Transcript Cellular Systems--Cellular Concepts

Cellular Systems--Cellular Concepts
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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
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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.
Cellular Concepts
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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 (cochannel cell) for transmission suffers from sufficient
attenuation. Factors need to be considered include:
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Geographical separation (path loss)
Shadowing effect
Cell Footprint
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The actual radio coverage of a cell is known
as the cell footprint.
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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.
Cell Footprint
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Coverage contour should be circular. However it is
impractical because it provides ambiguous areas
with either multiple or no coverage.
Due to economic reasons, the hexagon has been
chosen due to its maximum area coverage.
Hence, a conventional cellular layout is often
defined by a uniform grid of regular hexagons.
Cell Footprint
Frequency reuse
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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.
Terminology
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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.
Co-ordinates for hexagonal cellular
geometry
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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.
Co-ordinates for hexagonal cellular
geometry
Designing a cellular system
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N=19
(i=3, j=2)
Designing a cellular system
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The cluster size must satisfy: N = i2 + ij + j2
where i, j are non-negative integers.
Designing a cellular system
Designing a cellular system
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Can also verify that
where Q is the co-channel reuse ratio
Handover / Handoff
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Occurs as a mobile moves into a different cell
during an existing call, or when going from
one cellular system into another.
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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.
Handover / Handoff
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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.
Handover / Handoff
=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
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Dwell Time
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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.
Handover indicator
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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.
Prioritizing Handover
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Dropped call is considered a more serious event
than call blocking. Channel assignment schemes
therefore must give priority to handover requests.
A fraction of the total available channels in a cell is
reserved only for handover requests. However, this
reduces the total carried traffic. Dynamic allocation
can improve this.
Queuing of handover requests is another method to
decrease the probability of forced termination of a
call due to a lack of available channel. The time
span over which a handover is usually required
leaves room for queuing handover request.
Practical handover
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High speed users and low speed users have
vastly different dwell times which might cause
a high number of handover requests for high
speed users. This will result in interference
and traffic management problem.
Practical handover
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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.
Practical handover
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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.
Interference and System Capacity
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In a given coverage area, there are several cells
that use the same set of frequencies. These cells
are called co-channel cells. The interference
between signals from these cells is called cochannel 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’.
Interference and System Capacity
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Received power at a distance d from the
transmitting antenna is approximated by
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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′) .
Interference and System Capacity
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D’ is normally
approximated by
the base station
separation
between the two
cells D, unless
when accuracy is
needed. Hence
Interference and System Capacity
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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.
Interference and System Capacity
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If only first tier co-channel cells are
considered, then i0 = 6.
Unless otherwise stated, normally assuming
Di ≈ D for all i.
Outage probability
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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
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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
Outage probability
Outage probability
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Approximation
in distance has
been made on
the 2nd tier
onwards.
Outage probability
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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.
Coverage Problems
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Revision:
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Recall that the mean measured value,
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Measurement shows that at any value of d, the path loss
PL(d) at a particular location is random and distributed lognormally (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).
Boundary coverage
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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)
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where Q(x) is the standard normal distribution.
Cell coverage
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Proportion of locations within the area defined by the cell
radius R, receiving a signal above the threshold γ.
Cell coverage
Solution can be found using the graph provided. (n :
path loss exponent)
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Cell coverage
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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.
Cell coverage
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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.
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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.
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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.
Cell coverage --Cellular Traffic
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The basic consideration in the design of a
cellular system is the sizing of the system.
Sizing has two components to be considered.
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Coverage area
Traffic handling capability
After the system is sized, channels are
assigned to cells using the assignment
schemes mentioned before.
Cell coverage --Terminology in traffic
theory
Trunking : exploits the statistical characteristics of the
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users calling behaviour. Any efficient communication
system relies on trunking to accommodate a large
number of users with a limited number of channels.
Grade of service (GoS) : A user is allocated a channel
on a per call basis. GoS is a measure of the ability of a
user to access a trunked system during the busiest
hour. It is typically given as the likelihood that a call is
blocked (also known as blocking probability mentioned
before).
Trunking theory : is used to determine the number of
channels required to service a certain offered traffic at
a specific GoS.
Call holding time (H) : the average duration of a call.
Request rate (λ) : average number of call requests
perunit time.
Cell coverage --Traffic flow or intensity A
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Measured in Erlang, which is defined as the call
minute per minute.
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Total offered traffic for such a system is given as
A = λ ⋅H
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Exercise : There are 3000 calls per hour in a cell, each
lasting an average of 1.76 min. Offered traffic A =
(3000/60)(1.76) = 88 Erlangs
Cell coverage
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If the offered traffic exceeds the maximum possible
carried traffic, blocking occurs. There are two
different strategies to be used.
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Blocked calls cleared
Blocked calls delayed
Trunking efficiency : is defined as the carried traffic
intensity in Erlangs per channel, which is a value
between zero and one. It is a function of the number
of channels per cell and the specific GoS
parameters.
