, and then starts the link establishment process.

[0008] (2) The first node UE1 sends a direct communication request message DCRq containing its own security capability value to the second node UE2, stores and records the DCRq message together with the Layer2 IDs of UE1 and UE2, and then starts a timer T4100. It is specified that when the timer T4100 is running, UE1 cannot send a new DCRq message to UE2.

[0009] (3) After receiving the DCRq message from UE1, UE2 stores the Layer2 IDs of both communication parties in the DCRq message and checks the information contained in the DCRq to avoid duplicate link establishment. After passing the check, UE2 starts the direct security mode control process to respond to the received DCRq message, sets its own security capability value to the security capability value received from the DCRq message, and then sends a direct security mode message DSMCm to UE1. After that, it starts a timer T4111.

[0010] (4) After UE1 receives the DSMCm message from UE2, it will perform a message integrity check to verify whether the security capability value of UE2 in the DSMCm message is consistent with the security capability value that UE1 sent to UE2 in the DCRq message:

[0011] If the security capability values are consistent, UE1 will send a Direct Security Mode Complete message DSMCp to UE2, and from this point on, UE1 will use the new security mode to protect all signaling messages and user data and jump to step (11b);

[0012] If the security capability values are inconsistent, it is considered that a link establishment conflict has occurred. UE1 will send a Direct Security Mode Reject message DSMRj to UE2 and jump to step (6);

[0013] (5) UE2 performs corresponding processing on the received different messages

[0014] If the message received by UE2 is a Direct Security Mode Complete message DSMCp, it will jump to step (10b);

[0015] If the message received by UE2 is a Direct Security Mode Reject message DSMRj, it will send a Direct Communication Reject message DCRj to UE1 and execute step (6);

[0016] (6) After UE1 sends the DSMRj message or receives the DCRj message, it terminates the current link establishment process and records the backoff count;

[0017] (7) UE1 or UE2 determines whether the number of times of initiating the communication link establishment has reached the set maximum backoff count:

[0018] If the maximum count is reached, it will no longer initiate a link establishment request and wait for the timer to expire. UE1 executes step (11a) and UE2 executes step (10a);

[0019] Otherwise, UE1 or UE2 randomly selects a backoff time greater than the minimum backoff interval and executes step (8);

[0020] (8) UE1 or UE2 waits for the backoff time to end. After the backoff time ends, it retries to establish the communication link and executes step (9);

[0021] (9) UE1 or UE2 first determines whether there is a Layer2 ID of the communication partner node in the existing direct communication link:

[0022] If there is a Layer2 ID of the other party node in the existing direct communication link, it will not initiate the communication link establishment process,

[0023] Moreover, UE1 executes step (11a) and UE2 executes step (10a);

[0024] Otherwise, return to step (2) to initiate the communication link establishment process;

[0025] (10) UE2 ends the communication link establishment process:

[0026] 10a) UE2 waits for the T4111 timer to expire and ends the current communication link establishment process;

[0027] 10b) UE2 stops the timer T4111 and uses the new security mode to protect all signaling messages and user data.

[0028] Subsequently, UE2 sends a direct communication acceptance message DCA to UE1 to complete the current communication link establishment process;

[0029] (11) UE1 ends the communication link establishment process:

[0030] 11a) UE1 waits for the T4100 timer to expire and ends the current communication link establishment process;

[0031] 11b) UE1 waits for the direct communication acceptance message DCA. After receiving the DCA, it stops the timer T4100, and from this point on, UE1 uses the direct communication link established with UE2 to communicate and completes the current communication link establishment process.

[0032] Compared with the prior art, the present invention has the following advantages:

[0033] 1. The success rate of communication link establishment is improved.

[0034] In the existing standard, the nodes using the side chain link are divided into two types: relay nodes and remote nodes. Relay nodes do not send a direct link establishment request message DRCq to remote nodes. In the future ad hoc network scenario, the role identities of nodes need to be blurred, and there may be a situation where two nodes establish links with each other. The existing standard cannot handle the link establishment conflict caused by two nodes sending DRCq messages to each other, which will lead to link establishment failure.

