Impulse Noise In Digital Subscriber Line ...

Impulse Noise In Digital Subscriber Line Technologies -A Comprehensive ReviewBy Kenneth S. Schneider PhD Telebyte, Inc. 355 Marcus Blvd. Hauppauge, NY11788 [email protected]

I.

Introduction

Impulse Noises of various types have long been known as a deleterious effect impacting the performance of telecommunications. The principal slant of this paper is directed at Impulse Noises as they are encountered relative to Digital Subscriber Line (DSL) technology – the principal wire line technology for broadband access. This subject has already been addressed in detail in a number of published works the most excellent of which is given by Starr et al [1]. However, the emphasis of the present work is quite different in that it focuses mainly on several new subjects. The topics addressed are: • • • • • • • • •

II.

What is Impulse Noise? What are the physical origins of Impulse Noises? How do the various Impulse Noises couple into the DSL loop? How are Impulse Noises classified with respect to their spectrums? How have Impulse Noises been modeled and characterized? What Impulse Noises been used for Standard’s based testing? Mitigating Impulse Noise in Present, Implemented, and DSL technologies. The Problem of Impulse Noise in Vectoring Dealing with Impulse Noise in a Post Vectored environment.

What is Impulse Noise?

This is noise which is characterized, temporally, by transient short-duration disturbances- bursts. It is basically a non-stationary stochastic electromagnetic interference. That is, it is generally 1

characterized by non-overlapping transient disturbances, randomly occurring energy bursts, with potentially high peak power. These bursts may be fixed in duration or have a random duration. The burst amplitude may be either deterministic or random but in either case the extremes are large. The spectral behavior may vary considerably- all the way from being spectrally impulsive (RFI bursts) to white (bursts of Additive White Gaussian Noise) to being not well defined by second order statistics. The frequency of occurrence of these disturbances may also vary considerably. The impulse burst could be an isolated event. There could be a regular periodic or deterministically aperiodic burst train. There could be randomness in the inter-burst spacing. Impulse Noises are generally of short duration–from about 1 microsecond to a few tens of microseconds–with a fast rise time and moderately fast fall time. Impulse Noise is characterized statistically through its amplitudes, durations, inter-arrival times and its frequency spectrum (either Fourier Transform or Power Spectral Density or Energy Spectral Density). While there have been several attempts to characterize, probabilistically, the various amplitudes, durations and inter-arrival times, no single one applies in all cases. Within the classic service provider environment an Impulse Noise event is considered to have occurred if the noise voltage increased by at least 12 dB above message circuit noise for no more than 10 ms. The actual quantification of Impulse Noise is extremely difficult due to the many different coupling mechanisms. The time-domain picture of Impulse Noise in Figure 1 gives you some initial idea of what Impulse Noise looks like. However, this is only meant as an initial example and not meant as a total generalization.

Figure 1: Example of Impulse Noise as seen in the time domain- multiple Impulses shown

As is evident from this example Impulse Noise is very bursty and short-lived. It is often hard to see on a spectrum analyzer unless it is set to “peak hold.”

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III. What are the physical origins of Impulse Noises? Impulse Noises come from a variety of sources. They may often come from the electromechanical interference of heavy machinery and household appliances. While they can get into a manufacturing plant near an industrial park they can also enter from every home connected to the plant. Furthermore, because the electrical equipment must be running to generate Impulse Noise, it is an impairment that can come and go depending on the time of day. It is important to understand that there are many different origins for Impulse Noise. But, it is also important to understand that these various sources of Impulse Noise are statistically additive. On a given DSL connection each Impulse Noise source is produces noise spikes of different (possibly random) magnitudes, occurring at different (possibly random) times. These all sum together. The overall impulse noise observed is most likely the sum of many impulses of lesser magnitude and different phase. In this section we take a careful look at all of the sources of Impulse Noise. A taxonomy of physical origins of the different Impulse Noises is provided. The following classification system is employed: •

Classification 1: DSL Loop Coupling Point- CPE end or CO end

•

Classification 2: Frequency of Occurrence: ▪ ▪ ▪

•

Isolated Event-one time Impulse Deterministic Periodic Impulse Train Random Aperiodic Impulse Train

Classification 3: Cause of Impulse Generation ▪ ▪ ▪ ▪

Capacitive- discharge of a capacitor Inductive- break down of a magnetic field Arcing- arcing between 2 terminals (ionization) Radiation

