GSM

Introduction :-

What is GSM ?

GSM stands for Global System for Mobile Communication.It was originally called Groupe Spécial Mobile.  

Definition of GSM : 
GSM is combination of both TDMA and FDMA technologies.The GSM is a circuit-switched system that divides each 200kHz channel into eight 25kHz time-slots. 

GSM is a standard set developed by the European Telecommunications Standards Institute (ETSI) to describe technologies for second generation (2G) digital cellular networks.

The GSM standard is the most widely accepted standard and is implemented globally.

The GSM is owning a market share of more than 70 percent of the world's digital cellular subscribers.

A GSM digitizes and compresses data, then sends it down through a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHz or 1,800 MHz frequency band.

GSM bands :-

GSM-900

The term GSM-900 is used for any GSM system which operates in any 900 MHz band.

P-GSM-900

P-GSM-900 band is the primary band for GSM-900 Frequency band for primary GSM-900 (P-GSM-900) : 2 x 25 MHz

890 – 915 MHz for MS to BTS (uplink)

    935 – 960 MHz for BTS to MS (downlink)

E-GSM-900

In some countries, GSM-900 is allowed to operate in part or in all of the following extension band. E-GSM-900 (Extended GSM-900) band includes the primary band (P-GSM-900) and the extension band :

880 – 890 MHz for MS to BTS (uplink)

    925 – 935 MHz for BTS to MS (downlink)




R-GSM-900 
R-GSM-900 (Railway GSM-900) band includes the primary band (P-GSM-900) and the following extension band:
876 – 890 MHz for MS to BTS (uplink)
921 – 935 MHz for BTS to MS (downlink)
\ 
GSM-1800 
Frequency band: 2 x 75 MHz  
1710 – 1785 MHz for MS to BTs (uplink)  
1805 – 1880 MHz for BTS to MS (downlink)


The cell ranges are derived with propagation loss formulas such as Okumura-Hata, using inputs of maximum path loss, differences in the operating environments and the quality targets in different cell ranges.


The traffic capacity requirements have to be combined with the coverage requirements, by allocating frequencies. This also may have impact on the cell range.

Carrier Spacing and Channel Structure

Channel number – the carrier frequency is designated by the absolute radio frequency channel number (ARFCN). The frequency value of the carrier n in the lower band is called FL (n) while FU (n) is the corresponding frequency value in the upper band. Frequencies are in MHz

P-GSM-900:

Downlink Frequency for n-th Channel No. in MHz= 890 + 0.2 n

and   

Uplink Frequency for n-th Channel No. in MHz=  890 + 0.2 n  + 45

with 1 <  n  <  124

E-GSM-900:

FL  (n) = 890 + 0.2 x n  with 1 < n < 124

FL (n) = 890 + 0.2 x (n-1024)     with 975 <  n  <  1024

FU (n) = FL (n) + 45

R-GSM-900:

FL  (n) = 890 + 0.2 x n  with 1 < n < 124

FL (n) = 890 + 0.2 x (n-1024)     with 955 <  n  <  1024

FU (n) = FL (n) + 45

GSM-1800:

FL  (n) = 1710.2  + 0.2 x (n-512)  with 512 < n < 885

FU (n) = FL  (n) + 95 

Carrier spacing is 200 kHz

8 time slots per carrier

Coverage, Capacity, and Quality

Providing coverage is usually considered as the first and most important activity of a new cellular operator. For a while, every network is indeed coverage driven. However, the coverage is not the only thing. It provides the means of service and should meet certain quality measures.

The starting point is a set of coverage quality requirements.

To guarantee a good quality in both uplink and downlink direction, the power levels of BTS and MS should be in balance at the edge of a cell. Main output results of the power link budgets are:

Maximum path loss that can be tolerated between the MS and the BTS 

Maximum output power level of the BTS transmitter.

These values are calculated as a function of design constraints:

BTS and MS receiver sensitivity levels

MS output power level

Antenna gain

Diversity reception

Losses in combiners, cables, etc

The cell ranges are derived with propagation loss formulas such as Okumura-Hata, using inputs of maximum path loss, differences in the operating environments and the quality targets in different cell ranges.

The traffic capacity requirements have to be combined with the coverage requirements, by allocating frequencies. This also may have impact on the cell range.


Traffic  theory :-
Traffic is calculated using erlang calculation. The Erlong per subscriber is calculated using farmulae


Erlang=(BHCA X Avg Call Holding Time) / 3600


BHCA = Busy Hour Call Attempt