wireless communication 2

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SURESHA .V Professor,Dept. of E&C

e-mail: [email protected]

WIRELESS COMMUNICATION

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Introduction to Wireless Telecommunication Systems and Networks - Gary J. Mullett

Suresha V. Professor, Dept. of E&C, KVG College Of Engineering. Sullia, D.K - 574 327

Mullett

UNIT- THREE Wireless Network Architecture and Operation

Objectives

3 Suresha V. Professor, Dept. of E&C, KVG College Of Engineering. Sullia, D.K - 574 327

Discuss the cellular concept and explain the advantages of frequency reuse.

Draw a diagram of a typical cellular cluster and explain the meaning of frequency reuse number.

Discuss how the capacity of a cellular system may be expanded.

Explain the difference between cell splitting and sectoring.

Discuss the use of backhaul networks for cellular systems.

Explain the concept of mobility management and discuss the operations it supports.

Discuss the concepts of power management and network security.

1. The Cellular Concept


developed by Bell Labs 1960’s-70’s

areas divided into cells

a system approach, no major technological changes

a few hundred meters in some cities, 10s km at country

side

each served by base station with lower power transmitter

each gets portion of total number of channels

neighboring cells assigned different groups of channels,

interference minimized

hexagon geometry cell shape

1. The Cellular Concept


Introduction: The main objective of cellular concept is to allocate more users in a

limited allocated spectrum.

Multiple lower-power base stations that service mobile users within their coverage area and handoff users to neighboring base stations as users move. Together base stations tessellate the system coverage area.

The cellular Advantage


A large subscriber capacity

Efficient use of spectrum resources

Nationwide coverage

Adaptability to traffic density

Telephone service to both vehicle and portable user

terminals

Telephony but also other services including closed user

groups with voice dispatch operations

Toll quality

Affordability, which could eventually make it a mass-market

serviceCont….

The cellular Advantage


Power requirement for mobile is less due smaller cell and low power

transmitter

Longer battery life and smaller mobile station form factors.

Initial implement cost is large due to

◦ Deployment of large no. of low power stations

◦ Acquisition of lands for cell sites

◦ The associated hardware

◦ RBS TxrRXr and controller

◦ Antennas and towers

◦ MSC RBC and Radio spectrum.

But the cellular concept allows a large enough increase in capacity to make

these operations economically feasible.

Implementation of basic cellular architecture:


Instead of one base station covering an entire city, the city was broken up into cells, or smaller coverage areas.

Each of these smaller coverage areas had its own lower-power base station.

The radio channels must be allocated to these smaller cells in such way as to minimize interference but at the same time provide the necessary system performance to handle the traffic load within the cells.

Frequency Reuse


The key characteristic of a cellular network is the ability to reuse frequencies to increase both coverage and capacity

Extensive frequency reuse allows for many users to be supported at the same time.

Total spectrum allocated to the service provider is broken up into smaller bands.

A cell is assigned one of these bands. This means all communications (transmissions to and from users) in this cell occur over these frequencies only.

Frequency Reuse

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Neighboring cells are assigned a different frequency band.

This ensures that nearby transmissions do not interfere with each other.

The same frequency band is reused in another cell that is far away. This large distance limits the interference caused by this co-frequency cell.

Implementation of basic cellular architecture:

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Cluster: o It is a group of cell that make use of all the available radio spectrum. o Cluster has N cells with unique and disjoint channel

o Since adjacent cannot use the same frequency channels, the total frequency allocation is divided up over the cluster and then repeated for other clusters in the system .

o The number of cells in a cluster is known as the cluster size or the frequency reuse factor (1/N)

illustration of cellular system capacity

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Example: Consider service provider wants to provide cellular communications to

a particular geographic area. the provider is licensed = 5MHz.

Each system subscriber bandwidth(channel B.W)= 10KHz. If the service provider was to provide coverage from only one

transmitter site, the total theoretical number of possible simultaneous users = Total B.W/ Channel B.W= 5MHz/10kHz /user = 500 users.

