Power Line Communication

1. Power Line Communication Why not use electrical lines?

2. Power Line Communication • Power line communication or power line carrier (PLC), also known as Power line Digital Subscriber Line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), or Broadband over Power Lines (BPL) are systems for carrying data on a conductor also used for electric power transmission. • Power-line networks are promising mediums for network access technology by which broadband services can be offered

3. Power Line Communication Services that can be offered • remote meter reading • broadband Internet access • digital entertainment • home networking services

4. Overview of PLC network

5. Brief History of PLC • Power line communication has been around for quite some time, but has only been used for narrow band tele-remote relay applications, public lighting and home automation. • Broadband over PLC only began at the end of the 1990s: • 1950: at a frequency of 10Hz, 10kW of power, one-way: town lighting, relay remote control. • Mid 1980s: beginning of research into the use of the electrical grid to support data transmission, on bands between 5 - 500Khz, always in a one-way direction, • 1997: first tests for bidirectional data signal transmission over the electrical supply network and the beginning of research by Ascom (Switzerland) and Norweb (U.K.) • 2000: first tests carried out in France by EDF R&D and Ascom.

6. Power Line Communication • 60%+ U.S. homes have Internet access • 10% broadband • Inaccessible and costly • PLC = No new wires: • Emerging Technology • Use of Power grid for communication • Extensive infrastructure • "Every" building

7. Power Line Communication • Home-Plug 1.0 PLC networking protocol • Other services (in the house) • In-home entertainment • Internet home appliances • BPL : Broadband Power Line (outside house) • A "last-mile" technology

8. Power Line Communication • Need high frequencies: current lines designed @ 50-60 Hz to 400 Hz • Contaminated -- noise, unreliable • Legal restrictions on frequency bands limit data rates • Power Loss • Directly proportional to square of current • Proportional to distance • High, medium, & low voltage lines & customer premise lines

9. Attenuation in PLC • The decrease in the value of something • Decrease in amplitude of an electrical signal • Opposite of amplification • Dependant on Impedance

10. Impedance in PLC • The opposition a circuit presents to an electrical circuit (in ohms) • Maximum signal received only when impedance of all components (transmitter, power line, receiver) are match • Power line systems vary significantly • Also varies with signal frequencies, time, load pattern • Mismatches are destructive

11. Noise in PLC • Major source: Electrical Appliance • e.g. run vacuum cleaner when TV is on • those @ 60 Hz & 50 Hz • Also radio signals from broadcast, commercial, military, CB, amateur stations • 3 categories of appliance noise • Impulsive: on/off switches • Periodic impulsive: dimmers • Continuous impulsive: AC motors (vacuum, shavers, etc)

12. PLC Electromagnetic Compatibility • Power lines are leaky: radiate high-frequency electromagnetic signals • Interferes with nearby wireless devices • Need filters to prevent leakage • 802.11b wireless network protocol (WiFi)

13. Modulation • Process of varying a carrier signal to use the signal to convey information • Analog vs. digital • Amplitude, phase, frequency • Why? To allow different components compatible • Modem: modulate and demodulate

14. PLC Modulation SchemeNecessary Properties • Ability to overcome nonlinear channel characteristics • PLC is very nonlinear • Requires expensive, complex equalization to obtain more than 10 Mbps • Should overcome without highly involved channel equalization • Ability to cope with multipath spread • Impedance mismatch causes echo signal, delaying signal by ~1ms • Must handle delays

15. Modulation Properties 3. Ability to adjust dynamically • PLC changes with load • Must track without large overhead or complexity 4. Ability to mask certain frequencies • PLC uses unlicensed frequency band but regulations limit radiation in some sub & adjacent bands • Masking would help marketability of PLC

16. Orthogonal Frequency DivisionModulation Scheme - OFDM • OFDM -- Collection of transmission techniques • OFDM meets all desirable properties • Used in European digital audio broadcast (DAB) • Also in some variants of 802.11x -- wireless protocols • Used in Home Plug 1.0

17. PLC Protocols • Several for low-bandwidth digital services • Products for home automation & home network are based on these • Differ in modulation, frequency, channel access

18. X-10 Protocol • Oldest, uses ASK modulation • Originally unidirectional -- controller to devices • Some bidirectional products • Typically, signals over PL to receivers controlling lights & appliances • Poor bandwidth utilization • 60 bps on 60 Hz line • Poor reliability in noisy environments • Limited application

