Ambient OFDM Pilot-Aided Backscatter Communications: Concept and Design
Takanori Hara, Ryuhei Takahashi, Koji Ishibashi
Abstract
This paper investigates a backscatter communications system that exploits ambient <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pilot</i> symbols used in existing standards based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">orthogonal <i>frequency-division</i> multiplexing</i> (OFDM), such as IEEE802.11, in order to realize ultra-low power communications with longer transmission range and higher data rate than conventional ambient backscatter systems. Two modulation schemes, phase-shift keying (PSK) and a new approach named <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">delay-shift</i> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">keying</i> (DSK), are investigated for the proposed system, and the optimal design of DSK is provided based on the theoretical upper bound of the symbol error rate (SER) over double frequency-selective channels. An optimal maximum-likelihood (ML) detector is also developed for the proposed system along with a feasible transmission protocol including channel estimation. Computer simulation results reveal that, at a bandwidth efficiency less than 3 bits per channel use (bpcu), PSK achieves the lowest SER while DSK achieves the lowest SER at efficiencies greater than 2 bpcu. The performance of the proposed detector was comparable to that of the conventional joint ML detectors with a lower degree of complexity, even when a limited number of subcarriers were available as pilot symbols.