Efficient implementation of digital compensation in IQ modulators

Abstract

A flexible approach to the implementation of a radio transmitter is to synthesise base-band I and Q signals using a DSP (Digital Signal Processor) followed by a vector modulator to up-convert the base-band signals directly to RF. This approach can be limited, however, by analogue reconstruction filters that are necessary to attenuate digital image components in the base-band signal spectrum before transmission. Recently, methods have been proposed to digitally compensate for these shortcomings. While these methods have been shown to be effective in improving modulator performance they require special attention to numerical issues and they typically result in large filter structures that are computationally demanding to implement. In this paper we present the results of an investigation into reducing the computational load of a digital compensation method on the DSP. The investigation has resulted in a technique that not only significantly reduces the computational burden but also improves the numerical robustness of the solution.