Exploiting Cache-Free/Cache-Aided TWR-NOMA in Cognitive Hybrid Satellite-Terrestrial Networks

In this paper, we consider an underlay cognitive hybridsatellite-terrestrial network (CHSTN) which constitutes a primary satellite source communicating with its multiple terrestrial primary receivers and two non-orthogonal multiple access (NOMA) secondary terrestrial users exchanging their information with the help of a half-duplex decode-and-forward based secondary relay. We demonstrate there exists an inevitable inter-user interference (IUI) due to the NOMA scheme, which causes an adverse effect on the performance of the secondary network. Hence, to achieve the improved performance and subsequently the low latency requirements, the wireless content caching is employed, whereby the relay can store the most popular contents of both the NOMA users. Further, the pertinent hybrid channels are characterized by the shadowed-Rician fading and Nakagami-
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fading models. Hereby, exploiting the mutual interference between primary and secondary networks, and the realistic assumption of NOMA-based imperfect successive interference cancellation, we analyze the performance of CHSTN for the schemes viz., cache-free (CF) two-way relay (TWR) NOMA and cache-aided (CA) TWR-NOMA, while assessing the outage probability (OP), throughput, and average transmission time. Also, we carry out the asymptotic OP analysis at a high signal-to-noise ratio to present the insights on the attainable diversity orders. We manifest that zero diversity order results for both the schemes due to unavoidable IUI. However, one can anticipate the remarkable enhancement in the performance for CA TWR-NOMA scheme over its CF TWR-NOMA counterpart, owing to the reduced IUI and the efficient utilization of available spectrum resources.