GSM Technology

* GSM : 

>>     GSM( Global System For Mobile Communication) is an open, digital cellular technology used for transmitting mobile voice and data services.

   >>   GSM network is mainly used for speech.  Basic gsm planning is called Green field planning.These are two types :

•      Capacity planning : depends on traffic
•      Coverage planning: depends on coverage Area.

>> Capacity sites are divided into two:

1.  Micro  (1 sector, 2 sector site)
2.  Macro (3 sector site)

GPRS :  GPRS(General Packet Radio Services) is an extension of the GSM architecture. packet data runs on a new backbone Ip network and is seperate from the existing Gsm core network. that is used mainly for speech.

* EDGE : GSM networks upgraded with Enhanced Data rates for GSM Evolution (EDGE)  technology can provide up to three times the data capacity of GPRS. It is a new modulation method.Enabling higher data bit rates. 

GSM WORLD: 

>>   Basic cell planning process :

1.   Traffic and Coverage Analysis.
2.   Nominal cell plan
3.   Surveys
4.   Implementation ( installation, comissioning& Testing )
5.   System tuning.

>>  Different frequency bands Gsm ( 450/800/900/1800/1900 )

  In India following Bands are used :

                                 Uplink (MHz)                Down link(MHz)
GSM - 800                   8xx-8xx                       8xx-8xx
GSM - 1900                 17xx-17xx                   18xx-18xx
P-GSM   900                8xx-9xx                       9xx-9xx
E- GSM  900                8xx-9xx                        9xx-9xx



GSM ARCHITECTURE:






















* MSC ( Mobile Servicing Switching Center ) :
                             The MSC is responsible for set-up, routing and supervision of calls to and from mobile subscribers. other functions are also implemented in the Msc, such as authentication.

* VLR ( Visitor Location Register ) : 
                             VLR is a data base containing information about all MSc's that currently are located in the Msc service area. when MS roams into a new Msc Service area.the VLR connected to that Msc requests data about the MS from the HLR and stores it.

* SMS-GM SC(Short message service-Gateway Msc) :
                          A SMS-GMSC is capable of receiving a short message from a service center, interrogating an HLR for routing information and message waiting data and delivering the short message to MSC of the receiving MS.

* SMS- IWMSC (Short message service-interworking MSC) :
                         It is capable of receiving a mobile oriented message from Msc and submitting the message to the receipents sc.

* GMSC (Gateway Msc) : 
                        The GMSC supports the function for routing incoming calls to the Msc where the mobile subscriber is currently registered.

* IWU (Inter Working Unit) : 
                       The IWU provides an interface to various networks for data communication. Its main functions include modem and fax adapter. This functionality is implemented in DTI ( Data Transmission Interworking unit).

* HLR (Home Location Register) : 
                        HLR is a database holding information about all subscribers belonging to PLMN (Public Land Mobile Network). In PLMN there is one or more HLRs.

* AUC (Authentication center) : 
                       The AUC provides authentication and encryption parameters required for subscriber verification and to ensure call confidentially. AUC stores the 

• a RANDom number (RAND)
• a Signed RESponse (SRES)
• a Ciphering Key (Kc)

* FNR (Flexible Numbering Register ) : 
                          The number portability enables a subscriber to change an GSM operator without changing his MSISDN number.

* BSS (Base Station System ) : 
                         The base Station System is composed of two major components.

•        Base Station Controller (BSC)
•        Radio Base Station (RBS)

* BSC / TRC :
                         The BSC can manage entire radio network and performs the following funcs:
          

•          Handling MS connection and Handover
•          Radio Network Management
•          Trans coding and rate adaptation.
•          Traffic Concentration
•          Transmission management of the BTS
•          Remote Control of the BTS

*  BTS or RBS ( Base Transceiver Station ) : 
                             It includes all radio and transmission interface equipment needed in one cell. Each BTS operates at one or several pairs of frequencies. One frequention is used to Transmit signal to MS and one to receive signal from MS.


* CHANNEL CONCEPT : 

The Carrier Seperation in GSM, is 200 KHz.

• In GSM 900 band carriers are 124
• In GSM 1800 band carriers are 374
• In GSM 1900  band carriers are 299

>> Each carrier can be shared by eight MSs.

• The number of channels in Gsm 900 band 124*8 = 992 channels
• The number of channels in Gsm 1800 band 374*8 = 2952 channels

* Logical channels : 
                         There are eleven logical channels in the GSM system, two of which are used for Traffic and nine for control signaling.

