Ambient backscatter, an emerging green communication technology, has aroused great interest from both academia and industry. One open problem for ambient backscatter communication (AmBC) systems is channel estimation for a massive-antenna reader. In this paper, we focus on channel estimation problem in AmBC systems with uniform linear array (ULA) at the reader which consists of large number of antennas. We first design a two-step method to jointly estimate channel gains and direction of arrivals (DoAs), and then refine the estimates through angular rotation. Additionally, Cramer-Rao lower bounds (CRLBs) are derived for both the modulus of the channel gain and the DoA estimates. Simulations are then provided to validate the analysis, and to show the efficiency of the proposed approach.

This letter analyzes the outage probability of an ambient backscatter communication system with a pair of passive tag and reader. Specifically, exact expressions of the effective channel distribution and outage probability of the reader are derived rigorously. Since the outage expression is an infinite sum, a tight truncation error bound is provided to facilitate numerical evaluations. The relative truncation error drops rapidly even for very few terms. In addition, an asymptotic outage expression is derived for the high signal-to-noise ratio (SNR) regime, which matches well with the exact value even for the moderate SNR.

Cooperative ambient backscatter communication (CABC) is a promising and energy-efficient wireless technique, particularly relevant for green Internet of Things applications. Previous studies on CABC have often assumed that the tag can always perform backscatter operations. However, this assumption is not accurate since the tag needs sufficient power to support its circuit consumption for successful backscatter. In light of this, this article focuses on studying a CABC system with sensitivity constraints at the tag. The main contributions of this work are as follows. First, we derive the ergodic sum capacity of both primary transmission from the RF source to the reader and backscatter transmission from the tag to the reader. The resulting expressions are solved using Gaussian-Chebyshev quadrature (GCQ) to ensure accuracy and efficiency. Second, we present compact asymptotic results for low and high signal-to-noise ratios, enabling simplified calculations and analysis. Finally, we provide simulations to validate our analytical results. By considering sensitivity constraints at the tag, our study enhances the potential of CABC implementations in real-world scenarios.