Basics of GSM network

Figure: GSM Network Architecture.

The GSM network consists of the following components:

a. Mobile Station (MS)

b. Base Transceiver Station (BTS)

c. Base Station Controller (BSC)

d. Mobile Switching Center (MSC)

e. Home Location Register (HLR)

f. Visitor Location Register (VLR)

g. Authentication Center (AuC)

h. Equipment Identity Register (EIR)

Functions of Base Transceiver Station (BTS):

The BTS is a part of the Base Station Sub system (BSS) and it contacts Mobile Station (MS) through the radio interface. The BTS is in charge of management of simultaneous transmission and reception on the radio interface.

Functions of Base station Controller (BSC):

The BSC is connected on one side to one or several Base transceivers (BTS) and on the other side to the Mobile Switching Center (MSC). The main function of the Base Station Controller (BSC) is allocation and release of radio channels(or allocated ARFCH) and the handover (HO) management.

Functions of Mobile Switching Center (MSC)

The Mobile Switching Center (MSC) responsible for the basic function of switching. The main function of the Mobile Switching Center (MSC) is to co-ordinate the setting up of calls to and from GSM users and the external network The Mobile Switching Center (MSC) has interface with the Base Station Subsystem (BSS) on one side and the external network on the other side.

Functions of Home Location Register (HLR)

The Home Location Register (HLR) together with the Mobile Switching Center (MSC), provide the call ­routing and roaming capabilities of GSM. The Home Location Register (HLR) contains all the administrative information of each subscriber registered in the related to GSM network, along with the current location of the mobile.

Functions of Visitor Location Register (VLR):

The Visitor Location Register(VLR) contains roaming information. Once the visited system automatically detects the mobile, the Visitor Location Register (VLR) of that system enquires the Home location Register (HLR) to make sure you are a valid subscriber. It temporarily stores the last location area visited by the Mobile Station (MS), the power the mobile uses, the special services the Mobile Station (MS) is subscribed to and so on.

Functions of Equipment Identity Register (EIR):

The Equipment Identity Register (EIR) is a database that stores data related to the mobile equipment. It contains a list of all valid mobile equipment on the network, where each mobile station (MS) is identified by International Mobile Equipment Identity(IMEI). This is useful when searching for stolen mobile equipment or when monitoring misuse of Mobile Station (MS). If a Mobile operators want to prevent the misuse of Mobile phone; It's not too difficult for them.

Functions of Authentication Centers (AuC):

The Authentication Center is a database that stores a copy of the secret key stored in each subscriber's in Subscriber Identity Module (SIM) card, which is used for authentication and ciphering of the radio channel.

Find more Information:

• RF Planning and Optimization question (wimaxtec.blogspot.com)
• RF (Radio Frequency) Engineering (wimaxtec.blogspot.com)
• Implementation Of 4G network in Bangladesh (wimaxtec.blogspot.com)
• GSM Base Station KPI Analysis

RF Planning and Optimization question

Here is few question about RF planning and optimization, All the questions were collected from recruiter. these questions are frequently asked in viva board.

1। What is the basic procedure for RF planning using RF planning tool in GSM?

Answer:

• Analyze the system requirement and plan for data acquisition.

• Select proper Planning Tool

• Loading of digital map data of planning area on the planning tool

• Model tuning for prediction of network on the planning tool

• Nominal cell planning for the coverage area

• Frequency planning and interference calculation.

• Final system planning.
  

2। What are the different types of antennas used in a GSM network?

Answer:

Antennas may be categorize based on antenna beam width and gain, In GSM we have

• Space diversity antenna

• Cross polarized antenna

• Omni directional antennas.

3। How do the power, orientation, beam width, tilt and height of the antenna affect the coverage?

Answer:

Power

Antenna output power, always proportional to the coverage area. So power increases coverage increases.

Proper Orientation

Orientation of an antenna gives us coverage on specific direction of desired planning area, Antenna orientation tuning is a major treatment when we prescribe for a network optimization.

Beam width and tilt

Antenna beam width and tilt improves the coverage of the planned area. Proper tuning of beam width and tilt gives better coverage and performance.

Antenna height

Increase in antenna height improves the coverage.

4। What is EIRP? How is it calculated?

Answer:

Effective isotropic radiated power is the total output power of Base station

EIRP=BTS Output power + Antenna gain – {cable losses - other (combiner) losses}

5। What is frequency planning? Why is frequency reuse pattern used?

Answer:

Frequency planning is to be done for the GSM network for a given operator with A set of GSM frequency band .Frequency reuse pattern is used for capacity planning and for effective resource utilization.

6। What are all the patterns available? How is frequency planning done on planning tool ?

Answer:

7/21 pattern, 4/12 pattern, 3/9 pattern ।Automatic frequency planning can be done with planning tool।

7। What is co-channel interference and adjacent channel interference?

Answer:

The interference caused by using same channels in a network is called co - channel interference and interference caused by adjacent channels of is called adjacent channel interference. To distinguish actual signal from interface minimum power difference should be -9db.

8। How do you minimize co-channel interference?

Answer:

• Proper frequency planning

• Proper reuse pattern.

• Physical parameter tuning (Orientation, Height, Beam width, tilt etc)

• Finally resource addition ( Frequency and New BTS)

9. How do you minimize adjacent channel interference?
Answer:

• Proper frequency planning.

• Physical parameter tuning (Orientation, Height, Beam width, tilt etc)

10. What is model tuning? Why is it used? Okarama Hatta model?

Answer:

Model tuning is done for any network using planning tool to obtain proper prediction about network.

As there will be difference in actual and predicted data for planning tool. One of the Model is Okarama Hatta Model, generally known as Hatta model, is widely used which considers the signal propagation losses.