Hybrid Beamformer Design for High Dynamic Range Ambient Backscatter Receivers

Abstract

In bi-static Ambient Backscatter Communications (AmBC) systems, the receiver needs to operate at a large dynamic range because the direct path from the ambient source to the receiver can be several orders of magnitude stronger than the scattered path modulated by the AmBC device. In this paper, we propose a novel analog-digital hybrid null-steering beamformer which allows the backscatter receiver to detect and decode the weak AmBC-modulated signal buried in the strong direct path signals and the noise without requiring the instantaneous channel state information. The analog cancellation of the strong signal components allows the receiver automatic gain control to adjust to the level of the weak AmBC signals. This hence allows common analog-to-digital converters to be used for sampling the signal. After cancelling the strong components, the ambient source signal appears as zero mean fast fading from the AmBC system point of view. We use the direct path signal component to track the phase of the unknown ambient signal. In order to avoid channel estimation, we propose AmBC to use orthogonal channelization codes. The results show that the design allows the AmBC receiver to detect the backscatter binary phase shift keying signals without decoding the ambient signals and requiring knowledge of the instantaneous channel state information.Index Terms-Ambient backscatter, low-power receiver, hybrid analog-digital beamformer.

Reinforcement Learning for Scalable and Reliable Power Allocation in SDN-based Backscatter Heterogeneous Network

“…, where η denotes the energy harvesting efficiency, ρ E is the power-splitting ratio, and P f i is the transmit power of the small-cell BS. Generally, the transmission signal is known at the small-cell BS and, thus the small-cell BS can apply interference cancellation techniques to obtain the signal from monostatic backscatter tag [21].…”

Polarization Conversion-Based Ambient Backscatter System

“…In many practical AmBC deployments, the received power from the direct signal path is several orders of magnitude larger than that of the desired signal component of the AmBC system [14]. This imposes a challenge to the backscatter receiver design that needs to have a large dynamic range in order not to lose the backscatter message into the quantization noise of the analog-to-digital converter (ADC) or distortion products due to receiver amplifier saturation [15], [16]. A contemporary survey of AmBC systems is presented in [17] discussing issues and limitations of AmBC systems.…”

“…Several techniques have been pro-posed to mitigate the impact of the direct path interference, such as using long symbol duration to improve the signal-tonoise-plus-interference ratio (SINR) of the scattered signal at the receiver, shifting the backscattered signal to a different frequency band [18], [19] and using multiple receiver antennas [20]- [22]. More recently, hybrid beamforming techniques, machine-learning assisted detection approaches together with coding schemes have been applied to AmBC systems to promote the applications of AmBC in practice [15], [16].…”

Machine Learning-Assisted Detection for BPSK-Modulated Ambient Backscatter Communication Systems

“…This strong signal component of the legacy system drastically degrades the signal-to-interference-plus-noise ratio (SINR) of the AmBC signal 1 . Several methods have been proposed in literature to handle this issue, for instance, [4], [14], [16]- [18]. Authors in [4] have used two receive antennas to mitigate the unknown direct path.…”

“…In the theoretical analysis, the legacy signals are jointly decoded assuming that channel coefficients are available at the receiver [16]. The work [18] has designed a hybrid beamforming method to separate two paths in a multi-antenna scenario, which acquires spreading gain by using Hadamard codes [18]. Relying on the knowledge of the constellation set of the ambient RF signals at the AmBC receiver, [14] has applied EM-assisted methods to retrieve the OOK-modulated backscatter information.…”