Low-complexity QOSTBC processing with spatial modulation for D2D-assisted AR/VR communications

Augmented and virtual reality (AR/VR) services require high data rates, low latency, reliable links, and energy-efficient processing. This paper proposes a low-complexity device-to-device (D2D)-assisted transmission scheme for simultaneous multicast and unicast delivery when the direct source-to-far-node link is unreliable. Spatial modulation (SM) is applied at both hops through antenna-pair and power-state selection, allowing additional multicast and relay-originated information to be conveyed without requiring additional dedicated transmission resources. The unicast stream is transmitted through a full-rate quasi-orthogonal space–time block code (QOSTBC)-based virtual multiple-input multiple-output (MIMO) link. At the relay, channel-dependent amplify-and-forward (AF) processing is employed to improve quasi-orthogonal symbol separation without unicast symbol detection, reconstruction, or re-encoding, thereby removing the corresponding decode-and-forward (DF) processing stages. The antenna-pair/power-state levels and the useful-signal component of the relay output are normalized to maintain the same average useful-signal transmit power across all compared schemes. Monte Carlo BER results under Rayleigh fading show a limited performance degradation due to SM-based antenna-pair/power-state detection. A separate transmission structure analysis yields relative increases in the total per-hop spectral efficiency of 25%, 37.5%, and 50% for three, four, and five available transmit antennas with 4-QAM, respectively. Additional simulations under Rician fading and imperfect channel state information characterize the sensitivity of the scheme to LoS-dominant propagation and estimation errors. Overall, the results indicate a practical trade-off among reliability, aggregate spectral efficiency, and relay-side processing requirements, while the latency and energy implications remain implementation-dependent.