GSM Technology

            What is GSM Tech. Principle behind this?

Ans: - GSM (Global System for Mobile Communication.) It is a digital cellular mobile communication system. This system meets the criteria as follows.

1. Spectrum efficiency.
2. International Roaming.
3. Low mobile and base station cost.
4. Good Subjective Voice Quality.
5. Compatibility with other systems such as ISDN.
6. Ability to support new services.

MS	Mobile Station
BTS	Base Transceiver Station
BSC	Base Station Controller
BSS	Base Station Sub-system
MSC	Mobile services Switching Center
HLR	Home Location Register
VLR	Visitor Location Register
AuC	Authentication Center
EIR	Equipment Identity Register
OMC	Operations and Maintenance Center
NMC	Network Management Center
ADM	Administration Center

·         Mobile Station

The mobile station comes in a number of different forms, ranging from the traditional car-mounted phone operating at 20W, through transportables operating at 8W and 5W, to the increasingly popular hand portable units which typically radiate less than 2W. A fifth class for hand portables operating at 0.8W has been specified for Micro Cellular versions of the network.

One of the main factors governing the hand portable size and weight is the battery pack. Several features of the system are designed to allow this either to be smaller or to give a substantially longer life between charges. Chief among these is Discontinuous Receive (DRX). This allows the mobile to synchronize its listening period to a known paging cycle of the network. This can typically reduce the standby power requirements by 90%.

6.3.2.2. The Radio Sub-system

When the mobile user initiates a call, his equipment will search for a local base station. A base station sub-system (BSS) comprises a base station controller (BSC) and several base transceiver stations (BTS), each of which provides a radio cell of one or more channels. The BTS is responsible for providing layers 1 and 2 of the radio interface, that is, an error-corrected data path. Each BTS has at least one of its radio channels assigned to carry control signals in addition to traffic. The BSC is responsible for the management of the radio resource within a region. Its main functions are to allocate and control traffic channels, control frequency hopping, undertake handovers (except to cells outside its region) and provide radio performance measurements. Once the mobile has accessed, and synchronized with, a BTS the BSC will allocate it a dedicated bi-directional signaling channel and will set up a route to the Mobile services Switching Center (MSC).

6.3.2.3. The Switching Sub-system

The MSC routes traffic and signaling within the network and interworks with other networks. It comprises a trunk ISDN exchange with additional functionality and interfaces to support the mobile application. When a mobile requests access to the system it has to supply its IMSI (International Mobile Subscriber Identity). This is a unique number which will allow the system to initiate a process to confirm that the subscriber is allowed to access it. This process is called authentication. Before it can do this, however, it has to find where the subscriber is based. Every subscriber is allocated to a home network, associated with an MSC within that network. This is achieved by making an entry in the Home Location Register (HLR), which contains information about the services the subscriber is allowed. The HLR also contains a unique authentication key and associated challenge/response generators.

6.3.2.4. Mobility Management and Security

Whenever a mobile is switched on, and at intervals thereafter, it will register with the system; this allows its location in the network to be established and its location area to be updated in the HLR. A location area is a geographically defined group of cells. On first registering, the local MSC will use the IMSI to interrogate the subscriber's HLR and will add the subscriber data to its associated Visitor Location Register (VLR). The VLR now contains the address of the subscriber's HLR and the authentication request is routed back through the HLR to the subscriber's Authentication Center (AuC). This generates a challenge/response pair which is used by the local network to challenge the mobile. In addition, some operators also plan to check the mobile equipment against an Equipment Identity Register (EIR), in order to control stolen, fraudulent or faulty equipment.

The authentication process is very powerful and is based on advanced cryptographic principles. It especially protects the network operators from fraudulent use of their services. It does not however protect the user from eavesdropping. The TDMA nature of GSM coupled with its frequency hopping facility will make it very difficult for an eavesdropper to lock onto the correct signal however and thus there is a much higher degree of inherent security in the system than is found in today's analogue systems. Nevertheless for users who need assurance of a secure transmission, GSM offers encryption over the air interface. This is based on a public key encryption principle and provides very high security.

