Adaptive duplexer implemented using single-path and multipath feedforward techniques with BST phase shifters

As the complexity of modern modulation schemes increases, the noise levels inserted in the receive band by the transmitter are also increased. A technique to enhance the isolation of a surface acoustic wave duplexer is introduced, which reduces the noise levels in the receive band of the system. Feedforward techniques are used to create an adaptive null in the receive band, which can be targeted at any channel across the band. This null has been shown to give an improved isolation of 20 dB for the duplexer over a 2-MHz bandwidth. This feedforward system is implemented using a barium strontium titanate phase shifter to enable tunability of the enhancement null. Multipath feedforward techniques are also used to create extra nulls in the receive band and, hence, further improve isolation.