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Classification 1: DSL Loop Coupling Point- CPE end or CO end CPE end • Automobile self-starters, ignition noise • External home security systems- Passive Infrared Motion (PIR) lights • Electrical Pumps and Air-conditioning compressors • Household appliances and Power tools • Lightning Flashes • Motorized Industrial machines • Faulty or dusty insulation of high voltage power lines • Electric switches and switching on/off devices connected to a power line • Earth leakage fault-if it trips Ground Fault Interrupter (GFI) • Thermostats (Central heating, Immersion heaters) • Electrical power supply units (Laptops, Routers, Plasma TVs) • Industrial/Commercial power usage (Electric Railways, Electric fences) • Decorative electrical items (Christmas tree lights, Touch lights) • Digital Communication Receivers including set top boxes • Transients coupling from internal power cables to close telecom cables • Circuit breakers • Relays with and without contact bounce • Triac controlled light dimmers • Micro-Interruption generated transients coupled from one loop to another in the binder • Switching Power Supplies • Fluorescent lights, Halogen lamps • TV set (CRT based) • Amateur Radio- Morse Code-“Unfriendly Transitions”-not at zero crossings

CO end • Ring trip events • On-Hook/Off-Hook events • Abrupt changing of tip and ring voltages • Lightning • Atmospheric noise from electrical discharges and Sunspots • Dial-Pulsing • Insertion and removal of the Power Ringing signal. • Service Provider maintenance activities • Circuit breakers • Relays with and without contact bounce • Switching Power Supplies • Switching Battery sources in and out • Mechanical switches • Air- conditioning compressors • Service Provider maintenance activities • Circuit breakers • Relays with and without contact bounce • Switching Power Supplies • Switching Battery sources in and out • Mechanical switches • Air- conditioning compressors

Classification 2: Frequency of Occurrence 4

Isolated event- one time impulse • Automobile self-starters • Lightning Flashes • Faulty or dusty insulation of high voltage power lines • Electric switches (if not on a timer) • Switching on or off any device connected to a power line • Earth leakage fault –if it trips Ground Fault Interrupter (GFI) • Thermostats (Central heating, Immersion heaters) • Electrical power supply units (Laptops, Routers, Plasma TVs) surge on and off • Decorative electrical items (non-blinking Christmas tree lights, Touch lights) • Relays • Motor Arcing • Micro-Interruption generated transients coupled from one loop to another in the binder • Air-conditioning compressors • Halogen lamp (12V) with transformer/power supply • Coffee maker • Electric stove Deterministic Periodic Impulse Train- Constant Interpulse Spacing • Continuous duty and variable speed electric motors • Decorative electrical items- blinking Christmas tree lights • Triac controlled light dimmers • Switching Power Supplies • Fluorescent lights • TV Set CRT based • Washing machine Random Aperiodic Impulse Train- Random Interpulse Spacing • Switching on or off any device connected to a power line • Thermostats (Central heating, Immersion heaters) • Industrial/Commercial power usage (Electric Railways, Electric fences) • Decorative electrical items (blinking Christmas tree lights) • Circuit breakers • Relays with contact bounce • Ring trip events • On-Hook/Off-Hook events • Abrupt changing of tip and ring voltages • Lightning Flashes • Dial-Pulsing • Insertion and removal of the Power Ringing signal • Service Provider maintenance activities • Switching Battery sources at CO in and out • Amateur Radio- Morse Code-“Unfriendly Transitions”-not at zero transitions

“A Power Line Communication Tutorial- Challenges and Technologies” [2] provides the time behavior of Impulse Noises from several of these sources. These are shown below in Figure 2.

Figure 2a: Lamp Dimmer

Figure 2b: Electric Tooth Brush Stand 5

Figure 2c: Vacuum cleaner noise Figure 2: Impulse Noises from common sources

Classification 3: Cause of Impulse Generation Before giving this Classification it will be worthwhile give some “simplified” examples of the types of Impulse waveforms that are generated for the categories in this class. These are illustrated in Figure 3a, 3b, 3c and 3d. Some caveats are in order here. These waveforms are only meant to represent the Impulse event at the point of generation. They most certainly would be modified by propagation through a coupling circuit to the point of egress to a DSL loop.

