Interference Mitigation and Spectral Efficiency Enhancement in a Multi-BD Symbiotic Radio

This study presents a framework designed to mitigate direct-link interference (DLI) and inter-backscatter device interference (IBDI) in multi-backscatter orthogonal frequency division multiplexing (OFDM)-based symbiotic radio (SR) systems. The framework employs OFDM signal designs with strategic allocation of null subcarriers and incorporates two backscatter modulation techniques: on-off frequency shift keying (OFSK) and multiple frequency shift keying (MFSK) for symbiotic backscatter communication (SBC). Additionally, we propose Fully-Orthogonal and Semi-Orthogonal multiple access schemes to facilitate SBC alongside primary communication. The Fully-Orthogonal scheme maintains orthogonality between direct link and SBC signals, thereby ensuring interference-free SBC, albeit at a reduced spectral efficiency. In contrast, the Semi-Orthogonal schemes eliminate IBDI but permit partial DLI, striking a balance between reliability and spectral efficiency. To address the partial DLI inherent in Semi-Orthogonal schemes, successive interference cancellation (SIC) is employed at the receiver, enhancing SBC reliability. To tackle channel estimation challenges in SBC within the SR system, we implement non-coherent detection techniques at the receiver. The performance of the proposed system is evaluated based on average bit error rate (BER) and sum-rate metrics, demonstrating the effectiveness of our schemes. We provide analytical results for the system's detection performance under both proposed modulation techniques and multiple access schemes, which are subsequently validated through extensive simulations. These simulations indicate a notable error-rate reduction of up to $10^{-3}$ at $20$ dB with the Fully-Orthogonal scheme with MFSK.