If , however, the service provider implements a cellular system with 35 transmitter sites, located to minimize interference and provide total coverage of area, determine the new system capacity?.

Cont...

illustration of cellular system capacity

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Solution: Assume the cluster size N = 7 The allocated B.W/cell= system B.W/ Number of cells in a cluster

=5*106/7=714kHz Bandwidth per cell=714kHz. No. of cluster 35/7= 5. Each cell has a capacity =714kHz/10kHz/user=71 users Total system capacity =35 cells*71 users/cell=2485 users. This is a system capacity increase of =5 times.

Conclusion: Smaller cells higher M higher C+ Channel reuse higher capacity

+ Lower power requirements for mobiles◦ Additional base stations required◦ More frequent handoffs◦ Greater chance of ‘hot spots’

Cellular Hierarchy

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Fig: Relative coverage areas of different size cell

Cellular Hierarchy (Cont..)

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Sl. No.

Cell Type

Cell diameter

Operating Environment

1 Femtocells

< 10 mts Personal Area network(PAN)

2 Picocells

< 100 mt Indoor environment

3 Microcells

100-1000mts

Outdoor to indoor and pedestrian

4 Macrocells

>1000mts to Few Kms

Vehicular and high antenna environment

5 Megacells

Global coverage

Vehicular and high antenna environment

Note: All type uses different radio link propagation and different technical cell design concept

2 . CELL FUNDAMENTALS

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A Cell : is a small geographic area

within which each cellular base station

is allocated a group of radio channels

to be used.


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The Footprint: The actual radio coverage of a cell and is determined from field measurements or propagation prediction models.

Although Real footprint is formless in nature, a regular cell shape is needed for systematic system design.

Why circle can not be used to represent the coverage area of a base station? because adjacent circles can not be overlaid upon a map without leaving gaps or creating overlapping regions.

Thus, when considering geometric shapes which cover an entire region without overlap and with equal area, there are three sensible choices:

a square; an equilateral triangle; and a hexagon.


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Why Hexagonal model ?.

(1) A cell must be designed to serve the weakest mobiles within the footprint, and these are typically located at the edge of the cell. For a given distance between the center of a polygon and its farthest perimeter points, the hexagon has the largest area of the three.

(2) By using the hexagon geometry, the fewest number of cells can cover a geographic region

(3) The hexagon closely approximates a circular radiation pattern which would occur for an Omni-directional base station antenna and free space propagation.

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To gain the maximum reuse of the frequencies for a cellular system, cells are arranged in clusters.

To determine the minimum size cluster that can be used it is necessary to calculate the interference levels generated by the co-channel cells.

The reuse distance has been determined that relates cluster size N,

cell radius R and the reuse distance D. The frequency reuse distance can be calculated by:

D = R(3N)1/2

where R=cell radius and N=reuse pattern.

Values of N can only take on numbers calculated from the following expression:

N = i2 + ij + j2

where i and j are integers.

Reuse Number

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Reuse NumberFig: Various cellular reuse pattern

Conclusion: N D C Network cost Complexity

Handoff Interference

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Example 1 For mobile system cluster size of 7, determine the frequency reuse

distance if the cell radius is five kilometers. Repeat the calculation for the cluster size of 4.

Solution: Typical AMPS system

◦ cluster size of N=7 ◦ the reuse distance for cell is 3.

(figure 4-4) use the expression N= i2 + ij + j2, one can show that possible value for N is 7.

As shown in fig 4-4, the hexagons (cells) are arranged with one hexagon in the centre of a cluster and six other hexagons surrounding the middle hexagon. Adjacent clusters repeat the previous pattern. The re-use distance is found from the following equation :

D=R(3N)1/2

◦ Therefore, for a cluster size of 7, D=5(3*7)1/2 = 5(21)1/2 = 5(4.5823) = 22.913km.

For a cluster size of 4, the re-use distance is given by : D= 5(3*4)1/2 = 5(12)1/2 = 5(3.464) = 17.32km.

a smaller cluster size results in a smaller re-use distance.

