Cellular Network

Özge Bengür

Merih Miran

S.Burak Sarıgöl

The Cellular Network

The Cellular Concept

GSM Architecture

Capacity of a Cellular System

Cellular Network Today

The Cellular Concept

Made up of a number of radio cells

Uses multiple low-power transmitters (100W or less)

Coverage areas divided into cells

Implements Space Division Multiplex

A mobile phone

Two types of channesl avaible between mobile unit and Base Station Control Channels Traffic Channels

The Cellular Concept (con’t)

The Cellular Concept (con’t) The MS may be a vehicle or carried as a portable and

are assigned a duplex channel and communicate with an assigned Base Station

Base Station’s communicate simultaneously with all MS’s within their area of coverage and are connected to mobile switching centers

A MSC Controls a number of cells and are connected to PSTN

The Cellular Concept (con’t)

Each cell has allocated to it a number of channels which can be used for traffic or signalling

An active MS registers with an appropriate BS, the information is stored in MSC

When a call is set up either from of to the MS, the control and signalling system Assings a channel and Instructs the mobile to use the corresponding channel

Advantages of Cell Structures

Higher capacity, higher number of users

Less transmission power needed

More rebust, decentralized

Base Stations with interference, transmission area etc.

Frequency Reuse

Cellular phone networks use cellular frequency reuse.

In the cellular reuse concept, frequencies allocated to the service are reused in a regular pattern of areas, called "cells", each covered by one base station.

Frequency Reuse (con’t)

In mobile-telephone nets these cells are usually hexagonal.

Adjacent cells use different frequencies. However in cells that are separated further away, frequencies can be reused.

Principles of Cellular Frequency Reuse

Typical frequency reuse plan for 7 different radio frequencies, based on hexagonal cells.

In fact some problems in cellular frequency assignment are solved using map coloring theory.

Principles of Cellular Frequency Reuse (con’t)

Frequency 're-use' distance is the closest distance between the centers of two cells using the same frequency (in different clusters) is determined by the choice of the cluster size C and the lay-out of the cell cluster.

GSM

Global System for Mobile

A second generation cellular standard developed to cater voice services and data delivery using digital modulation

GSM (con’t)

The network behind the GSM system seen by the customer is large and complicated in order to provide all of the services which are required.

It is divided into a number of sections 1. Base Station Subsystem 2. Network and Switching Subsystem 3. GPRS Core Network 4. GSM services such as voice calls and SMS.

GSM System Architecture

Mobile Station (MS) Mobile Equipment (ME) Subscriber Identity Module (SIM)

Base Station Subsystem (BSS) Base Transceiver Station (BTS) Base Station Controller (BSC)

Network Switching Subsystem (NSS) Mobile Switching Center (MSC) Home Location Register (HLR) Visitor Location Register (VLR) Authentication Center (AUC) Equipment Identity Register (EIR)

GSM Specifications - I

RF Spectrum GSM 900

Mobile to BTS (uplink) : 890 – 915 MHz BTS to Mobile (downlink) : 935 – 960 MHz Bandwith : 2 * 25 MHz

GSM 1800 Mobile to BTS (uplink) : 1710 – 1785 MHz BTS to Mobile (downlink) : 1805 – 1880 MHz Bandwith : 2 * 75 MHz

GSM Specification - II Carrier Separation : 200 KHz

Duplex Distance : 45 MHz

No. of RF carriers : 124

Access Method : TDMA/FDMA

Modulation Method : GMSK

Modulation data rate : 270.833 Kbps

Outgoing Call1. MS sends dialed number to BSS

2. BSS sends dialed number to MSC

3. (4) MSC checks VLR if MS is allowed the requested service. If so, MSC asks BSS to allocate resources for call.

5 MSC routes the call to GMSC

6 GMSC routes the call to local exchange of called user

7 (8,9,10) Answer back (ring back) tone is routed from called user to MS via GMSC,MSC,BSS

Incoming Call1. Calling a GSM subscribers

2. Forwarding call to GMSC

3. Signal Setup to HLR

4. (5) Request MSRN from VLR

6. Forward responsible MSC to GMSC

7. Forward Call to current MSC

8. (9) Get current status of MS

10. (11) Paging of MS

12. (13) MS answers

14. (15) Security checks

16. (17) Set up connection

GSM Operation

Speech decoding

Channel decoding

De-interleaving

Burst Formatting

De-ciphering

DemodulationModulation

Ciphering

Burst Formatting

Interleaving

Channel Coding

Speech coding

Radio Interface

Speech Speech

13 Kbps

22.8 Kbps

22.8 Kbps

33.6 Kbps

33.6 Kbps

270.83 Kbps

Security in GSM

On air interface, GSM uses encryption and TMSI instead of IMSI.

SIM is provided 4-8 digit PIN to validate the ownership of SIM

3 algorithms are specified : A3 algorithm for authentication A5 algorithm for encryption A8 algorithm for key generation

Advantages of GSM

Capacity increases Reduced RF transmission power and longer

battery life. International roaming capability. Better security against fraud (through

terminal validation and user authentication). Encryption capability for information security

and privacy.

