Performance of an Adaptive Cooperative NOMA Scheme with Transmit Antenna Selection

In this paper, we investigate the performance of hybrid cooperative non-orthogonal multiple access (CNOMA) underlaid in a primary network. The CNOMA setup consists of a secondary near user (NU), provisioned to act as a full-duplex (FD) relay for a secondary far user (FU). We propose a novel mode switching mechanism, where the NU can choose to operate in either of the following two modes: 1) The Cooperative Mode (CM) and 2) The Non-Cooperative Mode (NCM). It is only when the NU achieves a desired performance of its own, it chooses to operate in CM, while it switches to NCM otherwise. Through this intelligent switching between modes, the NU retains the same performance as in a network without the FU. We employ a maximum received-power based transmit antenna selection (MRP-TAS) strategy and derive analytical expressions for the outage probability at the NU and FU under peak transmit power and peak interference power constraints. The interference temperature limit (ITL) imposed by the primary network is carefully apportioned between the secondary transmitter and the NU with an ITL apportioning parameter (ITLAP). An optimum choice of both ITLAP and NOMA power allocation parameter (NPAP) is proposed in closed-form, that results in minimum outage performance. The analytical results are validated by extensive simulation studies, and superiority of the proposed scheme over conventional NOMA and OMA schemes is established.