Cooperative Backscatter Communications with Reconfigurable Intelligent Surfaces: An APSK Approach

TL;DR

The paper targets the multiplicative fading challenge in cooperative backscatter communications by introducing RIS-CBC-APSK, a modulation framework that uses optimized APSK constellations to encode backscatter information onto PSK/unmodulated signals via passive or active RIS. It develops a RIS-based bit-mapping scheme that maximizes minimum Euclidean distance, derives closed-form SER upper bounds under Rician fading with ML detection, and proposes a low-complexity detector. An alternating-optimization approach extends RIS-CBC-APSK to MISO, jointly optimizing transmit beamforming and RIS reflection. Through extensive simulations, RIS-CBC-APSK demonstrates superior SER performance over state-of-the-art RIS-CBC schemes, including in unmodulated incident-signal scenarios, and establishes the practical viability of active/passive RIS configurations for low-power IoT and future wireless networks.

Abstract

In this paper, a novel amplitude phase shift keying (APSK) modulation scheme for cooperative backscatter communications aided by a reconfigurable intelligent surface (RIS-CBC) is presented, according to which the RIS is configured to modulate backscatter information onto unmodulated or PSK-modulated signals impinging on its surface via APSK. We consider both passive and active RISs, with the latter including an amplification unit at each reflecting element. In the passive (resp. active) RIS-CBC-APSK, backscatter information is conveyed through the number of RIS reflecting elements being on the ON state (resp. active mode) and their phase shift values. By using the optimal APSK constellation to ensure that reflected signals from the RIS undergo APSK modulation, a bit-mapping mechanism is presented. Assuming maximum-likelihood detection, we also present closed-form upper bounds for the symbol error rate (SER) performance for both passive and active RIS-CBC-APSK schemes over Rician fading channels. In addition, we devise a low-complexity detector that can achieve flexible trade-offs between performance and complexity. Finally, we extend RIS-CBC-APSK to multiple-input single-output scenarios and present an alternating optimization approach for the joint design of transmit beamforming and RIS reflection. Our extensive simulation results on the SER performance corroborate our conducted performance analysis and showcase the superiority of the proposed RIS-CBC-APSK schemes over the state-of-the-art RIS-CBC benchmarks.

Figures (9)

• Figure 1: Examples of $16$-APSK constellations: (a) $4+12$-APSK and (b) $4+4+8$-APSK.
• Figure 2: Block diagram of passive RIS-CBC-APSK in a SISO setting.
• Figure 3: Constellations of (a) $x$ and (b) $\mathcal{H} _{N_a}e^{j\psi}$, for $A=P=4$.
• Figure 4: The partition for $4+12$-APSK with $A=P=4$.
• Figure 5: Performance comparison among passive RIS-CBC-APSK, RIS-star-QAM I, and RIS-NM with $A=P=4$ (4 bpcu) and $A=P=8$ (6 bpcu): (a) SER of the active information, (b) SER of the backscatter information, and (c) SER of the active and backscatter information.