chapter 2 cellular systems--cellular concepts.ppt

7/28/2019 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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The cluster size must satisfy: N= i2 + ij+j2

where i,jare non-negative integers.


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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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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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Dis normallyapproximated by

the base station

separation

between the twocells D, unless

when accuracy is

needed. Hence


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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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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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splitting


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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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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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Sectorization


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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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Sectorization

T h


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Sectorization

S i T h i


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Sectorization


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S E i T h i Mi


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System Expansion Techniques --Micro

cells

chapter 2 cellular systems--cellular concepts.ppt

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