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