Wireless Communications
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Transcript of Wireless Communications
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Wireless Communications
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Course Material
• Wireless Communications: Principles and Practice by T. Rappaport
• Mobile Communications: Jochen Schiller
• References
– Wireless Communications and Networks by W. Stallings
– Wireless Communication by Roy Blake.
– Internet.
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CELLULAR CONCEPTS
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Single Cell ‘Network’
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History of Cellular Networks
• Why cellular networks?– To address requirement for greater capacity
– For efficient use of frequency
– To address the poor quality of non cellular mobile networks
and increases coverage
• replaces a large transmitter with smaller ones in cells
• smaller transmitting power
• each cell serves a small geographical service area
• each cell is assigned a portion of the total frequency
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Replacement of huge single cell by a number of small cells
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Why Hexagonal Cell Structure
No proper coverage of the area with theoretical circles.
Polygon near to the circleHexagon is selected for further technical
simplicity.
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Description of a Cell
• Approximated to be a hexagonal coverage
• best approximation of a circular area
• Served by a base station• low powered transceiver
• antenna system
• may be divided into 6 equilateral triangles
• length of base of each triangle = 0.5R (radius)
• different groups of channels assigned to base stations
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Mathematical Description of a Cell
• Area of a cell is:
• Perimeter of a cell = 6R
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RxAreacell
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Types of Mobile Communication Cells• The size of a cell is dictated by capacity demand
– Macro-cell
• large, covering a wide area
• range of several hundred kilometers (km) to ten km
• mostly deployed in rural and sparsely populated areas
– Micro-cell
• medium cell, coverage area smaller than in macro cells
• range of several hundred meters to a couple of meters
• deployed mostly in crowded areas, stadiums, shopping malls
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Types of Mobile Communication Cells Contd.• The size of a cell is dictated by capacity demand
– Pico-cell
• small, covering a very small area
• range of several tens of meters
• low power antennas
• can be mounted on walls or ceilings
• used in densely populated areas, offices, lifts, tunnels etc
– Mega-cell
-- These cells are formed by LEO and MEO
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Capacity Computations
• Assume there are N cells, each allocated k different frequency channels. These N cells are said to form a cluster. Total number of channels per cluster is given by
• S = k N • Total capacity associated with M clusters: C = M k N = M S • A cluster may be replicated more times in a given area if the cells
are made smaller (note that power needs to be reduced accordingly).
• Capacity of cellular system is directly proportional to “M”, number of times a cluster is replicated.
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Capacity versus interference for same size cell and power transmission
• Decrease N for More Capacity: If Cluster Size, N is decreased while cell size remains fixed,
more clusters are required to cover the area (M increases). Therefore, Capacity increases.
• Increase N for Less Interference: On the other hand, if N is increased (large cluster size)
means that co-channels are now farther than before, and hence we have will have less interference.
Value of N is a function of how much interference a mobile or a base station can tolerate.
We should select a smallest possible value of N but keeping S/I in the required limits.
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Means of Increasing System Capacity
• There are several approaches for increasing cellular system
capacity including:
– Cell clustering
– Sectoring of cells
– Cell splitting
– Frequency reuse
– Reduction of adjacent cell interference and co-channel
interference
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Cell Clusters• Service areas are normally divided into clusters of cells to
facilitate system design and increased capacity
• Definition
– a group of cells in which each cell is assigned a different
frequency
• cell clusters may contain any number of cells, but
clusters of 3, 4, 5, 7 and 9 cells are very popular in
practice
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Cell Clusters
• A cluster of 7 cells
• the pattern of cluster is repeated throughout the network
• channels are reused within clusters• cell clusters are used in frequency planning for the
network• Coverage area of cluster called a ‘footprint’
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Cell Clusters (1)
• A network of cell clusters in a densely populated Town
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Representation Of Cells Through BS
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Frequency Plan
• Intelligent allocation of frequencies used
• Each base station is allocated a group of channels to
be used within its geographical area of coverage
called a ‘cell’
– Adjacent cell base stations are assigned completely
different channel groups to their neighbors.
