Towards an optimal user association in heterogeneous cellular networks

We investigate how a heterogeneous cellular network should self-organize by proposing a load-aware user association scheme. This is an important consideration, in order to move traffic off congested cells and onto more lightly loaded cells. Although the network-wide optimal association problem is NP hard, a closely related utility maximization problem can be made convex by applying relaxations on the association metric. We then address a low-complexity distributed algorithm that converges to a near-optimal solution with theoretical guarantee on its performance, requiring limited information and no coordination. This is directly related to range extension and small-cell biasing, which is how cell associations are likely to work in practice. Our load-aware association scheme provides theoretical guidance on the best “biasing factor” for different tiers of base stations. Numerical results show a 3.5x throughput gain for cell-edge users and a 2x gain for median users relative to the standard max-SINR association where a mobile connects to the strongest base station.