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.

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	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	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)

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)

Figure 3 Clipped OFDM signal (clipped by 12dB)

OFDM signal (683kB)

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)

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)