Digital compensation in IQ modulators

We consider digital signal processor (DSP) based inphase/quadrature (IQ) modulators generating continuous phase frequency shift keying (CPFSK) signals. Departures from a flat-magnitude, linear phase frequency response in the pass bands of signal reconstruction filters in the I and Q channels cause ripple in the modulator output signal envelope. Amplitude modulation (AM) in the signal envelope function produces undesirable sidelobes in the FSK signal spectrum when the signal passes through nonlinear elements in the transmission path. A structure was developed for digitally compensating for the magnitude and phase characteristics of the signal reconstruction filters by Tuthill and Cantoni (see IEEE Trans. Comm., vol.47, no.10, p.1466-69, 1999) and by Tuthill, (see Australian PhD Dissertation, Telecommunications Research Institute, Curtin University of Technology, 2000). However, this approach results in finite impulse response (FIR) filters that have a large number of coefficients. Implementation of these filters on a DSP is computationally demanding. In this paper we have generalised the technique of Tuthill and Cantoni to include infinite impulse response (IIR) filters, which have fewer coefficients. We then investigate whether a reduction in the number of filter coefficients leads to a reduction in the computational load in implementation.