The USRP can increase or decrease the amplitude of signals before they reach the analog-to-digital converter. Gain values are in decibels (dB). The maximum effective gain for the USRP N210 is 31.5 dB. The USRP will accept higher gain values, but it will provide the higher gain by multiplying the digital samples. This does not help with signal quality, so it is usually best avoided.
The compression process on the FPGA calculates the overall amplitude in each frequency bin as the sum of the real amplitude squared and the imaginary amplitude squared. The maximum possible overall amplitude is approximately 1 billion. The FPGA sends to the host computer all frequency bins with overall amplitudes greater than the threshold.
Start with a relatively high gain. The higher the gain, the larger the signal amplitudes in the compression process will be. This will provide higher resolution when comparing amplitudes to the threshold.
Start with a threshold around 1000. Increase the threshold if you encounter overflow, and decrease the threshold if you are not receiving the signals you are interested in.
When the USRP tries to send more compressed data than the network or host computer can handle, it enters an overflow state and stops sending data.
When this happens, up to a second of signals will be missing from the output. If overflow is frequent, you will need to increase the threshold.
If the threshold is too high, the USRP may not send any data to the host computer. It will send only the average samples, which will result in a very small network traffic rate.
For Bluetooth signals sent from a device less than one meter from the receiver, a gain of 30 and a threshold of 10000 is a good place to start.