Coded modulation and equalization for highly bandwidth efficient communication on broadband wireless channels
Reliable communication over a wireless channel is an inevitable part of realizing ubiquitous connectivity—the goal of modern communication systems. Efforts to realize this goal is seriously hampered by distortion of the transmitted signal due to the presence of multipath, a phenomenon which results in the reception of multiple copies of the transmitted signal, albeit at different delays and scaled by different factors. Two broad methodologies generally employed to deal with multipath are the single-carrier (SC) and the multicarrier (MC) methodologies, where the MC system in the context of a wireless channel is often implemented as an orthogonal frequency division multiplexed (OFDM) system.
This dissertation provides a theoretical comparison of the performance of the two methodologies from a channel coding, equalization and modulation perspective, taking into account their implementation complexities. This comparison is particularly relevant in the context of the current debate in the broadcasting community as regards the comparative performance of the two systems.
Further, the dissertation attempts to fix some of the biggest problems of both the SC and the MC systems. In implementing an OFDM system the following problems are encountered: (1) the presence of large peak amplitudes in the envelope of the transmitted signal and (2) when both systems employ the same channel code, the OFDM system performs poorly as compared to the SC system. An approach to designing multidimensional signal sets that results in improved performance and a lower peak amplitude of the envelope of the transmitted signal compared to the conventional OFDM system, is presented. An alternate interpretation as a hybrid SC/MC system is presented, and is shown to perform better than a standard hybrid system formed by dividing the entire available frequency band into several smaller subbands and employing a SC system on each of these subbands.
One major drawback of blind equalization schemes employed in SC systems to combat multipath is the slow convergence of the coefficients of the filters constituting the equalizer to the optimal solution. The dissertation concludes with a novel approach to hasten the convergence of the coefficients of the filters of a predictive decision feedback equalizer.