CIS 6930
Powerline Communications
PHY Layer
(c) 2013 Richard Newman
PHY Layer/Modulation
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What does PHY layer do?
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Theoretical limits
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Impairments
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Modulation
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Analog

Digital
Conclusions
Physical Layer – What's in It?
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Mechanical
Medium
 Connectors

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Electrical/optical
Band
 Modulation
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Procedural
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Timing, etc.
Noise handling
Scrambling
 Channel coding
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Theoretical Limits
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Nyquist
Noiseless
 Dual of Sampling Theorem
C (bps) = 2H (Hz) log2 M
H = Hertz bandwidth = fmax – fmin
M = # symbol elements

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Shannon-Hartley
White noise (e.g., thermal)
C (bps) <= H (Hz) log2 (1+S/N)
S = signal power, N = noise power
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Decibels
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Logarithmic measure
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Addition of logs = multiplication
SNR in dB
SNR (dB) = 10 log10 (S/N)
Dividing Bands
SDM – space (different wire, direction,…)
 FDM - frequency
 TDM – time
 CDM – code division multiplexing (spread spectrum)
 MF/TDM – First divide frequency, then time
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TDM
FDM
time
frequency
chan 4
chan 1
chan 3
chan 2
chan 1
time
frequency
frequency
chan A
chan B
chan C
chan D
MF/TDM
A1 A2 A3 A1
B1 B2 B1 B2
C1
time
Electrical/Optical

Band
Range of frequencies used
 Amplitude range


amplitude
Modulation
Analog signal (AM, FM)
 Digital
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ASK
FSK
PSK
phase
Analog
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Carrier modeled as sine wave
c(t) = A sin (2πf + φ)
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AM – Amplitude is function of signal A(t) = s(t)
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FM – Frequency is function of signal f(t) = fc + s(t)
Animated GIF from WikiImages
Traditional Digital
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
Transmission is a sequence of discrete
symbols, each symbol one of a finite set
ASK – amplitude shift keying
–
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FSK – frequency shift keying
–
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Symbols differ in frequency, fixed ampl & phase
PSK – phase shift keying
–
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Symbols differ in amplitude, fixed freq & phase
Symbols differ in phase, fixed freq & ampl
QAM – combine PSK and ASK
Constellations
01
Q-PSK
Q-ASK
00 01 11 10
11
00
10
Note use of the Gray
Code to minimize
received bit errors in
the presence of noise
8-QAM
QPSK Timing Diagram
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I and Q are sine and cosine components
–
Odd bits coded on I, even bits coded on Q
Image from WikiImages
16-QAM Demo
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16 combinations of phase and amplitude
–
4 bits of data per symbol, but noise limits arity
Image from WikiImages
Impairments
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ISI – Intersymbol Interference
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Effect of multipath interference, delay distortion
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Also effect of band-limited channel
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Causes symbols to “smear” into each other
ACI – Adjacent Channel Interference
Power from one channel spills into adjacent channel – side
lobes, filtering
Noise
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White, colored, impulse
Interference
BPSK Eye Diagrams
BPSK Eye diagram
Previous symbol to left,
Current symbol in center,
Next symbol to right
Overlay of multiple transitions
Same BPSK Eye diagram with ISI
Multipath interference smears symbols
Increases noise
Limits arity of symbol elements
Special Digital
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PPM – pulse position modulation
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OFDM – Orthogonal Frequency Division Mux
–
Basic
–
Bit loading
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WM – wavelet modulation
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Spread Spectrum
–
FHSS – frequency hopping
–
DSSS – direct sequence
Pulse Position Modulation
One pulse per symbol frame
 Uses position of pulse in symbol frame
 If N positions, then log N bits/pulse
2
 Very power efficient

Symbol = position
…
0xA7
0x1F
…
Symbol Frame i
…
0x02
…
Symbol Frame i+1
Time
OFDM
2.5
Sub-carrier n – BPSK
2.3 Sub-carrier m – not used
Frequency
1.5 Sub-carrier l - QPSK
0.7 Sub-carrier k - BPSK
0.5 Sub-carrier j – 8-QAM
Time
Multiple subcarriers
Especially useful for frequency selective fading channels
Carriers orthogonal, so no ICI
Select modulation, which subcarriers to use, FEC rate
Bit loading – select modulation per subcarrier
Conclusions
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Need special techniques to deal with PL
channels – traditional approaches don't work
PPM appropriate for long distances when data
rate and bandwidth efficiency can be low
OFDM, WM are appropriate for high data rate,
broadband applications