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Lecture 4: Cellular Fundamentals

Chapter 3 - Continued

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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 channels

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actual filter response

desired filter response

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This affects both forward & reverse links 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.

Reverse Link → mobile-to-base interference @ base station Rx from nearby mobile

Tx when desired mobile Tx is far away from base station

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

urce is far away ACI is primarily from mobiles in the same cell

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 c

ontiguous band of frequencies for example, use channels 1, 4, 7, and 10 for a cell. no channels next to each other

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

hboring 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 (provi

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

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

Provider B is a traditional wireline operator 21 VC groups with ≈ 19 channels/group

at least 21 channel separation for each group

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

ell – 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 reuse is used for CC’s instead of reusing a CC every 7 cells, as for VC’s, r

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

CI

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

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)

choice of modulation schemes different modulation schemes provide less or more

energy outside their passband.

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

ived signal strength mobile Tx power varied (controlled) so that smalles

t Tx power necessary for a quality reverse link signal is used (lower power for the closer the mobile is to the base station)

also helps battery life on mobile

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dramatically improves adjacent channel S / I ratio, since mobiles in other cells only transmit at high enough power as transmitter controls (not at full power)

most beneficial for ACI on reverse link will see later that this is especially important for

CDMA 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 channels channel allocated on demand & returned to channel

pool 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 th

at 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 # of telephone lines

efficient use of equipment resources → savings disadvantage is that some probability exists that mo

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

ing queue for specified amount of time

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

elayed 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 hour

Table 3.3, pg. 78 → trunking theory definitions

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

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

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

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

eing theory)

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

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

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Erlang B formula Calls are either admitted or blocked

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

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

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capacities to support various GOS values

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

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Erlang C formulas blocked call delayed → BCD → put into holding q

ueue GOS is probability that a call will still be blocked e

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

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 2% 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 A (Traffic Intensity) from the figure?

So, how many users can be supported?

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

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

Table 3.4, pg. 79 → assume 1% GOS Assume Au = 0.2 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 trunked system The larger the trunking pool, the better the trunking

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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Note: Trunking efficiency is an issue both in FDMA/TDMA systems and in CDMA systems (where the capacity limit is the number of 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 Would like to upgrade the system instead of rebuild Would like to use existing towers as much as

possible

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Cell Splitting subdivide congested cell into several smaller cel

ls increases number of times channels are reused i

n an area must decrease antenna height & Tx power

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 heig

ht What about existing base stations?

If kept at the same power, they would overpower new microcells.

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

One solution: Use separate groups of channels. One group at the original power and another group at t

he lower power. New microcells only use lower power channels. 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 clust

er 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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cell channels broken down into sectored groups CCI reduced because only some of neighboring co-

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

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

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

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

apacity

What would the system capacity, Cnew, now be when 12

0° using 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. multi

ple new base stations for cell splitting primary disadvantage is that the available chann

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

y ↓ There are more channels per cell, because of smalle

r cluster sizes, but those channels are broken into sectors.

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other disadvantages: must design network coverage with sectoring decid

ed in advance can’t effectively use sectoring to increase capacity a

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

c ↑ like cell splitting More Handoffs More antenna, more cost

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

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HW-2

3-10, 3-13, 3-15, 3-22, 3-26