Wireless Communication Mobile Communications Lecture 6

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Wireless CommunicationMobile Communications
Lecture 6

• Tanvir Ahmad Niazi
• Tanvir.niazi_at_mail.au.edu.pk
• Air University, Islamabad

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• Overview of the Previous Lecture
• New Topics
• Trunking and Grade of Service
• Improving Coverage and Capacity in Cellular
  Systems
• Announcements

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Overview of the Previous Lecture

• Channel Assignment Strategies
• --Fixed, Dynamic, Channel borrowing
• Hand off Strategies
• --Prioritizing Handoffs
• --Practical Handoff Considerations
• Interference and System Capacity
• --Co-channel interference and System capacity
• --Channel Planning for Wireless Systems
• --Adjacent Channel Interference
• --Power Control for Reducing Interference

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Adjacent Channel Interference

• Interference from channels that are adjacent in
  frequency,
• The primary reason for that is Imperfect Receive
  Filters which cause the adjacent channel energy
  to leak into your spectrum.
• Problem is severer if the user of adjacent
  channel is in close proximity. ?Near-Far Effect
• Near-Far Effect The other transmitter(who may or
  may not be of the same type) captures the
  receiver of the subscriber.
• Also, when a Mobile Station close to the Base
  Station transmits on a channel close to the one
  being used by a weaker mobile The BS faces
  difficulty in discriminating the desired mobile
  user from the bleed over of the adjacent
  channel mobile.

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Near-Far Effect Case 1

• The Mobile receiver is captured by the
  unintended, unknown transmitter, instead of the
  desired base station

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Near-Far Effect Case 2

• The Base Station faces difficulty in recognizing
  the actual mobile user, when the adjacent channel
  bleed over is too high.

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Minimization of ACI

• (1) Careful Filtering ---- min. leakage or sharp
  transition
• (2) Better Channel Assignment Strategy
• Channels in a cell need not be adjacent For
  channels within a cell, Keep frequency separation
  as large as possible.
• Sequentially assigning cells the successive
  frequency channels.
• Also, secondary level of interference can be
  reduced by not assigning adjacent channels to
  neighboring cells.
• For tolerable ACI, we either need to increase the
  frequency separation or reduce the passband BW.

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Trunking and Grade of Service (GOS)
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Trunking and Grade of Service (GOS)

• Trunking
• A means for providing access to users on demand
  from available pool of channels.
• With trunking, a small number of channels can
  accommodate large number of random users.
• Telephone companies use trunking theory to
  determine number of circuits required.
• Trunking theory is about how a population can be
  handled by a limited number of servers.

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Terminology

• Traffic intensity is measured in Erlangs
• One Erlang traffic in a channel completely
  occupied. 0.5 Erlang channel occupied 30 minutes
  in an hour.
• Grade of Service (GOS) probability that a call
  is blocked (or delayed).
• Set-Up Time time to allocate a channel.
• Blocked Call Call that cannot be completed at
  time of request due to congestion. Also referred
  to as Lost Call.
• Holding Time (H) average duration of typical
  call.
• Load Traffic intensity across the whole system.
• Request Rate (?) average number of call requests
  per unit time.

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Traffic Measurement (Erlangs)
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Erlang C Model Blocked calls cleared

• A different type of trunked system queues blocked
  calls Blocked Calls Delayed. This is known as an
  Erlang C model.
• Procedure
• Determine Prdelaygt 0 probability of a delay
  from the chart.
• Prdelay ?gt t delay ? gt 0 probability that
  the delay is longer than t, given that there is a
  delay
• Prdelay ? gt t delay ?gt 0
  exp-(C-A)t /H
• Unconditional Probability of delay ? gt t
• Prdelay ?gt t Prdelay ?gt 0 Prdelay
  ?gt t delay ? gt 0
• Average delay time D Prdelay?gt 0 H/ (C-A)

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Erlang C Formula

• The likelihood of a call not having immediate
  access to a channel is determined by Erlang C
  formula

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Improving Capacity in Cellular Systems

• Cost of a cellular network is proportional to the
  number of Base Stations. The income is
  proportional to the number of users.
• Ways to increase capacity
• New spectrum expensive. PCS bands were sold for
  20B.
• Architectural approaches cell splitting, cell
  sectoring, reuse partitioning, microcell zones.
• Dynamic allocation of channels according to load
  in the cell (non-uniform distribution of
  channels).
• Improve access technologies. 3.7 Improving
  Capacity in Cellular Systems

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Cell Splitting

• Cell Splitting is the process of subdividing the
  congested cell into smaller cells
  (microcells),Each with its own base station and a
  corresponding reduction in antenna height and
  transmitter power.
• Cell Splitting increases the capacity since it
  increases the number of times the channels are
  reused.

