On the Performance of NOMA Enabled Caching in User-Centric Networks with Unreliable Backhauls

Caching at small-cell base stations (SBSs) reduces the traffic burden on macro-cell base stations (MBSs). Integrating non-coherent joint transmission (NC-JT) and non-orthogonal multiple access (NOMA) in cache-assisted systems can further boost system performance. In this paper, we consider a cache-assisted cooperative communication system. Cache-enabled SBSs prestore files to promptly respond to the request of user equipments (UEs). We propose a NOMA based joint transmission scheme to coordinate the SBSs, which takes advantage of the prestored file to compensate for unreliable backhauls. We study the received signal power distribution and derive the success delivery probability. Upon above analytical results, the outage spectral efficiency of the system is also investigated. Numerical results show that prestoring popular files significantly improves system performance. Moreover, with more SBSs deployed, the NOMA based scheme outperforms its orthogonal counterpart in a wider range of parameters, and the negative impact of unreliable backhaul can be effectively alleviated.