Wideband Tx leakage cancellation using adaptive delay filter at RF frequencies

Transmitter leakage is caused by limited isolation between Tx and Rx paths of an frequency-division duplexing (FDD) transceiver due to use of non-ideal duplexers. As the leakage can seriously impair the performance of the receiver, it is important to increase the isolation of the existing system. A portion of the leakage falling into the Rx band can´t be removed using conventional band-pass techniques, which leads to exploration of active cancellation methods. This paper presents an active Tx leakage cancellation structure with adaptive delay filter, together with a related design methodology. Based on S-parameters characterisation of a SAW duplexer, simulation results show that for 20 MHz bandwidth more than 25 dB cancellation can be achieved, for LTE band 8 carrier. The complexity of proposed cancellation structure is proportional to the characteristics of a duplexer used in the transceiver. Using finite number of elements, the number of auxiliary paths needed depends on how complex the characteristics of the duplexer are within the band of interest.