Adaptive modulation and predistortion techniques for nonlinear fading channels.

摘要

The radio spectrum available for wireless services is extremely scarce, while demand for these services is growing at a rapid pace and high speed wireless data transmission is requiring robust and spectrally efficient communication techniques. Hence, spectral efficiency is of primary concern in the design of future wireless communications systems. Adaptive modulation is a promising technique to achieve high average spectral efficiency over time-varying wireless channels by changing certain parameters based on channel conditions. However, unlike first and second generations, third and fourth generations wireless communication systems, due to power concern, are equipped with nonlinear High Power Amplifiers (HPA). As a result, these systems suffer from nonlinear distortions due to the existence of the HPAs. This is, of course, further exacerbated by the fading caused by the propagation channel. Predistortion is a powerful technique to overcome the nonlinear distortion by predistorting the transmitted data or by inverting the nonlinear characteristics of the HPA. In this dissertation, we have analyzed and implemented adaptive modulation and predistortion techniques for nonlinear fading channels.; We began with the general description of nonlinear communication system and its components. Performance of adaptive modulation technique over fading channels is also presented. Then we derived a general probability of a bounded Symbol Error Rate (SER) for circular and rectangular M-QAM constellations over nonlinear channels. Optimum constellation parameters for the M-QAM constellations have been calculated based on the SER performance and HPA's back off.; We then proposed two predistortion techniques to overcome the nonlinear distortion of the HPA. We applied the two proposed techniques for all M-QAM constellations. The total degradation performance of all M-QAM constellations has been calculated and compared among them. Moreover, we implemented the two techniques using Altera FPGA DSP board. The implementation has been done using Altera DSP builder blocks. In addition to that, we measured the symbol error rate performance of nonlinear communication system with and without predistortion technique as well as when the HPA is operating with optimum back off. The effects of the modeling error of the HPA have also been measured.; Finally, we compared the performance of adaptive modulation over linear and nonlinear channels, and with and without predistortion, for different normalized Doppler frequencies. The analysis is supported with simulation results. Results showed that the percentage time of using different constellations has smother distribution for linear channel case than nonlinear channel case. Results also showed that the adaptive modulation scheme over nonlinear channels allows significant spectral efficiency improvement over non adaptive system.