# Information in GFSK signal

I was measuring a Bluetooth LE Signal, particullary a Channel with center frequency 2402 MHz and Bandwidth of 2 MHz, 2401 - 2403 MHz.

Those measurement points are just the highest peaks. In the Bluetooth specifications it says, that the modulation is a GFSK with a bandwidth-bit period of BT = 0.5 and that the modulation index is between 0.45 and 0.55. Furthermore, it says that the frequency deviation is not less than 185kHz.

Can I, given these informations, calculate from where to where (Freq.) the transmitted data is?

EDIT: Maybe I should point out why I want to know this. I am downconverting this to the baseband and need to construct a filter. Normally I would filter this with a 0 to 1 MHz lowpass filter, but if the BW is less, it would be better for me.

EDIT 2: x-axis is frequency and y-axis power in dBm ;D.

• Theoretical calculation is difficult. It's easier just to simulate a noiseless signal in MatLab, Octave, or GNURadio; take the FFT; and compute the 99.0% or 99.9% energy bandwidth. – Andy Walls Nov 8 '17 at 2:19
• could you please explain why 99% and why this would help me in this case. – OcK Nov 8 '17 at 10:37
• 99% was arbitrary. You could go with 95% or 90% or 85% whatever, depending on the distortion and BER performance degradation you're willing to tolerate. Be advised that the tuned center frequency between the transmitter and receiver will never be exactly the same. You'll want your initial channel filter to be wider than necessary anyway to avoid distortion before you correct for carrier frequency offset. – Andy Walls Nov 8 '17 at 11:05
• BTW, a modulation index of 0.5 is GMSK. The table of % bandwidth for certain BTs, from Murata's classic paper on GMSK, is on slide 31 of this presentation: nuthal4u.files.wordpress.com/2014/03/unit3.ppt – Andy Walls Nov 8 '17 at 11:28
• But why does it say GFSK in the Bluetooth specification? – OcK Nov 8 '17 at 11:43