Research On The Fast Random Access Mechanism Of TD-LTE Trunking System

Trunking system has great advantages in dispatching command, secure communication and various other aspects. It plays an important role in real life. With the large-scale commercial use of TD-LTE in China, the demand for the combination of trunking system and TD-LTE is further expanded. Research on TD-LTE trunking system include many aspects such as the random access procedure, establishment of calls, handover process and resource allocation. Random access is the first step when trunking system user tries to access TD-LTE, because of that, whether user can quickly access TD-LTE system or not has great influence on the performance of trunking system. Base on that, this article lays emphasis on the random access procedure of TD-LTE trunking system: after thoroughly analyzed the characteristics and requirements of the random access procedure in TD-LTE and trunking system, this article designed a method which allows trunking system user to access TD-LTE faster. And to solve the weakness in system fairness of that method, this article introduced two different improvement of the method, respectively from the access protocol and back-off algorithm is improved and the single leading code fast random access method and adaptive retransmission channel backoff algorithm.This paper begins with an overview of the TD-LTE system and trunking communication system development and evolution. Then this paper analyzes random access technology in LTE system, summarizes the access process of trunking system, compares the similarities and differences of TD-LTE random access and trunking random access, analyzes their requirements for time limit of the acces process, and points out that according to the current LTE access process, if users of trunking system want to access the TD-LTE system, they must complete the random access as normal users do. Because the group of trunking system users is bound to occupation of a large amount of access channel resources in a short period of time in their access to the LTE network, which has the potential to cause access collision and delay and affects trunking users communication. Therefore, the method of traditional random access process can’t meet the needs of the user.To solve this problem, this paper proposes a fast random access method for users of TD-LTE trunking system. This paper changes the way of the users in the trunking group to complete the random access by using a proxy to represent the trunking group and communicate with the base station, to achieve the target that when a user complete the random access process, the whole group complete it, helping to solve the problem of users in the trunking group accessing simultaneously leading to the collision problem, shortening the time of access process, increasing the system throughput. And the paper verifies the validity of this method by simulation. Simulation results exposes a problem that although TD-LTE system trunking fast random access method can effectively shorten the time of the access process, but in an access time slot a lot of preamble code resources are occupied so that ordinary users access collision rate increased substantially and the fairness of the system greatly reduced.To solve the weakness mentioned above, from the perspective of the protocol, this article introduced a single preamble fast random access method, to inform the eNodeB the number of trunking users by increasing the interaction steps in order to save the preamble, reducing the average user collision probability, increase the fairness of the system. From the angle of back-off algorithm, proposed a new adaptive retransmission back-off algorithm. This algorithm based on the study of the classic back-off algorithm and LTE back-off mechanism, using TD-LTE’s multi-channel characteristic, divided the channels into new transmission channel and retransmission channel. In the retransmission channel using optimal access control algorithm, thus the channel can achieve the theoretical upper bound of the throughput. And by changing the proposion of the new channel and retransmission channel, improve the new channel throughput and reduce the colliding rate. By analyze the simulation result we can found that those two methods are both able to guarantee the access time of the trunking system users, reduce the access delay of ordinary users, and effectively improve the system fairness.