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CIS 6930 Powerline Communications PHY Layer

Discover the intricacies of the Physical Layer (PHY) in powerline communications, including theoretical limits, modulation techniques, impairments, and noise handling. Uncover the diverse aspects of the PHY layer, such as mechanical components, connectors, and noise reduction methods like scrambling. Learn about modulation types like analog and digital, and delve into the potential of OFDM and wavelet modulation for high-speed data applications.

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CIS 6930 Powerline Communications PHY Layer

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  1. (c) 2013 Richard Newman CIS 6930Powerline CommunicationsPHY Layer

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

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

  4. 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

  5. Decibels • Logarithmic measure • Addition of logs = multiplication • SNR in dB SNR (dB) = 10 log10 (S/N)

  6. Dividing Bands • SDM – space (different wire, direction,…) • FDM - frequency • TDM – time • CDM – code division multiplexing (spread spectrum) • MF/TDM – First divide frequency, then time TDM MF/TDM FDM chan A chan 1 chan 4 A2 A3 A1 A1 frequency frequency frequency chan 1 chan B chan 2 chan 3 B1 B2 B1 B2 chan C C1 chan D time time time

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

  8. 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

  9. 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

  10. 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

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

  12. 16-QAM Demo • 16 combinations of phase and amplitude • 4 bits of data per symbol, but noise limits arity Image from WikiImages

  13. 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

  14. 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

  15. 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

  16. 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 = position 0xA7 0x1F 0x02 … … … … Symbol Frame i Symbol Frame i+1 Time

  17. OFDM 2.5 Sub-carrier n – BPSK 2.3 Sub-carrier m – not used 1.5 Sub-carrier l - QPSK Frequency 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

  18. 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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