[0035] Starting from the reasons for the failure of side chain link establishment, the present invention studies a reasonable and effective solution to the communication link establishment conflict.

[0036] When there is a conflict problem caused by mutual link establishment, the two nodes will terminate the ongoing communication link establishment process and perform random backoff independently. If the backoff durations of the two nodes differ by at least one minimum backoff interval, the two nodes will definitely receive the DCRq message sent by the initiating node under ideal channel conditions, thus avoiding the conflict caused by mutual link establishment and reducing the probability of communication link establishment failure.

[0037] Meanwhile, since the present invention analyzes the size of the backoff interval based on the transmission time of the sidelink signaling in the 3GPP standard, the effectiveness of the random backoff process is ensured, and the success rate of communication link establishment is further improved.

[0038] 2. Reduces the link establishment delay caused by link establishment failure

[0039] In the current version of the standard, the initiation interval of link establishment is restricted. After a link establishment failure, it is necessary to wait for the timer to expire before starting the link establishment process again, which will cause a large delay.

[0040] Through the method of random backoff, the present invention selects a reasonable minimum backoff interval, shortens the initiation interval of link establishment on the premise of ensuring the link establishment success rate, and reduces the delay caused by link establishment conflicts. Brief Description of the Drawings

[0041] Figure 1 is the implementation flowchart of the present invention;

[0042] Figure 2 is the signaling interaction schematic diagram of the present invention

[0043] Figure 3 is the simulation comparison chart of the communication link establishment success rate between the present invention and the existing method in the standard;

[0044] Figure 4 is the simulation comparison chart of the communication link establishment delay between the present invention and the existing method in the standard. Detailed Embodiment

[0045] The following further elaborates on the embodiments and effects of the present invention with reference to the accompanying drawings.

[0046] To address the challenges brought by link establishment conflicts in the existing 3GPP sidelink standard, this example starts from the reasons for Sidelink link establishment failure and studies a reasonable and effective solution to communication link establishment conflicts, that is, for the problem that the existing D2D communication link establishment scheme in the 3GPP standard cannot solve the problem of link establishment failure caused by repeated link establishment between UEs, a D2D communication link establishment conflict solution based on random backoff is proposed.

[0047] Refer to Figure 1 and Figure 2 , the implementation steps of this example are as follows:

[0048] In this example, two nodes UE1 and UE2 are selected as examples.

[0049] Step 1, initialization

[0050] Two nodes, UE1 and UE2, initialize their respective side-chain functions, start the discovery process, broadcast discovery messages containing their own Layer2 IDs, and at the same time listen for discovery messages from surrounding nodes.

[0051] The Layer2 ID is the unique identity of each node in side-chain communication.

[0052] Step 2: Send a direct communication request message DRCq.

[0053] After UE1 receives the discovery message from neighbor node UE2, it obtains its Layer2 ID from the content of the discovery message, and sends a direct communication request message DRCq to the target node to start the link establishment process. The DRCq message contains its own security capability value. After sending, UE1 stores the DRCq message and the Layer2 IDs of both sides of the link, and then starts timer T4100, and specifies that when timer T4100 is running, this node cannot send a new DRCq message to UE2.

[0054] At the same time, UE2 is also listening for the discovery message sent by UE1. When it hears the discovery message sent by UE1 but has not received the direct communication request message DRCq sent by UE1, UE2 also sends a DRCq message to UE1 and starts timer T4100.

[0055] Step 3: Send a direct security mode message DSMCm.

[0056] After receiving the DRCq message sent by UE1, UE2 first stores the Layer2 IDs of both communication parties in the DRCq message, and then UE2 starts the direct security mode control process to respond to the received DRCq message, sets its own security capability value to the security capability value received from the DRCq message, and sends the direct security mode message DSMCm to UE1; then, starts timer T4111.

[0057] Similarly, after receiving the DRCq message sent by UE2, UE1 will also perform the same processing, store the information in the DRCq message, start the direct security mode control process, modify the security capability value, and reply to UE2 with the DSMCm message and start timer T4111.

[0058] Step 4: Process the direct security mode message DSMCm.

