Uplink capacity and user association for cooperative heterogeneous cellular networks

This paper aims at studying user association for uplink (UL) transmission in cooperative heterogeneous cellular networks, where small cell sum rate is limited by the backhaul (BH) capacity. The investigated architecture can be modeled as a multiple-access relay channel (MARC) channel where multiple users communicate with a final destination (the macro base station) with the help of a relay (the small cell base station). A network-coding-based MARC, which exploits both the direct and relayed transmissions, is introduced to deal with low-capacity BH. We assess the UL average system capacity for 1) the baseline association based on the strongest reference signal received power (RSRP) in downlink (DL) and 2) the optimized user association chosen to improve the overall network UL sum-rate. Our results show that the network-coding-based MARC scheme achieves significant gains with respect to the pure relaying strategy. We further show that the optimized user association provides significant gains in network UL sum-rate with respect to the RSRP association, for the same sum power constraint and without degrading DL performance, especially when the BH capacity is sufficiently large.