Research On Downlink Coordinated Multi-Point Transmission Based On Limited Backhaul Capacity

Coordinated Multi-Point(Co MP) transmission is put forward in the Long Term Evolution-Advanced(LTE-A) system to improve both average and cell-edge throughput. Co MP is generally considered to be a promising technology in LTE-Advanced system, to enhance system throughput by averting or even utilizing inter-cell interference(ICI). The downlink Co MP is classified into CS/CB and JP.Co MP-JP scheme can get larger cell-edge throughput and lower BER(bit error rate) than Co MP-CB scheme, but it also has higher complexity due to data sharing. In this paper, a hybrid coordinated strategy with the parameter β, which indicates proportion of users employing Co MP-JP scheme, is proposed to apply Co MP-JP scheme to improve the poorer communication quality of cell edge and employ Co MP-CB scheme for other users to enhance average throughput and spectral efficiency. This paradigm selected users defined by the certain threshold of SINR(signal to interference plus noise power ratio) corresponding to the parameter β to the Co MP-JP scheme. This paper compares the BER performance between the BD based precoding and the linear precoders by maximizing SLNR, and also indicates that SLNR based precoding algorithm gets lower BER than BD based precoding algorithm with certain signal-to-noise ratios(SNR). Finally, this paper discussed that the system performance is partially affected by the percentage of Co MP-JP users and concluded that 50% of users sorted to communicate under the Co MP-JP scheme will reach the best system performance.In many existing systems, backhaul link has limited capacity, which becomes the bottleneck to realize potential performance of downlink Co MP. In this paper, downlink Co MP resource allocation based on limited backhaul capacity is proposed, which takes a tradeoff between system throughput and fairness. And it is composed of cooperative set selection, resource allocation, Co MP transmissi on mode selection and power allocation. First of all, semi-dynamic cooperative set selection based on relative threshold is adopted, which dynamically determines cooperative set for each cell-edge user according to average interference strength from interf erence e Node Bs. Then, a fixed resource partition is chosen, and the whole available RB resource is divided into Co MP resource and non-Co MP resource, thus all the cells schedule users on the basis of above resource division. Resource allocation of proportional fairness based on querying table is proposed. In this scheme, it updates RB allocation matrix when center cell has completed resource allocation and delivers the matrix to adjacent cells, then adjacent cells allocate resource by taking center cell resource allocation into consideration. In addition, a joint hard switchover scheme is applied, which selects Co MP transmission mode by comparing the total data rate of cell-edge users when they respectively employ Co MP-JP and Co MP-CB mode. Finally, adaptive water-filling power allocation algorithm is proposed, which takes a tradeoff between system throughput and fairness.In this paper, performance of downlink Co MP based on limited backhaul capacity and single-point transmission is contrasted, and results indicate that according to different backhaul constraint the proposed scheme can get 59.2%~102.9% higher system throughput and spectral efficiency, compared with single-point transmission. Moreover, system throughput and spectral efficiency of adaptive water-filling power allocation are slightly lower than that of conventional water-filling algorithm, but adaptive water-filling obtains higher system fairness. Finally, this paper discussed that the system performance is partially affected by the percentage of Co MP resource, and results indicate that Co MP technology can reach 56.1% larger throughput and 52.85% higher Jain’s fairness index than single-point transmission, when 40% of RB resource is allocated to Co MP transmission.