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Wireless Communications T L Singal
Companies, Inc. All rights reserved. No part of this this PowerPoint slide may be displayed, displayed, reproduced reproduced or or PROPRIETARY MATERIAL. © 2010 The McGraw-Hill Companies, distributed in any form or by any means, without the prior written permission of the publisher, or used beyond the limited distribution to teachers and educators
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Principles of Cellular Communication
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Principles of Cellular Communication
Cellular Terminology
Cell Structure and Cluster
Frequency Reuse Concept
Cluster Size and System Capacity
Method of Locating Co-channel Cells
Frequency Reuse Distance
Co-channel Interference and Signal Quality
Co-channel Interference Reduction Methods
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Cellllul Ce ular ar Te Term rmin inol olog ogy y – A Cel Celll A
cell is the basic geographic unit of a cellular system.
It is also called a footprint.
A
cell is the radio area covered by a cellcellsite that is located at its center
A large geographical area is divided into a number of contiguous smaller geographical coverage areas called cells
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Cell Structure (Cell Shape) Determined by the desired received signal level by the mobile subscribers from its base station transmitter in its operating area
Ideal, actual and fictitious cell models R R
R
R R
(a) Ideal cell
(b) Actual cell
(c) Fictitious cell models
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A Cell with a CS CS and Mobile
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Cell Structure
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Geometric Cellular Structures
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Hexagonal Cellular Geometry Offers best possible non-overlapped cell radio coverage
Multiple hexagons can be arranged next to each other
For a given radius (largest possible distance between the polygon center and its edge), the hexagon has the largest area
Simplifies the planning and design of a cellular system
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A Cellular Cluster Cluster A
group of cells that use a different set of frequencies in each cell
Only selected number of cells can form a cluster Can be repeated any number of times in a systematic manner The cluster size is the number of cells within it, and designated by K
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Hexagonal Cluster Patterns
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Conventional Mobile Communication Conventional mobile communication service was structured in a fashion similar to television broadcasting: One very powerful transmitter located at the highest spot in an area would broadcast in a radius of up to 50 kilometers.
Limitations:
High power consumption
Low capacity
Large size of the mobile
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System Design Problem?
Main limitation of a conventional mobile wireless communication system is:
Limited availability of FREQUENCY of FREQUENCY SPECTRUM !!!
So the big challenge is … to serve large number of mobile users ..
within limited allocated frequency spectrum
with a specified system quality
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System Capacity & Spectrum Utilization? The need: • Opti Optimu mum m spectrum usage • More capacity • High High qua qualit lity y of service (QoS) • Low cost
System capacity at required QoS with conventional frequency plan Out of Capacity!!! Subscriber growth Time
How to increase capacity & Spectrum utilization ?
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Poss Po ssib ible le Sol Solut utio ion n – Fr Freq eque uenc ncy y Reuse Reuse
Reuse allocated RF spectrum or a given set of frequencies (frequency channels) in a given large geographical service area without increasing the interference
Divide the service area into a number of small areas called cells
Allocate a subset of frequencies to each cell
Use low-power transmitters with lower height antennas at the base stations
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Frequency Reuse Concept
Large coverage area, efficient spectrum utilization and enhanced system capacity are the major attributes of cellular communication
Frequency reuse is the core concept of cellular communications
The design process of selecting and allocating channel groups for all the cellular base stations within a system is called frequency reuse
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Illustration of Frequency Reuse
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Co-channel and Adjacent Channel Cells
Cells, which use the same set of frequencies, are referred to as cochannel cells
Co-channel cells are located sufficiently physically apart so as not to cause cochannel interference
The space between adjacent cochannel cells is filled with other cells that use different frequencies to provide frequency isolation
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Frequency Reuse & Spectrum Efficiency Suppose the system has RF spectrum for 100 voice channels 2
1
3
7
4
6 5
HPT – High High Power Power Transm Transmitte itterr
LPT LPT – Low Low Powe Powerr Tr Tran ansm smit itter ter
Scenario
1: A high power base station covering entire area – system capacity = 100 channels
Scenario
2: Divide spectrum into 4 groups of 25 channels each; cells (1, 7), (2, 4), (3, 5), 6 are assigned distinct channel groups – system capacity = 175 channels
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Technical Issues Technical issues for proper design and planning of a cellular network:
Selection of a suitable frequency reuse pattern
Physical deployment and radio coverage modeling
Plans to account for the expansion of the cellular network
Analysis of the relationship between the capacity, cell size, and the cost of the infrastructure
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Cluster Size and Cell Capacity
In a cellular system, the whole geographical service area is divided into a number of clusters having finite number of cells.
