On Spectrally Efficient Continuous Phase Modulation

Licentiate thesis, 2000

Spectrally efficient partial response continuous phase modulation (CPM) for wireless links is considered. Continuous phase modulation defines constant envelope phase codes, which are advantageous when using non-linear amplifiers. The channel is an additive white Gaussian noise (AWGN) channel and the spectrum requirement is a mask, which puts stringent requirements on the spectral main lobe and the spectral side lobes. The thesis consist of three papers. In the first paper, a simplified receiver is evaluated for high-complexity partial response CPM on the AWGN channel. The simplified receiver consists of an approximate filter bank, using simple integrate-and-dump filters, and a reduced state sequence detector (RSSD). The complexity reduction is significant with a very small degradation for the AWGN channel, with and without co-channel interference, compared to the optimum maximum likelihood (ML) detector. The simplified receiver is more sensitive to adjacent channel interference, than the maximum likelihood (ML) detector, but a large complexity reduction is still possible with a small performance degradation. In the second paper, convolutionally encoded partial response CPM is evaluated. A search for serially concatenated convolutional codes has been performed. The somewhat surprising result is that no coding gain is obtained compared to the best uncoded CPM schemes. In the last paper, a numerical optimization of the minimum Euclidean distance with the spectrum mask as constraint and the phase pulse shape and modulation index as free parameters, has been performed. A small gain in minimum Euclidean distance and an interesting property for some optimized phase pulses, that could be useful for simplified receivers, were obtained. There seems to be a saturation effect in CPM when using long phase pulses, thus restricting the possibilities to define more powerful constant envelope phase codes within CPM. In addition, complementary simulation results for the 'best' optimized CPM scheme, taking complexity into account, are provided in an appendix. The advantageous property for the optimized phase pulse over a standard phase pulse, when using a simplified receiver, is demonstrated.