A Full-Duplex Quadrature Balanced RF Front End With Digital Pre-PA Self-Interference Cancellation

This article presents a quadrature balanced radio-frequency (RF) front-end transmitter (TX) architecture assisted by a digital self-interference cancellation (SIC) algorithm for simultaneous transmission and reception in next-generation full-duplex radios. The proposed quadrature balanced power amplifier (QBPA) topology is a four-port circuit with an embedded passive primary isolation (PI) between the antenna and the receiver (RX) low-noise amplifier (LNA), along with a very low power SIC injection mechanism that reuses the TX’s gain. A transmit–receive (TR) isolation of >57 dB is measured for continuous-wave (CW) signals over an 80 MHz bandwidth (BW). A TR isolation of >50 and >56 dB is demonstrated for an 80 MHz 802.11ac orthogonal frequency-division multiplexing (OFDM) signal at 10- and 20 dB backoff from 30 dBm peak TX power ( $P_{\text {TX,max}}$ ), respectively, within the 2.4 GHz Wi-Fi band. A digital predistortion algorithm is applied to linearize the TX at the high output power case, achieving error vector magnitude (EVM) lower than −34.7 dB for concurrent TX-SIC operation. A simultaneous TR scenario is demonstrated, measuring an RX signal EVM of −22 dB, while $P_{\text {TX,max}}\,\,=$ 30 dBm. Measured RX loss is lower than 1.8 dB within the operating frequency band.