Figure 3a: Capacitive

Figure 3b: Inductive

Figure 3c: Arcing

Figure 3d: Radiation

Capacitive

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• • • • • • • • •

Transients coupling from internal power cables to close telecom cables Micro-Interruption generated transients coupled from one loop to another binder Switching power supplies Home appliances Ring trip events On-Hook/Off-Hook events Abrupt changing of tip and ring voltages Service Provider maintenance activities Switching Battery sources in and out

Inductive • • • • • • • • • • • • •

Automobile self-starters, ignition noise Electrical Pumps and Air-conditioning compressors Household appliances and Power tools Motorized Industrial machines Industrial/Commercial power usage (Electric Railways, Electric fences) Continuous duty electric motors Transients coupling from internal power cables to telecom cables running close together Circuit breakers Relays with and without contact bounce Switching Power Supplies Electrical power supply units (Laptops, Routers, Plasma TVs) Switching on or off any device connected to a power line Thermostats (Central heating, Immersion heaters)

Arcing • • • • • • • • • • • • • • • • • • • •

Automobile self-starters, ignition noise Faulty or dusty insulation of high voltage power lines Electric switches (if not on a timer) and Switching on or off any device connected to a power line Earth leakage fault- if it trips Ground Fault Interrupter (GFI) Thermostats (Central heating, Immersion heaters0 Electrical power supply units (Laptops, Routers, Plasma TVs) turning on and off Relays Motor Arcing Micro-Interruption generated transients coupled from one loop to another in the binder Continuous duty electric motors Decorative electrical items- blinking Christmas tree lights Triac controlled light dimmers Switching Power Supplies Fluorescent lights Industrial/Commercial power usage (Electric Railways, Electric fences) Circuit breakers Relays with contact bounce Ring trip events On-Hook/Off-Hook events Dial Pulsing

Radiation • Microwave ovens • Lightning Flashes • Amateur Radio- Morse Code-“Unfriendly Transitions”-not at zero crossings • TV Set CRT based • Coffee maker • Electric stove

Some discussion of these prototype Impulse waveforms in Figure 3 will be worthwhile. The Capacitive and Inductive waveforms are representative of discharge type mechanisms- falling exponentially with a time constant. The time constants are dependent upon the actual Impulse 7

event. By way of example, if they correspond to a mechanical mechanism- say a motor or a pump then the time constants are probably quite long. The Arcing waveform is based upon measured data. The Radiation prototype is representative of electric field measurements made in the vicinity of a thunderstorm where the spectral behavior of a “lightning pulse” falls as 1/f2 . This corresponds to a pulse time behavior which follows t e –αt . It should be mentioned at this point that new tools and instrumentation have been developed and are under development to assist in the further categorization of the plethora of Impulse Noise types. A new feature in the VDSL2 standard is the Impulse Noise Monitoring (INM) diagnostic tool. This provides a histogram-like report on the occurrence of Impulse Noise induced errors. Telebyte, Inc. has completed the development of a new product, the Field Noise Capture System, for monitoring, capturing and analyzing the different noises/interferences present in the Service Provider’s environment. It is ideal of for capturing and analyzing the various characteristics of different Impulse Noises- durations, periodicities and spectra.

IV. How do the various Impulse Noises couple into the DSL loop? CPE end Coupling at this end may either be radiative or capacitive. By way of example, Impulse Noise events caused by fluorescent lights or automobile ignitions would certainly be radiative. Capacitive coupling would occur due to the proximity of the DSL cabling to power lines, alarm system wiring and CATV cables.

CO end Here the Impulse Noise coupling mechanism is primarily a near-end phenomenon, due to the capacitive nature of the telephone cable, and may couple from other pairs in the cable, from adjacent circuits on a line card at the CO, from inadequate voltage regulation, and from common mode ground noise. All of these coupling mechanisms are reduced when the tip and ring switching voltages and currents are not allowed to change abruptly i.e. when band limiting is present.

V.

