Chapter 3 Cellular Concept - fju.edu.t€¦ · Practical handoff considerations •Mobile velocities –for high speed vehicles, a handoff may be never needed during a call, particularly

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Chapter 3Cellular Concept

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Wireless Communication

Chapter 3 - Cellular Concept 1 Dr. Sheng-Chou Lin

Objectives

To resolve spectral congestion and user capacity

To provide additional radio capacity radio capacity with no additionalincrease in radio

Methods•Large Cells small cells, High power low power•Handoff and interference go up•Frequency Reuse: result in CCI (Cochannel interference)

Frequency Planing•Selecting and allocating channel groups for all of the cellular base stations (Channel

assignment)•To reduce CCI,ADJ (Adjacent Interference)•Different groups of channels for neighboring base stations

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Cellular Structure

Geometric Shapes: Square, triangle, Hexagon•without overlap•with equal area•Hexagon: for a given distance (center to farthest point) the largest area

Cell Diagrams

o

oo

oo o o o

o

Imaginary Ideal Real

easy to draweasy to consider

free spaceisotropic antennas

The Real World



Frequency Reuse

Cluster size N•S = kN, S: Total channels available

for user–k: channels per cell,–N: N cells a cluster

•C = MkN = MS = The total number ofduplex channels.A cluster is replicated M times

•N = 4, 7, 12 typically Capacity and frequency reuse

•Capacity = Channels per cell = (totalchannels) / N

•Capacity goes up as N decreases,but CCI goes up

N=7

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4N=4

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Given our propagation model anddesired C/I, we determined D/R inthe last slide.

Now we can establish a channelassignment pattern using thesmallest number of cells which willseparate co-channel cells by atleast D/R. N cells are required.

N is determined from geometry, asshown at left:

Frequency ReuseD/R determines required minimum N

D =distance between twoco-channel transmitters

R = coverage radius where acell is the best server

Example Sketch demonstrates N=7

N = I + I x J + J2 2

N = ( )2 3DR /

D

f1f7

f2

f1

f3J=1

f6

f4

f2

f5

f4 f7

I=2

f1

R R

X+60

X

• Move I cellsalong anychain

• Turn 60counter-clockwise andmove J cells



An Example

Total BW = 33MHz (RX+TX channels)One channel BW = 25kHz( per simplex channel)2= 50k (per duplex channel)•Total channels = 33,000/50 = 660 channels•1 M for control channel, 1000/50 = 20 control channels, 660-20 =640 voice

channels.•For N = 7, case one

–4 (3 control channels + 92 voice channels) cells–2 (3 control channels + 90 voice channels) cells–1 (2 control channels + 92 voice channels) cells

•For N = 7, case two: one control channel each cell–4 (1 control channels + 91 voice channels) cells–3 (1 control channels + 92 voice channels) cells

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The Resource: AMPS SpectrumFrequencies and Channel Numbers

An operator authorized frequency block contains 416 channels In a frequency plan, we assign specific channels to specific cells,

following a reuse pattern which restarts with each Nth cell Uplink and downlink bands are paired mirror images

•A channel includes one uplink and one downlink frequency

Uplink (Reverse Path) Downlink (Forward Path)

Paired Bands

824 835 845 870 880 894

869

849

846.5825

890

891.5

Frequency, MHz

A (non-Wireline) B (Wireline) BAA7997166663331991-

1023 Channel Numbers334



Cellular Band

BAND-A BAND-B

BAND-AControl Ch.

BAND-BControl Ch.

Ch. No. 1 312 355 666 716 799 991 1023

Extended-B

Extended-A

21 2150 33

83

Band-A Voice Channels: 1 - 312 = 312 ch.667 - 716 = 50 ch.991 - 1023 = 33 ch.

Tot. No. of Voice Ch. = 395

Band-A Control Channels: 313 - 333 = 21 Ch

Band-B Voice Channels: 355 - 666 = 312 ch.717 - 799 = 83 ch.

Tot. No. of Voice Ch. = 395

Band-B Control Channels: 334 - 354 = 21 ch.

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Capacity Improvement

How to improve capacity•To minimize required C/I•Strategy Channel assignment

Channel Assignment: minimize interference•Fixed: Voice channels are predetermined for each cell

–Borrowing strategy: All of its channels are already occupied.•Dynamic: MSC (Mobile switch allocates a channel to the required following an

algorithm:–Future blocking, frequency of use of the candidate channel, reuse

distribution of the channel, other cost functions.–MSC correct real-time data on channel occupancy, traffic distribution,

radio signal strength indications (RSSI)–Advantages: channel utilization , Blocking , trunking capacity .–Disadvantages: Memory storage , Computational load .



Channel AssignmentIn channel assignment, we dole out

the channels to the cells, muchlike a dealer in a card game dealsout cards from the deck until everyplayer has a set.

A channel set is a collection ofchannels which could be assignedat one cell

Channels in a channel set normallyare N channels apart, where N isthe reuse factor

Channels in a set must meetcombiner minimum frequencyspacing requirements

Notice that Sets 1 and 3 (i.e., 1 andN) are adjacent frequencies

If N=3, for example:

1 2 3 4 5 6 7 8 9 10 12Channels

Freq.

Channel Set 1 1, 4, 7, 10, . . .Channel Set 2 2, 5, 8, 11, . . .Channel Set 3 3, 6, 9, 12, . . .

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31 1

1

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32

N=3

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Handoff

Mobile automaticallytransfers the call to anew channel of the newchannel base station.

Handoff threshold isslightly stronger than aparticular signal(minimum signal foracceptable voicequality)

Mobile SwitchingCenter (MSC)

Local TelephoneExchange

Call Starts

Cell-1

t 2

Call Ends

Cell-2

t 1

t 3

Cell-3

Hand-off Required atBoundary Crossings



Handoff Factors Not due to momentary fading

•unnecessary handoffs are avoided•monitor the signal level for a certain period of time average measurement•propagation fading effect•the length of average time depends on vehicle speed short term fading

Dwell time: a call may be maintained within a cell without handoff•Factors: propagation, interference, distance between BS and MS, time varying

effects•Statistics: speed, type of radio coverage (Highway , Microcell ).

