Expert: Cezar L. Palconet - 4/12/2011

Question
Hi,

I would like to know why the GSM bitrate is set to 270.833 kbits/s. Given the BT of the Gaussian filter is 0.3, wouldn't it be possible to use a different bitrate and therefore a different T (instead of 81kHz) that keeps BT=0.3?

Thanks!

Answer
Hi, Curro

First of all, the Modulation GMSK is  form of MSK, and MSK type of modulation exhibits a number of side lobes extending well beyond the datarate, thus contributory to ISI, and ISI is undesirable, because it makes detection more difficult much more with real world scenarios like multipathing. For wireless systems which require more efficient use of RF channel BW, which is 200KHz. In the case of GSM, it is necessary to reduce the energy of the  side-lobes, the only practical and realistic approach to this is the use of a Gaussian filter, thus  the GMSK was adopted.
BT – refers to the filter’s -3dB BW and data rate,  if bit are spread over more than 1 bit period,  this will give rise to ISI.
The BT 0.3 was chosen because it provides the best compromise between bit rate and ISI
About 99% of the RF power for GMSK are confined to 250kHz (+/- 25kHz margin from the signal), this is virtually no side lobes outside this frequency band, and the ISI is nearly zero or negligible.
Anything is possible on paper, when exploring new possibilities, however real world applications, field test result evaluations,  and where production and efficiency are a primary considerations due to cost, it is always the simple and best compromise that takes the lead.

For refference: Haykin, S. 2001: “Communication Systems”. 4th ed. New York, NY. John Wiley & Sons.


Best Regards