Downlink Cell Association and Load Balancing for Joint Millimeter Wave-Microwave Cellular Networks

The integration of millimeter-wave base stations (mmW-BSs) with conventional microwave base stations (μW-BSs) is a promising solution for enhancing the quality-of-service (QoS) of emerging 5G networks. However, the significant differences in the signal propagation characteristics over the mmW and μW frequency bands will require novel cell association schemes cognizant of both mmW and μW systems. In this paper, a novel cell association framework is proposed that considers both the blockage probability and the achievable rate to assign user equipments (UEs) to mmW-BSs or μW-BSs. The problem is formulated as a one-to-many matching problem with minimum quota constraints for the BSs that provides an efficient way to balance the load over the mmW and μW frequency bands. To solve the problem, a distributed algorithm is proposed that is guaranteed to yield a Pareto optimal and two-sided stable solution. Simulation results show that the proposed matching with minimum quota (MMQ) algorithm outperforms the conventional max-RSSI and max-SINR cell association schemes. In addition, it is shown that the proposed MMQ algorithm can effectively balance the number of UEs associated with the μW-BSs and mmW-BSs and achieve further gains, in terms of the average sum rate.