The document discusses key concepts in cellular network design including: 1) The cellular concept divides a large service area into smaller cells served by low-power base stations to improve capacity compared to single transmitter systems. 2) Frequency reuse planning involves assigning different channel groups to neighboring cells to minimize interference while maximizing frequency reuse. 3) Handoff strategies are used to transfer calls between cells as users move, and guard channels and queuing can help reduce dropped calls. 4) Techniques like cell splitting, sectoring, and smaller cell zones can help improve coverage and capacity in congested areas without requiring additional spectrum.

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Submitted to:
Mr. Sumit Singh
Dhanda
CELLULAR CONCEPTS

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Topics
• System design fundamentals
• Frequency reuse channel assignment strategies
• Hand off strategies, Interference and system
capacity
• Improving coverage and capacity in cellular
system.

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Basics
 Early mobile telephony systems were not cellular. Coverage over a large
area was provided by a high powered transmitter mounted on a tall tower.
That resulted in very low capacity.
 The cellular concept arose from the need to re-structure the radio telephone
system with the increase in demand. The increase in demand could not be
satisfied just by additional spectrum allocations.
 Cell – a geographical area covered by a BS.
 Base station antennas are designed to cover specific cell areas.
 For convenience, the cells are shown with a hex pattern. A hex pattern
is the simplest pattern that can tessellate an area.
 simple model for easy analysis → circles leave gaps
• In practice, cells are not hexagonal
• Base station location
– cell center → omni-directional antenna (360° coverage)
– not necessarily in the exact center (can be up to R/4 from the ideal
location) 3

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The Cellular concept is a system level idea which calls for replacing,
a single, high power transmitter with many low power transmitters,
each providing coverage to only small portion of the service area.
Cellular Concept

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 It is a design process of selecting & allocating channel groups
of cellular base stations.
 Co-channel cells → cells that share the same set of frequencies
 VC & CC traffic in co-channel cells is an interfering source to
mobiles in several different cells
 Two competing/conflicting objectives:
—maximize frequency reuse in specified area
—minimize interference between cells
Frequency
Reuse/Planning

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Frequency reuse
pattern

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Contd...
 Cells labeled with the same letter use the same group of
channels.
 Cell Cluster: group of N cells using complete set of available
channels
 Each cell allocated a % of the total number of available
channels
 Nearby (adjacent) cells assigned different channel groups to
prevent interference between neighboring base stations and
mobile users.
 As frequency reuse↑ → # possible simultaneous users↑→ #
subscribers ↑→ but system cost ↑ (more towers)
 To increase number of users without increasing radio
frequency allocation, reduce cell sizes (more base stations)
↑→ # possible simultaneous users ↑

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 S : total # of duplex channels available for use in a
given area; determined by:
– amount of allocated spectrum
– channel BW → modulation format and/or standard
specs. (e.g. AMPS)
 k : number of channels for each cell (k < S)
 N : cluster size → # of cells forming cluster
 S = k N
System Capacity

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Contd...
 M : number of times a cluster is replicated over a
geographic coverage area
 System Capacity = Total # Duplex Channels
 C = M S = M k N
 If cluster size (N) is reduced and the geographic area for
each cell is kept constant:
– The geographic area covered by each cluster is smaller,
so M must ↑ to cover the entire coverage area (more
clusters needed).
– S remains constant.
– So C ↑
– The smallest possible value of N is desirable to
maximize system capacity.

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Contd...
 Frequency reuse factor = 1 / N
– each frequency is reused every N cells
– each cell assigned k ≒ S / N
N cells/cluster
– connect without gaps
– specific values are required for hexagonal
geometry
• N = i2 + i j + j2 where i, j ≧ 1
• Typical N values → 3, 4, 7, 12; (i, j) = (1,1),
(2,0), (2,1), (2,2)

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Contd...
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To find the nearest co-
channel neighbors of a
particular cell
−Move i cells along any
chain of hexagons, then
−turn 60 degrees and move
j cells.

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 Goal is to minimize interference & maximize use of capacity
– lower interference allows smaller N to be used → greater frequency
reuse → larger C
 Two main strategies: Fixed or Dynamic
 Fixed
– each cell allocated a pre-determined set of voice channels
• calls within cell only served by unused cell channels
• all channels used → blocked call → no service
– several variations
• MSC allows cell to borrow a VC from an adjacent cell
• donor cell must have an available VC to give
Channel Assignment
Strategies

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Contd...
Dynamic
− channels NOT allocated permanently
− call request → goes to serving base station → goes to MSC
− MSC allocates channel “on the fly”.
allocation strategy considers:
– likelihood of future call blocking in the cell
– reuse distance (interference potential with other cells that are
using the same frequency)
– channel frequency

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Contd...
 Advantage: reduces call blocking (that is to say, it
increases the trunking capacity), and increases voice
quality
 Disadvantage: increases storage & computational
load at MSC
– requires real-time data from entire network related
to:
• channel occupancy
• traffic distribution