Relationship between cluster size (N) and reuse distance(D).

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Additionally, more complex calculation can yield the signal to interference ratio for a particular cluster size, N.

The signal to interference ratio(S/I or SIR) gives an indication of the quality of the received signal much like the time honored signal to noise ratio (SNR) measurement.

Using a fairly simple mathematical model for S/I ratio calculations involving unidirectional cells yield the results tabulated in the table 4-1 for several common values of N :

Table 4-1 Signal to interference ratio for various cluster sizes.

Cellular interference issues (S/I)

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Smaller cluster sizes will yield a larger possible subscriber

but, as shown in table 4-1, the trade-off is a lowered S/I ratio and the corresponding decrease in the radio link quality.

The AMPS system did not yield usable voice quality radio links unless an S/I ratio exceeding 18dB was available.

This value of S/I was only possible for a cluster of size 7 and up. Therefore, the typical AMPS system was deployed with a cluster size of N=7.

Example 2: The example show a possible distribution of channels for an AMPS system

with a cluster size of N=7. Solution: For this situation, the 416 radio channel are divided by the 7 cells per cluster to yield 59+ channels per cell site.

Each cell can have three control channels and some 56+ traffic channels.

Cellular interference issues(cont….)

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Table 4-2 A possible assignment of AMPS channels for a cluster size of 7.

Note : Each cell has a channel spacing of 7*30kHz=210kHz

Cellular interference issues(cont….)

Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7

Control channels

1 2 3 4 5 6 7

8 9 10 11 12 13 14

15 16 17 18 19 20 21

Traffic Channels

22 23 24 25 26 27 28

29 30 31 32 33 34 35

36 37 -- -- -- -- --

-- -- -- -- -- -- 401

402 403 404 405 406 407 408

409 410 411 412 413 414 415

416

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Cellular capacity is a number of users in a cell. It is a concern in any wireless communications system.

High demand for cellular service, especially in large urban markets, has created a need to serve a greater number of users in a limited amount of frequency space.

Interference is the major limiting factor in the performance and capacity of cellular radio systems. It is a major bottleneck in increasing capacity and is often responsible for dropped calls.

Sources of interference 1. Another mobile in the same cell

2. A call in progress in a neighboring cell

3. Any non-cellular system which inadvertently leaks energy into the cellular frequency band.

4. Other base stations operating in the same frequency band.

3. CAPACITY EXPANSION TECHNIQUES

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The approaches to capacity expansion are either architecturally or technologically enabled. They are1. Cell splitting 2. Cell sectoring 3. Overlaid Cells4. Channel allocation5. Lee’s Microcell Technology6. Smart Antenna Technology7. Migration to Digital Technology

3. CAPACITY EXPANSION TECHNIQUES ***

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The process of subdividing a congested cell into smaller cells. (each with its own base station and a corresponding reduction in antenna height and transmitter power)

By defining and installing new cells which have a smaller radius than the original cells (microcells).

Cell splitting preserves the geometry of the architecture and therefore simply scales the geometry of the architecture

The increased number of cells would increase the number of clusters which in turn would increase the number of channels reused, and capacity

Cell splitting

Fig: Increase capacity by cell splitting

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3.1 Cell splitting

• Figure shown above assume that Cell A has become saturated and is unable to support its traffic load. • Using cell splitting, six new smaller cells with approximately one-quarter the area of the larger cells are inserted into the system around A in such a way as to be halfway between two co channel cells. • These smaller cells will use the same channels as the corresponding pair of larger co channel cells. • In order that the overall system frequency reuse plan be preserved, the transmit power of these cells must be reduced by a factor of approximately 16 or 12dB.

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Cell splitting will work quite well on paper however, in practice many times the process is not as smooth as one would desire.

Very often due to the difficulty of acquiring appropriately located cell sites, the conversion process will be prolonged and different size cell size will exist in the same area.

