Cooperative NOMA-Based Spectrum Leasing With Multiple Secondary Users

Abstract

This paper proposes a new cooperative nonorthogonal multiple access-based spectrum leasing (CNOMA-SL) framework for enhanced network performance. Herein, the system is comprised of a primary network and a secondary network with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$n$</tex-math></inline-formula> -transmitting users. Primary and secondary users exploit an underlay spectrum leasing scheme to transmit signals to a relay station (RS), which forwards the signals to the corresponding receivers. Two phases of data transmission are assumed. In Phase I, the secondary transmitters employ uplink NOMA to transmit the symbols. The RS then uses downlink NOMA to broadcast the received symbols in Phase II. We employ the maximum ratio combining decoding scheme and propose a successive-interference-cancellation concept with ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$n+1$</tex-math></inline-formula> ) stages to decode the symbols at the primary and secondary receiving ends, respectively. Considering a transmission power constraint imposed at the secondary transmitters, we derive the closed-form expressions for achievable rate and outage probability for both primary and secondary users. Simulation results manifest that, compared with the conventional cooperative orthogonal multiple access-based spectrum leasing scheme, the proposed CNOMA-SL exhibits improved performance in terms of both achievable rate and outage probability. It is also revealed that the system performs better when the RS takes the lease of the whole spectrum. Furthermore, if more interference power is allowed to the primary receiver, the secondary users reach higher achievable rates.