OFDM communications over peak-limited channels
Baxley, Robert John
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Orthogonal frequency division multiplexing (OFDM) has become a popular modulation method in high-speed wireless communications. By partitioning a wideband fading channel into flat narrowband channels, OFDM is able to mitigate the detrimental effects of multipath fading using a simple one-tap equalizer. However, in the time domain OFDM signals suffer from large envelope variations, which are often characterized by the peak-to-average ratio (PAR). High PAR signals, like OFDM, require that transmission amplifiers operate at very low power efficiencies to avoid clipping. In this dissertation, we explore the problems associated with transmitted OFDM signals through peak limited channels. A large part of this work deals with analyzing different distortion metrics and determining which metrics are most useful. We find that the signal-to-noise-plus-distortion ratio (SNDR) is one of the most important metrics in assessing distortion in nonlinear channels. As part of this analysis, we compare sample-based SNDR and symbol-based SNDR and find that using the more comprehensive symbol-based metric as the objective in SNDR maximization algorithms leads to only marginal SNDR improvements. The SNDR perspective is also applied to existing PAR-reduction techniques to compare existing schemes and proposed new schemes. Part of this work involves deriving a SNDR maximizing adaptation of the popular PAR-reduction scheme, selected mapping (SLM). We also compare another popular PAR-reduction method, partial transmit sequence (PTS), to SLM through a variety of metrics including SNDR and found that for any given amount of complexity or side information SLM provided better performance. The next major piece of work in this dissertation addresses synchronization and channel estimation in peak-limited channels for OFDM. We build off of existing work that shows that embedded synchronization energy is a more bandwidth efficient means of synchronization than preamble-base methods. With this, we demonstrate a method for generating embedded sequences that have low PAR, and thus minimize the PAR of the combination OFDM symbol/embedded sequence among all embedded sequences. Next, we extend this work to sequences called joint synchronization-pilot sequences (JSPSs) by deriving the symbol-estimate mean squared error (MSE) pilot placements for the JSPSs and by showing how the JSPSs can be used with SLM for blind detection. Finally, the dissertation concludes with a derivation of the SNDR-optimal transmitter/receiver pairs. Using functional analysis, we show that the SNDR-optimal receivers for peak-limited transmitters are not linear. Instead they follow non-linear functions that depend on the noise and signal distributions.