GSM signal structure

[Bob Graham]

I am trying to understand how information is encoded within a GSM
signal. All I have found is as summarized in the link below.

Can anyone please explain briefly how and where the voice is inserted?

http://www.privateline.com/mt_gsmhistory/06_multiple_access_and_channel_structure/

Thank you for any insights.

Bob Graham

 

[Bob Graham]

"This digital signal is modulated onto the analog carrier frequency
using Gaussian-filtered Minimum Shift Keying (GMSK)"

http://www.privateline.com/mt_gsmhistory/06_multiple_access_and_channel_str=
ucture/v_channel_coding_and_modulation/

Read that and still a bit hazy.

Is the following interpretation correct please?

The "900MHz" analog carrier is actually one of the 124 200KHz-spaced
FDMA frequencies.

As I understand, these are hopped to minimize interference. BTW what
is the usual hopping rate?

For TDMA, the selected carrier is pulsed at a 217Hz rep rate, with
every 25th and 26th pulse omitted.

The pulse duration is 0.5ms, meaning 8 separate real time messages can
be fit into one TDMA slot. And riding within that time slot is the
GMSK.

Is that about it?

Just wondering ... if all eight 0.5ms time slots are filled,
presumably by 8 transmitters does the transmitted signal more or less
resemble a continuous 900MHz carrier transmision? MSK aside of couse.

Bob Graham

 

[Bob Graham]

If it's eight different transmitters, then the amplitude and delay
within each slot will vary. If it's eight different sources all going
through the same transmitter, then I think there'll still be 'holes' for
the guard times in TDMA.

I hadn't read before that each transmitter could be assigned a space
WITHIN one of the 8 slots, eg. by differing delay and amplitude as you
mention.

I thought each BTS just had a different slot, and the point of the low
duty cycle time was to maintain amplitude, being signal strength..

It would be appreciated if you can please explain your comment further
so I do not misunderstand.

Bob Graham

 

[Bob Graham]

The complete specification is on the web, for you to read. It will
clarify all.

I looked and could not find anything about delays or amplitude
variations within a _single_ TDMA time slot that you seem to have
referred to.

http://www.gsmfordummies.com/tdma/tdma.shtml

Any chance of a more definitive answer?

Bob Graham