C03-The Cellular Concept

7/29/2019 C03-The Cellular Concept

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The Cellular Concept


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Important Definitions

Mobile Station (MS): is the part of a mobile communication system

that changes its position as time passes. Cellular phones are a type of

mobile stations.

Base Station (BS): is the part of a mobile communication system that

is stationary (does not move). The base station communicates with all

mobile stations and takes a central position surrounded by mobile

stations. Cellular towers are a type of base stations. Full Duplex Systems: are communication systems in which

transmission between the mobile and base stations occurs in both

directions at the same time (transmit and receive at the same time) such

as cellular phone systems. The regular phone at your house is a type of

full duplex systems because you can talk and listen to other side talking

at the same time. Half Duplex Systems: are communication systems in which

transmission between the mobile and base stations occurs at different

times (transmit and receive at different times) such as pushtotalk

systems.


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Simplex Systems: are communication system in which transmission of

information occurs in one direction only such as a garage door opening

system. Forward Channel: is the communication channel used to transmit

information from the base station to the mobile station.

o Forward Control Channel (FCC): is the channel used by the base station

to inform mobile stations of a call directed to them, and to instruct mobile

stations of the voice channels they should use to send and receive

information.o Forward Voice Channel (FVC): is the channel used by the base station to

transmit the voice signal to the mobile station.


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Reverse Channel: is the communication channel used to transmit

information from the mobile station to the base station.Reverse Control Channel (RCC): is the channel used by the mobile

station to request from a cellular tower to initiate a phone call.

Reverse Voice Channel (RVC): is the channel used by the mobile

station to transmit the voice signal to the base station.

Multiple Access Techniques: are methods by which multiple mobile

stations in a communication system request that part of the limitedspectrum of the system be reserved for its communication and then

release the reserved spectrum once the communication is

completed.


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Cellular Telephone Systems

Achieve a large coverage area by using a simple, high powered transmitter.

Put BS on top of mountains or tall towers, so that it could provide

coverage for a large area.

So good coverage, but it was impossible to reuse those same frequencythroughout the system


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The Bell mobile system in New York City in the1970s could only support a maximum of

twelve simultaneous calls over a thousand

square miles.


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EXAMPLE 1: Using a typical analog system, each channel needs to have a

bandwidth of around 25 kHz to enable sufficient audio quality to be

carried, as well as allowing for a guard band between adjacent signals toensure there are no undue levels of interference. Using this concept, it is

possible to accommodate only forty users in a frequency band 1-MHz

wide. Even if 100 MHz were allocated to the system, this would enable

only 4000 users to have access to the system. Today cellular systems have

millions of subscribers, and therefore a far more efficient method of using

the available spectrum is needed.


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Introduction to CellularConcept

Solves the problem of spectral congestion and user capacity

Cellular Concept replacing a single, high power transmitter (large cell)

with many low power transmitters (small cells) and each providing

coverage to only small portion of the service area

Each base station is allocated a portion of the total number of channels

available channels to the entire system

Neighboring base stations are assigned different group of channels, so that

interference b/w base stations is minimized


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Frequency Reuse

Cellular radio systems rely on an intelligent allocation and reuse of channelsthroughout a coverage region

Each cellular base station is allocated a group of radio channels within asmall geographic area called a cell

Neighboring cells are assigned different channel groups

By limiting the coverage area to within the boundary of the cell, thechannel groups may be reused to cover different cells

Keep interference levels within tolerable limits

Frequency reuse or frequency planning


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Frequency Reuse


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Base Station Location

Base station location:

At the center of the cell (Omni-directional antenna)

At the vertices of three cells (directional antennas)

Practical considerations usually do not allow base stations to be placed

exactly as they appear in the hexagonal layout (~1/4 cell radius away from

the ideal location)


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How many calls does a cellular tower

typically carry (1s, 10s, 100s, 1000s,

10000s)?

The number of calls a cellular tower can serve

at any time is called the tower capacity. Acellular tower typically can serve around 100

to 200 customers at any time. Different

configurations can increase or decrease thetower capacity.


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Most Efficient Cell Shapes to Cover

Large Regions For proper cell shapes, let us observe the following points:

Boarders of cells are straight lines and cell shapes are

polygons (Polygons are geometric shapes with all edges being

straight lines like triangles, rectangles, pentagons, ).

