modeling of electrical power supply systems as communication channels: the in-home and in-vehicle...

Modeling of Electrical Power Supply Systems

as Communication Channels:

the In-Home and In-Vehicle Cases

Dr. Stefano GalliTelcordia Technologies, Inc.Room: RRC-1D258One Telcordia DrivePiscataway, NJ 08854Tel. : (732) 699-2219Fax : (732) 336-7026Email: [email protected]

Copyright © 2005 Telcordia Technologies. All Rights Reserved.

IEEE International Symposium on Power Line Communications, ISPLC, Vancouver, April 2005IEEE International Symposium on Power Line Communications, ISPLC, Vancouver, April 2005

Telcordia Technologies Proprietary - Copyright 2005.

Outline of presentation

1) Beyond broadband access

2) Wiring and grounding practices

3) Overview of indoor modeling approaches

4) A complicated in-vehicle case: the NASA Space Shuttle

5) Transceiver optimization considerations

6) Conclusions


Beyond Broadband Access… Home Networking

Home Networks (HN) will likely flourish:– Success of broadband access– Internet access within multi-computer homes– HDTV (YG in 7/04: 60 M by 2008)

• PLC-based networks give additional motives for HN success:– As complexity moves to the edge of the network, the in-home LAN

becomes a factor in QoS PLCs can ensure bit rate availability with QoSPLCs can ensure bit rate availability with QoS

– Access line will no longer terminate on a single devicePLCs can support a plethora of networked devices PLCs can support a plethora of networked devices

– Once the bitpipe is in place, applications will multiplyPLCs allow for every “powered” device to be a network nodePLCs allow for every “powered” device to be a network node

– Wired and wireless networks will coexist in the homePLCs PLCs andand wireless can provide truly ubiquitous home networking wireless can provide truly ubiquitous home networking


• PLCs allows for easy in-vehicle networking:– In any vehicles (from automobiles to ships, from aircraft to space

vehicles), separate cabling is used to establish the PHY of a local command and control network which is becoming broadband

– The in-vehicle power distribution network may well perform double-dutydouble-duty, as an infrastructure supporting both power delivery and broadband digital connectivity.

– Weight, space and cost savings.– “Plug & Play”

• PLCs as the enabler for truly ad-hoc ad-hoc networks:– Wireless ad-hoc networks do notnot scale (Gupta-Kumar, T-IT’00)

– Wireless routing requires exchange of too much overhead

– Just look around… power is everywhereTraffic lights, lamp posts, etc. can easily become network nodes

– …. and we know how to optimize wired networks!

Home Networking …. and beyond


Wiring and Grounding Practices

• The power from the local distribution transformer comes in three common configurations:

– Single phaseSingle phase: hot and neutral connectors (sometimes also separate “earth” wire)

– USA: 120V AC

– Europe: typical for residences 240V (UK) or 220V (rest of EU), but harmonization process towards 230 V (±10%)

– Two phaseTwo phase: two hot conductors (opposite polarity) with one neutral conductor.

– USA: typical 120V AC (120/240V AC split phase), but sometimes two legs of 120/208 wye (apartment complexes)

– Europe: not common

– Three phase:Three phase: three hot wires and one return

– USA: 120/208 V, but rare for homes– Europe: 230V/400V (typical for homes in Germany, Sweden and Finland), but sometimes 127/220V (Finland and Belgium), and 230V and no neutral in the supply - outlets are wired between two phases (Scandinavia)



• Varieties of inside wiring:– UniversalUniversal: wiring system uses a star (e.g., a single cable feeds all

of the wall outlets in one room only) or tree arrangement

– EuropeEurope:

– two wire (ungrounded) or three wire (grounded) outlets – If three phase supply is used, separate rooms in the same apartment may be on different phases

– UK exceptionsUK exceptions:

– special rings: a single cable runs all the way round part of a house interconnecting all of the wall outlets; a typical house will have three or four rings.– neutral not grounded in the home

– New buildsNew builds: three phase with four of five wires (neutral, ground)

– Problematic old wiringProblematic old wiring: two-wire 1 phase, neutral and ground share common wire



• Wiring and grounding come in many flavors, and this makes modem design much more challenging.

• However, international harmonization is happening:

– Typical outlets have three wires: hot, neutral and ground

– Classes of appliances (light, heavy duty appliances, outlets, etc.) fed by separate circuits

– Neutral and ground separate wires within the home, except for the main panel where they are bonded

Although complex topologies may exist, today’s regulations can simplify analysis of signal transmission


SERVICEPANELFEED

LIGHTING CIRCUITSNon-symmetric geometry

for B&W

RECEPTACLE CIRCUITS15-20 amps, branching, and

symmetric geometry for B&W

GROUNDBONDING

EMBEDDED APPLIANCES50 amps, non-branching, and symmetric

geometry for B&W



RSB

BLK BLK

WHT WHT

GND GND

RTN HOT

RSB

L3 L2

CIRCUIT BREAKERS

SERVICE TRANSFORMER

SERVICE DROP


Typical service panel, showing Typical service panel, showing bonding between the neutral and bonding between the neutral and

the ground cable through the ground cable through RRSBSB. .

