Communications Blockset

Discrete Multitone Signaling Demo

The Discrete Multitone Signaling demo, dmt_sim, models a modulation technique that is part of the asymmetric digital subscriber line (ADSL) technology for transmitting data and multimedia information over telephone lines. The discrete multitone (DMT) signaling technique divides the channel into many subchannels and modulates each one individually. This document highlights these aspects of the demo:

• Structure of the demo, and use of Communications Blockset blocks
• An alternative model for the DMT technique, dmt_sim_alt

Structure of the Demo

This demo uses various Communications Blockset blocks to model DMT signaling. To see how the modulation or demodulation blocks are arranged, first open the DMT Modulator or DMT Demodulator systems at the top level of the model, and then look under the mask of the Modulator Bank or Demodulator Bank subsystems. Notice that each of the 16 Modulator Bank icons represents a set of 16 Rectangular QAM Modulator Baseband blocks. The DMT technique allocates different numbers of bits to different subchannels. Each copy of the modulator block acts as a distinct subchannel, and uses the 256-element vector b in the MATLAB workspace to determine the M-ary number parameter that is appropriate for that subchannel.

The demo also includes:

• A plot of the number of bits that each of the 256 subchannels transmits. To see this plot, double-click on the icon labeled, "Load and Plot Bit Allocation Vector."
• A plot of the spectrum of the transmitted signal.
• A display icon that shows the bit error rate, the number of bit errors, and the total number of bits processed.
• Frame-based processing, so that the simulation processes many bits in each time step. The double connector lines between blocks indicate frame-based signals.

Learning More About the Gray Coding Demo

Discrete Multitone Signaling Demo, Alternative Form