Wide band TX Leakage Cancellation for FDD Transceivers

Frequency division duplex (FDD) is the most widely used full duplex technique for modern wireless communication systems. A device called a duplexer is needed in order to separate the transmitter (TX) and the receiver (RX) signals. To avoid RX being saturated and desensitized, the isolation between the TX and RX ports of the duplexer needs to be as large as possible. However, such a duplexer is less desirable for a base station due to its bulky size, high cost and design challenges. Relaxing the isolation performance requirement of the duplexer allows TX signal to appear in the RX path. Such a signal is known as TX leakage and consists of in-band and out-of-band portions. Due to the fact that the in-band leakage can’t be filtered out with conventional filtering techniques, an active cancellation system is required. This thesis will present a novel wide band TX leakage cancellation approach along with two new modelling techniques. The cancellation system is based on the analog finite impulse response (FIR) filter structure and consists of several cancellation paths (also known as taps). In each of the taps, there is an attenuator, phase shifter and delay line. These components are utilized to recondition the reference signal which is the replica of the output signal of the high power amplifier (PA) in the TX path. The output of the cancellation system (the cancellation signal) in theory should have the same amplitudes, but 180� phase offsets from the leakage signal within the band of interest. By combining the cancellation signal and the leakage, the latter can thus be suppressed. The two novel modelling techniques, named as the modified adaptive delay filter using direct cross-correlation (MADF-DC) and the modified compressed sampling matching pursuit (MCoSaMP) are proposed in this thesis. The two algorithms are developed from the classic adaptive delay filter (C-ADF) and the classic compressed sampling matching pursuit (C-CoSaMP), respectively. These modelling techniques can provide the optimization of not only the amplitudes and phase, but the delays. They are designed for the cancellation system in order to achieve wide band cancellation with the minimum hardware resources. The evaluation of the cancellation system was carried out utilizing a test bench, which represented a typical base station. Experimental results show that greater than 15 dB of cancellation has been achieved for 35 MHz bandwidth at the carrier frequency of 897.5 MHz using four taps. In contrast, such result is about 7 dB less than the simulated cancellation performance. The reasons of such performance degradation have been analysed and will be presented in this thesis. vi