Linearization techniques for wideband, low-noise, CMOS software-defined radio receivers

Abstract

A Communication revolution is happening with mobile communication equipment getting cheaper and feature rich, thanks to the continuing scaling of the digital CMOS process technology. In contrast, spectrum as a resource is becoming increasingly scarce, fragmented and crowded. Each country has its own spectrum allocation laid out for the various services making it challenging to design communication equipment that can operate across the globe. Especially, receiving weak wanted signals in the presence of strong out-of-band (OOB) interference which are quite close to the wanted signal band edge poses a challenging problem. This problem is traditionally mitigated by using dedicated and expensive high-quality band-pass filters in front of the receivers. In multi-standard mobile phones, these filters increase the cost and size of the phones. This is because these filters are not tunable, i.e. they work in a fixed frequency band, which makes them unusable if many radio standards in many different frequency bands need to be supported.Software-Defined Radio (SDR) receivers reduce the cost and form-factor of the receivers by using a re-configurable wide-band RF front-end while avoiding dedicated filters. However, this requires very linear front-end circuits and ways to implement the filtering on chip. To reject an OOB interferer, filtering requires separation in frequency of the interferer from the wanted signals. This was traditionally achieved by allocating guard bands between the frequency bands in which no emissions were allowed. However, to improve spectral efficiency these guard bands are made small or are completely eliminated. Furthermore, newer standards support wider bandwidths for higher data rates. In such a wideband receiver, even with some guard bands, the OOB intereferers are so close to the wanted in-band signals that they are hardly filtered. Therefore, they are treated as effectively in-band signals. To improve the receiver's linearity, frequency translated filtering such as N-path techniques can be used to reject OOB interference but this is less effective to handle close-in OOB interference particularly for wideband radios.In this thesis, linearization techniques for wideband, low-noise, CMOS SDR receivers are presented for both in-band or close-in OOB as well as far OOB interference. Existing SDR receiver techniques are systematically reviewed to arrive at a receiver architecture using a frequency translated noise cancellation technique which is suitable for wideband and Low-Noise v vi ABSTRACT performance.In such a receiver, the RF transconductor which forms a part of the Low-Noise Transconductance Amplifier (LNTA) is the linearity bottle-neck for far OOB interference. A CMOS inverter as a transconductor can potentially achieve good linearity but relies on distortion cancellation making its linearity performance sensitive to process, voltage and temperature (PVT) variations. Therefore, a new PVT robust RF transconductor which uses resistive degeneration in combination with a "floating battery" techniqu...