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CIS 6930 Powerline Communications PHY Layer (c) 2013 Richard Newman

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PHY Layer/Modulation What does PHY layer do? Theoretical limits Impairments Modulation Analog Digital Conclusions

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Physical Layer – What's in It? Mechanical Medium Connectors Electrical/optical Band Modulation Procedural Timing, etc. Noise handling Scrambling Channel coding

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Theoretical Limits Nyquist Noiseless Dual of Sampling Theorem C (bps) = 2H (Hz) log 2 M H = Hertz bandwidth = f max – f min M = # symbol elements Shannon-Hartley White noise (e.g., thermal) C (bps) <= H (Hz) log 2 (1+S/N) S = signal power, N = noise power

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Decibels Logarithmic measure Addition of logs = multiplication SNR in dB SNR (dB) = 10 log 10 (S/N)

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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 time frequency chan A chan 1 chan B A1 A2A3A1 C1 chan C chan D chan 2chan 3 chan 1 chan 4 B1B2B1B2 FDM TDMMF/TDM

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Electrical/Optical Band Range of frequencies used Amplitude range Modulation Analog signal (AM, FM) Digital ASK FSK PSK phase amplitude

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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

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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

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Constellations Q-ASK Q-PSK 8-QAM Note use of the Gray Code to minimize received bit errors in the presence of noise

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QPSK Timing Diagram I and Q are sine and cosine components – Odd bits coded on I, even bits coded on Q Image from WikiImages

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16-QAM Demo 16 combinations of phase and amplitude – 4 bits of data per symbol, but noise limits arity Image from WikiImages

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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

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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

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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

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Pulse Position Modulation One pulse per symbol frame Uses position of pulse in symbol frame If N positions, then log 2 N bits/pulse Very power efficient Symbol Frame i ………… 0xA70x1FSymbol = position Time Symbol Frame i+1 0x02

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OFDM Frequency Time Sub-carrier k - BPSK Sub-carrier j – 8-QAM Sub-carrier m – not used Sub-carrier l - QPSK1.5 Sub-carrier n – BPSK2.5 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 Multiple subcarriers

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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

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