CN102014516A - Random access method in (LTE)-A system - Google Patents

Abstract

The invention relates to the field of random access technology of wireless communication, in particular to a random access method in an LTE-A system. The edge user equipment sends a random access preamble to the base station of the camping cell and the base station of the candidate serving cell respectively; when receiving a random access response within a specified time window, it sends a MSG3 message to the corresponding base station; the edge user equipment is sending After the MSG3 message, judge whether the contention resolution message is received within the predetermined time, if received, judge whether the identification in the contention resolution message matches the identification in the corresponding sent MSG3, if not received, perform backoff according to the priority After the process, re-initiate an access request to the corresponding base station; if the identifier in the contention resolution message matches the identifier in the corresponding sent MSG3, a HARQ confirmation message is sent to complete the random access. The invention improves the success rate of random access of edge users and reduces the time delay of random access.

Description

Random access method in LTE-A system

technical field

The invention relates to the field of random access technology of wireless communication, in particular to a random access method in an LTE-A system.

Background technique

IMT-2000 (International Mobile Telecommunications-2000) is a global standard for third-generation wireless communications defined by the International Telecommunication Union. An IMT-Advanced (IMT-A for short) system is a mobile system with new capabilities beyond the capabilities of IMT-2000. The system is capable of providing a wide range of telecommunication services: increasingly advanced mobile services based on packet transmission supported by mobile and fixed networks.

Random access is the access process of the terminal before starting to communicate with the network, and is a decisive link to ensure the establishment of communication. In the LTE (Long Term Evolution, long-term evolution technology) system, random access is mainly used for initial user registration, uplink synchronization, and application for user resource requirements. The random access process directly affects the performance of the system. The realization method of the random access process is simple, and is suitable for multiple access when the system load is light. When the system load increases, the number of user terminals accessing the system is large, and the capacity of the system will be severely damaged due to frequent collisions between users. The LTE-A system requires seamless services and free wireless access. In order to achieve this goal, the node access technology is required to ensure a high access success rate and low access delay, and to meet QoS (Quality of Service, network service quality) requirements of different services. At the same time, the LTE-A system pays more attention to the performance of edge users, and takes improving the performance of edge users as one of the important indicators of the system.

At present, many improved random access methods have been proposed in the prior art, such as: the algorithm based on slotted ALOHA; the algorithm based on resource reservation and the algorithm based on dynamic allocation of random access channel resources; Although the method of two consecutive random access preambles can improve the random access success rate and guarantee the QoS of different services to a certain extent, it cannot be applied to the coordinated multi-point transmission mode; some access protocols based on cooperative transmission use cooperative The multi-point diversity feature solves the collision problem on the physical level and improves the access success rate, but this type of access strategy does not consider the QoS requirements of different services. At the same time, the existing access priority determination method is only determined according to the service type, without considering the purpose of access.

In the existing LTE system, the contention-based random access process is shown in Figure 1.

Step 101: Random Access Preamble

The main purpose of the preamble transmission is to indicate to the base station the occurrence of a random access attempt and to allow the base station to estimate the time delay between the base station and the mobile terminal. The delay estimate will be used in step 102 to adjust uplink timing.

There are 64 available preambles per cell. The mobile terminal equipment UE selects one of the 64-Ncf contention-based signatures used for PRACH (Physical Random Access Channel, Physical Random Access Channel), where Ncf is eNodeB (evolved NodeBase, that is, evolved base station, abbreviated as eNB) Reserved as the number of signatures used for non-contending. Sequences for contention-based signatures are further subdivided into two subgroups. The mobile terminal equipment UE randomly selects a preamble from the corresponding random access preamble group A or random access preamble group B according to the size of the random access MSG3 message, and according to the initial transmission power defined by the protocol, in the corresponding sent out in the random access channel. Since the UE randomly selects the preamble, different UEs may select the same random access preamble and send it to the base station on the same random access channel.

