Transcript LWANSlides2

The Physical Layer
The Theoretical Basis for Data
Communication
•
•
•
Fourier analysis
Niquist chriterium for bandwidth-limited
channel
Shannon maximum data rate of a noisy
channel
Fourier Transform
• Periodic signals with period T=2π/w


n 0
n 0
s(t )   an cos( nwt )   bn sin( nwt )
T
T
2
2
an   s(t ) cos( nwt )dt , bn   s(t ) sin( nwt )dt
T 0
T 0
• Non-periodic signals
1
s (t ) 
2

jwt
S
(
jw
)
e
dw



S ( jw) 
 jwt
s
(
t
)

e
dt


Bandwidth-Limited Signals
A binary signal and its root-mean-square Fourier amplitudes.
(b) – (c) Successive approximations to the original signal.
Bandwidth-Limited Signals
(d) – (e) Successive approximations to the original signal.
Bandwidth-Limited Signals
Relation between data rate and harmonics.
Band-Limited Channel
• Fourier transform of a typical signal
S ( jw) 
 
 jwt
a
g
(
t

nT
)

e
dt 
s
 n
 n 0


n 0

  an  g (t  nTs )  e
 jwt
-1/Ts
sin( wTs / 2)  
dt 
an C ( n )

( wTs / 2) n 0
1/Ts
2/Ts
w
Niquist Theorem
• If the signal bandwidth has width of W,
then it can be reconstructed by taking 2W
samples per second.
• Maximum data rate is
R  2W log 2 V
where V is the number of different symbols
Niquist Chriterium
 n  sin 2W (t  n / 2W )
X (t )   X 

 2W  2W (t  n / 2W )


Power Spectrum Density
• Autocorrelation function of signal or
noise
 ( )  E ( X (t )  X (t   ))
• Power spectrum density

 ( w)    ( )e  jw d

Shannon Theorem
• If the signal bandwidth has width of W,
and S/N is the signal-to-noise ratio, then the
maximum data rate is
R  W log 2 (1  S / N ),
S  S (0) 


s pS ( s )ds 
2


N   N (0)   | n |2 p N (n)dn 

Wc
 S (w)dw
Wc
Wc
 N (w)dw, where N (w)  N / 2W
c
Wc
Filtering
• Channel behaves as a filter

y (t )   h(t   ) x( ),

Y ( w)  H ( w) X ( w),
 Y ( w)  H ( w)  X ( w)
2
• When the noise is white (uncorrelated)
Gaussian optimum filter has transfer
function H(w)=X*(w).
Modulation schemes
(a) QPSK.
(b) QAM-16.
(c) QAM-64.
Guided Transmission
• Twisted Pair
• Coaxial Cable
• Fiber Optics
Twisted Pair
(a) Category 3 UTP 16 MHz.
(b) Category 5 UTP 100MHz.
Issues of Twisted-Pair Transmission
•
•
•
•
Attenuation
Distortion
Cross-talk
Impulse noise
Coaxial Cable
A coaxial cable 1GHz.
Fiber Optics
n1  sin 1  n2  sin 1
(a) Three examples of a light ray from inside a silica fiber impinging on the
air/silica boundary at different angles.
(b) Light trapped by total internal reflection.
Light Propagation
•
From Maxwell equations:
Ez r ,    AJ ur e j , Ez r ,    BK wr e j
u  k n  , w   k n
2
•
2 2
0 1
2
2
2
2 2
0 2
From boundary conditions:
2 2


1
1


2
2
 2
 ju  kw  n1 ju  n2 jw   2 
2 
( wa)  k0
 (ua)

