wireless communication architecture

8/9/2019 wireless communication architecture

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Wireless Network

Architecture andOperation


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The cellular concept

Limited number of frequencies => limited channels Single high power antenna => limited number of users Smaller cells => frequency reuse possible => more number of users

Base stations (BS): implement space division multiplex Each BS covers a certain transmission area (cell) Each BS is allocated a portion of the total number of channels available Cluster: group of nearby BSs that together use all available channels

Mobile stations communicate only via the base station FDMA, TDMA, CDMA may be used within a cell

As demand increases (more channels are needed) Number of base stations is increased Transmitter power is decreased correspondingly to avoid interference


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Cell size: 100 m in cities to 35 km on the country side (GSM) even less for higher frequencies Umbrella cell: large cell that includes several smaller cells

Avoid frequent handoffs for fast moving traffic

Cell shape: Hexagonal is useful for theoretical analysis Practical footprint (radio coverage area) is amorphous

BS placement: Center-excited cell: BS near center of cell

omni-directional antenna

Edge-excited cell: BSs on three of the six cell vertices sectored directional antennas


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Advantages: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally

Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells: co-channel, adjacent-channel

Important Issues: Cell sizing

Frequency reuse planning Channel allocation strategies

Bottom line: Attempt to maximize availability of channels in an area


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Cellular Geometries

Hexagonal pattern is preferred coz it supportsequidistant to all adjacent cells.


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Factors:

Equal area

No overlap between the cells

For a given S, A3>A1 and A3>A2.

Here A3 covers maximum area for a given value of S.

By using hexagon geometry, the fewest number of cells

covers a given geographic region.


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Ideal Cell Hexagonal cell

The hexagon is closely

approximates a circularradiation pattern whichwould occur for an omnidirectional base stationantenna and free space

propagation


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Channel Capacity

Let a cellular system have total of S duplex channels

for use. If S channels are divided into N cells (in a

cluster) into unique and disjoint channel groups which

eachhas the same number of channels,

total number of available radio channels is:

S = KNWhere, K is the number of channels / cell.

If a cluster is replicated M times within the

system, the total number of duplex channels, C, or thecapacity is

C = MKN = MSCluster size N = 4, 7 or 12


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Design of cluster size N

In order to connect without gaps between adjacent

cells

N= i2 + ij + j2

Where i and j are non non-negative integers

Example i = 2, j = 1, then N=7

i, j = 0,1,2,3,. Then the possible values of N are1,3,4,7,9,.


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Cellular interference issues

The signal to interferenceratio (SIR or S/I) gives anindication of the quality ofthe received signal.

Smaller the cluster sizes,closer the reuse distanceand therefore larger thesystem capacity or total

number of possible users.But increases cost,complexity of the network,lower SIR and hencedecrease in radio link

quality.Co channel cells for cluster N=7


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S= Signal power from designated base station

Ii= Interference power caused by the ith interfering co-channel


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Problems

If a particular FDD cellular telephone system has a total bandwidth of

33 MHz, and if the phone system uses two 25 KHz simplex channels to

provide full duplex voice and control channels. Compute the number of

channels per cell if N = 4, 7, 12.

Solution

Total bandwidth = 33 MHz

Channel bandwidth = 25 KHz x 2 = 50 KHz

Total avail. channels = 33 MHz / 50 KHz = 660N = 4, Channel per cell = 660 / 4 =165 channels

N = 7, Channel per cell = 660 / 7 =95 channels

N = 12, Channel per cell = 660 / 12 =55 channels


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The Cell Structure for N = 7


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Cell Structure for N = 4


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Cell Structure for N = 12


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Reuse number

Maximize frequency reuse Determine minimum size cluster

To calculate interference levels D = R(3N)1/2

R = cell radiusN = reuse pattern= i2 + ij + j2

D = reuse distance If N is small, closer is reuse distance (D), larger is system

capacity If D is reduced, increases subscribers that can be

handled, increases cost of network hardware


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N = i2 + ij + j2

N = 22 + 2*1 + 12

N = 4 + 2 + 1N = 7

D = R(3N)1/2

D = 4.58R

The cluster size is specified in terms of the offset of thecenter of a cluster from the center of the adjacent cluster


