Cellular concepts

Ch t 3Chapter 3

The Cellular Concept - System Designp y gFundamentals

I. Introduction

Goals of a Cellular System High capacity L Large coverage area Efficient use of limited spectrum

Large coverage area - Bell system in New York City g g y yhad early mobile radio Single Tx, high power, and tall tower Low cost Low cost Large coverage area - Bell system in New York City had 12

simultaneous channels for 1000 square miles S ll # Small # users Poor spectrum utilization

What are possible ways we could increase the number

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p yof channels available in a cellular system?

Cellular concept Frequency reuse pattern

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Cells labeled with the same letter use the same group of channels.

C ll Cl f N ll i l f Cell Cluster: group of N cells using complete set of available channels

Many base stations lower power and shorter Many base stations, lower power, and shorter towers

Small coverage areas called “cells”g Each cell allocated a % of the total number of

available channels Nearby (adjacent) cells assigned different channel

groups t t i t f b t i hb i b

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to prevent interference between neighboring base stations and mobile users

Same frequency channels may be reused by cells a “reasonable” distance away reused many times as long as interference between same reused many times as long as interference between same

channel (co-channel) cells is < acceptable level As frequency reuse↑→ # possible simultaneous

users↑→ # subscribers ↑→ but system cost ↑ (moreusers↑→ # subscribers ↑→ but system cost ↑ (more towers)

To increase number of users without increasing radio frequency allocation reduce cell sizes (more basefrequency allocation, reduce cell sizes (more base stations) ↑→ # possible simultaneous users ↑

The cellular concept allows all mobiles to be f d h f f imanufactured to use the same set of freqencies

*** A fixed # of channels serves a large # of users by reusing channels in a coverage area ***

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by reusing channels in a coverage area ***

II. Frequency Reuse/Planning

Design process of selecting & allocating Design process of selecting & allocating channel groups of cellular base stations

Two competing/conflicting objectives: Two competing/conflicting objectives:1) maximize frequency reuse in specified area2) minimize interference between cells2) minimize interference between cells

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Cells base station antennas designed to cover specific cell

area hexagonal cell shape assumed for planning simple model for easy analysis → circles leave gaps actual cell “footprint” is amorphous (no specific shape) where Tx successfully serves mobile unit where Tx successfully serves mobile unit

base station location cell center → omni-directional antenna (360° coverage) cell center → omni directional antenna (360 coverage) not necessarily in the exact center (can be up to R/4

from the ideal location)

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cell corners → sectored or directional antennas on 3 corners with 120° coverage. very commom Note that what is defined as a “corner” is

somewhat flexible → a sectored antenna coverssomewhat flexible → a sectored antenna covers 120° of a hexagonal cell.

So one can define a cell as having three antennas gin the center or antennas at 3 corners.

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III. System Capacity

S : total # of duplex channels available for use in a given area; determined by:g ; y amount of allocated spectrum channel BW → modulation format and/or standard

specs. (e.g. AMPS) k : number of channels for each cell (k < S)( ) N : cluster size → # of cells forming cluster S = k N S k N

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M : # of times a cluster is replicated over a geographic coverage area

System Capacity = Total # Duplex Channels = C System Capacity Total # Duplex Channels C

C = M S = M k N( i l MN ll ill h )(assuming exactly MN cells will cover the area)

If cluster size (N) is reduced and the geographic area for each cell is kept constant:p The geographic area covered by each cluster is smaller, so

M must ↑ to cover the entire coverage area (more clusters needed).)

S remains constant. So C ↑ The smallest possible value of N is desirable to maximize

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The smallest possible value of N is desirable to maximize system capacity.

Cluster size N determines: distance between co-channel cells (D)( ) level of co-channel interference A mobile or base station can only tolerate so much y

interference from other cells using the same frequency and maintain sufficient quality.

large N → large D → low interference → but small M and low C !

T d ff i lit d l t i Tradeoff in quality and cluster size. The larger the capacity for a given geographic area,

the poorer the quality

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the poorer the quality.

