Load balancing with almost blank subframe control in heterogeneous cellular networks

In heterogeneous cellular networks, the network capacity can be significantly enhanced via user offloading between macro- and pico-tier. Due to the strong cross-tier interference from macrocells to picocells in co-channel deployment, the user offloading needs to be jointly optimized with macrocells´ transmit power nulling, i.e., almost blank subframes (ABSs) in 3GPP LTE-A systems. In this paper, we discuss the network-wide utility maximization problem where the cell association and the number of ABSs are jointly optimized. Due to the NP-hardness of the formulated problem, an online heuristic algorithm is proposed where (i) user load is re-distributed based on users´ expected data rate by handover for the current number of ABSs, and (ii) the current number of ABSs is changed (increased or decreased) by estimating the possible users to be offloaded based on users´ expected data rate by handover and ABS change in a gradient descent manner. Through simulations, we demonstrate that the proposed algorithm not only improves the average data rate of users, but also achieves better fairness among users.