1. Ma Z, Zhang Z Q, Ding Z G, et al. Key techniques for 5G wireless communications: network architecture, physical layer, and MAC layer perspectives. Sci China Inf Sci, 2015, 58: 041301
2. Cui Q M, Wang H, Hu P X, et al. Evolution of limited feedback CoMP systems from 4G to 5G. IEEE Veh Tech Mag, 2014, 9: 94–103
3. Liu Y J, Lu L, Geoffrey L, et al. Joint user association and spectrum allocation for small cell networks with wireless backhauls. IEEE Wirel Commun Lett, 2016, doi: 10.1109/LWC.2016.2593465
4. Fei Z S, Ding H C, Xing C W, et al. Performance analysis for range expansion in heterogeneous networks. Sci China Inf Sci, 2014, 57: 082305
5. Jo H S, Sang Y J, Xia P, et al. Heterogeneous cellular networks with flexible cell association: a comprehensive downlink SINR analysis. IEEE Trans Wirel Commun, 2012, 11: 3484–3495