[0059] After receiving the DSMCm message from UE2, UE1 will check whether the security capability value of UE2 in the DSMCm message is consistent with the security capability value that UE1 sent to UE2 in the DRCq message:

[0060] If the security capability values are the same, UE1 will send a Direct Security Mode Complete message (DSMCp) to UE2. From this point on, UE1 will use the new security mode to protect all signaling messages and user data. UE1 will then jump to step (10), and UE2 will execute step (5) after receiving the DSMCp message.

[0061] If the security capability values are different, it is considered that a duplicate link establishment has occurred. UE1 will send a Direct Security Mode Reject message (DSMRj) to UE2. UE1 will execute step (6), and UE2 will execute step (5) after receiving the DSMRj message.

[0062] Meanwhile, after receiving the DSMCm message sent by UE1, UE2 also detects the existence of a link establishment conflict and sends a DSMRj message to UE1. UE2 will then execute step (6), and UE1 will execute step (5) after receiving the DSMRj message sent by UE2.

[0063] Step 5: Process the Direct Security Mode Complete message (DSMCp) or the Direct Security Mode Reject message (DSMRj).

[0064] UE2 and UE1 nodes will perform corresponding processing on the different messages they receive:

[0065] If the received message is the Direct Security Mode Complete message (DSMCp), jump to step (10);

[0066] If the received message is the Direct Security Mode Reject message (DSMRj), reply with a Direct Communication Reject message (DCRj) and execute step (6);

[0067] Step 6: Terminate the current link establishment process.

[0068] After sending the DSMRj message or receiving the DCRj message, UE1 will terminate the current link establishment process, perform backoff, and count the number of backoffs. After the backoff ends, UE1 will initiate a request to establish a new communication link.

[0069] Meanwhile, after sending the DSMRj or receiving the DCRj message, UE2 will also perform the same operations, that is, terminate the current link establishment and record the number of backoffs.

[0070] Step 7: Determine whether to terminate the current link establishment based on the number of backoffs.

[0071] UE1 determines whether the number of attempts to initiate the establishment of the communication link has reached the set maximum number of backoffs:

[0072] If the maximum number is reached, no more link establishment requests will be initiated. Wait for the timer to expire and jump to step (9) to terminate the current communication link establishment process;

[0073] Otherwise, the node randomly selects a backoff time greater than the minimum backoff interval and waits for the random backoff to end;

[0074] Meanwhile, UE2 also performs the same operation. First, it determines that the maximum backoff count has not been reached, then randomly selects a backoff interval, and waits for the random backoff to end;

[0075] If the backoff durations of UE1 and UE2 differ by at least one backoff interval, then one of these two nodes will definitely be able to receive the DCRq message sent by the other under ideal channel conditions.

[0076] The minimum backoff interval T delay should be calculated based on the average value of the side-chain signaling transmission delay:

[0077] T delay = 1.5T SL - 0.5T pscch - 0.5T pssch % 8

[0078] where T SL represents the period of the side-chain link, T pscch represents the period of the Physical Side-chain Control Channel (PSCCH), and T pssch represents the period of the Physical Side-chain Shared Channel (PSSCH).

[0079] Step 8, retry establishing the communication link.

[0080] After the backoff time ends, UE1 first checks whether the Layer 2 ID of UE2 exists in the existing direct communication link:

[0081] If it exists, it does not initiate the communication link establishment process and jumps to step (9);

[0082] Otherwise, it returns to step (2) and initiates the communication link establishment process.

[0083] In this example, UE1 finishes the backoff first. At this time, UE1 checks that the Layer 2 ID of UE2 does not exist in the existing direct communication link and returns to step (2) to send a direct link establishment request message DRCq to UE2; UE2 receives the DRCq message from UE1 before the backoff time ends and returns to step (3). When the backoff time ends, since the Layer 2 ID of UE1 exists in the existing direct communication link of UE2, it does not initiate the communication link establishment process.

[0084] Step 9, the node ends the communication link establishment process according to the timer.