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An Example

• The area covered by a circle with radius R is
  four times the area covered by the circle with
  radius R/2
• The number of cells is increased four times
• The number of clusters the number of channels and
  the capacity in the coverage area are increased
• Cell Splitting does not change the co-channel
  re-use ratio Q D/R

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Transmit Power

• New cells are smaller, so the transmit power of
  the new cells must be reduced
• How to determine the transmit power?
• The transmit power of the new cells can be found
  by examining the received power at the new and
  old cell boundaries and setting them equal
• Pr(at the old cell boundary) is proportional to
• Pr(at the new cell boundary) is proportional to

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Transmit Power
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Application of cell splitting

• Not all cells are split at the same time.
• Larger transmit power
• Some of the channels would not be sufficiently
  separated from the co-channel cells.
• Smaller transmit power --parts of the larger
  cells left uncovered
• Two groups
• one that corresponds to the smaller cell and the
  other for larger cell reuse requirements

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Application of cell splitting (cont.)

• The sizes of these two groups depend on the stage
  of the splitting process
• At the beginning, fewer channels will be there in
  the smaller power group. As the demand grows,
  smaller groups would require more channels
• Cell splitting continues until all the channels
  are in the smaller power group
• Antenna Down tilting
• To limit the radio coverage of microcells

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Cell Overlay

• Its a relatively novel technique
• Cells used by A are divided into
• Channels used by a those are used by A only
  within radius R/2 from center.
• Channels not used by a no restrictions on
  their use in A.

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Cell sectoring

• Another way to reduce the number of cells in a
  cluster and hence, to reduce Interference is
  sectoring. Sectoring refers to the use of
  directional rather than omni antennas. Three (3)
  120 degrees sectors are shown as an example
• Analysis mobile in center cell will experience
• interference from only 2 cells (not 6).
• Improvement of 6dB in S/I. Alternatively,
• try to reduce the reuse factor. Sectoring
  entails
• reduced trunking efficiency.

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Example of Cell Sectoring

• With omin directional antennas
• Where we assumed that the power attenuation n
  4.
• For N 4, we obtain S 13.8 dB.
• For N 4 and with 3 sectors, we get S 19. 9
  dB

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Microzones

• Multiple zones and a base station make up a cell
• As a mobile travels within the cell, it is served
  by the zone with the strongest signal
• This technique is superior to sectoring because
  antennas are placed at the outer edges of the
  cell, and any base station channel can be
  assigned to any zone by the base station

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Microzoning
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ADVANTAGES

• No handoffs is required at the MSC
• The base station radiation is localized and
  interference is reduced. A given channel is
  active only in the particular zone in which the
  mobile is traveling
• The co-channel interference is also reduced

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• Decreased co-channel interference improves signal
  quality which leads to an increase in capacity
  without any degradation in trunking efficiency
  caused by sectoring
• For example
• We know an (S/I) of 18dB is required for
  satisfactory system performance in narrowband FM

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EXAMPLE

• If a system with N7 and (D/R)4.6,it can
  achieved a (S/I) of 18dB
• For a microcell zone system, since transmission
  at any instant is confined to a particular zone,
  this implies that a (Dz/Rz) of 4.6 can achieve
  the required performance
• where,
• Dz minimum distance between active
  co-channel zones and
• Rz zone radius

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EXAMPLE (cont.)
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Repeaters for Range Extension

• Repeaters are radio re-transmitters used to
  provide coverage for hard-to-reach areas, such as
  within buildings or in valleys or tunnels
• Repeaters are bidirectional. Upon receiving
  signals from base station, then amplifies and
  reradiates the base station signals to the
  specific coverage region. Also it will send
  signals to the serving base station.
• The repeaters do not add capacity to the
  system-it simply serves to reradiate the base
  station signal into specific locations

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Repeaters for Range Extension
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Summary for chapter 3

• Concepts of handoff, frequency reuse, trunking
  efficiency and frequency planning have been
  presented
• The capacity of a cellular system depends on
  several factors and the methods to increase the
  capacity
• The overriding objective of these methods is to
  increase the number of users in the system

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Announcements

• Problems 1.3, 1.13, 1.9, 1.10 and 1.18
• Problems 3.1, 3.2, 3.4, 3.5 and 3.8
• Due date
• 14th March, 2008