K' wa 
kw 
wa K wa 

J ' ua 
ju 
ua J ua 
Light Propagation
V  k0 a n12  n22 
2a

n12  n22
Transmission of Light through Fiber
Attenuation of light through fiber in the infrared region.
Bands 25-30THz, and last two bands have attenuation less than 5%/km
Fiber Cables
(a) Side view of a single fiber.
(b) End view of a sheath with three fibers, diameter 8-10μm.
Transmission Devices
•
•
•
•
•
Light emitting diode (LED)
Semiconductor lasers
Mach-Zehnder external modulator
EDFA
Photodiode
Optical Transmitters
A comparison of semiconductor diodes and LEDs as light sources.
Fiber Optic Networks
A fiber optic ring with active repeaters.
Fiber Optic Networks
A passive star connection in a fiber optics network.
Wireless Transmission
•
•
•
•
•
The Electromagnetic Spectrum
Radio Transmission
Microwave Transmission
Infrared and Millimeter Waves
Lightwave Transmission
Wireless Transmission
• Relationship between wavelength and frequency:
f  c
• 100MHz waves are about 3m long, 1000MHz
waves are 0.3m long.
• An object distracts those waves, whose length is
smaller or equal to the object dimension.
The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication.
Radio Transmission
(a) In the VLF, LF, and MF bands, radio waves follow the
curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
Issues in Wireless Transmission
• Radio signals are omnidirectional, and
penetrate through objects. Throughput is low.
• HF radio and microwave signals are
directed. Suffer from multipath fading, and are
reflected against the buildings.
• Above 4GHz, signals are absorbed by the
rain.
Politics of the Electromagnetic Spectrum
• Goverments allocate frequencies, through
contests, lottery, auctions.
• In ISM only the power is specified, used for
household devices. Infrared is available.
The ISM bands in the United States.
Lightwave Transmission
Convection currents can interfere with laser communication systems.
A bidirectional system with two lasers is pictured here.
Fog and rain are disruptive too.
Communication Satellites
•
Geostationary Satellites
•
•
Several kWs. 40 transponders with 80MHz.
TDMA.
Medium-Earth Orbit Satellites
•
•
24 GPS satellites.
Low-Earth Orbit Satellites
•
Iridium project started by Motorola
Communication Satellites
Communication satellites and some of their properties,
including altitude above the earth, round-trip delay time
and number of satellites needed for global coverage.
Communication Satellites
The principal satellite bands.
Geostationary Satellites
Very Small Aperature Terminals (VSATs) using a hub.
Low-Earth Orbit Satellites
Iridium
(a) The Iridium satellites from six necklaces around the earth.
(b) 66 satellites, 1628 moving cells cover the earth, 253000
channels.
Iridium and Globalstar
(a) Relaying in space (Iridium).
(b) Relaying on the ground (Globstar).
Public Switched Telephone System
•
•
•
•
•
Structure of the Telephone System
The Politics of Telephones
The Local Loop: Modems, ADSL and Wireless
Trunks and Multiplexing
Switching
Structure of the Telephone System
(a) Fully-interconnected network.
(b) Centralized switch.
(c) Two-level hierarchy.
Structure of the Telephone System
10km
Coax,
micorwave,
fiber
A typical circuit route for a medium-distance call.
Major Components of the
Telephone System
•
Local loops

•
Twisted pairs going to houses and businesses
Trunks

•
Fiber optics connecting the switching offices
Switching offices

Where calls are moved from one trunk to another
The Politics of Telephones
164
LATAs
The relationship of LATAs, LECs, and IXCs. All the
circles are LEC switching offices. Each hexagon
belongs to the IXC whose number is on it.
The Local Loop: Modems,
ADSL, and Wireless
The use of both analog and digital transmissions for a computer to
computer call. Conversion is done by the modems and codecs.
Modems
(a) A binary signal
(b) Amplitude modulation
(c) Frequency modulation
(d) Phase modulation
Modems
(a)
(a) V.32 for 9600 bps.
(b) V32 bis for 14,400 bps.
(b)
Higher Bit-rate Modems
•
•
•
•
35kbps is the Shannon limit
Line from a local office to an ISP is
digitalized.
V90 35kbps upstream, 56kbps
downstream
V92 48kbps upstream, 56kbps
downstream
Digital Subscriber Lines
Bandwidth versus distanced over category 3 UTP for DSL.
Digital Subscriber Lines
Operation of ADSL using discrete multitone modulation.
Up to 8Mbps downstream, and up to 1Mbps upstream
Digital Subscriber Lines
A typical ADSL equipment configuration.
Wireless Local Loops
50km
198MHz at
2.5GHz;
2km
1.3GHz at
28GHz
Architecture of an LMDS system.
Frequency Division Multiplexing
(a) The original bandwidths.
(b) The bandwidths raised in frequency.
(b) The multiplexed channel.
Wavelength Division Multiplexing
Wavelength division multiplexing.
Time Division Multiplexing
The T1 carrier (1.544 Mbps).
Time Division Multiplexing
Multiplexing T1 streams into higher carriers.
Time Division Multiplexing
Two back-to-back SONET frames.
Time Division Multiplexing
SONET and SDH multiplex rates.
Circuit Switching
(a) Circuit switching.
(b) Packet switching.
Message and Packet Switching
(a) Circuit switching (b) Message switching (c) Packet switching
Switching Comparison
?
?
A comparison of circuit switched and packet-switched networks.
The Mobile Telephone System
•
First-generation mobile phones: Improved
mobile telephone system (IMTS)
•
•
23 channels, 150-450MHz
Second-generation mobile phones:
Advanced mobile telephone system
(AMTS)
•
•
Cellular system
Third-generation mobile phones: 3G
•
Voice and data
Advanced Mobile Phone System
10km
(a) Frequencies are not reused in adjacent cells.
(b) To add more users, smaller cells can be used.
Channel Categories
•
•
•
•
The 832 30kHz channels in bands 824849MHz and 869-894Mhz (45 per cell), are
divided into four categories:
Control (base to mobile) to manage the system
Paging (base to mobile) to alert users to calls
for them
Access (bidirectional) for call setup and
channel assignment
Data (bidirectional) for voice, fax, or data
D-AMPS
Digital Advanced Mobile Phone System
Bands 1850-1910, and 1930-1990MHz
(a) A D-AMPS channel with three users.
(b) A D-AMPS channel with six users.
GSM
Global System for Mobile Communications
GSM uses 124 200kHz frequency channels, each of
which uses an eight-slot TDM system
GSM
A portion of the GSM framing structure.
CDMA – Code Division Multiple Access
IS-95
g1(t)
r1(t)
t
X
 ( )dt
0
g2(t)
r2(t)
t
r(t)
X
 ( )dt
0
gK(t)
rK(t)
t
X
 ( )dt
0
Walsh-Hadamard Sequences
 a11 a12
a
a22
21

