CMOS front-end techniques for in-band full-duplex radio

In-band full-duplex wireless communication (FD), i.e. transmission and reception at the same time at the same frequency, is an emerging research topic, driven by the increasing demand for mobile data traffic in the crowded radio spectrum. Besides promising up to twice the spectral efficiency, additional advantages are being explored. The main issue in FD is strong self-interference (SI) from the transmitter (TX) into the local receiver (RX). In typical links, over 90dB total SI-rejection is required to fully compete with half-duplex links. This rejection is achieved by combining SI-isolation and cancellation in different domains. This work shows that impairments in the radio components limit the achievable SI-cancellation in the digital domain, typically requiring at least 40dB SI-rejection in the analog and RF domain. Implementing such analog cancellation paths to achieve competitive link budgets with CMOS integration potential, small form factor, limited complexity and low power consumption remains challenging. This work studies the feasibility of FD using a custom designed CMOS front-end. Full implementation details and analysis are presented of a 65-nanometer mixer-first front-end with a vector modulator (VM) downmixer for SI-cancellation. Using this front-end, the impact of several transceiver impairments on its full-duplex operation were experimentally investigated, such as distortion, phase noise, image rejection and transmitter impairments. The receiver was found to have over 90dB linear link budget potential in a 16.25 MHz bandwidth, when combined with only 20dB worst-case antenna isolation, thanks to its highly linear rejection of SI present at the receiver input. An improved second front-end was developed in 65nm CMOS, targeting over 100dB linear link budget, supporting a significantly higher transmit power, increased receiver linearity and reduced noise floor. Transistor-level simulations of the full system showed the reduced NF, increased linearity and expected cancellation behavior. Preliminary measurements on the stand-alone power amplifier are also presented. Complete measurements are outside the time span of this thesis work. Overall, this thesis demonstrates that vector-modulator downmixers are a promising building block for SI-cancelling full-duplex front-ends, enabling highly integrated full-duplex radios that can compete with relaxed half-duplex links.