A Multimode 5–6 GHz FD/HD/FDD Quadrature Balanced CMOS Front End With Transmit–Receive Cross-Modulation Distortion Correction

Abstract

We introduce a multimode and wideband quadrature balanced CMOS RF front end (RFFE) supporting full-duplex (FD), half-duplex (HD), and frequency-division duplex (FDD) operation at the 5-6 GHz band. The proposed RFFE is enhanced by nonlinear digital algorithms that maintain optimal performance at high transmit (Tx) power levels. A theoretical model of the transmit-receive (TR) cross-modulation (CM) distortion mechanism stemming from instantaneous variations in the power amplifiers (PAs) output impedance is laid out and verified in simulation and measurement. A 2-D digital postdistortion (2-D DPoD) algorithm that utilizes the known baseband Tx signal and the downconverted distorted Rx signal is proposed to linearize the CM-distorted Rx signal, in conjunction with wideband digital RF and baseband cancellation of the Tx self-interference at the receiver input and 1-D digital predistortion to linearize the Tx signal. The system was characterized in simultaneous TR operation using 802.11ac VHT Wi-Fi signals at 10 dBm average Tx power. In the FD mode using 20 MHz signals, TR isolation of 66 dB was achieved, and the 2-D DPoD corrected Rx error vector magnitude (EVM) by 11 dB to a value of -33 dB. In the FDD mode with 800 MHz frequency separation, 14 dB improvement was demonstrated in Rx EVM with 2-D DPoD to the levels of -35 and -31 dB for 20 and 160 MHz signals, respectively. Similar EVM values were measured in the HD Rx mode at -40 dBm average Rx power, along with Rx insertion loss lower than 1 dB without using a TR switch.