GSM Applications

Mobile telephony

GSM-R

Telemetry System Fleet management Automatic meter reading Toll Collection

Value Added Services

Handover

When a mobile terminal moves outside the coverage area of its base station, the network management is assumed to take appropriate measures.

A 'handover' or 'handoff' to another base station is required to ensure sufficient quality of reception, including acceptable interference power levels.

A mobile user experiences the worst link quality if the terminal is located at the boundary of two cells where the distances to base stations are maximum.

Handover (con’t)

Different strategies for handover exist:

1. Centralized methods, as for instance used in GSM.

2. Decentralized methods, as for instance used in DECT.

Co-Channel Interference (CCI) CCI arises in cellular systems where the available

frequency channels are divided into different sets.

Each set being assigned to a specific cell and with several cells in the system using the same set of frequencies.

CCI limits the system capacity

This interference generally happens in places where population is high.

The Capacity of Cellular Network

Why do we need more capacity?

Reach more users at the same time

Share more information throughout the network.

New technologies will require more complex solutions and these solutions can be achieved with maximum space available.

The capacity of cellular systems can be increased by;

Frequency borrowing

Cell splitting

Cell sectoring

Microcells

The Capacity of Cellular Network (con’t)

Frequency Borrowing

RF bandwidth is the most important constraint in wireless systems.

So to increase the capacity, frequency of Radio Signals and wireless systems shall be increased.

To do this, frequencies are taken from adjacent cells by congested cells.

Cell Splitting

The unit area of RF coverage for cellular network is called a cell.

In each cell, a base station transmits from a fixed cell site location, which is often centrally located in the cell.

In base stations where the usage of cellular network is high, these cells are split into smaller cells.

Cell Splitting (con’t) The radio frequencies are reassigned, and

transmission power is reduced. A new cell site must be constructed when a cell is

split

Cell splitting is one of the easy and less costly solution when increasing the capacity of cellular network.

Splitting the cells into smaller ones also lead to a new solution called cell sectoring.

Cell Sectoring

Sectorization consists of dividing an omnidirectional (360 degree) view from the cell site into non-overlapping slices called sectors.

When combined, sectors provide the same coverage but they are considered to be separate cells.

Also considered as one of easy and inexpensive capacity increasing solution.

Cell Sectoring

Microcells

As the splitting of cell idea evolves, the usage of smaller cells become efficient and it leads the creation of microcells.

The aim of creating microcells are increasing the capacity of cellular network in areas where population is high.

Microcells (con’t)

Typical comparison can be made like this;

Cells typically range in size from two to twenty kilometers in diameter.

Microcells range from about a hundred meters to a kilometer in diameter.

The Cellular Network Past

AMPS

Two Standards IS-95 (CDMA) IS-136(D-AMPS)

The Cellular Network Today

The Cellular Network Today

Present Day

Becoming increasingly popular

Mobile phones are extremely common

More devices will use cellular network

World becomes smaller and more united

2G and 3G Systems

The Cellular Network in Future

The Cellular Network in Future

Next Generation 3G System Use of common global frequencies for all cellular

networks. Worldwide roaming. Standardization of radio interfaces. High data transmission rates for both circuit and

packet switched data. Efficient spectrum utilization schemes.

From GSM to UMTS Upgrade to provide better data transmission

Conclusion Useful and cheap service

Services provided by network operators

High speed data networks

Industry working on mobile communication technologies

The voice and data services the ideal communication

References http://www.acm.org/crossroads/xrds7-2/cellular.html http://wireless.per.nl:81/reference/chaptr04/cellplan/cellular.htm http://www.bsi.de/literat/doc/gsm/index_e.htm http://www-dse.doc.ic.ac.uk/~nd/surprise_96/journal/vol1/pr4/arti

cle1.html#one http://en.wikipedia.org/wiki/Cellular_network http://www.wipo.int/pctdb/en/wo.jsp?

wo=2004068777&IA=WO2004068777&DISPLAY=CLAIMS http://users.ece.utexas.edu/~jandrews/ee381k/EE381KTA/

article6.pdf http://www.leapforum.org/published/internetworkMobility/split/

node33.html http://en.wikipedia.org/wiki/GSM CellularRadioSystems-M.Şafak

Questions

Question1

How can Cellular network capacities will be improve in the future?

A: There are lots of solutions for improving the capacity of the Network. But the one of the most logical one is, using the logical solution cell in the sector with adaptive antennas. And using more cells where the number of subscriber is bigger.

Question2

Why Base stations of GSM operators trying to expand around the Turkey?

A: The communication between mobiles and base stations is provided with radio signals. More base stations amplify the radio signal and make a strong communication over the mobiles. For example when you water the tribunes from the middle of a stadium, you cannot reach every point of the tribunes. But when you water the tribunes from the corners, you can reach every point of the tribunes. Hence the GSM operators in the Turkey are trying to reach

every point for creating a powerful signal area all over the country.

Question3

Why we need the frequency reuse? What are the reasons?

A: We need frequency reuse because we have a bandwidth. If we use same frequency in every cell, the other cells make interference. Hence the specific frequency is trying to not use by the other cells.