Fundamentals of Cellular Concept

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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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