Coexisting Passive RIS and Active Relay-Assisted NOMA Systems

Abstract

A simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted downlink (DL) active and uplink (UL) backscatter communication (BackCom) framework is proposed. More particularly, a full-duplex (FD) base station (BS) communicates with the DL users via the STAR-RIS's transmission link, while exciting and receiving the information from the UL BackCom devices with the aid of the STAR-RIS's reflection link. Non-orthogonal multiple access (NOMA) is exploited in both DL and UL communications for improving the spectrum efficiency. The system weighted sum rate maximization problem is formulated for jointly optimizing the FD BS active receive and transmit beamforming, the STAR-RIS passive beamforming, and the DL NOMA decoding orders, subject to the DL user's individual rate constraint. To tackle this challenging non-convex problem, we propose an alternating optimization (AO) based algorithm for the joint active and passive beamforming design with a given DL NOMA decoding order. To address the potential high computational complexity required for exhaustive searching all the NOMA decoding orders, an efficient NOMA user ordering scheme is further developed. Finally, numerical results demonstrate that: i) compared with the baseline schemes employing conventional RISs or space division multiple access, the proposed scheme achieves higher performance gains; and ii) higher UL rate gain is obtained at a cost of DL performance degradation, as a remedy, a more flexible performance tradeoff can be achieved by introducing the STAR-RIS.

Outage Performance of Multiple Hybrid Active Relays and RISs-Assisted NOMA Networks

“…k and e jϕ (t,r) k,m are the amplitude and phase shift adjustments imposed on the incident signal by the k th STAR-RIS, respectively, i.e., β (t,r) k ∈ [0, 1], and ϕ (t,r) k,m ∈ [0, 2π) for m ∈ {1, ..., M }. Further, β (t) k + β (r) k = 1 should be satisfied 4 . Hence, if a STAR-RIS is selected, the received signal at U i is…”

“…Evidently, the working principle of RISs is similar to classical relaying technologies [4], and it is useful to scrutinize these analogies to gain a holistic understanding of the possibilities and obstacles associated with their coexistence in future communication systems. Unlike active relays, RISs are unable to amplify incoming signal magnitudes due to their natural passive design, thereby limiting the achievable performance improvements.…”

“…In this context, few important contributions have investigated the possibilities of integrating both RIS and relay technologies in NOMA networks. For instance, a downlink transmission framework for coexisting passive RIS and active relay is proposed in [4], wherein both the RIS and relay contribute to assist communication between the AP and a pair of NOMA users. In [9], a RIS-aided cooperative multiple-input single-output (MISO)-NOMA system involving two users is investigated.…”

“…Although acquiring perfect CSI may not be achievable in practical scenarios, this letter focuses on theoretically evaluating the proposed system performance in order to understand its theoretical boundaries 4. Note that, fixed β(t) k and β (r) k are considered here.…”