Demonstrate the use of a communications layer using simple modulation and bursts to send user datagrams over the air. This example requires SDR hardware with timed burst support and was demonstrated on a single Blade RF -- looping-back through the antennas.

Find the resource files for the modem demonstrations here:

• https://github.com/pothosware/PothosDemos/tree/master/modem_examples

The transmit chain modules a user-provided payload from an input chat box. The modulation is QPSK (2 bits per transmit symbol).

• The transmitter chain first converts the payload from a packet of bytes into a packet of symbols.
• The packet of symbols is transferred into the stream domain to be processed by blocks that operate on stream-oriented data.
• The symbol mapper converts a stream of symbols into QPSK constellation points.
• The frame inserter applies a frame header so that the receiver can correct for frequency, phase, and timing error
• The FIR filter performs pulse shaping and interpolation to 4 samples per symbol.
• Finally the burst timer schedules burst transmission for the SDR sink block.

The receiver chain demodulates the QPSK and sends the user's payload to the output chat box to be displayed.

• The frame sync searches for a frame header and applies phase and timing recovery to the output payload.
• The symbol slicer re-maps the constellation points (with error) to our original 2-bit symbol set.
• The stream of symbols are converted into the packet domain as a packet of symbols.
• The packet of symbols is converted into a packet of bytes containing the user's message.