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N = i2 + ij + j2

N = 22 + 2 * 0 + 02

N = 4 + 0 + 0

N = 4

D = R(3N)1/2

D = 3.46R i

D

R

The Frequency Re-Use for N = 4


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D

L * iL * j

r

In this case: j=2,

i=1

ComputeD basedon

law ofcosineNijjirD

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Capacity expansion techniques

Due to more users

Techniques Additional frequency spectrum

Expensive Change in architecture

Sectoring

Cell splitting

Overlaid cell schemes

Channel allocation scheme Traffic parameters

Next generation technology adaptation


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Cell splitting


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Microcell Zone

The Problems of sectoring can be addressed byMicrocell Zone Concept

A cell is conceptually divided into microcells or zones Each microcell (zone) is connected to the same base

station (fiber/microwave link) Doing something in middle of cell splitting and sectoring byextracting good points of both

Each zone uses a directional antenna Each zone radiates power into the cell.

MS is served by strongest zone As mobile travels from one zone to another, it retains thesame channel, i.e no hand off

The BS simply switches the channel to the next zone site


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G

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D

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DE

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New cells

Cell splitting


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Cell splitting Divides a large cell into smaller regions called microcells

Large-power antenna is replaced with multiple low-power antennas It rescales system by reducing cell size Advantages:

Allows orderly growth of the system The number of cells in an area increases Number of clusters increases Capacity increases Maintain the co-channel reuse ratio

More frequent handoffs between cells specially for high mobilityusers

More equipment: Cost issues with buying the equipment. A cell sitecosts in the range of $650,000-$800,000

Power Remote generation equipment required that is a costand a security problem


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Large cellsLarge cells

Medium cellsMedium cells

Small cellsSmall cells

R/4R/4

R/2R/2Suppose original congestedarea is originally covered by 5

cells each with 80 channelsCapacity=5x80=400 usersAfter cell splitting, Rnew= R/2We now have 24 cells

New capacity = 24x80 = 19200usersFor n = 4,Transmit Power

of New BS is 12 dB lower thanoriginal


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Cell sectoring

By subdividing a cell into sectors, we canincrease the capacity

3-sector cell (each sector is 1200)

6-sector cell (each sector is 600)


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Cell sectoring basic idea

Replace an omni-directional antenna withdirectional antennas

Point them to reduce co-channelinterference

Sectors are based on either 1200 or 600

sectoring

Divide cell channel pool among thesectors in the cell


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Tri-Sector antenna for a cell


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Cell sectoring basic idea

Antennas are placedat the outer edges ofthe cell

Any channel may beassigned to any zoneby the base station

Mobile is served bythe zone with thestrongest signal.


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Cell sectoring advantage and

disadvantage

Advantages: Improves the SIR of the system A gain of ~ 7 dB is achieved over

omnidirectional systems when 1200sectoring is used.

Problems: Increased handoff requirements

Multiple antennas are required in a BS Decreased trunking efficiency (less

number of channels per sector)


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Overlaid cells

2 methods to increasecapacity For split band analog systems

Using macrocell, microcells

Requires dual mode mobilesystems

For GSM or TDM

Helps migrating to othersystems Use same base stations

Tiering Subcell in large cell

D3

D2

D1

B3A3C3

A2B2

C2

A1B1

C1

B2A2

C2B3

A3

C2

B1A1

C1

B1A2

C2

1 group ofoverlaid

3/9 plan

underoverlaid

4/12 plan

Part ofanother

Group ofAn overlaid

3/9 plan


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Overlaid cells


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Channel allocation

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

Portable cellular sites COW (cell on wheels)

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


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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 cellswill 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

systems assigned channels


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Other capacity expansion schemes

