I am developing software for transmitting data from a computer to a mobile device using audio. Specifically, from the computer's speaker to the device's microphone. (I am developing both the sender and the receiver software.) This is one channel, and the sample rate is a standard 44.1Khz. It needs to be VERY resilient to noise - it will be used in environments with street noise, people talking, background music, etc., which may be 10dB louder than the signal itself.
Knowing almost nothing about DSP, thus far I have apparently implemented frequency-division multiplexed on-off keying, without realizing it until now :) Basically each of 96 frequencies corresponds to a bit, and on the receiving end, if the frequency is present, the bit is on. I then use some custom error correction code to whittle those 96 bits down to 8 very reliable bits.
My system works very well and is very robust, but unfortunately has an EXTREMELY low data rate (~80 bits/sec). I have some ideas about how to improve bandwidth, like using more than 96 frequencies, and using the phase component to encode additional data. Also it appears I should look into frequency-shift keying and phase-shift keying.
So - my question is simply this:
What are the options for encoding data in a noisy broadcast audio signal, and what kind of bandwidth and error rates can be expected with each technique?