Modulation Predistortion Technique for Polar Transmitter

Orthogonal Frequency Division Multiplexing (OFDM) has become a cornerstone of modern wireless communication due to its robustness against multipath fading and high spectral efficiency. However, its practical deployment in polar transmitters is challenged by two critical impairments: the high peak-to-average power ratio (PAPR) of amplitude and abrupt discontinuities in the phase trajectory. These effects lead to power amplifier inefficiency, spectral regrowth, and degraded modulation accuracy. This thesis investigates digital signal processing techniques for mitigating these impairments in the context of an all-digital polar modulator. The first part focuses on amplitude-domain Crest Factor Reduction (CFR) through a window-based peak cancellation approach, compared against amplitude clipping. MATLAB simulations demonstrate that the proposed CFR scheme achieves substantial PAPR reduction while maintaining lower error vector magnitude (EVM) degradation and superior compliance with the IEEE 802.11a spectral mask. The second part addresses phase-domain discontinuities by introducing a phase smoothing algorithm based on excess redistribution across a short sample window. This method effectively suppresses instantaneous frequency spikes while preserving accumulated phase integrity. Performance evaluation highlights the trade-off between spectral containment and modulation accuracy, with an intermediate threshold providing the best balance. Both algorithms were implemented in fixed-point arithmetic and translated into fully pipelined Verilog RTL architectures, including a CORDIC-based Cartesian-to-polar converter. Comprehensive MATLAB and RTL co-simulation confirmed bit-true equivalence, identical EVM and PAPR results, and full spectral compliance. The outcomes of this work demonstrate that the integration of CFR and phase smoothing in a polar transmitter significantly improves spectral efficiency and power amplifier compatibility without compromising modulation fidelity.