Frequency reuse factor = 1 / N each frequency is reused every N cellsq y y each cell assigned k S / N

N cells/cluster N cells/cluster connect without gaps specific values are required for hexagonal geometryspecific values are required for hexagonal geometry N = i2 + i j + j2 where i, j ≧ 1 Typical N values → 3, 4, 7, 12; (i, j) = (1,1), (2,0),

(2,1), (2,2)

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To find the nearest co-channel neighbors of a particular cell (1) Move i cells along any chain of hexagons, then (2)

turn 60 degrees and move j cells.

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IV. Channel Assignment Strategies

Goal is to minimize interference & maximize use of capacity l i t f ll ll N t b d t lower interference allows smaller N to be used → greater

frequency reuse → larger C

Two main strategies: Fixed or Dynamicg y Fixed each cell allocated a pre-determined set of voice channels calls within cell only served by unused cell channels all channels used → blocked call → no service

several variations several variations MSC allows cell to borrow a VC (that is to say, a FVC/RVC

pair) from an adjacent cell

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donor cell must have an available VC to give

Dynamic channels NOT allocated permanently call request → goes to serving base station → goes

to MSC MSC allocates channel “on the fly” allocation strategy considers: lik lih d f f t ll bl ki i th ll likelihood of future call blocking in the cell reuse distance (interference potential with other cells

that are using the same frequency) channel frequency

All frequencies in a market are available to be used

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Advantage: reduces call blocking (that is to say, it increases the trunking capacity), and g p y),increases voice quality

Disadvantage: increases storage & g gcomputational load @ MSC requires real-time data from entire network related q

to: channel occupancy traffic distribution Radio Signal Strength Indications (RSSI's) from all

channels

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channels

V. Handoff Strategies

Handoff: when a mobile unit moves from one cell to another while a call is in progress, the p g ,MSC must transfer (handoff) the call to a new channel belonging to a new base station new voice and control channel frequencies very important task → often given higher priority

than new call It is worse to drop an in-progress call than to deny a

new onenew one

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Minimum useable signal level Minimum useable signal level lowest acceptable voice quality call is dropped if below this levelc s d opped be ow s eve specified by system designers typical values → -90 to -100 dBmyp

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Quick review: Decibels

S = Signal power in WattsPower of a signal in decibels (dBW) is Psignal = 10 log10(S)

Remember dB is used for ratios (like S/N)dBW is used for Watts

dBm = dB for power in milliwatts = 10 log10(S x 103)dBm = 10 log10(S) + 10 log10(103) = dBW + 30

90 dBm = 10 log10(S x 103)-90 dBm = 10 log10(S x 103)10-9 = S x 103

S = 10-12 Watts = 10-9 milliwatts-90 dBm = -120 dBW90 dBm 120 dBW

Signal-to-noise ratio:N = Noise power in Watts

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pS/N = 10 log10(S/N) dB (unitless raio)

choose a (handoff threshold) > (minimum useable signal level)g ) so there is time to switch channels before level

becomes too low as mobile moves away from base station and

toward another base station

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Handoff Margin △ △ = Phandoff threshold - Pminimum usable signal dB carefully selected △ too large → unnecessary handoff → MSC loaded down △ too small → not enough time to transfer → call dropped! △ too small → not enough time to transfer → call dropped!

A dropped handoff can be caused by two factors not enough time to perform handoffg p delay by MSC in assigning handoff high traffic conditions and high computational load on MSC

can cause excessive delay by the MSCcan cause excessive delay by the MSC no channels available in new cell

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Handoff Decision signal level decreases due to signal fading → don’t handoff mobile moving away from base station → handoff

t it i d i l t th i d must monitor received signal strength over a period of time → moving average

time allowed to complete handoff depends on t e a owed to co p ete a do depe ds omobile speed large negative received signal strength (RSS) slope →

high speed → quick handoffhigh speed → quick handoff statistics of the fading signal are important to

making appropriate handoff decisions → Chapters

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g pp p p4 and 5

1st Generation Cellular (Analog FM → AMPS) Received signal strength (RSS) of RVC measuredReceived signal strength (RSS) of RVC measured

at base station & monitored by MSC A spare Rx in base station (locator Rx) monitors

