CIS 6930 Powerline Communications PHY Layer (c) 2013 Richard Newman
Jan 07, 2016
CIS 6930Powerline Communications
PHY Layer
(c) 2013 Richard Newman
PHY Layer/Modulation What does PHY layer do? Theoretical limits Impairments Modulation
Analog Digital
Conclusions
Physical Layer – What's in It? Mechanical
Medium Connectors
Electrical/optical Band Modulation
Procedural Timing, etc.
Noise handling Scrambling Channel coding
Theoretical Limits Nyquist
Noiseless Dual of Sampling Theorem
C (bps) = 2H (Hz) log2 M
H = Hertz bandwidth = fmax
– fmin
M = # symbol elements
Shannon-Hartley White noise (e.g., thermal)
C (bps) <= H (Hz) log2 (1+S/N)
S = signal power, N = noise power
Decibels
Logarithmic measure Addition of logs = multiplication
SNR in dBSNR (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
time
frequency
time
frequency
timefrequency
chan A
chan 1
chan BA1 A2 A3 A1
C1chan C
chan Dchan 2
chan 3
chan 1
chan 4B1 B2 B1 B2
FDM TDM MF/TDM
Electrical/Optical
Band Range of frequencies used Amplitude range
Modulation Analog signal (AM, FM) Digital
ASK FSK PSK
phase
amplitude
Analog Carrier modeled as sine wave
c(t) = A sin (2πf + φ) AM – Amplitude is function of signal A(t) = s(t)
FM – Frequency is function of signal f(t) = fc + s(t)
Animated GIF from WikiImages
Traditional Digital
Transmission is a sequence of discrete symbols, each symbol one of a finite set
ASK – amplitude shift keying
– Symbols differ in amplitude, fixed freq & phase FSK – frequency shift keying
– Symbols differ in frequency, fixed ampl & phase PSK – phase shift keying
– Symbols differ in phase, fixed freq & ampl QAM – combine PSK and ASK
Constellations
00
01
11
10
00 01 11 10
Q-ASK Q-PSK
8-QAMNote use of the GrayCode to minimizereceived bit errors in the presence of noise
QPSK Timing Diagram
I and Q are sine and cosine components
– Odd bits coded on I, even bits coded on QImage from WikiImages
16-QAM Demo
16 combinations of phase and amplitude
– 4 bits of data per symbol, but noise limits arityImage from WikiImages
Impairments
ISI – Intersymbol Interference Effect of multipath interference, delay distortion Also effect of band-limited channel Causes symbols to “smear” into each other
ACI – Adjacent Channel Interference Power from one channel spills into adjacent channel – side
lobes, filtering
Noise White, colored, impulse
Interference
BPSK Eye Diagrams
BPSK Eye diagramPrevious symbol to left, Current symbol in center,Next symbol to rightOverlay of multiple transitions
Same BPSK Eye diagram with ISIMultipath interference smears symbolsIncreases noiseLimits arity of symbol elements
Special Digital
PPM – pulse position modulation OFDM – Orthogonal Frequency Division Mux
– Basic
– Bit loading WM – wavelet modulation 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 log2 N bits/pulse Very power efficient
Symbol Frame i
… … … …
0xA7 0x1FSymbol = position
TimeSymbol Frame i+1
0x02
OFDM
Frequency
Time
Sub-carrier k - BPSK
Sub-carrier j – 8-QAM
Sub-carrier m – not used2.3
0.7
0.5
Sub-carrier l - QPSK1.5
Sub-carrier n – BPSK2.5
Especially useful for frequency selective fading channelsCarriers orthogonal, so no ICISelect modulation, which subcarriers to use, FEC rateBit loading – select modulation per subcarrier
Multiple subcarriers
Conclusions
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