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Chapter 10

Cellular wireless Networks

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Single Cell ‘Network’

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History of Cellular Networks

Why cellular networks?

To address requirement for greater capacity

For efficient use of frequency

To address the poor quality of non cellular mobile

networks and increases coverage

– replaces a large transmitter with smaller ones in cells

– smaller transmitting power

– each cell serves a small geographical service area

– each cell is assigned a portion of the total frequency

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Replacement of huge single cell by a number of small cells

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Why Hexagonal Cell Structure

No proper coverage of the area with theoretical circles.

Polygon near to the circle Hexagon is selected for further technical

simplicity.

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Description of a Cell

Approximated to be a hexagonal coverage

– best approximation of a circular area

Served by a base station

– low powered transceiver

– antenna system and it

– may be divided into 6 equilateral triangles

– length of base of each triangle = 0.5R (radius)

– different groups of channels assigned to base stations

R

RR

87.02

3

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Mathematical Description of a Cell

Area of a cell is:

Perimeter of a cell = 6R

22

598.22

33

23

26 R

RRx

RxAreacell

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Types of Mobile Communication Cells

The size of a cell is dictated by capacity demand

Macro-cell

– large, covering a wide area

– range of several hundred kilometers (km) to ten km

– mostly deployed in rural and sparsely populated areas

Micro-cell

– medium cell, coverage area smaller than in macro cells

– range of several hundred meters to a couple of metrrs

– deployed mostly in crowded areas, stadiums, shopping malls

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Types of Mobile Communication Cells Contd.

The size of a cell is dictated by capacity demand

Pico-cell

– small, covering a very small area

– range of several tens of meters

– low power antennas

– can be mounted on walls or ceilings

– used in densely populated areas, offices, lifts, tunnels etc

Mega-cell

-- These cells are formed by LEO and MEO

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Capacity Computations

Assume there are N cells, each allocated k different frequency channels. These N cells are said to form a cluster. Total number of channels per cluster is given by

S = k N Total capacity associated with M clusters: C = M k N = M S A cluster may be replicated more times in a given area

if the cells are made smaller (note that power needs to be reduced accordingly).

Capacity of cellular system is directly proportional to “M”, number of times a cluster is replicated.

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Capacity versus interference for same size cell and power transmission

Decrease N for More Capacity: If Cluster Size, N is decreased while cell size

remains fixed, more clusters are required to cover the area (M increases). Therefore, Capacity increases.

Increase N for Less Interference: On the other hand, if N is increased (large cluster

size) means that co-channels are now farther than before, and hence we have will have less interference.

Value of N is a function of how much interference a mobile or a base station can tolerate.

We should select a smallest possible value of N but keeping S/I in the required limits.

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Means of Increasing System Capacity

There are several approaches for increasing

cellular system capacity including:

Cell clustering

Sectoring of cells

Cell splitting

Frequency reuse

Reduction of adjacent cell interference and co-channel

interference

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Cell Clusters

Service areas are normally divided into clusters of

cells to facilitate system design and increased

capacity

Definition

a group of cells in which each cell is assigned a different

frequency

– cell clusters may contain any number of cells, but clusters

of 3, 4, 5, 7 and 9 cells are very popular in practice

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Cell Clusters

A cluster of 7 cells

the pattern of cluster is repeated throughout the network

channels are reused within clusters cell clusters are used in frequency planning for

the network Coverage area of cluster called a ‘footprint’

1

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Cell Clusters (1)

A network of cell clusters in a densely populated Town

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Representation Of Cells Through BS

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Frequency Plan

Intelligent allocation of frequencies used

– Each base station is allocated a group of channels to be

used within its geographical area of coverage called a

‘cell’

Adjacent cell base stations are assigned completely

different channel groups to their neighbors.

base stations antennas designed to provide just the

cell coverage, so frequency reuse is possible

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Frequency Reuse Concept

Assign to each cluster a group of radio channels to be

used within its geographical footprint

ensure this group of frequencies is completely different from

that assigned to neighbors of the cells

Therefore this group of frequencies can be reused in a

cell cluster ‘far away’ from this one

Cells with the same number have the same sets of

frequencies

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Frequency Reuse Factor

Definition

When each cell in a cluster of N cells uses one of N

frequencies, the frequency reuse factor is 1/N

frequency reuse limits adjacent cell interference

because cells using same frequencies are separated

far from each other

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Factors Affecting Frequency Reuse

Factors affecting frequency reuse include:

Types of antenna used

--omni-directional or sectored

placement of base stations

-- Center excited or edge excited.

