Wideband OFDM Backscatter with Limited-Bandwidth Antennas for WiFi-6 and Future High-Data-Rate Backscatter Systems

Recent efforts toward achieving interoperability between backscatter communication devices and existing wireless communication standards such as IEEE 802.11 (WiFi) have driven the need for ever-wider bandwidth backscatter signals, up to 20 MHz or more for WiFi-6 for example. In this paper we consider a key problem in the generation of wideband orthogonal frequency division multiplexing (OFDM) backscatter signals as used in the IEEE 802.11ax standard. Backscatter modulation is achieved by presenting a carefully controlled reflection coefficient at the terminals of the device antenna. This reflection coefficient depends on the choice of backscatter modulator component values as well as the frequency-varying impedance of the backscatter device's antenna. For wideband backscatter systems, the frequency-varying antenna impedance thus presents a critical design tradeoff in the design of the backscatter modulator. We analyze the variation in backscatter signal power and the error vector magnitude (EVM) of backscatter signals across wideband channels given the measured antenna impedance variation of an example ceramic chip antenna in the 2.4 GHz industrial, scientific, and medical (ISM) frequency band. The analysis presented here is also applicable to narrowband signals that frequency-hop across a wide bandwidth, as is the case for Bluetooth and Bluetooth Low Energy (BLE) signals.