[0085] Since the link initiated by UE1 to UE2 and the link initiated by UE2 to UE1 can be regarded as two different links, the same node can be the initiating node of one link and the target node of another link. That is, when UE1 initiates a direct communication link request to UE2, for this link, UE1 is the initiating node and UE2 is the target node. There is a T4100 timer regarding this link in the initiating node UE1, and a T4111 timer regarding this link in the target node UE2. When UE2 initiates a direct communication link request to UE1, for this link, UE2 is the initiating node and UE1 is the target node. There is a T4100 timer regarding this link in the initiating node UE2, and a T4111 timer regarding this link in the target node UE1.

[0086] Accordingly, the implementation of the two nodes UE1 and UE2 to end the communication link according to the timer is as follows:

[0087] If the target node UE2 has not received the DSMCp message all the time, it waits for the T4111 timer to expire, and the target node UE2 ends the establishment process of this communication link;

[0088] If the initiating node UE1 has not received the direct communication acceptance message DCA replied by UE2 all the time, it waits for the T4100 timer to expire, and the initiating node UE1 ends the establishment process of this communication link.

[0089] Step 10, complete the establishment of the communication link.

[0090] The two nodes UE1 and UE2 complete the establishment of the communication link according to the signaling messages they receive:

[0091] After the target node UE2 receives the DSMCp message, it actively stops the timer T4111, and uses the new security mode to protect all signaling messages and user data, and then sends the direct communication acceptance message DCA to UE1;

[0092] When the initiating node UE1 receives the direct communication acceptance message DCA replied by UE2, it stops the timer T4100 and completes the establishment process of this communication link.

[0093] The effect of the present invention can be further illustrated by the following simulation:

[0094] I. Simulation conditions

[0095] 1. Simulation scenario: The present invention is simulated based on the open-source system-level simulation software ns-3 platform.

[0096] 2. Communication resource allocation: The resource allocation mode adopted for signaling and data transmission is resource allocation mode 2 specified in 3GPP TS 36.300.

[0097] 3. Path loss model: The calculation of the closed-loop path loss uses the model designed for the D2D scenario in TR 36.843. The path loss of this model is: PL_B1_tot(d) = max(PL freespace (d), PL_B1(d)), where d represents the distance between UEs, and PL freespace (d) represents the free space path loss, and PL_B1(d) represents the path loss in the line-of-sight transmission scenario defined by the Winner+B1 model.

[0098] 4. The remaining default simulation parameters are given in Table 1.

[0099] Table 1. Default simulation parameter settings for the communication link establishment process

[0100] parameter numerical value UE transmission power 23dBm <![CDATA[Side chain period T SL > 40ms <![CDATA[PSCCH period value T pscch > 8ms <![CDATA[PSSCH period value T pssch > 32ms side-chain communication transmission resource block 5RB <![CDATA[System simulation time T s > 1515s number of communication link establishments n 300 <![CDATA[System frequency f c > 1930GHz <![CDATA[UE height h BS h MS > 1.5m <![CDATA[Minimum backoff interval T delay > 32ms

[0101] 5. The existing technology used for comparison in the simulation: The direct communication link establishment process defined in 3GPP standard TS36.213.

[0102] II. Simulation Content and Results

[0103] Simulation 1: Under the above simulation parameters, the communication links are established using the present invention and the existing method in the standard respectively, and the success rates of communication link establishment under different UE intervals are compared between the two methods.

[0104] During the simulation period T s , first, the total number of communication link establishment requests initiated by UEs, n, is counted, and whether the communication link is established successfully or not, M j , in the j-th period is determined. Then, the communication link establishment success probability is calculated as follows: where M j is taken as 0 to indicate establishment failure, and M j is taken as 1 to indicate establishment success. The success rate results of communication link establishment under different UE intervals for the two methods are obtained, as shown in Figure 3 .

[0105] From Figure 3 , it can be clearly seen that when the sidelink function of two UEs is turned on simultaneously, using the communication link establishment process specified in the 3GPP standard, the success rate of communication link establishment only remains at 12%. As the distance between UEs increases continuously, the SINR will decrease continuously until the sidelink signaling cannot be correctly received, resulting in the communication link establishment success rate dropping to 0 when the UE distance is greater than 170 m.