Signal strength measurement (RSSI): made by BS MSC•reverse link strength•first generation analog cellular•locator receiver: monitor the signal strength of users in neighboring cells•BS measurement load MAHO (mobile assisted handoff)

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Handoff considerations

MAHO (Mobile Assisted Handoff) v.s. RSSI•Measurement are made by MS•to measure the received power from surrounding BS•to report result to the serving BS A handoff is initiated

–power (neighboring cell) > power (current BS)•Second generation (Digital TDMA)•Much faster since BS load •be suited for microcellular, where handoffs are more frequent

Practical handoff considerations•Mobile velocities

–for high speed vehicles, a handoff may be never needed during a call,particularly in microcell.

•Another feature: other than signal strength, CCI and ADJ may be measurement C/I handoff



Hand-off Mechanism

Base Station RF Antenna

Adjacent Cell

Adjacent Cell

Adjacent Cell

Adjacent Cell

Adjacent Cell

Adjacent Cell

1. Base Station continuously measure RSSI [C/I]2. Based on this measurements decide the Handoff request.3. Once Handoff request is identified, asks adjacent cells to measure the

RSSI on that mobile and send the measurements.4. Identifies the candidate cell for Handoff5. Starts Handoff

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For Call Continuation

•to avoid dropping call as mobileleaves coverage range of theserving cell

To Avoid Interference

•maintain desired C/I ratio

•avoid giving, receivinginterference in other cells

For Operational Reasons

•For Load Balancing,Maintenance on VCH, etc.

Basic Cellular Call Processing:Why Handoff?

AB

CD

Distance, km

A B

RSSI,dBm

-120

-50

C DSites

C/I

RSSI

Drop



Basic Cellular Call Processing:Mechanics of the Handoff Process

Conditions Trigger system to attempt handoff•signal strength too low• interference or bit error rate too high

Measure, Screen alternatives•choose surrounding cells to monitor mobile

strength, report results–if TDMA, use MAHO - the mobile can

measure & report Analyze Measurements, Select Target Cell

•Is a better choice available?•Is changing worthwhile?

Implement the Handoff•MTX sets up new voice path trunking•handoff order sent via blank-and-burst•mobile acknowledges and jumps to new voice channel•Conversation continues

Time

RSSI

Trigger

A -95B -75C -100 Voice Channels

Handoff

Trigger

Screen

Select

Handoff!

DMS-MTX

A

B

C

DMS-MTX

A

B

C

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CDMA vs. AMPS/TDMA Handoffs Soft handoff: unique handoff capability provided by CDMA, since

•spread spectrum shares the same channel in every cell.• Its ability to select between the instantaneous received signals from a variety of BS

AMPS/TDMA Handoffs Break-before-make AMPS takes approximately 200 ms TDMA takes between 400-600 ms Can diminish call quality Increased chance of dropped calls

CDMA Handoffs Make-before-break Directed by the mobile not the base

station Undetectable by user Improves call quality

CellSiteA

HANDOFF

CellSite

B

MAKE

AMPSTDMA

BREAK

CellSite

A

CellSite

B

CellSite

A

CellSiteB

CDMA

Soft handoff can only be used between CDMAchannels having identical frequency assignments.

Soft handoff provides diversity of Forward and ReverseTraffic Channel paths on the boundaries between basestations.



Soft Handoff Considerations

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Soft handoff

Soft Handoff : the mobile station starts communications with atarget base station without interrupting communications with thecurrent serving base station.

Can involve up to three cells simultaneously and use all signals

Mobile station compares frames from each cell, and uses thebest one

Eliminates “Ping-Pong”effect and chances of dropped calls

CellSite

A

CellSite

BMTX

BSC

PSTN



Softer Handoff

alpha

beta

gamma

Handoff is betweensectors of the same cell

Communications aremaintained across bothsectors until the mobilestation transition hascompleted

May happen frequently

MTX is aware but doesnot participate

All activities are managedby the cell site

Signals received at bothsectors can be combinedfor improved quality

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Interference

The major limiting factor in the performance of cellular radio•Frequency Plan Restrictions Effects•A major bottleneck in increasing capacity

Major types and sources•Adjacent interference (ADJ)

–mobile in the same cell–mobile in a neighboring cell

•Cochannel Interference (CCI)–other BS operating in the same

frequency (co-channel cells)–Multiple access interference (MAI)

• Intermodulation Product (IM)–Due to out-of-band users

• In band interference–Noncellular syetem: leaks energy

into band.

Effects•become severe due to near-far

effect.•Cross talk on voice•miss and block calls on control

channel•be responsible for dropped calls•difficult to control in practice due

to random propagation effects(Fading phenomenon)



Cochannel interference and Systemcapacity

C/I (Carrier-to-Interference Ratio)•C/I is kept the same even C increases

–CCI can not combated by simply increasingthe carrier, unlike S/N

•be independent of TX power•A function of radius of cell (R) and the

distance to the center of nearest cochannelcell (D) (D/R)

•Cochannel reuse ratio = Q=D/R=SQRT(3N), N: Clutter size

•Q, N,C/I, Quality, CapacityQ, N,C/I, Quality, Capacity

C/I caluclation(optimistic result)•Propagation loss•D/R•number of cochannel cells

D =distance between twoco-channel transmitters

R = coverage radius where acell is the best server

Example Sketch demonstrates N=7

D

f1f7

f2

f1

f3J=1

f6

f4

f2

f5

f4 f7

I=2

f1

R R

X+60

X

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Frequency ReuseImplications of N

N is the number of cells in thefrequency reuse pattern.

N is a very important factor, since itdetermines:

Capacity of A CellChannels per cell =

(total channels) / N•As N goes up, capacity becomes

progressively worse Interference

•As N goes up, interferencebecomes progressively better

Channelsper Cell* D/R

395198132997966564944

1.7322.4493.0003.4643.8734.2434.5834.8995.196

40 5.47736 5.745

N

123456789101112 33 6.000

*Assuming use of 395 voice channels includingexpanded spectrum



SIR v.s.System capacity Calculation (1)

Signal - to - Interference

S

I=

I =1

i0

Ii

SS : desired signal power

Ii : nth cochannel interference

SitesPo

Pr

do

P

d

Pr = Po ( d / do ) -n

Average received power ata distance d from TX

Pr (dBm) = Po (dBm) - 10 n × log( d / do )

n ~ 2 - 4 in urban cellular

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I =1

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Di-n

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Mobile is on the border

dBm = 10log (P/1mW)dBW = 10log (P/1W)

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optimistic result


=

I =1

Di-n

R -n

•All interferong signals are equal.