M
 ...
...

aM 1 aM 2
a1M 
... a2 M 
... ... 

... aMM 
...
H M
 1  1
H2  
H 2M  

 1  1
H M
HM 

 HM 
M Sequences and Gold Code
z -1
am-1
z -1
z -1
X
X
a1
am-2
z -1
X
X
a0
+
• Autocorrelation function of m-sequence
j0
n
 ( j)  
 1 1  j  n  1
•
Gold sequence is a sum of two m-sequences such that it has
cross-correlation values {-1,-2(m+2-mod(m,2))+1,2(m+2-mod(m,2))-1)
CDMA – Code Division Multiple Access
IS-95
(a) Binary chip sequences for four stations
(b) Bipolar chip sequences
(c) Six examples of transmissions
(d) Recovery of station C’s signal
Multiuser Detection
g1(t)
r1(t)
t
X
 ( )dt
0
g2(t)
r2(t)
t
r(t)
X
 ( )dt
gK(t)
rK(t)
t
X
Maximize
Metrics
C(rk,bk)
0
 ( )dt
0
Decorrelating Detector
•
Information is detected according to the formula:
bˆ K  sgn( R s 1rK ),
R s  {rij }KxK
T

  g i (t ) g j (t )dt 
0

Decorrelating Detector, K=2
•
Information is detected according to the formula:
1
Rs1 
1  2
 1
 

