Optimising energy efficiency of non-orthogonal multiple access for wireless backhaul in heterogeneous cloud radio access network

This paper studies the downlink problem of a cloud-based central station (CCS) to multiple base stations (BSs) in a heterogeneous cellular network sharing the same time and frequency resources. We adopt non-orthogonal multiple access (NOMA) and propose power allocation for the wireless downlink in the heterogeneous cloud radio access network (HCRAN). Taking into account practical channel modelling with power consumptions at BSs of different cell types (e.g. macro-cell, micro-cell, etc.) and backhauling power, we analyse the energy efficiency (EE) of the practical HCRAN utilising NOMA. Simulation results indicate that the proposed NOMA for the HCRAN outperforms the conventional orthogonal frequency division multiple access (OFDMA) scheme in terms of providing higher EE of up to four times. Interestingly, the results reveal a fact that the EE of the NOMA approach is not always an increasing function of the number of BSs but varies as a quasiconcave function. This motivates us to further introduce an optimisation problem to find the optimal number of BSs that maximises the EE of the HCRAN. It is shown that, with a low power supply at the CCS, a double number of micro BSs can be served by HCRAN providing an improved EE of up to 1.6 times compared to the macro BSs and RRHs, while they achieve the same EE performance with high-power CCS

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

On the energy efficiency of NOMA for wireless backhaul in multi-tier heterogeneous CRAN

This paper addresses the problem of wireless backhaul in a multi-tier heterogeneous cellular network coordinated by a cloud-based central station (CCS), namely heterogeneous cloud radio access network (HCRAN). A non-orthogonal multiple access (NOMA) is adopted in the power domain for improved spectral efficiency and network throughput of the wireless downlink in the HCRAN. We first develop a power allocation for multiple cells of different tiers taking account of the practical power consumption of different cell types and wireless backhaul. By analysing the energy efficiency (EE) of the NOMA for the practical HCRAN downlink, we show that the power available at the cloud, the propagation environment and cell types have significant impacts on the EE performance. In particular, in a large network, the cells located at the cloud edge are shown to suffer from a very poor performance with a considerably degraded EE, which accordingly motivates us to propose an iteration algorithm for determining the maximal number of cells that can be supported in the HCRAN. The results reveal that a double number of cells can be covered in the urban environment compared to those in the shadowed urban environment and more than 1.5 times of the number of microcells can be deployed over the macrocells, while only a half number of cells can be supported when the distance between them increases threefol

On the performance of regenerative relaying for SWIPT in NOMA Systems

As a potential access strategy in 5G mobile communication systems, non-orthogonal multiple access (NOMA) has been proposed as a supplement to the traditional orthogonal multiple access (OMA). This paper investigates simultaneous wireless information and power transfer (SWIPT) in a NOMA relaying system. The data is transferred from a source to two end terminals among which the one close to the source acts as a relay employing decode-and-forward protocol to assist the far-end one. In order to simultaneously harvest the energy and information processing at relay node, power-splitting relaying (PSR) and time switching-based relaying (TSR) protocols are sequentially considered. Outage probability and ergodic rate of both protocols are firstly analyzed to realize the impacts of various parameters including energy harvesting time, power splitting ratio, energy harvesting efficiency, source data rate, and the distance between the source and the relay node. Numerical results are then provided to validate the analytical findings. It is shown that the PSR outperforms the TSR at normal SNR regime in terms of throughput and ergodic rate

On the performance of NOMA in SWIPT systems with power-splitting relaying

This paper presents a decode-and-forward (DF) relaying protocol, namely power-splitting relaying (PSR), employed at relay nodes in NOMA technique. The PSR is considered for simultaneous wireless information and power transfer (SWIPT) systems. The relaying node is both energy harvesting from the received radio frequency (RF) signal and information forwarding to the destination. The outage performance and ergodic rate of the PSR are analyzed to realize the impacts of energy harvesting time, energy harvesting efficiency, power splitting ratio, source data rate, and the distance between the source and relay nodes. The simulation results show that NOMA schemes have the lower outage probability compared to the that of the conventional orthogonal multiple access (OMA) schemes at the destination node. Numerical results are provided to verify the findings