Chapter 3 Cellular Concept - fju.edu.t€¦ · Practical handoff considerations •Mobile...
Transcript of Chapter 3 Cellular Concept - fju.edu.t€¦ · Practical handoff considerations •Mobile...
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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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Wireless Communication
Chapter 3 - Cellular Concept 2 Dr. Sheng-Chou Lin
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
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Chapter 3 - Cellular Concept 3 Dr. Sheng-Chou Lin
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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Chapter 3 - Cellular Concept 4 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 5 Dr. Sheng-Chou Lin
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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Wireless Communication
Chapter 3 - Cellular Concept 6 Dr. Sheng-Chou Lin
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
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Chapter 3 - Cellular Concept 7 Dr. Sheng-Chou Lin
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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Wireless Communication
Chapter 3 - Cellular Concept 8 Dr. Sheng-Chou Lin
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 .
Wireless Communication
Chapter 3 - Cellular Concept 9 Dr. Sheng-Chou Lin
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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N=3
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Chapter 3 - Cellular Concept 10 Dr. Sheng-Chou Lin
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
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Chapter 3 - Cellular Concept 11 Dr. Sheng-Chou Lin
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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Chapter 3 - Cellular Concept 12 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 13 Dr. Sheng-Chou Lin
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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Chapter 3 - Cellular Concept 14 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 15 Dr. Sheng-Chou Lin
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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Wireless Communication
Chapter 3 - Cellular Concept 16 Dr. Sheng-Chou Lin
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.
Wireless Communication
Chapter 3 - Cellular Concept 17 Dr. Sheng-Chou Lin
Soft Handoff Considerations
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Wireless Communication
Chapter 3 - Cellular Concept 18 Dr. Sheng-Chou Lin
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
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Chapter 3 - Cellular Concept 19 Dr. Sheng-Chou Lin
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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Wireless Communication
Chapter 3 - Cellular Concept 20 Dr. Sheng-Chou Lin
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)
Wireless Communication
Chapter 3 - Cellular Concept 21 Dr. Sheng-Chou Lin
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
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Chapter 3 - Cellular Concept 22 Dr. Sheng-Chou Lin
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
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Chapter 3 - Cellular Concept 23 Dr. Sheng-Chou Lin
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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Mobile is on the border
dBm = 10log (P/1mW)dBW = 10log (P/1W)
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Chapter 3 - Cellular Concept 24 Dr. Sheng-Chou Lin
optimistic result
SIR v.s.System capacity Calculation (2)
=
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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Chapter 3 - Cellular Concept 25 Dr. Sheng-Chou Lin
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
SIR v.s.System capacity Calculation (3)
=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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Wireless Communication
Chapter 3 - Cellular Concept 26 Dr. Sheng-Chou Lin
Outage Probability
Outage Probability: P(SIR > SIRo): SIRo Threshold•(N=7, 120o, l = 8dB and r=4)
Wireless Communication
Chapter 3 - Cellular Concept 27 Dr. Sheng-Chou Lin
Outage Probability
Outage Probability: P(SIR > SIRo): SIRo Threshold•(N=7, 120o, l = 8dB and r=4)
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Wireless Communication
Chapter 3 - Cellular Concept 28 Dr. Sheng-Chou Lin
Percentage of area
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Chapter 3 - Cellular Concept 29 Dr. Sheng-Chou Lin
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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Wireless Communication
Chapter 3 - Cellular Concept 30 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 31 Dr. Sheng-Chou Lin
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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Wireless Communication
Chapter 3 - Cellular Concept 32 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 33 Dr. Sheng-Chou Lin
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
1
1
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Wireless Communication
Chapter 3 - Cellular Concept 34 Dr. Sheng-Chou Lin
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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Wireless Communication
Chapter 3 - Cellular Concept 35 Dr. Sheng-Chou Lin
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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Wireless Communication
Chapter 3 - Cellular Concept 36 Dr. Sheng-Chou Lin
15
23
4
15
2
12
12
31
5
23
4
15
4
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
•Each cell has 4 adjacent channelneighbors
