Design of Downlink Hybrid NOMA Transmission

TL;DR

The paper tackles the problem of integrating non-orthogonal multiple access into downlink communications by proposing a hybrid NOMA scheme that augments a legacy TDMA/OMA framework, first for SISO and then for MISO with near-field beam sharing.For SISO, it derives the SIC-based decoding, per-slot data-rate expressions, and a multi-objective energy minimization approach; it demonstrates that, with ordered channels and equal time-slot durations, the downlink hybrid NOMA solution outperforms OMA and can achieve a Pareto-optimal power allocation.For MISO, it introduces beam sharing between SDMA groups, develops a near-field channel model to illustrate feasibility, and formulates an energy-minimization problem solved via successive convex approximation (SCA) with a tractable OMA benchmark.Simulation results validate the analytical findings, showing substantial energy savings of downlink hybrid NOMA over conventional OMA, and reveal nuanced effects of beam sharing, antenna count, and near-field conditions on performance.The work suggests future directions, including extending to more than two user groups and incorporating practical non-idealities such as non-line-of-sight paths and hybrid beamforming.

Abstract

The aim of this paper is to develop hybrid non-orthogonal multiple access (NOMA) assisted downlink transmission. First, for the single-input single-output (SISO) scenario, i.e., each node is equipped with a single antenna, a novel hybrid NOMA scheme is introduced, where NOMA is implemented as an add-on of a legacy time division multiple access (TDMA) network. Because of the simplicity of the SISO scenario, analytical results can be developed to reveal important properties of downlink hybrid NOMA. For example, in the case that the users' channel gains are ordered and the durations of their time slots are the same, downlink hybrid NOMA is shown to always outperform TDMA, which is different from the existing conclusion for uplink hybrid NOMA. Second, the proposed downlink SISO hybrid NOMA scheme is extended to the multiple-input single-output (MISO) scenario, i.e., the base station has multiple antennas. For the MISO scenario, near-field communication is considered to illustrate how NOMA can be used as an add-on in legacy networks based on space division multiple access and TDMA. Simulation results verify the developed analytical results and demonstrate the superior performance of downlink hybrid NOMA compared to conventional orthogonal multiple access.

Figures (6)

• Figure 1: Illustration of the key idea of hybrid NOMA transmission.
• Figure 2: The total energy consumption realized by the considered transmission schemes in the SISO case.
• Figure 3: An illustration of the properties of hybrid NOMA power allocation. $h_m=m$, $1\leq m \leq M$, and $R=2$ nats per channel use (NPCU).
• Figure 4: Verification for the optimality of the hybrid NOMA solution shown in Lemma \ref{['lemma1']}, where $h_m=m$, $1\leq m \leq M$, and $R=2$ NPCU.
• Figure 5: The total energy consumption realized by the considered transmission schemes in the MISO case. The users in $\mathcal{G}_1$, ${\rm U}_m^{\rm G1}$, are uniformly located in the half-ring with radius $10$ m and $50$ m. The users in $\mathcal{G}_2$, ${\rm U}_k^{\rm G2}$, are at fixed locations $200$ m away from the base station, and their angles are equally spaced between $-\frac{\pi}{3}$ and $\frac{\pi}{3}$. $K=3$ and $P^{\rm G1}=10$ dBm.

Theorems & Definitions (4)

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