This slide is covering cellular concepts like Frequency reuse, splitting handoff etc

1. UNIT-3
THE CELLUAR CONCEPT
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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2. Cellular Systems--Cellular Concepts
 The cellular concept was a major breakthrough in solving the
problem of spectral congestion and user capacity. It offered very
high capacity in a limited spectrum allocation without any major
technological changes.
 The cellular concept has the following system level ideas
 Replacing a single, high power transmitter with many low power
transmitters, each providing coverage to only a small area.
 Neighboring cells are assigned different groups of channels in order to
minimize interference.
 The same set of channels is then reused at different geographical
locations.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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3.  When designing a cellular mobile communication system, it is
important to provide good coverage and services in a high
user-density area.
 Reuse can be done once the total interference from all users in
the cells using the same frequency (co-channel cell) for
transmission suffers from sufficient attenuation. Factors need
to be considered include:
 Geographical separation (path loss)
 Shadowing effect
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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4. Cell Footprint
 The actual radio coverage of a cell is known as the
cell footprint.
Irregular cell structure and irregular placing of the
transmitter may be acceptable in the initial system
design. However as traffic grows, where new cells
and channels need to be added, it may lead to
inability to reuse frequencies because of cochannel interference.
For systematic cell planning, a regular shape is
assumed for the footprint.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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6. Frequency reuse
 Cellular system depends upon intelligent allocation and reuse
of channels.
 Each BS is allocated separate group of channels to be used in
small geographic region called as CELL.
 Adjacent cells are allocated separate group of channels.
 BS antenna are designed to provide coverage to particular
cell.
 By doing this same group of channels can be used again in
separate cells physically at large distance from cell containing
those channels by very well keeping interference within
tolerable limits.
 This design process of selecting and allocating the channels of
CBS within system is called as frequency reuse.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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7.  A cellular system which has a total of S duplex
channels.
 S channels are divided among N cells, with each cell
uses unique and disjoint channels.
 If each cell is allocated a group of k channels, then
S=kN.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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8. Terminology
• Cluster size : The N cells which collectively use the
complete set of available frequency is called the
cluster size.
• Co-channel cell : The set of cells using the same set
of frequencies as the target cell.
• Interference tier : A set of co-channel cells at the
same distance from the reference cell is called an
interference tier. The set of closest co-channel cells is
call the first tier. There is always 6 co-channel cells in
the first tier.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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9. Co-ordinates for hexagonal cellular geometry
• With these coordinates, an array of
cells can be laid out
so that the center of
every cell falls on a
point specified by a
pair of integer coordinates.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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11. Designing a cellular system
• N=19
• (i=3, j=2)
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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12. Designing a cellular system
• The cluster size must satisfy: N = i2 + ij + j2
where i, j are non-negative integers.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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13. • Can also verify that
where Q is the co-channel reuse ratio
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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14. Problem
•
•
Total 33 MHz b/w allocated to a FDD cellular system which uses 25 kHz simplex
channels to provide full duplex voice and control channels. Find the number of
channels available per cell if a system uses: a) four-cell reuse, b) 7-cell reuse. If 1
MHz of the allocated spectrum is dedicated to control channels, find an equitable
distribution of control channels & voice channels.
Total b/w = 33 MHz
–
–
•
For N=4,
–
•
•
Total no of ch per cell k = 660/7 = 95 channels
1MHz for control channels ie. 1000/50 = 20 control channels. So only 640 channels
(660-20) would be allotted for voice
For N = 4,
–
•
Total no of ch per cell k = 660/4 = 165 channels
For N = 7,
–
•
Channel b/w = 2 X 25khz = 50 khz
Total available channels S = 33, 000/50 = 660 channels
5 control ch + 160 voice ch per cell
For N =7,
–
–
4 cells with (3 control ch + 92 voice ch) & 2 cells with (3 control + 90 voice ch) & 1 cell with (2 control
ch + 92 voice channels)
Each cell with 1 control ch and 4 cells with 91 voice ch and 3 cells with 92 voice ch

15. Handover / Handoff
 Occurs as a mobile moves into a different cell
during an existing call, or when going from
one cellular system into another.
It must be user transparent, successful and not
too frequent.
Not only involves identifying a new BS, but also
requires that the voice and control signals be
allocated to channels associated with the new BS.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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16.  Once a particular signal level Pmin is specified as the
minimum usable signal for acceptable voice quality at the
BS receiver, a slightly stronger signal level PHO is used as
a threshold at which a handover is made.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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17. • =handoff threshold Minimum acceptable
signal to maintain the call
•
too small:
– Insufficient time
to complete handoff
before call is lost
– More call losses
•
too large:
– Too many handoffs
– Burden for MSC
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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18. Dwell Time
 The time over which a user remains within one cell is
called the dwell time.
 The statistics of the dwell time are important for the
practical design of handover algorithms.
 The statistics of the dwell time vary greatly,
depending on the speed of the user and the type of
radio coverage.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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19. Handover indicator
 Each BS constantly monitors the signal strengths of all of
its reverse voice channels to determine the relative
location of each mobile user with respect to the BS. This
information is forwarded to the MSC who makes
decisions regarding handover.
 Mobile assisted handover (MAHO) : The mobile station
measures the received power from surrounding BSs and
continually reports the results of these measurements to
the serving BS.
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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20. Practical handover
• The Umbrella Cell approach
will help to solve this
problems. High speed users
are serviced by large
(macro) cells, while low
speed users are handled by
small (micro) cells.