Step 102: Random Access Response (Random Access Response, RAR)

The eNodeB sends a random access response message on the physical downlink shared channel, including random access preamble identification, timing advance information, initial uplink grant, and assigned temporary cell radio network temporary identification (T-C-RNTI, Temporary-Cell-RNTI) .

If multiple UEs collide due to selecting the same preamble in the same time-frequency resource, they will each receive a RAR (Random Access Response). If the UE does not receive a RAR within the set time window, it will perform backoff and re-initiate the random access process.

Step 103: MSG3 message

The UE receives the random access response message, and if the random access preamble identifier contained in the random access response message matches the sent random access preamble, the UE receives the random access time-frequency resource allocated in the random access response message Select a subframe, and send a MSG3 message on the selected subframe, carrying the contention resolution identifier. The MSG3 message is an exact random access request message, such as RRC (Radio Resource Control) link request, tracking area update, or scheduling request. The message includes the temporary C-RNTI (Cell Radio Network Temporary Identifier, cell radio network temporary identifier) on the RAR in step 102 and the UE identifier. If different users transmit the same preamble on the same time-frequency resource in step 101, a collision occurs. The conflicting UEs will receive the same RAR and thus obtain the same uplink time-frequency resources of MSG3 messages, causing the MSG3 messages of the conflicting users to interfere with each other in the uplink time-frequency resources, so that the eNodeB cannot decode them correctly.

Step 104: Contention Resolution Message

The base station receives the MSG3 message, performs contention judgment, and returns a contention resolution message to the UE.

The contention resolution message includes the ID of the successfully accessed UE. If the UE detects that the UE ID in the contention resolution message matches its own ID, it knows that the access has been successful; otherwise, the UE considers that a collision has occurred, performs a backoff process, and re-initiates a new random access process.

It can be seen from the above process that in the existing contention-based random access process, if different UEs select different random access preambles and initiate random access on the same random access time-frequency resource, due to the Based on the orthogonality, the base station can correctly distinguish UEs for different preambles, and allocate different access time-frequency resources for different UEs in the random access response message returned to the UE. Then the UEs respectively transmit their random access MSG3 messages on their allocated access time-frequency resources. Due to the different access time-frequency resources, the MSG3 messages sent by different UEs will not collide with each other, and the base station can correctly receive and demodulate the MSG3 messages of different UEs, so as to allocate data transmission requirements for different UEs. Data transmission time-frequency resources, so as to successfully complete the process of random access. However, if different UEs select the same random access preamble and initiate random access on the same time-frequency resource, the base station will only detect that a UE has sent the random access preamble, and cannot distinguish between sending the random access preamble. Access different UEs corresponding to the preamble, and allocate the same random access time-frequency resources for different UEs for the random access preamble, and return the same random access response message; after the UE receives the random access response message , different UEs will simultaneously transmit their respective MSG3 messages on the same random access time-frequency resource, resulting in transmission collisions between MSG3 messages, so that the base station cannot correctly receive and demodulate the MSG3 messages of different UEs, As a result, the random access fails this time, resulting in a low random access success rate of the UE. For some important UEs, the low random access success rate will greatly affect the transmission of their data services.

Gradually, people began to use the back-off algorithm in LTE to solve the transmission collision problem. The back-off algorithm is an important part of the random access process, and it is another effective means to improve the success rate of user access. The backoff algorithm generally changes the retransmission delay adaptively according to the business load, and is applied to dynamically changing network conditions to improve system performance. In the backoff algorithm, the backoff time is generally randomly selected between zero and the maximum delay time, which is called the backoff interval. The backoff interval is dynamically controlled by the backoff algorithm. The back-off algorithm plays an important role in reducing packet collision, improving the success rate of sending, and improving the channel throughput.

The backoff algorithm in the existing LTE adopts a centrally controlled backoff algorithm, and the backoff parameters are determined by the eNB. The UE side uses the window method to retransmit the preamble, and the minimum value of the window is fixed, and the maximum value of the retransmission window is notified by the eNB as a backoff parameter. In step 102 of competing for random access, the eNB transmits a backoff parameter BI (Backoff Index) to the UE. Its initial value is set to 0, that is, if the eNB does not pass the backoff parameter to the UE, the preamble will not be backed off and will be transmitted in the next valid random access resource.

