The operation of Terahertz (THz) communication can be significantly impacted by the interaction between the transmitted wave and the molecules in the atmosphere. In particular, it has been observed experimentally that the signal undergoes not only molecular absorption, but also molecular re-radiation. Two extreme modeling assumptions are prevalent in the literature, where the re-radiated energy is modeled in the first as additive Gaussian noise and in the second as a scattered component strongly correlated to the actual signal. Since the exact characterization is still an open problem, we provide in this paper the first comparative study of the performance of a reconfigurable intelligent surface (RIS) assisted THz system under these two extreme models of re-radiation. In particular, we employ an RIS to overcome the large pathloss by creating a virtual line-of-sight (LOS) path. We then develop an optimization framework for this setup and utilize the block-coordinate descent (BCD) method to iteratively optimize both RIS configuration vector and receive beamforming weight resulting in significant throughput gains for the user of interest compared to random RIS configurations. Our results reveal that a slightly better throughput is achieved under the scattering assumption for the molecular re-radiation than the noise assumption.

This paper focuses 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 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-antenna (SA) and multi-antenna (MA) receivers, and demonstrate the robustness of the new architecture to timing errors. Our results demonstrate that while the direct-link (DL) interference from the ambient power source leads to a BER floor in the SA receiver, the MA receiver can remove this interference by estimating the angle of arrival (AoA) of the DL. The analysis further quantifies the effect of improved angular resolution on the BER as a function of the number of receive antennas. A key intermediate result of our analysis is the derivation of a new concentration result for a general sum sequence that is central to the derivation of the conditional distributions of the received signal.

Due to their inherent ability to operate without the channel state information (CSI), non-coherent receivers are highly suitable for ambient backscatter devices, allowing them to co-exist alongside the non-cooperative primary users. In this work, we analyze the performance of an ambient backscatter system employing non-coherent detection in a time-selective fading channel (modeled using the first order autoregressive (AR) process). We characterize the bit error rate (BER) of a multiantenna (MA) receiver by deriving the expression for antenna gain achievable after the direct link (DL) cancellation. In addition, the analysis also captures the additional angular resolution in a MA receiver with more than two antennas, which allows us to ensure reasonable performance even when the angles of arrival (AoAs) of the DL and backscatter link (BL) are almost similar.

Non-coherent detection is an important component of ambient backscatter communication due to the energy constrained nature of the backscatter devices. This paper provides the first comprehensive performance analysis of non- coherent detection under a fast-varying wireless channel for ambient backscatter communication. In particular, we evaluate the bit error rates (BERs) for two data encoding mechanisms under a new receiver architecture that is based on the direct averaging of the received signal samples. Our results concretely demonstrate the existence of BER floor for a single antenna receiver, which results in poor performance. We further show that a multi-antenna receiver can overcome this drawback by eliminating the interference created by the direct link from the ambient power source that considerably improves the BER of this receiver. This multi-antenna receiver exploits the fact that the time duration of the variation in angle of arrival (AoA) of the communication links in the fast-varying channel is much larger than the coherence period of the small-scale fading, allowing it to track the AoA of the direct link.

The success of Internet-of-Things (IoT) relies on enabling the reliable exchange of data at low-rate and low-power among billions of battery-operated energy-constrained IoT devices. Ambient backscattering, with its technological capability of simultaneous information and energy transfer is quickly emerging as an appealing solution for this communication paradigm. In this paper, we investigate the detection of binary data transmitted using ambient backscatter at a receiver tracking the channel state information (CSI) of a flat-fading Rayleigh channel, and characterize the corresponding performance in terms of the bit-error probability. A binary hypothesis testing problem is formulated for the received signal and the performance of the receiver under mean threshold (MT) detection technique is analyzed. Two main contributions of the analysis that distinguish this work from the prior art are the characterization of the average signal energy in terms of the exact conditional density functions, and the characterization of average bit error rate (BER) expression 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 magnitudes of channel gains that is needed for the BER analysis.