2.2 HARDWARE AND HARDWARE AND SOFTWARE VIEWS OSOFTWARE VIEWS OF CELLULAR F CELLULAR NETWORK:NETWORK:
– Hardware view of a cellular network
• Serving areas
• Cells
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Fig 2.4 Hardware view of cellular network
– Software view of a cellular network
• Location area identity
• Cell global identity
• Mobile country code and network code
Fig 2.5 Software view of Cellular system
2.3
2.3 3G 3G Cellular Cellular System System ComponentsComponents – Core network
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– Radio network controller – Radio base station
Fig 2.6 The 3G radio network controller
2.4
2.4 Cellular Component IdentificationCellular Component Identification – Subscriber device identification
• Mobile station ISDN identification number – North American version
– The rest of the world
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Fig 2.6 Formation of MSISDN number
• Cellular Component Identification
– International mobile subscriber identity
Fig 2.7 Formation of IMSI number
– International mobile equipment identity
Fig 2.8 Formation of IMEI number
Cellular system component addressing
• Location area identity
• Cell global identity
• Radio base station identity code
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• Location numbering
• Addressing cellular network switching nodes
• Global title and global title translation
• 2.5 Call Establishment 2.5 Call Establishment
– Mobile-terminated call
• PSTN messages
• GMSC operations
• MSC/VLR operations
• BSC operations
Fig 2.9 Mobile terminated call operations
Mobile-srcinated call
• Mobile operations
• Radio base station operations
• Base station controller operations
• MSC operations
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Fig 2.10 Mobile srcinated call operations
– Call release
• Connection management operations
• Radio resource operations
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Fig 2.11 Call release
The above figure shows the operation during release of a mobile call through MSC . the steps involved as shown in detail which is self explanatory.
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UNIT - 3 UNIT - 3
Wireless network architecture and operation, Cellular concept Cell fundamentals, Capacity expansion techniques, Cellular backbone networks, Mobility management, Radio resources and power management Wireless network security
6 Hours 6 Hours
TEXT BOOK:
TEXT BOOK:
1. Wireless Telecom Systems and networksWireless Telecom Systems and networks, Mullet: Thomson Learning 2006.
REFERENCE BOOKS:
REFERENCE BOOKS:
1. Mobile Cellular TelecommunicationMobile Cellular Telecommunication, Lee W.C.Y, MGH, 2002.
2. Wireless communicationWireless communication - D P Agrawal: 2nd Edition Thomson learning 2007.
3. Fundamentals of Wireless CommunicationFundamentals of Wireless Communication, David Tse, Pramod Viswanath, Cambridge 2005.
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3.1 The Cellular ConceptThe Cellular Concept
Solves the problem of spectral congestion and user capacity,Offer very high capacity in a limited spectrum without major technological changes,Reuse of radio channel in different cells.Enable a fix number of channels to serve an arbitrarily large number of users by reusing the channel throughout the coverage region.Simplex and duplex
Each cellular base station is allocated a group of radio channels within a small geographic area called acell .Neighboring cells are assigned different channel groups.
By limiting the coverage area to within the boundary of the cell, the channel groups may be reused to cover different cells.Keep interference levels within tolerable limits.
Frequency reuse or frequency planning seven groups of channel from A to G.footprint of a cell - actual radio coverage ,omni-directional antenna v.s. directional antenna
Steps for frequency reuse:
Consider a cellular system which has a total ofS duplex channels.
• Each cell is allocated a group ofchannels, k .
• TheS channels are divided among N cells.
• The total number of available radio channels
• The N cells which use the complete set of channels is calledcluster .
• The cluster can be repeated M times within the system. The total number of channels,C , is used as a measure of capacity
• The capacity is directly proportional to the number of replication M .
• The cluster size, N , is typically equal to 4, 7, or 12.
• Small N is desirable to maximize capacity.
• The frequency reuse factor is given by
• Hexagonal geometry has
– exactly six equidistance neighbors
– the lines joining the centers of any cell and each of its neighbors are separated by multiples of 60 degrees.
• Only certain cluster sizes and cell layout are possible.
• The number of cells per cluster, N , can only have values which satisfy
• Co-channel neighbors of a particular cell, ex,i=3 and j=2.
