Multiplex communications – Generalized orthogonal or special mathematical techniques – Fourier transform
Reexamination Certificate
1997-07-31
2001-01-16
Olms, Douglas W. (Department: 2732)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Fourier transform
C370S203000
Reexamination Certificate
active
06175551
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a system and method for transmitting digital data by multiplexing a number of closely spaced adjacent carriers, such as, Orthogonal frequency division multiplex (OFDM) and Orthogonal Code Division Multiplex (OCDM) systems, and more particularly to techniques for reducing the peak-to-average power ratio of signals transmitted in such systems.
BACKGROUND OF THE INVENTION
Orthogonal frequency division multiplex (OFDM) signals have been proposed for various communications applications. An OFDM signal consists of a number of adjacent carriers, closely spaced in frequency, and each modulated with data. With OFDM, a data stream of &khgr; bps is transmitted over N subcarriers, each carrying &khgr;/N bps. An advantage of OFDM technology is that the symbol rate of the data stream is decreased by a factor of N, which gives a proportional increase in delay spread tolerance. That is, delay spread or inter-symbol interference due to signal dispersion can be reduced or even eliminated by inserting a guard time interval between the transmission of subsequent symbols, thus avoiding use of an equalizer as required in single carrier systems. The guard time allows delayed copies of each symbol to die out before the succeeding symbol is received. Thus, OFDM is attractive because of its ability to overcome the adverse effects of multi-channel transmission without the need for equalization. Orthogonal Code Division Multiplex (OCDM), also known as multi-code CDMA is similar to OFDM, with a difference that instead of N subcarriers, N codes are transmitted simultaneously.
A disadvantage with the transmission of both OFDM and OCDM signals is that the maximum possible peak power is N times larger than the average transmitted power. This large peak-to-average power (PAP) ratio reduces the efficiency of the corresponding transmitter power amplifier. In order to avoid distortion of the transmitted signal, a backoff (which is the reduction in average transmitted power relative to the maximum possible output power) has to be used which is about the same as the PAP ratio of the input signal. There is a certain tradeoff between distortion and power efficiency. That is, efficiency can be improved by decreasing the backoff, allowing for more distortion. The amount of distortion that can be tolerated, however, depends on spectral regulations and required error rate.
Due to the tradeoff which exists between power efficiency and distortion, a need exists for a system which maximizes power efficiency while at the same time minimizing distortion.
SUMMARY OF THE INVENTION
The present invention teaches a system and method to reduce the peak-to-average power ratio of systems transmitting parallel channels. Examples of such systems are Orthogonal Frequency Division Multiplex (OFDM) and Orthogonal Code Division Multiplex (OCDM), also known as multi-code CDMA. Both systems have the disadvantage of a large peak-to-average power (PAP) ratio, which degrades the efficiency of the power amplifier of the transmitter. The present invention reduces the PAP problem by canceling the large signal peaks through subtraction of an appropriate reference function. That is, a time shifted and scaled reference function is subtracted from a sampled signal interval or symbol, such that each subtracted reference function reduces the peak power of at least one portion or signal sample of the symbol. In accordance with the present invention, an appropriate reference function with approximately the same bandwidth as the transmitted signal is selected. In this way it is assured that the peak power reduction does not cause any out-of-band interference. One example of a suitable reference signal for use in the present invention is a sinc function. The present invention also describes a variety of systems for implementation of the peak reduction technique.
In accordance with another embodiment of the present invention, forward error correction coding is applied across several OFDM/OCDM symbols, in order to correct for symbols with low average power resulting from the peak reduction. In case of packet transmission, a different data scrambling is applied for each retransmission, in order to ensure that the PAP ratios and error probabilities are uncorrelated for initial packets and retransmissions.
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Awater Geert Arnout
de Wild Arnout Hendrik
van Nee Richard D.
Gibbons Del Deo Dolan Griffinger & Vecchione
Lucent Technologies - Inc.
Olms Douglas W.
Pizarro Ricardo M.
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