RSS of RVC's in neighboring cells Tells Mobile Switching Center about these mobiles and

their channelstheir channels Locator Rx can see if signal to this base station is

significantly better than to the host base stationg y MSC monitors RSS from all base stations &

decides on handoff

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2nd Generation Cellular w/ digital TDMA (GSM, IS-136) Mobile Assisted HandOffs (MAHO) important advancement The mobile measures the RSS of the FCC’s from

adjacent base stations & reports back to serving base stationstation

if Rx power from new base station > Rx power from serving (current) base station by pre-determined margin for a long enough time period → handoff initiated by MSC

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MSC no longer monitors RSS of all channels d t ti l l d id bl reduces computational load considerably enables much more rapid and efficient handoffs imperceptible to user imperceptible to user

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A mobile may move into a different system controlled by a different MSC Called an intersystem handoff What issues would be involved here?

Prioritizing Handoffs I P i d G d f S i (GOS) i Issue: Perceived Grade of Service (GOS) – service

quality as viewed by users “quality” in terms of dropped or blocked calls (not q y pp (

voice quality) assign higher priority to handoff vs. new call request a dropped call is more annoying than an occasional

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a dropped call is more annoying than an occasional blocked call

Guard Channels % of total available cell channels exclusively set % of total available cell channels exclusively set

aside for handoff requests makes fewer channels available for new call

requests a good strategy is dynamic channel allocation (not

fixed) adjust number of guard channels as needed by demand so channels are not wasted in cells with low traffic so channels are not wasted in cells with low traffic

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Queuing Handoff Requests use time delay between handoff threshold and y

minimum useable signal level to place a blocked handoff request in queue

a handoff request can "keep trying" during that time period, instead of having a single block/no block decisiondecision

prioritize requests (based on mobile speed) and handoff as needed

calls will still be dropped if time period expires

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VI. Practical Handoff Considerations

Problems occur because of a large range of Problems occur because of a large range of mobile velocities pedestrian vs. vehicle userpedestrian vs. vehicle user

Small cell sizes and/or micro-cells → larger # handoffshandoffs

MSC load is heavy when high speed users are passed between very small cellspassed between very small cells

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Umbrella Cells Fig. 3.4, pg. 67g , pg use different antenna heights and Tx power levels to

provide large and small cell coverage multiple antennas & Tx can be co-located at single

location if necessary (saves on obtaining new tower li )licenses)

large cell → high speed traffic → fewer handoffs ll ll l d t ffi small cell → low speed traffic example areas: interstate highway passing thru

urban center office park or nearby shopping mall

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urban center, office park, or nearby shopping mall

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Cell Dragging low speed user w/ line of sight to base station (very strong

signal) strong signal changing slowly user moves into the area of an adjacent cell without handoff user moves into the area of an adjacent cell without handoff causes interference with adjacent cells and other cells Remember: handoffs help all users, not just the one which is

h d d ffhanded off. If this mobile is closer to a reused channel → interference -

for the other user using the same frequency So this mobile needs to hand off anyway, so other users

benefit because that mobile stays far away from them.

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Typical handoff parameters Analog cellular (1st generation) Analog cellular (1st generation) threshold margin △ ≈ 6 to 12 dB total time to complete handoff ≈ 8 to 10 sec

Digital cellular (2nd generation) total time to complete handoff ≈ 1 to 2 sec l th h ld i △ 0 t 6 dB lower necessary threshold margin △ ≈ 0 to 6 dB enabled by mobile assisted handoff

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benefits of small handoff time greater flexibility in handling high/low speed g y g g p

users queuing handoffs & prioritizing more time to “rescue” calls needing urgent

handoff f d d ll GOS i d fewer dropped calls → GOS increased

can make decisions based on a wide range of metrics other than signal strengthmetrics other than signal strength such as also measure interference levels can have a multidimensional algorithm for

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can have a multidimensional algorithm for making decisions

Soft vs. Hard Handoffs Hard handoff: different radio channels assigned

when moving from cell to cell all analog (AMPS) & digital TDMA systems (IS-136,

GSM, etc.)GS , e c.) Many spread spectrum users share the same

frequency in every cell CDMA → IS-95 Since a mobile uses the same frequency in every cell, it

can also be assigned the same code for multiple cells when it is near the boundary of multiple cells.