Full coverage of the whole region is necessary without leaving

any uncovered spots.

We will assume that all cells have the same shape.

Cells should have some symmetry (cells can be rotated in place

at angles less than one complete rotation without affecting cells

layout)


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Pentagon?


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Cell Shape

Ex. hexagon geometry cell shape Designed to serve the weakest mobiles within the footprint (typically located at

the edge)

The hexagon has the largest area of the three regular shapes

Simplistic model, Universally adopted

Fewest number of cells can cover a geographic region

Approximate circular shape

no gaps

no overlap

equal area


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Do Cells in Reality have the

Hexagonal Shape? The answer is certainly NO. It is very rare that you see a cell that is close to

hexagonal because of many reasons:

1) Geographical features such as mountains and valleys alter the shape of

a cell significantly. Even small variations in height around the cellular

tower affect the shape of the cell.

2) The inability of a cell phone company to place the cell towers in exactly

the desired location due to geographical features or buildings.

3) The inefficiency of insuring hexagonal cells as sometimes the

population density within the coverage area may vary making it more

efficient to place more towers in regions with high population and lesstowers in regions with low population.


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Why Do We Study Hexagonal Cells

and not Non-Hexagonal Cells?

Because hexagonal cells are easier to analyze

and they give a good understanding of the

analysis techniques for nonhexagonal cells

without the complication of irregularly shapedcells. So, we will limit our discussion to

hexagonal cells only.


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Are Cells Sometimes Intentionally

Made Non-Hexagonal?

Yes. Two of the most spread nonhexagonal

cell shapes are the (1) highway style coverage

and (2) the Manhattan style coverage.


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How Often Are Frequencies

Reused (Frequency Reuse Factor)?

The frequency reuse factor is defined as 1

over the number of cells in the cluster of the

system.

Valid clusters are those that result in 6 cells

with the same frequency of a particular cell

located at equal distance from it.


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1-Cell Frequency Reuse Cluster

(Frequency Reuse Factor = 1)

Whole band of frequency is used in the cell

and reused in

Each of the adjacent

Cells


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(Frequency Reuse Factor = 1/3)


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What Makes a Cell Frequency

Pattern Valid or Invalid? It is not wither you can stack clusters near each other to cover

the whole desired coverage area or not.

For example, 2Cell and 5cell frequency reuse clusters can

cover the whole area without gaps. However,

if you look at either the 2 or 5, you note that to each cell thereare some close cochannel cells (not equal to 6) and there are

some cochannel cells at a farther away distance (also not

equal to 6).

This makes the interference be dominated by the closecochannel cells. So, we are splitting the frequency

band into smaller regions in the hope of reducing the

interference but we are not necessarily getting the

benefits of this.


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What do We Gain What do

We Loose with Frequency Reuse? The higher the number of divisions of the spectrum over cells

(higher cellreuse factor), the lower the capacity of the

network but the further away cells with similar frequency

allocations are located resulting in lower interference.

The lower the number of divisions of the spectrum over cells

(Lower cellreuse factor), the higher the capacity of the

network but the closer cells with similar frequency allocations

are located resulting in higher interference.


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Frequency Allocation Concepts

Assume that the total frequency band allocated for

a cellular system is B Hz, and that each halfduplex

channel requires WHz, the number of fullduplex

channels Sthat the total band supports (onechannel for transmission and one for reception) is

S=B/2W

Let the total number of fullduplex channels be

divided equally among N cells (in an NCell

Frequency reuse system). The total number of

channels kassigned to each cell becomes


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K=S/N

with a frequency reuse factor FRF given by

FRF=1/N The N cells over which the complete frequency band has been

divided is called a CLUSTER. Ifthis cluster is repeated Mtimes

over the coverage area, this gives a total number of

fullduplex channels in the coverage area C equal to


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Cluster Size and System Capacity

Assume the following system parameters:K Number of channels in a cell

N Number of cells/cluster (Cluster size)

M Number of times the cluster is repeated

S = KN Number of channels in a cluster

C Total number of channelsC = MkN = MS


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Example

If a total of 33 MHz of bandwidth is allocated to a particular FDDcellular telephone system which uses two 25 kHz simplex channelsto provide full duplex voice and control channels,

(1) compute the number of channels available per cell if a systemuses

(a) 4-cell reuse, (b) 7-cell reuse (c) 12-cell reuse.