6.3.2.5. Call Set-up

Once the user and his equipment are accepted by the network, the mobile must define the type of service it requires (voice, data, supplementary services etc.) and the destination number. At this point a traffic channel with the relevant capacity will be allocated and the MSC will route the call to the destination. Note that the network may delay assigning the traffic channel until the connection is made with the called number. This is known as off-air call set-up, and it can reduce the radio channel occupancy of any one call thus increasing the system traffic capacity.

GSM Recievers.

1.                  GSM 900 in Europe and Asia Pacific.With 890 MHZ – 915 MHZ Uplink and 935 MHZ – 960 MHZ Downlink Frequencies. There are 124 carriers per channel and carrier width is 200 KHZ, Bandwidth 25 MHZ, Wavelength 33cm. and Channel separation 20 MHZ.   ( Freq MHz = 890 + 0.2 * n )  where 1≤ n ≤124

2.                  GSM 1800 in Europe, Asia Pacific and Australia. With 1710 – 1785 MHZ Uplink and 1805 – 1880 MHZ Downlink. The carrier width is 200 khz , Band width 75 Mhz, and channel Separation is 20 Mhz. There are 375 carriers per channel.

Freq. MHz = 1710 + 0.2 * (n – 512) , where 512 ≤ n  ≤ 885

                             

3.                  GSM 1900 US , Canada, Latin America and Africa.With 1850 – 1910 MHZ Uplink and 1930 – 1990 MHZ Downlink. There are 300 Careers per channel, 60 MHZ Band width, Channel Separation is 20 MHZ.

Modulation Used in GSM 900 is GMSK (Gaussian Minimum Shift Keying)

Principle Behind this is the Frequency Reuse. A Geographical area is divided into several hexagonal cells. Each cell has some specific radius and having a set of frequencies. The frequencies allotted to each cell in such a way that after some distance these frequencies can again be reuse by other cells without interfering each other.

2.Q      How many channels used in GSM .Explain each ?

Ans: - Data burst for traffic

            Data burst for control

Two types of channels Physical and Logical.

Physical channel is combination of Time slot and Carrier freq. One RF channel supports eight physical channels in time slots 0, 1, 2, ----7.

A logical channel carries information of specific type.

Traffic channel (TCH) carries digitally encoded user speech or data and have same function in both forward and reverse link.

Control channel carries signaling and synchronizing commands between BS and MS. Certain type of control channels defined for forward and reverse link.

TCH Traffic Channel Full rate and Half rate.

When transmitted as full rate user data is contained within 1 time slot per frame. 22.8 Kbits/ps.

When transmitted as half rate user data is mapped onto same time slot but in alternate frames. 11.4 Kbits/ps.

Four types of control channels.

1.                  Broadcast Control Channels.

2.                  Associated control Channels.

3.                  Dedicated Control Channels.

4.                  Common Control Channels.

Broadcast Channels: - operates on forward link and transmit data on first time slot. It Contains.

1.                  SCH (Synchronization Channel) it is used to identify the serving BS and allowing each mobile to frame synchronize with the BS. The frame no. is sent with the BSIC during SCH burst. And also 6 bit BSIC.

2.                  FCCH (Frequency Correction Channel) The FCCH allows each MS to synchronize its internal freq. with exact freq. of the BS.

3.                  BCCH (Broadcast Control Channel) It carries information’s such as cell and network identity. It also broadcast a list of channels that are currently in use within a cell.

4.                  CBCH (Cell Broadcast Channel) Used to transmit short alphanumeric text msg. to all MS within a cell.

Common Control Channels (CCCH): - CCCH helps to establish the call from the MS. Three different types of CCCH are defined.