The MSC simultaneously monitors reverse link signal at several base stations

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MSC dynamically decides which signal is best and then listens to that oneand then listens to that one Soft Handoff passes data from that base station on to the PSTNp

This choice of best signal can keep changing. Mobile user does nothing for handoffs except ob e use does ot g o a do s e cept

just transmit, MSC does all the work Advantage unique to CDMA systemsg q y As long as there are enough codes available.

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VII. Co-Channel Interference

Interference is the limiting factor in performance of all cellular radio systems

What are the sources of interference for a mobile receiver?

Interference is in both voice channels

l h l control channels Two major types of system-generated

interference:interference:1) Co-Channel Interference (CCI)2) Adjacent Channel Interference (ACI)

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2) Adjacent Channel Interference (ACI)

First we look at CCI Frequency Reuse Frequency Reuse Many cells in a given coverage area use the same

set of channel frequencies to increase system q ycapacity (C)

Co-channel cells → cells that share the same set of frequencies

VC & CC traffic in co-channel cells is an interfering source to mobiles in Several differentinterfering source to mobiles in Several different cells

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Possible Solutions?1) Increase base station Tx power to improve radio ) p p

signal reception? __ this will also increase interference from co-channel

ll b hcells by the same amount no net improvement

2) Separate co channel cells by some minimum2) Separate co-channel cells by some minimum distance to provide sufficient isolation from propagation of radio signals? if all cell sizes, transmit powers, and coverage patterns

≈ same → co-channel interference is independent of Tx power

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Tx power

co-channel interference depends on: co channel interference depends on: R : cell radius D : distance to base station of nearest co-channel cell

if D / R ↑ then spatial separation relative to cell coverage area ↑ i d i l i f h l RF improved isolation from co-channel RF energy

Q = D / R : co-channel reuse ratio hexagonal cells → Q = D/R = 3N hexagonal cells → Q = D/R = 3N

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Fundamental tradeoff in cellular system design: small Q → small cluster size → more frequency

reuse → larger system capacity great But also: small Q → small cell separation →

increased co channel interference (CCI) → reducedincreased co-channel interference (CCI) → reduced voice quality → not so great

Tradeoff: Capacity vs. Voice Quality Tradeoff: Capacity vs. Voice Quality

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Signal to Interference ratio → S / I, ____________

S : desired signal powerg p Ii : interference power from ith co-channel cell io : # of co-channel interfering cells

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Approximation with some assumptions

Di : distance from ith interferer to mobile Rx power @ mobile ( ) n

iD

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n : path loss exponent free space or line of sight (LOS) (no obstruction) →

n = 2 urban cellular → n = 2 to 4, signal decays faster

ith di t f th b t tiwith distance away from the base station having the same n throughout the coverage area

means radio propagation properties are roughly themeans radio propagation properties are roughly the same everywhere

if base stations have equal Tx power and n is the q psame throughout coverage area (not always true) then the above equation (Eq. 3.8) can be used.

