Transcript KVIV course, part 2

Telecommunications
Concepts
Chapter 1.5
Communications
Media
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Contents
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Optical fibers
Coaxial cables
Twisted pairs
Wireless communications
09-07- K.Steenhaut & J.Tiberghien - VUB
Contents
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Optical fibers
Coaxial cables
Twisted pairs
Wireless communications
09-07- K.Steenhaut & J.Tiberghien - VUB
Snell’s Law
sin 1
sin 2
2
n2
n1
n2

n1
1
=
n2
n1
n2
n1
c
>c
n2 < n1
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Optical Fibers
(step index)
n2 < n1
n2
n1
Protective coating
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Multimode Fiber
Diameter : > 50 
Step index fiber
Graded index fiber
Low cost but limited bandwidth * distance
due to multimode dispersion
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Multimode Dispersion
t
t
Step index fiber :
< 50 MHz.Km
Graded Index Fiber : < 1000 MHz.Km (1990)
< 5000 MHz.Km (2000)
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Monomode Fiber
Diameter : < 5 
Only one propagation mode possible
Higher cost due to end equipment
but enormous bandwidth*distance product
10 Gb/s over 500 Km optical sections (1995)
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Wave Domain Multiplexing
Each color can carry an independent data flow.
In 2000
40 colors carrying each 10 Gb/s or
80 colors carrying each 2.5 Gb/s
were commercially available
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Optical amplifiers
Pump
laser
Erbium
doped fiber
Erbium atoms are pumped into a higher energy
state by the light of the pump laser,
they fall back in synchronism with the incoming
light, amplifying it.
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Optical Switching
From IEEE Com.Mag.V39,N1, Jan 2001.
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Contents
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Optical fibers
Coaxial cables
Twisted pairs
Wireless communications
09-07- K.Steenhaut & J.Tiberghien - VUB
Coaxial Cables
Insulator
Conductor
Conductor
Protective coating
Monomode propagation for all data applications
Transmission rates up to some Gb/s
Distance limited by electrical attenuation
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Contents
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Optical fibers
Coaxial cables
Twisted pairs
Wireless communications
09-07- K.Steenhaut & J.Tiberghien - VUB
Twisted Pairs
Performance highly
dependant on cable
quality
Transmission speed up
to several 100 Mb/s
for distances of up to
100 m.
with better cables
(class 5 or 6)
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Contents
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Optical fibers
Coaxial cables
Twisted pairs
Wireless communications
09-07- K.Steenhaut & J.Tiberghien - VUB
Wireless Communications
Why ?
Mobile terminals
Cost of wiring
Why Not ?
Lower data rates
Lower reliability
Potential Lack of Security
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Wireless Communications
Main restrictions:
• Limited available bandwidth
• Uncontrolled sources of noise
Some solutions:
• Displace some heavy users (TV)
• Reuse of frequencies at different locations
(Cellular radio, Point to point links, …)
• Sharing of a set of frequencies
(spread spectrum radio)
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Reuse of Frequencies
At some distance, a transmitter can no longer be
received and the same frequency can be reused
Pr/Sa = Pt/r2
r
transmitter
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Pr = Power at receiver
Sa = Area of receiver antenna
Pt = Power at transmitter
r = Distance
09-07- K.Steenhaut & J.Tiberghien - VUB
Cellular Radio
Ideally, 3 different frequency sets are sufficient
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The Mobile Access Network
Second Generation Handover
When a receiver is between two cells, the receiver
has to disconnect from one cell and connect into the
next one. Circuit routing has to be adapted
accordingly.
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Cellular Radio
k = number available frequencies per cell
S = Area of a cell = *r2
n = Number of simultaneous calls per km2
pt = Power of transmitter
P0/Sa= Minimal field strength at receiver input
n =k/S
pt = p0/Sa * r2
With smaller cells,
- more antenna sites are needed ...
- more simultaneous calls are possible
- transmitted power can be reduced
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Cellular Radio
in practice
Flanders
Ardennes
Propagation conditions depend heavily on geography
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Cellular Radio
In practice, seven or more different sets of
frequencies are needed
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Digital Cellular Phones
Name
DECT
GSM
DCS
1800
Freq.(MHz)
1880-1890
890-915
1710-1785
935-960
1805-1880
# rad.ch.
12
124
248
P.max.(W)
0.25
2
1
r.max.(Km)
0.2
35
8
Voicerate (Kb/s)
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13
13
Capacity (E/Km2)
10 000
1000
2000
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Cellular Radio
and frequency hopping
Problem :
Propagation conditions are extremely variable
in function of location and frequency,
especially in cities
Solution :
Use a set of different frequencies
and switch at a high rate between them.
e.g. In GSM every 20 mS frequencies change.
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Wireless Communications
Main restrictions:
• Limited available bandwidth
• Uncontrolled sources of noise
Some solutions:
• Displace some heavy users (TV)
• Reuse of frequencies at different locations
(Cellular radio, Point to point links, …)
• Sharing of a set of frequencies
(spread spectrum radio)
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Wireless Interference Margins
Cause considerable loss in transmission capacity
Frequency
• Considerable room for
improvements by
controlling interferences
– Signal hardening
– Signal recovery
– Signal expansion
= Third generation mobile
networks (UMTS)
Time
Space
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The Mobile Access Network
Fast hopping Spread Spectrum
Data
n b/s
Modulated signal
Large bandwidth ≈
m times bandwidth
needed for data
xor
Pseudo- m * n b/s
random
sequence
HF carrier
• Data combined with known higher frequency pseudo random
sequence
• Resulting modulated radio signal has high bandwidth
• Shannon : low data rate combined with high bandwidth =
excellent noise margins!
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The Mobile Access Network
Spread Spectrum and CDMA
D1
Tx1
S2
xor
S1
HF
HF
D2
Tx2
S1
xor
S2
HF
HF
Correl
-ator
D2
Rx2
Correl
-ator
D1
Rx1
For radio link Tx1-Rx1, emission by Tx2 is just
another source of noise
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The Mobile Access Network
Multi-path Interference
Different paths have different
lengths and different delays
GSM : interference = noise
UMTS : correlator adds similar input signals with
appropriate delays so that they reinforce each other
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The Mobile Access Network
Third Generation Handover
When a receiver is between two cells, both
transmitters send the same signal. These two signals
reinforce each other, as multipath propagation does.
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Multi-path Interference
Different paths have different
lengths and different delays
2nd generation (GSM) : interference = noise
3rd generation (UMTS) : add similar input signals with
appropriate delays so that they reinforce each other
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Wireless Local Loop
Radio is sometimes cheaper than digging the streets !
Used for telephony
and for
Internet access (WiMax)
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Microwave Point to Point Links
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Highly directive antennas limit spatial spreading
High transmission capacity (several Mb/s)
transmission impaired by heavy rain
Cost effective for line of sight communications
09-07- K.Steenhaut & J.Tiberghien - VUB
Satellite Communications
Geostationary
36000 Km
Round trip Delay = 240 ms
High power ground stations
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Satellite Communications
Low Orbit
Short round trip delays
Low power ground stations
In fact, a cellular system with mobile base- stations
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Introduced concepts
• Optical communications are becoming dominant.
– Low cost, high throughput fixed communications.
• Wireless communications are growing:
– For replacing local wiring
– For mobile communications
• Geostationary satellite communications:
– One way broadcasting
– low traffic point to point but high delays
• Low Orbit satellites:
– Cellular system for global mobile application.
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