Chapter 2 Cellular Systems--Cellular Concepts.ppt
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Transcript of Chapter 2 Cellular Systems--Cellular Concepts.ppt
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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.
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Cellular Concepts
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 fromall 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
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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 systemdesign. However as traffic grows, where new cells
and channels need to be added, it may lead to
inability to reuse frequencies because of co-
channel interference. For systematic cell planning, a regular shape is
assumed for the footprint.
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Cell Footprint
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Frequency reuse
A cellular system which has a total ofS
duplex channels.
S channels are divided among Ncells, with
each cell uses unique and disjoint channels.
If each cell is allocated a group ofkchannels,
then
S = k N.
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Terminology
Cluster size : The Ncells which collectivelyuse the complete set of available frequency iscalled the cluster size.
Co-channel cell : The set of cells using thesame set of frequencies as the target cell.
Interference tier : A set of co-channel cells atthe same distance from the reference cell is
called an interference tier. The set of closestco-channel cells is call the first tier. There isalways 6 co-channel cells in the first tier.
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Co-ordinates for hexagonal cellular
geometry
With these co-
ordinates, an
array of cells can
be laid out so thatthe center of every
cell falls on a point
specified by a pairof integer co-
ordinates.
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Co-ordinates for hexagonal cellular
geometry
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Designing a cellular system
N=19
(i=3, j=2)
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Designing a cellular system
The cluster size must satisfy: N= i2 + ij+j2
where i,jare non-negative integers.
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Designing a cellular system
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Designing a cellular system
Can also verify that
where Q is the co-channel reuse ratio
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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 nottoo 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.
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Handover / Handoff
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.
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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 handoveralgorithms.
The statistics of the dwell time vary greatly,
depending on the speed of the user and thetype of radio coverage.
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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 resultsof these measurements to the serving BS.
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Prioritizing Handover
Dropped call is considered a more serious eventthan call blocking. Channel assignment schemestherefore must give priority to handover requests.
A fraction of the total available channels in a cell isreserved only for handover requests. However, this
reduces the total carried traffic. Dynamic allocationcan improve this.
Queuing of handover requests is another method todecrease the probability of forced termination of acall due to a lack of available channel. The time
span over which a handover is usually requiredleaves room for queuing handover request.
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Practical handover
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.
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Practical handover
The Umbrella Cell
approach will help to
solve this problems.
High speed users areserviced by large
(macro) cells, while low
speed users are
handled by small (micro)
cells.
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Practical handover
A hard handover does break beforemake, ie. The old channel connection isbroken before the new allocated channelconnection is setup. This obviously cancause call dropping.
In soft handover, we do make beforebreak, ie. The new channel connectionis established before the old channelconnection is released. This is realized inCDMA where also BS diversity is used to
improve boundary condition.
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Interference and System Capacity
In a given coverage area, there are several cellsthat use the same set of frequencies. These cellsare called co-channel cells. The interferencebetween signals from these cells is called co-channel interference.
If all cells are approximately of the same size andthe path loss exponent is the same throughout thecoverage area, the transmit power of each BS isalmost equal. We can show that worse case signal
to co-channel interference is independent of thetransmitted power. It becomes a function of the cellradius R, and the distance to the nearest co-channelcell D.
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Interference and System Capacity
Received power at a distance dfrom the
transmitting antenna is approximated by
Useful signal at the cell boundary is the weakest,given by Pr(R). Interference signal from the co-
channel cell is given to be Pr(D) .
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Interference and System Capacity
Dis normallyapproximated by
the base station
separation
between the twocells D, unless
when accuracy is
needed. Hence
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Interference and System Capacity
For the forward link, a very general case,
where Diis the distance of the ith interfering
cell from the mobile, i0 is the total number of
co-channel cells exist.
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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.
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Outage probability
The probability that a mobile station does notreceive 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 meetthe SIR requirements for GSM
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Outage probability
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Outage probability
Approximation
in distance has
been made on
the 2nd tieronwards.
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Outage probability
More accurate SIR can be
obtained by computing the
actual distance.
Our computation of outageonly based on path loss. For
more accurate modeling,
shadowing and fast fading
need to be taken intoconsideration. This will not
be covered in this course.
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Coverage Problems Revision:
Recall that the mean measured value,
Measurement shows that at any value ofd, the path lossPL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value.
Pr(d)dB = Pr(d)dB +X
whereX is a zero-mean Gaussian distributed randomvariable (in dB) with standard deviation (in dB).
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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 . ( is usually the receiver sensitivity)
where Q(x) is the standard normal distribution.
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Cell coverage
Proportion of locations within the area defined by the cellradius R, receiving a signal above the threshold .
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Cell coverage
Solution can be found using the graph provided. (n :path loss exponent)
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Cell coverage
Example: if n=4, =8 dB, and if the boundary is tohave 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, =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 coveragecriteria. This is typically the 90% rule 90% of a
given geographical area must be covered for 90% ofthe time.
