Does any one know the reason why a 217Hz pulse train was selected for TDMA in a GSM signal?
IOW what is the functional advantage of this particular frequency over any other.
Secondly, by what rationale was it determined that every 26th pulse be omitted? Again, instead of every 25th, 27th, etc.
No doubt there are technical reasons which would be of interest to the less informed of us, as well as being a slice of telecommunications history.
Thanks for any insights.
Bob Griffin
The 217 Hz repetition rate with ~500us bursts are quite optimal for the frequency hopped spread spectrum in the urban environment.
The GSM multiframe usually consists of 26 frames. Typically, 24 frames are used for traffic, and the remaining two for control and other technical purposes. That division has to do with the expected amount of control data vs traffic payload. However, there could be many other frame arrangements.
The GSM protocol stack is very involved. It has its roots in SS7/E1 and ISDN. The original concept of GSM was to put E1 in the air.
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
Thank you for your informative reply. However, I am still grappling with the TDMA frames..
As I understand, each 4.62ms frame is divided into 8 time slots of
0.5mS each. And each pulse contained therein undergoes MSK, being four possible phase postions, represented as half sinusoids. And within this the data is digitially encoded. Is that correct?Bob Griffin
What you described is the normal traffic burst. There are also other types of bursts used for access, synchronization, frequency adjustment, etc.
The original modulation is GMSK BT = 0.3, which is partial response. No exact phases, no half sinusoids; think of it as a filtered FSK. The 3g+ modes can operate in 8-PSK mode also.
There is at least a dozen of different formats for the different data encoding in GSM, used in the different logical/physical channels.
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
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