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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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The cellular concept
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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The cellular concept
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
ijjiLD
jiLjLiLD
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3)(3/
)(
)5.0(2
)3/2cos())((2)()(
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rL 3!
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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
B
D
C
E
F
DE
A
G
F
B
C
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