How are Impulse Noises classified with respect to their spectrums? 8

In this section consideration is given to the characterization of the different Impulse Noise types with respect to frequency. The typical method for characterizing the frequency dependence of commonly encountered noise types is to use the Power Spectral Density (PSD). However, really, this only strictly defined for stationary signals over an infinite time period. Impulse Noise events are definitely not in this category. An Impulse Noise event does not really possess a PSD. On a realization by realization basis it is possible to take an Impulse Noise event and perform a Fast Fourier Transform (FFT) and get a spectrum. But, this is an Energy Spectral Density (ESD) not a PSD. This subject is of interest because it provides an indication of which Impulse Noise types provide significant interference with which DSL technologies. Along another line, an understanding of the spectral characteristics can determine whether frequency diversity techniques can be employed to ameliorate the deleterious effects of Impulse Noise events. Such techniques are discussed in Section IX -Dealing with Impulse Noise in a Post Vectoring environment. In order to make, concrete, technically unchallengeable statements about the spectral characteristics one should really have available the ESD’s of each of the Impulse Noise types enumerated in the classifications provided in Section III. Furthermore, these spectral characteristics should have been obtained from measured experimental field data. An examination of the literature does not provide any such listing or anything close to it for the entire spectral range embracing all DSL technologies. This is unfortunate. It indicates that some research effort should be undertaken, perhaps organized by a Standards organization, for obtaining this “complete data base” of Impulse Noise spectral characteristics. What can be found in the literature is a “patchwork” of isolated cases. What is more, often these are presented with only anecdotal statements to support them- not measured experimental data. Nonetheless, it is worthwhile presenting some of these because some general conclusions can be drawn from them. Dohrenburg Systems reported [3] sources of ADSL Noise. Table 1 is extracted from a material provided in that article as applied to Impulse Noises enumerated in the classification previously carried out. Table 1: Sources of ADSL Noise reported by Dohrenburg Systems

Frequency Range

Origin Computers, monitors, printers Fluorescent lights Light dimmers, night lights Electrical motors Inverters and battery chargers Electric ranges Microwave ovens Electric water/air heaters Atmospheric Phenomena

60 Hz and harmonics, refresh rates 60 Hz and harmonics 60 Hz and harmonics 60 Hz and harmonics 60 Hz and harmonics 60 Hz and harmonics 60 Hz and harmonics, microwave radiation(2.45 GHz) 60 Hz and harmonics DC to 40 MHz

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Reviewing Table 1 it is evident that the Impulse Noises are all “low frequency” phenomena with the sole exception of “Atmospheric Phenomena.” Even considering “harmonics” these “low frequency” spectra at most might affect ADSL loops but would have little effect on VDSL2. Lightning flashes are generally of more importance to electrical protection than as a source of Impulse Noise events in communications. However, they may well generate electromagnetic pulses which may well couple into a DSL loop via a ground wave. The subject of Spectral and statistical characteristics of the electric field perturbations in the vicinity of thunderstorm clouds is dealt with in [4]. This article which is based upon measured experimental data provides some insight into the spectral characteristics of Impulse Noise caused by Lightning flashes. Figure 4 provides the ESD of electric field perturbations in the vicinity of thunderstorm clouds taken with electrostatic flux meters. The abscissa at the top indicates the time of observation. The authors partition it into “3 Bands” as follows: Band 1- time periods exceeding 20 minutes, Band 2- time periods extending from 1 minute to 20 minutes, Band 3- time periods less than 1 minute. Impulse Noise from a Lightning flash would fall within Band 2 and Band 3. The authors indicate that the frequency dependence in Band 2 falls exponentially while in Band 3 (which they relate to a “Pulse Process” the ESD has a power law dependence f-2. In either case this indicates essentially a process where the ESD is only significant in the low frequency region. However, it is plain from Figure 3 that “low frequency” here can mean frequencies in excess of 15 MHz. In other words based upon these measured results Impulse Noise events derived from lightning flashes could well affect VDSL2 receptions. Interestingly, this is consistent with the “Atmospheric Phenomena” of Table 1.

Figure 4: ESD of electric field perturbations in the vicinity of thunderstorm clouds There is further discussion of lightning induced noise in the web based article “Sources and Characteristics of Low Frequency Radio Noise,” Chris Trask, Sonoran Radio Research.

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The time domain behavior of a typical pulse is provided in this article and shown below as Figure 5.

