Cellular networks address the problems of spectral congestion and limited user capacity by replacing single high-power transmitters with many low-power transmitters. This allows for frequency reuse, where the same frequencies can be used in cells farther apart due to lower transmission powers. Key aspects of cellular networks include frequency reuse patterns, cell types and sizes, co-channel interference management through techniques like sectorization and microcell deployment, and balancing capacity gains from smaller cells and frequency reuse against infrastructure costs. Cellular networks provide major improvements in spectral efficiency and user capacity over traditional wireless networks.

1. Cellular Concept - 1
By
Dr. Muhammad Moinuddin

2. Outline
 Frequency Reuse and the Cellular Concept
 Cellular Architecture
 Co-Channel Reuse Factor
 Cell Types
 Advantages and Disadvantages of Cellular System
 Spectral Efficiency of a Cellular System
 Interference
 System Capacity
 Capacity enhancement
 Cell Sectoring
 Micro cell deployment
 Examples related to system capacity evaluation
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3. Cellular Concept
 Cellular concept is a major breakthrough
in solving problems of
 Spectralcongestion
 User Capacity
 Cellular concept is a system level idea to
replace a single high power transmitter
with many low power transmitters.
 Concept of “Frequency reuse”
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4. Definitions / Terminology
 Frequency reuse
 Frequency reuse factor=1/N, N=cluster size
 A cell / sector is a logical network element that uses
given resources.
 A cell site (or site or base station) can include multiple
of cells/sectors at the same physical location.
 Cell coverage is defined as geographical area where
the cell is likely to serve mobiles.
 Network coverage refers to sum of individual cell
coverage.
 Actual radio coverage of a cell is called footprint and is
determined from the field measurement or propagation
prediction models.

5. Examples of frequency reuse

6. Cellular Architecture
 In practice the cells are not regular hexagons, but
instead are distorted and overlapping areas.
 The hexagon is an ideal choice for representing
macrocellular coverage areas, because it closely
approximates a circle and offers a wide range of
tesellating (a regular tiling of polygons) reuse cluster
sizes.
 A tesellating reuse cluster of size N can be constructed
if,
 where i and j are non-negative integers, and i ≥ j. It follows that
the allowable cluster sizes are N=1,3,4,7,9,12,…..
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7. Commonly used cellular reuse
structures
 Examples of 3-, 4-, and 7-cell reuse
clusters are shown
 The reuse clusters are tesellated to form a
frequency plan.
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8. Macrocellular deployment using
7-cell reuse pattern
 A simplified 7-cell frequency reuse plan is
shown where similarly marked cells use
identical sets of carrier frequencies.
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9. 19-cell reuse example (N=19)
 Figure shows method of locating co-channel cells in a cellular
system. In this example, N = 19 (i.e., i = 3, j = 2).
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10. Co-channel Reuse Factor
 The co-channel reuse factor D / R (sometimes
also referred as Q), is defined as the ratio of the
co-channel reuse distance D between cells
using the same set of carrier frequencies and
the radius of the cells R.
 For hexagonal cells, the reuse cluster size N and
the co-channel reuse factor D / R are related by,
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11. Smaller N is greater capacity
 Co-channel Reuse Ratio:
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12. Microcellular Deployment
 For microcellular systems with lower BS antenna
heights, regular hexagons are no longer
appropriate for approximating the radio
coverage zones.
 Typical microcell BSs use an antenna height of
about 15 m, well below the skyline of any
buildings that might be present.
 For microcells, the choice of cell shape depends
greatly upon the particular deployment.
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13. Microcellular deployment along a
highway with a 3-cell reuse pattern
 The linear cells may provide a more
accurate model of highway microcells
that are deployed along a highway with
directional antennas.
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14. Microcellular deployment in an
urban canyon
 In an area with urban canyons, the buildings act as wave guides to
channel the signal energy along the street corridors.
 Figure shows a typical Manhattan microcell deployment that is
often used to model microcells that are deployed in city centers.
Base stations are deployed
at every intersection in a dense
urban area with a 2-cell
reuse pattern
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15. Summary of cellular concept

16. Advantages
 Higher capacity
 More frequent resource utilization increases the capacity
 Less transmission power
 Reduced cell sizes, less power needed to cover the cell area
 Relaxed power amplifier specs at base stations
 Longer life-time for mobile station batteries
 Localized interference
 Due to smaller service areas of cells, interference is as well localized to
a smaller area
 Robustness
 In case that one cell is down, overlapping of cells guarantees that a
mobile is able to get connected through other base stations
 No technological challenges in deployment
 Major problems related to minimizing the implementation and
operational expenses of the system
 Technological challenges related to capacity improvement methods

