Uplink Non-Orthogonal Multiple Access over Mixed RF-FSO Systems
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In this paper, we consider a relay-assisted uplink non-orthogonal multiple access (NOMA) system where two radio frequency (RF) users are grouped for simultaneous transmission, over each resource block, to an intermediate relay which forwards the amplified version of the users' aggregated signals in the presence of multiuser interference to a relatively far destination. In order to cope with the users' ever-increasing desire for higher data rates, a high-throughput free-space optics (FSO) link is employed as the relay-destination backhaul link. Closed-form expressions for the outage probability and ergodic capacity formulas are derived in the case of independent Rayleigh fading for the user-relay access links when the FSO backhaul link is subject to Gamma-Gamma turbulence with pointing error. Moreover, the outage probability and ergodic capacity analysis are extended to the conventional RF-backhauled systems in the presence of multiuser interference to both relay and destination nodes, and Rician fading for the relay-destination RF link. Dynamic-order decoding is employed at the destination to determine the priority of the users based on their instantaneous channel state information (CSI). Extensive numerical results are carried out to illustrate the dependence of the performance of mixed RF-FSO and dual-hop RF/RF NOMA systems to various design parameters. Our results reveal the superiority of FSO backhauling for high-throughput and high-reliability NOMA systems compared to RF backhauling. This work can be regarded as the general analysis of dual-hop uplink NOMA systems as well as first attempt to incorporate power-domain NOMA in mixed RF-FSO systems.
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