Capacity Increase Techniques and Calculations

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Capacity Increase Techniques and Calculations
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Increasing capacity of cellular systems

• Capacity of a cellular system can be enlarged
  using frequency reuse
• Capacity can also be improved using cellular
  layout and antenna design techniques such as
• Cell splitting
• Antenna sectoring

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Cell splitting calculations

• Cell splitting subdivides a congested cell into
  smaller cells, each with its own base station.
• Original large cell with radius R is split into
  medium cells with radius R/2.
• Medium cell is further split into small cells
  with radius R/4.

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Cell splitting from radius R to R/2 and R/4
R
R/2
Large cells
R/4
Medium cells
Small cells
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System capacity increase using cell splitting

• Example
• Consider a R - R/2 cell splitting system with R
  1 km.

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. . .System capacity increase using cell splitting

• Suppose each base station is allocated 60
  channels regardless of cell size. Find the
  number of channels contained in a 3 x 3 km2 area
  around (small ) cell A for the following cases
• Without cell splitting (i.e. just the original
  large cells)
• With cell splitting (using the small cells)

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Cell splitting example with R 1 km
R
A
R/2
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Solution

• To cover a 3 x 3 km2 area centered as around
  cell A, we need to cover 1.5 km to right, left,
  top and bottom.

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. . . Solution

• Number of large cells in this 3 x 3 km2 area
  (approx.) 4
• Number of small cells in this 3 x 3 km2 area
  x number of large cells 4 x 4 16 small
  cells
• With base stations, the number of channels
  equals 4 x 60 240
• With 16 small cells, the number of channels in
  the square, with cell splitting equals 60 x 15
  900

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System capacity increase using directional
antennas (sectoring)

• In basic form, antennas are omnidirectional
• Directional antennas can increase the system
  capacity relative to that of omnidirectional
  antennas

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. . . System capacity increase using direction
antennas (sectoring)
(
)
S I
1 6
qk

, where
omni
D/R
k path loss exponent
q

• Sectorization can be done in multiples of 60

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. . . System capacity increase using direction
antennas (sectoring)

• For N 7 cell reuse

3
120
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Antenna sectorization
2
1
1
3
2
3
6
4
2
1
1
3
5
2
6
4
3
5
a. 3 sectors of 120 each
b. 6 sectors of 60 each
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Worst case scenario in 120 sectoring
D 0.7R
Mobile
D
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Example

• For N 7, base stations using omnidirectional
  antennas cannot satisfy the 18 db
  requirement.
• Determine if the use of 120 sectoring and N 7
  would satisfy the 18 db requirement for path loss
  exponent k 4.

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Solution
4.6
3 x 7
q


(
)
1 2
1 2
285 ? 24.5 db
qk


4.6
4
120

• Since this is greater than 18 db, it will work.

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Handoff management

• MSC is an appropriate device to oversee the
  handoff operation
• Transfer of mobile control from current base
  station BS to new target BS
• Initiation phase
• Employs a decision making strategy based on
  measured received signal.

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. . . Handoff management

• Execution phase
• Involves the allocation of new channels to
  mobile, and exchange of control messages.
• MSC obtains status information for all base
  stations periodically
• Intraswitch handoff between cells controlled
  by same MSC
• Interswitch handoff between cells controlled
  by different MSCs.

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Handoff Strategies

• Mobile controlled handoff (MCHO)
• Desirable since it does not burden the network.
• However, it increases complexity of mobile
  terminal.

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. . . Handoff Strategies

• Network controlled handoff (NCHO)
• BSs or APs (Access points) monitor signal
  quality from mobile.
• MSC then chooses the candidate BS or AP and
  initiates handoff.
• Mobile plays passive role in process.

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. . . Handoff Strategies

• Mobile assisted handoff (MAHO)
• Employed by GSM system
• Mobile records signal levels from various BSs
  using a periodic beacon generated by BSs Mobile
  relays power levels from different BSs to MSC via
  current BS
• MSC makes handoff decision.

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Types of handoff

• Hard handoff - (break before make)
• Mobile has radio link with only one BS at
  anytime
• Old BS connection is terminated before new BS
  connection is made.

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. . . Types of handoff

• Soft handoff (make before break)
• Mobile has simultaneous radio link with more
  than one BS at any time
• New BS connection is made before old BS
  connection is broken
• Used by CDMA systems

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. . . Types of handoff

• Backward handoff
• Handoff is predicted and initiated via the
  existing BS link
• Loss of power in existing BS link is a problem
• Forward handoff
• Handoff is begun via the new BS radio link
• Delay is a problem

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Intraswitch handoff process
MSC
Fixed Terminal
AP_1
AP_0
Link beforehandoff
Link afterhandoff
MS
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Signaling sequence for intraswitch handoff
MSC
(3) SEQ_PKT (8) UP_READY

• NEW_AP_READY
• (4) HO_MUST

(2) LAST_PKT (7) NO_MORE
(6) LAST_UP
AP_0
AP_1
(5) UP_NO_MORE
FBK
(5) READY

• NEW_AP_READY
• (4) HO_MUST

MS
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Example

• Mobile is currently located in cell served by
  AP_0 and is moving toward the cell being served
  by AP_1.
• When mobile reaches the cell boundary of AP_0,
  the MSC initiates and executes handoff algorithm.
  
• MSC knows that AP_1 has a channel available to
  accept the handoff.

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. . . Example

• Step 1 MSC directs the mobile to handoff to
  AP_1 in the following steps
• Send the message
• (1) NEW_AP_READY
• To the mobile via AP_0, with identity of
  candidate AP_1 included.

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. . . Example

• Step 2 AP_0 responds with message
• (2) LAST_PKT to the MSC
• Message contains the sequence number of packet
  sent to the mobile
• MSC send the sequence number of the following
  downlink packet to AP_1 using the message
• (3) SEQ_PKT

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. . . Example

• Step 3 the message
• (4) HO_MUST indicates the last downlink packet
  from MSC to AP_0
• When AP_0 receives this message, it flags the
  termination of connection by sending the message
  (5) VP_NO_MORE to MSC

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. . . Example

• Step 4
• Mobile switches its operating frequency and
  sends the message (6) READY to AP_1
• This message contains the sequence number of the
  last packet correctly received by mobile.

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. . . Example

• Step 5
• AP_1 starts downlink transmission and buffers
  all uplink packets from mobile
• It also sends the message (7) LAST_UP to MSC
  requesting approval of uplink transmission

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. . . Example

• Step 6 MSC waits for message(8) NO_MORE from
  AP_0
• Than MSC switches uplink connection from AP_0 to
  AP_1 and sends the message(9) UP_READY to AP_1