[0106] The link establishment success rate of the present invention can reach 90%, because the UE has one more opportunity to initiate the D2D communication link establishment process compared with the no-backoff scheme. As the maximum number of backoffs increases to 3, the communication link establishment success rate also reaches 100%. As the communication distance between UEs increases, the communication link establishment success rate also begins to decline, but it does not drop to 0 until the UE spacing is greater than 220m, indicating that the present invention can support the side chain link establishment process over a longer distance.

[0107] Simulation 2: Under the above simulation parameters, the communication link is established using the present invention and the existing method in the standard respectively, and the delays of the communication link establishment under different UE intervals are compared between the two methods.

[0108] During the simulation period T s first, the communication link establishment delay T i of each successful communication link establishment is statistically counted, and the parameter M j indicating whether the communication link is successfully established in the j-th cycle is counted. Then, the average communication link establishment delay is calculated as follows: where i represents the i-th time, M j takes 0 to indicate establishment failure, and M j takes 1 to indicate establishment success. The delay results of the communication link establishment under different UE intervals for the two methods are obtained as shown in Figure 4 .

[0109] From Figure 4 it can be seen that on the premise of successful link establishment, when using the communication link establishment process specified in the 3GPP standard, the average communication link establishment delay is approximately maintained at 153ms. If the link establishment fails, it is necessary to wait for the timer to expire, and the delay can reach more than 1000ms. However, the average communication link establishment time of the present invention can be stably within 400ms.

[0110] In summary, the present invention can effectively improve the communication link establishment success rate on the premise of ensuring the communication link establishment delay.

Claims

1. A method for establishing a communication link in Sidelink based on random backoff, characterized in that, It includes the following steps: (1) Two nodes UE1 and UE2 communicate through side chains and each initialize, start the discovery process, broadcast discovery messages containing their own Layer2ID, the first node UE1 discovers the neighbor, i.e., the second node UE2, through the discovery message, obtains its Layer2ID, and then starts the link establishment process; (2) The first node UE1 sends a direct communication request message DCRq containing its own security capability value to the second node UE2, stores and records the DCRq message together with the Layer2ID of UE1 and UE2, starts timer T4100, and limits that UE1 cannot send a new DCRq message to UE2 while timer T4100 is running; (3) After receiving the DCRq message from UE1, UE2 stores the Layer2ID of both communication parties in the DCRq message and checks the information contained in the DCRq to avoid duplicate link establishment; after passing the check, UE2 starts the direct security mode control process to respond to the received DCRq message, sets its own security capability value to the security capability value received from the DCRq message, and then sends the direct security mode message DSMCm to UE1; then, starts timer T4111; (4) After receiving the DSMCm message from UE2, UE1 will perform a message integrity check to verify whether the security capability value of UE2 in the DSMCm message is consistent with the security capability value that UE1 sent to UE2 in the DCRq message: If the security capability values are consistent, UE1 will send a direct security mode completion message DSMCp to UE2, and from this moment on, UE1 will use the new security mode to protect all signaling messages and user data, and jump to step (11b); If the security capability values are inconsistent, it is considered that a link establishment conflict occurs, UE1 sends a direct security mode rejection message DSMRj to UE2, and jumps to step (6); (5) UE2 performs corresponding processing on the received different messages If the message received by UE2 is the direct security mode completion message DSMCp, jump to step (10b); If the message received by UE2 is the direct security mode rejection message DSMRj, send a direct communication rejection message DCRj to UE1 and execute step (6); (6) After sending the DSMRj message or receiving the DCRj message, UE1 terminates the current link establishment process and records the backoff count; (7) UE1 or UE2 determines whether the number of times of initiating the communication link establishment has reached the set maximum backoff count: If the maximum number is reached, no more link establishment requests will be initiated and wait for the timer to expire. UE1 executes step (11a) and UE2 executes step (10a); Otherwise, UE1 or UE2 randomly selects a backoff time greater than the minimum backoff interval and executes step (8); (8) UE1 or UE2 waits for the backoff time to end. After the backoff time ends, attempts to establish the communication link again and executes step (9); (9)UE1 or UE2 first determines whether there is a Layer2ID of the communication partner node in the existing direct communication link: If there is a Layer2ID of the other node in the existing direct communication link, the communication link establishment process is not initiated, and UE1 executes step (11a), and UE2 executes step (10a); Otherwise, return to step (2) to initiate the communication link establishment process; (10)UE2 ends the communication link establishment process: 10a)UE2 waits for the T4111 timer to expire and ends this communication link establishment process; 10b)UE2 stops the timer T4111 and uses the new security mode to protect all signaling messages and user data, Subsequently, UE2 sends a direct communication acceptance message DCA to UE1 to complete this communication link establishment process; (11)UE1 ends the communication link establishment process: 11a)UE1 waits for the T4100 timer to expire and ends this communication link establishment process; 11b)UE1 waits for the direct communication acceptance message DCA. After receiving the DCA, it stops the timer T4100, and from this point on, UE1 uses the direct communication link established with UE2 for communication to complete this communication link establishment process.

