Cross Layer Association Control for Throughput Optimization in Wireless LANs with inter-AP Interference

In this paper, we study the problem of determining the optimal association in multi-cell WLANs in the presence of hidden terminals and inter-AP collisions. Unlike most work in this area which deal with networks without inter-AP interference, we reveal that association control alone is not sufficient to achieve fair throughput allocation and load balancing across APs. Instead, we advocate the joint association control, rate control and contention control to improve network performance. Based on this, we formulate a cross-layer optimization problem whose objective is to allocate downlink throughput according to the proportional fairness principle. As the problem turns out to be a non-convex mixed integer programming problem, which is known to be NP-hard, we relax it into a continuous convex problem and propose a distributed algorithm to solve it. We then design a simple yet effective distributed approximation algorithm to construct an solution that fulfills the discrete integral association constraint. The output of the algorithm provides the optimal association, the maximum achievable rate for each downlink flow and each AP's optimal average backoff time. Numerical experiments and simulation results show that our algorithm converges rapidly and works effectively.