If 1 MHz of the allocated spectrum is dedicated to control channels,

(2) determine an how many control channels and voice channelsin each cell for each of the three systems.

l d


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Cluster Size and SystemCapacity

Cluster size N (with cell size const) more clusters are required tocover a given area C and hence more capacity

Co-channel cells become closer

Cluster size N (with cell size const) the ratio between cell size and thedistance between co-channel cells is large

Design Objectives for Cluster Size

1. High spectrum efficiency

many users per cell

small cluster size gives much bandwidth per cell

2. High performance

Little interference

Large cluster sizes

l d


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Cluster Size and SystemCapacity

There are only certain cluster sizes and cell layout which are

possible in order to connect without gaps between adjacent cells

N = i2 + ij + j2 , where i and j are non-negative integers

Example i = 2, j = 1

N = 22 + 2(1) + 12 = 4 + 2 + 1 = 7

Typical Cluster Sizes

N = 1, 3, 4, 7, 9, 12, 13, 16, 19, 21


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Frequency Reuse Again


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Frequency Reuse


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Nearest co-channel To find the nearest co-channelneighbors of a particular cell:

Move i cells along any chainof hexagon

Then turn 60 degree counter-

clockwise and move j cells


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Nearest co-channel

b h l ll


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Distance between Co-Channel Cell

Centers


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Geometry of a Hexagon


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Channel AssignmentStrategiesFixed Channel Assignments

Each cell is allocated a predetermined set of voice channels

If all the channels in that cell are occupied, the call is blocked, and

the subscriber does not receive service

Variation includes a borrowing strategy: a cell is allowed to borrow

channels from a neighboring cell if all its own channels are occupied

This is supervised by the Mobile Switch Center: Connects cells to

wide area network; Manages call setup; Handles mobility


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Channel AssignmentStrategiesDynamic Channel Assignments

Voice channels are not allocated to different cells permanently

Each time a call request is made, the serving base station requests achannel from the MSC

MSC then allocates a channel to the requested call according toalgorithm taking into account different factors: frequency re-use of

candidate channel and cost factors

Dynamic channel assignment is more complex (real time), butreduces likelihood of blocking


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HandoffThere are two base station

antennas that are transmitting a

signal of equal power to the phone

The primary base station of the cellin which the car is moving and a

secondary base station in the

neighboring cell the car is

approaching

The signal from the secondarystation causes interference with the

signal from the primary station

resulting a degradation of the cell

phones capabilities

Thus the power of the signal

received by the cell phone varies

as the car moves along

Signal-to-interference ratio

OrCarrier-to-interference ratio

Mobile moves from one cell to another cellwhile a conversation is in progress



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Once we know the height of the antennas and the distance between base

stations, both thepower of the received signals and hence the signal-to-

interference ratio can be computed

Fig 1 Fig 2

Fig 3


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Handoff (Contd.)

Designers must specify an

optimum signal level at which to

initiate a handoff.

Margin () is defined,

= handoff threshold - Minimumacceptable signal to maintain the call

If too small:

Insufficient time

to complete handoff

before call is lost

More call losses

If too large:

Too many handoffs

Burden for MSC


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Call Dropped

Handoff is not made and call is dropped if:

Large delayby the MSC in assigning a handoff

Threshold margin () is set too small for the handoff time in the

system.

Excessive delays may occur during high traffic condition due to

computational loading at the MSC

No channels are available on any of the nearby base stations (thus

forcing the MSC to wait until a channel in a nearby cell becomes

free)


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Dwell Time

It is the time over which a call may be maintained within a

cell, without handoff

Depends on:

Propagation, interference, distance between the subscriber and the

base station, and other time varying effects. (the speed of the user

and the type of radio coverage)

Even a stationary subscriber may have a random and finite dwell time due to

fading effect.