•TX power of each BS is equal

•n is the sameS

I=

(D/R) n

i0

= ( 3N ) n

i0

i0 = 18.7 dB

For n = 4 and N = 7

= 18 dB ( n = 4 and N =6.49)For AMP cellular (S/I)min= 18dB for sufficientvoice quality

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Worse case: mobile unit is at thecell boundary (rarely occurs)•D-R from the two nearest CCI cells•(D+R/2, D. D-R/2, D+R) from other interfering

cells


=R -4S

I (D-R)-4 + (D-R/2)-4+ + (D+R/2)-4 + (D+R)-4 + 2(D)-4

=R -4

2(D-R)-4 + 2(D+R)-4 + 2(D)-4 (Approx.)

= 49.56 = 17dB for N = 7

For n = 4

Slightly less than 18dB, N 9, 7/9 capacity reduction can not be tolerable

(precise)

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Outage Probability

Outage Probability: P(SIR > SIRo): SIRo Threshold•(N=7, 120o, l = 8dB and r=4)



Outage Probability

Outage Probability: P(SIR > SIRo): SIRo Threshold•(N=7, 120o, l = 8dB and r=4)

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Percentage of area



Interference will not allow firstadjacent channels to be used at samecell site•typical receiver IF bandwidth too broad•worst case is at cell site: if adjacent

channel is much stronger than desiredsignal (Near-Far effect)

•adjacent user 10 KHz. signaling tonetroublesome–false terminations, etc.

1st. Adjacent channels OK for use inadjacent cells•effective handoffs required, never allow

adjacent signal to be stronger thandesired

2nd., 3rd., etc. adjacent channels OKin same cell

Adjacent-Channel Interference (ADJ)

Frequency

First-Adjacent ChannelsNot Feasible in Same Cell

Second-Adjacent ChannelsFeasible in Same Cell

First-Adjacent ChannelsFeasible in Adjoining Cells

if handoffs effective

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CDMA Multiple access interference (MAI) How can CDMA work on a negative signal-to-noise ratio

(i.e., noise higher than signal)? “Processing Gain”

Signal & interference are bothnarrow band

After spreading, signal & interferenceboth become wide band

After de-spreading, signal become narrowband but interference remain wide band

After passing a narrow band filter most of the wideband interference energy get filtered out

signal

interference

signal

interference



Near-Far Effect

Interference becomes severdue to near-far effect•Adjacent interference (ADJ)•Multiuser access interference

(MAI): CDMA•Intermodulation Product (IM)

PiPs

dnear

Pr

dfar

Interference

Thermal Noise

Eb

Nt

MAI becomessevere

ADJ becomes severe

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An Example of ADJ Why adjacent channel interference

• Imperfect receiver filter•Can be particular serious for Near-Far effect•N, ADJ, C/I

Solutions to minimize ADJ•Careful filtering•Channel assignment

EX: d (far) = 20 d (near), n = 4,

•SIR (near-Far) = (Pf / Pn)= (10)-n - 52dB• If filter of RX = 20 dB / octave• If (SIR)min = 0dB, total S/ADJ = 0+52dB•A separation of 6 channels is required

• If (SIR)min = 18dB, total S/ADJ = 18+52dB•A separation of 11 channels is required

PnPf

dnear

P

dfar

B/2 B 2B 4B

20dB

2 52/206.06

2 70/2011.3



A Tour of Reuse Factor N

N=1: Lethal•awful C/I: every neighbor is cochannel•every neighbor cell is adjacent channel too!•center 1/3 of each cell OK, rest is lost in

horrible interference

N=2: Better, but still lethal•Each cell still has 2 cochannel neighbors•Each cell has 4 adjacent channel neighbors

1

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1

1

1

1 21

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N = 3 : Better, but still lethal•Cochannel neighbors are now spaced at

D/R of 3.0 - better, but not 18 dB....•Each cell has 6 adjacent channel

neighbors - all the neighbors are adjacent!!

N = 4 : Better, but still lethal•Cochannel neighbors are now spaced at

D/R of 3.464•Each cell has 4 adjacent channel

neighbors

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N = 7 : The first arrangement that works inmost propagation environments, giving18+ dB C/I•Cochannel neighbors farther away

•Six at D/R of 4.58•Each cell has 2 adjacent channel

neighbors

N = 8 : Better, but not worthwhile•Cochannel neighbors farther away

•Four at D/R of 4.58•Two at D/R of 6.0•Two at D/R of 6.93

•Each cell has 2 adjacent channelneighbors

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15 N = 5 : Better, but not good enough

•Cochannel neighbors farther away•Two at D/R of 3.0•Four at D/R of 4.58


N = 6 : Better, but not by much•Cochannel neighbors farther away

•Two at D/R of 3.464•Two at D/R of 4.58•Two at D/R of 6.0


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Intermodulation Distortion

Imagine a non-linear device which is being fed two signals as input•Input: frequency 1 and frequency 2.

Because the device is non-linear, its output includes the two inputfrequencies and additional signals due to intermodulation distortion•frequencies of the intermod distortion products are of the form nf1+ mf2 and nf1-mf2

where n,m=1,2,3,....•The sum n + m is called the the order of the intermod products

Example:•3rd Order Components: 2f1- f2 , 2f2- f1 , 2f1+ f2 , 2f2+ f1•5th Order Components: 3f1- 2f2 , 3f2- 2f1 , 3f1+ 2f2 , 3f2+ 2f1

ff1 f2

f3f1-2f2 3f2-2f1f1 f2

2f2-f12f1-f2

Non-Linear DeviceInput Output

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Intermodulation Distortion

Intermodulation distortioncan turn a good-soundingcellular system

into a sea of phantominterferers, dropped calls,and intermittent crosstalk.