* Traffic Channels (TCH) : 
                         Two types of TCHs are used :

•  Full rate channel, Bm-  This channel is used for full rate speech
•  Half rate channel, Lm -This channel can be used for half rate speech.

 CONTROL CHANNELS :

 * Broad Cast Channel (BCH) : 

• FCCH ( Frequency correction channel ) : It is used for frequention correction of the MS, downlink only.
• SCH ( Synchronization Channel ) : It carries information about TDMA frame number and BSIC ( Base station identity code ) of the BTS, downlink only.
• BCCH (Broad cast control channel ) : Broad casts cell specific information to MS, dwonlink only.

 * Common Control Channels ( CCCH ) : 

• PCH ( Paging channel ) : It is used to page the MS, downlink only.
• RACH ( Random Access Channel ) : Used by the MS to request SDCCH or an access to MS call origination / registration.......etc. up link only.
• AGCH ( Access Grant Channel ) : It is used to allocate SDCCH to an MS, downlink only.

 * DCCH ( Dedicated Control Channel ) :

• SDCCH ( Stand alone Dedicated Control Channel ) : It is used for singing during the call   set-up or registration, up and downlink.
• SACCH ( SlowAssociated Control Channel ) : This control channel associated with TCH or an SDCCH, up and down link. on this channel the measurement reports are sent on up link and timing advance and power orders on the down link.
• FACCH ( Fast  Associated Control Channel ) : This control channel associated with TCH. up and down link. FACCH works in bit stealing mode. It is used during handover when SACCH signaling is not fast enough.

                               Several logical channels can share the same physical channel or Timeslot                 (TS). on TSO the BCH and CCCH are multiplexed

  *  HYPER FRAMES, SUPER FRAME, MULTI FRAME : 
    
        1 bit duration ~ 3.69 Micro sec.

        1 Time slot = 156.25 bit durations ~ 0.57 ms.

        1 TDMA Frame = 8 time slots ~ 4.615 ms.

        1 ( 26-frame ) multiframe = 26 TDMA frames ~ 120 ms
            It carries TCH, SACCH, FACCH.

        1 ( 51- Frame ) multiframe = 51 TDMA frames ~ 235.4 ms.
            It carries FCCH, SCH, BCCH, CCCH, SDCCH, SACCH and CBCH.
       
        1 Super frame = 1,326 TDMA frames ~ 6.12 sec

        1 Hyper frame = 2048 super frames = 2,715, 648 TDMA frames duration ~ 3h 28m 53 sec         760 us.

* EQUIPMENT CHARACTERISTICS : 

    * MOBILE STATION : power classes for Gsm 800/900

       MS power class         output power (dbm)            sensitivity (dbm)
                2                                    39                                        - 106
                3                                    37                                        - 106
                4(hand held)                  33                                        - 104
                5(hand held)                  29                                        - 104
      
       the sensitivity is - 102 dbm for small MSs, from leading mobile manufacturers shows that            sensitivity is 2-4 dB higher.
                                               MS transmitted power off set : -2 dB
                  No loss or antenna gain should be used for the MSs.
  
      power classes for Gsm 1800

        Ms power class               output power (dbm)            Sensitivity (dbm)
               1                                         30                                           - 104
               2                                         24                                           - 104

 * ANTENNA NEAR PARTS :

    Feeders and jumpers : When calculating power budget, the feeder loss must be taken             into account. the most commonly used feeder type is 7/8 ".

           feeder type                     800/900 (dB/100M)              1800/1900(dB/100m)
             LCF 1/2"                           7.0/7.2                                         10.5
             LCF 7/8"                           4.0                                                6.5
             LCF 1- 1/4"                       3.3                                                5.3
             LCF 1- 5/8"                       2.6                                                4.2
         
        Apart from feeder loss, an additional loss arises in jumpers and connectors. Typical               values are :
                0.5 db  loss for each jumper
                0.1 db loss for each connector.

   External filters : Duplex filters make it possible to use the same antenna for transmission      and reception. The loss of duplex in both up link and down link is 0.5 db. Attennation Tx to      Rx is more than 30 dB.Diplex filters make it possible to use the same feeders for Gsm 900      and 1800 in dual band site. A typical loss in diplex filter is around 0.3 db.

  Tower Mounted Amplifiers (TMA) : To improve sensitivity on the plink, a TMA can be         used. The purpose of the TMA is to amplify the received signal before it is further                   attenuated in the antenna feeder.
  Basically TMAs are used in rural areas, to reach a balanced power budget. But TMAs is         getting more important in all areas due to that new MSs usually are transmitting on lower         output power.