21. Practical handover
• A hard handover does “break before make”,
ie. The old channel connection is broken
before the new allocated channel
connection is setup. This obviously can cause
call dropping.
• In soft handover, we do “make before break”,
ie. The new channel connection is
established before the old channel
connection is released. This is realized in
CDMA where also BS diversity is used to
improve boundary condition.

22. Interference and System Capacity
• In a given coverage area, there are several cells that use
the same set of frequencies. These cells are called cochannel cells. The interference between signals from
these cells is called co-channel interference.
• If all cells are approximately of the same size and the
path loss exponent is the same throughout the coverage
area, the transmit power of each BS is almost equal. We
can show that worse case signal to co-channel
interference is independent of the transmitted power. It
becomes a function of the cell radius R, and the distance
to the nearest co-channel cell D’.
• On control channel I/f leads to missed or block calls.

23. In urban areas more severe due high RF noise floor
The 2 major types are

Co-Channel interference

Adjacent channel interfernce
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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24. Interference and System Capacity
Q- Co-Channel reuse ratio is given by:-
Let i0 is the no of co- channel nterfering cells than
12/24/2013
Dr.Vrince Vimal, MIT, MIET GROUP, MEERUT
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25. – Received power at a distance d from the
transmitting antenna is approximated by
– Useful signal at the cell boundary is the weakest,
given by Pr (R). Interference signal from the cochannel cell is given to be Pr (D′) .

26. Interference and System Capacity
– D’ is normally
approximated by
the base station
separation
between the two
cells D, unless
when accuracy is
needed. Hence

27. Interference and System Capacity
• For the forward link, a very general case,
where Di is the distance of the ith interfering
cell from the mobile, i0 is the total number of
co-channel cells exist.

28. Interference and System Capacity
• If only first tier co-channel cells are considered,
then i0 = 6.
Unless otherwise stated, normally assuming Di
≈ D for all i.

29. Outage probability
• The probability that a mobile station does not receive a
usable signal.
• For GSM, this is 12 dB and for AMPS, this is 18 dB. If
there is 6 co-channel cells, then
• Exercise : please verify this
– For n=4, a minimum cluster size of N=7 is needed to meet the
SIR requirements for AMPS.
– For n=4, a minimum cluster size of N=4 is required to meet the
SIR requirements for GSM

30. Outage probability

31. Outage probability
• Approximation
in distance has
been made on
the 2nd tier
onwards.

32. Outage probability
• More accurate SIR can be
obtained by computing the
actual distance.
• Our computation of outage
only based on path loss. For
more accurate
modeling, shadowing and fast
fading need to be taken into
consideration. This will not be
covered in this course.

33. •
Coverage Problems
Revision:
– Recall that the mean measured value,
– Measurement shows that at any value of d, the path loss PL(d)
at a particular location is random and distributed log-normally
(normal in dB) about this mean value.
Pr (d)dB = Pr (d)dB + Xσ
where Xσ is a zero-mean Gaussian distributed random variable
(in dB) with standard deviation σ(in dB).

34. Boundary coverage
• There will be a proportion of locations at distance R (cell radius) where a
terminal would experience a received signal above a threshold γ. (γ is
usually the receiver sensitivity)
• where Q(x) is the standard normal distribution.

35. Cell coverage
• Proportion of locations within the area defined by the cell
radius R, receiving a signal above the threshold γ.

36. Cell coverage
Solution can be found using the graph provided. (n : path
loss exponent)


37. Cell coverage
• Example: if n=4, σ=8 dB, and if the boundary is to have
75% coverage (75% of the time the signal is to exceed the
threshold at the boundary), then the area coverage is
equal to 94%.
• If n=2, σ=8 dB, and if the boundary is to have 75%
coverage, then the area coverage is equal to 91%.
• 􀂉 An operator needs to meet certain coverage criteria.
This is typically the “90% rule” – 90% of a given
geographical area must be covered for 90% of the time.

38. Cell coverage
• The mean signal level at any distance is determined by path
loss and the variance is determined by the resulting fading
distribution (log-normal shadowing, Rayleigh fading,
Nakagami-m, etc). In this course, we will deal with log-normal
shadowing only.
• The proportion of locations covered at a given distance (cell
boundary, for example) from BS can be found directly from
the resultant signal pdf/cdf.
• The proportion of locations covered within a circular region
defined by a radius R (the cell area, for example) can be found
by integrating the resultant cdf over the cell area.