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Excitation of Cells

Once a frequency reuse plan is agreed upon overlay the

frequency reuse plan on the coverage map and assign

frequencies

The location of the base station within the cell is referred

to as cell excitation

In hexagonal cells, base stations transmitters are either:

– centre-excited, base station is at the centre of the cell or

– edge-excited, base station at 3 of the 6 cell vertices

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Finding the Nearest Co-Channel

After selecting smallest possible value of N we should see that N should

follow the following eq. N= i2+j2+ij

(1) Move i cells along any chain of hexagons

(2) Turn 600 counter-clockwise and move j cells, to reach the next cell

using same frequency sets

this distance D is required for a given frequency reuse to provide enough

reduced same channel interference

ie, after every distance D we could reuse a set of frequencies in a new cell

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Freq Reuse ( N=7 , i=2 j=1)

A

B

E

DF

CG

A

B

E

DF

CGA

B

E

DF

CGi

j

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Freq Reuse ( N=19 , i=3 j=2)

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How frequency Reuse Increases Capacity

Example: A GSM communication system uses a

frequency reuse factor of 1/7 and 416 channels available.

If 21 channels are allocated as control channels, compute

its system capacity. Assume a channel supports 20 users

Channels available for allocation = 416 - 21 = 395

Number of cells = 395 / 7 = 57

Number of simultaneous users per cell = 20 x 57 = 1140

Number of simultaneous users in system = 7 x 1140 = 7980

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To satisfy the user, a channel needs to be available on request.

Reasonable probability of call blockage (GOS) is 2%. GOS fluctuate with location and time. The goal is to keep

a uniform GOS across the system. Reduction of variations in GOS allow more users – an

increase in capacity.

Three types of algorithms for channel allocation: Fixed channel allocation (FCA) Channel Borrowing Dynamic channel allocation (DCA)

Channel Allocation Techniques Targets to achieve through the different

channel allocation techniques.

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Available spectrum is W Hz and each channel is B Hz. Total number of channels:

Nc = W/B For a cluster size N, the number of channels per cell:

Cc = Nc/N To minimize interference, assign adjacent channels to

different cells.

Fixed Channel Allocation Techniques

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FCA is the optimum allocation strategy for uniform traffic across the cells.

A non uniform FCA strategy, when it is possible to evaluate GOS in real time and adjust the FCA accordingly. This requires a more complex algorithm.

Features of Fixed Channel Allocation Techniques

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Channel Borrowing Borrow frequencies from low traffic cells to high traffic

cells. Temporary channel borrowing: channel is returned after

call is completed. If channels from cell E are borrowed by cell A, then

neighboring cells E cannot use those channels.

C

AG

BE

A

GC

F

G

C

B

D

B

D

G

F

a

DA

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Dynamic Channel Allocation All channels are placed in a pool, and are assigned to new

calls according to the reuse pattern. Signal is returned to the pool, when call is completed.

Issues related to channel allocation are still under research.

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Comparison of Channel Allocation Techniques

Fixed Channel Allocation Advantages:

--- Less load on MSC

--- Simple Disadvantages:

Blocking may happen

Dynamic Channel Allocation Advantages:

Voice channels are not allocated permanently. That is, resource is shared on need-basis

Disadvantages:

--- Requires MSC for processing---burden on MSC

--- May be very complicated

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Handoff Do we need hand off in old mobile telephony

and why. No because of single huge cell covering the whole

service area. What is handoff. When a mobile moves into a different cell while a conversation

is in progress,the MSC automatically transfers the call to a new channel belonging to the new base station

What are important considerations to design a handoff process.

