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 thesis we review the most popular OFDM PAR-reduction techniques and demonstrate that selected mapping (SLM) is a particularly promising reduction technique. In a SLM system, an OFDM symbol is mapped to a set of quasi-independent equivalent symbols and then the lowest-PAR symbol is selected for transmission. The tradeoff for PAR reduction in SLM is computational complexity as each mapping requires an additional inverse fast fourier transform (IFFT) operation in the
transmitter.
In additional to an overview of current SLM work, we present a thorough analysis of SLM as well as several novel SLM proposals. First, we derive the closed-form expression for the expected PAR in an SLM system. The expected PAR can be thought of as a metric of PAR reduction capability. Second, we provide a power analysis of SLM to determine if the computational power costs outweigh the power saved through PAR reduction. Through this analysis, we show that SLM is capable of several Watts of net power savings when used in a wireless transmission system. Third, we propose that a PAR threshold should be set in SLM. Such thresholding leads to significant complexity decreases. Fourth, we derive the maximum likelihood (ML) and maximum extit{a posteriori} (MAP) detection metrics for blind SLM (BSLM) and threshold BSLM respectively. Lastly, we demonstrate that by using monomial phase sequences in SLM blind phase sequence detection is possible with a single FFT operation in the receiver.
| Identifer | oai:union.ndltd.org:GATECH/oai:smartech.gatech.edu:1853/6922 |
| Date | 20 April 2005 |
| Creators | Baxley, Robert John |
| Publisher | Georgia Institute of Technology |
| Source Sets | Georgia Tech Electronic Thesis and Dissertation Archive |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Format | 932592 bytes, application/pdf |
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