1G PCS

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(No Transcript)
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Cellular Network Architecture
LocationRegister (Database)
Radio Network
Mobile Switching Center
Base Station Controller
MSC
Backbone Wireline Network
Mobile Terminal
Base Station
Cell
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Mobility Management

• Enables telecomm networks to
• Locate roaming MSs for call delivery
• Maintain connections as the MSs move between
  different cells
• Involves two operations
• Location Management
• Handoff Management
• Involves two types of mobility
• Terminal mobility
• Personal mobility

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Types of Mobility

• TERMINAL MOBILITY
• (Network should route calls to the MT
• regardless of its point of attachment)
• PERSONAL MOBILITY
• (Users should access the network wherever
  they are UPT (Universal Pers. Tel ))
• SERVICE PROVIDER MOBILITY
• (Allow user to roam beyond regional networks).

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Mobility Management

• Location Management

• Handoff Management

Base Station
A
MT A is receiving a call ! How will the
network deliver the call to A ?
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Location Management
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Cost Tradeoff
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Solution

• Local Areas (GSM) Registration Areas (IS-41)

Registration Area Boundary
Center Cell
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Handoff Types
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Mobility ManagementLocation Management

• Location management enables the system to track
  the locations of MTs between consecutive
  communications
• Trade-off between the costs of location update
  and paging design optimal location
  management schemes to reduce the overall cost

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Mobility ManagementHandoff Management

• Handoff management an MT keeps its connection
  active when it moves from one access point to
  another one
• Four types of handoffs Network Controlled
  Handoff (NCHO), Mobile Controlled Handoff (MCHO),
  Network Assisted Handoff (NAHO), and Mobile
  Assisted Handoff (MAHO)

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Location Management
Home Location Register
(HLR)
BACKBONE TELEPHONE NETWORK
Visitor Location Register
(VLR)
Mobile Switching Center
MSC
(MSC)
VLR
Mobile Terminal (MT)
Local Signaling
Long Distance Signaling
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Location Registration
MT enters a new LA, and transmits location update
to new BS
New LA is under same VLR. VLR updates the LA ID
for the MT.
VLR determines address of HLR, and sends location
registration message
BS forwards update to MSC, which queries VLR
HLR authenticates and registers MT by updating
the VLR ID for the MT. Then, HLR cancels former
VLR.
Does the MT have an existing record?
Yes
No
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Location Registration

• BS keeps broadcasting Location Area (LA) ID.
• MT listens to broadcast and will perform a
  location update when
• Powering up
• Crossing LA boundaries
• After a defined period of time

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Location Registration
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Call Delivery
The MSC gives the MT a Temporary Local Directory
Number, and forwards the TLDN back to the HLR
Incoming call for roaming MT reaches an MSC
The calling MSC determines the address of the
MTs HLR, and sends a location request message to
the HLR.
The HLR forwards this message to the calling MSC,
which sets up a route to the MT at its current
MSC.
Finally, the current MSC tells all of the BSs in
the MTs LA to send a polling signal to page the
MT. When the MT responds, the call is connected.
The HLR sends a route request message to the VLR,
which forwards the message to its MSC
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Call Delivery
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How is a Cell Phone Call made?

• When a mobile originates a call, a call
  initiation request is sent on the control channel
  to BS.
• With this request the mobile transmits its tel
  number (MIN
• Mobile ID Number), electronic serial number (ESN)
  and tel number of the called party.
• Base station receives this data and sends it to
  the MSC.
• MSC validates the request make connection to the
  called party through the PSTN and instructs base
  station and mobile to use an idle forward and
  reverse voice channel to allow conversation to
  begin.
• (in AMPS - 10-60 voice channels one control
  channel in each cell base station).

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Research Areas in Location Management

• Database Architectures
• Paging Techniques
• Multi-network location management
• Location Area Design

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Database Architectures

• Centralized
• Increase database hierarchy
• Cache user locations at switching points
• Replicate user profiles at more than one database
• Use pointers to follow a path of VLRs to the MTs
  current location
• Distributed
• Database Trees
• Partitioning

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Paging Techniques

• Blanket paging
• Paging the MS in all cells belonging to an LA
  simultaneously.
• Advantage The delay of the response to paging is
  kept at a minimum.
• Disadvantage Paging has to be done in several
  cells.
• Closest-cells first
• The cell where the MS was last seen is paged
  first followed by subsequent equidistant ring of
  cells.
• Several rings may be polled simultaneously in a
  paging cycle to keep delay low.
• Sequential paging
• Subsequent pages are performed in most likely
  locations based on past history and distance.

