14-08-2012, 10:18 AM
Effects of Clipping and Filtering on the Performance of OFDM
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INTRODUCTION
ORTHOGONAL frequency division multiplexing
(OFDM) is a very attractive technique for the
transmission of high-bit-rate data in a radio environment
[1]. However, any multicarrier signal with a large number
of subchannels is burdened with a large crest factor
(CF peak voltage/rms voltage). When passed through a
nonlinear device, such as a transmitter power amplifier, the
signal may suffer significant spectral spreading and in-band
distortion. The conventional solutions to this problem are to
use a linear amplifier or to backoff the operating point of a
nonlinear amplifier; both approaches resulting in a significant
power efficiency penalty.
Several alternative solutions have been proposed to reduce
the crest factor (CF) of the signal input to the amplifier [2]–[5].
One of these approaches, and the simplest, is to deliberately
clip the OFDM signal before amplification. In particular,
since the large peaks occur with very low probability [5],
clipping could be an effective technique for CF reduction.
However, clipping is a nonlinear process and may cause
significant in-band distortion, which degrades the bit-errorrate
(BER) performance, and out-of-band noise, which reduces
the spectral efficiency. Filtering after clipping can reduce the
spectral splatter but may also cause some peak regrowth. Here,
we investigate, through extensive computer simulations, the
effects of clipping and filtering on the performance of an
OFDM wireless communication system.
SIMULATION RESULTS
An initial study of the cumulative distribution function
(CDF) of the OFDM signal amplitudes [5] reveals that most
of the signals are concentrated at low amplitudes, indicating
that clipping could significantly reduce the CF with minimal
performance degradation. In the following, we concentrate on
three performance measures: power spectral density (PSD),
crest factor (CF), and bit-error rate (BER).
SUMMARY
In this letter, we investigated the effects of clipping and
filtering on the performance of OFDM. Instead of using an
absolute CF, the CF’s at various percentiles of the CDF are
used to better characterize the “peakiness” of an OFDM signal.
With a clipping ratio around 1.4 and filtering, the CF of a
bandpass OFDM signal with 128 tones at the 99.999% point
is reduced from 13 to about 9 dB, which is comparable to the
absolute CF of a raised-cosine pulse-shaped bandpass QPSK
signal. This is at the expense of only a 1-dB degradation in
the received SNR. We conclude that clipping and filtering is a
promising technique to reduce the CF of OFDM signals using
realistic linear amplifiers.