[time-nuts] Re: Austron Oscillator Products - GSM

[john.haine at haine-online.net]

TVB in an email was asking about the numerology of GSM.

The channel spacing in GSM is 200kHz = 13MHz/65, and all subsequent
standards have channel centre frequencies that are on the same 200kHz raster
though the channels are now much wider.  200kHz can also be divided by 25 to
8kHz and locked to 2048kHz divided by 256 hence lock to the E1 rate.

The channel symbol rate is 13MHz/48 = 270.8333.. kbit/s.  

From
https://www.rfcafe.com/references/electrical/gsm-specs.htm#:~:text=A%20GSM%2
0Multiframe%20is%20the,Bit%20being%203.69231%20%CE%BCs%20long.:

"A GSM Multiframe is the basic unit, and is 120 ms long. There are 26 Frames
in each Multiframe, with each Frame being 4.61538 ms long (120 ms/26).
Within each Frame are 8 Timeslots at 576.92 μs per Timeslot (577 μs in round
numbers). Finally, there are 156.25 Bits per Timeslot, each Bit being
3.69231 μs long."

So every handset needed a 13MHz oscillator that had to be within 1ppm before
lock and was then pulled in to lock using feedback from an "f burst" in each
downlink timeslot.  VCTCXOs were used initially so low cost very small
components became available based on hundreds of millions of devices being
made.  Eventually digitally locked oscillators took over I believe.  The
specs for 4G and 5G are considerably tighter I think.



-----Original Message-----
From: John via time-nuts <time-nuts at lists.febo.com> 
Sent: Saturday, March 2, 2024 7:27 AM
To: time-nuts at lists.febo.com
Cc: John <reid.john at hotmail.com>
Subject: [time-nuts] Re: Austron Oscillator Products - GSM

Ericsson GSM products did lock their master oscillator to the 2.048 mbps
stream.

The master oscillator was 13 MHz, and I understand the RF was locked to
that. As well as lots of timing bibs and bobs..

John


From: <john.haine at haine-online.net>

> Subject: [time-nuts] Re: Austron Oscillator Products
> To: "'Discussion of precise time and frequency measurement'"
> 	<time-nuts at lists.febo.com>
> Message-ID: <011201da6afe$e7bea780$b73bf680$@haine-online.net>
> Content-Type: text/plain;       charset="utf-8"
>
> GSM standards development started in 1982 though there was prior work in
several labs.  Basic phy layer parameters were probably fixed by 1985 as the
development cycle was quite long drawn out (especially for handsets as the
"long pole" went through the tester!).  Networks were launched in 1991 so
devices must have been in procurement several years before then.  When were
these devices made?  I think they might have locked BTS clocks to the
incoming PCM 2.048 Mbit/s links, not sure about the RF, though given the way
all the numbers are interrelated they might have had to.
>
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