Bonding has been largely ignored in indoor PLC modeling


B

0.3 FREQUENCY ( MHz) 30.0

LOS

S

3dB

/DIV

Ground bonding introduces non negligible resonant modes due to pair-mode excitation.

Effects of Bonding on Signal Propagation

Same topology with bonding

Topology without bonding


B

0 5 10 15 20 25 30-25

-20

-15

-10

-5

0

5

Frequency in MHz

Mag

nitu

de o

f Tra

nsfe

r Fu

nctio

n (d

B)

LOS

S

4dB

/DIV


Effects of Bonding on Signal Propagation

Current Models(no bonding)

Measurements(when bonding present)


Overview of the Main Modeling Approaches

• Many efforts have been devoted to the modeling of the power line channel (PLC), both for the indoor and outdoor cases.

• There are two main approaches

• Time-domain: multipath

• Frequency-domain: two/multi-conductor transmission lines

• Both approaches allow us to unveil the main characteristics of the transfer function of the PLC:

• it depends on the topology of the network, and on the wiring and grounding practices• it depends on the input impedance of appliances and devices that are plugged in the network• it is time-varying• it is isotropic, regardless of topology, under mild conditions


Multipath ModelBarnes – ISPLC’98,

Phillips, and Dostert & Zimmermann - ISPLC’99

• The multipath nature arises from the presence of several branches and impedance mismatches that cause reflections.

• Good model, but it has some limitations:

Overview of Channel Modeling Approaches

• modeling is based on parameters that can be estimated only after the actual channel transfer function has been measured• wiring and grounding practices not explicitly accounted for, but “phenomenologically” included• computational cost in estimating the delay, amplitude and phase associated with each path (time-domain model) drawback for some indoor/in-vehicle channels.


Two-Conductor Transmission Line Model Hooijen - ISPLC’98

• Straightforward approach, follows TPC/coax modeling• Frequency domain model: dual of multipath model• Transfer function can be computed a priori• Limited computational complexity if topology has many branches

• Limitations:• Whole topology is needed • Accuracy of results depend on accuracy of cable models• Incomplete model, presence of third wire not included so that wiring and grounding practices not explicitly accounted for• Some aspects of signal propagations cannot be explained with this model



Multi-Conductor Transmission Line ModelGalli & Banwell - ISPLC’01; IEEE Trans. on Power Delivery, April and July 2005

• Based on Multi-conductor Transmission Line Theory and Modal Decomposition: can take into account multi-conductor nature of PL cables, as well as wiring and grounding practices

• First published work that takes into account grounding practices• Transfer function can be computed a priori• Frequency domain model (limited computational complexity)• Allows to unveilunveil interesting and useful properties of the PLC, e.g.

superposition of resonant modes, isotropy of channel

• Limitations:• Whole topology is needed• MTL is not easy to handle• Accuracy of results depend on accuracy of cable models



B

0 5 10 15 20 25 30-25

-20

-15

-10

-5

0

5

Frequency in MHz

Mag

nitu

de o

f Tra

nsfe

r Fu

nctio

n (d

B)

LOS

S

4dB

/DIV


MTL Approach: bonding can be accounted for

Current models (no bonding)MTL model (Galli, Banwell)


The In-Vehicle Case

• Not as much hype as access and HN, but products in the automotive market should come out this year (Valeo)

• No literature available about modeling in-vehicle PLC !!• Are the indoor models sufficient?• Can they be adapted to accurately model the in-vehicle PLC?

• Let’s look at an extreme case: explore the possibility of re-using in-vehicle power cabling for providing an “additional” LAN connection on board the NASA Shuttle.

– “Little cost” redundancy for increased safety– Support proliferation of sensing devices across the Shuttle

Goal of this preliminary study (Galli et al., IEEE VTC’04): – Analysis of on-board wiring and grounding practices

– Analyze applicability of existing indoor PLC models

– Calculate a “typical” frequency transfer function between nodes (e.g., sensors, computers, etc.) communicating on the vehicle.


1-wire conductors (35 )

FC FC FC

DC buses

Shuttle structure (grounding)

DC/AC converter

3-wire conductors (75 )AC bus

RPC


1-wire conductors (50 )RPC

Nos

e se

ctio

n

Tai

l sec

tion

FC: Fuel cellRPC: remote power controller

An “non-typical” in-vehicle case: the NASA OrbiterPower Distribution


A B20 ft 16 ft 46 ft 62 ft 21 ft

24 ft

19 ft

19 ft

18 ft

10 ft

9 ft

8 ft

10 ft

10 ft

17 ft

10 ft15 ft

15 ft

14 ft

13 ft22 ft

12 ft

33 ft

5 ft

5 ft

5 ft

Z=35

Z=50

Z=100

An “non-typical” in-vehicle case: the NASA OrbiterWiring and Grounding Practices





0 5 10 15 20 25 30 35 40 45 50-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Frequency in MHz

20*l

og10

(abs

(H(f

)))

