Downlink Ergodic Rate Analysis for Virtual Cell Based Cloud Radio Access Networks

Cloud Radio Access Network (CRAN) is a promising approach to cope with the challenges of next generation mobile communications. This paper studies the downlink ergodic rate of a (N, K) virtual cell-based CRAN, in which each mobile station (MS) selects the N closest remote radio heads (RRHs) to form its virtual cell and the K (K ≤ N) closest RRHs will serve the target MS cooperatively with a total transmit power constraint. Using the stochastic geometry, the locations of RRHs and MSs are modeled as two independent Poisson point processes, respectively. Then, a computationally tractable integral expression is derived for the downlink ergodic rate of the (N, K) virtual cell-based CRAN. The analytical expression indicates that in the interference limited regime, the ergodic rate only relies on the MS to RRH intensity ratio and is independent with the individual intensity of either the RRH or the MS. Therefore, without deteriorating the downlink ergodic rate significantly, the number of simultaneously served MSs can be improved by increasing the RRH intensity. Specially, when there is only one RRH in each virtual cell, the analytical expression reduces to the ergodic rate expression for the conventional cellular networks. Numerical results indicate that the cooperation among multiple RRHs within each virtual cell can improve the downlink ergodic rate significantly.