17. Disadvantages
 Massive infrastructure
 Number of base stations increases, if more capacity is required
 Lots of infrastructure needed also after radio air interface
(location registers, switches, management servers etc.)
 More complex mobility management
 Seamless connections throughout the network has to be
provided Handovers needed
 Management of handover procedures might get complicated
 Resource planning and management
 Tight resource planning strategy needed (time slots,
frequencies)
 Resource management needed to support resource planning

18. Spectral Efficiency of a Cellular
System
 Main factors that determine the spectral efficiency of a cellular
system.
 The cell sizes,
 The ability of radio links to withstand interference, and
 The ability of the cellular system to react to variations in traffic.
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19. Requirement for High Spectral
Efficiency
 High spectral efficiency require:
 effective cellular architecture,
 fast and accurate link quality measurements,
 rapid control in all types of environments,
 installation of BSs to provide radio coverage virtually
everywhere, and
 power and bandwidth efficient air interface schemes that can
mitigate the harsh effects of the propagation environment and
tolerate high levels of noise and interference.
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20. Spectrum Allocation
 In cellular systems, the available spectrum is partitioned
among the BSs with two types of channels
 Voice Channels
 Control Channels
 A given frequency is reused at the closest possible
distance that the radio link will allow.
 Smaller cells have a shorter distance between reused
frequencies, and this results in an increased spectral
efficiency and traffic carrying capacity.
 Example 3.1
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21. Interference and system capacity
 Interference limits the system capacity and the performance of a
single radio link.
 The higher is the interference level, the lower is the system capacity
and poorer is the quality of communication links.
 Interference can origin from
 Mobiles under the same cell
 Mobile under different cell
 Base stations operating at the same frequency (or in general
using simultaneously the same resources)
 From non-cellular systems
 Co-channel interference is the most problematic.
 Also adjacent channel interference affect the system capacity.
 However, adjacent channel interference is also filtering problem,
which can be decreased by assigning channels intelligently.

22. Interference and system capacity
 Hence, the target of frequency (or resource)
allocation is to plan frequency usage in such a
manner that system capacity is maximized (i.e.,
frequencies should be utilized as frequently as
possible).
 However, simultaneously the interference should
be kept at adequate level.
 For cellular planning point of view, this can be
achieved by providing as much isolation as
possible between co-channel cells.

23. SIR
 Signal strength from the serving cell is denoted as S and
co-channel interference as I. The resulting relation is
signal-to-interference ratio (SIR) that defines the quality
of the signal.
 where Ij is the co-channel interference received from jth
co-channel cell. For simplicity, if transmission powers (P)
of all cells is assumed to be the same as well as radio
signal attenuation equal from all cells, SIR can be written
as

24. SIR
 where d0 is the distance from the serving cell and dj from
the jth cochannel cell.
 Propagation exponent (n) varies according to
environment (in free space n=2, in dense urban
environment n=4)
 In simple scenario, where R is the distance from the
serving cell, and D is the distance from the first tier of
interfering co-channel cell, SIR can be expressed

25. Capacity and quality
enhancement methods
 System capacity can be improved by decreasing
the frequency reuse factor and quality by
improving SIR.
 System capacity or SIR can be improved by
 Introducing multiple antennas at the base station site
sectoring
 Minimization of out-of-cell interference (base station
antenna characteristics)
 Decreasing antenna height
 Microcell deployment

26. Sectoring
 Sectoring implies to increasing the number of logical
cells belonging to a single base station.
 Hence, one physical location (base station) includes
multiple of antennas of the cells.
 Co-channel interference can be thus reduced.
 However, omni-directional antennas (1-sectored base
station) has to be changed into directional ones.
 More intra-cell handovers

27. Sectoring
 Illustration of reduction of co-channel interference in N=4 using 3-
sectored base stations.
 Example 3.2

28. Microcell deployment
 A network having antennas above
roof top level is referred to as
macrocellular network.
 Deploying antennas below roof top
level reduces the frequency reuse
factor microcellular network.
 Building prevent the signal
propagation co-channel
interference reduced significantly.
 Simultaneously, cell coverage is
reduced and more cells are needed
to cover certain area.
 Frequency reuse factor can be 5-7.
 Microcell deployment is expensive
solution.