However, the existing back-off algorithm whose back-off parameters are determined by the eNB cannot react quickly according to system load changes and adjust the back-off parameters in a timely manner. Unnecessary delays affect the access speed.

Contents of the invention

The present invention overcomes the above-mentioned shortcomings, provides a random access method in a long-term evolution system, improves the success rate of random access for high-priority users at the cell edge, and reduces the access delay.

The technical scheme that the present invention solves its technical problem is: a kind of random access method in the LTE-A system, comprises the steps:

The edge user equipment sends a random access preamble to the camping cell base station and the candidate serving cell base station respectively;

When the edge user equipment receives the random access response within the specified time window, it sends a MSG3 message to the corresponding base station;

After the edge user equipment sends the MSG3 message, it judges whether it receives the contention resolution message within a predetermined time, and if it receives it, it judges whether the identification in the contention resolution message matches the identification in the MSG3 sent correspondingly, and if it does not receive After the backoff process is executed, the access request is re-initiated to the corresponding base station;

The judgment result of whether the identifier in the contention resolution message matches the identifier in the corresponding sent MSG3, if they match, send a HARQ confirmation message to complete random access, if they do not match, perform a backoff process, and then re-initiate to the corresponding base station Access request.

When the edge user equipment receives a random access response within a specified time window, in the step of sending a MSG3 message to the base station, if the received random access response is the only random access response received by the edge user equipment, Then the MSG3 message is directly sent to the corresponding base station at the time indicated in the random access response.

When the edge user equipment receives the random access response within the specified time window, in the step of sending the MSG3 message to the base station, if the received random access response is the second random access response, it is judged that the current random access response Whether the time of the MSG3 message indicated by the incoming response overlaps with the time of sending MSG3 indicated by the first random access response. Send MSG3 message to both base stations.

After the step of sending the random access preamble to the base station of the camping cell and the base station of the candidate serving cell respectively by the edge user equipment, if the edge user equipment does not receive a random access response within the specified time window and does not reach If the upper limit of back-off times is preset, after the back-off process is executed, an access request is re-initiated to the corresponding base station, and if the upper limit of back-off times has been reached, the access to the base station fails.

The backoff process is specifically:

The edge user equipment determines the size of the backoff window according to the priority level;

A backoff delay is randomly selected in the backoff window, and a new round of random access process is reinitiated after the backoff delay ends.

The backoff window size is

$\mathrm{<span\; class="notranslate">\; WinSize</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; [</span>\frac{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; BPV</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; ]</span>$

Wherein BPV is the backoff time obtained in the random access response, N is the number of priorities, n is the priority of the current edge user equipment, and the square brackets indicate rounding down.

The priority level of the random access is determined according to the access purpose and the requested service type of the edge user equipment.

Before the step of sending the random access preamble to the base station of the camping cell and the base station of the candidate serving cell respectively, the edge user equipment further includes judging whether the edge user is a user equipment with a set priority, and if it is to camp on The base station of the cell and the base station of the candidate serving cell send a random access preamble.

The present invention aims at marginal users in LTE-A system, adopts back-off strategy and multiple random access method according to access priority users, and is applied in the case of high load and users access the network through contention random access process, improving the The success rate of random access reduces the delay of random access. In addition, in the present invention, the access priority is divided according to the purpose of random access and the service QoS level, and the division method is more in line with the actual system application.

Description of drawings

FIG. 1 is a flow chart of contention-based random access in the prior art;

FIG. 2 is a schematic diagram of a scene where the present invention is applied;

Fig. 3 is a flowchart of a preferred embodiment of the present invention.

