Gsm Technology

SpeakersSpeakers

Hasan Mahmood 061-19-324Shamim Jahan 061-19-346Nafiur Rahman 061-19-347

What’s a Cellular Phone ?What’s a Cellular Phone ?

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Why it Calls Cellular ?Why it Calls Cellular ?

Uses hexagonal Cells

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Why Use Cells ?Why Use Cells ?

If there was no cell

Limited frequency spectrum for available Mobile Communication.

Frequency utilization is inefficient.

Single high power antenna causes to be the limited number of users.

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Why Use Cells ?Why Use Cells ?

Advantages of cell structures:

More capacity due to frequency reuse.Less transmission power needed.Deals interference, transmission area locally.

Problems:

Fixed network needed for the base stations.Handover necessary.Interference with other cells.

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Frequency ReuseFrequency Reuse

Adjacent cells are assigned different frequencies to avoid interference or crosstalk.

The same frequency is reused in different areas for after a certain distance.

In GSM networks a frequency reuse pattern with

k = 3, 4, 7, 12 or 21. (k= Frequency reuse factor)

Each ‘k’ number of cells make a cluster.

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Different Frequency ReuseDifferent Frequency Reuse

Fixed frequency assignment:

certain frequencies are assigned to a certain cell.problem: different traffic load in different cells.

Dynamic frequency assignment:

base station chooses frequencies depending on the frequencies already used in neighbor cells.more capacity in cells with more traffic.

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Frequency Reuse 3 & 4Frequency Reuse 3 & 4

Reuse factor = 33 cell Cluster

Reuse factor = 44 cell Cluster

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Frequency Reuse 7Frequency Reuse 7

Reuse factor = 77 cell Cluster

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Adjacent Channel InterferenceAdjacent Channel Interference

Comes from imperfect filters that allow frequency leakage into the band

Serious problem if interferer is nearby, near-far effect

Nearby mobile transmits on a frequency near to that of a weak mobile

Base station receivers need high-Q filters to reject adjacent channel interference.

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Adjacent Channel InterferenceAdjacent Channel Interference

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Co Channel InterferenceCo Channel Interference

One of the primary forms of man-made signal degradation associated with digital radio, co-channel interference occurs when the same carrier frequency reaches the same receiver from two separate transmitters.

In a fully equipped hexagonal-shaped cell system, there is always 6 co channel cell.

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Co Channel InterferenceCo Channel Interference

F1 F1

F1

F1

F1MS

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6 Co Channel Cell in 16 Co Channel Cell in 1stst Tier Tier

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Co Channel InterferenceCo Channel Interference

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MS HandoffMS Handoff

Max Handoff Gap: 150 ms in the case of inter cell.

When the mean signal (over some predetermined time) is below some threshold

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Handoff ProcedureHandoff Procedure BTS provides list to MS of available channels in

neighboring cells via BCCH (Broadcast Control Channel)

MS monitors RSS from the BCCHs of the neighboring cells and reports it to MSC using SACCH

BTS also monitors RSS of MS. Proprietary algorithms are used to decide when to

handoff. MSC negotiates new channel with the new BSS and

indicates to the MS to initiate a handoff . Upon completion of handoff MS informs the MSC. Old BSS is informed to release the resources.

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Signal StrengthSignal Strength

Cell i

-60

-70-80

-90

-100

Cell j

-60-70

-80-90

-100

Signal Strength (in dB)

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High speed vehicles can cross many “small” cells in a short time.

Umbrella cell. Large cell with a powerful tower to handle high speed vehicles

Another problem is called cell dragging.

Happens when the user moves slowly away from the cell and the tower didn’t recognize it due to strong average signal.

Problems of HandoffProblems of Handoff

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Between two BTS under same BSS

Between two BTS under different BSS but same MSC

Between two BSS under different MSCs(Old MSC continues to handle the call)

Between two MSCs in different networks (providers)

Types of HandoffTypes of Handoff

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Next SpeakerNext Speaker

Shamim JahanShamim Jahan

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Sectored CellsSectored CellsTo reduce the adjacent channel interference & co-

channel interference a cell can be divided into 3 or 6 sectors according to the demand.

