Semi-Coherent Detection and Performance Analysis for Ambient Backscatter System

Abstract

We study a novel communication mechanism, ambient backscatter, that utilizes radio frequency (RF) signals transmitted from an ambient source as both energy supply and information carrier to enable communications between low-power devices. Different from existing non-coherent schemes, we here design the semi-coherent detection, where channel parameters can be obtained from unknown data symbols and a few pilot symbols. We first derive the optimal detector for the complex Gaussian ambient RF signal from likelihood ratio test and compute the corresponding closed-form bit error rate (BER).To release the requirement for prior knowledge of the ambient RF signal, we next design a suboptimal energy detector with ambient RF signals being either the complex Gaussian or the phase shift keying (PSK). The corresponding detection thresholds, the analytical BER, and the outage probability are also obtained in closed-form. Interestingly, the complex Gaussian source would cause an error floor while the PSK source does not, which brings nontrivial indication of constellation design as opposed to the popular Gaussian-embedded literatures. Simulations are provided to corroborate the theoretical studies.

Non-Coherent Signal Detection and Bit Error Rate for an Ambient Backscatter Link under Fast Fading

“…It can be easily observed that the expectation of this sum grows asymptotically of the order of N , meaning E [S N ] = Θ(N ). Using this, the expectation of M N = S N N can be shown to be a constant, whose value is given in (10). The variance of the sum sequence S N can first be simplified as given below:…”

Symbol Detection of Ambient Backscatter Systems With Manchester Coding

“…We consider the elementary ambient backscatter system as in [15][16][17], which consists of three nodes, namely the ambient RF source S, the reader R, and the tag T, as shown in Fig.1. We assume the frequency flat block fading scenario, such that all channels remain unchanged within each coherence interval, but vary independently in different coherence intervals.…”

“…The above expression indicates that, as the SNR increases, the BER reaches an error floor, which is determined by the sampling rate N and the relative channel difference (RCD) of the path, i.e., RCD ||h1| 2 −|h0| 2 | √ |h0| 4 +|h1| 4 . We now compare the BER performance of the proposed detector with that of the semi-coherent detector proposed in [15]. Recalling the BER of the semi-coherent detector proposed in [15], it can be expressed as…”

“…We now compare the BER performance of the proposed detector with that of the semi-coherent detector proposed in [15]. Recalling the BER of the semi-coherent detector proposed in [15], it can be expressed as…”

“…Hence, theoretically, the BER performance of the proposed SeCoMC detector is inferior. The main reason is that the semicoherent detector in [15] is based on the absolute symbol energy, while the proposed detector is based on the energy difference of the first and second half of the entire symbol interval, which may causes some information loss. However, it is worth highlighting that P CG b can only be achieved when the threshold is perfect.…”