Digital compensation techniques for in-phase quadrature (IQ) modulator

Khang Chuang Lim

Research output: ThesisDoctoral Thesis

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[Formulae and special characters can only be approximated here. Please see the pdf version of the abstract for an accurate reproduction.] In In-phase/Quadrature (IQ) modulator generating Continuous-Phase-Frequency-Shift-Keying (CPFSK) signals, shortcomings in the implementation of the analogue reconstruction filters result in the loss of the constant envelope property of the output signal. Ripples in the envelope function cause undesirable spreading of the transmitted signal spectrum into adjacent channels when the signal passes through non-linear elements in the transmission path. This results in the failure of the transmitted signal in meeting transmission standards requirements. Therefore, digital techniques compensating for these shortcomings play an important role in enhancing the performance of the IQ modulator. In this thesis, several techniques to compensate for the irregularities in the I and Q channels are presented. The main emphasis is on preserving a constant magnitude and linear phase characteristics in the pass-band of the analogue filters as well as compensating for the imbalances between the I and Q channels. A generic digital pre-compensation model is used, and based on this model, the digital compensation schemes are formulated using control and signal processing techniques. Four digital compensation techniques are proposed and analysed. The first method is based on H2 norm minimization while the second method solves for the pre-compensation filters by posing the problem as one of H∞ optimisation. The third method stems from the well-known principle of Wiener filtering. Note that the digital compensation filters found using these methods are computed off-line. We then proceed by designing adaptive compensation filters that runs on-line and uses the “live” modulator input data to make the necessary measurements and compensations. These adaptive filters are computed based on the well-known Least-Mean-Square (LMS) algorithm. The advantage of using this approach is that the modulator does not require to be taken off-line in the process of calculating the pre-compensation filters and thus will not disrupt the normal operation of the modulator. The compensation performance of all methods is studied analytically using computer simulations and practical experiments. The results indicate that the proposed methods are effective and are able to provide substantial compensation for the shortcomings of the analogue reconstruction filters in the I and Q channels. In addition, the adaptive compensation scheme, implemented on a DSP platform shows that there is significant reduction in side-lobe levels for the compensated signal spectrum.
Original languageEnglish
QualificationDoctor of Philosophy
Publication statusUnpublished - 2004


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