Enhanced diversity for MIMO-OFDM via multidimensional code-spreading
Abstract
Orthogonal Frequency Division Multiplexing (OFDM) is a popular technique used to combat frequency selective fading in multipath channels. When spectral nulls are present in the channel, they can severely degrade or cancel out OFDM tones, resulting in an irreducible error rate. To combat this, standards typically employ forward error correction (FEC) techniques. Code-Spread OFDM (CS-OFDM) combines the characteristics of OFDM and Code Division Multiple Access (CDMA) to create a more robust modulation scheme which provides substantial performance improvements relative to standard OFDM. In CS-OFDM, each sinewave carries a weighted sum of all the information symbols being transmitted in an OFDM block interval. The MMSE estimator is derived for each symbol for a number of cases, including when the number of carriers is greater than the number of symbols. To enhance the BER performance while maintaining a low receiver complexity, subcarrier grouping is combined with the Local Maximum Likelihood (LML) method of Rugini et al. This dissertation expands on idea of code spreading for MIMO-OFDM and proposes a space-time-frequency coding scheme that achieves full rate for an arbitrary number of antennas with complex-valued information symbols. In addition, the proposed scheme provides full diversity with ML receivers. In the new scheme, the spatial and frequency spreading is followed by a temporal spreading that facilitates simple separation of the respective signals sent by the various transmit antennas. This temporal spreading enables perfect separation of the superimposed transmit signals at the receiver prior to channel estimation. Simulations are presented demonstrating the efficacy of the new scheme and is compared to Orthogonal Space Time Block Code (OSTBC) based schemes and the performance of MMSE, ML, and LML receivers are also noted.
Degree
Ph.D.
Advisors
Zoltowski, Purdue University.
Subject Area
Electrical engineering
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