In these cases, it is necessary to form two groups of channels in the old cell:◦ one group that corresponds to the small-cell frequency reuse requirements and ◦ Another group that corresponds to the old-cell reuse requirements.

Usually the larger cell channels are reserved for highly mobile traffic and therefore will have fewer handoffs than the smaller cells.

Cell splitting

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As the splitting process moves toward completion the number of channels in the small cells will increase until eventually all the channels in the area are used by the lower-power group of cells and the original Cells A has had its power reduced and also joins the new smaller cluster.

As traffic increases in other areas of the system this process may be repeated over again, Eventually the entire system will be resealed with smaller cells in the high-traffic areas and larger cells on the outskirts of the system or in areas of low traffic or low population density.

Cell splitting effectively increases system capacity by reducing the cell size and therefore reducing the frequency reuse distance thus permitting the use of more channels.

Cell splitting

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Advantages: Increases the system capacity Reduces the cell size, frequency reuse distance. Increases the number of channels Disadvantages: Difficult to acquire appropriately located cell sites prolonged conversion process, different cell size exist in the same area. Larger cell channels are reserved for highly mobile traffic and

therefore will have fewer hand offs ,than smaller cells. As traffic increases in the nearby areas of small cells, larger cells join

the smaller cluster. therefore entire system gets concentrated with small cells

No. of base station increases Trunking efficiency decreases Handoff process increases

Cell splitting

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Popular method It increase capacity is to keep the cell radius unchanged and seek methods to

decrease the D /R ratio. Uses directional antennas by replacing a single omni-directional antenna at

the base station Antennas split the cell in to 3 new cells Antennas 120o apart Provides interference reduction S/I ratio increases Does not require new cell sites Additional antennas and triangular

mounting only

Demerits: Increased network system architecture complexity

3.2 Sectoring

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3.2 Sectoring

Fig: Interference reduction due to cell sectoring

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As shown in Figure the sectoring of a cell results in a reduction in the amount of interference that the sector experiences from its cochannel neighbors in adjacent clusters and conversely the amount of interference that the sector supplies to its cochannel neighbors.

Before sectoring, for a cluster size of 7, a cell receives and gives interference to six other nearest cochannel cells in other clusters.

Now, as shown by Figure for Cell AO, the number of interfering cells has been reduced to two (AI and A2).

This results in a higher S/I ratio for that sector and its companion sectors in other clusters.

3.2 Sectoring

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Table below tabulates these new values for a three-sector scheme for some common values of cluster size.

3.2 Sectoring

Cluster Size N S/I Ratio in dB

3 16.08

4 18.58

7 23.44

12 28.12

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Comparison Between Cell splitting and Cell Sectoring

Cell Splitting Cell Sectoring

1. Cell splitting achieves capacityimprovement by essentially rescaling the system.

1.It is an antenna technique to increase the system capacity

2.By decreasing the cellradius R and keeping the co-channel reuse ratio D/R unchanged, cell splitting increases the number of channels per unit area.

2. It increase capacity is to keep the cell radius R unchanged and seek methods to decrease the D /R ratio

3.Here the number of handoffs decreases 3. Here the number of handoffs increases

4.It increase in trunking efficiency 4. It decrease in trunking efficiency

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This method can be used to expand the capacity of cellular systems in two ways. ◦ For split band analog systems

Using macrocell, microcells Requires dual mode mobile systems

◦ For GSM or TDM Helps migrating to other systems

◦ Use same base stations Tiering

◦ Subcell in large cell

3.3 Overlaid Cells

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Overlaid cells in a split-band system Using overlaid cell operational wideband analog system could be upgraded to

increase its capacity by overlaying and analog system with narrower bandwidth requirements over it.

In such a split-band overlay system, channels are divided between a larger macrocell (using AMPS or TACS) and the overlaid microcell (using NAMPS or

NTACS) that is contained in its entirety within the macrocell.

