A generalized prefix for multi-carrier modulation

Torrey Frank, Purdue University

Abstract

Wireless communication systems are pervasive today. There is an ever increasing need to increase throughput and to make more efficient use of the available spectrum for data transmission (i.e. the wireless channel). Orthogonal Frequency Division Multiplexing is a modulation technique that assists in achieving these goals. This technique allows multiple information symbols to be transmitted on orthogonal sub-carrier frequencies and then recombines them to recover the transmitted information at the receiver. To separate the OFDM information symbols at the receiver and avoid Inter-Symbol Interference (ISI), one approach is to add an information buffer or prefix before each OFDM symbol. In most practical wireless systems, a cyclic prefix is used for this purpose. Although the cyclic prefix prevents ISI, it does not prevent loss of data in frequency-selective fading channels. In such a channel with spectral nulls or deep fades, data transmitted along affected frequencies is lost and the bit error rate (BER) has an error floor. In this research, a generalized prefix is advanced which prevents ISI as well as transforms frequency-selective fading channels into channels without spectral nulls thus obliterating the error floor which constrains systems using the conventional cyclic prefix. The proposed system is comparable in complexity to systems implementing the cyclic prefix and as such has similar associated cost and power requirements while offering significant improvements in BER performance in frequency-selective fading channels. This is the case because the generalized prefix repairs any available channel with spectral nulls in a Single-Input Single-Output (SISO) system and transforms the existing channel into one allowing lower BER. This research also investigates the impact of the generalized prefix on communication systems which already exploit diversity gain techniques. The diversity technique to be examined is spatial or antenna diversity in which multiple antennas are used to mitigate the effects of a poor channel by providing alternate paths to the receiver. In examination of this technique, Space-Time Block Coding (STBC) will be employed to encode and decode data in a wireless system. The generalized prefix approach, therefore, produces a wireless system that is of comparable complexity with the cyclic prefix scheme and offers significantly better BER performance in channels with spectral nulls or deep fades. In addition, with similar complexity to the conventional approach, the generalized prefix method offers a practical alternative to existing systems.

Degree

M.S.E.

Advisors

Cooklev, Purdue University.

Subject Area

Electrical engineering

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