Interference-Free Backscatter Communications for OFDM-Based Symbiotic Radio

This study proposes an orthogonal frequency division multiplexing (OFDM) based scheme to achieve interference-free backscatter communications (BC) in a symbiotic radio system. In this scheme, the backscatter device performs frequency shift keying (FSK) modulation to shift the primary signal, i.e., the OFDM symbols transmitted from a base station, in the frequency domain to transmit its information. Symbiotically, the base station empties specific subcarriers within the band so that the received frequency-shifted signals from the backscatter device and the primary signal are always orthogonal. Based on the empty subcarrier placement and the corresponding FSK modulation, we propose three schemes including on-off keying (OOK), FSK-1, and FSK-2. Specifically, the first scheme integrates FSK into OOK modulation while the second and the third schemes are based on the conventional FSK modulation with different in-band null-subcarrier allocation. To address the channel estimation challenge for the signals arriving from a backscatter device, we consider a non-coherent detector for obtaining the information from the backscatter signal at the receiver. We derive the bit-error rate performance of the detector theoretically. The comprehensive simulations show that the proposed approach achieves a lower bit-error rate up to 10-4 at 30 dB with BC by eliminating direct link interference.