Symbiotic Backscatter Communication: A Design Perspective on the Modulation Scheme of Backscatter Devices

TL;DR

This paper investigates low-complexity modulation schemes for Symbiotic Backscatter Communication, deriving achievable rates and optimizing phase parameters to enhance primary transmitter performance, with a focus on practical, low-cost device implementations.

Contribution

It introduces and analyzes MASK and MPSK modulation schemes for SBC, deriving rate expressions and optimal phases, addressing the gap of practical low-complexity signal models.

Findings

Optimal phase adjustments can improve the PT's rate.

Low-order ASK modulation outperforms low-order PSK for BDs.

BD's rate is unaffected by the phase of the modulation scheme.

Abstract

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.