3.3 Overlaid Cells (cont…)

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Fig2:Overlaid cells in a reduced cluster size system

3.3 Overlaid Cells(cont…)

B2A2

C2

B3A3

C2

B1A1

C1B1

A2

C2

1 group of overlaid 3/9 plan

under overlaid 4/12 plan

Part of anotherGroup of An overlaid 3/9 plan

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Overlaid cells in a reduced cluster size system The second method of using overlaid cells may be applied to GSM or

NA-TDMA systems. As an example of this method, consider a system with a cluster size of

N=4. On the top of this system, a cluster of overlaid cells is applied with a

cluster size of 3. If the channels for the overlaid cell cluster are taken from the

underlaid cluster, the system capacity increases since the area needed for the overlaid system for the overlay system, the greater the increase in system capacity.

This type of expansion allows operators to migrate their systems using the same base station and mobile station equipment. See Figure 2 for an illustration of this technique

3.3 Overlaid Cells(cont…)

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Traffic in each cell is dynamic◦ Eg- sporting events, rock concerts, natural disasters◦ Change with time

Portable cellular sites ◦ COW (cell on wheels)◦ It is deployed during natural disasters to restore disrupted

communication Channel allocation techniques

To avoid non-availability of service◦ Blocking◦ Configure entire network capacity◦ Should be less than 2%

Stabilizes temporal fluctuations of blockage◦ Minimize call blocking probability◦ Serve subscribers effectively

3.4 Channel allocation

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3 methods to achieve efficient channel allocation◦ Fixed channel scheme

Fine tune the system where needed Instead of equally dividing up channels over cells, some cells will receive

larger channel allocations. Periodically update

◦ Channel borrowing High traffic cells borrow channels from low traffic cells Other cells in the cell lose that frequency Channel returned after traffic is cleared

◦ Dynamic channel allocation (DCA) Available channel are placed in channel pool Each channel assigned new call based on Signal to interference statistics Channel used until SIR is met Complex Every cell site must be capable of transmitting every one of system’s

assigned channels

3.4 Channel allocation(cont…)

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Lee’s Microcell technology◦ Sectoring increases handoffs increasing loads on switching elements

◦ Use zones instead of sectors

◦ Reduce number of hand offs

◦ Uses 3 antennas in a cell connected to same RBS

◦ Antenna with best reception used for uplink and downlink

Other capacity expansion schemes

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Fig: Lee’s microcell concept for capacity expansion

Other capacity expansion schemes

RBS

Antenna patterns

High speed communication links

Zoneantennasite

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As shown in FIG, this technique employs three antennas that provide coverage by “looking” into the microcell.

All three antennas are connected to the same base station by high-speed microwave or fiber links.

The antenna with the best reception of the mobile is used for both the uplink and downlink.

As the mobile travels with in the microcell the same channel can be used and there is no need for handoff operations.

As the mobile moves into another zone the base station simply switches the channel to a different zone.

1.Lee’s Microcell technology

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Not yet implemented

Proposed for 3G techniques

A BS could direct a narrow beam of radio wave at a particular mobile station

Reuse the same channel over another narrow beam (smart antenna use phased array technology,i.e, Adaptive steered antenna are used).

This technology is referred to as SDMA (Space Division Multiple Access)

Many systems have space diversity to select the best signal out of 2 or more signals.

2. Smart Antenna Technology

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1G S/I=18dB AMPS 2G S/I=12dB GSM,NA-TDMA Newer technology is based on digital modulation technique e.g. 3G services 2G systems use TDMA and CDMA to achieve greater capacity

TDMA vs. CDMA

3.Migration to newer technology

TDMA CDMA

1.Multiple time slots per multiple users.

1.Multiple users use the same channel simultaneously.

