Digital signal injection technique for cancellation of receive-band spurious emissions in FDD cellular transmitters

In this paper, we demonstrate a new technique for cancelling the deterministic spurious emissions from the power amplifier into the corresponding receive band in Frequency-Division Duplex (FDD) systems. Some cellular bands such as band 31 (TX: 455MHz and RX: 465MHz) have very narrow duplex spacing (only 10MHz). In these bands, power amplifier emissions due to nonlinearities are strong enough to severely desensitize the receiver even with 60-70dB transmit signal rejection in the duplexer. We have developed a fully digital technique for spurious cancellation, based on accurate modeling of PA receive band emissions, characterizing the transmitter chain in receive band, and injecting into the transmitter at baseband a cancelling signal located at the duplex spacing offset from the main transmit signal. Experimental results show receiver sensitivity improvement by more than 10dB.