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Modeling of Electrical Power Supply Systems as Communication Channels: the In-Home and In-Vehicle Cases Dr. Stefano Galli Telcordia Technologies, Inc. Room: RRC-1D258 One Telcordia Drive Piscataway, NJ Tel.: (732) Fax: (732) Copyright © 2005 Telcordia Technologies. All Rights Reserved. IEEE International Symposium on Power Line Communications, ISPLC, Vancouver, April 2005

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Telcordia Technologies Proprietary - Copyright 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

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Telcordia Technologies Proprietary - Copyright 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 QoS PLCs can ensure bit rate availability with QoS –Access line will no longer terminate on a single device PLCs can support a plethora of networked devices –Once the bitpipe is in place, applications will multiply PLCs allow for every powered device to be a network node –Wired and wireless networks will coexist in the home PLCs and wireless can provide truly ubiquitous home networking

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Telcordia Technologies Proprietary - Copyright 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 double-duty –The in-vehicle power distribution network may well perform double-duty, as an infrastructure supporting both power delivery and broadband digital connectivity. – Weight, space and cost savings. – Plug & Play ad-hocPLCs as the enabler for truly ad-hoc networks: not –Wireless ad-hoc networks do not scale (Gupta-Kumar, T-IT00) –Wireless routing requires exchange of too much overhead –Just look around… power is everywhere Traffic lights, lamp posts, etc. can easily become network nodes –…. and we know how to optimize wired networks! Home Networking …. and beyond

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Telcordia Technologies Proprietary - Copyright Wiring and Grounding Practices The power from the local distribution transformer comes in three common configurations: –Single phase –Single 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 phase –Two 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)

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Telcordia Technologies Proprietary - Copyright Wiring and Grounding Practices Varieties of inside wiring: –Universal –Universal: wiring system uses a star (e.g., a single cable feeds all of the wall outlets in one room only) or tree arrangement –Europe –Europe: – 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 exceptions –UK 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 builds –New builds: three phase with four of five wires (neutral, ground) –Problematic old wiring –Problematic old wiring: two-wire 1 phase, neutral and ground share common wire

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Telcordia Technologies Proprietary - Copyright Wiring and Grounding Practices 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, todays regulations can simplify analysis of signal transmission

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Telcordia Technologies Proprietary - Copyright SERVICE PANEL FEED LIGHTING CIRCUITS Non-symmetric geometry for B&W RECEPTACLE CIRCUITS amps, branching, and symmetric geometry for B&W GROUND BONDING EMBEDDED APPLIANCES 50 amps, non-branching, and symmetric geometry for B&W Wiring and Grounding Practices

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Telcordia Technologies Proprietary - Copyright Wiring and Grounding Practices Typical service panel, showing bonding between the neutral and the ground cable through R SB. Bonding has been largely ignored in indoor PLC modeling

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Telcordia Technologies Proprietary - Copyright B 0.3 FREQUENCY ( MHz) 30.0 LOSS 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

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Telcordia Technologies Proprietary - Copyright B LOSS 4dB/DIV 0.3 FREQUENCY ( MHz) 30.0 Effects of Bonding on Signal Propagation Current Models (no bonding) Measurements (when bonding present)

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Telcordia Technologies Proprietary - Copyright 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

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Telcordia Technologies Proprietary - Copyright Multipath Model Barnes – ISPLC98, Phillips, and Dostert & Zimmermann - ISPLC99 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.

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Telcordia Technologies Proprietary - Copyright Two-Conductor Transmission Line Model Hooijen - ISPLC98 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 Overview of Channel Modeling Approaches

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Telcordia Technologies Proprietary - Copyright Multi-Conductor Transmission Line Model Galli & Banwell - ISPLC01; 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) unveilAllows to unveil 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 Overview of Channel Modeling Approaches

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Telcordia Technologies Proprietary - Copyright B LOSS 4dB/DIV 0.3 FREQUENCY ( MHz) 30.0 MTL Approach: bonding can be accounted for Current models (no bonding) MTL model (Galli, Banwell)

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Telcordia Technologies Proprietary - Copyright 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? Lets 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 VTC04): –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.

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Telcordia Technologies Proprietary - Copyright wire conductors (35 ) FC DC buses Shuttle structure (grounding) DC/AC converter 3-wire conductors (75 ) AC bus RPC 2-wire conductors (100 ) 1-wire conductors (50 ) RPC Nose section Tail section FC: Fuel cell RPC: remote power controller An non-typical in-vehicle case: the NASA Orbiter Power Distribution

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Telcordia Technologies Proprietary - Copyright A B 20 ft16 ft46 ft 62 ft 21 ft 24 ft 19 ft 18 ft 10 ft 9 ft 8 ft 10 ft 17 ft 10 ft 15 ft 14 ft 13 ft 22 ft 12 ft 33 ft 5 ft Z=35 Z=50 Z=100 An non-typical in-vehicle case: the NASA Orbiter Wiring and Grounding Practices 1-wire conductors (35 ) 1-wire conductors (50 ) 2-wire conductors (100 )

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Telcordia Technologies Proprietary - Copyright Example of transfer function on the Shuttle Two transfer functions, before and after an RPC commutes

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Telcordia Technologies Proprietary - Copyright 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

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Telcordia Technologies Proprietary - Copyright 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

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Telcordia Technologies Proprietary - Copyright 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 result: the capacity of a channel with or without interference is the same if the interference is known at the transmitter (Costa, T-IT 1983).A surprising result: the capacity of a channel with or without interference is the same if the interference is known at the 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

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Telcordia Technologies Proprietary - Copyright 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-COM02): 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-IT05): The derivative of the mutual information with respect to the SNR is equal to half the MMSE, regardless of the input statistics. This relationship holds for both scalar and vector signals, as well as for discrete-time and continuous-time noncausal MMSE estimation. Receiver Optimization – Soft Estimators

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Telcordia Technologies Proprietary - Copyright 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 same with or without feedback The capacity of an erasure channel is the same with or without feedback 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 decoded from any set of K encoded packets, for K only slightly larger than K

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Telcordia Technologies Proprietary - Copyright 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 ?

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Telcordia Technologies Proprietary - Copyright SUB-PANEL FEED 1:3 SPLIT 1WAY 2×3WAY 4-WAY RELAY SOME CIRCUITS WILL ALWAYS BE DIFFICULT!! (I vote for robustness, maybe) (FEED) 14-2 Epilogue…

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