Detailed ways

As shown in Figure 2, it is the application scenario of the present invention, including adjacent cell base stations C1, C2, C3, and edge user equipment UE1, UE2. In combination with this scenario, taking the access of edge user equipment UE1 as an example, The content of the present invention is described in detail.

A preferred embodiment of the present invention is mainly realized through the following stages, and the specific implementation process is as shown in Figure 3:

In the first stage, the user terminal equipment searches and camps in a cell, as in step 301, and then judges whether it is at the edge of the cell, that is, whether it is an edge user equipment, as in step 302. If the current user terminal equipment is not an edge user equipment, perform random access according to the existing random access method in the long-term evolution system (not marked in the figure); otherwise, go to step 303 and execute the second stage.

The present invention adopts the SINR (Signal to Interference plus Noise Ratio, SINR, SINR for short) threshold determination method to determine whether the current user terminal equipment is an edge user equipment. First, a SINR threshold is predefined, and the user terminal device determines its own type according to its SINR in the cell where it resides: if the measured SINR is higher than this threshold, the device is a central user device, otherwise it is an edge user device (cell-edge UE). The specific formula is as follows: (where 1 represents the edge user and 0 represents the central user)

$\mathrm{<span\; class="notranslate">\; UE</span>}<span\; class="notranslate">\; .</span>\mathrm{<span\; class="notranslate">\; type</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; SINR</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\left\{\begin{array}{cc}\mathrm{<span\; class="notranslate">\; 1</span>}& \mathrm{<span\; class="notranslate">\; SINR</span>}<span\; class="notranslate">\; <</span>{\mathrm{<span\; class="notranslate">\; SINR</span>}}_{\mathrm{<span\; class="notranslate">\; threshold</span>}}\\ \mathrm{<span\; class="notranslate">\; 0</span>}& \mathrm{<span\; class="notranslate">\; SINR</span>}<span\; class="notranslate">\; \&Greater\; Equal;</span>{\mathrm{<span\; class="notranslate">\; SINR</span>}}_{\mathrm{<span\; class="notranslate">\; threshold</span>}}\end{array}\right.$

A reasonable SINR threshold can be given through simulation or actual measurement to determine which users are edge users.

In the second stage, the edge user equipment UE judges whether it is a user equipment with a set priority, that is, selects an edge user equipment with a set priority from among the edge user equipments. As in step 303, in this embodiment, it is judged whether the user is a high-priority user ranked among the top three. If it is not a high-priority user, perform random access according to the existing random access method in the long-term evolution system, that is, for user equipment with low access requirements, only perform access according to the existing random access method; Otherwise, if it is a set marginal high-priority user, go to step 304 and execute the third stage.

The UE's priority determination can be determined from two aspects of random access purpose and service category. According to the LTE protocol, the purpose of random access can be divided into the following five categories:

1) Random access requesting initial access;

2) Random access in the radio resource control (RRC, Radio Resource Control) link reestablishment process;

3) random access during handover;

4) When the uplink is in the "asynchronous" state, when the UE is in the radio resource control_connected (RRC_CONNECTED) state, the random access initiated by the arrival of downlink data;

5) In the uplink "asynchronous" state, when the UE is in the RRC_CONNECTED state, the uplink data arrives and initiates random access.

On the other hand, 3GPP (Third Generation Partnership Project) divides service categories into four categories according to QoS for mobile networks: conversational, streaming, interactive, and background. The main basis for classification is the sensitivity of services to delay. The session class is very sensitive to delay, descending in order, and the background class is the least sensitive to delay. Conversational services and streaming media services are mainly used to carry real-time service streams, among which the session class is used to transmit services that are very sensitive to delay (such as VOIP: Voice over Internet Protocol, that is, Internet telephony), and the streaming media class is used for time synchronization Services with weaker sensitivity to delay (such as VOD: Video On Demand, that is, video on demand technology). Interactive services and background services are mainly used for traditional Internet services and other business data services. Since they have low requirements on delay, the reliability of data can be improved through channel coding error correction and retransmission mechanism. Interactive services require maintaining the basic request-response mode, such as WWW browsing, while background services have no interactive requirements and can be used to send and receive E-Mail and download data. For the basic features and application examples of each business category, please refer to the table below.