Receivers

Duplexer

Combiners

Transmitters

IM products everywhere! Frequency

IM under control Frequency

Amplitude



Intermodulation Distortion Every device (amplifier, etc.)

has a relationship betweeninput and output

Normally, the output is alinear replica of the input,except•when the input is so weak it is

lost below the noise floor•when the expected output is

stronger than the capabilities ofthe amplifier, and compressionoccurs

Even seemingly passivedevices (cables, connectors,antennas) have noise floorsand compression points

Power Transfer Characteristicsof a typical amplifier or other device

Noise Floor

Input Power (dBm)

PredictedPower

3dBCompression

point

1dB Compressionpoint

OutputPower(dBm)

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Concept: Third-Order IM Intercept Point

Strength of 3rd-Order Intermod Products:P3 = 2P1 + P2 - 2P3i (dBm) (for 2f1 +/- f2)P3 = 2P2 + P1 - 2P3i (dBm) (for 2f2 +/- f1)Where:P1 = Output power @ f1P2 = Output power @ f2P3i = 3rd order intercept point

The third-order IM intercept is a signal level defining the

interference-free dynamicrange of an amplifier or device

defined as the intersectionpoint of:•the slope of normal signals and•the slope of third-order IM

products–notice the power of 3rd

order intermod productsincreases 3 times fasterthan the original signal (itsslope is 3 times steeper)

Input Power (dBm)

Power Transfer Characteristicsof typical amplifier or other device

Noise Floor

OutputPower(dBm)

Third orderintercept

point

Third Orderintermodulation

products

Predictedpower

INPUT

Output port



Intermodulation Problem ExamplesCase 1: User Receiver Fundamental Overload

GIVEN:Input power at f1 = - 20 dBmInput power at f2 = - 25 dBm1 dB. Compression @ -35 dBm3rd order intercept pt = -15 dBmSOLUTION:P1 = - 20 dBm, P2 = -25 dBmP3 (f1) = 2 (-20) + (-25) - 2(-15) = - 35 dBmP3 (f2) = 2 (-25) + (-20) - 2(-15) = - 40 dBm

Imagine a cellular handheld is beingused on system “B”at a signal level of-85 dBm.

Next door, there is a cell site ofsystem “B”with just two channels up,delivering -20 and -25 dbm,respectively.

What are the 3rd-order IM levelsgenerated at the front end of thehandheld?

From calculations at right, the 3rd-order IM products are at -35 and -40dbm, respectively.

If one of these IM products happens tofall on the system B channel, it will be45 or 50 db stronger than System“B”!!

C/I (system B) = - 85-(- 35) = - 50dB

f1 @ -20 dBmf2 @ -25 dBm

f3 @-85 dBm

AMP

CDMA(system B)

Power control cannot work due tointerference fromdifferent system

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Intermodulation due to Near-Far Effect(Reverse link)

Intermodulation Interference (IM) becomessevere

Dynamic Power control can be applied atthe mobile transmitter to reduce IM

PiPs

dneardfar

f1 @ -20 dBm

f2 @ -25 dBm

f3 @

-85 dBm

fi = f3 @

-35 or -40 dBm

Powercontrol

f1 f2 fi f3



Intermodulation Problem ExamplesCase 2: Mixing in Corroded Receiving Antenna

GIVEN:Input power at f1 = -10 dBmInput power at f2 = -10 dBmEquiv. 3rd order intercept pt= +25 dBmSOLUTION:P1 = - 10 dBm, P2 = -10 dBmP3 (f1) = 2 (-10) + (-10) - 2(+25) = - 80 dBmP3 (f2) = 2 (-10) + (-10) - 2(+25) = - 80 dBm

Suppose Antenna B at a certain cell site hascorrosion in one element or in a connectoron the coax jumper.•The corrosion acts as a non-linear

conductor with an equivalent 3rd orderintercept as shown.

Only 10 feet away (8.8@870 MHz.) isAntenna A, transmitting frequencies f1 andf2 each at +40 dBm input.•Isolation from Antenna A to Antenna B is

approx. -50 db. IM products of -80 dBm are generated at the

antenna and fed into the cell site receivers! Solutions

•Increase isolation between antennas•Add a filter at receiver B

AntennaA

AntennaB

f1,2 @ +40 dBmCell SiteTransmitters

Cell SiteReceivers

Isolation50 dB

corrosion

f1 f2f3

filter

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Intermodulation Problem ExamplesCase 3: Mixing in Corroded Transmit Antenna

Suppose Antenna A at a certain cell sitehas corrosion in one element or in aconnector on the coax jumper.•The corrosion acts as a non-linear

conductor with an equivalent interceptas shown

IM products of +20 dbm are generatedand radiated

Only 10 feet away (8.8@870 MHz.) isAntenna B, with spacing isolation of 50db

IM products arrive at Antenna B with alevel of --30 dBm and are fed into the cellreceivers

Solutions•Increase isolation between antennas•Add a filter at receiver B•Reduce TX power

AntennaA

Antenna B

f1,2 @ +40 dBmCell SiteTransmitters

Cell SiteReceivers

Isolation50 dB

corrosion

GIVEN:Input power at f1 = +40 dBmInput power at f2 = +40 dBmEquiv. 3rd order intercept pt= +50 dBmSOLUTION:P1 = +40 dBm, P2 = +40 dBmP3 (f1) = 2 (+40) + (40) - 2(+50) = +20 dBmP3 (f2) = 2 (+40) + (40) - 2(+50) = +20 dBm

f1 f2f3

TX Filter



An measurement of IntermodulationMini-circuit mixer 15542 XP-2

Input IM3 = P1 + P/2 P3 = 2P1 + P2 - 2P3i

P = P2 –P3, then input intercept point

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Dynamic Power Control- (DPC) InterferenceReduction

Dynamic Power Control reducestransmitter power when the pathis short:•Prevents IM due to receiver

overload by strong signals on shortpaths

•Reduces interference (CCI+ADJ),since on average, the interferingtransmitters are likely to bepowered down

Power control can be applied atthe mobile transmitter, at the cellsite transmitter, or at both

There are 7 power steps of 4 dBeach in dynamic power control,0(max) to 7 (min) –TDMA system

DPCTH

DPCTL

Distance from Cell

RSSI

DPC disable

DPC enable

• Smallest power to maintain a good qualityon the reverse link

• Prolong battery life• Reduce reverse channel interference• Especially important for CDMA



CDMA Power Control (Up & down link)