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Cell coverage
The mean signal level at any distance is determined bypath loss and the variance is determined by the resultingfading distribution (log-normal shadowing, Rayleighfading, Nakagami-m, etc). In this course, we will dealwith log-normal shadowing only.
The proportion of locations covered at a given distance(cell boundary, for example) from BS can be founddirectly from the resultant signal pdf/cdf.
The proportion of locations covered within a circularregion defined by a radius R(the cell area, for example)can be found by integrating the resultant cdf over the cellarea.
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Cell coverage --Cellular Traffic
The basic consideration in the design of a
cellular system is the sizing of the system.
Sizing has two components to be considered. Coverage area
Traffic handling capability
After the system is sized, channels areassigned to cells using the assignment
schemes mentioned before.
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Cell coverage --Terminology in traffic
theory Trunking : exploits the statistical characteristics of theusers calling behaviour. Any efficient communicationsystem relies on trunking to accommodate a largenumber of users with a limited number of channels.
Grade of service (GoS) : A user is allocated a channelon a per call basis. GoS is a measure of the ability of a
user to access a trunked system during the busiesthour. It is typically given as the likelihood that a call isblocked (also known as blocking probability mentionedbefore).
Trunking theory : is used to determine the number ofchannels required to service a certain offered traffic ata specific GoS.
Call holding time (H) : the average duration of a call.
Request rate () : average number of call requestsperunit time.
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Cell coverage --Traffic flow or intensityA
Measured in Erlang, which is defined as the call
minute per minute.
Total offered traffic for such a system is given as
A = H
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
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Cell coverage
If the offered traffic exceeds the maximum possiblecarried traffic, blockingoccurs. There are twodifferent strategies to be used. Blocked calls cleared
Blocked calls delayed
Trunking efficiency: is defined as the carried trafficintensity in Erlangs per channel, which is a valuebetween zero and one. It is a function of the numberof channels per cell and the specific GoS
parameters. Call arrival process: it is widely accepted that calls
have a Poisson arrival.
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Channel Assignment Strategies Channel allocation schemes can affect the
performance of the system. Fixed Channel Allocation (FCA) :
Channels are divided in sets.
A set of channels is permanently allocated to each cell
in the network. Same set of channels must beassigned to cells separated by a certain distance toreduce co-channel interference.
Any call attempt within the cell can only be served bythe unused channels in that particular cell. The serviceis blocked if all channels have used up.
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Channel Assignment Strategies (FCA)
Most easiest to implement but least flexibility. An modification to this is borrowing scheme. Cell
(acceptor cell) that has used all its nominal channelscan borrow free channels from its neighboring cell(donor cell) to accommodate new calls.
Borrowing can be done in a few ways: borrowing fromthe adjacent cell which has largest number of freechannels, select the first free channel found, etc.
To be available for borrowing, the channel must notinterfere with existing calls. The borrowed channel
should be returned once the channel becomes free.
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Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA) :
Voice channels are not allocated to any cell permanently. Allchannels 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 thelikelihood 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.
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Channel Assignment Strategies
Flexible channel assignment
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 schemesare scheduled and predictive.
Channels need to be assigned to users to accommodate
new calls
handovers
with the objective of increasing capacity and minimizing
probability of a blocked call.
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System Expansion Techniques
As demand for wireless services increases, thenumber of channels assigned to a cell eventuallybecomes insufficient to support the required numberof users. More channels must therefore be made
available per unit area. This can be accomplished by dividing each initial cell area
into a number of smaller cells, a technique known as cell-splitting.
It can also be accomplished by having more channels per
cell, i.e. by having a smaller reuse factor. However, to havea smaller reuse factor, the co-channel interference must bereduced. This can be done by using antenna sectorization.
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System Expansion Techniques--Cell
splitting 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 growthcapability. 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.
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System Expansion Techniques--Cell
splitting
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System Expansion Techniques--Cell
splitting For new cells to be smaller in size, the transmit
power must be reduced. Ifn=4, then with areduction of cell radius by a factor of 2, the transmitpower should be reduced by a factor of 24 (why?)
In theory, cell splitting could be repeated indefinitely. In practice it is limited
By the cost of base stations
Handover (fast and low speed traffic)
Not all cells are split at the same time : practical problemsof BS sites, such as co-channel interference exist
Innovative channel assignment schemes must bedeveloped to address this problem for practical systems.
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System Expansion Techniques--Cell
splitting
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System Expansion Techniques --
Sectorization
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 a60 or a 120 sector.
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System Expansion Techniques --
Sectorization
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System Expansion Techniques --
Sectorization
If we partition a cell into three 120 sectors, thenumber of co-channel cells are reduced from 6 to 2in the first tier.
Using six sectors of 60, we have only one co-
channel 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 decreasein trunking efficiency and an increase in the number
of required antennas. But how can the increase in system capacity be
achieved?
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System Expansion Techniques --
Sectorization
T h
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System Expansion Techniques --
Sectorization
S i T h i
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System Expansion Techniques --
Sectorization
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S E i T h i Mi
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System Expansion Techniques --Micro
cells