Therefore, the method for judging the user access priority adopted by the present invention can be obtained from the above: (Priority 1 is the highest priority, descending in turn, and priority 6 is the lowest priority). 1, 2, 3 edge user devices.

In the third stage, the edge user equipment UE selects a candidate cell, as in step 304 . The edge user equipment selects a cell from neighboring cells as a candidate cell according to a candidate cell selection algorithm.

Specific steps are as follows:

1) Taking UE1 as an example, measure the signal strength of adjacent cell base stations C1, C3, etc. around base station C2, and select the cell base station C1 with the strongest signal strength;

2) Read the system broadcast message of the adjacent cell base station C1, which includes the registration information, access service level and some related information of the base station physical layer in the system broadcast message, where the registration information and access service level information record Information related to the UE1 registration authority and access service priority of the adjacent cell base station C1;

3) Based on the registration information, access service priority and physical layer related information, judge whether the adjacent cell meets the requirements according to the following three judgment conditions:

是否允许位置注册登记；

Whether to allow location registration;

接入服务优先级是否满足要求；

Whether the access service priority meets the requirements;

用物理层相关信息计算出来的信号路径损耗是否满足要求。其中路径损耗准则为：

Whether the signal path loss calculated by using the relevant information of the physical layer meets the requirements. The path loss criterion is:

C1 = RXLEV-RXLEV_ACCESS_MIN-Max((MS_TXPWR_MAX_CCH-P), 0).

In the above formula, RXLEV is the average receiving level of UE1; RXLEV_ACCESS_MIN is the minimum receiving level that allows UE1 to access; MS_TXPWR_MAX_CCH is the maximum transmit power level that UE1 can use when accessing the system; P is the maximum output power of UE1.

If the adjacent cell C1 satisfies the above three conditions, then perform step 4); otherwise, return to step 1) and select the adjacent cell C3 with the second strongest signal strength to restart steps 2), 3);

4) The adjacent cell C1 is determined as a candidate cell, and UE1 maintains relevant information required for accessing the candidate cell.

In the fourth stage, the edge user equipment UE1 initiates two independent random access procedures to the camping cell base station C2 and the candidate cell base station C1 at the same time. Access to the network is successful as long as at least one random access procedure is successful. In this phase, if there is a need to back off and retransmit, the back off algorithm will be executed according to the priorities identified in the second phase.

The detailed access process is shown in steps 305-315 in Figure 3,

Step 305: UE1 sends random access preambles to the main serving cell, that is, the camping cell base station C2 and the candidate serving cell base station C1, respectively in adjacent subframes, and starts two random access procedures.

Step 306: If UE1 receives a RAR response within a specified time window, go to step 307; if not received a RAR response and the preset backoff times upper limit is not reached, go to step 312, and if it has reached If the upper limit of backoff times is set, the access to the base station will fail.

Step 307: UE1 receives a RAR response containing a feedback uplink authorization indication, instructing UE1 to send a MSG3 message to the base station; if the RAR response is the first RAR response received by UE1, and only this RAR response is received within the RAR receiving window A RAR response, then directly send the MSG3 message to the corresponding base station at the time indicated in the RAR; otherwise, if UE1 receives the second RAR response within the RAR receiving window, it is judged that the RAR response instructs UE1 to send the MSG3 message Whether the time overlaps with the time when the RAR is received for the first time indicating that UE1 sends MSG3, if yes, go to 308, if not, go to 309.

Step 308: UE1 only sends the MSG3 message to the base station C2 of the primary serving cell, and cancels the preparation for sending the MSG3 message to the base station C1 of the candidate serving cell.

Step 309: UE1 sends MSG3 messages to the two base stations C1 and C2 respectively at the times indicated by the two RAR responses.