Lees Microcell technology

Sectoring increases handoffs increasing loadson 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


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Cellular backhaul networks 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


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RBS

RBS

RBS

BSCMSC

IWF

PSTN

PDN

CDMA Cellular system data network connections


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N

GPR

S

PLM

N

GSM

PLMN

BSC

To PDN

To PSTN

GSM cellular system data network connection

GSM andGPRS coverage

area


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Mobility management

Characteristics/features

Provide mobility to user

Contrasting wireless and PSTN network

Location management

Location update

Paging messages

Handoff management


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Location management Keeping track of users present location

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

Basic functions Location updating paging messages transmission of location information between network elements


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Location updating

Performed by MS

MS attached to a base station and islocated initially

Periodically checked for changes

MS sends update message every time itchanges point of access in a network

Exchange information for handoff If a connection fails, systems page group

of surrounding stations to track a MS


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BSC A MSC/VLR A

Update infoTo VLR

Cell A1 in LAI 15

MS in instructed to

Activate an SDCCH

MS requests radio resource

BTS A1

MS performs authenticationAnd updating proceduresMS

LAI 15

Radioresourcerequest

SDCCH stand alone dedicated control channel

LAI Location area ID

Cellular location updating


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Location updating

Balance required between number of updatemessages and number of cells to be paged

Greater degree of certainty in locating the MS

Call blocking due to frequent paging

2 updating schemes

Static Geographic layout determines updating requirements

Dynamic Users mobility determines updating requirements


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Location update in BSS


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Paging messages 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

Parameters measured

RSS (received signal strength) BER (bit error ratio) Symbol Block error rate Parameters can undergo fluctuations due to signal fading


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Paging messages

Handoff initiated when power from currentRBS drops

Reduce ping-pong effect

Handover to and fro between a cell pairfrequently

Solution is to define threshold

Fine tuning algorithm to improve system

performance Provide required QOS continuity during

handoff


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Radio resources and power management

Transmission powers represent a key degree of

freedom in the design of wireless networks Interference management:

Due to the broadcast nature of wireless communication, signalsinterfere with each other. Power control helps ensure efficientspectral reuse and desirable user experience.

Energy management:

Due to limited battery power in mobile stations, handhelddevices, or any nodes. Power control helps minimize a keycomponent 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 transmitterand estimate the channel state. Power control helps maintainlogical connectivity for a given signal processing scheme.


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Power control

D

esign issues making it desirable toinclude dynamic power control in acellular system

Received power must be sufficiently above

the background noise for effectivecommunication

Desirable to minimize power in thetransmitted signal from the mobile

Reduce co-channel interference, lessenhealth concerns, save battery power

Energy efficient hardware and software


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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 quickerto 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


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Power control Achieve SIR tolerance with good quality

communications

Must constantly adjust to change in signalstrength 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 SIRforall radio links


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Power saving schemes 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


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Radio resources and power management

Radio resource management (RRM) is thesystem level control of co-channel interferenceand other radio transmission characteristics inwireless communication systems

Types Static RRM:

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

Dynamic RRM:Adaptively adjust the radio network parameters to thetraffic load, user positions, quality of servicerequirements, etc.


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Radio resource management

Provide functional improvements for RFoperation

Implement system power control to reduceinterference

Maximize capacity from above concept

Best available radio channel selection

Use wireless radio resource management

scheme to enable handoff operations


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Wireless network security

Wireless medium has certain limitationsover 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


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Security Issues In Cellular Networks

Authentication Integrity Confidentiality Access Control Operating Systems Web Services Location Detection

Viruses And Malware Downloaded Contents Device Security


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Wireless network security requirements

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


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Network security requirements

Identification

Authentication

Billing Maintenance

All-IP network

Increased management issues Prevent hacking of systems

Software virus prevention


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Network security

Techniques

Encryption

Scrambling using key

Secret key algorithms

Prevent threat from global terrorism

wireless communication architecture

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