# Gaussian Minimum Phase Shift keying

Discussion in 'Homework Help' started by abdulsamad4321, Jun 26, 2014.

Nov 24, 2013
38
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Can You tell why There are 8 time slots in Gaussian Minimum Phase Shift keying(gmsk)?
Thank you.

2. ### Papabravo Expert

Feb 24, 2006
11,136
2,174
Because 8 is a power of 2.

Nov 24, 2013
38
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Why don't we use 16 or 4 ?
And What with the power of 2?
thank you.

4. ### Papabravo Expert

Feb 24, 2006
11,136
2,174
Well it was a guess based on the occurrence of powers of 2 in other phase shift keying applications and digital modulation/decoding methods. From a brief look at the material I wasn't able to determine how "time slots" relate to the construction of the filter. Perhaps you could provide a more detailed description of how your question relates to the actual implementation.

5. ### dougc314 Member

Dec 20, 2013
38
11
The question actually makes no sense. GMSK is a modulation method, meaning MSK with some gaussian shaping of the baseband in order to control the spectral rolloff. The number of time slots in a TDMA frame has nothing to do with the modulation method. However I'm pretty sure you are thinking of the original GSM system, that used GMSK at about 300 kBits.

I think your question should be why are there 8 timeslots in GSM, which happens to use GMSK?

Think of them as being parameters of different layers of the communications protocol, they are unrelated.

The reason why there is a specific number of time slots in a TDMA frame has to do with concerns such as time slot payload, (user rate), Over all rate, periodicity of the frame (latency) and doubtless many more issues.

For further reading you can try the wikipedia article on "Physical Layer".

In the case of GSM I suspect there was an overall occupied bandwidth that they had to meet, (in order to be compatible with other cellular services.) With the decision to use GMSK that defined the raw channel bitrate. Then considering framing overhead, error correction overhead (and whatever else removed bits from the subscriber) the system designers determined what fraction of the available bit rate they wanted to offer to a subscriber, and still have a competitive service.

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6. ### Papabravo Expert

Feb 24, 2006
11,136
2,174
I was struggling to find a connection between "time slots" and either Gaussian Filters or IQ Modulators.

Nov 24, 2013
38
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Yes in TDMA why their are eigth time slots and tdma is used by gsm. I specifically want to know why there are time slots from 0-7? What is the reason behind it? Can't we use 4 ,or 16 or any other no of time slots?
thank you

Nov 24, 2013
38
1
Thank you.

9. ### Papabravo Expert

Feb 24, 2006
11,136
2,174
How things get divided into frames and time slots may not have any particular reasoning behind it. The major constraint is how mach data can be packed into a time slot and how many time slots you need per second in order to recreate a voice signal for example. The selection of 8 may have been chosen arbitrarily from among a number of equally good alternatives.

Nov 24, 2013
38
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Can we say we conventionally use it?

Last edited: Jul 3, 2014
11. ### Papabravo Expert

Feb 24, 2006
11,136
2,174
Sure. I have no problem with that statement.