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Abstract
The thesis aims to investigate the gains that could be achieved if adaptive modulation schemes are used instead of the conventional systems employing fixed modulation, for different fading environments. Also in the present research, simulation tests have been made to study the performance of adaptive modulation under realistic and practical system conditions. It is found that much latency can make adaptive modulation of no beneficial use. However, suggestions, along with simulation results, that have the ability to deal with these practical issues, are presented rendering adaptive modulation feasible, realistic and efficient. It should be mentioned that Personal Communication Standards and real data from different sources are used throughout the thesis.
It is shown, that the main problem facing the adaptive modulation schemes is the signaling delay inherent to such systems. A technique has b~en proposed by Duel¬Hallen et al. [10] [24] based on long-range channel prediction for the narrowband case to alleviate the dramatic effects that this delay might have when static design is used. In this thesis we extend the use of this channel prediction algorithm to the more general case of wideband adaptive modulation systems. Significant improvement in performance was found from the simulation results of adaptive modulation assisted with channel equalization (suitable for wideoand channels) and multi-step channel prediction.
When adaptive modulation is employed with OFDM systems, significant benefits for high-speed wireless data transmission are achieved. In this thesis several bit-loading algorithms (namely, Chin Wang [17], Campello [33], Chow [15] and the Bisection algorithm [16]) were employed for adaptation purpose in the simulation process. Earlier, the use of these algorithms was limited to the wired DMT applications. Then, the TDD-AOFDM assisted with MMSE channel prediction system proposed by Munster [18] is simulated in this research. However, channel prediction was efficiently performed using the adaptive channel predictor developed by Schathuber el al. [19] for the coded OFDM systems.
vi A Brief summary of the contents of each chapter is presented below:
Chapter (I) is a general introduction about adaptive modulation and its motivations. Also, the thesis organization is presented with detailed infonnation about the contents of each chapter.
In chapter (2) a study of the perfonnance of adaptive modulation in narrowband fading environments with perfect channel state infonnation (CSI) is presented.
In chapter (3), the latencies of the adaptive modulation scheme over narrowband fading channels are investigated and channel prediction is presented as a promising tool ensuring the system feasibility.
Chapter (4) presents a study of the perfonnance of adaptive modulation assisted with channel equalization for the more general case of wideband fading channels.
In chapter (5), a more realistic adaptive wideband scheme is considered. The system limitations are investigated and suggestions to treat the latent factors are provided.
Chapter (6) presents a study of the perfonnance of adaptive multicarrier modulation (AOFDM). Adaptation is carried out by means of bit-loading algorithms available in the context ofDMT applications.
In chapter (7), adaptive multicarrier modulation with imperfect CSI is considered. Remedies for the degrading parameters are presented.
Finally, Chapter (8) presents the thesis general conclusions and provides suggestions for future work.