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• Picocells
• Microcells
• Macrocells
• Megacells and femtocells
Fig 3.1 Cellular concept 3.2 Cell Fundamentals
3.2 Cell Fundamentals
– The use of hexagons – Reuse number
• Cellular reuse patterns
Fig 3.2 Frequency reuse concept
• Frequency reuse scheme – increases capacity
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– minimize interference
• Channel assignment strategy – fixed channel assignment – dynamic channel assignment
• Fixed channel assignment
– each cell is allocated a predetermined set of voice channel – any new call attempt can only be served by the unused channels – the call will beblocked if all channels in that cell are occupied
• Dynamic channel assignment
– channels are not allocated to cells permanently.
– allocate channels based on request.
– reduce the likelihood of blocking, increase capacity.
• Cell Fundamentals – Reuse number
• Frequency reuse distance
– The reuse distance can be calculated by using the equation:
Fig 3.3 Frequency Reuse number
• Cell Fundamentals
– Cellular interference issues
• Signal-to-interference ratio
• Channel assignments
Fig 3.4 Cellular calculations
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• Split congested cell into smaller cells.
– Preserve frequency reuse plan.
– Reduce transmission power.
• Transmission power reduction from to Examining the receiving power at the new and old cell boundary
• If we taken = 4 and set the received power equal to each other
• The transmit power must be reduced by 12 dB in order to fill in the srcinal coverage area.
• Problem: if only part of the cells are splited
– Different cell sizes will exist simultaneously
• Handoff issues - high speed and low speed traffic can be simultaneously accommodated
Fig 3.5 cell splitting
• Capacity Expansion Techniques – Cell sectoring
• Sectoring concept
• Decrease theco-channel interferenceand keep the cell radius R unchanged – Replacing single omni-directional antenna by several directional antennas – Radiating within a specified sector
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Fig 3.6 Cell sectoring
• Capacity Expansion Techniques – Overlaid cells
• Overlay concept
Fig 3.7 Cell overlaid
• Capacity Expansion Techniques – Channel allocation
– Other capacity expansion schemes
• Lee’s microcell technology
• Smart antenna technology
• Migration to digital technology
•
3.4 Cellular Backhaul Networks 3.4 Cellular Backhaul Networks
– Introduction
– Standards for PSTN carriers
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Fig 3.8 cellular backhaul network
Fig 3.9 cellular backhaul network 3.5 Mobility Management
3.5 Mobility Management
– Location management
• Need
• Frequency
• Location updating
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Fig 3.10 Location management in cellular network
• Mobility Management – Paging messages
– Different paging schemes
– Transmission of the location information between network elements
• Mobility Management – Handoff management
• Handoff control
• Handoff operation
• Handoff algorithm
• When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station.
• Handoff operation
– identifying a new base station
– re-allocating the voice and control channels with the new base station.
• Handoff Threshold
– Minimum usable signal for acceptable voice quality (-90dBm to -100dBm) – Handoff margin cannot be too large or too small.
– If it is too large, unnecessary handoffs burden the MSC
– If it is too small, there may be insufficient time to complete handoff before a call is lost.
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Fig 3.10 Mobility management in cellular network
• Handoff must ensure that the drop in the measured signal is not due to momentary fading and that the mobile is actually moving away from the serving base station.
• Running average measurement of signal strength should be optimized so that unnecessary handoffs are avoided.
– Depends on the speed at which the vehicle is moving.
– Steep short term average -> the hand off should be made quickly
– The speed can be estimated from the statistics of the received short-term fading signal at the base station
• Dwell time: the time over which a call may be maintained within a cell without handoff.
• Dwell time depends on – propagation – interference – distance – speed
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• Handoff measurement
– In first generation analog cellular systems, signal strength measurements are made by the base station and supervised by the MSC.
– In second generation systems (TDMA), handoff decisions are mobile assisted, called mobile assisted handoff (MAHO)
• Intersystem handoff: If a mobile moves from one cellular system to a different cellular system controlled by a different MSC.
• Handoff requests is much important than handling a new call.
• Different type of users
High speed users need frequent handoff during a call.
Low speed users may never need a handoff during a call.
• Microcells to provide capacity, the MSC can become burdened if high speed users are constantly being passed between very small cells.
• Minimize handoff intervention
– handle the simultaneous traffic of high speed and low speed users.
• Large and small cells can be located at a single location (umbrella cell) – different antenna height
– different power level
• Cell dragging problem: pedestrian users provide a very strong signal to the base station
– The user may travel deep within a neighboring cell
Handoff for first generation analog cellular systems ,10 secs handoff time, is in the order of 6 dB to 12 dB,Handoff for second generation cellular systems, e.g., GSM 1 to 2 seconds handoff time, mobile assists handoff , is in the order of 0 dB to 6 dB Handoff decisions based on signal strength, co-channel interference, and adjacent channel interference.
