Optimal OFDM design for time-frequency dispersive channels

• Published in: IEEE transactions on communications Vol. 51; no. 7; pp. 1111 - 1122
• Main Authors: Strohmer, T., Beaver, S.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.07.2003; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Channels; Communication channels; Design engineering; Dispersion; Dispersions; Doppler effect; Frequency division multiplexing; Interference; Intersymbol interference; Lattices; OFDM; Optimization; Orthogonal Frequency Division Multiplexing; Protection; Pulse shape; Pulse shaping methods; Robustness; Shape; Studies; Time frequency analysis
• ISSN: 0090-6778; 1558-0857
• DOI: 10.1109/TCOMM.2003.814200

Transmission over wireless channels is subject to time dispersion due to multipath propagation and to frequency dispersion due to the Doppler effect. Standard orthogonal frequency-division multiplexing (OFDM) systems, using a guard-time interval or cyclic prefix, combat intersymbol interference (ISI), but provide no protection against interchannel interference (ICI). This drawback has led to the introduction of pulse-shaping OFDM systems. We first present a general framework for pulse shape design. Our analysis shows that certain pulse shapes proposed in the literature are, in fact, optimal in a well-defined sense. Furthermore, our approach provides a simple way to adapt the pulse shape to varying channel conditions. We then show that (pulse-shaping) OFDM systems based on rectangular time-frequency lattices are not optimal for time- and frequency-dispersive wireless channels. This motivates the introduction of lattice-OFDM (LOFDM) systems which are based on general time-frequency lattices. Using results from sphere packing theory, we show how to design LOFDM systems (lattice and pulse shape) optimally for timeand frequency-dispersive channels in order to minimize the joint ISI/ICI. Our theoretical analysis is confirmed by numerical simulations, showing that LOFDM systems outperform traditional pulse-shaping OFDM systems with respect to robustness against ISI/ICI.