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Styles of Handoff

Network Controlled Handoff (NCHO)- In first generation cellular system, each base station constantly monitorssignal strength from mobiles in its cell

- Based on the measures, MSC decides if handoff necessary

- Mobile plays passive role in process

- Burden on MSC

Mobile Assisted Handoff (MAHO)

- Mobile measures received power from surrounding base stations and reportto serving base station

- Handoff initiated when power received from a neighboring cell exceedscurrent value by a certain level or for a certain period of time

- Faster since measurements made by mobiles, MSC dont need monitor signalstrength


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Intersystem Handoff

If a mobile moves from one cellular system to different cellular

system controlled by a different compatible MSC

When a mobile signal becomes weak in a given cell and the MSC

cannot find another cell within its system to which it can transfer the

call in progress


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Prioritizing Handoff

Dropping a call is more annoying than line busy

Guard channel concept (Decrease the probability of forced termination

due to lack of available channels)

Reserve some channels for handoffs

Waste of bandwidthBut can be dynamically predicted

Queuing of handoff requests (due to lack of available channels)

There is a finite time interval between time for handoff and time to

drop (signal goes below the handoff threshold)

Better tradeoff between dropping call probability and network traffic

Practical Handoff


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Practical HandoffConsiderations

(1) Practically, several problems arise when attempting to design for a wide

range of mobile velocities

High speed vehicles pass through the coverage region of a cell within a

matter of seconds, whereaspedestrian users may never need a handoff

during a call

Particularly with the addition of microcells to provide capacity, the

MSC can quickly become burdened ifhigh speed users are constantly

beingpassed between very small cells

(2) Another practical limitation is the ability to obtain new cell sites. In

practice it is difficult for cellular service providers to obtain new physical

cell site locations in urban areas


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The umbrella Cell Solution

Is used to provide large area coverage to high speed users while providing

small area coverage to users traveling at low speeds

By using different antenna heights (often on the same building or tower)

and different power levels, it is possible to provide large and small cells

which are co-located at a single location

# handoffs is minimized for high speed users and provides additional

microcell channels for pedestrian users

If a high speed user in the large umbrella cell is approaching the base

station, and its velocity is rapidly decreasing, the base station may decide to

hand the user into the co-located microcell, without MSC intervention


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Umbrella Cell Approach


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Cell Dragging

As the user travels away from the base station at a very slow speed, the

average signal strength does not decay rapidly

Even when the user has traveled well beyond the designed range of the cell,

the received signal at the base station may be above the handoff threshold,

thus a handoff may not be made

Interference and traffic management problem, since the user has meanwhile

traveled deep within a neighboring cell

To solve this problem, handoff thresholds and radio coverage

parameters must be adjusted carefully


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Interference and SystemCapacity Interference is the major limiting factor in performance of cellular radio

systems

Sources of interference:

Mobiles in same cell

A call in progress in a neighboring cell Other base stations operating in the same frequency band

Non-cellular system leaking energy into the cellular frequency band

Effect of interference:

Cross talk in voice channels

For control channels missed or blocked calls

The two main types are:

co-channel interference

adjacent channel interference


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Co-channel Interference

Co-channel cells: Cells that use the same set of frequencies

Unlike thermal noise which can be overcome by increasing the signal-to-noise ration (SNR), co-channel interference cannot be combated by

simply increasing the carrier power of a transmitter

To reduce co-channel interference, co-channel cells must be physically

separated by a minimum distance to provide sufficient isolation due topropagation


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Co-channel Interference

When the size of each cell is the same, and the BSs transmit the same

power, the co-channel interference ratio depends on:

The radius of the cell (R)

The distance between centers of the nearest co-channel cells (D)

Co-channel reuse ratio:

Q increases Interference decreases Q decreases Interference increases (cluster size N decreases and

system capacity increases)


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h l


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Co-channel Reuse Ratio


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Signal-to-Interference Ratio

The signal-to-interference ratio (S/I or SIR) for a mobile receiver whichmonitors a forward channel (Down Link Channel) =

S : The desired signal power from the desired base station

Ii : The interference power caused by the ith interfering cell base station.

i0 : The number of interfering cells.


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Co-channel InterferenceAssumptions

The interference is due to co-channel base stations.

The transmit power of each base station is equal

The path loss exponent (n) is the same throughout the coverage area,

S/I for a mobile can be approximated as


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Adjacent ChannelInterferenceOrigin: Arising from signals which are adjacent in frequency to the

desired signal

Become serious by

Imperfect receiver filters which allow nearby frequencies to leak into thepassband (near-far-effect)

The Adjacent Channel Interference that a receiver A experiences from a

transmitter B is the sum of power that B emits into A's channel ( which iscalled the unwanted emission and represented by the ACLR (AdjacentChannel Leakage Ratio)

B emitting power into A's channel is called Adjacent Channel Leakage, orunwanted emissions