Step 310: UE1 starts the contention resolution timer after sending the MSG3 message each time, and monitors whether the contention resolution message is timed out. If the contention resolution message is not received within the predetermined time, go to step 312. To contention resolution message, go to step 311.

Step 311 : UE1 judges whether the identifier in the received contention resolution message matches the identifier in the corresponding sent MSG3 , if not, go to step 312 , if they match, go to 313 .

Step 312: According to the priority-based backoff algorithm, after the backoff process is executed, an access request is re-initiated to the corresponding base station.

Step 313: UE1 judges whether it has sent the HARQ acknowledgment message, if it has sent, go to step 314, otherwise, go to step 315.

Step 314: UE1 has connected to a base station, indicating that the access process has been completed, and the message received at this time is ignored.

Step 315: UE1 sends a HARQ confirmation message to the corresponding base station that has received the contention resolution message, and this access process is completed.

Wherein, if the contention resolution message received by UE1 in step 311 is from the primary serving base station C2, then in step 315, a HARQ confirmation message is sent to the primary serving base station C2; if the contention resolution message received in step 311 is from the candidate base station C1, then In step 315, a HARQ acknowledgment message is sent to the candidate base station C1.

The LTE-based priority-based backoff algorithm involved in steps 317 and 318 of the present invention can treat various types of random access differently, so it can meet the delay requirements of various types of random access, making the access performance of the system be further improved. Its specific algorithm is as follows.

Assume that the random access priority of the LTE system has 1, 2, ..., N, a total of N priority levels (N is 6 in this embodiment), wherein priority 1 is the highest priority, descending in order, and priority Level N is the lowest priority. At the same time, it is assumed that the UE has obtained the backoff index BI (Backoff Index) determined by the eNB according to the load condition of the cell in the previous Random Access Response (RAR). Thus, the UE can obtain the backoff parameter value BPV (Backoff Parameter value), which is the backoff time, according to the BI.

After the UE randomly accesses the network and fails due to collision, for example, in steps 317, 312, and 313, it reads the backoff index BI to determine whether to execute the backoff algorithm. If the value of BI is 0 at this time, it means that the load of the cell is relatively small at this time, and the UE does not need to execute the back-off algorithm, and can directly re-initiate the random access process at the next available random access time. If the value of BI is non-zero, it means that the load of the cell is not small, and the following back-off steps need to be performed:

First, the UE determines the priority level of random access according to the purpose of random access and the requested service type. The UE that is about to back off performs a priority decision according to the type of the previously failed random access (the decision method has been described above), so as to obtain the priority level n of the random access.

Then, the UE determines the size of the backoff window according to the priority level n. The formula for calculating the size of the backoff window is as follows:

(中括号表示向下取整)

(square brackets indicate rounding down)

It can be seen from the formula that the higher the priority of random access, the smaller its backoff window, which can make its backoff delay relatively small, so that it can quickly access the network. For random access with low priority, its backoff window is relatively large, so its backoff time may be relatively large, so that the channel can be given up to random access with high delay requirements, thus satisfying various types of random access. Access delay requirements.

Finally, back off is performed. Similar to the existing back-off method in LTE, a back-off delay is randomly selected in the window determined in the previous step, and a new round of random access process is re-initiated after the back-off time is over.

In the present invention, for the edge users in the LTE-A system, according to the access priority users, the back-off strategy and the random access method of multiple transmissions are adopted, that is, access requests are sent to two cell base stations at the same time, and the application is applied when the load is high and the user uses In the case of accessing the network during a competitive random access process, the success rate of random access is improved.

Moreover, in the present invention, the eNB detects whether the user equipment collides, and then transmits the detection result to UEs participating in the competition through contention random access. If the UE detects ID information that matches its own ID in the transmitted information, it considers that the access is successful, and proceeds to the following steps. If no ID is detected or the detected ID does not match your own. Then it is considered that the competition has failed and backoff is performed. In order to respond quickly to the system load, adjust back-off parameters in time to adapt to load changes, thereby improving system performance. LTE chooses to transmit the backoff parameter in the random access response phase of the contention random access rather than through system information, which reduces the delay of random access.

