rad1o-id

Harnessing the awesome power of the cyberspectrum and the rad1o badge

Purpose

This project has the purpose the of showcasing the fact that you can do arbitrary stuff if you have an SDR frontend.

The idea is that two people meet, bring their rad1os into proximity and exchange a virtual business card.

Architecture

Development happens in multiple steps:

1. Software implementation of both sides of the transaction in GNU Radio
2. Porting of the transmitter side to rad1o firmware
3. If that makes sense later on, porting of the receiver side, too.

Theory

specific words might be worth reading up on Wikipedia if you don't know what they mean.

This is software defined radio. So you can't just send bits to the transmit antenna and hope that they trickle out of the receiver antenna; that's not how physics works.

So what happens is that you take the input (vCard file) and map the bits to complex numbers; these then go through pulse shaping, to give you a digital complex baseband signal. That's what the DAC on the rad1o can convert to an analog I/Q baseband signal and what the frontend chip can mix/upconvert (direct conversion) to the RF bandpass signal, which gets amplified and sent to the antenna.

On the receiver, the signal coming from the antenna goes through the downconverter and then the ADC, which gives us complex digital baseband again. Feeding that to a filter that is the time-inverse of the TX pulse shape FIR filter eliminates a lot of noise, so the SNR is maximized. This is called matched filtering.

Now we again have stream of complex numbers. These are related, but not the same as the transmitted complex numbers: Since we don't know the time the RF signal was "in flight" and also not the offset between the oscillators used in mixing or the timing offset between the DAC and ADC, there is a unknown phase offset. But that is nothing more than a complex constant multiplied (exp(j*2pi phi)). So we either need some way of measuring that, or we need a mapping/demapping from complex numbers to bits that is invariant to phase shifts. We choose the latter, by using differential encoding, where only the difference of the argument of the complex numbers matters. Hurray!

To let the receiver know that a transmission starts, we need to prepend our data with a preamble, and look for that on the receiver. That inherently gives our transmission a packet structure. Now, if we do the preamble anyway, we can also add a length field right after the preamble, and a CRC for the data. Now, that's what you'd usually call a header :)