1.                  The Paging Channel (PCH). It is used to alert the MS of an incoming call.

2.                  The Random Access Channel (RACH). Is used by MS to access the network.

3.                  The Access Grant Channel (AGCH). Is used by the Base Station to inform the MS that which channel it should use.

Dedicated Control Channels (DCCH): - These channels are used for message exchange between several mobiles or a mobile and network. Two types of DCCH are there.

1.                  Stand Alone Dedicated Control Channels (SDCCH). Authentication, Registration, Location area updation, SMS etc. needed for setting up a TCH.

2.                  Slow Associated Control Channels (SACCH).

  

Associated Control Channels: - Associated with the TCH.

1.                  Slow Associated Control Channel (SACCH). Associated with TCH, Channel quality, Signal power level.

2.                  Fast Associated Control Channel (FACCH). Uses time slots from TCH, Handover info.

4.Q      What is Timing Advance ?

Ans.    

 Timing Advance

The timing of the bursts transmissions is very important. Mobiles are at different distances from the base stations. Their delay depends, consequently, on their distance. The aim of the timing advance is that the signals coming from the different mobile stations arrive to the base station at the right time. The base station measures the timing delay of the mobile stations. If the bursts corresponding to a mobile station arrive too late and overlap with other bursts, the base station tells, this mobile, to advance the transmission of its bursts.

1 TA = 554m. 

Calculation is given below.

Timing Advance:

T ´ T (bit) = (2d) ¤  c

Where T= Timing Advance

 C = vel.of light 3´10^5 m /ms

T (bit) = 1 ¤  270.833

(Raw bit rate per carrier is 270.833 Kbps. Each carrier is shared by 8 users in TDMA Fashion.

There for bit rate for one user or one time slot is 1 / 270.833 Kbps ).

Now d = T ((T (bit) ´ c)  ¤  2)

            = T ((1  ¤  270.833) ´ 3 ´ 10^5)  ¤  2)

Now after calc. d= T ´ 554 m

TA is from 0 to 63.

                       

5.Q  What type of modulation used in GSM ?

Ans.

 Digital Modulation

The modulation chosen for the GSM system is the Gaussian Minimum Shift Keying (GMSK). Figure 4 illustrates a GMSK modulator.

Q.        What is handover? Explain it.

Ans.

Handover

The user movements can produce the need to change the channel or cell, especially when the quality of the communication is decreasing. This procedure of changing the resources is called handover. Four different types of handovers can be distinguished:

• Handover of channels in the same cell.
• Handover of cells controlled by the same BSC.
• Handover of cells belonging to the same MSC but controlled by different BSCs.
• Handover of cells controlled by different MSCs.

Handovers are mainly controlled by the MSC. However in order to avoid unnecessary signaling information, the first two types of handovers are managed by the concerned BSC (in this case, the MSC is only notified of the handover).

The mobile station is the active participant in this procedure. In order to perform the handover, the mobile station controls continuously its own signal strength and the signal strength of the neighboring cells. The list of cells that must be monitored by the mobile station is given by the base station. The power measurements allow deciding which the best cell is, in order to maintain the quality of the communication link. Two basic algorithms are used for the handover:

• The minimum acceptable performance algorithm. When the quality of the transmission decreases (i.e. the signal is deteriorated), the power level of the mobile is increased. This is done until the increase of the power level has no effect on the quality of the signal. When this happens, a handover is performed.
• The “power budget” algorithm. This algorithm performs a handover, instead of continuously increasing the power level, in order to obtain a good communication quality.

• Decibell Relation:
• db and dbm :-

1W = 30 dbm

2W = 33 dbm 

dbm = 10 * log ( Pwr in Watts  * 1000 )   OR     10  * log  (power in Watts) / 1 mW

• dbi and dbd : -

1 dbd = 2.14 dbi

dbi = dbd – 2.14              

           

• Grade of Service  GoS :

How much traffic can one cell carry? That depends on the

Number of traffic channels available and the acceptable

Probability that the system is congested, the so called Grade of

Service (GoS).