– base stations antennas designed to provide just the cell
coverage, so frequency reuse is possible
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Frequency Reuse Concept
• Assign to each cluster a group of radio channels to be used
within its geographical footprint
– ensure this group of frequencies is completely different from
that assigned to neighbors of the cells
• Therefore this group of frequencies can be reused in a cell
cluster ‘far away’ from this one
– Cells with the same number have the same sets of
frequencies
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Frequency Reuse Factor
• Definition
– When each cell in a cluster of N cells uses one of
N frequencies, the frequency reuse factor is 1/N
– frequency reuse limits adjacent cell interference
because cells using same frequencies are
separated far from each other
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Factors Affecting Frequency Reuse
• Factors affecting frequency reuse
include:
Types of antenna used
--omni-directional or sectored
placement of base stations
-- Center excited or edge excited.
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Excitation of Cells– Once a frequency reuse plan is agreed upon overlay the frequency
reuse plan on the coverage map and assign frequencies
– The location of the base station within the cell is referred to as cell
excitation
– In hexagonal cells, base stations transmitters are either:
• centre-excited, base station is at the centre of the cell or
• edge-excited, base station at 3 of the 6 cell vertices
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Finding the Nearest Co-Channel
After selecting smallest possible value of N we should see that N should follow
the following eq. N= i2+j2+ij
(1) Move i cells along any chain of hexagons
(2) Turn 600 counter-clockwise and move j cells, to reach the next cell using
same frequency sets
– this distance D is required for a given frequency reuse to provide enough
reduced same channel interference
– ie, after every distance D we could reuse a set of frequencies in a new
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Freq Reuse ( N=7 , i=2 j=1)
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Freq Reuse ( N=19 , i=3 j=2)
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How frequency Reuse Increases Capacity
Example: A GSM communication system uses a
frequency reuse factor of 1/7 and 416 channels
available. If 21 channels are allocated as control
channels, compute its system capacity. Assume a
channel supports 20 users
Channels available for allocation = 416 - 21 = 395
Number of cells = 395 / 7 = 57
Number of simultaneous users per cell = 20 x 57 = 1140
Number of simultaneous users in system = 7 x 1140 = 7980
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To satisfy the user, a channel needs to be available on request.
Reasonable probability of call blockage (GOS) is 2%.
GOS fluctuate with location and time. The goal is to keep a uniform GOS across the system.
Reduction of variations in GOS allow more users – an increase in capacity.
Three types of algorithms for channel allocation: Fixed channel allocation (FCA) Channel Borrowing Dynamic channel allocation (DCA)
Channel Allocation TechniquesTargets to achieve through the different
channel allocation techniques.
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Available spectrum is W Hz and each channel is B Hz. Total number of channels:
Nc = W/B For a cluster size N, the number of channels per
cell:Cc = Nc/N
To minimize interference, assign adjacent channels to different cells.
Fixed Channel Allocation Techniques
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FCA is the optimum allocation strategy for uniform traffic across the cells.
A non uniform FCA strategy, when it is possible to evaluate GOS in real time and adjust the FCA accordingly. This requires a more complex algorithm.
Features of Fixed Channel Allocation Techniques
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Channel Borrowing Borrow frequencies from low traffic cells to high
traffic cells. Temporary channel borrowing: channel is
returned after call is completed. If channels from cell E are borrowed by cell A,
then neighboring cells E cannot use those channels.
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Dynamic Channel Allocation All channels are placed in a pool, and are
assigned to new calls according to the reuse pattern. Signal is returned to the pool, when call is completed.
Issues related to channel allocation are still under research.
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Comparison of Channel Allocation Techniques
Fixed Channel Allocation Advantages:
--- Less load on MSC--- Simple
Disadvantages: Blocking may happen
Dynamic Channel Allocation Advantages:
Voice channels are not allocated permanently. That is, resource is shared on need-basis
Disadvantages: --- Requires MSC for processing---burden on MSC
--- May be very complicated
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