Topics in orthogonal frequency -division multiplexing
Abstract
Orthogonal Frequency Division Multiplexing (OFDM) is a flexible transmission scheme which has been proposed for digital broadcast and the downlink of PCS systems. This thesis focuses on increasing the efficiency of OFDM, and also introduces a novel multiple access scheme for use with OFDM. The efficiency of OFDM is limited by length of the cyclic prefix needed to mitigate interference due to delay spread, and the number of pilot symbols needed to track channel variations due to Doppler spread, in fading multipath channels. In OFDM the cyclic prefix length is normally chosen longer than the channel impulse response, which can result in considerable overhead. We analyze the effect of reducing the length of the cyclic prefix. Also, in OFDM the use of large constellations (e.g., 16QAM versus DEQPSK) to increase capacity, requires that the channel be estimated, which is typically done with pilot based estimators. We show how to reduce the number of pilots by using a concatenated coding scheme with iterative decoding and decision directed pilot-assisted channel estimation. The above analysis and algorithms are validated by simulation on UMTS channels. When OFDM is used for multiple access, the conventional scheme is to assign each user a small subset of channels. We develop and analyze a multi-access scheme for OFDM in which all users multiplex their data onto all the subchannels. This allows each user to better exploit the diversity of the channel. We show this performs better, even when frequency hopping is used with the conventional scheme.
Degree
Ph.D.
Advisors
Gelfand, Purdue University.
Subject Area
Electrical engineering
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