Research On Coordinated Multi-point Interference Suppression Strategies In Ultra Dense Small Cell Systems

With the rapid development of the mobile Internet, the demand of data services growths explosively. High spectral reuse factor and hundreds of times capacity improvement can be achieved in ultra dense small cell systems via densely deployed base stations. Ultra dense small cell deployment is one of the key technologies of the fifth generation mobile communication system to meet the goal of 1000 times capacity improvement. But with the decrease of cell radius, the coverage area of the small cell overlaps, severe inter base station co-channel interference has been seen as one of main factors to restrict the system capacity improvement. Interference suppression strategies in time domain, frequency domain and space domain are effective measures to solve the problem. Coordinated multi-point (CoMP) technology, which works by sharing the user’s data and the channel state information between base stations and jointly designing multiple small cell downlink precoding, is able to utilize space domain resources to suppress inter base station interference and dramatically improve the system throughput. The shared data is transmitted through the backhaul networks. But the cost of network deployment will increase tremendously in ultra dense small cell systems, if all the small cells are equipped with wired backhaul. Wireless backhaul, which does not require wired cable, can improve the flexibility of base station deployment, reduce the deployment cost and satisfy the flexibility and low cost requirements of ultra dense small cell systems.This paper mainly focus on designing coordinated multi-point interference suppression strategies to maximize the spectral and energy efficiency of ultra dense small cell systems with wireless backhaul.Firstly, an interference suppression strategy is proposed for in-band self-backhaul small cell systems, which adopts a user-centric clustering partial cooperation model to suppress co-channel interference. Considering the same radio frequency band is shared between access link and backhaul link, this paper formulates the joint optimization problem for resource allocation of backhaul link and precoding design of access link to maximize the access link throughput. The problem is difficult to find the optimal solution, so based on convex relaxation and block coordinated decent method, a suboptimal iterative algorithm is proposed. Motivated by reducing the computational complexity, we prove the upper bound of the fraction of resources used for backhauling and propose a simplified heuristic algorithm. Simulation results reveal that the proposed algorithms can significantly improve the access link throughput and increase the system spectral efficiency.Secondly, an interference suppression strategy is proposed for out-band wireless backhaul small cell systems, which adopts a user-centric clustering partial cooperation model to reduce the energy consumption of backhaul network, and turns some low-loaded small cell into sleep mode to reduce the energy consumption of base stations. This paper formulates the joint optimization problem for the design of precoding, base station sleep strategy and cooperation strategy to minimize the total system energy consumption subject to QoS constraints of the users. This optimization problem is a non-convex nonlinear mixed integer programming problem, and the optimal solution cannot be obtained directly. The optimal solution can be founded through a two-step optimization algorithm. But its complexity is extremely high. Therefore, this paper proposes an iterative suboptimal algorithm based on the semi definite programming relaxation and successive convex approximation. Simulation results reveal that the proposed algorithm can reduce the power consumption and improve the energy efficiency of ultra dense small cell systems.