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CDMA Power Control



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6

Cochannel Interference LocationsUplink/Reverse Path

Cochannel interferencecan occur on eitheruplink, downlink, or both

On uplink, interferenceoccurs at the cell sitereceiver, from mobiles insurrounding cochannelcells

Dynamic Power Controlof mobile can give C/I ahelpful boost sincemobiles in adjacent cellsstatistically will averagelower, while desired userstill is poweredadequately

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6

Cochannel Interference LocationsDownlink/Forward Path

On the downlink,interference occurs atthe mobile user’sreceiver due to signalsfrom BS in surroundingcochannel cells

Dynamic Power Controlof cell voice channelscan give C/I a verybeneficial extra “boost”since statistically, theinterferer is likely to bepower down, whiledesired user still ispowered adequately



Adjacent-Channel Interference LocationsUplink/Reverse Path

Interference occurs at the cell site receiver•interference user location is the preliminary variable

in determining severity of interferenceOther important factors

•Dynamic power control for mobiles can reducelikelihood that interferer will be stronger thandesired user— Statistically reduces contribution of interfering

mobiles except in worst-case near the commonedge of the two cells

•Handoffs: Keep them tight!!— don’t let interfering mobile drag into Cell A!— Will be at full power and closer than desired user,

who may be dragging into cell B!

Uplink adjacent channel interference casesare less frequent than on downlink, but whenthey occur, they can be more severe.

Cell B

Cell B

Desiredmobile

Page 27



Adjacent-Channel Interference LocationsDownlink/Forward Path

Interference occurs at the mobile receiver Handoff is the primary factor in controlling

downlink adjacent-channel interference If there is a cell that is do much stronger

than the serving cell, why isn’t IT theserving cell?

Cases of downlink adjacent-channelinterference are more frequent but lesssever than uplink cases•Since both cells transmit continuously; they

naturally will be equal in strength at theboundary

•If sever interference occurs, it’s a sign of calldragging: handoffs are too loose.

Cell B

Cell B

Desiredmobile



Frequency Plan ConstraintsWhat are the ground rules?

Ideally, we would like to use every cellularchannel in every cell.Why can we?

Practical Cell Hardware Restrictions•combining multiple transmitters into an

antenna - some inconveniences Interference Restrictions

•Adjacent-channel Interference–Can’t use adjacent channels in the

same cell•Co-channel Interference

–Can’t use same channel for multipleconversations in same cell

–must provide some physical spacingbetween cells using same channel

1Cell395voice

channels

What’s wrong withthis beautiful picture?

Page 28



Frequency Plan ConstraintsTransmitter Combining Restrictions

The number of antennas at a cell site islimited•cost and space considerations•zoning, aesthetic considerations

It is desirable to combine as manytransmitters as possible into oneantenna. Restrictions:•power-handling capability of the antenna

(typically 500 watts)–25-50 channels typical limit

• intermod considerations:– transmitter isolation is required

Solution: various types of combiners• input port for each transmitter• low attenuation thru to antenna port•high attenuation back into other transmitter

ports

Receivers

Duplexer

Combiners

Transmitters

Antennas 21

RF Block Diagram of a Cell Site



Frequency Plan ConstraintsTypes of Transmitter Combiners

Tuned Cavity - manual•minimum frequency separation 21

ch. required for 17 db isolation• requires manual tuning; time-

consuming process• insertion loss 1.5 db for up to 16

inputs Tuned Cavity - Auto-Tune

•minimum frequency separation 21ch. required for 17 db isolation

• fast automatic tuning• insertion loss 1.5 db for up to 16

inputs

Hybrid•any frequency separation OK•Cost of freedom: insertion loss

3.5 db for 2 inputs7 db for 4 inputs10.5 db for 8 inputs14 db for 16 inputs

INPUT1

OUTPUT

INPUT2

-3db

-3db

n4IN

OUT

n4IN

OUT

fc fc+21fc-21frequency

db

0

Tuned CavityFrequency vs. Attenuation

Page 29



C/I is Carrier-to-Interference Ratio AMPS modulation characteristics

require 18 dB co-channel C/I oversingle interferer, for good quality (17dbover multiple interferers )

Between a pair of sites using same channel,three C/I regions exist:•Site A C/I better than 18 dB•neither site gives usable C/I•Site B C/I better than 18 dB

Other sites are needed toserve the region whereneither A nor B has good C/I

Rate of signal decay determines how close the next co-channel site can be,and how many additional sites on other channels are needed between

By careful inspection of this scenario, it is possible to determine therequired separation between co-channel sites to avoid interference

Frequency Plan RestrictionsCo-channel Interference

-120

-110

-100

-90

-80

-70

-60

Distance, km1 3 5 7 9 11 13 15 17 19 21 23 250

Site A Site B-50

C/I = 18 db C/I = 18 db

GoodService

GoodServiceInterference

RSSI,dBm

Frequency Reuse ScenarioOther sites



Frequency ReuseDetermining Required D/R Ratio

Setting up a co-channel cell as close aspossible without interference

When laying out a new system or a coverageexpansion, propagation prediction and/ormeasurement data are used to develop amodel for the coverage of an average cell(Okumura, etc.)

At some distance R from the cell A, the signaldrops to the minimum acceptable level forcoverage. R = coverage radius

By the distance dINTERF, the signal hasdropped an additional number of db equal tothe required C/I (18 dB)

If a new cell B on the same channel is distantfrom Cell A by the amount R + dINTERF, thedesired C/I will exist for Cell A all the way outto the distance R.

Distance D = R + dINTERF is the smallestusable separation for co-channel sites in thispropagation environment.

-120

-110

-100

-90

-80

-70

-60

Distance0

Site A Site B-50

RSSI,dBm

Frequency Reuse Scenario

R

dINTERF

dINTERF

D = R + dINTERF

C/I

R = Radius of Serving CellD = smallest usable distance

to co-channel Cell

Page 30



Frequency Planning Implications forTDMA

Digital System AcceptablePerformance under Combined

ACI and C/I

9 11 13 15 17 19 21 23

-16

2

0

-2

-4

-8

-10

-12

-14

Area of AcceptablePerformance

C/I dB

ACIdB

Frequency plans which work well foranalog systems generally willprovide good performance on TDMAsystems.

However, TDMA and digital systemsin general have definite bit error ratethresholds which must not beexceeded.