19. CE Bus Protocol • Peer-to-peer communication • Avoids collisions via CSMA/CRCD protocol (carrier sense multiple access/collision resolution & collision detection) • Physical layer: spread spectrum technology patented by Intellon Corp. • Sweeps thru frequencies rather than hopping • From 100 to 400 kHz • Sweep called chirp -- used for synchronization, collision resolution, data transmission • Data rate ~10kb/s • Frequency used limits use in North America

20. Lon Works Protocol • Peer-to-peer communication • Developed by Echelon Corp. • Uses CSMA • Narrow band spread spectrum modulation • 125 to 140 kHz • Patented noise cancellation technique • Preserves data in presence of noise • Can be used in N. America and Europe • Due to narrow band

21. Home Plug 1.0 Protocol • Achieves Ethernet class network on-site using existing electrical wiring • Has been introduced in American market • Mitigates unpredictable noise • Splits bandwidth into many small sub channels • Masks noisy ones & others • Maintain 76 for use in U.S. market • Data rate: 1 to 14 Mbps • Nodes estimate each 5 sec. & adapt to optimal data rate

22. Home Plug 1.0(cont) • But, still use DSL or cable for Internet into home • CSMA/CA -- collision avoidance • MAC -- medium access control • Avoidance • PHY layer detects preamble of frame • MAC layer maintains virtual timer • Each frame is preceded by contention period, if none transmits during, it can

23. Home Plug 1.0(cont)

24. PL & Last Mile Broadband Internet • Home Plug 1.0 -- still depends on DSL or cable to bring Internet into home • 18 million miles of power lines in U.S. • Developing countries have PL but not DSL/cable • Techniques/devices being developed for this

25. PLC Security • PLC is shared channel (like WiFi) • Robust security is serious issue • Encryption necessary: security vs. complexity • Home Plug: DES - 56 bit key • Intrusion & interference from adjacent subnets • e.g. Apartments - contention, degradation • Decoupling filters - isolate circuits at meter • Can also "separate" power line with router

26. Indoor Power Line Communication • Deliver telecom services to every corner of household through existing electrical wiring

27. Indoor Power Line Communication Typical MIMO-OFDM System

28. MIMO OFDM • MIMO communication is a well established technique in radio transmission system and can be equally applicable to PLC by replacing transmit and receive antennas with signal feed and receive ports, and the wireless channel with electrical wiring. • solution to multipath • An effective modulation approach to compromise the ISI

29. Why is OFDM so popular fornew broadband systems? • Most broadband systems are subject to multipath transmission • !Conventional solution to multipath is an equalizer in the receiver high data rates - equalizers too complicated • With OFDM there is a simple way of dealing with multipath • relatively simple DSP algorithms

30. PLC-MIMO Channel

31. PLC Block Diagram

32. QPSK Mapping • QPSK uses four points on the constellation diagram, equi-spaced around a circle. • With four phases, QPSK can encode two bits per symbol, shown in the diagram with gray coding to minimize the bit error rate (BER)

33. Coding & Decoding Algorithms • Channel coding is a viable method to reduce information rate through the channel and increase reliability. • Achieved by adding redundancy to the information symbol vector resulting in a longer coded vector of symbols Example of Coding that can be used: • Quasi-Cyclic Low Density Parity Check Coding • Space Time Coding • Space Frequency Coding

34. Low Density Parity Check Coding • a linear error correcting code • a method of transmitting a message over a noisy transmission channel

35. Space Frequency Coding • spreads the source symbols across space and frequency.

36. IFFT • Parallel data streams are used as inputs to an IFFT • IFFT output is sum of signal samples • IFFT does modulation and multiplexing in one step Typical IFFT Output Samples

37. Cyclic Prefix • refers to the prefixing of a symbol with a repetition of the end. • As a guard interval - it eliminates the intersymbol interference from the previous symbol.

38. PLC Channel Zimmerman Model • each path as a weighting factor gi≤ 1 representing the product of the reflection and transmission factors along the path • di is the length of path • with c speed of light • r the dielectric constant.

39. Noise in PLC Channel • suffers impulsive noise interference that is generated from connected electrical appliances • Middleton’s Class A noise model is one of the appropriate models for impulsive noise environment

40. OFDM Receiver

41. BER to SNR