T
 
,    g1 (t ) g 2 (t )dt

1 
0
r (t )  E1 b1 g1 (t )  E2 b2 g 2 (t )  n(t )
 E1 b1   E2 b2  n1 
r2  

  E1 b1  E2 b2  n2 
2

E
b

(
n


n
)
/(
1


)
1
1 1
1
2
R s r2  
2 
 E2 b2  (n2  n1 ) /(1   )
Third-Generation Mobile Phones:
Digital Voice and Data
Basic services an IMT-2000 network should provide
•
•
•
•
High-quality voice transmission
Messaging (replace e-mail, fax, SMS, chat, etc.)
Multimedia (music, videos, films, TV, etc.)
Internet access (web surfing, w/multimedia.)
3G
•
•
•
•
W-CDMA or universal mobile
telecommunication system (UMTS)
compatible with GSM, uses 5MHz.
CDMA2000 extension of IS-95 uses 5Mhz
Enhanced data rates for GSM evolution
(EDGE) uses more bits per baud
General radio packet servise (GPRS) is overlay
packet network over D-AMPS or GSM
Cable Television
•
•
•
•
•
Community Antenna Television
Internet over Cable
Spectrum Allocation
Cable Modems
ADSL versus Cable
Community Antenna Television
An early cable television system.
Internet over Cable
Cable television
Internet over Cable
The fixed telephone system.
Spectrum Allocation
Frequency allocation in a typical cable TV system
used for Internet access
Cable Modems
Typical details of the upstream and downstream
channels in North America.
Community Antenna Television (CATV)
HOME
antenna
oo
oo
oo
oo
oo
HEADEND
oo
oo
oo
RF Spectrum:
AM-VSB signals
55 MHz
Sheryl Woodward, AT&T Labs-Research
oo
Long chains of RF amplifiers:
limited bandwidth,
poor reliability.
350 MHz
Linear Lightwave Revolution
Hybrid-Fiber-Coax Architecture: Improved reliability and performance,
BUT to transmit 80 channels of AM-VSB, an optical link must operate
near fundamental limits.
oo
oo
oo
oo
oo
HEADEND
RF Spectrum:
oo
Fiber
Node
oo
oo
oo
80 AM-VSB channels
oo
HOME
55 (E 85)MHz
Sheryl Woodward, AT&T Labs-Research
350 MHz
550 (E 606)MHz
challenge:
HFC plant is HYBRID
Coax -limited bandwidth,
-good SNR.
M-QAM standard digital
transmission format.
Fiber -huge bandwidth,
-difficult to
maintain high SNR.
On-Off Keying is the preferred
transmission format.
Quick optical transmission format comparison.
On/Off keying:
>40 Gbps / λ
>1Tbps on a single fiber.
Sheryl Woodward, AT&T Labs-Research
256-QAM:
5 Gbps / λ requires
RIN<-135 dB/Hz.
Fiber non-linearities limit
number of λ per fiber.
Compressed Digital Video
a)
b)
c)
d)
MPEG-3 compresses a video channel to <5 Mbps.
Quadrature Amplitude Modulation (QAM) can be used to transmit
multiple television channels in a single 6 (E 8)MHz
RF channel. Around 38Mbps can be transmitted through this
channel.
A much lower Carrier-to-Noise Ratio (CNR) is required to transmit
these QAM signals than is required by AM-VSB.
A set top box is required to receive these channels.
RF Spectrum:
80 AM-VSB channels
55 (E 85)MHz
Sheryl Woodward, AT&T Labs-Research
30 QAM channels
(~150 video channels)
350 MHz
550 (E 603)MHz 750 (E 862)MHz
QAM capacity limitations
Max. Number of Channels
10000
From top to bottom
RIN = -140dB/Hz
-135dB/Hz
-130dB/Hz
64-QAM
1000
RIN= -135dB/Hz
616
18.5 Gbps
123
4.9 Gbps
100
256-QAM
5MHz per channel
10
20
25
30
35
40
45
50
SNR (dB)
C.F. Lam, "A Simplified Model for Estimating the Capacity Limit of an Optical Link in
Transporting Multi-channel M-QAM Singals," to appear in IEEE Photonics Technology
Letters, Nov. 2000
Sheryl Woodward, AT&T Labs-Research
Upstream Transmission
a)
Can now offer interactive services
HOME
oo
oo
oo
oo
oo
HEADEND
oo
Fiber
Node
oo
oo
oo
RF Spectrum:
80 AM-VSB channels
5-40 (E 65)MHz
Sheryl Woodward, AT&T Labs-Research
oo
30 QAM channels
(~150 video channels)
350 MHz
550 (E 603)MHz 750 (E 862)MHz
Upstream Transmission
a)
b)
c)
d)
e)
f)
g)
h)
i)
RF band is 5-42 (E 65)MHz, this band can carry multiple RF channels.
Modulation schemes are QPSK or 16QAM
5-15 MHz is plagued with ingress noise.
All frequencies suffer from the funnel effect.
Up to 10 Mbps transmission per RF channel is provided in the standard, but
a peak rate of ~3 Mbps is more realistic.
Bandwidth is shared.
Services can be segregated by RF frequency.
For data the standard is DOCSIS (Data Over Cable Service Interface
Specification).
Telephony can be carried over DOCSIS 1.1.
A cable modem or set top box resides in the home, a CMTS, which
coordinates traffic, resides in the headend.
Sheryl Woodward, AT&T Labs-Research
Upstream Transmission-Frequency Stacking
oo
HEADEND
oo
oo
oo
1
2
HOME
3
oo
Fiber
Node
4
oo
oo
oo
RF Spectrum
on upstream fiber:
1
2
3
5-40 MHz
Sheryl Woodward, AT&T Labs-Research
4
300 MHz
oo
To improve upstream capacity
divide the fiber node serving area
into quadrants. Frequency stack
these signals, then transmit them
to the HE.
Scientific Atlanta’s Baseband Digital
Return
oo oo
HOME
digital Tx Fiber
Node
oo
Mux
HEADEND
A-D
oo
oo
To avoid needing analog links,
digitization (not demodulation) is
performed at the Fiber Node.
RF transparency is preserved.
Sheryl Woodward, AT&T Labs-Research
oo
A-D
oo
oo
oo
A Sample HFC System
Downstream: 500 MHz shared by ~50,000 (broadcast)
200 MHz by 1200 (narrowcast)
Upstream: ~37 MHz shared by 300
oo
up
b
n
(4n/fiber)
Secondary Hub
oo
HOME
oo
oo
Fiber
Node
oo
oo
oo
oo
oo
RF Spectrum on coax:
return
80 broadcast channels
broadcast
5-42 MHz
Sheryl Woodward, AT&T Labs-Research
30 QAM channels
(~150 video channels)
narrowcast
550 MHz
750 MHz