The figure at right shows therelationship of adjacent-channelinterference (ACI) and co-channelinterference (C/I) which should beobserved for TDMA systems.•Note that negative ACI indicates the

adjacent channel interferer is stronger thanthe desired signal



Summary of Frequency Planning Rules

Can not use adjacent channels at thesame cell•Adjacent channels OK in adjacent cells so

long as prompt handoff available

Two cells using the same channel (co-channel cells) must be separatedgeographically to preserve at least 18 dBC/I at their service boundaries•determine required geographic separation D/R

from propagation analysis

Channels used by transmitters feeding thesame antenna must be separated infrequency sufficiently to allow combinersto provide isolation•or, hybrid combiners or other non-frequency-

critical technique must be used

DR

D/R

Receivers

Duplexer

Combiners

Transmitters

Antennas 21

fcfc+21

fc-21 frequency

0

F

Page 31



Capacity Improvement

Cell splitting: increases the number of base stations Sectoring: relies on BS antenna placement to reduce CCI Zone microcell: relies on BS antenna placement to reduce CCI

Cell splitting Sectoring Zone microcell



Sectorization Advantages

•Cell radius unchanged•CCI D/R , cluster size N , frequency

reuse , capacity •Omni-directional antenna Directional

antenna•Three 120o sectors, six 60o sectors•For N=7 and 120o sectors, number of CCI

decreases from 6 to 2. C/I = 24.2dB.•C/I =18 dB < (sectoring C/I = 24.2dB, N=7). =

N 12 (omni worst case), Increase incapacity 12/7

•Downtilting the sector antenna to reduce CCI Disadvantages

•Number of antenna •Trunking efficiency , channels 3 groups•Handoffs , not a major concerns since it

occurs within the same cell withoutintervention from MSC

Page 32



Radiation PatternsKey Features and Terminology

Radiation patterns of antennas are usuallyplotted in polar form

The Horizontal Plane Pattern showsthe radiation as a function of azimuth(i.e.,direction N-E-S-W)

The Vertical Plane Pattern shows theradiation as a function of elevation(i.e., up, down, horizontal)

Antennas are often compared bynoting specific features on theirpatterns:•-3 db (“HPBW”), -6 db, -10 db points•front-to-back ratio•angles of nulls, minor lobes, etc.

Typical ExampleHorizontal Plane Pattern

0 (N)

90(E)

180 (S)

270(W)

0

-10

-20

-30 db

Notice -3 dB points

Front-to-back Ratio

10 dbpoints

MainLobe

a MinorLobe

nulls orminima



Antenna DowntiltWhat’s the goal?

Downtilt is commonly used fortwo reasons:

1. Reduce Interference•reduce radiation toward a distant

co-channel cell•concentrate radiation within the

serving cell 2. Prevent overshoot

•Improve coverage of nearbytargets far below the antenna–otherwise within “null”of

antenna pattern

Are these good strategies? How is downtilt applied?

Scenario 2

Cell A

Scenario 1

Cell B

Page 33



Sector Antenna

Dallas, Texas, USA



Sector Antenna

FJU, Taiwan Lin-Ko, Taiwan

Page 34



Rationale for Sectorization

Sectorization is a tool for moretightly controlling frequencyutilization in a cellular system

We saw that if the number ofchannels remains constant,sectorization actually reduces thecapacity of a cell

So, why would anyone want tosectorize?•In hope of being able to reduce N•To substantially improve C/I, even if N

is not changed•To gain flexibility to control traffic

distribution and reduce interference attroublesome boundaries where largecells and small cells meet

45

1515

15

35.61erlangs

27.03erlangs

N=7 Omni

N=4Sector?



Comparison of Typical CoverageUsing Omni and Sector Antennas

-95 dBm

-113 dBm

Miles0 5 10 15 20 25

Coverage ComparisonUsing Sector and Omni Antennas

• ERP = 100w• Ant. Ht. 150 ft.• DB-833 vs Omni Whip

The figure shows computed coveragein miles for•an omnidirectional collinear vertical

antenna and•a panel antenna typically used for sector

applications•Computation used Okumura-Hata

formula from Lesson 3 -95 dbm is typical design limit for

edge of a cell -113 dbm is interfering contour which

would deliver 18 dB C/I at a distantcell edge (-95 dbm)

Notice how substantially bothcoverage and interference aresuppressed off the back of the sectorantenna

Page 35



Sectorization Improves Reuse Density In a system where N=7 omni works well,

N=4 120 sector may be feasible•possible 55% increase in capacity

Problems and additional considerations:•increased system complexity•handoffs & handovers very critical to achieving

acceptable performance•possibility of specific local propagation

conditions unsuitable for sectorization•cost of sectorization

N=7 Omni Plan

N=4 120ºSector Plan

-95 dBm-113 dBm

C/I = 18 dB

N=7Omni

N=4 120ºSector

Voice Channels

Total voice channels 312( ignoring expanded spectrum )

45/cell 26/sector78/cell

Capacity, Erlangs 35.6 18.4/sector55.2/cell

N=7/N=4Comparison



N = 7, 120 DEG. Sectorized Cell Plan

CHARACTERISTICS:

A CLUSTER OF 7 CELLS

3 SECTORS / CELL

TOTAL NUMBER OF SECTORS = 7 x 3 = 21

REQUIRES 21 FREQUENCY GROUPS

USES DIRECTIONAL ANTENNAS

1a

1c

1b

2a

2b

2c3a

3b

3c

4a

4b

4c5a

5b

5c

6a

6b

6c

7a

7b

7c

Page 36



A-BAND N=7 CHANNEL SETSChannel Set 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21Designations A1 B1 C1 D1 E1 F1 G1 A2 B2 C2 D2 E2 F2 G2 A3 B3 C3 D3 E3 F3 G3

Control Ch. 333 332 331 330 329 328 327 326 325 324 323 322 321 320 319 318 317 316 315 314 313

Voice 312 311 310 309 308 307 306 305 304 303 302 301 300 299 298 297 296 295 294 293 292Channels 291 290 289 288 287 286 285 284 283 282 281 280 279 278 277 276 275 274 273 272 271