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Paging
VLR
(MSC)
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Location Area Design

• Tradeoff
• Location Updates versus Terminal Paging
• Goal Improvements to tradeoff
• Geographical
• Fixed versus Dynamic
• User-based versus Global definition
• Network-specific

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Dynamic Location Update Schemes

• Movement-based
• The MT performs an update each time it crosses a
  certain movement threshold, where one movement is
  made by crossing a cell boundary.
• Distance-based
• The MT performs an update when its distance from
  the cell where it performed its last update
  surpasses a certain distance threshold.
• Time-based
• The MT performs an update at a constant time
  threshold, deltaT.

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Example

• A MT is moving through the cellular network (R
  km) as shown in the figure at a rate of
  30km/hour.
• Label the cell IDs where the MT will perform its
  updates for
• Movement-based (T3)
• Distance-based (T6km)
• Time-based (30 minutes)

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Example Figure

• A MT is moving through the cellular network (R
  km) at a rate of 30km/hour.
• Where will updates be performed for
• Movement-based (T3)
• Distance-based (T6km)
• Time-based (30 minutes)

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Answer-Movement-based
K
A
P
F
L
B
T 3, 2 h 3 Update at S and G
Q
G
M
C
R
H
D
N
S
I
E
O
J
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Answer-Distance-based
K
A
F
P
L
B
G
Q
M
C
T 6, 2 h 3
R
H
D
N
S
I
Update at O, and near the M/G border
O
E
J
T6km
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Answer-Time-based
K
A
F
P
L
B
G
Q
M
C
R
H
D
N
S
I
Update only at M
O
E
J
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Group Problem

• Design a location update and paging scheme.
• Provide a diagram with numbered steps.
• Explain how your scheme reduces the signaling
  overhead.

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Handoff

• The transfer of a mobile terminals active
  connection(s) from one channel to another.
• Hard handoffs vs. soft handoffs
• Hard handoff break old connection, then form new
  connection.
• Soft handoff Connect to several BSs
  simultaneously.
• In CDMA, handoff does not change the physical
  channel, it just changes the BS that handles the
  channel.

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SIGNAL STRENGTH

• Cellular systems depend on the radio signals
  received by an MS throughout the cell and on the
  contours of signal strength emanating from the
  BSs of two adjacent cells i and j.

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Signal Strength
Signal strength (in dB)
Cell i
Cell j
-60
-60
-70
-70
-80
-80
-90
-90
-100
-100
Select cell j on right of boundary
Ideal Boundary
Select cell i on left of boundary
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Signal Strength (2)
Signal Strength (in dB)
Cell j
Cell i
-60
-70
-60
-80
-70
-90
-80
-90
-100
-100
Signal strength contours indicating actual cell
tiling. This happens because of terrain, presence
of obstacles and signal attenuation in the
atmosphere.
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SIGNAL STRENGTH

• Signal strength goes down as a mobile terminal
  moves away from the BS.
• As the mobile terminal moves away from the BS of
  the cell, the signal strength weakens and the
  so-called HANDOFF occurs.
• This implies a radio connection to another
  adjacent cell.

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Handoff Region
Signal strength due to BSj
Signal strength due to BSi
Pj(x)
Pi(x)
E
Pmin
BSi
BSj
MS
X1
X3
X4
X2
X5
Xth
Pz(x) (for z1,2) denote the power received at MS
from BS z. By looking at the variation of signal
strength from either base station it is possible
to decide on the optimum area where handoff can
take place.
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HANDOFF REGION

• At X1 the received signal from BSj is close to 0
  and the signal strength at the mobile terminal
  could be primarily attributed to BSi.
• Similarly, at distance X2 the signal from BSi is
  negligible.
• To receive and interpret signals correctly at
  mobile, the received signal must be at a minimum
  power level Pmin (X3 and X4), i.e., between X3
  and X4 the mobile terminal can be served either
  by BSi or BSj.

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HANDOFF REGION

• The area between X3 and X4 is called HANDOFF AREA
  or HANDOFF REGION.
• Where to perform HANDOFF depends on many factors.
• Do handoff at X5 where two BSs have equal
  signal strength.
• HARD HANDOFF!!!!!

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HANDOFF REGION

• Avoid Ping-Pong Effect, if the mobile moves
  back and forth between BSi and BSj
• SOLUTION? SOFT HANDOFF!!!!
• Continue to maintain both links with BSi and
  BSj until the signal strength from BSj exceeds
  that of Bi by some pre-specified threshold value
  E as shown by point X in Figure.

