[Paper Review] Achievable information rates of ambient backscatter communications

Donatella Darsena, Giacinto Gelli+1 more|arXiv (Cornell University)|May 16, 2016

Energy Harvesting in Wireless Networks27 references3 citations

TL;DR

This paper analyzes the theoretical capacity limits of ambient backscatter communications in broadband fading channels, showing that legacy multicarrier systems can exploit backscatter-induced interference as multipath diversity to improve performance. Under realistic conditions, backscatter systems achieve significant data rates—up to 360 kbps over 800 m—especially when the backscatter receiver is co-located with the legacy transmitter.

ABSTRACT

Ambient backscatter is an intriguing wireless communication paradigm that allows small devices to compute and communicate by using only the power they harvest from radio-frequency (RF) signals in the air. Ambient backscattering devices reflect existing RF signals emitted by legacy communications systems, such as digital TV broadcasting, cellular or Wi-Fi ones, which would be otherwise treated as harmful sources of interference. This paper deals with the ultimate performance limits of ambient backscatter systems in broadband fading environments, by considering different amounts of network state information at the receivers. After introducing a detailed signal model of the relevant communication links, we study the influence of physical parameters on the capacity of both legacy and backscatter systems. We find that, under reasonable operative conditions, a legacy system employing multicarrier modulation can turn the RF interference arising from the backscatter process into a form of multipath diversity that can be suitably exploited to noticeably increase its performance. Moreover, we show that, even when employing simple single-carrier modulation techniques, the backscatter system can achieve significant data rates over relatively short distances, especially when the intended recipient of the backscatter signal is co-located with the legacy transmitter, i.e., they are on the same machine.

Motivation & Objective

• To determine the ultimate performance limits of ambient backscatter communications in terms of information-theoretic metrics like ergodic and outage capacity.
• To investigate how backscatter transmission affects both the backscatter system and the underlying legacy communication system.
• To evaluate the impact of network state information, signal constellations, and spatial configurations on achievable rates.
• To explore whether ambient backscatter can turn RF interference into a performance-enhancing diversity gain for legacy systems.

Proposed method

• Develops a detailed signal model for legacy and backscatter communication links in frequency-selective fading environments.
• Analyzes ergodic and outage capacity under symbol variance and amplitude constraints for both systems.
• Uses multicarrier modulation for the legacy system and single-carrier modulation for the backscatter system.
• Derives upper and lower bounds on backscatter system capacity using stochastic geometry and channel state information models.
• Evaluates performance under varying distances, angles, and signal-to-noise ratios (SNR) between backscatter transmitter, legacy transmitter, and backscatter receiver.
• Considers both co-located and spatially separated configurations of legacy and backscatter receivers.

Experimental results

Research questions

• RQ1 Can ambient backscatter systems achieve significant data rates over short to moderate distances under realistic propagation conditions?
• RQ2 How does the presence of a backscatter transmitter affect the performance of a legacy multicarrier system?
• RQ3 Under what conditions can the legacy system exploit backscatter-induced interference as a form of multipath diversity?
• RQ4 What is the optimal placement of the backscatter transmitter relative to the legacy transmitter and receiver to maximize backscatter data rate?
• RQ5 How do different signal constellations (e.g., QPSK, PSK) impact the achievable capacity of the backscatter system?

Key findings

• The backscatter system can achieve up to 360 kbps over a distance of 800 m when the legacy transmitter is a 6 MHz TV tower and the SNR at the backscatter receiver is -10 dB.
• When the backscatter receiver is co-located with the legacy transmitter, the backscatter system achieves its highest data rates, with performance peaking at close proximity.
• The legacy system can benefit from backscatter transmission by exploiting the induced interference as a form of multipath diversity, improving its own performance.
• The backscatter system's capacity is multimodal when the angle between the backscatter transmitter and the legacy transmitter is not in the favorable set, showing local maxima and minima depending on distance.
• Capacity saturation occurs at low noise levels due to residual interference from the legacy system on the backscatter link, limiting further gains.
• PSK constellations outperform other signal constellations in terms of cut-off rate when symbols are equiprobable, especially in low-SNR regimes.