1) Handoff must be performed successfully.2) Handoff must occur infrequently.3) Handoff should be smooth and the users must not be able to

detect it.

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Handoff Contd. Arrangements to fulfill the requirements of

a successful handoff. Handoff must be given priority over call initiation

request . Minimum usable signal for voice quality at the base

station receiver is normally from –90 dbm to –100 dbm.

Optimum signal level at which to initiate a handoff should be taken in such away that should not be too large or too small. Where

= Pr(handoff) – Pr(Min.usable)

If is too large----- more handoffs-----Burden on MSC If is too small ----- more call drops.

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Handoff Contd. When to handoff. Signal level drop is not due to momentary fading

and mobile is actually moving away from the BS. For this the BS monitors the signal level for a certain

period of time before a handoff is initiated.This period of time varies with the speed of MS.

Perfect relationship is required between speed of signal drop and required handoff time.

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Handoff Contd. Who detects the need for handoff.1) Network Initiated Handoff: Signal measurements by BS & supervised by MSC. BS monitors its all RVCs to determine the relative

location of each mobile user with respect to the BS. Mostly implemented in 1G systems. This type of handoff takes almost 10 sec as in AMPS. Two separate receivers on BS(i) One is used to measure RSSI(Radio Signal Strength Indication)

of calls in progress with in the cell.(ii) Second is used to scan and determine signal strengths of

mobile users which are in neighboring cells.

(iii) Both the signals are monitored by the MSC which decides when to handoff.

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Handoff Contd Who detects the need for handoff.2) Mobile Assisted Handoff( MAHO) Every mobile station measures the received power from

surrounding base stations and continually reports the results of these measurements to the serving base station.

A handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from the current base station (serving base station) by a certain level or for a certain period of time.

This handoff technique has been implemented in most of the 2G systems.

This type of handoff takes almost 1- 2sec as in GSM. Preferred for microcellular system

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Handoff Contd. What are the types of handoff. Hard Handoff Soft Handoff What are different levels of handoff. (1) Intra Cell (2) Inter cell (3) Inter

system Importance of handoff. When no priority to handoff call blocking

would be equal for call initiation and call handoff.

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Priority To Handoff

There are two strategies to give a priority to handoff.

(1) Guard Channel: 100% guaranty for successful handoff but It will cause low trunking efficiency.

(2) Queuing Of Handoff Request: There can be unsuccessful handoffs due to long delay in queue.

--“Probability of forced termination” decreases at the cost of reduced Total Carried Traffic.

-- Queuing is possible because of the time available between the Threshold power level and the Hand off power level.

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Practical Handoff Problems

(1)Problem Caused by high speed mobility: More handoffs are required to handle high

speed mobility of MS during a call. It will cause load on the system as well as call drops.

(2) Problem Caused by low speed mobility: Cell dragging:

In the line of sight and smooth area signal does not drop sharply for pedestrian users so user goes on using the frequency of the previous cell in to the new cell. This causes increase in the co-channel interference.

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Solution For More Handoffs

Umbrella Cell Approach: Micro cells inside A macro cell.

---- Macro cell is defined by high power and lengthy tower. ---- Micro cells are defined inside the macro cell with less power and less height towers. ---- High speed MS are handled by macro cell and low speed subscribers are handled by micro cells. ---- This strategy increases the no of capacity channels per unit area and decreases the no of handoffs.

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Solution For Cell Dragging

Handoff threshold ----and radio coverage parameters must be adjusted carefully according to the environment .

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Assignment No.1

There are 50 cells in the city. We have only 36 carrier frequencies .

Perform freq planning using

N=4/12 and N= 4/12 + 8 + 8+8

Discuss and compare the capacity and interference in both the cases.

• Given : Freq. hopping is allowed in for TCH but not for BCCH.

Note: If you are supposing some thing else mention it.

Due Date: Before 1/2/05

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INTERFERENCE AND SYSTEM CAPACITY

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Interference

It is a major limiting factor in the performance of cellular radio systems. (In comparison with wired comm. Systems, the amount and sources of interferences in Wireless Systems are greater.)

