Phd Defence Dirk-Jan van den Broek

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 ever increasing demand for mobile data traffic in the crowded radio spectrum. Besides a theoretical doubling of the spectral efficiency, the inherent channel reciprocity is an attractive physical layer aspect. In higher network layers, additional advantages are being explored such as collision prevention, low latency, security and simplified frequency planning.


The main issue in FD is strong self-interference (SI) from the transmitter (TX) into the local receiver (RX). In typical links, the transmitted signal is in excess of 90dB above the system noise floor, necessitating over 90dB total SI-rejection to fully compete with half-duplex links. This rejection is achieved by combining isolation at the antenna interface with SI-cancellation in different domains.

Impairments in the radio components limit the amount of achievable SI-cancellation in the digital domain. As a result, a system-level study shows that the SI should be rejected by at least 40dB in the analog and RF domain. In most scenarios, this cannot be achieved by antenna isolation alone, and requires analog SI-cancellation paths that adapt to a varying antenna environment. In conclusion, SI-cancellation techniques across multiple domains have to be combined, and achieving 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, as opposed to using commercially available components. 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 the implemented front-end as a research vehicle, several transceiver impairments that may limit 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 21.5dBm effective IIP3 for SI present at the receiver input. It offers up to 27dB cancellation of up to -16.4dBm SI at the RX input without generating distortion above the noise floor.  The co-integrated transmitter was found to have almost sufficient performance to support this link budget, requiring minor linearization by e.g. pre-distortion. Transmitter and SI-cancelling receiver operate jointly over a wide range of center frequencies from 0.15 to 3.5GHz. Implementing a complete point-to-point link with digital cancellation is beyond the scope of this thesis.

An improved second front-end was developed in 65nm CMOS to investigate the limits of the proposed architecture and to support future research at system level. It targets over 100dB linear link budget, supporting a significantly higher transmit power (from 3.6 to 16dBm average output) and reduced noise floor (from 12.3 to 8dB worst-case noise figure). This required re-design of the VM and RX mixer for an effective IIP3 in the range of 30-40dBm. A two-stage class-AB power amplifier with partial on-chip matching was implemented to support the higher transmit power over a wide bandwidth of 1.5-4GHz. Also, the baseband section was revised to provide higher gain and lower noise.

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.

Overall, this thesis demonstrates that vector-modulator downmixers are a promising building block for SI-cancelling full-duplex front-ends. Owing to its high linearity, the proposed architecture can improve upon a low and varying isolation at antenna-level without generating SI-induced distortion above the noise floor. This enables highly integrated full-duplex radios that can compete with relaxed half-duplex links.