Cooperative downlink transmission mode selection under limited-capacity backhaul

Coordinated multi-point transmission (CoMP) has been widely recognized as a promising technique for improving spectral efficiency in future cellular systems. However, it may require high-capacity backhaul links, which can increase expense. In this paper, we study the cooperative transmission method with limited-capacity backhaul. Downlink CoMP transmission requires the exchange of information between base stations, depending on the type of CoMP schemes. Either channel state information (CSI) information of all users (for coordinated beamforming), or both data and CSI (for joint processing) need to be shared among all cooperative base stations. The latter method exhibits better performance, but requires high capacity backhaul links. For this reason, we propose in this paper a transmission mode selection method under capacity constrained backhaul. We start by analyzing the performance of two kinds of CoMP transmission modes: joint processing and coordinated beamforming with various types of user scheduling. Then we propose a closed-form decision rule, which depends on the strongest average channel gains of the users co-scheduled in the same time-frequency resource. Simulation results show that the proposed transmission mode selection outperforms the single-mode CoMP transmission.