FDMA and TDMA

[beadCraze]

Hi,
THe GSM network uses a combination of FDMA , TDMA and frequency hopping to share the radio spectrum.
The allocated 25 MHz bandwidth is divided into 124 carrier frequencies. And within each carrier frequency, it's divided in time into 8 burst, hence giving a total of 992 channels.
Can anyone tell me if this 992 channels are enough to support the number of subscribers worldwide? (or am i getting the facts right??)if not, how else does it cope with the large number of subscribers all over the world? ALso, how does frequency hopping fit into the picture?

beadCraze

 

[johnwiss]

The number of channels needs to be large enough to support the number of users in a given geographical area. The frequencies are reused at 1:7 rate I think. This means a cellular network can reallocate the same frequency when you are 2R away from a base station where R is the cellular radius. You can fit 6 circles around a 7th if they are the same size. The phones have limited range so the 992 number is for one cell--not the whole world. Also, there are two main GSM bands, one around 900 MHz and one around 1900 Mhz. Frequency hopping randomizes the channel so that if one frequency is in a fade it won't persist providing a more reliable link.

 

[beadCraze]

Thanks johnwiss.
I would have thought 992 is to be distributed among the cluster of 7 cells, and have that 7 cells pattern repeated. So approximately 193 in a cell? I m not sure myself.. just trying to figure it out.


beadCraze

 

[johnwiss]

I think that there are multiple users (8) on a frequency so I think the max # users are about 193 X 8 or about 1600 actual calls/cell on the 193 frequencies as you correctly computed.

 

[JohnFortier]

Presumably the multiple users on a frequency are time division multiplexed onto the frequency. Does anyone know how upstream timing is handled, given that different users will be different distances from the transponder and there will be a differing time delay from each user's transmission due to C.

To assign each user a varying time slot would be extremely complex, so presumably there is a guard time between transmissions. Will this not cut into bandwidth significantly, or are my assumptions wrong? (won't be the first time!)

Also, mobile companies are advertising internet connection over mobile links. Any ideas about the actual bandwidth (B/s) offered here?

John

 

[johnwiss]

The network synchronization is definitely interactive with the base station and mobile units--the time slots are defined for a call and the mobile needs to transmit data at the correct absolute slot during one hop period (GSM frequency hops relatively slowly). The guard time is kept small during regular commun. by the interactive syncronization which involves sounding, reporting results, and a series of overhead synch. channels.

Check it out on the web for details--here is a newsgroup that may help

http://www.erlang.com/forum/

Don't ever rely on only one place (like eng-tips) for all your interactive news-grouping.

 

[beadCraze]

To simplify the design of the mobile, an offset of three time slots are used between the base station and mobile timing.THus avoid the necessity for the mobile to transmit and receive simultaneously. So everytime the mobile transmits at a certain time slot, there's a three time slot gap before receiving from the base station.

To overcome the propagation delays (when transmitting from the mobile to the base station), timing advance is used to help with the timing synchronisation. Mobiles at different location in a cell will have different propagation delay since their distance from the base station varies. The base station caters for this problem by telling the mobile to advance its timing. This timing advance is then superimposed onto the 3 time-slots.

This is how the burst are transmitted and received accurately within the 'real-time' time slots.