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Results and Conclusions

Module by: Brian Viel. E-mail the author

Summary: The outcome of our DMT transmission system

Results

Unfortunately, our microphone-speaker system was not successful in transmitting a text message. The measured transfer function seemed reasonable since it modeled a low-pass filter. But it was ineffective in equalizing our received signal. This is most likely because the channel added far too much noise, in addition to attenuating many of the frequencies beyond recovery.

Since we were unable to acquire the desired results on bit-rate maximization and error minimization in the acoustic channel, we created an artificial channel, using our observed frequency response plus Gaussian noise. Modeling this channel in Matlab produced notable results. See the figures below.

Figure 1: This graph illustrates the fact that data rate increases as we increase block length. It also shows that a 4-bit scheme has twice the data rate of a 2-bit scheme, as one would expect.
Bit-rate vs Block Length (2 and 4 bit)
Bit rate.
Figure 2: This graph illustrates the fact that as block length increases so will the amount of errors. Also we see that the 4-bit scheme has a much greater amount of errors than the 2-bit.
Percent Error vs Block Length (2 and 4 bit)
percent.

These figures indicate that both bit-rate and error-rate go up as block length increases. This makes sense since increasing the block length increases the number of channels (Taking the iFFT of a longer signal produces more unique sinusoids.). Squeezing more sinusoid carriers over the same bandlimited channel (0-22kHz) should result in more errors in demodulation, while transmitting more bits at the same time. It also makes sense that the 4 bit constellation mapping yielded higher bit-rates and error percentages since each sinusoid carries more information, yet can more easily be approximated to the wrong constellation point.

The next project dealing with Discrete Multi-Tone modulation in the acoustic channel should certainly involve a more professional recording system.

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Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

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