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OFDM Audio Tests

By Eric Lawrey, Copyright 1997

This page shows some OFDM examples to demonstrate the effect of noise on the OFDM signal and several other tests. The aim was to be able to demonstrate these effects on the OFDM signal.


OFDM Setup

Parameter Value
FFT size 2048
Number of Carriers 800
Guard Period 512 samples
Guard Type Half cyclic extension, half zero amplitude
Sample rate of OFDM signal 44.1kHz
Bandwidth 17.5kHz

Original Sound Transmission (Music by the Corrs)


OFDM Noise Tolerance

The small sound clip was used as the data source for the OFDM transmission. The wavefile was transfered across the simulated link as channel noise was varied. This shows effect of the channel noise of the data being transfered.


QPSK Noise Tolerance

Figure 1 Complete OFDM signal for sound transmission (using QPSK)

OFDM signal (683kB)
Parameter Value
Multipath None
Peak Power Compression None
Input Data Wave Format 4.33sec, Mono, 11kHz, 8bit
OFDM signal length 15.26sec

Parameter Result
SNR 12dB
Bit Error Rate 0.000277
RMS phase error 12.1 degrees
Total Number of errors 53
Received Data SNR12dB_QPSK.wav (47kB)

Parameter Result
SNR 8dB
Bit Error Rate 0.02145
RMS phase error 19.37 degrees
Total Number of errors 4105
Received Data SNR8dB_QPSK.wav (47kB)

Parameter Result
SNR 4dB
Bit Error Rate 0.1591
RMS phase error 33.31 degrees
Total Number of errors 3.044e+004
Received Data SNR4dB_QPSK.wav (47kB)

256PSK Noise Tolerance

Figure 2 Complete OFDM signal for sound transmission (using QPSK)

OFDM signal (203kB)

The 256PSK modulation was achieved by sending each 8bit sample of the input data sound file, was sent as one carrier on one symbol. Using this, the amplitude of the input wave file is mapped to a proportional phase angle, thus any phase error occuring in the transmission results in noise on the data soud file.

Parameter Value
Multipath None
Peak Power Compression None
Input Data Wave Format 4.33sec, Mono, 11kHz, 8bit
OFDM signal length 4.702sec

Parameter Result
SNR 12dB
Bit Error Rate 0.9534
RMS phase error 11.89 degrees
Total Number of errors 4.561e+004
Received Data SNR12dB_256PSK.wav (47kB)

Parameter Result
SNR 8dB
Bit Error Rate 0.9716
RMS phase error 19.35 degrees
Total Number of errors 4.648e+004
Received Data SNR8dB_256PSK.wav (47kB)
Parameter Result
SNR 4dB
Bit Error Rate 0.982
RMS phase error 33.05 degrees
Total Number of errors 4.698e+004
Received Data SNR4dB_256PSK.wav (47kB)

256PSK wuth Data Averaging

4 repeats of each symbol were sent using 256PSK modulation. This gives the same data rate and bandwidth as QPSK. The phase angle of the 4 repeats of each symbol were averaged at the receiver, effectively reducing the noise by 6dB. This results in the signal having better performance then QPSK for SNR conditions <~9dB.

SNR 12dB
Received Data SNR12dB_4Avg.wav (47kB)

SNR 4dB
Received Data SNR4dB_4Avg.wav (47kB)

Peak Power Clipping

Figure 3 Clipped OFDM signal (clipped by 12dB)

OFDM signal (683kB)
Note: The clipped OFDM signal is much louder as the peak to RMS power of the signal is much lower, resulting in more signal energy of the same maximum amplitude. The signal was rescaled back up in amplitude after clipping.

In this test the OFDM signal is clipped so that the peak power of the signal is reduced. Clipping of the OFMD signal causes intermodulation distortion leading to an increase in the bit error rate. This test also shows the peak to RMS power ratio for the OFDM signal before and after clipping. The non-clipped OFDM signal has a peak to RMS power ratio of 13.8dB, which is quite high. This could explain why clipping the signal by up to 9dB has little effect on the error rate.

Close up of 12dB clipped OFDM signal

Click on the image for larger version

This shows a close up of the clipped OFDM signal. It can be seen that the signal is still very complex, however the signal is more dense the the original signal

Close up of original OFDM signal

Click on the image for a larger version


This is a close up of the OFDM signal. It shows that the signal simply looks like white noise.

The next table shows the effect of clipping a normal wave file. The original sound file was clipped in the same way as the OFDM signal, to give an idea of how much distortion is caused by clipping a signal. Clipping the signal doesn't have as much effect as you would expect. The music is still quite understandable.

Peak Power Compression 12dB 20dB
Clipped Original Wave File (47kB) (47kB)

Parameter Value
Multipath None
Input Data Wave Format 4.33sec, Mono, 11kHz, 8bit
OFDM Max Signal Level 0.0889
OFDM RMS Signal Level 0.0182
OFDM Peak to RMS power ratio 13.7635dB
OFDM signal length 15.36sec

The results in the above table are for the original OFDM signal before any clipping.

Parameter Result
Peak Power Compression  12dB
Bit Error Rate 0.003543
RMS phase error 15.14 degrees
Total Number of errors 678
Max Signal Level 0.02174
RMS Signal Level 0.01393
Peak to RMS power ratio 3.867dB
Received Data COMP12dB_QPSK.wav (47kB)

Parameter Result
Peak Power Compression  20dB
Bit Error Rate 0.1205
RMS phase error 29.59 degrees
Total Number of errors 2.306e+004
Max Signal Level 0.008691
RMS Signal Level 0.007097
Peak to RMS power ratio 1.76dB
Received Data COMP20dB_QPSK.wav (47kB)

Copyright © 2001 Sky DSP