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Department of Computer Science

DCS

COMSATS Institute of

Information Technology

Cellular Networks

Rab Nawaz JadoonAssistant ProfessorCOMSATS IIT, Abbottabad

Pakistan

Mobile Communication


2G- Cellular Systems

▲ The second-generation (2G) systems, the successors of 1G systems,

are digital [i.e., they convert speech into digital code (a series of pulses) which results in a clearer signal] and thus they overcome the deficiencies of 1G systems mentioned above.

It may be noted that the user traffic (computer data which is inherently digital or digitized voice) must be converted into radio waves (analog signals) before it can be sent (between MT and BS).

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2G Standards

▲ A 2G system is called personal communications services (PCS) in the marketing literature.

There are several 2G standards followed in various parts of the world.

Some of them are Global System for Mobile Communications (GSM) in Europe,

Digital-AMPS (DAMPS) in United States, and

Personal Digital Cellular (PDC) in Japan.

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GSM

▲ GSM is an extremely popular 2G system, which is fully digital, used across over 100 countries.

GSM originally stood for Groupe Speciale Mobile, the name of the working group that designed it.

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GSM Variants•This first variant reuses

the spectrum intended for Europe's analog total access communication system (TACS)]

Operating around 900 MHz

•Licensed in Europe specifically for GSM.

•This variant is sometimes called digital communications network (DCN)]

Operating around 1,800 MHz

•Used in United States for several different digitalnetworks)

Operating around 1,900 MHz

•Latest variant for replacing aging analog networks based on NMT system)

Operating around 450 MHz

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GSM Services

▲ Apart from voice service, GSM also offers a variety of data services.

The modulation scheme used in GSM is Gaussian minimum shift keying (GMSK).

GSM uses frequency duplex communication, and each call is allotted a duplex channel.

The duplex channels are separated by 45 MHz.

Every channel is of 200 KHz bandwidth. Thus, GSM uses FDM to separate the channels.

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Channels used in GSM

▲ The downlink (BS-MT)

Channels are allotted 935-960 MHz

▲ Uplink (MT-BS)

Channels are on 890-915 MHz

The uplink "frame" of eight slots is shifted by a delay of three slots from the downlink frame.

Why???

Because that the MT does not have to send and receive at the same time.

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Adaptive frame Alignment

▲ A procedure called adaptive frame alignment is used to account for propagation

delay.

An MT which is far away from the BS starts its frame transmission slightly ahead of the actual slot commencement time, so that the reception at the BS is synchronized to the frame structure.

The exact advancement is instructed by the BS tothe MT, with MTs that are farther away from the BS requiring a greater advancement.

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GSM Frequency Band and TDMA Frame

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Channel Division

▲ Each physical channel is divided into eight periodic time slots

(Each of which is 0.577 ms duration), which are time-shared between as many as eight users (logical channels) using TDMA.

A complete cycle of eight time slots is called a (TDMA) frame.

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Parts of TDMA Slot

▲ Four parts,

Header and footer

(These are empty space at the beginning and end of the slot to separate a slot from its neighbors and to negate the effects of propagation delay for distances up to 35 Km from the BS).

Training sequence

(This is to help a receiver lock on to the slot)

Stealing bits

These identify whether the slot carries data or control information)

Traffic

(This part carries user traffic (voice/data) as well as control information and error correction).

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Cont…

▲ Users cannot use all frames; for every 24 GSM frames that carry voice/data, one is "stolen" forsignaling and another reserved for other types of traffic (such as short text messages orcaller line identification).

Thus eight slots make up a TDMframe and 26 TDM frames make up a multiframe.

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Control channel classification

▲ 3 main categories are

The broadcast control channel (BCCH)

is a downlink channel that contains the BS's identity andchannel status. All MTs monitor the BCCH to detect if they have moved into a new cell.

The dedicated control channel (DCCH)

is used for call-setup, location updates, and all call-management related information exchange.

Every call has its own allotted DCCH.

The information obtained on the DCCH is used by the BS to keep a record of all the MTs currently in its footprint (coverage area).

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Cont…

Common control channel(CCCH),

consists of the downlink paging channel, which is used to page any MT to alert it for an incoming call.

As each MT is assigned only one slot within each frame, the maximum speed for data services is around 34 Kbps (1/8 of the 270.8 Kbps capacity of a 200 KHz GSM carrier).

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SIM

▲ Important feature of GSM is

the subscriber identity module (SIM), a smart card which is pluggable into a GSM phone (mobile handset).

The SIM stores information such as

the subscriber's identification number,

the networks and countries where the subscriber is entitled to service, and other

user-specific information.

By inserting the SIM card into another handset, the user is able to use the new handset to make/receive calls while using the same telephone number.

Thus SIM provides personal mobility.

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Call handling strategies in GSM

▲ GSM requires the call to always be routedthrough the home network of the mobile.

This causes a circuitous route for calls to bedelivered to roaming subscribers.

The HLR of the destination mobile is first contacted,and then pointers are followed to reach the MT's current cell, even if it is the same cell as that of the originating MT.

This could be avoided by routing directly to the foreign network, once the location of the receiving mobile is established.

.

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Data over Voice Channel

▲ Cellular systems were primarily designed to support voice calls only.

But the demand for supporting other data services, ranging from short messages to Web-browsing, slowly emerged.

It was realized that cellular networks have to be modified to support data services.

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Problems

▲ The main problems in using a voice network for data transmission are listed below,

Signal distortion:

Speech encoders are being used on all links to exploit the redundancy in speech and compress it, in order to conserve bandwidth. However, data cannot be expected to have such redundancy, and the data receivers cannot interpolate data the way a human listener can interpolate even with degradation in speech quality.

Handoff error:

Handoffs introduce a certain delay in transfer of the call from one cell to another, which may lead to loss of data.

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Cont…

Interfacing with fixed network:

A PSTN modem expects a 2,100 Hz tone from thesource when a data call is initiated.

On the other hand, PSTN networks do not indicate non-voice service.

So the cellular network should be able to differentiate between a data call and a voice call.

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Cont…

▲ The main problem is in making the network recognize a data call, and handle it differently from a voice call, by probably disabling some optimizations made on voice calls.

Some possible solutions suggested as under,

A control message could be transmitted all along the path of the call, to indicate a data call so that voice coding can be disabled.

A two-stage dial-up operation can be used, first to the cellular carrier and then to the subscriber. The carrier has different numbers for each service it offers. For

example, to set up a data call with an MT, the cellular carrier is first dialed, and it then informs the MT of a data call.

A subscriber could be assigned separate subscriber numbers for each service he/she opts for.

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Department of Computer Science 21

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