The K number of cells in a cluster (cluster size) use the complete set of available frequency channels, N.
Each cell in the cluster contains J = (N/K) number of channels only Therefore, N = J x K;
where J ≤ N
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Cluster Size and System Capacity
The cluster can be replicated many times to cover the desired geographical area by a cellular communication system
Let M be the number of clusters in the system, then overall system capacity, C is given as C=MxN
→
C=MxJxK
(N = J x K)
When K is reduced, J is proportionally increased since N = J x K is constant.
To increase C, M is increased. But reducing K will increase co-channel interference!
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Co-channel Cells Cells
which use the same set of frequencies are referred to as cochannel cells. The
interference between cochannel cells is referred to as cochannel interference. The
space between adjacent cochannel cells are filled with cells using different frequencies.
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Shift Parameters i and j in a Hexagonal Geometry The
shift parameters i and j are separated by 60 degrees in a hexagonal geometry It
can have any integer value 0, 1, 2, …. These
can be used to determine the location of cochannel cells
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Method to Locate Co-channel Cells
Rules for determining the nearest cochannel cell cell using using “Shift “Shift para paramet meters ers”” (i, j) to lay lay out out a cellular system is:
Step 1: Move i cells along any side of a hexagon.
Step 2: Turn 60 degrees anticlockwise
Step 3: Move j cells.
where i and j are shift parameters and can have integer value 0, 1, 2, 3, and so on …
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Cochannel Cells for i =3, j = 2
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Co-channel Hexagonal Geometry Distance
between two adjacent cells,
d = √3 R Let
D be the distance between two cochannel cells. D = √{3R2(i2+j2+ixj)} D2 /(3R2) = i2+j2+i x j
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A Larger Hexagon Hexagon in First Tier By joining the centers of the six nearest cochannel cells, a large hexagon is formed with radius equal to D, which is also the cochannel cell separation.
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Cluster Size, K =
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2 i
+
2 j
+ixj
2 Asmal smalll hex hex = (3√3/2)xR 2 Ala larg rge e he hex x = (3√3/2)xD
Number of cells in large hexagon, L = D2 ⁄ R2 Number of cells enclosed by large hexagon, L = K+6x[(1 ⁄ 3)xK] = 3 x K
K = D2 ⁄ (3 x R2)
K = i2 + j2 + i x j
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Reuse Pattern and Cluster Size
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Cell Structure for K = 4 and K = 7
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The Cellular Structure for K = 12 9 8
9 10
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11 3
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Frequency Reuse Distance, D
Reusing an identical frequency channel in different cells is limited by cochannel interference between cells.
The cochannel interference can become a major problem in cellular communication.
It is desirable to find the minimum frequency reuse distance D in order to reduce this cochannel interference.
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Factors which Influence `D’
The number of cochannel cells in the vicinity of the center cell
The type of geographic terrain contour
The antenna height
The transmitted power at each cell site
NOTE: As long as the cell size is fixed, cochannel interference is independent of transmitter power of each cell.