Creates bottleneck in increasing capacity

Sources of interference are:1. Mobile Stations2. Neighboring Cells

3. The same frequency cells 4. Non-cellular signals in the same spectrum

Interference in Voice Channels: Cross-Talk

Urban areas usually have more interference, because of:a)Greater RF Noise Floor, b) More Number of Mobiles

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Major Types Of Interference

1) Co-Channel Interference (CCI)

2) Adjacent Channel Interference (ACI)

3) Other services: like a competitor cellular service in the same area

1) Co-Channel Interference and System Capacity

The cells that use the same set of frequencies are called co-channel cells.

The interference between signals from these cells is called Co-Channel Interference (CCI).

Cannot be controlled by increasing RF power. Rather, this will increase CCI.

Depends on minimum distance between co-channel cells.

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In constant cell size and RF power, CCI is a function of Distance between the co-channel cells(D), and the size of each cell (R).

Increasing ratio D/R, CCI decreases.

Define Channel Reuse Ratio = Q = D/R

The yellow cells use the same set of frequency channels, and hence, interfere with each other.

In case of N=7, there are 6 first-layer co-channels.

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Signal-to-interference ratio

S is the power of the signal of interest and Ik is the power of kth interference.

The signal strength at distance d from a source is

SIR =S

Ikk

1

6

S d n

That is, received signal power is inversely related to nth power of the distance.

where n = path loss exponent

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For hexagonal geometry, D/R can be calculated:

NQ 3

NRDQ 3/

Smaller Q provides larger capacity, since that would

mean smaller N. (Capacity 1/N).

Larger Q improves quality, owing to less CCI.

for N=3, Q=3,

N=7, Q=4.58,

N=12, Q=6,

N=13, Q=6.24

R D

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Then we can express the SIR in terms of distance

where the denominator represents the users in neighboring clusters using the same channel.

Let D k=D be the distance between cell centers. Then

Note how C/I improves with the frequency reuse N.

Analog systems: U.S. AMPS required C/I ~= 18dB For n = 4, the reuse factor for AMPS is N 6.49, so N = 7.

Now, let us consider the worst case for a cluster size of N= 7. The mobile is at the edge of the cell. Express C/I as a function of actual distances.

C / I SIR =

R

D

n

kn

k 1

6

( / ) ( 3 )C/I

6 6

n nD R N

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Worst Case Design

Worst case carrier-to-interference ratio

Let n = 4 and D/R = q,

Let reuse N = 7, then

Compute C/I and get C/I = 17.3 dB

For a system with omni-directional antenna, N =7 is not sufficient. We need to increase N = 12

2( ) 2 2( )

n

n n n

C R

I D R D D R

4 4 4

1

2( 1) 2 2( 1)

C

I q q q

q 3 7 4 6.

D + R

D + R

D -R

D -R

D

D

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Increasing N from 7 to 12, brings C/I above the 18dB level. However, the system capacity is decreased.

Reduction of capacity by 7/12 for taking care of the worst case situation when CIR ~= 17.4 dB is not justified because this situation will occur very rarely.

Conclusion: Co-Channel Interference controls the link performance which then decides Frequency Reuse Plan, and System Capacity.

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If S/I min = 15 dB, what is the capacity for n = 4, n = 3

(a) n = 4, N = 7

N =7 can be used

(b) n = 3, N = 7

E

0 0

/ 3 7 4.58

( / ) ( 3 ) (4.58)18.66 dB>15 dB

6

n n n

D R

S D R N

I i i

SI

N

D R

SI

4 586

12 05

3 12 6 0

66

36 1556

3

3

..

/ .

.

dB<15 dB

Need larger

dB>15 dB

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(2) Adjacent Channel Interference

Interference from channels that are adjacent in frequency,

The primary reason for that is Imperfect Receiver Filters which cause the adjacent channel energy to leak into your spectrum.

Problem is severer if the user of adjacent channel is in close proximity. Near-Far Effect

Near-Far Effect: The other transmitter(who may or may not be of the same type) captures the receiver of the subscriber.