Call arrival process: it is widely accepted that calls
have a Poisson arrival.
Channel Assignment Strategies
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Channel allocation schemes can affect the
performance of the system.
Fixed Channel Allocation (FCA) :
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Channels are divided in sets.
A set of channels is permanently allocated to each cell
in the network. Same set of channels must be
assigned to cells separated by a certain distance to
reduce co-channel interference.
Any call attempt within the cell can only be served by
the unused channels in that particular cell. The service
is blocked if all channels have used up.
Channel Assignment Strategies (FCA)
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Most easiest to implement but least flexibility.
An modification to this is ‘borrowing scheme’. Cell
(acceptor cell) that has used all its nominal channels
can borrow free channels from its neighboring cell
(donor cell) to accommodate new calls.
Borrowing can be done in a few ways: borrowing from
the adjacent cell which has largest number of free
channels, select the first free channel found, etc.
To be available for borrowing, the channel must not
interfere with existing calls. The borrowed channel
should be returned once the channel becomes free.
Channel Assignment Strategies (DCA)
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Dynamic Channel Allocation (DCA) :
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Voice channels are not allocated to any cell permanently. All
channels are kept in a central pool and are assigned dynamically
to new calls as they arrive in the system.
Each time a call request is made, the serving BS requests a
channel from the MSC. It then allocates a channel to the
requested cell following an algorithm that takes into acount the
likelihood of future blocking within the cell, the reuse distance of
the channel and other cost functions ⇒ increase in complexity
Centralized DCA scheme involves a single controller selecting a
channel for each cell. Distributed DCA scheme involves a number
of controllers scattered across the network.
For a new call, a free channel from central pool is selected based
on either the co-channel distance, signal strength or signal to
noise interference ratio.
Channel Assignment Strategies
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Flexible channel assignment
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Divide the total number of channels into two groups, one of
which is used for fixed allocation to the cells, while the
other is kept as a central poor to be shared by all users.
Mix the advantages the FCA and DCA, available schemes
are scheduled and predictive.
Channels need to be assigned to users to accommodate
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new calls
handovers
with the objective of increasing capacity and minimizing
probability of a blocked call.
System Expansion Techniques
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As demand for wireless services increases, the
number of channels assigned to a cell eventually
becomes insufficient to support the required number
of users. More channels must therefore be made
available per unit area.
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This can be accomplished by dividing each initial cell area
into a number of smaller cells, a technique known as cellsplitting.
It can also be accomplished by having more channels per
cell, i.e. by having a smaller reuse factor. However, to have
a smaller reuse factor, the co-channel interference must be
reduced. This can be done by using antenna sectorization.
System Expansion Techniques--Cell
splitting
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Cell splitting increases the number of BSs in order to
increase capacity. There will be a corresponding
reduction in antenna height and transmitter power.
Cell splitting accommodates a modular growth
capability. This in turn leads to capacity increase
essentially via a system re-scaling of the cellular
geometry without any changes in frequency
planning.
Small cells lead to more cells/area which in turn
leads to increased traffic capacity.
System Expansion Techniques--Cell
splitting
System Expansion Techniques--Cell
splitting
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For new cells to be smaller in size, the transmit
power must be reduced. If n=4, then with a
reduction of cell radius by a factor of 2, the transmit
power should be reduced by a factor of 24 (why?)
In theory, cell splitting could be repeated indefinitely.
In practice it is limited
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By the cost of base stations
Handover (fast and low speed traffic)
Not all cells are split at the same time : practical problems
of BS sites, such as co-channel interference exist
Innovative channel assignment schemes must be
developed to address this problem for practical systems.
System Expansion Techniques--Cell
splitting
System Expansion Techniques -Sectorization
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Keep the cell radius but decrease the D/R
ratio. In order to do this, we must reduce the
relative interference without increasing the
transmit power.
Sectorization relies on antenna placement
and directivity to reduce co-channel
interference. Beams are kept within either a
60° or a 120° sector.
System Expansion Techniques -Sectorization
System Expansion Techniques -Sectorization
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If we partition a cell into three 120° sectors, the
number of co-channel cells are reduced from 6 to 2
in the first tier.
Using six sectors of 60°, we have only one cochannel cell in the first tier.
Each sector is limited to only using 1/3 or 1/6 of the
available channels. We therefore have a decrease in
trunking efficiency and an increase in the number of
required antennas.
But how can the increase in system capacity be
achieved?
System Expansion Techniques -Sectorization
System Expansion Techniques -Sectorization
System Expansion Techniques -Sectorization
System Expansion Techniques --Micro
cells
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Micro cells can be introduced to alleviate
capacity problems caused by “hotspots”.
By clever channel assignment, the reuse
factor is unchanged. As for cell splitting, there
will occur interference problems when macro
and micro cells must co-exist.
System Expansion Techniques --Micro
cells