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.
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)
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.
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) 
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.
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 nonclipped 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) 
