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Wireless Communications: System Design

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Title: Wireless Communications: System Design


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Wireless Communications System Design
  • Dr. Mustafa Shakir

3
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.

4
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
7
  • 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

9
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

12
  • 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
15
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.

17
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.
19
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.

20
  • Power Control in Mobile Com

21
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

22
Why power control ?
  • Near-far effect
  • Mechanism to compensate for channel fading
  • Interference reduction,
  • prolong battery life

23
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.

24
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.

25
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

28
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

29
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.

30
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

32
33
Cell Sectoring
34
Example for sectoring
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Explanation For Cell Sectoring
36
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.

37
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.

38
Repeaters For Range Extension
39
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

44
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.

45
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.

46
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.

47
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

48
The Erlang B Chart
49
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.

50
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.

51
  • 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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