2.Immune to noise and interference

2.Interference handling capacity is inherent

3.Provide service with low S/I

3. Good S/I ratio

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Backhaul – “Getting data to the network backbone“ or transmitting from a remote site or network to a central or main site

1G◦ Voice voice + data◦ Change in requirements for PSTN and PDN◦ Separate facilities for voice and data networks

2G◦ Voice band signals are transcoded (compressed and reformatted) at BSC◦ Fiber optic cables between MSC and PSTN◦ Minimized costs◦ CDMA had IWF for data but same connection maintained for voice

2.5G, 2.5+G◦ Own private wideband networks to backhaul both voice and data between

MSC and BS GSM

◦ Packet switched networks◦ GPRS, PLMN added◦ Access web sites through private servers

3G◦ High speed data services◦ All IP network, ATM◦ SONET/SDH

4. Cellular backhaul networks

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CDMA Cellular system data network connections

4. Cellular backhaul networks(cont…)

RBS

RBS

RBS

BSCMSC

IWF

PSTNPSTN

PDNPDN

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GSM cellular system data network connection

4. Cellular backhaul networks(cont…)

GPRSPLMN

GPRSPLMN

GSM PLMN

GSM PLMN

BSC

To PDN

To PSTN

GSM andGPRS coverage area

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It is important characteristics of wireless communication system is the ability to provide mobility to the user.

It explains how the network knows where the subscriber is ( LOCATION MANAGEMENT) and how it keeps track of and in contact

with the mobile station as the user moves from one cell to another (HAND OFF MANAGEMANT)

(Mobility = location + hand off )management Characteristics/features

◦ Provide mobility to user◦ Contrasting wireless and PSTN network◦ Location management◦ Location update◦ Paging messages◦ Handoff management

5. Mobility management ***

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It is important characteristics of wireless communication system is the ability to provide mobility to the user.

It explains how the network knows where the subscriber is ( LOCATION MANAGEMENT) and how it keeps track of and in contact

with the mobile station as the user moves from one cell to another (HAND OFF MANAGEMANT)

(Mobility = location + hand off )management Characteristics/features

◦ Provide mobility to user◦ Contrasting wireless and PSTN network◦ Location management◦ Location update◦ Paging messages◦ Handoff management

5. Mobility management ***

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It is the process of Keeping track of the present or last known location of the mobile station and delivery of both voice and data to it is move around.◦ Eg. Voice call through PSTN

WorkingWhen a call is made that passes through PSTN,

◦ Dedicated traffic channel set up from BS to MS◦ PSTN sets up circuit over fixed part of network◦ Wireless network allocates radio channels for air interface◦ For this MS location must be known

Objectives ◦ Provide continuous radio link◦ Direct the packet in a network◦ Determine MS status in network◦ Check availability of the MS 3 basic functions performed by Location management

1. Location updating2. Paging messages3. Transmission of location information between network elements

5.1 Location management

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Performed by MS

After initial power up registration, the MS and BS will exchange their identification information.

MS attached to a base station and is located initially

Periodically checked for changes

MS sends update message every time it changes point of access(AP) in a network

Exchange information for handoff

If a connection fails, systems page group of surrounding stations to track a MS

It tuned on with new registration

1.Location Updating

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Balance required between number of update messages and number of cells to be paged

Greater degree of certainty in locating the MS

Call blocking due to frequent paging

Two types of updating schemes◦ Static

Geographic layout determines updating requirements

◦ Dynamic User’s mobility determines updating requirements

1.Location Updating

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1.Location Updating

• Static Method: Geographic layout is used for dating requirements. It is most of cellular provider use this technique

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1.Location Updating

• Static Method: Geographic layout is used for dating requirements. It is most of cellular provider use this techniqueDraw back: PING-PONG Effect: It’s a draw back faced by a static location area ID scheme. This effect can occur if the mobile is moving in a path that takes it back and forth between the borders of 2 LA s. o location updating procedure (at BSS)

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Dynamic location updating: ◦ It is not a popular with in the wireless industry.◦ These scheme are typically based on the status or state of the

mobile.◦ Some of the typical measures used to determine the mobile’s

status and hence determine the need to perform the updating algorithm are elapsed time, total distance travelled, call patterns, number different LAs entered, and so on..

