These screen-shots are taken from a simulation for LDPC-codes with a rate of 0.25. In the figure to the left you can see the setup of the radio. Only one of the two radio-parts are shown. The chain to the left is the synchronisation chain, that is a set of modules used to generate a synchronisation signal and some initialisation-data to be used by the second (not shown) radio.
The second chain shows the LDPC-receiver together with an error-counting module. By the time of this snapshot, the SNR was down to -1.05 dB (Es/N0). As you can see on the 'test_data_rcv' module, there were 150 wrong data-bits received.
Using the visualisation-tool included in the software-radio, it is possible to draw a graph of a set of measurements. In the following image, you can see the distribution of the error with respect to the SNR (Eb/N0). You could even export it either as postscript file or as matlab-function to do further data-processing.
Lots of data-points from a simulation.
Just to give an idea what -1.05 dB means to a signal, the next screenshot shows the I/Q symbols received at this signal-to-noise ratio. As you can see, it's barely visible that there is anything else than noise. And still the LDPC-decoder is capable of decoding most of the bits.
The I/Q-diagram of the received signal at -1.5dB SNR!