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Frequency Reuse Ratio, q C1 R
C1 D
R
q = D ⁄ R The frequency reuse ratio, q is also referred as
the cochannel reuse ratio
the th e co coch chan anne nell reus reuse e fact factor or
cochannel interference reduction factor
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q = D/R Ratio D
6 6 R 5
5 1
7 4
7 4
2 3
1 2 3
The real power of the cellular concept is that Interference is not related to the absolute distance between cells but related to the ratio of the distance between cochannel (same frequency) cells to the cell radius
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Frequency Reuse Factor, q and Cluster Size, K The frequency reuse factor, q can be determined from the cluster size K as
q = D / R = (3K)
The frequency reuse factor determines the minimum distance for repeating a set of frequencies
The spectrum efficiency is most significantly influenced by the frequency reuse factor
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Frequency Reuse Ratio and Cluster Size
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What is Optimum Cluster Size?
Theoretically, a large value of D is desired to reduce co-channel interference. Large D can be obtained by choosing large cluster size K. However,
when K is too large, the number of channels assigned to each of K cells becomes small since N is fixed. As
K increases, system capacity per cell and hence spectrum efficiency reduces significantly.
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How to estimate D and K?
Now the challenge is to obtain the smallest K which can still meet the desired system performance requirements in terms of capacity per cell, spectrum utilization efficiency and received signal quality (minimum cochannel interference). This involves estimating cochannel interference and selecting the minimum frequency reuse distance D to reduce co channel interference (and hence small K).
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Freq Fr eque uenc ncy y Reu Reuse se – Pr Pros os & Con Cons s
Advantage: The frequency reuse system can drastically increase the spectrum efficiency, thereby, increasing the system capacity. Disadvantage:
If the system is not properly designed, co-channel interference may occur due to the simultaneous use of the same channel. Co-channel
Interference is the major concern in the concept of frequency reuse.
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Interference in Cellular System Interference is a major limiting factor in the performance of cellular radio system
Sources of Interference Another
mobile in the same cell
A call in progress in a neighboring cell
Other base stations operating in the same frequency band or Any
non cellular system which inadvertently inadvertently leaks energy into the cellular frequency band
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Effects of Interference On Control Channels Interference leads to missed calls and blocked calls due to error in the digital signaling. On Voice Channels
Causes cross talk where the subscriber hears interference in the background due to an undesirable transmission .
Interference is more pronounced in urban areas due to greater RF noise floor and the large number of base stations and mobiles. Capacity cannot be increased.
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Cochannel Interference and Sign Si gna al Qu Qual alit ity y (T (Tes estt 1) 1)
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Interference Measurement at Mobile A channel scanning mobile receiver records three received signals while moving in any one cochannel cell, under following conditions:
When only serving cell transmits (signal recorded is termed as C)
Cell-sites of all six cochannel cells only transmit (signal recorded is termed as I)
No transmission by any cell site (signal recorded is termed as N)
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Interference Measurement Analysis
If C/I > 18 dB in most of the area being served by a cell, the system is said to be properly designed.
If C/I < 18 dB and C/N > 18 dB in some areas, the system is said to have co-channel interference problem.
If both both C/I C/I and and C/N C/N < 18 dB and C/I is approximately same as C/N in a given area, the system is said to have radio coverage problem.
If bo both th C/I C/I and and C/N C/N ar are e < 18 dB dB and C/I < C/N in a given area, the system is said to have bo both th co co-c -cha hann nnel el interference as well as radio coverage problem.
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Cochannel Interference and Sign Si gna al Qu Qual alit ity y (T (Tes estt 2) 2)
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Interference Measurement at Cell-site
When only the mobile unit in the serving cell transmits (signal recorded is termed as C)
Up to six interference levels are obtained at the serving cell site from six mobile units transmitting in six cochannel interfering cells. (the statistical average signal recorded is termed as I)
No transmission by any mobile unit (signal recorded is termed as N)
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Cochannel Interference Reduction
Increasing the separation between two cochannel cells, D
Lowering the antenna heights at the cell site
Using directional antennas at the cell site
Use of diversity scheme at receiver
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Summary
The essential principles of cellular communication include frequency reuse, cochannel cells, and cochannel interference
Various cell parameters based on regular hexagonal cellular pattern include cluster, frequency reuse distance, reuse factor
Cochannel interference reduction methods aim at maintaining desired signal quality and cell capacity
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