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.