The success of Internet-of-Things (IoT) paradigm relies on, among other things, developing energyefficient communication techniques that can enable information exchange among billions of batteryoperated IoT devices. With its technological capability of simultaneous information and energy transfer, ambient backscatter is quickly emerging as an appealing solution for this communication paradigm, especially for the links with low data rate requirement. In this paper, we study signal detection and characterize exact bit error rate for the ambient backscatter system. In particular, we formulate a binary hypothesis testing problem at the receiver and analyze system performance under three detection techniques: a) mean threshold (MT), b) maximum likelihood threshold (MLT), and c) approximate MLT.Motivated by the energy-constrained nature of IoT devices, we perform the above analyses for two receiver types: i) the ones that can accurately track channel state information (CSI), and ii) the ones that cannot. Two main features of the analysis that distinguish this work from the prior art are the characterization of the exact conditional density functions of the average received signal energy, and the characterization of exact average bit error rate (BER) for this setup. The key challenge lies in the handling of correlation between channel gains of two hypotheses for the derivation of joint probability distribution of magnitude squared channel gains that is needed for the BER analysis.

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This paper is focused on the non-coherent detection in ambient backscatter communication, which is highly appealing for systems where the trade-off between signaling overhead and the actual data transmission is very critical. Modeling the time-selective fading channel as a first-order autoregressive (AR) process, we consider two data encoding schemes at the transmitter and propose a new receiver architecture based on the direct averaging of the received signal samples for detection, which departs significantly from the energy averaging-based receivers considered in the literature. For the proposed setup, we characterize the exact asymptotic bit error rate (BER) for both single and multi antenna receivers. Our results demonstrate that while the direct interference received from the ambient power source leads to a BER floor in the single antenna receiver, the multi-antenna receiver can efficiently remove this interference by estimating the angle of arrival (AoA) of the direct link from the power source. A key intermediate result of our analysis is a new concentration result for a general sum sequence (including the asymptotic growth rates of its expectation and variance) that is central to the derivation of the conditional distributions of the signal at the receiver. Index TermsAmbient backscatter, non-coherent detection, auto-regressive model, time-selective fading, bit error rate.

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This paper introduces a novel line-of-sight (LoS) β −γ terahertz (THz) channel model that closely mirrors physical reality by considering radiation trapping. Our channel model provides an exhaustive modeling of the physical phenomena including the amount of re-radiation available at the receiver, parametrized by β, and the balance between scattering and noise contributions, parametrized by γ, respectively. Our findings indicate a nontrivial relationship between average limiting received signal-to-noise ratio (SNR) and distance emphasizing the significance of γ in THz system design. We further propose new maximum likelihood (ML) thresholds for pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM) schemes, resulting in analytical symbol error rate (SER) expressions that account for different noise variances across constellation points. The results confirm that the analytical SER closely matches the true simulated SER when using an optimal detector. As expected, under maximum molecular re-radiation, the true SER is shown to be lower than that produced by a suboptimal detector that assumes equal noise variances.

Simplifying the detection procedure and improving the bit error rate (BER) performance of a non-coherent receiver in ambient backscatter is vital for enhancing its ability to function without the channel state information (CSI). In this work, we analyze the BER performance of Manchester encoding which is implemented at the transmitter for data transmission, and demonstrate that the optimal decision rule is independent of the system parameters. Further, through extensive numerical results, it is shown that the ambient backscatter system can achieve a signal-to-noise ratio (SNR) gain with Manchester encoding compared to the commonly used uncoded direct on-off keying (OOK) modulation, when used in conjunction with a multiantenna receiver employing the direct-link cancellation.