Example of transfer function on the Shuttle

Two transfer functions, before and after an RPC commutes


An “non-typical” in-vehicle case: the NASA Orbiter

Lessons learnt: Current cable layout differs significantly from well-known

topologies found in telecom and residential power lines:– many levels of multiple branching– RPCs may control several branches at the same time, topology

may change abruptly and significantly over time– structural grounding– 1-wire to 2-wire cable splicing

Common PLC models inadequate:– Multipath models

Too many paths to account for Difficult to make measurements to fit parameters

– Transmission Line models 1-wire to 2-wire splicing difficult to model Ground as signal return path


System Optimization

• We have today a good understanding of the PLC

• Transfer function of the power line channel more deterministic than originally thought, if grounding and wiring practices are taken into account

• Better understanding of channel characteristics can help in transceiver optimization, but more work needs to be done

• Here we just give few examples of possible research directions:

• optimal precoding

• iterative decoding

• coding for erasure channels


• The transfer function of the power line channel, regardless of its topology, is isotropic (same transfer function from either side) under the sole condition that the output impedance of the TX is the same as the input impedance of the RX (Galli & Banwell, IEEE Trans. on Power Delivery, July 2005).

• This is equivalent to say the transfer function is known at the transmitter: when B receives data from A and estimates the channel from A to B, transmission from B to A can be optimized because the channel from B to A is known by B.

• A surprising resultA surprising result: the capacity of a channel : the capacity of a channel withwith or or withoutwithout interference is the interference is the samesame if the “interference” is known at the if the “interference” is known at the transmitter (Costa, T-IT 1983). transmitter (Costa, T-IT 1983).

• ISI can be seen as a form of interference, therefore knowledge of channel at the transmitter allows to use optimal pre-coding.

• Intense research also in the wireless domain for assessing the capacity regions of the broadcast and multiple access channels.

Transmitter Optimization – Symmetry of the Channel


• The transfer function is symmetric but what about the noise?

• It might be stronger at one node if special appliances are present (drill, vacuum, etc.), and weaker at another node: interference cannot be removed completely.

• Nonlinear detection and estimation techniques based on iterative soft decoding can alleviate this problem (Galli, T-COM’02):

• optimal “soft” decision are the NL-MMSE filtered and fixed-lag smoothed estimates of the transmitted symbols• NL-MMSE filtered and fixed-lag smoothed estimates of the transmitted symbols can be obtained via the APP vector, that are computed in Homeplug A/V for turbo-decoding

• Nonlinear MMSE estimation is today receiving growing attention (Guo, Shamai, Verdu on T-IT’05):

“The derivative of the mutual information with respect to the SNR is The derivative of the mutual information with respect to the SNR is equal to half the MMSE, regardless of the input statistics. This equal to half the MMSE, regardless of the input statistics. This relationship holds for both scalar and vector signals, as well as for relationship holds for both scalar and vector signals, as well as for discrete-time and continuous-time noncausal MMSE estimation.”discrete-time and continuous-time noncausal MMSE estimation.”

Receiver Optimization – Soft Estimators


System Optimization – Coding for Erasure Channels

• The power line channel can be seen as an erasure channel:• number of retransmissions grows with erasure probability• in a broadcast scenario, retransmissions grow even larger

• Another interesting result: The capacity of an erasure channel is the The capacity of an erasure channel is the samesame with with or or without without

feedbackfeedback

• Classical approach: Reed-Solomon codes, but there are some disadvantages in PLCs

• erasure probability is unknown a priori• erasure probability is time-varying

• More recently, Luby (’98) and Shokrollahi (’03) developed a new family of sparse-graph rate-less codes that achieve capacity on erasures channels: Fountain (LT) code, and Raptor codes

Regardless of the statistics of erasures, the source data can be Regardless of the statistics of erasures, the source data can be decoded from any set of K’ encoded packets, for K’ only slightly decoded from any set of K’ encoded packets, for K’ only slightly larger than Klarger than K


Conclusions

• We have today a good understanding of the PLC

• Better models are probably needed for the in-vehicle environment

• PLC more deterministic than originally thought

• Plethora of grounding and wiring practices, but harmonization of regulations can simplify analysis of signal transmission

–Wiring and grounding practices must be taken into account !

• How do we characterize the “average” link?– Following practice from DSL and wireless, we can:

– A) define a family of topologies and grounding practices that can be considered representative of what is found in the field– B) associate the above set of representative topologies to their transfer functions using the MTL-based channel model– C) Modulation and coding techniques should be tested against the above set of transfer functions• Harmonization and known channel characteristics can help in

transceiver optimization, but... what should we optimize?• Throughput, BER, etc.?• Robustness ?


8-3SUB-PANEL

FEED

1:3 SPLIT

1WAY2×3WAY

4-WAYRELAY

SOME CIRCUITS WILL ALWAYS BE DIFFICULT!!

(I vote for robustness, maybe)

14-2 14-2-214-2-2

12-314-3

14-212-2

12-2 (FEED)14-2

Epilogue…

modeling of electrical power supply systems as communication channels: the in-home and in-vehicle...

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