              TMA products for Gsm 800/900

  TMA products                      Duplexers                          TMA downlink loss (db)

  TMA 900 Simplex                       0                                               0
  TMA 800/900 Dual duplex          2                                               0.3

               TMA products for Gsm 1800

  TMA Product                        Duplexers                      TMA downlink loss (db)

  TMA 1800 Simplex                     0                                                0
  TMA 1800 Duplex                      1                                              0.3
  TMA 1800 Dual duplex               2                                             0.3


 With TMA, the receiver sensitivity will not be affected by the loss in the feeder, as long as it  doesn't exceeds 4db.
                
                        Loss (db)                                   Sensitivity (db)
                           <4                                                  0
                             6                                                 0.5
                             8                                                 1.5
                            10                                                2.5

DIVERSITY : One way of reducing the influence of multipath fading in the up link is to use antenna diversity. The antenna diversity gain will depend on the correlation between the fading of two antenna signals as well as the efficiency in power reception of the two separate antennas. There are two different types of antenna diversity :

1) Space diversity : Space diversity means that two Rx antennas positioned at a certain minimum distance from each other are used. Space diversity improves the uplink by 3.5 db.

2) Polarization diversity : Polarization diversity offers relief by allowing two space diversity antennas separated by several meters to be replaced by one dual polarized antenna. This antenna has normal size but contains two antenna arrays with different polarization The uplink diversity gain with polarization diversity is also typically 3.5 db.
In urban and sub-urban areas the correlation between the two Rx Signals is very low due to better multipath fading conditions, resulting in minimum antenna diversity gain. There fore a higher diversity gain could be used in an Urban/sub-urban area compared rural areas.
The effective diversity gain obtained from polarization diversity is slightly less than the gain from space diversity. The isolation between Two polarization planes needs to be 30 db.


CELL COVERAGE : 

Required signal strength (SSreq) :

 1)  SSreq  =  MSsens + RFmarg + IFmarg + BL 

                MSsens  -  MS sensitivity
                RFmarg  -  Rayleigh fading margin
                IF marg   -  Interference margin
                BL           -  Body loss.

Design level : The design level can be calculated from

2) SSdesign = SSreq + LNFmarg(o) MS outdoor
3) SSdesign = SSreq + LNFmarg(o) + CPL MS in - car
4) SSdesign = SSreq + LNFmarg(o+i) + BPL mean MS indoor

where
LNF marge(o)  -  Outdoor log -normal fading margin
LNF marge(0+i)  -  Outdoor + indoor log - normal fading margin.
CPL  -   Car penetration loss.
BPL mean -  Mean building penetration loss.

Rayleigh Fading : Reyleigh fading is due to multipath interference and occurs especially in urban areas. The distance between two adjacent fading clips is approx 1/2. Antenna diversity reduces the effect of Rayleigh fading, but in a different way than frequency hopping.
( RF marg ) Rayleigh fade margin - 3db

INTERFERENCE : The interference margin depends on the frequention reuse, the traffic load and the desired percentage of area coverage. Frequention hopping, dynamic power control, and particularly DTX reduce the interference level. In normal system, an interference margin is 2dB
                   
                   IF margin = 2dB

BODY LOSS :  The human body affects the MS performance in a number of ways, compared to a free standing mobile 
* head absorbs energy
* The antenna efficiency of some MSs may be reduced
* Other effects may be a change of the lobe direction and the polarization.
         The body loss is less for higher.
Frequencies than lower
         Body loss (BL) = 5 dB ( 800/900 MHz)
         Body loss (BL) = 3 dB (1800/1900 MHz)

Car penetration loss (CPL) : When MS is situated in a car without an external antenna, an extra margin must be added. to the penetration loss of the car. This extra margin is approx 6dB.
            
                CPL = 6dB

Design levels : The signal strength is calculated as the sum of the required signal strength (SSreq) and various margins.
SSreq = MSsens + RF marg + IF marg + BL
           = - 104 + 3 + 2 + 5 = -94 dBm

Outdoor and In- Car coverage : 

            SSdesign = SSreq + LNF marg(0) MS Outdoor.
            SSdesign = SSreq + LNF marg (0) + CPLMS in car.

The fading will be represented by its standard deviation LNF (0) and depends on area type. Typical values of LNF(0) are :

            Area Type                             LNF (0) dB
              
             Deuse urban








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