Wireless Communications: System Design

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Wireless Communications System Design

• Dr. Mustafa Shakir

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Issues in cell to cell moving

• What are different levels of handoff.
• (1) Intra Cell (2) Inter cell (3) Inter
  system
• Importance of handoff.
• When no priority to handoff call blocking would
  be equal for call initiation and call handoff.
• There are two strategies to give a priority to
  handoff.
• (1) Guard Channel 100 guaranty for successful
  handoff but It will cause low trunking
  efficiency.
• (2) Queuing Of Handoff Request There can be
  unsuccessful handoffs due to long delay in queue.
  
• --Probability of forced termination
  decreases at the cost of reduced Total Carried
  Traffic.
• -- Queuing is possible because of the time
  available between the Threshold power level and
  the Hand off power level.

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Umbrella Cell Approach
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Solution For More Handoffs

• Umbrella Cell Approach
• Micro cells inside A macro cell.
  ---- Macro cell is defined by high
  power and lengthy tower.
  ----
  Micro cells are defined inside the macro cell
  with less power and less height towers.
  ---- High speed MS are handled by macro cell
  and low speed subscribers are handled by micro
  cells.
  ---- This strategy
  increases the no of capacity channels per unit
  area and decreases the no of handoffs.

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Umbrella Cell Approach
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• INTERFERENCE AND SYSTEM CAPACITY

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Interference

• It is a major limiting factor in the performance
  of cellular radio systems. (In comparison with
  wired comm. Systems, the amount and sources of
  interferences in Wireless Systems are greater.)
• Creates bottleneck in increasing capacity
• Sources of interference are 1. Mobile
  Stations 2. Neighboring Cells 3. The
  same frequency cells 4. Non-cellular signals in
  the same spectrum
• Interference in Voice Channels Cross-Talk
• Urban areas usually have more interference,
  because of a)Greater RF Noise Floor, b)
  More Number of Mobiles

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Major Types Of Interference

1. Co-Channel Interference (CCI)
2. Adjacent Channel Interference (ACI)
3. Other services like a competitor cellular
   service in the same area

• The cells that use the same set of frequencies
  are called co-channel cells.
• The interference between signals from these cells
  is called Co-Channel Interference (CCI).
• Cannot be controlled by increasing RF power.
  Rather, this will increase CCI.
• Depends on minimum distance between co-channel
  cells.

1) Co-Channel Interference and System Capacity
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The yellow cells use the same set of frequency
channels, and hence, interfere with each
other. In case of N7, there are 6 first-layer
co-channels.

• In constant cell size and RF power, CCI is a
  function of Distance between the co-channel
  cells(D), and the size of each cell (R).
• Increasing ratio D/R, CCI decreases.
• Define Channel Reuse Ratio Q D/R

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• Signal-to-interference ratio
• S is the power of the signal of interest and Ik
  is the power of kth interference.
• The signal strength at distance d from a source
  is

• That is, received signal power is inversely
  related to nth power of the distance.
• where n path loss exponent

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• For hexagonal geometry, D/R can be calculated

• Smaller Q provides larger capacity, since that
  would mean smaller N. (Capacity ? 1/N).
• Larger Q improves quality, owing to less CCI.

• for N3, Q3,
• N7, Q4.58,
• N12, Q6,
• N13, Q6.24

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• Then we can express the SIR in terms of distance
• where the denominator represents the users in
  neighboring clusters using the same channel.
• ?Let D kD be the distance between cell centers.
  Then
• Note how S/I improves with the frequency reuse N.
• Analog systems U.S. AMPS required S/I 18dB
  For n 4, the reuse factor for AMPS is N ? 6.49,
  so N 7.
• Now, let us consider the worst case for a cluster
  size of N 7. The mobile is at the edge of the
  cell. Express C/I as a function of actual
  distances.

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Worst Case Design Worst case carrier-to-interferen
ce ratio Let n 4 and D/R q, Let reuse N
7, then Compute C/I and get C/I 17.3 dB
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If S/I min 15 dB, what is the capacity for
n 4, n 3 (a) n 4, N 7 N 7 can
be used (b) n 3, N 7
E
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(2) Adjacent Channel Interference

• Interference from channels that are adjacent in
  frequency,
• The primary reason for that is Imperfect Receiver
  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
Unintended Tx
Strong bleed over
BS as Tx
Mobile User Rx
Weaker signal
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
BS as Rx
Weaker signal
Strong bleed over
Desired Mobile Tx
Adjacent Channel Mobile Tx
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

• Careful Filtering ---- min. leakage or sharp
  transition
• 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 pass band BW.

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• Power Control in Mobile Com

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What is power control ?

• Both the BS and MS transmitter powers are
  adjusted dynamically over a wide range.
• Typical cellular systems adjust their transmitter
  powers based on received signal strength.
• TYPES OF POWER CONTROL
• Open Loop Power Control
• It depends solely on mobile unit, not as
  accurate as closed loop, but can react quicker
  to fluctuation in signal strength. In this there
  is no feed back from BS.
• Closed Loop Power Control
• In this BS makes power adjustment decisions and
• communicates to mobile on control channels

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Why power control ?

• Near-far effect
• Mechanism to compensate for channel fading
• Interference reduction,
• prolong battery life

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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, microcell zones.
• Dynamic allocation of channels according to load
  in the cell (non-uniform distribution of
  channels).
• Improve access technologies.