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Now if we consider only the first layer (or tier) of co-channel cells assume only these provide significant interference

And assume interfering base stations are And assume interfering base stations are equidistant from the desired base station (all at distance ≈ D) then)

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What determines acceptable S / I ? voice quality → subjective testingq y j g AMPS → S / I 18 dB (assumes path loss exponent

n = 4) Solving (3.9) for N

Most reasonable assumption is io : # of co-channel interfering cells = 6

N 7 ( h i f AMPS)

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N = 7 (very common choice for AMPS)

Many assumptions involved in (3.9) : same Tx powerp hexagonal geometry n same throughout areag Di ≈ D (all interfering cells are equidistant from the

base station receiver) optimistic result in many cases propagation tools are used to calculate S / I when

i lidassumptions aren’t valid

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S / I is usually the worst case when a mobile is at the cell edge low signal power from its own base station & high

interference power from other cells more accurate approximations are necessary in those cases more accurate approximations are necessary in those cases

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4 4 42( ) 2( ) 2S RI D R D R D

4 4 42( ) 2( ) 2I D R D R D

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N =7 and S / I ≈ 17 dB

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Eq. (3.5), (3.8), and (3.9) are (S / I) for forward link only, i.e. the cochannel base Tx interfering with desired base station transmission to mobile unit so this considers interference @ the mobile unit

What abo t reverse link co channel interference? What about reverse link co-channel interference? less important because signals from mobile antennas (near

the ground) don’t propagate as well as those from tall base station antennas

obstructions near ground level significantly attenuate mobile energy in direction of base station Rxenergy in direction of base station Rx

also weaker because mobile Tx power is variable → base stations regulate transmit power of mobiles to be no larger than necessary

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than necessary

I. Adjacent Channel Interference

Two major types of system-generated interference:

1) Co-Channel Interference (CCI) – discussed in last lecture

2) Adjacent Channel Interference (ACI) Adjacent Channel Interference (ACI) Imperfect Rx filters allow energy from adjacent

channels to leak into the passband of other h lchannels

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d i d fil desired filter response

actual filter response actual filter response

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This affects both forward & reverse links Forward Link → base-to-mobile Forward Link → base to mobile interference @ mobile Rx from a ______ Tx

(another mobile or another base station that is not (the one the mobile is listening to) when mobile Rx is ___ away from base station.

signal from base station is weak and others are somewhat strong.

R Li k bil t b Reverse Link → mobile-to-base interference @ base station Rx from nearby mobile

Tx when desired mobile Tx is far away from base

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Tx when desired mobile Tx is far away from base station

Near/Far Effect interfering source is near some Rx when desired g

source is far away ACI is primarily from mobiles in the same cellp y some cell-to-cell ACI does occur as well → but a

secondary source Control of ACI don’t allocate channels within a given cell from a

contiguous band of frequencies for example, use channels 1, 4, 7, and 10 for a cell. h l t t h th

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no channels next to each other

maximize channel separation separation of as many as N channel bandwidths separation of as many as N channel bandwidths some schemes also seek to minimize ACI from

neighboring cells by not assigning adjacentneighboring cells by not assigning adjacent channels in neighboring cells

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Originally 666 channels, then 10 MHz of spectrum was added

666+166 = 832 channels 395 VC plus 21 CC per service provider

(providers A & B)395*2 = 790, plus 42 control channels

Provider A is a company that has not traditionally provided telephone service

P id B i t diti l i li t Provider B is a traditional wireline operator 21 VC groups with ≈ 19 channels/group t l t 21 h l ti f h

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at least 21 channel separation for each group

for N = 7 → 3 VC groups/cell For example, choose groups 1A, 1B, and 1C for a p , g p , ,

cell – so channels 1, 8, 15, 22, 29, 36, etc. are used. ≈ 57 channels/cell at least 7 channel separation for each cell group

to have high quality on control channels, 21 cell g q yreuse is used for CC’s instead of reusing a CC every 7 cells, as for VC’s,

reuse every 21 cells (after every three clusters) greater distance between control channels, so less

CCI

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CCI

use high quality filters in base stations better filters are possible in base stations since they p y

are not constrained by physical size and power as much as in the mobile Rx

makes reverse link ACI less of a concern than forward link ACI also true because of power control (discussed below) also true because of power control (discussed below)

choice of modulation schemes diff t d l ti h id l different modulation schemes provide less or more

energy outside their passband.