1.Location Updating

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Incoming call/message to MS initiates paging of mobile Consists of

◦ Broadcasting message To bring response from a single particular mobile Starts communication processing Required if exact cell of mobile not known This information not available always

◦ Blanket paging Broadcast to all cells in a location area Initiates MS to respond

◦ Sequential paging Paged to the cell where it was last registered

2.Paging messages

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Handoff initiated when power from current RBS drops

Reduce ping-pong effect◦ Handover to and fro between a cell pair frequently

Solution is to define threshold

Fine tuning algorithm to improve system performance◦ Provide required QOS continuity during handoff

2.Paging messages(cont…)

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For location updating to work correctly in a wireless network, there must exist several data bases where mobile station information can be stored and accessed by the network as needed.

Wkt when MS is register (i.e. Mobile ID numbers are stored) in a home location

register (HLR) maintained by this subscriber’s home network.

This HLR data base is usually located with the MSC and also store the user’s profile, which includes permanent data about the subscribers, including call plan supplementary services, location information and authentication parameters.

VLR is also maintained by the home network and also usually colocated with the MSC (MSC/VLR).

The home VLR will temporarily store information about MS that has registered itself with the home network.

Therefore, if an MS is turned on by a subscriber’s home network area, the home VLR will temporarily store the uses information.

3. Transmission of the location information between network elements

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With in a particular network there are usually several to many MSCs used to support network’s operation.

Depending up on the particular network topology, each MSC may contain HLR and VLR data base function are alternately single HLRs (configured as a MSC/HLR/VLR) might service a group of MSC/VLRs .

For a small system another possibility is that a gateway MSC (GMSC) might house the HLR function for a group of integrated MSC/VLRs. A gate way MSC is an MSC that interface the mobile network with other network such as PSTN.

3. Transmission of the location information between network elements

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let us examine several possible scenarios that could occur during the operation of a wireless network. ◦ The firs possibility the user turned on a mobile within his or her home area.

◦ The mobile registers with the VLR for the home area.

◦ The colocated or system HLR confirms that the subscribers has network privileges Subscriber has network privileges.

◦ Communication between remote HLR and MSC/VLR occurs using particular signalling protocol over an SS7 network.

◦ The second case would occur when the user is away from his or her home location.

◦ Now mobile registers with VLR of another MSC or a ‘’foreign’’ network.

◦ The first possibility refers to the fact that subscribers still connecting to his or her own service provider’s network but different MSC/VLR is covering the area where the subscribers is now located.

3. Transmission of the location information between network elements(cont….)

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◦ Whereas, a foreign network belongs to a different service provider (this type of connection is called roaming). In these situations, the MSC/VLR must send a message to the subscribers HLR to verify authentication information about the mobile.

◦ The HLR will respond to the request by transmitting the information back to the requesting MSC/VLR over SS7 signalling network.

A few comments about the communications between MSC/VLRs and VLRs are appropriate here.

For a GSM cellular system and most other modern, the SS7 system is used to communicate these massages.

The signalling done over this network is accomplished using massage transfer part (MTP) as the common platform and signalling connection part (SCCP) to provide the additional functionality to connect network data bases (HLRs and MSC/VLRs) without any speech connection occurring during this operation .

3. Transmission of the location information between network elements(cont..)

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Handoff, Handoff control, Handoff algorithm ,Handoff operation Handoff: If a subscriber moves from one cell to another, the cellular system

must have the ability to reconfigure the connection to the mobile from the current base station to the new BS in the new cell. This connection hand over process is called hand-off.

Handoff control

◦ The algorithm used to determine when to made a handoff can be located in a network element or in a mobile terminal.

◦ Two major types : Network controlled handoff or NCHO Mobile-controlled handoff or MCHO

Parameters measured/used in the handoff algorithms ◦ RSS (received signal strength)◦ System path loss.◦ S/I ratios◦ BER (bit error ratio)◦ Symbol/Block error rate◦ Parameters can undergo fluctuations due to signal fading

Handoff management

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Fig: Typical cellular handoff operation

Handoff management

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Handoff management

Fig: Typically handoff logarithms using RSS measurents

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Transmission powers represent a key degree of freedom in the design of wireless networks◦ Interference management:

Due to the broadcast nature of wireless communication, signals interfere with each other. Power control helps ensure efficient spectral reuse and desirable user experience.