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Example

If a mobile is 20 times as close to the base station as another mobile and has

energy spill out of its passband, the signal-to- interference ratio at the base

station for the weak mobile (before receiver filtering) is approximately

For a path loss exponent n = 4, this is equal to -52 dB

Trunking and Grade of


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Trunking and Grade ofService

In cellular mobile communication, the two important aspect that has to be

considered with more care are,

1) Trunking2) Grade of Service (GOS)

These aspects has to be well planned so that it will lead to a better system

performance

Trunking


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Trunking Trunking deals with accommodation of large number of mobile users in

minimum radio spectrum

By using this Trunking concept it is possible to allow many users to share

smaller number of mobile channels in a cell

It is done by assigning channels on demand basis and allocating a channel

from a pool of channels available

That is if a user want to access a channel for establishing a call, then from

the pool of channels the required channel will be assigned to the user

If call got terminated, then the channel used so far will return to the pool

and will be ready for next call

The trunking concept finds application in telephone circuitry, mobile radio

communication in a large way

Trunking Theory


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Trunking Theory

Important to design trunked radio systems that can handle a specificcapacity at a specific grade of service, GOS

Trunking theory was developed by Erlang

Erlang based his studies of the statistical nature of the arrival and the lengthof calls. The measure of traffic intensity bears his name

One Erlang represents the amount of traffic intensity carried by a channelthat is completely occupied

For example, a radio channel that is occupied for 15 minutes during an

hour carries 0.25 Erlangs of traffic

Traffic Intensity = = 0.25 Erlangs

Th G d f S i (GOS)


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The Grade of Service (GOS)

The grade of service (GOS) is a measure of the ability of a user to access

a trunked system during the busiest hour

It is used to define the desired performance of a particular trunked system

GOS is typically given as the probability that a call is blocked, or the

probability of a call experiencing a delay, greater than a certain queuing

time

Some Definitions used in Trunking Theory


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Some Definitions used in Trunking Theory

Set-up Time: The time required to allocate a trunked radio channel torequesting user

Blocked Call: Call which cannot be completed at the time of request

Holding Time (H): Average duration of a typical call (H in seconds)

Traffic Intensity (A): Measure of channel time utilization (averagechannel occupancy measured in Erlangs)

Load: Traffic intensity across the entire trunked radio system (Erlangs)

Grade of service (GOS): A measure of congestion which is specified asthe probability of a call being blocked, or the probability of a call beingdelayed beyond a certain amount of time

Request Rate (): The average number of call request per unit time

T ffi I i (A)


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Traffic Intensity (A)

T f t k d t


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Types of trunked systems

E l B f l


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Erlang B formula

Improving Coverage &


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Improving Coverage &Capacity Increasing

C ll litti


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Cell splitting

C ll litti


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Cell splitting

E l


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Example

The base stations are placed at corners of the cells

The original base stationA is surrounded by six new microcells

In this example the smaller cells added in such a way as to preserve the

frequency reuse plan of the system

Each microcell base station isplaced half way between two larger stations

utilizing the same channel

Cell splitting simply scales the

geometry of the cluster

The radius of each new microcell

is half that of the original cell

i l bl i C ll li i


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Practical problems in Cell splitting

Channel Assignment

Not all cells are split at the same time

It is often difficult to find real estate that is perfectly situated for cell

splitting

Different cell sizes will exist simultaneously

Special care needs to be taken to keep the distance between co-channel

cells at the required minimum, and hence channel assignments become

more complicated

Handoff:

High speed and low speed traffic should be simultaneously accommodated(the umbrella cell approach is commonly used).

P i l bl i C ll li i


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Practical problems in Cell splitting

If the larger transmit power is used for all cells, some channels used by the

smaller cells would not be sufficiently separated from co-channel cells

If the smaller transmit power is used for all the cells, there would be parts

of the larger cells left unserved

In practice different cell sizes will exist simultaneously

C ll S t i


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Cell Sectoring

Reduction of Co-channel interference


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Reduction of Co channel interference

using sector antennas

Cell Sectorin


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Cell Sectoring

The S/I improvement is achieved at the cost of thenumber of antennas at

each base station

Sectoring decreases trunking efficiency due to channel sectoring at the

base station

Since sectoring reduces the coverage area of a particular group of channels,

the number of handoffs increases

Handed off from sector to sector within the same cell without intervention

from the MSC

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C03-The Cellular Concept

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