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Handoff Control

• NCHO (Network-Controlled Handoff)
• All close-by BSs monitor signal strength from the
  mobile terminal.
• MSC collects data from BSs, decides best
  candidate BS for the mobile terminal, and
  initiates the MSs handoff (CT-2, AMPS).
• Results in heavy signaling load, handoff delay of
  many seconds.
• MAHO (Mobile-Assisted Handoff)
• MT monitors signal strength from nearby BSs and
  reports the measurements back to the BS/MSC
  (twice per second).
• MSC decides best candidate BS and initiates the
  handoff (GSM)
• MCHO (Mobile-Controlled Handoff)
• MT monitors signal strength from nearby BSs,
  decides best candidate BS, and initiates handoff
  (DECT)

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Handoff Management
Radio Link Transfer
(Detection Decision)
Channel Assignment
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Handoff Management

• Initiation (Detection/Decision)
• The user, the network, or changing channel
  conditions detect the need for handoff.
• New connection generation (Channel Assignment)
• The network must find new resources for the
  handoff call
• The network must also perform any needed routing
  operations.
• Data flow control (Radio Link Transfer)
• Delivery of the data from the old path to the new
  path is maintained according to agreed-upon
  service guarantees.

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Handoff Initiation

• A balance of user movement versus network
  conditions
• Goals
• Keep user connected
• Minimize network signaling
• Minimize ping-pong handoffs

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Handoff InitiationWhat criteria should cause
handoff?
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Handoff Management

• Handoff in cellular telephony
• Transfer of a voice call from one BS to another
• Handoff in WLANs
• Transfer of a connection from one AP (Access
  Point) to another
• Handoff in hybrid networks
• From a BS to another, from an AP to another, from
  a BS to an AP, or vice versa

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Handoff Decision Time Algorithms

• Traditional algorithms employ thresholds
• Channel measurements
• Received Signal Strength (RSS)
• Measures the co-channel interference power and
  noise
• Alternatively to RSS or in conjunction
• Path loss
• Carrier-to-interference ratio (CIR)
• Signal-to-interference ratio (SIR)
• BER
• Block error rate (BLER)
• Symbol error rate (SER)
• Etc.

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Handoff Decision Time Algorithms

• Goals
• Keep user connected
• Minimize network signaling
• Minimize ping-pong handoffs
• In order to avoid the ping-pong effect,
  additional parameters are used such as hysteresis
  margin, dwell timers, and averaging windows.

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Handoff Decision Time Algorithms

• Received Signal Strength (RSS) The BS whose
  signal is received with the largest strength is
  selected.
• RSS Threshold If the RSS of a new BS exceeds
  that of the current one and the signal strength
  of the current BS is below a threshold.
• RSS Hysteresis If the RSS of a new BS is
  greater than that of the old BS by a hysteresis
  margin.
• RSS Hysteresis Threshold If the received
  signal strength of a new BS exceeds that of the
  current one by a hysteresis margin and the signal
  strength of the current BS is below a threshold.
• Algorithms Dwell Timer A timer is started at
  the instant when the condition in the algorithm
  is true. The handoff is performed is the
  condition continues to be true until the timer
  expires.

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Traditional Handoff Algorithms
What kind of handoff is Happening in A, B, C, D?
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Sample RSS seen by MS traveling in a straight
line between them
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Performance of Handoff Algorithms

• Performance measures (related to voice
  connections)
• Call blocking probability
• Handoff blocking probability
• Delay between handoff request and execution
• Call dropping probability
• Objective Minimize unnecessary handoffs
• Overlooked issues
• Throughput maximization
• Maintaining QoS guarantees during and after
  handoff

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Generic Handoff Management Process

• Decision to handoff is made (network-controlled,
  mobile-assisted or controlled)
• MT registers with visiting database via a handoff
  announcement
• New visiting database communicates with home
  database for authentication and subscriber
  profile
• Home database responds with authentication. Both
  databases are updated.
• Home database communicates with old visiting
  database to clear registration information for
  the MT
• The old visiting database flushes or redirect
  packets to the new visiting database and removes
  the MT form its list.

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Problem

• Show the handoff times for
• RSS
• RSS threshold of -60 dBm
• RSS hysteresis of 10 dB
• RSS hysteresis of 5 dB threshold of -55 dBm

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Problem

• Show the handoff times for
• RSS
• RSS threshold of -60 dBm
• RSS hysteresis of 10 dB
• RSS hysteresis of 5 dB threshold of -55 dBm

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Problem

• Show the handoff times for
• RSS
• RSS threshold of -60 dBm
• RSS hysteresis of 10 dB
• RSS hysteresis of 5 dB threshold of -55 dBm

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Problem

• Show the handoff times for
• RSS
• RSS threshold of -60 dBm
• RSS hysteresis of 10 dB
• RSS hysteresis of 5 dB threshold of -55 dBm

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Problem

• Show the handoff times for
• RSS
• RSS threshold of -60 dBm
• RSS hysteresis of 10 dB
• RSS hysteresis of 5 dB threshold of -55 dBm

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Problem Solution
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Exercise

• Show the handoff times for
• RSS
• BS1-BS?-BS?-BS?-BS?
• RSS threshold of -60 dBm BS1-BS?-BS?-BS?-BS?
• RSS hysteresis of 5 dB
• BS1-BS?-BS?-BS?-BS?