An energy-efficient NOMA for small cells in heterogeneous CRAN under QoS constraints

This paper investigates downlink performance of wireless backhaul in a heterogeneous cloud radio access network (HCRAN) consisting of a cloud-based central station (CCS) and multi-tier small cells. Non-orthogonal multiple access (NOMA) is adopted for the downlink from the CCS to multiple small cells of different types (e.g. microcells, picocells and femtocells). Taking into account practical power consumption at small cells operating within various propagation environment models, we first develop a power allocation for the NOMA, which allows us to derive the energy efficiency (EE) of the wireless backhaul in the practical HCRAN downlink. It is shown that the NOMA is superior to the conventional OFDMA scheme achieving a higher EE of up to six times with the deployment of small cells. The propagation environment is also shown to have a significant impact on the EE performance with a big gap between different cell types when the number of cells is large. Particularly, the EE of the NOMA is shown to not always increase or decrease as a function of the number of cells, while the throughput performance at the cloud edge is strikingly degraded as the number of cells increases. This accordingly motivates us to propose a two-stage algorithm for determining the optimal number of various cells that maximises the EE of the HCRAN while still maintaining the QoS requirement at the cloud edge. Simulation results show that, to meet a target cloud-edge throughput, the same number of femtocells and picocells can be used; however, the femtocells are favourable to the picocells in achieving the maximal EE.