Also, when a Mobile Station close to the Base Station transmits on a channel close to the one being used by a weaker mobile: The BS faces difficulty in discriminating the desired mobile user from the “bleed over” of the adjacent channel mobile.

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Unintended

Tx

Mobile User Rx

BS as Tx

Weaker signal

Strong “bleed over”

The Mobile receiver is captured by the unintended, unknown transmitter, instead of the desired base station

Near-Far Effect: Case 1

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Adjacent Channel

Mobile Tx

Desired Mobile Tx

BS as Rx

Weaker signal

Strong “bleed over”

The Base Station faces difficulty in recognizing the actual mobile user, when the adjacent channel bleed over is too high.

Near-Far Effect: Case 2

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Minimization of ACI

(1) Careful Filtering ---- min. leakage or sharp transition(2) Better Channel Assignment Strategy

Channels in a cell need not be adjacent: For channels within a cell, Keep frequency separation as large as possible.

Sequentially assigning cells the successive frequency channels.

Also, secondary level of interference can be reduced by not assigning adjacent channels to neighboring cells.

For tolerable ACI, we either need to increase the frequency separation or reduce the pass band BW.

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Power Control

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What is power control ?

Both the BS and MS transmitter powers are adjusted dynamically over a wide range.

Typical cellular systems adjust their transmitter powers based on received signal strength.

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Why power control ?

Near-far effect Mechanism to compensate for “channel

fading” Interference reduction, prolong battery life

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Types Of Power Control

Open Loop Power ControlIt depends solely on mobile unit, not as accurate as

closed loop, but can react quicker to fluctuation in signal strength. In this there is no feed back from BS.

Closed Loop Power Control

In this BS makes power adjustment decisions and communicates to mobile on control channels

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Improving Capacity in Cellular Systems

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Improving Capacity in Cellular Systems

Cost of a cellular network is proportional to the number of Base Stations. The income is proportional to the number of users. Ways to increase capacity: New spectrum – expensive. PCS bands were sold

for $20B. Architectural approaches: cell splitting, cell

sectoring, microcell zones. Dynamic allocation of channels according to load

in the cell (non-uniform distribution of channels).

Improve access technologies.

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Cell Splitting

Cell Splitting is the process of subdividing the congested cell into smaller cells (microcells),Each with its own base station and a corresponding reduction in antenna height and transmitter power.

Cell Splitting increases the capacity since it increases the number of times the channels are reused.

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Cell splitting diagram 1

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An Example

The area covered by a circle with radius R is four times the area covered by the circle with radius R/2 The number of cells is increased four times

The number of clusters the number of channels and the capacity in the coverage area are increased Cell Splitting does not change the co-channel re-use ratio Q =D/R

●

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Transmit Power

New cells are smaller, so the transmit power of the new cells must be reduced

How to determine the transmit power? The transmit power of the new cells can be

found by examining the received power at the new and old cell boundaries and setting them equal

Pr(at the old cell boundary) is proportional to

Pt1 * R-n

Pr(at the new cell boundary) is proportional to

Pt2 * (R/2)-n

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Transmit Power

● Take n=4, we get Pt2 = Pt1/16 ● We find that the

transmit power must be reduced by 16 times or 12 dB in order to use the microcells to cover the original area. While maintaining the same S/I.

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Application of cell splitting

Not all cells are split at the same time. Larger transmit power Some of the channels would not be sufficiently

separated from the co-channel cells. Smaller transmit power --parts of the larger cells left uncovered Two groups: one that corresponds to the smaller cell and the other for larger cell reuse requirements

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Application of cell splitting (cont.)

● The sizes of these two groups depend on the stage of the splitting process ● At the beginning, fewer channels will be there in the smaller power group.As the demand grows, smaller groups would require more channels ● Cell splitting continues until all the channels are in the smaller power group ● Antenna Downtilting To limit the radio coverage of microcells

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Cell Overlay

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Cell Sectoring

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Example for sectoring

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Explanation For Cell Sectoring

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Example Of Cell Sectoring

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Microzones

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Advantages Of Zoning

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Example Of Microzoning

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