User-centric virtual cell design for Cloud Radio Access Networks

The revolutionary Cloud Radio Access Network (C-RAN) enables real-time physical-layer coordination over a large number of distributed remote radio heads (RRHs). Connected via high-bandwidth low-latency optics, RRHs can cooperate dynamically and seamlessly according to user locations or traffic loads. Without limitations on the RRH cooperation, the traditional cellular structure that associates mobile users with cells centered around base stations (or RRHs in C-RAN) needs to be revamped. This paper proposes a novel concept, namely, user-centric virtual cell, to associate each mobile user with a set of cooperative RRHs. In particular, a virtual cell is configured with a mobile user at the cell center and its serving RRHs located in a circular area around the user.With this concept, we are interested in the optimal radius of virtual cells that maximizes the system downlink capacity. In contrast to previous works, the active RRHs in CRAN not only cluster around mobile users, but also have correlated transmit powers due to efficient power allocation. We first characterize the distribution of the interference based on the mean and variance. We then apply the results to obtain the optimal cell radius and discuss its dependence on various system parameters such as user separation distance, RRH density, etc. Our work here provides an important guideline to the cell plan of C-RAN in future 5G wireless networks.