Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1998-01-30
2001-10-23
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S285000, C375S260000
Reexamination Certificate
active
06307892
ABSTRACT:
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, attenuated and 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 utilize 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 information on multiple sub-channels, the communication device comprising: means for encoding the information for each of the multiple sub-channels with a first coding scheme, said first coding scheme incorporating a second coding scheme and a transformation derived from redundancy in the second coding scheme, said first coding scheme producing codewords having pairwise Euclidean distance properties corresponding to those of the same information encoded by said second coding scheme alone, and said transformation being selected so that a modulated composite signal envelope derived from said codewords has a peak-to-mean envelope power ratio (PMEPR) reduced relative to a PMEPR for correspondingly modulated information encoded by said second coding scheme alone; and modulation means for modulating said sub-channels in accordance with said codewords to produce a composite signal envelope.
The encoding means may comprise: first encoding means for encoding the information for each of the multiple sub-channels with said second coding scheme to produce channel encoded information; and second encoding means for encoding the channel encoded information for each of the multiple sub-channels with said transformation, said transformation transforming the channel encoded information into codewords having pairwise Euclidean distance properties corresponding to those of the channel encoded information.
In a preferred embodiment, the transformation is a function of the information encoded by said second coding scheme alone and a set of training vectors which is obtained from an associative map having a ranking in ascending order of peak envelope power; and the transformation may comprise symbol-wise addition modulo-v of a constant mask vector to information encoded by said second coding scheme, where 0, 1, . . . , v−1 represent possible values for each symbol in the encoded information.
In another aspect of the present invention there is provided a method of simultaneously transmitting information on multiple channels comprising the steps of: encoding the information for each of the multiple sub-channels with a first coding scheme, said first coding scheme incorporating a second coding scheme and a transformation derived from redundancy in the second coding scheme, said first coding scheme producing codewords having pairwise Euclidean distance properties corresponding to those of the same information encoded by said second coding scheme alone, and said transformation being selected so that a modulated composite signal envelope derived from said codewords has a peak-to-mean envelope power ratio (PMEPR) reduced relative to a PMEPR for correspondingly modulated information encoded by said second coding scheme alone; and modulating the sub-channels in accordance with said codewords to produce a composite signal envelope.
The encoding step may comprise: encoding the information for each of the multiple sub-channels with said second coding scheme to produce channel encoded information; and encoding the channel encoded information for each of the multiple sub-channels with said transformation, said transformation transforming the channel encoded information into codewords having pairwise Euclidean distance properties corresponding to those of the channel encoded information.
The present invention advantageously provides a mechanism which can achieve significant improvements in PMEPR by encoding the transmitted sequence in such a way as to avoid excessive PEPs.
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Jones Alan
Wilkinson Timothy Alan
Chin Stephen
Fan Chieh M.
Juffernbruch Daniel W.
Mancini Brian M.
Motorola Inc.
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