Modulation in the Air: Backscatter Communication over Ambient OFDM Carrier (1704.02245v2)

Abstract: Ambient backscatter communication (AmBC) enables radio-frequency (RF) powered backscatter devices (BDs) (e.g., sensors, tags) to modulate their information bits over ambient RF carriers in an over-the-air manner. This technology also called "modulation in the air", thus has emerged as a promising solution to achieve green communications for future Internet-of-Things. This paper studies an AmBC system by leveraging the ambient orthogonal frequency division multiplexing (OFDM) modulated signals in the air. We first model such AmBC system from a spread-spectrum communication perspective, upon which a novel joint design for BD waveform and receiver detector is proposed. The BD symbol period is designed to be in general an integer multiplication of the OFDM symbol period, and the waveform for BD bit 0' maintains the same state within a BD symbol period, while the waveform for BD bit1' has a state transition in the middle of each OFDM symbol period within a BD symbol period. In the receiver detector design, we construct the test statistic that cancels out the direct-link interference by exploiting the repeating structure of the ambient OFDM signals due to the use of cyclic prefix. For the system with a single-antenna receiver, the maximum-likelihood detector is proposed to recover the BD bits, for which the optimal threshold is obtained in closed-form expression. For the system with a multi-antenna receiver, we propose a new test statistic, and derive the optimal detector. Moreover, practical timing synchronization algorithms are proposed, and we also analyze the effect of various system parameters on the system performance. Finally, extensive numerical results are provided to verify that the proposed transceiver design can improve the system bit-error-rate (BER) performance and the operating range significantly, and achieve much higher data rate, as compared to the conventional design.

• The paper proposes a novel joint waveform and receiver design for ambient backscatter communication over ambient OFDM signals, effectively mitigating direct-link interference in both single and multi-antenna systems.
• The proposed transceiver design significantly improves bit-error-rate performance and operating range compared to conventional methods, validated by extensive numerical simulations.
• This ambient backscatter design, requiring only partial channel knowledge, offers a sustainable and scalable solution for energy-efficient low-power Internet of Things applications.

Overview of "Modulation in the Air: Backscatter Communication over Ambient OFDM Carrier"

This paper presents a thorough investigation into ambient backscatter communication (AmBC) systems, specifically focusing on leveraging ambient orthogonal frequency division multiplexing (OFDM) modulated signals. AmBC systems allow RF-powered backscatter devices (BDs) to modulate their data over existing OFDM signals without the need to generate RF carriers, thus providing a low-cost and energy-efficient solution for the Internet of Things (IoT).

Contributions and Methodology

The paper begins by modeling an AmBC system using a spread-spectrum (SS) communication framework. The authors propose a novel joint design for both the BD waveform and the receiver detector, addressing the challenges that arise from the unknown and time-varying nature of ambient signals and the strong direct-link interference from the RF source.

For single-antenna receiver systems, the paper presents a strategy to construct test statistics that effectively eliminate direct-link interference by leveraging the cyclic prefix in OFDM signals. The authors derive a maximum-likelihood detector, optimizing the detection threshold in a closed-form expression, eliminating the need for extensive channel knowledge.

In systems with multiple antennas, the paper introduces a test statistic that combines per-antenna test statistics into an optimal detector capable of efficiently processing the received signals jointly. This reduces implementation complexity as it requires only the estimation of backscatter channel strength.

The authors additionally address practical concerns by proposing timing synchronization algorithms that avoid reliance on known preambles in ambient signals. The paper concludes with an analysis of various parameters affecting system performance, such as cyclic prefix length and multi-path spread, alongside extensive numerical simulations validating the proposed designs' efficacy.

Implications and Future Directions

The results demonstrate that the proposed transceiver design significantly enhances bit-error-rate (BER) performance and operating range compared to conventional methods, highlighting its suitability for IoT applications. By only requiring partial channel knowledge and successfully mitigating direct-link interference, this AmBC design promotes more sustainable and scalable IoT networks.

Looking forward, future work may explore adaptive modulation schemes that further optimize data transmission rates and reliability or investigate the incorporation of machine learning techniques for dynamic channel estimation and interference cancellation. Such developments could broaden the application scope of AmBC systems and enhance their integration within complex IoT ecosystems.

In summary, this paper provides a sophisticated approach to backscatter communication over ambient OFDM carriers, presenting a compelling case for its adoption in next-generation green communication systems.