◦ Energy management:Due to limited battery power in mobile stations, handheld devices, or any “nodes”. Power control helps minimize a key component of the overall energy expenditure.

◦ Connectivity management:Due to uncertainty and time variation of wireless channels, the receiver needs to be able to maintain a minimum level of received signal so that it can stay connected with the transmitter and estimate the channel state. Power control helps maintain logical connectivity for a given signal processing scheme.

6. Radio resources and power management

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Design issues making it desirable to include dynamic power control in a cellular system◦ Received power must be sufficiently above the background noise for

effective communication◦ Desirable to minimize power in the transmitted signal from the mobile◦ Reduce co-channel interference, lessen health concerns, save battery

power◦ Energy efficient hardware and software

Types of power control Open-loop power control

◦ Depends solely on mobile unit◦ No feedback from BS◦ Not as accurate as closed-loop, but can react quicker to fluctuations in signal

strength Closed-loop power control

◦ Adjusts signal strength in reverse channel based on metric of performance◦ BS makes power adjustment decision and communicates to mobile on control

channel

Power control

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Achieve SIR tolerance with good quality communications

Must constantly adjust to change in signal strength caused by fading or mobility of MS

Usual Power control algorithm has 2 phases◦ Phase I:

MS registers with BSS Determine minimum output power Avoid possibility of a call drop

◦ Phase II: Additional measurements to reduce power Output power of RBS is adjusted

Use complex algorithms achieve maximum SIR for all radio links

Power control

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Discontinuous transmission (DTX)◦ Transmit during speech only◦ Extra over head◦ Compensate low-power background during silence◦ Adopted by MS, TRC, BSC also

Sleep mode◦ No activity◦ RF circuitry is powered off◦ Periodical awakening

Energy efficient designs◦ Semiconductor technologies◦ Power efficient modulation schemes◦ Software/hardware design◦ DSP technology

Power saving schemes

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Radio resource management (RRM) is the system level control of co-channel interference and other radio transmission characteristics in wireless communication systems

Types◦ Static RRM:

Involves manual as well as computer aided fixed cell planning or radio network planning.

◦ Dynamic RRM:Adaptively adjust the radio network parameters to the traffic load, user positions, quality of service requirements, etc.

Provide functional improvements for RF operation◦ Implement system power control to reduce interference◦ Maximize capacity from above concept◦ Best available radio channel selection◦ Use wireless radio resource management scheme to enable

handoff operations

Radio resources management

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Wireless medium has certain limitations over the wired medium ◦ Open access◦ Limited bandwidth ◦ Systems complexity

3G networks have a packet switched core◦ Connected to external networks like Internet ◦ vulnerable to new types of attack

Security Issues In Cellular Networks◦ Authentication◦ Integrity◦ Confidentiality◦ Access Control◦ Operating Systems ◦ Web Services◦ Location Detection◦ Viruses And Malware◦ Downloaded Contents◦ Device Security

7. Wireless network security

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Limitations Security issues GSM security

◦ Global control equipment identity register (CEIR) Database in Dublin, Ireland List of handsets approved for GSM White/Black listed

◦ GSM cellular operators employ an EIR Keep track of handsets to be blocked Registered user of CEIR share database

◦ CEIR creates master black list for operator Identification Authentication Billing Maintenance All-IP network

◦ Increased management issues◦ Prevent hacking of systems◦ Software virus prevention

Wireless network security requirements

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Network security Techniques

◦ Encryption Scrambling using key Secret key algorithms

Prevent threat from global terrorism

Wireless network security requirements

THE END OF UNIT - 3

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Keep going

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Mercedes Horse

Date post:	31-Oct-2014
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