Cellular Mobile Communication Concept and GSM Frequency Band

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Cellular Mobile Communication Concept & GSM Frequency Band

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Shuvo Chakraborty

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What is Mobile Phone?

The mobile phone or cell phone is a long-range, portable electronic device used for mobile communication.

What a mobile containsPCB(Printed Circuit Board)

RF Amplifier 

Microphone

Antenna

Speaker

Battery

Flash/ROM

What is Mobile Communication?

A wireless form of communication in which voice and data information is emitted, transmitted and received via microwaves.

Example: cellular and digital cordless telephones, pagers, satellite based communication etc.

Related Terms

• MS - Mobile Station• BTS - Base Transceiver

Station• MSC - Mobile Switching

Center• PSTN - Public Switching

Telephone Network

How a Mobile Works?

Working Procedure of Mobile Phone

• At first the mobile signal generates from Mobile Station

• Then the signal is received by Base Station where the Base Transceiver Station is responsible for carrying out radio communications between the network and the Mobile Phone.

• After that the signal is transferred to Mobile Switching Center. The MSC is the heart of the GSM network. It handles call routing, call setup, and basic switching functions.

• Then the signal is received by PSTN(Public Switch Telephone Network) & then again received by Base Station Antenna and the connect to the Mobile Station.

What is Adaptive Power Control?

• Adaptive power control is a unique feature of a mobile phone which provides a facility to control the transmit power of a mobile phone

If a phone is far from a mobile tower, the phone will transmit maximum signal to be in range of the tower.The closer it gets to the mobile tower it lessens the transmit power.

This process is called Adaptive power control.

Special codes of Mobile phone• Mobile phone has 3 special codes• • 1)ESN(Electronic Serial Number) - A unique

32-bit number programmed into the phone when it is manufactured 2)MIN(Mobile Identification Number) - A 10-digit number derived from user’s phone's number 3)SID(System Identification Code) - A unique 5-digit number that is assigned to each carrier by the Federal Communications Commission (FCC)

SIM card

• A SIM card, also known as a subscriber identity module, is a smart card inside a cellular phone, carrying an identification number unique to the owner & storing personal data

What a SIM Number contain?

A SIM Number Contains : • Country Code - First 3 Digit ; Network Code- Next 2 Digit ; Mobile Station Identification-Around 10 Digit.

Control Channel for call

• In radio communication, a control channel is a central channel that initiates calls.

• In GSM networks, Control Channels can be broadly divided into 3 categories; ->Broadcast Control Channel (BCCH)->Common Control Channel (CCCH ) ->Dedicated Control Channels (DCCH).

Traffic channel for data Communication :

Traffic channel in GSM carry user speech or data as other technology.•It is used both in the uplink and downlink

• In cellular telecommunications, the term handover or handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another channel.

•

Different Frequency Bands

Am Radio

300 Hz – 3400 Hz

3 KHz – 300 GHz

88 – 108 MHz

540 – 1600 KHz

GSM (Global System for Mobile Communication)

• GSM is a circuit-switched system that divides each 200 kHz channel into eight 25 kHz time-slots

Frequency Bands in GSM System :

Mainly 4 frequency used in GSM

GSM 1800 MHz

GSM 400 MHz

GSM 900 MHz

GSM 1900 MHz

GSM operates on the mobile communication bands 900 MHz and 1800 MHz in most parts of the world. In GSM 900MHZ there are 2 part UL(uplink)– 890-915 MHzDL(Downlink)– 935-960MHz The difference between them is 45 MHz

Why ul(Uplink) frequency is lower than dl(Download) frequency?

• It is because a mobile is operated on a battery with low power and therefore it sends signals on low(uplink) frequencies whereas a base station is driven by high power and can easily send signals on high(downlink) frequencies. This can be verified by Friis equation which says Transmitted power and Transmitted frequency are directly proportional.

FDMA in GSM• The FDMA part involves the division by frequency of the (maximum)

25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. 25 MHz is divided by 200 KHz.

Total Frequency = 25MHz/200KHz = 125

1 125

100KHz 100KHz• From total 125 KHz, 1 KHz is used for guard band.• Remaining usable frequency = 125 – 1 = 124

• If the remaining 124 frequencies are divided into 3 operators then each will get –> 124/3 = 41.3 F

• The overall process is done by FDMA.

TDMA in GSM• In GSM TDMA is used in combination with FDMA to overcome

capacity issue due to the problem of limited frequency spectrum. • In GSM networks on one carrier frequency, that all users in the

same cell are using, TDMA allocate 8 timeslots. • Each slot contains uplink and downlink.

• 0 1 2 3 4 5 6 7• 1 slot works for 577 microsecond• Total slots works for 4.615 millisecond.

ul

Dl

Example• If there are 41 frequencies then the time slot • allocation using TDMA will be like below: 13

1 41 12

1 frequency is used for guard band Remaining frequency = 29 - 1 = 28F