Multi-RF and Generalized Single-RF Combination Models for Spatial Modulation and NOMA Technologies

Inspired by the individual advantages of spatial modulation (SM), space shift keying (SSK), and non-orthogonal multiple access (NOMA) technologies, this research designs NOMA-SM/SSK amalgamation models with a general number of users. Based on the number of RF chains, they are classified into multi-RF (MRF) and single-RF (SRF) models. We first present a spectral-efficient MRF model and then propose a new energy-efficient generalized SRF model denoted as SRFGen. In SRFGen, all users jointly choose a shared transmit antenna (TA) and independently select their modulation symbols. The SRFGen's flexibility produces several special cases covering the literature on SRF models and resolves their limits. Among the scenarios, an SRF NOMA-SSK supports multi-SSK signals and is proposed to reduce interference and detection processes. Then, introduce a low complexity TA index detector and an adaptive and fair power allocation algorithm. The spectral efficiency (SE) of the MRF and SRFGen is analyzed with lower bounds for both Gaussian and finite-alphabet inputs. Using the SE analysis and bit-error-rate (BER) simulation, the NOMA-SM/SSK models are evaluated regarding users' rates, sum-rate, ergodic capacity, and BER. The results show that the MRF grants high SE, whereas the SRFGen provides flexibility in trading-off features like SE, BER, users' fairness, and detection complexity. The outcomes also reveal the superior performance of the presented SRF models compared to the literature SRF models. Besides, the results reinforce the superiority of NOMA-SM/SSK over OMA-SM.