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 Handoff: when a mobile unit moves from one cell to another
while a call is in progress, the MSC must transfer (handoff) the
call to a new channel belonging to a new base station
– new voice and control channel frequencies
– very important task → often given higher priority than new
call
 choose a (handoff threshold) > (minimum useable signal level)
– so there is time to switch channels before level becomes
too low
– as mobile moves away from base station and toward
another base station
Handoff Strategies

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Contd...
 A dropped handoff can be caused by two factors
– not enough time to perform handoff
a. delay by MSC in assigning handoff
b. high traffic conditions and high
computational load on MSC can cause
excessive delay by the MSC
– no channels available in new cell

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Contd...
 2nd Generation Cellular digital TDMA (GSM, IS-136)
a. Mobile Assisted Hand Offs (MAHO)
− important advancement
− The mobile measures the RSS of the FCC’s from
adjacent base stations & reports back to serving base
station
− if Receiver power from new base station > Rx power
from serving (current) base station by pre-determined
margin for a long enough time period → handoff
initiated by MSC
 A mobile may move into a different system controlled by a
different MSC
– Called an intersystem handoff

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Guard channel
% of total available cell channels exclusively set
aside for handoff requests
makes fewer channels available for new call
requests
a good strategy is dynamic channel allocation
(not fixed)
− adjust number of guard channels as needed by demand
− so channels are not wasted in cells with low traffic

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Queuing Handoff
Requests
use time delay between handoff threshold and
minimum useable signal level to place a blocked
handoff request in queue
a handoff request can "keep trying" during that
time period, instead of having a single block/no
block decision
prioritize requests (based on mobile speed) and
handoff as needed
calls will still be dropped if time period expires

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 Problems occur because of a large range of mobile
velocities
– pedestrian vs. vehicle user
 Small cell sizes and/or micro-cells → larger number
of handoffs
 MSC load is heavy when high speed users are passed
between very small cells
Practical Handoff
Considerations

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use different antenna heights and Tx power levels
to provide large and small cell coverage
large cell → high speed traffic → fewer handoffs
small cell → low speed traffic
example areas: interstate highway passing through
urban center, office park, or nearby shopping mall
Umbrella Cells

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Cell Dragging
low speed user with line of sight to base station
(very strong signal)
strong signal changing slowly
user moves into the area of an adjacent cell
without handoff
causes interference with adjacent cells and other
cells

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Co-Channel
Interference
 Interference is the limiting factor in performance of all
cellular radio systems
 What are the sources of interference for a mobile
receiver?
 Interference is in both
– voice channels
– control channels
 Two major types of system-generated interference:
1) Co-Channel Interference (CCI)
2) Adjacent Channel Interference (ACI)

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Contd...
 Possible Solutions:
1) Increase base station Tx power to improve radio
signal reception
− this will also increase interference from co-channel
cells by the same amount
2) Separate co-channel cells by some minimum
distance to provide sufficient isolation from
propagation of radio signals
− if all cell sizes, transmit powers, and coverage patterns
≈ same → co-channel interference is independent of Tx
power

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Contd...
 co-channel interference depends on:
– R : cell radius
– D : distance to base station of nearest co-channel
cell
 if D / R ↑ then spatial separation relative to cell
coverage area ↑
– improved isolation from co-channel RF energy
 Q = D / R : co-channel reuse ratio
– hexagonal cells → Q = D/R =√3N

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 When the demand for wireless service increases, the number
of channels allotted to a cell becomes insufficient to support
the required number of users.
 Certain design techniques are used to provide more number of
channels per unit coverage area, thus increasing the capacity:
– Cell Splitting : It allows an orderly growth of the cellular
system
– Sectoring : It uses Directional antennas to control the
interference and frequency reuse of channels.
– Zone Microcell: It distributes the coverage of the Cell.
– More bandwidth
– Borrow channel from nearby cells
Improve Capacity

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 It is the process of subdividing the congested cell into smaller
cells.
 Each of the smaller cells will have their own base station with a
reduction in antenna height and transmitted power.
 The smaller cells are known as Microcells.
 Cell Splitting increases the capacity of the cellular system as it
increases the number of times the channels are reused
 The increased number of cells would increase the number of
clusters over the coverage region which in turn increase the
number of channels and thus capacity in the coverage area
 Cell Splitting allows the system to grow by replacing large cells
with smaller cells without changing the co-channel re-use ratio
(Q).
Cell Splitting

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Cells are split to add channels with no new spectrum
usage

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Sectoring
 The co-channel interference in a cellular system can be
decreased by replacing the Omni directional antenna at
the base station by several directional antennas, each
radiating within a specified sector.
 The process of reducing the co-channel interference
and thus increasing the capacity of the system by using
directional antennas is known as Sectoring.
 In general a cell is partitioned into three 120 degree
sectors or six 60 degree sectors.
 When sectoring is employed, the channels used in a
particular cell are broken down into sectored groups
and are used only in a particular sector.

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THANK YOU
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