Subcarrier power adjustment technique for peak-to-average power ratio reduction of OFDM systems
MILCOM'06 Proceedings of the 2006 IEEE conference on Military communications
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The demand for wireless networks has been growing rapidly over the recent past due to improved reliability, higher supported data rates, seamless connectivity between users and the access point, and low deployment costs relative to wireline infrastructure. This increase in demand started with the popular IEEE 802.11b wireless local area network standard. Many recent wireless network standards are now employing multicarrier modulation in their design. Multicarrier modulation reduces the system's susceptibility to the frequency-selective fading channel, due to multipath propagation, by transforming it into a collection of approximately flat subchannels. As a result, this makes it easier to compensate for the distortion introduced by the channel. However, standardized wireless modems, such as the ETSI HiperLAN/2 and the IEEE 802.11a standards, employ the same operating parameters across all subcarriers, and thus do not exploit all the advantages offered by the multicarrier framework. This dissertation investigates techniques to further enhance system throughput performance by tailoring several operating parameters on a per-subcarrier basis. These parameters are subcarrier modulation schemes, power levels, and equalizer lengths. The idea of tailoring modulation schemes and power levels, known as bit allocation and power allocation, has been studied for many years and for many applications. This work proposes two novel discrete bit allocation algorithms that strive to reach the optimal solution in a low computational complexity fashion, while constrained to a specified error performance. A novel power allocation algorithm is proposed that satisfies regulatory requirements by obeying a frequency interval power constraint. Investigation of the third parameter, subcarrier equalizer lengths, has not been conducted before in the literature. Two algorithms are proposed that vary the lengths of the subcarrier equalizers such that the overall distortion is reduced to some specified amount, while the number of equalizer taps used by the system are kept small. Finally, the use of bit allocation is extended to the case when multiple antennas are employed by the wireless modems. Four algorithms are proposed that perform generalized antenna selection diversity at both the transmitter and receiver, in tandem with discrete bit allocation. Results show that employing two transmit and two receive antennas with discrete bit allocation can achieve an average increase in throughput of up to 33% when compared to a system without bit allocation.