Combined Wideband Channel Estimation and Direct Link Interference Mitigation in Bistatic Backscatter Systems

In this paper, we present a novel algorithm for simultaneous direct link interference (DLI) mitigation and channel state information (CSI) estimation in wideband single-input single-output (SISO) backscatter (BS) systems. First, we establish a time-domain signal model to derive a method to separate the weak backscatter signal from the strong DLI by exploiting the masking property of the binary on–off keying (OOK) modulation of the backscatter device. Based on the time-domain model, we derive an estimator for the unknown parameters of the OOK modulation signal, which incorporates inherent cancellation of the direct link signal. With the obtained estimate of the modulation parameters, the impulse responses of the DLI and BS channels are estimated, using a masked maximum likelihood approach. The obtained algorithm is evaluated using a standard-compliant orthogonal frequency-division multiplexing (OFDM) transmission for the interrogation system and a radio frequency identification (RFID) preamble for the BS modulation. Exploiting the inherent spreading gain due to the wideband OFDM signal, we are able to recover the parameters of the BS signal at an SNR as low as -20 dB. Furthermore, we perform a proof-of-concept measurement in a wired BS setup using a pseudo-noise transmit signal and use the received signal to test the obtained algorithm.