I/Q modulator image rejection through modulation pre-distortion

I/Q modulators have been widely used to perform digital modulation such as QPSK, QAM, GMSK and MCM. In practice, there exists a small non-90° phase error between the I and Q channels of an I/Q modulator. Also, the I and Q channels may not always have the same gain. The phase error and gain imbalance cause unwanted image signals that are generated in-band and consequently, degrades the system performance. This paper presents an I/Q modulator image rejection algorithm through modulation signal pre-distortion. The presented algorithm is simple and lends itself well towards a digital signal processor (DSP) implementation. Due to its simplicity, the implementation results in almost no increase in modulator complexity. Experimental results shows that for an MCM modulation system, this algorithm can reduce the image by at least 10 dB. For portable products like modems and handhelds, this feature is specially attractive because low complexity implies low cost, small product size and/or long battery life