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
•Each cell has 2 adjacent channelneighbors
1 56
23
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1 56
23
4
1 56
4
16
2
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3
61
23
4
5
1
A Tour of Reuse Factor N
Wireless Communication
Chapter 3 - Cellular Concept 37 Dr. Sheng-Chou Lin
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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Wireless Communication
Chapter 3 - Cellular Concept 38 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 39 Dr. Sheng-Chou Lin
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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Wireless Communication
Chapter 3 - Cellular Concept 40 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 41 Dr. Sheng-Chou Lin
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
Page 22
Wireless Communication
Chapter 3 - Cellular Concept 42 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 43 Dr. Sheng-Chou Lin
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
Page 23
Wireless Communication
Chapter 3 - Cellular Concept 44 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 45 Dr. Sheng-Chou Lin
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
Page 24
Wireless Communication
Chapter 3 - Cellular Concept 46 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 47 Dr. Sheng-Chou Lin
CDMA Power Control (Up & down link)
Page 25
Wireless Communication
Chapter 3 - Cellular Concept 48 Dr. Sheng-Chou Lin
CDMA Power Control
Wireless Communication
Chapter 3 - Cellular Concept 49 Dr. Sheng-Chou Lin
1
2
3
4
5 6
7
4
6
1
1
1
1
1
14
7 2
7
2
5
4
7
3
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
Page 26
Wireless Communication
Chapter 3 - Cellular Concept 50 Dr. Sheng-Chou Lin
1
2
3
4
5 6
7
4
6
1
1
1
1
1
14
7 2
7
2
5
4
7
3
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
Wireless Communication
Chapter 3 - Cellular Concept 51 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 52 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 53 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 54 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 55 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 56 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 57 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 58 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 59 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 60 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 61 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 62 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 63 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 64 Dr. Sheng-Chou Lin
Sector Antenna
Dallas, Texas, USA
Wireless Communication
Chapter 3 - Cellular Concept 65 Dr. Sheng-Chou Lin
Sector Antenna
FJU, Taiwan Lin-Ko, Taiwan
Page 34
Wireless Communication
Chapter 3 - Cellular Concept 66 Dr. Sheng-Chou Lin
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?
Wireless Communication
Chapter 3 - Cellular Concept 67 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 68 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 69 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 70 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 71 Dr. Sheng-Chou Lin
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
48N=9 120o Sector 38.4 clean Moderate
78N=4 120o Sector 55.2 clean Moderate
78N=4 60o Sector 44.4 clean Very Difficult
96N=3 60o Sector 59 clean Very Difficult
Page 37
Wireless Communication
Chapter 3 - Cellular Concept 72 Dr. Sheng-Chou Lin
A comparison ofRegular antenna and smart antennas
Wireless Communication
Chapter 3 - Cellular Concept 73 Dr. Sheng-Chou Lin
Several kinds of intelligence insmart antennas
Page 38
Wireless Communication
Chapter 3 - Cellular Concept 74 Dr. Sheng-Chou Lin
Gain in Smart Antenna
SIR is boost in general ( Experiment has show 10dB increase ) givepossibility for reduce frequency reuse distanc
Wireless Communication
Chapter 3 - Cellular Concept 75 Dr. Sheng-Chou Lin
Smart Antenna Block
Page 39
Wireless Communication
Chapter 3 - Cellular Concept 76 Dr. Sheng-Chou Lin
Multi-beams antennas
Wireless Communication
Chapter 3 - Cellular Concept 77 Dr. Sheng-Chou Lin
Smart Antenna Demo
Page 40
Wireless Communication
Chapter 3 - Cellular Concept 78 Dr. Sheng-Chou Lin
Smart Antenna Components
Multi-LinePhase Shifter
Combiner/Divider
Wireless Communication
Chapter 3 - Cellular Concept 79 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 80 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 81 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 82 Dr. Sheng-Chou Lin
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
$
Wireless Communication
Chapter 3 - Cellular Concept 83 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 84 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 85 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 86 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 87 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 88 Dr. Sheng-Chou Lin
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.
Wireless Communication
Chapter 3 - Cellular Concept 89 Dr. Sheng-Chou Lin
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
Wireless Communication
Chapter 3 - Cellular Concept 90 Dr. Sheng-Chou Lin
Erlang-C
Wireless Communication
Chapter 3 - Cellular Concept 91 Dr. Sheng-Chou Lin
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
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Wireless Communication
Chapter 3 - Cellular Concept 92 Dr. Sheng-Chou Lin
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%
Wireless Communication
Chapter 3 - Cellular Concept 93 Dr. Sheng-Chou Lin
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%
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Wireless Communication
Chapter 3 - Cellular Concept 94 Dr. Sheng-Chou Lin
Lesson 3 Complete