Sending data Through an Acoustic Channel

This blog post is still a work in progress

In this project for my DSP class, we used matlab (huhhhh take me back to python and scipy) for running some lab experiments about transmitting pictures through the acoustic channel. In other words, we learn how to make loud noises that sound horrible but somehow gives pretty pictures

Matlab

This project was using matlab and my goodness it is huhhh. While it is a wonderful piece of software that makes development and translation from math to code really more trivial, I can help but hate that added level of abstractions. To be fair, my opinion is totally biased since I come from the Software world so python has been my goto for the past 5 years.

In Matlab, there is a function for everything which is a blessing and a curse. While it made some bits of this project more simple since we didn’t have to reinvent the wheel, it also added some abstraction and less flexibility and we had to be a bit more creative.

Simulink, on the other hand is a treasure, I really envy this to Matlab and I think the python community should develop a similar tool which would be the absolute Matlab killer.

The project

Enough rant about my love-hate relationship with a piece of code. The project was about using OFDM on QAM symbol to send data through the air which is heavily used in Wi-Fi (with beamforming, optimization algorithm, Reed-Solomon, … but it is not the main subject of this course). The idea behind Orthogonal Frequency Division Multiplexing is quite simple and ingenious, we use various symbol that won’t interfere with each other.

We have multiple subcarrier that are designed in a way that the peak of one is located at the lobe of all of the others avoiding some interferences.

The cherry on top is the use of QAM symbol which are complex value symbols and so previously, we had to rely on sending $cos$ and $sin$ wave to mimick the quadrature and in phase parts. Which is prone to error, to frequency shift, … Modulating those QAM Symbols by using OFDM and some clever math makes each symbol a sort of raised cosine.

From complexe to sound

As you all know, sending complexe value symbol is not really a thing so we can only rely on sending real valued data so the speaker can reproduce it and transmit the data. But how to come from a QAM to a real value symbol ? DFT and some symmetry !