270 269 268 267 266 265 264 263 262 261 260 259 258 257 256 255 254 253 252 251 250249 248 247 246 245 244 243 242 241 240 239 238 237 236 235 234 233 232 231 230 229228 227 226 225 224 223 222 221 220 219 218 217 216 215 214 213 212 211 210 209 208207 206 205 204 203 202 201 200 199 198 197 196 195 194 193 192 191 190 189 188 187186 185 184 183 182 181 180 179 178 177 176 175 174 173 172 171 170 169 168 167 166165 164 163 162 161 160 159 158 157 156 155 154 153 152 151 150 149 148 147 146 145144 143 142 141 140 139 138 137 136 135 134 133 132 131 130 129 128 127 126 125 124123 122 121 120 119 118 117 116 115 114 113 112 111 110 109 108 107 106 105 104 103102 101 100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 8281 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 6160 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 4039 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 1918 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

Expanded 1023 1022 1021Spectrum A' 1020 1019 1018 1017 1016 1015 1014 1013 1012 1011 1010 1009 1008 1007 1006 1005 1004 1003 1002 1001 1000

999 998 997 996 995 994 993 992 991

Expanded 716 715 714 713 712 711 710 709 708 707 706 705Spectrum A" 704 703 702 701 700 699 698 697 696 695 694 693 692 691 690 689 688 687 686 685 684

683 682 681 680 679 678 677 676 675 674 673 672 671 670 669 668 667

Set Channel Count Summary 416Control 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Normal A 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 14 14 14A" 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 2 2 2A' 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 2 2 2 2

Total Voice 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 18 18 18 18



Comparison Summary ofPopular Frequency Plans

Voice ChannelsPer CellFrequency Plan Capacity

Erlangs per CellAdjacent

Channels? Complexity

44N=7 Omni 34.7 Yes. GoodHandoffs!! Simple

33N=9 Omni 34.7 clean Simple

42N=7 120o Sector 32.8 clean Moderate



78N=4 60o Sector 44.4 clean Very Difficult

96N=3 60o Sector 59 clean Very Difficult

Page 37



A comparison ofRegular antenna and smart antennas



Several kinds of intelligence insmart antennas

Page 38



Gain in Smart Antenna

SIR is boost in general ( Experiment has show 10dB increase ) givepossibility for reduce frequency reuse distanc



Smart Antenna Block

Page 39



Multi-beams antennas



Smart Antenna Demo

Page 40



Smart Antenna Components

Multi-LinePhase Shifter

Combiner/Divider



Microcell zone

Structures•Three zone sites•A single BS•Share the same equipment•A mobile is shared by the zone with the

strongest signal•Any channel may be assigned to any zone•MS from one zone to another retains the

same channel•Useful along high way or urban traffic

corridors

Advantages•Handoff is not required•BS is localized similar to sectoring CCI

, TX power , Capacity •Capacity without reducing trunking

efficiency

Page 41



A Microcell Zone Example

Each group of threehexagons represents a cell

Omni-cell: N=7, D/R=4.6, S/I =18dB

For a constant value of SIR =18dB.•Microcell zone Dz/Rz = 4.6

–S/I 20dB in worst case•Omni-cell D/R = 3 N=3•Capacity increase 7/3=2.33

No loss in trunkingefficiency

Are adopted in many cellularand PCS systems

One zone

One cell



Trunking and GOS

Trunking concepts•Allow a large number of users to share the relatively small number of channels•Statistical behavior of users•Determine the number of telephone circuits in designing cellular radio•Handle a specific capacity at a specific “grade of service”based on

–Trunking Theory–Queuing Theory

GOS (Grade of Service)•A measure of the ability to access a trucked system during the busiest hour

–Specified as the blocking probability. Ex: 2%

Objectives: wireless designer’s job is to estimate maximum requiredcapacity and allocate the proper number of channels

Page 42



Traffic Engineering ObjectivesTraffic engineering is the intelligent art of

keeping both system customers andaccountants happy.

Traffic engineering finds answers toquestions at every stage in thedevelopment of a cellular system

In Initial Design:•How many cells are needed?•What size switching resources?•How many T-1s, how much microwave?

Ongoing during Operation:•How many radios for each cell or sector?•When are new cells needed for capacity?

7

8

9

1

3

2

1

3

24

5

6

7

8

9

7

8

9

1

3

2

6

4

6

10

11

$



Walking a Fine LineThe traffic engineer must walk a fine

line between two problems:Overdimensioning

•too much cost•insufficient resources to construct•traffic revenue is too low to support

costs•system operator may fail or get new

traffic engineer Underdimensioning

•blocking•poor technical performance

(interference)•capacity for billable revenue is low•revenue is low due to poor quality•users unhappy, cancel service•system operator may fail or get new

traffic engineer

Page 43



Basics of Traffic EngineeringTerminology & Concept of a Trunk

Traffic engineering in telephony is focused on the voice paths whichusers occupy. They are called by many different names:•trunks•circuits•radios (remember in TDMA, a radio may carry up to 3 Circuits?

Some other common terms are:•trunk group

–a trunk group is several trunks going to the same destination, combinedand addressed in switch translations as a unit , for traffic routingpurposes

•member–one of the trunks in a trunk group



Traffic Engineering

Erlang: Amount of traffic intensity carried by a channel that compleelyoccupied•Example: 0.5 Erlang of traffic A radio channel is occupied for 30 minutes during an

hour•Traffic intensity (each user) = call request rate call holding time

i.e. Au = H (Erlangs)•Total traffic intensity (U users) = A = u Au

•Traffic intensity per channel = Ac = u Au / C (C channels)

Maximum possible carried traffic is the number of channels C in Erlang AMP cellular: GOS = 2% blocking = 2 out of 100 calls will be blocked

during the busiest hour Trunking Efficiency: Number of users which can be offered a particular

GOS with a particular configuration of fixed channels.

Page 44



Basics of Traffic EngineeringOffered Traffic and Call Duration

A = C x TA = Offered Traffic (Erlangs)C = Average number of calls

per unit of timeT = Average call duration

Offered traffic is theamount of traffic usersattempt to transmitthrough the system.

N Trunks

1 hour

Offered Traffic,Erlangs

Example:C = 1000 call attempts in the busy hourT = 150 seconds average call durationWhat’s the offered traffic?

