Electrical Balance Duplexer Field Trials in High-Speed Rail Scenarios

Electrical balance duplexers (EBDs) present a potential alternative to the fixed-frequency duplexing filters used for frequency division duplexing in cellular handset radio frequency front ends. However, the transmit-to-receive (Tx-Rx) isolation can be affected by interaction between the antenna and the environment, and therefore, the EBDs balancing impedance must adaptively track time-domain antenna impedance variation. A rail scenario presents a potentially demanding use case for an EBD, as fast moving trains in the vicinity of the antenna may cause dynamically changing reflections, which can be received as self-interference. In this paper, measured dynamic antenna reflection coefficients at 745 and 1900 MHz from train mounted antennas are included in the EBD circuit simulations in order to investigate the resulting variation in Tx-Rx isolation, and determine requirements for balancing impedance adaptation. This paper also presents the results from rail-based field trials of a hardware prototype EBD, which implements real-time antenna impedance tracking. Results show that the rail scenario does result in variation in Tx-Rx isolation, but that rebalancing the EBD at the intervals of 5 ms was sufficient to maintain >50 dB isolation for ~95% of the time.