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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
  number of clusters over coverage region would be
  increased thus increasing the number of channels.
• New cells added having smaller radius than
  original cells and by installing these smaller
  cells (called microcells ) between existing cells
  , capacity increases due to additional number of
  channels per unit area.

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Cell splitting diagram 1
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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
• Pt1 R-n
• Pr(at the new cell boundary) is proportional to
• Pt2 (R/2)-n

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Transmit Power

• Take n4, we get
• Pt2 Pt1/16
• We find that the transmit power must be reduced
  by 16 times or 12 dB in order to use the
  microcells to cover the original area. While
  maintaining the same S/I.

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Application of cell splitting

• When there are two cell sizes one cant simply
  use the same transmit power for all cells. If
  larger transmit power used for all cells some
  smaller cells would not be sufficiently separated
  from co channel cells. Using smaller Pt the
  larger cells might be left unserved.
• So old channel broken to two channel groups
  corresponding to smaller and larger cell reuse.
• Larger cell for less frequent hand off.
• Antenna down tilting focusing radiated energy
  from base station to the ground to limit radio
  coverage of newly formed cells.

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Cell Sectoring

• Co channel interference may be reduced by
  replacing omni directional antenna by several
  directional antennas.
• Given cell will receive interference and would
  transmit with fraction of available co channel
  cells.
• Each sector uses directional antenna at the B.S
  and assigned a set of channels.
• Partitioning into three 120 deg. sectors or six
  60 deg. sectors.
• Amount of CCI reduced by number of sectors.
• Reduced Tx Power

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Cell Sectoring
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Example for sectoring
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Explanation For Cell Sectoring
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Effects of Sectoring

• Reduction in interference offered by sectoring
  would enable to reduce the cluster size N and
  additional degree of freedom in channel
  assignment.
• Increased number of antennas with shrinking
  cluster size and decrease in trunking efficiency
  due to channel sectoring at base station.
• Since sectoring reduces the coverage area of a
  particular group of channels the number of
  handoffs increases
• Available channels subdivided and dedicated to a
  specific antenna thus making up of several
  smaller pools contributing to decrease in
  trunking efficiency.

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Repeaters

• To provide dedicated coverage for hard to reach
  areas
• Radio retransmitters for range extension.
• Upon receiving signals from base station forward
  link the repeater amplifies and reradiates the
  base station signals to specific coverage region.
• In building wireless coverage by installing
  Distributed Antenna Systems.
• Repeaters must be provisioned to match the
  available capacity from the serving base station.

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Repeaters For Range Extension
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Microcell Zone

• The increased number of handoff as a result of
  sectoring would result in an increased load on
  switching and control link elements of the mobile
  system.
• Division into microcell zones and each of the
  three are connected to a single base station and
  share the same radio equipment.
• Zones connected by a coaxial cable, fiber optic
  cable or microwave link to the base station.
• Handoff not required while mobile travels between
  zones within cell.
• Channel switching and a channel active only
  within zone of travelling.

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Scenario

• In Micro cell zone scenario each hexagon
  represents a zone while the group of three
  hexagons represent a cell.
• Zone Radius Rz is one hexagon radius.
• Capacity of Microcell is directly related to
  distance betw. Cochannel cells and not zones.
• No handoffs is required at the MSC.
• The base station radiation is localized and
  interference is reduced

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• Trunking Grade Of Service

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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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Terminologies

• Erlang
• One Erlang
• When a circuit is busy for one hour it handled a
  traffic of one erlang.
• Grade of Service (GOS)
• probability that
  a call is blocked (or delayed).
• Set-Up Time
• Traffic intensity is measured in Erlangs
• 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.

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Terminologies Contd.

• 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)

• Traffic per user Au ?H? where ? ?is the request
  rate and H is the holding time.
• For U users the load is A U Au
• If traffic is trunked in C channels, then the
  traffic intensity per channel is Ac UAu /C
• Erlang B

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The Erlang B Chart
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Example

• Example An urban area has 2 million residents.
  Three competing cellular systems provide service
  
• System A 394 cells x 19 channels/cell.
• System B 98 cells x 57 channels/cell.
• System C 49 cells x 100 channels/cell.
• For each user ? ? 2 calls/hr, H 3min, GOS 2
  blocking. Find the number of users that can be
  supported by each system. Note that these are not
  simultaneous users.
• System A
• Au ? H 2 x 3/60 0.1 Erlangs.
• From the curve for GOS 0.02 and C 19 gt A
  12 Er.
• Users per cell (U) A/Au 12/0.1 120 users
• 120 users/cell x 394 cells 47,280 users can be
  served.
• Market penetration 2.36.

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No. of subscribers

• System C
• Prob Blocking 2 0.02
• C 100
• Au ? H 2 x 3/60 0.1 Erlangs.
• From table, A 88 Erlangs.
• Users per cell U A/Au 88/0.1 880 users
• 880 users/cell x 49 cells 43,120.
• Market penetration 2.156.
• System B
• Prob Blocking 2 0.02
• C 57
• Au ? H 2 x 3/60 0.1 Erlangs.
• From table, A 45 Erlangs
• Users per cell U A/Au 45/0.1 450 users
• 450 users/cell x 98 cells 44,100.
• Market penetration 2.21.

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• Total No. of supported users 47,280 44,100
  43,120
• 134,500 users.
• Total market penetration for 3 systems 6.725

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