• Key Performance Indicator ( KPI )

D1 ( Droop 1 ) or SD Droop < 1%

D2 ( Droop 2 ) or TCH Drop < 1.5%

SD Blocking < 0.5%

TCH Blocking < 0.5%

Congestion on SDCCH < 0.5%

Congestion on TCH < 1.5%

HOSR (Handover Success Rate) > 95%

TCH ASSR (TCH Assignment Success Rate) > 97%

CSSR (Call Setup Success Rate) > 98%

Setup Time = 3.5 Sec.

Availability = 99.9 %

CCR (Call Completion Rate) or CSR (Call Success Rate) > 96%

• Received Signal at MS and Path Loss:

= BTS (EIRP) – BTS to MS Path Loss + Donner Antenna Gain (G1) – Feeder Loss +  

Serving Antenna Gain (G2) – Avg. Fading Margin.

Where as Path Loss (db) = 20 log ( 4 Π d f / c )

Where ( d = Distance between antennas of BTS and MS.)

MS sensitivity = -102 dbm

BTS sensitivity = -104 dbm

Diversity Gain at BTS = 3.5 dbi

Antenna Gain at MS = 0.0 dbi

Slant Polarization Loss = 1.5 db

MS o/p Power = 2W or up to 0.8 W

EIRP = 53.7 db

Transmitted Power at BTS = 41 to 45 db

Duplex Loss at BTS = 0.8 db

Feeder loss and Jumper Loss at BTS = 3.00 db

Rayleigh fade margin without hopping = 3.4 db

Interference margin = 3.00 db

Car Loss = 6.00 db

Body Loss = 3.00 db

Dense urban loss = 6.00 db

• Erlang Traffic Theory :

Assuming that one cell has two carriers, corresponding typically to 2x8-2=14 traffic

channels (two physical channels are needed for signaling) and a GoS of 2% is acceptable, the traffic that can be offered is A=8.20 E. See the table in Figure 3-1.

This number is interesting if an estimate on the average traffic per subscriber can be obtained. Studies show that the average traffic per subscriber during the busy hour is typically 15-20 or in special cases 40 - 50 mE. (this can correspond to e.g. one call, lasting 54-72 seconds, per hour). Dividing the traffic that one cell can offer, Acell=8.20 E, by the traffic per subscriber, here chosen as Asub=0.025 E, the number of subscribers one cell can support is derived as 8.20/0.025 = 328 subscribers.

When half rate is used it will theoretically double the number of available traffic channels. In practice, however, live networks will most likely consist of a mixture between half rate mobiles and full rate mobiles.

Half rate will affect the SDCCH dimensioning since more Signalling will be required when the number of TCHs is increased. An important dimensioning factor is therefore the half rate penetration, i.e. the percentage of half rate mobiles in the network.

When half rate TCH capacity calculations are done it is assumed that the half rate mobiles are evenly spread among the cells, i.e. all cells have the same half rate penetration. The TCH capacity calculations made in this guideline are best illustrated with an example:

If for example a 2 TRX cell is used, it can accommodate 14 full rate TCHs, i.e. 14 subscribers if one SDCCH/8 is used for Signalling. A half rate penetration of 10 % would mean that 10 % of the 14 subscribers would be using a half rate connection, i.e. 1.4 subscribers (after been rounded up = 2 subscribers). This would result in 13 timeslots used for full rate and 1 timeslot used for half rate, resulting in 13 full rate TCHs and 2 half rate TCHs, i.e. 15 TCHs in total. The TCH capacity is then calculated for 15 TCHs with an Erlang B table with appropriate blocking figure.

Knowing the SDCCH holding times, with a given number of performances during busy hour for every procedure, the generated SDCCH traffic per subscriber can be calculated as follows:

For each type of procedure, multiply the number of performances per busy hour and subscriber by the holding time of the channel. By dividing the result by 3.6, the procedures contribution to the SDCCH load in mErlang/subscriber is achieved.