Pulse or digital communications – Systems using alternating or pulsating current – Angle modulation
Patent
1998-06-21
2000-10-03
Chin, Stephen
Pulse or digital communications
Systems using alternating or pulsating current
Angle modulation
375285, 375296, H04L 2710, H04B 1500, H04K 102
Patent
active
061283517
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates, in general, to multicarrier communication systems, such as an Orthogonal Frequency Division Multiplexed (OFDM) communication system, and is particularly applicable to a mechanism for controlling the peak-to-mean envelope power ratio (PMEPR) for transmissions in such systems.
SUMMARY OF THE PRIOR ART
Multicarrier transmission schemes, such as OFDM, have been proposed for many different types of communication system, including Digital Audio Broadcasting (DAB) and Broadband Wireless Local Area Networks (LANs). The advantage of such schemes is that unlimited transmission rates are theoretically possible in highly time dispersive channels that arise from a summation of multiple-delayed, phase-shifted paths for a signal, and which therefore display a distorted characteristic. Unfortunately, the composite signal envelope produced by OFDM exhibits a high PMEPR (which term is also commonly referred to as "the crest factor"). Moreover, in order to mitigate against the effects of distortion and spectral spreading (e.g. adjacent channel splatter) in multicarrier systems, a linear (and consequently inefficient) transmit amplifier is required for amplification of this composite signal envelope.
In addition to the foregoing disadvantages, the average power of a multicarrier signal (for a specified Peak Envelope Power (PEP) limit) is considerably lower than that for a constant envelope, single carrier signal (such as a Gaussian Minimum Shift-Keyed (GMSK) signal used in cellular communication systems, for example). Consequently, the selection of a multicarrier transmission scheme for a system does not currently utilise the available power range to a maximum extent.
As such, there is a desire to reduce the PMEPR of multicarrier transmission schemes in order to obtain the inherent advantages associated with the use of multicarrier signals in the limited frequency spectrum available to communication systems, generally.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a communication device for simultaneously transmitting independent information on multiple channels as a composite signal envelope, the communication device comprising: a modulator for generating complex waveform shapes in response to data sequences applied thereto; and at least two matched filters each having a predetermined filter characteristic that is a time-reversed, complex conjugate of a complex waveform shape produced by the modulator in response to predetermined codeword vectors applied to the modulator; whereby a combined effect of the predetermined filter characteristics reduces a peak-to-mean envelope power ratio (PMEPR) of the composite signal by causing relatively large excursions in the complex waveform shapes produced by the data sequences to be unmatched in the at least two filters, while relatively small excursion in the complex waveform shapes are matched and therefore enhanced.
Typically, the predetermined codeword vectors include vectors representative of extreme complex waveform shapes generated by the modulator in response to data sequences applied thereto.
The present invention advantageously provides a mechanism which can achieve significant improvements in the PMEPR by avoiding excessive PEPs, while enhancing small excursion in complex waveform shapes generated by the modulator.
In a second aspect of the present invention there is provided a receiver for receiving the composite signal envelope of claim 1, wherein the receiver comprises a frequency domain filter having reciprocal filter characteristics to those of the at least two filters of claim 1.
In another aspect of the present invention there is provided a filter for a communication device arranged to simultaneously transmit independent information on multiple channels as a composite signal envelope, the communication device comprising a modulator for generating complex waveform shapes in response to data sequences applied thereto, the filter having a predetermined characteristi
REFERENCES:
Jones et al "Block Coding Schemes for Reduction of Peak to Mean Envelope Power Ratio of Multicarrier Transmission Schemes", Electronic Letters, Dec. 8, 1994, vol. 30, No. 25, Dec. 8, 1994.
Enright et al, "OFDM Modem with Peak-to-Mean Power Ratio Reduction using Adaptive Clipping", HF Radio Systems and Techniques, Conference Publication, Jul. 1997.
Kim et al, "Clipping Noise Mitigation for OFDM by Decision-Aided Reconstruction", IEEE, vol. 3, No. 1, Jan. 1999.
Golding Paul
Jones Alan
Chin Stephen
Haas Kenneth A.
Motorola Inc.
Park Albert
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