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Power Control technique to minimize ACIq base station & MSC constantly monitor mobile

received signal strength mobile Tx power varied (controlled) so that

smallest Tx power necessary for a quality reverse li k i l i d (l f th l thlink signal is used (lower power for the closer the mobile is to the base station)

also helps battery life on mobile also helps battery life on mobile

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dramatically improves adjacent channel S / I ratio, since mobiles in other cells only transmit t hi h h t itt t lat high enough power as transmitter controls

(not at full power) most beneficial for ACI on reverse link most beneficial for ACI on reverse link will see later that this is especially important for

CDMA systemsCDMA systems

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III. Trunking & Grade of Service (GOS)

Trunked radio system: radio system where a large # of users share a pool of channelslarge # of users share a pool of channels channel allocated on demand & returned to channel

pool upon call terminationpool upon call termination exploit statistical (random) behavior of users so that

fixed # of channels can accommodate large # of users Trade-off between the number of available channels

that are provided and the likelihood of a particular userthat are provided and the likelihood of a particular user finding no channels available during the busy hour of the day.

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trunking theory is used by telephone companies to allocate limited # of voice circuits for large # ofallocate limited # of voice circuits for large # of telephone lines

efficient use of equipment resources → savingsq p g disadvantage is that some probability exists that

mobile user will be denied access to a channel blocked call : access denied → “blocked call cleared” delayed call : access delayed by call being put into

holding queue for specified amount of timeholding queue for specified amount of time

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GOS : measure of the ability of user access to a trunked system during the hourtrunked system during the _______ hour specified as probability (Pr) that call is blocked or

delayedy designed to handle the busiest hour → typically

______ Erlang : unitless measure of traffic intensity e.g. 0.5 erlangs = 1 channel occupied 30 minutes

during 1 hourduring 1 hour Table 3.3, pg. 78 → trunking theory definitions

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“Offered” Traffic Intensity (A) Offered? → not necessarily carried by system y y y

(some is blocked or delayed) each user Au=λH Erlangs (also called ρ in queueing

theory) λ = traffic intensity (average arrival rate of new calls,

in new requests per time unit say calls/min)in new requests per time unit, say calls/min). H = average duration of a call (also called 1/ µ in

queueing theory) system with U users → A = UAu = UλH Erlangs capacity = maximum carried traffic = C Erlangs =

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(equal to total # of available channels that are busy all the time)

Erlang B formula Calls are either admitted or blocked

A = total offered traffic C = # channels in trunking pool (e.g. a cell)g p ( g )

AMPS designed for GOS of 2% blocked call cleared (denied) → BCC

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( )

capacities to support various GOS values capacities to support various GOS values

N h i h i h h

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Note that twice the capacity can support much more than twice the load (twice the number of Erlangs).

Erlang C formulas blocked call delayed → BCD → put into holding y p g

queue GOS is probability that a call will still be blocked

even if it spends time in a queue and waits for up to t seconds

ti (3 17) t (3 19) i b k equations (3.17) to (3.19) in book

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Graphical form of Erlang B formulas

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Graphical form of Erlang C formulas

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Example: Find how many users can be supported in a cell containing 50 channels for a pp g2% GOS (Blocked Calls Cleared) if the average user calls twice/hr with an average call duration of 5 minutes.

What is the corresponding C from the figure? What is the corresponding C from the figure?

What is A (Traffic Intensity) from the figure?What is A (Traffic Intensity) from the figure?

So, how many users can be supported?

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, y pp

Trunking Efficiency measure of the # of users supported by a specific

configuration of fixed channels, efficiency in terms of users per available channel = U / C

Table 3 4 pg 79 → assume 1% GOS Table 3.4, pg. 79 → assume 1% GOS Assume Au = 0.2 1 group of 20 channels: 1 group of 20 channels:

2 groups of 10 channels, with equal number of users per group:

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the allocation of channel groups can substantially change the # of users supported by y g pp ytrunked system The larger the trunking pool, the better the trunking g g p g

efficiency. as trunking pool size ↓ then trunking efficiency

↓ What is the relationship between trunking pool size,

trunking efficiency, received signal quality, and cluster size?