IS-95 CDMA spread spectrum cellular system ,Mobiles share the channel in every cell.No physical change of channel during handoff ,MSC decides the base station with the best receiving signal as the service station Handoff within a cell, No channel re-assignment, Switch the channel to a different zone site, Reduce interference, Low power transmitters are employed
• Frequency reuse - there are several cells that use the same set of frequencies – co-channel cells
– co-channel interference
• To reduce co-channel interference, co-channel cell must be separated by a
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– co-channel interference is independent of the transmitted power – co-channel interference is a function of
• R: Radius of the cell
• D: distance to the center of the nearest co-channel cell
•
• Increasing the ratioQ=D/R, the interference is reduced.
• Q is called the co-channel reuse ratio
Fig 3.11 Handoff management
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Fig 3.12 analysis of handoff operation
3.6 Radio Resources and Power Management – Power control
– Power saving schemes
• Discontinuous transmission
• Sleep modes
• Energy efficient designs – Radio resource management
• Need
• Schemes 3.7 Wireless Network Security
– Wireless network security requirements – Network security requirements
– Network security
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UNIT - 4 UNIT - 4
GSM and TDMA techniques, GSM system overview, GSM Network and system Architecture, GSM channel concepts, GSM identifiers
6 Hours 6 Hours
TEXT BOOK:
TEXT BOOK:
1. Wireless Telecom Systems and networksWireless Telecom Systems and networks, Mullet: Thomson Learning 2006.
REFERENCE BOOKS:
REFERENCE BOOKS:
1. Mobile Cellular TelecommunicationMobile Cellular Telecommunication, Lee W.C.Y, MGH, 2002.
2. Wireless communicationWireless communication - D P Agrawal: 2
nd
Edition Thomson learning 2007.
3. Fundamentals of Wireless CommunicationFundamentals of Wireless Communication, David Tse, Pramod Viswanath, Cambridge 2005.
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Unit-4 Unit-4 GSM AND TDMA
GSM AND TDMA TECHNOLOGIESTECHNOLOGIES 4.1 Introduction to GSM and TDMA
Global System for Mobile Communications (GSM) services
Global System for Mobile Communications (GSM) services are a standard collection of applications and features available to mobile phone subscribers all over the world. The GSM standards are defined by the 3GPP collaboration and implemented in hardware and software by equipment manufacturers and mobile phone operators. The common standard makes it possible to use the same phones with different companies' services, or even roam into different countries. GSM is the world's most dominant mobile phone standard.
The design of the service is moderately complex because it must be able to locate a moving phone anywhere in the world, and accommodate the relatively small battery capacity,
limited input/output capabilities, and weak radio transmitters on mobile devices.
In order to gain access to GSM services, a user needs three things:
A billing relationship with a mobile phone operator. This is usually either where services are paid for in advance of them being consumed (prepaid), or where bills are issued and settled after the service has been consumed (postpaid).
A mobile phone that is GSM compliant and operates at the same frequency as the operator. Most phone companies sell phones from third-party manufacturers.
A Subscriber Identity Module (SIM) card, which is activated by the operator once the billing relationship is established. After activation the card is then programmed with the subscriber's Mobile Subscriber Integrated Services Digital Network Number (MSISDN) (the telephone number). Personal information such as contact
numbers of friends and family can also be stored on the SIM by the subscriber.
After subscribers sign up, information about their identity (telephone number) and what services they are allowed to access are stored in a "SIM record" in the Home Location Register (HLR).
Once the SIM card is loaded into the phone and the phone is powered on, it will search for the nearest mobile phone mast (also called a Base Transceiver Station/BTS) with the strongest signal in the operator's frequency band. If a mast can be successfully contacted, then there is said to be coverage in the area. The phone then identifies itself to the network through the control channel. Once this is successfully completed, the phone is said to be attached to the network.
The key feature of a mobile phone is the ability to receive and make calls in any area where coverage is available. This is generally called roaming from a customer perspective, but also called visiting when describing the underlying technical process. Each geographic area has a database called the Visitor Location Register (VLR), which contains details of all the
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the VLR record and will be used during a process called paging when the GSM network wishes to locate the mobile phone.
Every SIM card contains a secret key, called the Ki, which is used to provide authentication and encryption services. This is useful to prevent theft of service, and also to prevent "over the air" snooping of a user's activity. The network does this by utilising the Authentication Center and is accomplished without transmitting the key directly.