2. The method according to claim 1, wherein The discovery message in step (1) contains the node's own Layer2ID, message type, declaration information, and status indicator code. When the node enables the side chain function, the discovery message will be periodically broadcast outward on the side chain communication discovery channel PSDCH.

3. The method according to claim 1, wherein In step (4), the node UE1 uses the new security mode to protect all signaling messages and user data, which means that UE1 associates this security capability value with this communication link so that when it receives signaling with a different security capability value, it will not affect this communication link.

4. The method according to claim 1, wherein The direct communication request message DCRq in step (2) is a side chain signaling in the 3GPP standard, which includes the logical channel ID, Layer2IDs of both communication parties, the security capability value of the initiating node UE1, message type, signaling sequence number, IP configuration, maximum inactive period, and ultimate service code. It is used by the initiating node UE1 to initiate a side chain link establishment request to the target node UE2.

5. The method according to claim 1, wherein The direct security mode message DSMCm in step (3) is a side chain signaling in the 3GPP standard, which includes the Layer2IDs of both communication parties, logical channel ID, message type, signaling sequence number, security capability value of the target node UE2, IP configuration, maximum inactive period, and ultimate service code. The direct security mode message DSMCm is used by the target node UE2 to respond to the side chain establishment request of the initiating node UE1.

6. The method according to claim 1, wherein The link establishment conflict in step (4) means that UE1 has sent a direct communication request message DCRq to UE2, and before UE2 receives this message, UE2 also sends a direct communication request message DCRq to UE1, that is, both parties initiate link establishment requests to each other, resulting in the failure of link establishment.

7. The method according to claim 1, characterized in that, The direct security mode complete message DSMCp in step (5) is a sidelink signaling in the 3GPP standard. It contains the Layer2ID, logical channel ID, message type, signaling sequence number, algorithm selection, and user information of both communication parties. This direct security mode complete message DSMCp is used by the initiating node UE1 to declare successful link establishment to the target node UE2.

8. The method according to claim 1, wherein The direct security mode reject message DSMRj in step (5) is a sidelink signaling in the 3GPP standard. It contains the Layer2ID, logical channel ID, message type, signaling sequence number, and cause of the event of both communication parties. This direct security mode reject message DSMRj is used by the initiating node UE1 to declare failed link establishment to the target node UE2.

9. The method according to claim 1, characterized in that The direct communication accept message DCA in step (10) is a sidelink signaling in the 3GPP standard. It contains the Layer2ID, logical channel ID, message type, signaling sequence number, IP configuration, and local IPv6 address of both communication parties. This direct communication accept message DCA is used by the target node UE2 to declare successful link establishment to the initiating node UE1.

10. The method according to claim 1, characterized in that In step (7), UE1 or UE2 randomly selects a backoff time greater than the minimum backoff interval. The formula is as follows: T delay = 1.5T SL -0.5T pscch -0.5T pssch %8 where T delay represents the minimum backoff interval, T SL represents the period of the side-chain link, T pscch represents the period of the physical side-chain control channel PSCCH, T pssch represents the period of the physical side-chain shared channel PSSCH.

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