Research Of Downlink Packet Scheduling Algorithm For High Moving Speed Users In LTE System

As a quasi-4G technology, LTE has a flat network structure and OFDM, MIMO, link adaptation, fast packet scheduling, interference suppression and other key technologies in order to meet the requirement of IMT-Advanced in peak rate, delay, throughput, user mobility, spectrum efficiency, etc. As the core of radio resource management, fast packet scheduling is responsible for the allocation of resources and determines the performance of the system.At the same time, with the rapid development of high-speed railway and expressway, future mobile communication will face more high mobility environment. In high mobility environment, mobile communication system will produce large Doppler frequency shift, and channel will change rapidly. These changes can severely degrade the performance of mobile communication system, especially for cell edge users with serious inter-cell interference. The main research direction is to improve the throughput of cell edge high moving speed users effectively.First of all, this thesis introduced the system architecture, requirements, time-frequency resource structure and key techniques of LTE system, then highlighted LTE packet scheduler structure and several classical scheduling algorithms (RR algorithm, MaxCoI algorithm, PF algorithm). These algorithms were simulated to compare their throughput performance and fairness through three respects of the number of PRB user assigned, probability of user scheduled and relation between user position and scheduled probability. The simulation results show that RR algorithm has the best fairness but the lowest system throughput, MaxCoI algorithm maximizes system throughput with poor fairness, and PF algorithm gets a tradeoff between system throughput and user fairness.Secondly, this thesis studied the fairness in mix speed mode, and found cell edge high moving speed users suffered unfairness. To solve this problem, this thesis proposed a time domain TTI Allocation algorithm and frequency domain soft frequency reuse algorithm facing high moving speed users. Time domain TTI allocation algorithm allocates extra time domain TTI resources to cell edge high moving speed users. Specific implementation is to allocate one specific TTI to cell edge high moving speed users every certain number TTIs, and in the specific TTI only cell edge high moving speed users will be scheduled. Frequency domain soft frequency reuse algorithm facing high moving speed users is based on traditional soft frequency reuse, and increases the priority of cell edge high moving speed users on the band with higher transmit power. Specific implementation is that in frequency domain scheduler, the priority of cell edge high moving speed users is multiplied by a coefficient greater than1on the band with higher transmit power to improve their priority. The system level simulation results of the two algorithms show that the two algorithms can effectively improve the throughput of cell edge high moving speed users, and improve user fairness.