Pulse or digital communications – Systems using alternating or pulsating current – Antinoise or distortion
Reexamination Certificate
1998-12-31
2002-09-03
Chin, Stephen (Department: 2734)
Pulse or digital communications
Systems using alternating or pulsating current
Antinoise or distortion
C375S259000, C375S267000, C375S278000, C370S205000, C370S206000
Reexamination Certificate
active
06445747
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to communication systems with multi-tone transformation and, in particular, communication systems in which the multi-tone transformed signals are phase shifted in order to reduce their peak to average power ratios.
BACKGROUND OF THE INVENTION
Communication systems with multi-tone transformation are well known in the art. The multi-tone transformation is also referred as a discrete multi-carrier modulation or orthogonal frequency division multiplexing (OFDM).
Multi-tone transformation has widespread practical communication applications including communication systems using twisted pairs of wires which provide high speed wireline communication connections. Further, its hardware implementation lends itself to discrete Fourier Transformation (DFT) structures which are both inexpensive and well understood.
FIG. 1
illustrates a conventional multi-tone transformation transmitter
101
which includes a channel coder
103
, a constellation mapper
105
, an Inverse Discrete Fourier Transform (IDFT) device
107
and an RF amplifier
109
. Illustratively, the output of RF amplifier
109
is provided on a pair of twisted wires
111
or over a wireless communication channel. The channel coder
103
receives a plurality of input bits. The combination of the channel coder
103
and the constellation mapper
105
converts the received input bits to constellation symbols. The constellation symbols are input to the IDFT device
107
, and its output signal is fed to the RF amplifier
109
. With certain combinations of the constellation symbols, the output signal from IDFT device
107
has peak signals having larger signal magnitudes than that of average signals. This requires that the RF amplifier
109
operate linearly for a wide range of magnitudes in order to avoid clipping the peak signals. Amplifiers that have a wide dynamic range are expensive. Furthermore, practical amplifiers with reasonable dynamic range saturate at a certain peak and may attenuate the peak signals. This is known as the peak to average signal power ratio problem.
A number of possible solutions to the peak to average signal power ratio problem have been suggested. For example, U.S. Pat. No. 5,835,536 (May), which is incorporated herein by reference, teaches a communication system which generates time-domain discrete multi-tone symbols. Its peak to average power requirements are reduced by (1) comparing the magnitude of the time-domain discrete multi-tone symbols with a magnitude threshold and (2) clipping the magnitude of those time-domain discrete multi-tone symbols which have larger magnitudes than that of the magnitude threshold. However, by clipping the magnitudes, it loses information.
In another example, U.S. Pat. No. 5,838,732 (Carney) discloses a multi-channel wireless communication system which produces digital baseband modulated signals by modulating digital baseband channel signals according to its desired interface standard. In order to reduce its peak to average variance, the digital baseband modulated signals are then phase shifted by digital phase shifters.
SUMMARY OF THE INVENTION
The present invention provides a simplified and efficient method and system for reducing the peak to average power ratio (PAPR) in communication systems having multi-tone transformation. This is achieved without changing the error correction capability of the overall system, the encoding rate of its transmitter, and the decoding complexity of its receiver.
The method of the present invention includes the steps of receiving at least one block of bits of information and then generating a codeword which includes an N number of digital multi-tone symbols based on the block of input bits, wherein N is a positive integer. The method further includes the steps of calculating a plurality of phase shift values for the digital multi-tone symbols that minimizes the PAPR and phase shifting the digital multi-tone symbols by the phase shift values, to thereby reduce the PAPR.
The system of the present invention includes an encoder configured to receive a plurality of input bits and to generate a plurality of coded symbols based on the plurality of input bits and a phase shift calculator configured to calculate a plurality of phase shift values for the coded symbols that minimizes the peak to average power ratio. The system further includes an encoding phase shifter coupled to the encoder and the phase shift calculator, configured to receive the phase shift values from the phase shift calculator and configured to phase shift the plurality of coded symbols to generate a plurality of phase shifted symbols and an amplifier coupled to the encoding phase shifter and configured to amplify a plurality of symbols it receives.
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Jones et a., Combined Coding for Error Control and Increased Robustness to System Nonlinearities in OFDM, IEEE, 46th Vehicular Technology Conference, vol. 2, pp. 904-908, 1996.*
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Jafarkhani Hamid
Tarokh Vahid
AT&T Corporation
Chin Stephen
Liu Shuwang
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