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 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. • Adjacentcells 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 • Base Station Subsystem • Network and Switching Subsystem • GPRS Core Network • 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 Call • MS sends dialed number to BSS • BSS sends dialed number to MSC • (4) MSC checks VLR if MS is allowed the requested service. If so,MSC asks BSS to allocate resources for call. • MSC routes the call to GMSC • GMSC routes the call to local exchange of called user • (8,9,10) Answer back(ring back) tone is routed from called user to MS via GMSC,MSC,BSS
Incoming Call • Calling a GSM subscribers • Forwarding call to GMSC • Signal Setup to HLR • (5) Request MSRN from VLR • Forward responsible MSC to GMSC • Forward Call to current MSC • (9) Get current status of MS • (11) Paging of MS • (13) MS answers • (15) Security checks • (17) Set up connection
Speech Speech Speechdecoding Speech coding 13 Kbps Channel Coding Channeldecoding 22.8 Kbps Interleaving De-interleaving 22.8 Kbps BurstFormatting Burst Formatting 33.6 Kbps Ciphering De-ciphering 33.6 Kbps Radio Interface Modulation Demodulation 270.83 Kbps GSM Operation
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: • Centralized methods, as for instance used in GSM. • 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 Network (con’t) • The capacity of cellular systems can be increased by; • Frequency borrowing • Cell splitting • Cell sectoring • Microcells
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.
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 • 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 • 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/article1.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
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.