In addition, in order to meet the requirement of minimum latency in the LTE random access process, the backoff algorithm is not used when the probability of user collision is small or there is no collision in the light load or non-contention random access process. That is to say, in LTE, the back-off algorithm is only applied when the load is high and the user accesses the network using the contention random access process, thereby greatly reducing the time delay of the light load or non-contention random access process.

The random access method in the LTE-A system provided by the present invention has been introduced in detail above. In this paper, specific examples are used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only used to help understand the present invention. The method of the invention and its core idea; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation and scope of application. In summary, the content of this specification should not be understood To limit the present invention.

Claims (8)

1. a random access method in an LTE-A system, characterized in that, comprising the steps: The edge user equipment sends a random access preamble to the camping cell base station and the candidate serving cell base station respectively; When the edge user equipment receives the random access response within the specified time window, it sends a MSG3 message to the corresponding base station; After the edge user equipment sends the MSG3 message, it judges whether it receives the contention resolution message within a predetermined time, and if it receives it, it judges whether the identification in the contention resolution message matches the identification in the MSG3 sent correspondingly, and if it does not receive After the backoff process is executed, the access request is re-initiated to the corresponding base station; The judgment result of whether the identifier in the contention resolution message matches the identifier in the corresponding sent MSG3, if they match, send a HARQ confirmation message to complete random access, if they do not match, perform a backoff process, and then re-initiate to the corresponding base station Access request. 2. The random access method in the LTE-A system according to claim 1, characterized in that: when the edge user equipment receives a random access response within a specified time window, the step of sending a MSG3 message to the base station , if the received random access response is the only random access response received by the edge user equipment, the MSG3 message is directly sent to the corresponding base station at the time indicated in the random access response. 3. The random access method in the LTE-A system according to claim 1, characterized in that: when the edge user equipment receives a random access response within a specified time window, the step of sending a MSG3 message to the base station In, if the received random access response is the second random access response, it is judged whether the time of the MSG3 message indicated by the current random access response overlaps with the time of sending MSG3 indicated by the first random access response, If yes, the MSG3 message is sent only to the base station in the cell where the cell resides; otherwise, the MSG3 message is sent to the two base stations at the times indicated by the two random access responses. 4. The random access method in the LTE-A system according to claim 1, characterized in that: after the step of sending the random access preamble to the camping cell base station and the candidate serving cell base station respectively by the edge user equipment , if the edge user equipment does not receive a random access response within the specified time window and does not reach the preset upper limit of backoff times, it will re-initiate an access request to the corresponding base station after performing the backoff process, and if it has reached If the upper limit of backoff times is set, the access to the base station will fail. 5. The random access method in the LTE-A system according to any one of claims 1 to 4, characterized in that: the back-off process is specifically: The edge user equipment determines the size of the backoff window according to the priority level; A backoff delay is randomly selected in the backoff window, and a new round of random access process is reinitiated after the backoff delay ends. 6. The random access method in the LTE-A system according to claim 5, characterized in that: the size of the backoff window is $\mathrm{<span\; class="notranslate">\; WinSize</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; [</span>\frac{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; *</span>\mathrm{<span\; class="notranslate">\; BPV</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; ]</span>$ Wherein BPV is the back-off time obtained in the random access response, N is the priority number, n is the priority of the current edge user equipment, and the square brackets indicate rounding down. 7. The random access method in the LTE-A system according to claim 5, characterized in that: the priority level of the random access is determined according to the access purpose and the requested service type of the edge user equipment. 8. The random access method in the LTE-A system according to claim 1, characterized in that: before the step of sending the random access preamble to the base station of the camping cell and the base station of the candidate serving cell respectively, the edge user equipment It also includes judging whether the edge user is a user equipment with a set priority, and if so, sending a random access preamble to the base station of the camping cell and the base station of the candidate serving cell.

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