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This module shows the results and conclusions for our DMT project.


In order to test our DMT system, we investigated how the probability of bit error changed with the signal-to-noise ratio of the transmitted signal in our channel. In particular, we compared how these PE - SNR curves changed as the cyclic pad length was varied. As a comparison, we also looked at how DMT stacked up with a simple model of single-carrier modulation in these same regards.

Dmt vs single - carrier (sc)

We ran our tests of the system with the following constraints:

  • Bitstream length: 512
  • Block-size: 128 (DMT), 1 (SC)
  • AWGN amplitude: .00001 - 1
  • Channel impulse response length: 1 (wire), 16
  • Cyclic prefix length (DMT): 0, 8, 16
  • Impulse noise: 0
  • Constellation size: 16-point
The results of our tests were as expected. For wire channel, SC and DMT were comparable. They both demonstrated a Q-function behavior, dropping to zero error once the SNR was high enough and leveling off to about 0.5 (i.e. a coin flip) for very low SNR. For impulse response length L less than cyclic prefix C , DMT dropped to a error fell from 0.5 as SNR increased, but leveled off once ISI began to have a more prominent effect. Single-carrier also demonstrated this same behavior, though there was no cyclic prefix involved. This was a little strange, but this project's goal was to study DMT, not SC. Once the cyclic pad length was larger than the impulse response length, DMT again displayed Q-function behavior as ISI was no longer a problem.

L = 1

This is shows how bit error probabilities compare for DMT (red) and SC (blue) with a wire channel impulse response (length L = 1).


This is shows how bit error probabilities compare for DMT (red) and SC (blue) with a length L = 16 channel impulse response and a DMT cyclic prefix of length C = 8.


This is shows how bit error probabilities compare for DMT (red) and SC (blue) with a length L = 16 channel impulse response and a DMT cyclic prefix of length C = 16.


From our results and experience implementing our DMT system, we reached the following conclusions:

  • It is possible to implement a working DMT system in MATLAB. It looks as though a more complicated extension of the system, possibly involving variable frequency allocation, a time-varying channel freq. response, more complicated constellations or a transmitter-receiver synchronization process, should also be quite feasible.
  • DMT's advantages over other modulation schemes are difficult to see with such a simple system; they would be more apparent in a system including a more realistic model of data assignment to the various frequencies (instead of evenly distributing all data over the first 256).
  • The need for a cyclic prefix is readily verifiable; as soon as the prefix length is correct, bit error drops to zero at high SNR.
  • Communications engineering and DSP are fun (except when you have to stay up all night writing connexions modules)! Looking forward to 431 next semester!

Home | Previous: Implementation | Next: Group Members

Questions & Answers

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Source:  OpenStax, Ece 301 projects fall 2003. OpenStax CNX. Jan 22, 2004 Download for free at http://cnx.org/content/col10223/1.5
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