As cluster size decreases

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As cluster size decreases…

Note: Trunking efficiency is an issue both in FDMA/TDMA t d i CDMA tFDMA/TDMA systems and in CDMA systems (where the capacity limit is the number of possible codes and the interference levels)possible codes and the interference levels).

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IV. Improving Cellular System Capacity

A cellular design eventually (hopefully!) becomes insufficient to support the growing number of users. Need to provide more channels per unit coverage

area Would like to have orderly growth

ld lik d h i d f b ild Would like to upgrade the system instead of rebuild Would like to use existing towers as much as

possiblepossible

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Cell Splitting subdivide congested cell into several smaller subdivide congested cell into several smaller

cells increases number of times channels are reused

in an area must decrease antenna height & Tx powerg p so smaller coverage per cell results and the co-channel interference level is held

constant

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each smaller cell keeps ≈ same # of channels as the larger cell, since each new smaller cell uses the same number of frequencies this means that we keep that same cluster size

capacity ↑ because channel reuse ↑ per unit area smaller cells → “micro-cells”

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Illustration is for towers at the corners

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advantages include: only needed for cells that reach max. capacity → not

all cells implement when Pr [blocked call] > acceptable GOS system capacity can gradually expand as demand ↑

disadvantages include: # handoffs/unit area increases umbrella cell for high velocity traffic may be needed more base stations → $$ for real estate, towers, etc.

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complicated design process new base stations use lower power and antenna

height What about existing base stations? If kept at the same power they would overpower new If kept at the same power, they would overpower new

microcells. If reduced in power, they would not cover their own

llcells. One solution: Use separate groups of channels. One group at the original power and another group at O e g oup e o g powe d o e g oup

the lower power. New microcells only use lower power channels. As load growth continues more and more channels are

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As load growth continues, more and more channels are moved to lower power.

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Sectoring cell splitting keeps D / R unchanged (same

cluster size and CCI) but increases frequency reuse/area

alternate way to ↑ capacity is to _____ CCI (increase S / I ratio)

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replace omni-directional antennas at base station with several directional antennas 3 sectors → 3 120° antennas 6 sectors → 6 60° antennas

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ll h l b k d i d cell channels broken down into sectored groups CCI reduced because only some of neighboring co-

channel cells radiate energy in direction of main cellchannel cells radiate energy in direction of main cell center cell labeled "5" has all co-channel cells

illustratedillustrated only 2 co-channel cells will interfere if all are using

120° sectoring only 1 co-channel cell would interfere when using

60° sectoring If the S/I was 17 dB for N = 7 and n = 4, what is the

S / I now with 120° sectoring? 24 2 dB

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24.2 dB

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How is capacity increased? sectoring only improves S/I which increases voice

quality, beyond what is really necessary by reducing CCI, the cell system designer can choose

smaller cluster size (N ↓) for acceptable voice qualitysmaller cluster size (N ↓) for acceptable voice quality smaller N → greater frequency reuse → larger system

capacity

What would the system capacity, Cnew, now be when using 120° sectoring, as compared to the old capacity, Cold ?

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much less costly than cell splitting only require more antennas @ base station vs. y q @

multiple new base stations for cell splitting primary disadvantage is that the available p y g

channels in a cell are subdivided into sectored groups trunked channel pool ↓, therefore trunking

efficiency ↓ There are more channels per cell, because of

smaller cluster sizes, but those channels are broken into sectors

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into sectors.

other disadvantages: other disadvantages: must design network coverage with sectoring

decided in advance can’t effectively use sectoring to increase capacity

after setting cluster size N can’t be used to gradually expand capacity as

traffic ↑ like cell splittingd ff More Handoffs

More antenna, more cost

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Next topic: Mobile Radio Propagation - Large-scale path loss, small-scale fading, and p , g,multipath Free space propagation lossp p p g Reflections 2-ray model Diffraction Fading Multipath

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