I'm trying to generate a GMSK signal in accordance to the GSM specifications. To do so, I'm using the GNU Radio software and the following parameters:
*Samples/Symbol = 200
*3dB Time-Bandwidth Product = 0.3
*Pulse Duration (Symbols) = 156
To set these parameters, I considered (i) a sufficiently high base-band sampling rate (samples/symbol) in order to avoid aliasing; (ii) the well-known 0.3 BT product and (iii) a 156 bits GSM pulse, equivalent to 1 time slot (I considered that GMSK provides 1 bit/symbol).
Nevertheless, my signals have a bandwidth which is much larger than the 200 kHz requirement.
What am I doing wrong?
You're confusing the GSM burst of 156 bits with the Gaussian pulse duration of the GMSK modulator, i.e. how many adjacent symbols are affected by (controlled) interference (leakage) from the current symbol caused by filtering the modulator input with the Gaussian filter (pulse). Another observation would be that your oversampling rate is huge (impractical, I'd argue).
Thanks a lot,!
could you explain how you determined the oversampling rate?