Investigating multi-antenna RFID systems by means of time-varying scattering parameters

Recently, the use of multi-antenna techniques in backscatter modulation-based systems, mostly known from the application of RFID, has come into the focus of research due to the increasing demand for higher data rates or the extension of the achievable range or reliability. Thus, this contribution describes a measurement testbed for the investigation of multiantenna backscatter setups, which is based on a vector network analyzer. In contrast to other publications, the VNA is exploited as a multi-channel vector signal analyzer. By evaluating the time-varying scattering parameters of the system the data transmission on the reverse link can be assessed. Together with a tag emulator incorporating a 4-QAM backscatter modulator, a 1 × 1 × 2 test system is built (1 reader TX, 1 tag scattering and 2 reader RX-antennas). Measurement results in terms of the symbol error ratio with two signal processing methods in conjunction with maximal ratio combing are presented for various signal-to-noise levels at a center frequency of 5.8 GHz.