Fambirai Takawira | Searcharxiv

arXiv2026

Beamforming Design for RIS-Assisted Backscatter NOMA in Underlay Cognitive Radio

This paper investigates a reconfigurable intelligent surface (RIS)-assisted backscatter non-orthogonal multiple access (NOMA) network operating as the secondary system of an underlay cognitive radio network. A multi-antenna base station serves several user clusters through the RIS, each containing a backscatter device (BD). We maximise the weighted sum rate of the secondary network by jointly optimising the transmit beamformers and the RIS reflection matrix, subject to the power budget, the interference-temperature limit and minimum rate requirements. The non-convex problem is solved by alternating optimisation, using semidefinite relaxation and sequential rank-one constraint relaxation. Interference-normalised user ordering makes the successive interference cancellation constraints redundant. A closed-form equal-rate power split is derived; it agrees closely with simulation and costs about 4.5% of the sum rate. Supporting the BDs reduces the NOMA sum rate by 8.7% relative to a no-BD design, while a larger BD reflection coefficient raises the backscatter rate with little NOMA loss.

eess.SP↗

arXiv2012

Time-Varying Space-Only Codes for Coded MIMO

Multiple antenna (MIMO) devices are widely used to increase reliability and information bit rate. Optimal error rate performance (full diversity and large coding gain), for unknown channel state information at the transmitter and for maximal rate, can be achieved by approximately universal space-time codes, but comes at a price of large detection complexity, infeasible for most practical systems. We propose a new coded modulation paradigm: error-correction outer code with space-only but time-varying precoder (as inner code). We refer to the latter as Ergodic Mutual Information (EMI) code. The EMI code achieves the maximal multiplexing gain and full diversity is proved in terms of the outage probability. Contrary to most of the literature, our work is not based on the elegant but difficult classical algebraic MIMO theory. Instead, the relation between MIMO and parallel channels is exploited. The theoretical proof of full diversity is corroborated by means of numerical simulations for many MIMO scenarios, in terms of outage probability and word error rate of LDPC coded systems. The full-diversity and full-rate at low detection complexity comes at a price of a small coding gain loss for outer coding rates close to one, but this loss vanishes with decreasing coding rate.

cs.IT↗