Large cell (low density)

Small cell (high density)

Smaller cell(higher density)

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3 sectors per cell3 cells per cluster

1

2

3

2

3

1

2

3

2

3

2

3

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3 sectors per cell4 cells per cluster

3

12

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2

41

3

12

4

3

12

4

41

25

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6 sectors per cell

2 cells per cluster

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121

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2

22

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Original View of SectorsOriginal View of Sectors

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FDM & FDMAFDM & FDMA

Time

Fre

qu

enc

y

F3

F2

F1

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TDM & TDMATDM & TDMA

Time

Am

pli

tud

e

T3T2T1

Frequen

cy

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CDM & CDMACDM & CDMA

Time

Cod e

Code3

Code2

Code1

Frequen

cy

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Frequency SpectrumFrequency Spectrum

Down link 935~960MHz

Up link 890~915MHz

MS

BTS

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ChannelsChannels

GSM 900 uses 25 MHz frequency spectrum with 124 channels for uplink & downlink.

Each channels have 200 KHz bandwidth.

Each two channels are separated by 1.6 KHz guard band.

Each channel have 8 time slots which allows 8 subscribers to use same frequency by TDMA.

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ChannelsChannels

1 2 4 5 6 123 1243

1 42 3 5 86 7

Channels with 200 KHz bandwidth

Time slots

4.615 ms

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Cell TypesCell Types

Cells can be classified into three types

according to cell size

Macro cellMicro cellPico cell

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Cell Types based on RangeCell Types based on Range

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Cell Types based on RangeCell Types based on Range

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Next SpeakerNext Speaker

Nafiur RahmanNafiur Rahman

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Types of AntennaTypes of Antenna

There are three Types of antennas:

Omni-directional

Semi-directional

Highly-directional

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Omni-Directional AntennaOmni-Directional AntennaThe dipole is an omni-directional antenna, because

it radiates its energy equally in all directions around its axis. The signal from an omni-directional antenna radiates in a 360-degree horizontal beam.

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Dipole Doughnut Dipole Side View

Omni-Directional AntennaOmni-Directional Antenna

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Semi-Directional AntennaSemi-Directional Antenna

Semi-directional antennas often radiate in a hemispherical or cylindrical coverage pattern. Semi-directional antennas types frequently used with wireless LANs are Patch, Panel, and Yagi antennas.

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Usage of Semi-Directional AntennaUsage of Semi-Directional Antenna

Semi-directional antennas are ideally suited for short & medium range bridging. A corridor, or a large room, a semi-directional antenna would be a good choice to provide the proper coverage.

Coverage area of semi directional antenna.

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Semi Directional AntennaSemi Directional Antenna

Semi directional antenna

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Directional AntennaDirectional Antenna

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Highly-Directional AntennaHighly-Directional Antenna

Highly-directional antennas emit the most narrow signal beam of any antenna type & have the greatest gain of these three groups of antennas. Highly-directional antennas are typically concave, dish-shaped devices.

Highly Directional Parabolic Dish Antenna & Grid Antenna45

Usage of Highly-Directional AntennaUsage of Highly-Directional Antenna

These antennas are used for point-to-point communication links, and can transmit at distances up to 25 miles. Highly directional antennas have a very narrow beam width and must be accurately aimed at each other.

Radiation Pattern of Highly Directional Antenna46

LOS (Line of Sight)LOS (Line of Sight)

Directional antenna always propagate signal through LOS. Basically LOS are two types:

1.Visual LOS

2.RF LOS

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LOSLOS

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Fresnel ZoneFresnel Zone

Typically 40% fresnel zone blockage introduce a manageable amount of interference into the link.

But more then 40% is means the link will be unreliable.

Allowing no more then 20% blockage of fresnel zone. When system is design that time we have to keep it 0%.

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Fresnel ZoneFresnel Zone

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BSS Base Station System

BTS Base Transceiver Station

BSC Base Station Controller

MS Mobile Station

NSS Network Sub-System

MSC Mobile-service Switching Controller

VLR Visitor Location Register

HLR Home Location Register

AuC Authentication Server

GMSC Gateway MSC

GSM 2G ArchitectureGSM 2G Architecture

BTS

BSCMSC

VLR

HLRAuC

GMSC

BSS

PSTN

NSS

AE

CD

PSTNAbis

B

H

MS

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Data Transfer Over GSMData Transfer Over GSM

Four enhancements to GSM for data

HSCSD: High Speed Circuit Switched Data GPRS: General Packet Radio Service EGPRS: Enhanced GPRS EDGE: Enhanced Data rates for the GSM Evolution

Technically EGPRS and EDGE are same.

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