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UBC Theses and Dissertations

Differentially detected GMSK systems in the presence of adjacent channel interference and nonlinearities Toor, Jagdeep S.


In this thesis, the performance of differentially detected Gaussian Minimum Shift Keying (GMSK) systems operated in the presence of adjacent channel interference (ACT), modulation errors, amplifier nonlinearities and additive white Gaussian noise (AWGN) is investigated. We first evaluate by means of computer simulation the bit error rate (BER) per formance of conventional and decision feedback 1- and 2-bit differential receivers in the presence of static and Rayleigh faded ACT. The obtained BER performance evaluation results indicate that the decision feedback receivers outperform the con ventional differential receivers. For the static ACT channel, it was found that the best BER performance was achieved by the 2-bit decision feedback differential receivers. For example, at a BER=13Cr and at a carrier-to-interference ratio C/IA, these receivers resulted in gains in excess of 6 dB as compared to the conventional 2-bit differential receivers. For Rayleigh faded ACT channels, the BER performance evaluation results have indicated that the decision feedback differential receivers provide gains in the form of error floor reduction. Secondly, we have investigated, again by means of computer simulations, the effects on the BER of the cascade of an imperfect GMSK quadrature modulator followed by a nonlinear amplifier. We have considered a generic model for the imperfect modulator and have adopted two different sets of operating conditions (one typical and one extreme). In addition, we have considered two types of nonlinearities (one mild and one strong). For all the results obtained, it was found that the decision feedback differential receivers perform better than the conventional receivers for both typical and extreme values of the quadrature modulator errors as well as for both nonlinearities considered. It is also found that for the system operation under extreme operating conditions, the 1-bit decision feedback differential receiver outperforms all other receivers considered in this thesis. For example, it offers a gain of 8 dB over 2-bit decision feedback receiver at BER=10 3and C/IA=20 dB. However, for system operation under typical operating conditions 2-bit decision feedback receiver has the best performance when compared to the other receivers considered in this thesis. Finally, in order to experimentally verify the effectiveness of the decision feed back differential receivers, we have designed, implemented and tested a prototype GMSK system. Various experimental BER performance evaluation results are re ported for receivers employing the 1-bit conventional and decision feedback dif ferential detection and operated in the presence of static and Rayleigh faded ACT. The obtained experimental BER results are in agreement with equivalent computer simulated BER results.

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