Spectral Efficiency Analysis of Uplink Cell-Free Massive MIMO Symbiotic Radio
Feiyang Li, Qiang Sun, Xiaomin Chen, Jiayi Zhang
Abstract
This article considers the uplink of a cell-free massive multiple-input–multiple-output (MIMO) symbiotic radio (CF-mMIMO-SR) system. Conventional combining schemes cannot be used directly to detect the signal of the direct link due to its heavy suppression for the backscatter link. To this end, we propose two hybrid combining schemes, including the hybrid maximum ratio (MR) and hybrid local minimum mean square error (L-MMSE) combining schemes, which take into account the superposition of the two local channel estimation vectors at the access points (APs). When the number of APs goes to infinite, the asymptotic spectral efficiency (SE) of CF-mMIMO-SR with different combining schemes is analyzed. We prove that the conventional combining schemes tend to cause the effective signal of the backscatter link to disappear while the hybrid combining schemes can obtain good performance on the backscatter link. Meanwhile, the performance gap between the two on the direct link is small. In addition, we derive closed-form expressions with the conventional MR combining and hybrid MR combining schemes over independent Rayleigh fading channels. Moreover, we derive the achievable uplink SE expressions with an effective signal-to-interference-and-noise ratio (SINR) for a finite number of antennas. Simulation results verify our theoretical analysis and demonstrate that hybrid combining schemes perform much better on the backscatter link than conventional combining schemes. Specifically, compared to the hybrid MR scheme, the hybrid L-MMSE scheme offers a huge improvement in 95% likely SE, and has negligible performance loss on the direct link.