Digital signal processing for reducing the effects of RF imperfections in radio devices — An overview

Building compact and low-cost yet flexible and reconfigurable radios for future wireless systems is generally a challenging task. On one hand, the needs for flexibility and re-configurability prevent using dedicated hardware particularly designed and optimized for only a single application or part of the radio spectrum. And on the other hand, to keep the overall size and cost of the radio equipment feasible, especially in multi-antenna multi-radio scenarios, the cost and size of individual radios are strongly limited. As a result, various imperfections and impairments are expected to take place in the used radio transceivers, especially in the radio frequency (RF) analog electronics. Good examples of such imperfections are, e.g., mirror-frequency interference due to I/Q imbalance, non-linear distortion due to mixer and amplifier nonlinearities, timing jitter and non-linearities in sampling and analog-to-digital (A/D) converter circuits, and oscillator phase noise. These impairments, if not properly understood and taken into account, can easily become a limiting factor to the quality and performance of the radio device and thereon of the whole wireless link. This is even more so, when more complex and more sensitive high-order modulated wideband communications waveforms are being deployed in the future systems. This article gives an overview of the essential RF impairments in modern radio communication context. Furthermore, different conceptual alternatives for reducing the effects of RF impairments in radio transmitters and receivers utilizing digital signal processing are described.