Every GSM phone contains a unique identifier (different from the phone number), called the International Mobile Equipment Identity (IMEI). This can be found by dialing *#06#.
When a phone contacts the network, its IMEI may be checked against the Equipment Identity Register to locate stolen phones and facilitate monitoring.
TDMA
It can be easily adapted to the transmission of data and voice communication.
TDMA offers the ability to carry data rates of 64 kbps to 120 Mbps (expandable in multiples of 64 kbps). This enables operators to offer personal communication-like services including fax, voiceband data, and short message services (SMSs) as well as bandwidth-intensive applications such as multimedia and videoconferencing.
It will not experience interference from other simultaneous transmissions Unlike spread-spectrum techniques which can suffer from interference among the users all of whom are on the same frequency band and transmitting at the same time, TDMA’s technology, which separates users in time, ensures that they will not TDMA is the only technology that offers an efficient utilization of hierarchical cell structures (HCSs) offering pico, micro, and macrocells. HCSs
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needs. By using this approach, system capacities of more than 40-times AMPS can be achieved in a cost-efficient way. TDMA allows service compatibility with the use of
dual-mode handsets because of its inherent compatibility with FDMA analog systems.
4.2
4.2 GSM Network and System ArchitectureGSM Network and System Architecture
Mobile station
• Subscriber identity module Base station system
– Network switching system
• SMS gateway
• Flexible numbering register
– Operation and support system and other nodes
• Administrative and control system
Fig 4.1 components of GSM network GSM network interfaces and protocols
• GSM interfaces – Abis interface
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– Um interface
– Layered structure/OSI model
Fig 4.2 interfaces in GSM
GSM network interfaces and protocols
• GSM protocols and signaling model – Um interface
– Abis interface – A interface – Ater interface
The network structure is defined within the GSM standards. Additionally each interface between the different elements of the GSM network is also defined. This facilitates the information interchanges can take place. It also enables to a large degree that network elements from different manufacturers can be used. However as many of these interfaces were not fully defined until after many networks had been deployed, the level of standardisation may not be quite as high as many people might like.
1. Um in terf ace The "air" or radio interface standard that is used for exchanges between a mobile (ME) and a base station (BTS / BSC). For signalling, a modified
version of the ISDN LAPD, known as LAPDm is used.
2. Abis in terf ace This is a BSS internal interface linking the BSC and a BTS, and it has not been totally standardised. The Abis interface allows control of the radio equipment and radio frequency allocation in the BTS.
3. A in terf ace The A interface is used to provide communication between the BSS and the MSC. The interface carries information to enable the channels, timeslots
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The messaging required within the network to enable handover etc to be undertaken is carried over the interface.
4. B in terf ace The B interface exists between the MSC and the VLR . It uses a protocol known as the MAP/B protocol. As most VLRs are collocated with an
MSC, this makes the interface purely an "internal" interface. The interface is used whenever the MSC needs access to data regarding a MS located in its area.
5. C in terf ace The C interface is located between the HLR and a GMSC or a SMS-G.
When a call srcinates from outside the network, i.e. from the PSTN or another mobile network it ahs to pass through the gateway so that routing information required to complete the call may be gained. The protocol used for communication is MAP/C, the letter "C" indicating that the protocol is used for the "C" interface. In addition to this, the MSC may optionally forward billing information to the HLR after the call is completed and cleared down.
6. D in terf ace The D interface is situated between the VLR and HLR. It uses the MAP/D protocol to exchange the data related to the location of the ME and to the management of the subscriber.
7. E in terf ace The E interface provides communication between two MSCs. The E interface exchanges data related to handover between the anchor and relay MSCs using the MAP/E protocol.
8. F inte rf ace The F interface is used between an MSC and EIR. It uses the MAP/F protocol. The communications along this interface are used to confirm the status ofthe IMEI of the ME gaining access to the network.
9. G in terf ace The G interface interconnects two VLRs of different MSCs and uses the MAP/G protocol to transfer subscriber information, during e.g. a location update procedure.
10. H inte rf ace The H interface exists between the MSC the SMS-G. It transfers short messages and uses the MAP/H protocol.
11. I inte rf ace The I interface can be found between the MSC and the ME. Messages exchanged over the I interface are relayed transparently through the BSS.
Although the interfaces for the GSM cellular system may not be as rigorously defined as many might like, they do at least provide a large element of the definition required, enabling the functionality of GSM network entities to be defined sufficiently.
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Fig 4.3 GSM network interfaces and protocols