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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 MHz and 1900 MHz from train mounted antennas are included in EBD circuit simulations in order the investigate the resulting variation in Tx-Rx isolation, and determine requirements for balancing impedance adaptation. This paper also presents 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 re-balancing the EBD at intervals of 5 ms was sufficient to maintain >50 dB isolation for ~95% of the time.
- Electrical Balance Duplexer
- Self-interference cancellation
- 5G enabling technologies