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Klevo
2010-06-10, 01:37 AM
Hi friends,

I am currently working on a thesis and I need some ideas that why we cannot (it is not convenient) use QPSK modulation in GSM (air) communication.

Can you please provide some drawbacks???

I have checked the comments in google but I would like to hear the opinions of engineers.

thank you for your help.

RF engineer
2010-06-10, 03:39 AM
[QUOTE=Klevo;87782]Hi friends,

I am currently working on a thesis and I need some ideas that why we cannot (it is not convenient) use QPSK modulation in GSM (air) communication.

Can you please provide some drawbacks???

I have checked the comments in google but I would like to hear the opinions of engineers.

thank you for your help.[/QUOTE
It is related to bandwidth efficiency as GMSK has higher efficiency but the draw back is that need high power than QPSK. and GMSK using filter to reduce intersymbol interference.
BR
RF

atif
2010-09-24, 04:56 PM
QPSK requires more digital signal processing at baseband level and its also more complex as compared to GMSK

cooltyger
2010-09-24, 05:07 PM
Hi,

I feel that for voice which is something like a real time application, GMSK which uses 0 or 1 is more robust and reliable than QPSK which is symbol based and susceptible to more data loss than GMSK due to various RF factors and the real time experience may be lost!!! :D

BRs
CT

s52d
2010-09-24, 05:09 PM
[QUOTE=Klevo;87782]Hi friends,

I am currently working on a thesis and I need some ideas that why we cannot (it is not convenient) use QPSK modulation in GSM (air) communication.

Can you please provide some drawbacks???

I have checked the comments in google but I would like to hear the opinions of engineers.

thank you for your help.[/QUOTE
It is related to bandwidth efficiency as GMSK has higher efficiency but the draw back is that need high power than QPSK. and GMSK using filter to reduce intersymbol interference.
BR
RF
On top of adjacent ch interference (leakage into neighbor 200 kHz ch),
GMSK has constant envelope and thus it is much easier to make decent transmitter.
But, GMSK is history: with EDGE, even GSM uses high order modulations so we had to
replace TRXes with more complex tramsitters and more DSP CPU capacity for more demanding RX algorithms.

BR
s52d

boring
2010-09-25, 03:02 AM
Hi friends,

I am currently working on a thesis and I need some ideas that why we cannot (it is not convenient) use QPSK modulation in GSM (air) communication.

Can you please provide some drawbacks???

I have checked the comments in google but I would like to hear the opinions of engineers.

thank you for your help.


GMSK signal has a constant envelope (i.e. signal's amplitude does not change) so we can operate PA's at their saturation point and this means they are highly efficient in terms of power wastage.

Running a PA at the saturation point results in the generation of undesired harmonics which destroy the output signal through the increase of intersymbol interference. GMSK does not have this problem because of the signal's constant envelope but QPSK suffers from this harmonics generation. the typical solution is to back-off the operating point of the PA. this means that the PA is not as efficient as before but the intersymbol interference is reduced. a 3-dB back-off is typical in most implementations