A New Technique for Digital Pre-compensation in Iq Modulator

In DSP based IQ modulators generating CPFSK signals, departures from a flat-magnitude, linear phase frequency response in the pass bands of analogue reconstruction filters in the I and Q channels cause ripples in the modulator output signal envelope. These ripples then produce undesirable side-lobes in the output signal spectrum when the signal passes through non-linear elements in the transmission path. In this paper, we present a new technique of designing digital FIR pre-compensation filters in IQ modulators. These FIR filters are found by minimizing the mean square error (MSE) between the desired channel response and the response from the low-pass analogue reconstruction filter. This technique does not require the estimation of the I and Q channel analogue reconstruction filter impulse responses. Furthermore, we propose a time-domain approach based on decimation and interpolation to overcome the problem of ill-conditioned matrix and to reduce the computational load of the long FIR filters placed on the DSP. These techniques have shown to be effective in improving the modulator performance by achieving substantial reduction on the output envelope ripples. Key Words, Digital compensation, IQ modulator, Decimation, Interpolation