I used an RF generator transmitting FSK packets with a datarate of 50 kbps. The deviation was 12.5 kHz. With this combination of datarate and deviation the modulation index is 0.5. I found that my device undertest had an Rx sensitivity of ~102 dB. The IF BW is approximately 100 kHz.
I left DUT settings untouched. I changed the deviation of the signal sent by the generator from 12.5 kHz to 25 kHz. Datarate remained 50 kbps. This equals to modulation index = 1.
I found out that the sensitivity was a few dB's better. Someone told me that it is because the symbols are further apart from each other in the freq plane, and thus it is easier for the radio to demodulate it. My question is, why the wider deviation is easier for the radio?
The radio uses low IF receiver. RF is fed to an I/Q- down conversion mixer. The I/Q signals are sampled by IF ADC.