Contents I. Introduction
In fifth-generation wireless communication networks, Non-Orthogonal Multiple Access (NOMA) has attracted much attention in both academic and industrial fields because of its higher spectral efficiency in comparison with orthogonal multiple access [1–3]. Recently, NOMA was extended to co-operative transmission to enhance the transmission reliability for the users with poor channel conditions [4–6]. In particular, a cooperative NOMA scheme was proposed in [4] by selecting the users with better channel conditions as relays for assisting the others. A two-stage relay selection strategy for NOMA was also proposed in [5] for cooperative NOMA schemes with dedicated relays. In addition, cooperative NOMA schemes with dedicated relays were extended to systems with multiple users equipped with multiple antennas [6], in which relay selection based on the maximal instantaneous signal-to-noise-ratio (SNR) was analyzed. The aforementioned works on cooperative NOMA schemes usually assume that there are no direct links between the base station (BS) and the users, and all the users cannot communicate with the BS without the help of dedicated relays. However, for typical scenarios of small cells in 5G networks [7], some users can directly communicate with the BS while some cannot, thus the research on cooperative NOMA schemes taking into both direct and indirect links users is also addressed. In particular, a recent work addressed in [8] shows that the spectral efficiency is remarkably improved when coordinated direct and decode-and-forward relaying was employed in NOMA scheme, and the analysis for achievable outage probability and ergodic sum capacity was performed for a cooperative NOMA system with a dedicated full-duplex relay [9]. Despite the aforementioned progress on cooperative NOMA schemes, the results for amplify-and-forward (AF) relaying NOMA systems is barely addressed, which motivates the study of this letter.
