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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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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

cell

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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