Solution:A = C x T

= 1000 x ( 150 / 3600 )= 41.667 Erlangs



Basic Trucked systems

Erlang B Formula (Table 2.4, Fig 2.6): No queuing•No setup (allocate a channel) time•Immediate Acess to a channel if one is available•The call is blocked if no channels are available try again later•Is called blocked calls cleared•Based on M/M/m queue formula using Poisson and others

Erlang C Formula(Fig. 2.7): A queue is provided to hold blocked calls•Call request may be delayed until a channel becomes available•Is called blocked call delayed•GOS = Pr(delay > t)=Pr (delay>0) Pr(delay>t | delay>0)

=Pr(delay>0|)exp(-c(C-A)t/H)•Average delay D = Pr(delay >0) H/ (C-A)•GOS = Pr (delay>0) with Erlang C table

Page 45



Equation behind the Erlang-B Table

Pn(A) =

An

n!

1 + + ... +A1!

An

n!

Pn(A) = Blocking Rate (%)with n trunksas function of traffic A

A = Traffic (Erlangs)n = Number of Trunks

Offered Trafficlost due toblocking

Numberof

Trunks

time

max # oftrunks

average# of busychannelsOffered

Traffic,A

The Erlang-B formula is fairly simple to implement on hand-held programmable calculators, in spreadsheets, or popularprogramming languages.



Erlang-B

EX: Channels aregrouped

GOS = 0.01

• 10 channels

A = 4.46

• 2 x 5 channels

A= 2 x 1.36

= 2.72

• 60% Loss

• Total users

Page 46



Erlang-C



Erlang-B Traffic TablesAbbreviated - For P.02 Grade of Service Only

#Trunks Erlangs #Trunks Erlangs #Trunks #Trunks Erlangs #Trunks Erlangs #Trunks Erlangs #Trunks Erlangs #TrunksErlangs

1 0.0204 26 18.4 51 41.2 76 64.9 100 88 150 136.8 200 186.2 250 235.82 0.223 27 19.3 52 42.1 77 65.8 102 89.9 152 138.8 202 188.1 300 285.73 0.602 28 20.2 53 43.1 78 66.8 104 91.9 154 140.7 204 190.1 350 335.74 0.109 29 21 54 44 79 67.7 106 93.8 156 142.7 206 192.1 400 385.95 1.66 30 21.9 55 44.9 80 68.7 108 95.7 158 144.7 208 194.1 450 436.16 2.28 31 22.8 56 45.9 81 69.6 110 97.7 160 146.6 210 196.1 500 486.47 2.94 32 23.7 57 46.8 82 70.6 112 99.6 162 148.6 212 198.1 600 587.28 3.63 33 24.6 58 47.8 83 71.6 114 101.6 164 150.6 214 200 700 688.29 4.34 34 25.5 59 48.7 84 72.5 116 103.5 166 152.6 216 202 800 789.310 5.08 35 26.4 60 49.6 85 73.5 118 105.5 168 154.5 218 204 900 890.611 5.84 36 27.3 61 50.6 86 74.5 120 107.4 170 156.5 220 206 1000 999.112 6.61 37 28.3 62 51.5 87 75.4 122 109.4 172 158.5 222 208 1100 109313 7.4 38 29.2 63 52.5 88 76.4 124 111.3 174 160.4 224 21014 8.2 39 30.1 64 53.4 89 77.3 126 113.3 176 162.4 226 21215 9.01 40 31 65 54.4 90 78.3 128 115.2 178 164.4 228 213.916 9.83 41 31.9 66 55.3 91 79.3 130 117.2 180 166.4 230 215.917 10.7 42 32.8 67 56.3 92 80.2 132 119.1 182 168.3 232 217.918 11.5 43 33.8 68 57.2 93 81.2 134 121.1 184 170.3 234 219.919 12.3 44 34.7 69 58.2 94 82.2 136 123.1 186 172.4 236 221.920 13.2 45 35.6 70 59.1 95 83.1 138 125 188 174.3 238 223.921 14 46 36.5 71 60.1 96 84.1 140 127 190 176.3 240 225.922 14.9 47 37.5 72 61 97 85.1 142 128.9 192 178.2 242 227.923 15.8 48 38.4 73 62 98 86 144 130.9 194 180.2 244 229.924 16.6 49 39.3 74 62.9 99 87 146 132.9 196 182.2 246 231.825 17.5 50 40.3 75 63.9 100 88 148 134.8 198 184.2 248 233.8

Erlangs

Page 47



Examples:

Ex. 1 Erlang B: GOS = 0.5% boloking, Number of trunked channels =10•Each user generates 0.1 Erlangs of traffic Au = 0.1 Erlangs•A= 3.96 from Erlang-B, Total number of users = A/Au=3.96/0.139 users

Ex 2 Erlang C: A hexagonal cell within a 4-cell system, radius = 1.387km. Total number of channels = 60. The load per user = 0.029Erlangs, call rate = 1 call/hour, GOS= 5%•R= 1.387 km Area = 5km2

•N= 4, channels per cell = 60/4=15 channels•GOS = 5% probability of delay with C= 15, A= 8.8 Erlangs•Number of users = 8.8/0.029 = 303 users = 302/ 5km2 = 60/ km2

•= 1, call holding time = H = 0.029/1 hour=104.4 seconds•Pr(delay>10|delay>0)=exp(-(c-A)t/H)=exp(-(15-8.8)10/104.4)=52.22%•GOS = 0.05 = Pr(delay>0) Pr(delay>10) = 0.05 52.22=2.76%



Trunking EfficiencyAn Important Cellular Application

Busy cellular systems often usesectorized cells• A cell coverage area is divided into

several sectors using directional antennas–3-sector (120-degrees)–6-sector (60-degrees)

•radio channels assigned per sector Capacity of a sectorized cell is less

than capacity of an omni cell withsame total number of channels•45 channels: 35.61 Erlangs•3 x 15 channels: 3 x 9.01 erl.

= 27.03 ErlangsWhy would anyone sectorize?

• Sectorization eases frequency reusemore than it hurts capacity

45 channels

15 ch.15 ch.

15 ch.

blocking probability: 2%

Page 48



Lesson 3 Complete

Documents

Chapter 3 Cellular Concept - fju.edu.t€¦ · Practical handoff considerations •Mobile velocities –for high speed vehicles, a handoff may be never needed during a call, particularly