A Direct Mode Operation Design And Implement For TD-LTE Trunking Communication System

A direct mode operation applied to TD-LTE trunking communication system is a function that used in the off-net area. The function should obey TD-LTE physical layer specifications, use TD-LTE frequency band and can finish half duplex, multi hops trunking communications.The design solved three problems:(1) The design used relative synchronization to solve the synchronization issue in the off-net area, thus it can meet the conditions to the trunking system’s half-duplex communication;(2) Through dividing the frequency band into public channel and data channel, the design improved the channel utilization;(3) Through adopting random collision avoidance protocol, the design solved the collision issue between relays. In terms of the physical layer, the design used SC-FDMA in both up-link and down-link.Through simulating the signaling process and data process of the design respectively, it was concluded as follows. When the average success rate was more than 80%, within 20 people system, the signaling process should adopt 50 ms to 70 ms random collision avoidance length in order to achieve the fast channel set up speed and the data process should adopt 25 ms to 35 ms random collision avoidance length to achieve small data delay. For 20 to 40 people system, the signaling process should adopt 70 ms to 90 ms random collision avoidance length and the data process should adopt 35 ms to 45 ms random collision avoidance length. For more than 40 people system, the signaling process should adopt over 90 ms random collision avoidance length and the data process should adopt over 45 ms random collision avoidance length to achieve higher average success rate of the system.During implementing the physical layer key techniques of the design over the FPGA, there are two innovations:(1) In coding and decoding of Turbo code, the system stored the decoding results of all stripes in the decoding process. When decoding finished, the system compared hamming distance among the stripes and found the least one, then it output the stored results directly which eliminated the path back time.(2) In the synchronous sequence detection, the complexity of hardware implementation decreased by using the sampling values in the correlation computing, using the absolute value of real and imaginary part on behalf of length and setting detection threshold. Through implementing SC-FDMA over ML605 platform, the realizable of the design over hardware was proved.