  Characterizing user to Remote Radio Head (RRH) association strategies in
cloud radio access networks (C-RANs) is critical for performance optimization.
In this letter, the single nearest and N--nearest RRH association strategies
are presented, and the corresponding impact on the ergodic capacity of C-RANs
is analyzed, where RRHs are distributed according to a stationary point
process. Closed-form expressions for the ergodic capacity of the proposed RRH
association strategies are derived. Simulation results demonstrate that the
derived uplink closed-form capacity expressions are accurate. Furthermore, the
analysis and simulation results show that the ergodic capacity gain is not
linear with either the RRH density or the number of antenna per RRH. The
ergodic capacity gain from the RRH density is larger than that from the number
of antennas per RRH,which indicates that the association number of the RRH
should not be bigger than 4 to balance the performance gain and the
implementation cost.
