Symbiotic Backscatter Communication (SBC) has emerged as a spectrum-efficient and low-power communication technology, where backscatter devices (BDs) modulate and reflect incident radio frequency (RF) signals from primary transmitters (PTs). While previous studies have assumed a circularly symmetric complex Gaussian (CSCG) distribution for the BD's signal, this assumption may not be practical because the high complexity of generating CSCG signals is not supported by the low-cost BD. In this paper, we address this gap by investigating SBC for two low-complexity modulation schemes, i.e., $M$-ary amplitude-shift keying (MASK) and $M$-ary phase-shift keying (MPSK), where BD's signals inherently deviate from CSCG distribution. Our goal is to derive the achievable rate of the PT and BD under the MASK/MPSK and to design MASK/MPSK modulation scheme for maximizing the PT's rate. Towards this end, we first derive the expressions of both the PT's rate and BD's rate. Theoretical results reveal that whether or not the BD improves the PT's rate depends on the phase of MASK/MPSK modulation, while the BD's rate is independent of this phase. We then formulate two optimization problems to maximize the PT's rate by adjusting the phase under the MASK and MPSK modulation schemes, respectively, and derive the optimal phases for each modulation scheme in closed forms. Simulation results demonstrate that the optimal phase of MASK/MPSK can ensure an improvement in the PT's rate, and reveal that a low-order ASK modulation is better than a low-order PSK for the BD in terms of improving PT's rate, especially when the direct link is not significantly weaker than the backscatter link in SBC.

翻译：共生反向散射通信（SBC）作为一种频谱高效且低功耗的通信技术而兴起，其中反向散射设备（BD）对来自主发射机（PT）的入射射频（RF）信号进行调制与反射。尽管先前研究假设BD信号服从循环对称复高斯（CSCG）分布，但该假设在实际中可能难以成立，因为生成CSCG信号的高复杂度无法被低成本BD所支持。本文通过研究两种低复杂度调制方案——即$M$进制幅移键控（MASK）与$M$进制相移键控（MPSK）下的SBC系统来填补这一空白，其中BD信号本质上偏离CSCG分布。我们的目标是推导MASK/MPSK调制下PT与BD的可达速率，并设计能最大化PT速率的MASK/MPSK调制方案。为此，我们首先推导了PT速率与BD速率的表达式。理论结果表明，BD能否提升PT速率取决于MASK/MPSK调制的相位，而BD速率则与该相位无关。随后，我们分别构建了在MASK与MPSK调制方案下通过调整相位以最大化PT速率的两个优化问题，并以闭合形式推导了各调制方案的最优相位。仿真结果表明，MASK/MPSK的最优相位能够确保PT速率的提升，并揭示在改善PT速率方面，低阶ASK调制优于低阶PSK调制，这一优势在SBC系统中直射链路强度不显著弱于反向散射链路时尤为明显。