25-09-2013, 03:43 PM
PAPR Reduction Technique Using PTS Scheme in OFDM
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Introduction
OFDM is a powerful modulation technique being used in many new and emerging broadband communication systems.
OFDM is a multi channel modulation system employing FDM of orthogonal sub carriers.
OFDM is a combination of Modulation and Multiplexing.
OFDM has developed into a popular scheme for wide band digital communications.
Peak-to-Average Ratio of OFDM Signals
An OFDM signal consists of a number of independently modulated subcarriers, which can give a large peak-to-average power ratio.
High peak-to-average power ratio
Problem 1. It increased complexity of the analog-to-digital and digital-to-analog converters
Problem 2. It reduced efficiency of the RF power amplifier
The PAPR reduces the efficiency in the sense that a higher input back off factor is needed before the peaks in the signal experience significant distortion due to power amplifier nonlinearity.
The effect of High PAPR
High PAPR implies that the peak values of the signal is greater than the RMS value.
The Frequency of peak occurrence is low.
Due to the large number of sub-carriers in typical OFDM systems, the amplitude of the transmitted signal has a large dynamic range, leading to in-band distortion and out-of-band radiation.
In AM Systems:
DAC/Power Amplifier with larger dynamic range
Increased Power consumption, component cost.
In FM Systems:
Less Average Deviation
Low SNR at the FM receiver
Considerations on PAPR reduction
In order to improve the system performance, PAPR should predict the amount of distortion introduced by the nonlinearity
PAPR increases with the number of subcarriers in the OFDM signal.
The distortion term and the uniform attenuation and rotation of the constellation only depend on the back-off.
The effect of a nonlinearity to an OFDM signal is not clearly related to its PAPR
There will only be a BER performance improvement when the effect of reducing the in-band distortion becomes noticeable and more important than the loss of power efficiency.
Space Frequency Block Code
A number of approaches have been proposed for the OFDM PAPR Problem
SFBC is a simple and powerful technique for OFDM PAPR Problem
Advantages:
To reduce the PAPR of SFBC MIMO-OFDM signals, Which could provide good PAPR reduction with low computational complexity.
The improved AMS method without side information for 4-QAM and 16-QAM modulations.