US 6,175,551 B1 · Transmission system and method employing peak cancellation to reduce the peak-to-average power ratio

Abstract

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 the sampled signal, such that each subtracted reference function reduces the peak power of at least one signal sample. In accordance with the present invention, an appropriate reference function with approximately or exactly 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. 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.

Claims20

Dependent claims

A method of reducing a peak-to-average power ratio of a multi-carrier multiplexed signal to be transmitted, said method comprising the steps of:

coding a digital input data stream into blocks of N data streams, where N represents a number of subcarriers in said signal;
multiplexing data of said N data streams and modulating said subcarriers to provide symbols of modulated data;
detecting peak power amplitudes for said symbols and determining whether said peak amplitudes exceed a given threshold;
and
scaling said peak power amplitudes, if said peak amplitudes exceed said given threshold, wherein said step of scaling includes:
generating impulses having phases substantially equal to a peak phase of a corresponding one of said symbols and having an amplitude substantially equal to a peak amplitude of said corresponding one of said symbols minus a desired maximum amplitude;
lowpass filtering said impulses on a symbol by symbol basis;
and
subtracting a reference function from a corresponding symbol, wherein said subtracted reference function reduces said peak power amplitude of at least one signal sample of said corresponding symbol and said signal to be transmitted.

6 more elements

A transmitter apparatus for transmitting a multi-carrier multiplexed signal to be transmitted and reducing a peak-to-average power ratio of said signal, said apparatus comprising:

a coder for accepting an input data stream and coding said data stream into blocks of N data streams, where N represents a number of subcarriers in said signal;
a multiplexing circuit for multiplexing data of said N data streams to modulate said subcarriers and provide symbols of modulated data;
a peak cancellation circuit coupled to said multiplexing circuit for subtracting a reference function from said symbols having a peak power amplitude exceeding a predetermined threshold, such that subtraction of said reference function reduces peak power of at least one signal sample in a symbol;
and
a transmission circuit coupled to said peak cancellation circuit for transmitting said symbols representative of said signal;
wherein said peak cancellation circuit includes:
a peak detection circuit for detecting if said peak power of a symbol exceeds said threshold:
an impulse generator for generating impulses having phases substantially equal to a peak phase of a corresponding one of said symbols and having an amplitude substantially equal to a peak amplitude of said corresponding one of said symbols minus a desired maximum amplitude:
and
a lowpass filter for filtering said impulses on a symbol by symbol basis.

7 more elements

Dependencies are inferred from the claim text.

Transmission system and method employing peak cancellation to reduce the peak-to-average power ratio

Life of this patent

Transmission system and method employing peak cancellation to reduce the peak-to-average power ratio

Life of this patent

Abstract

Claims20