QoS-Aware User Association for Load Balancing in Heterogeneous Cellular Networks

In this paper, we propose a load-aware and QoS- aware user association strategy that jointly considers the load of each BS and user´s achievable rate instead of only utilizing the latter, and formulate it as a network-wide weighted utility maximization problem to reveal how a heterogeneous cellular network should self-organize. This is a nonlinear mixed-integer optimization problem, and its optimum solutions are very difficult to be obtained when it is large scale one. To solve the proposed problem, we relax association indicator variables and adopt a gradient descent method to find optimum solutions. Then, each user is associated with some BS with a maximum association indicator taken from solutions of the relaxed optimization problem. Experimental results show that, compared with the best power association and range expansion association, our strategy has a lower call blocking probability and a higher load balancing level.