Figure 5: Lightning pulse It is pointed out in this article that when measured across sea water, the frequency spectra for lightning return strokes has an f -1 frequency dependence from 100 kHz to 2 MHz, an f -2 dependence between 2 and 10 MHz, and an f -5 dependence above 10 MHz. True, the measurement across sea water is not directly applicable to DSL loops but it does provide an indication of the frequency dependence. What this says is that an Impulse Noise event derived from lightning may have impact up to the middle of the VDSL2 operating frequency range. Work carried out in an in-depth study by C. Collobert and colleagues at France Telecom is referenced in [1] and detailed in [5].“Impulse Noise Classification by a Non-Supervised Training Method,” Rolland, Bardouil, Clerot and Collobert, France Telecom Research and Development Report, December 21, 2000. In this work neural networks were employed to classify Impulse Noise types affecting ADSL in terms of their spectral density. Noise types were partitioned into 5 classes. The ESD of an exemplar Impulse for one of these classes is shown in Figure 3.9 of “DSL Advances.’ It clearly shows the ESD to being significant at frequencies below 1.5 MHznot surprising since the focus was on Impulse noise affecting ADSL. British Telecom (BT) has spent a considerable amount of effort characterizing Impulse Noise events in their operating environment. Typically, in this environment both telecom cables and power cables travel together. Furthermore, the particular manner by which electrical power is distributed in residences gives rise to repetitive and high level impulse trains which couple into DSL connections. BT has carried out extensive field measurements which have resulted in an accurate characterization of this Impulse Noise which they refer to as REpetitive High level Impulse Noise (REIN). REIN are bursts of pseudo random noise of 100μs duration. REIN Impulses are repetitive impulses that occur at twice the AC frequency. Consequently, the repetition rate is typically 120 Hz in North America and 100 Hz in Europe. The REIN spectrum is essentially significant at 11

frequencies below 2.2 MHz and trails off thereafter. REIN-ESD is again a “low frequency” spectrum. For a realization the dominant power is in the ADSL band. At higher frequencies it trails off quite fast. An Impulse modulated by Radio Frequency Interference (RFI) is a pervasive Impulse noise generated by the ignition system of modern vehicles. This is true of both spark, and compression ignited engines. The reason is simple. Ignition and/or injection systems contain a coil of wire in the form of an electromagnet. When the field collapses (after the coil is shut off), an EMP (electromotive pulse) is generated. It is this pulse which generates the spark across the spark plug's gap, and generates the majority of the RFI modulated Impulses. A goodly portion of the RFI is radiated not only by the secondary wiring in the vehicle, but into the primary wiring as well. With respect to Ignition Noise it has also been reported on the web that-At the same RPM, a V6 engine will fire three times with each revolution. At 60 MPH, that is a frequency of 5,400 Hz. Because the ignition arc occurs when the field of the ignition coil relaxes, the rise time is very fast. As a result, as significant amount of impulses modulated by RFI are generated well into the VHF spectrum- hence overlapping DSL spectral allocations. Modern engines universally incorporate electromechanical fuel injection. The electromagnetic coil is energized for a very precise period of time by the engine control computer. When the field collapses, a fast rise time pulse is generated. Here too the resulting RFI modulated impulses extend into the VHF spectrum-overlapping DSL spectral allocations. Modern diesels are not much better with respect to Impulse Noise as they too use electromagnetic injectors or an electromagnetic shuttle system. In fact, the current used to drive the coils tends to be higher than gas engines so more RFI is generated. Impulse Noise events resulting from Ignition Noise may well affect the entire VDSL2 band up to 30 MHz. The ESD of Vacuum cleaner noise is illustrated in Figure 6 taken from [2].

Figure 6: Vacuum cleaner noise spectrum

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This indicates that Impulse Noise form a vacuum cleaner is probably concentrated below 1 MHz. Dr. Kenneth J. Kerpez dealt with the subject of the measured spectra of Impulses Noise in considerable depth and reported results in [6], [7], [8]. He summarized the results of a collection of Impulses measured in the New York City area during a NYNE X impulse noise survey and previously reported in [9], [10]. “Figure 7 indicates an “average” ESD of Impulses measured in the NYNEX survey. Note that this indicates the ESD is essentially in the very low frequency region, below 1 MHz. Dr. Kerpez considered the subject of testing ADSL receivers in the presence of Impulse Noise and provided Impulse waveforms recorded at three residential locations, Fair Haven, Mountain View and Farmingdale. Figure 8, Figure 9 and Figure 10 are taken from this reported work. They provide both the temporal behavior and the ESD of test Impulses found at these locations. These figures again show the Impulse Noise ESD is concentrated at significant values only in the low frequency region, well below 1 MHz.

Figure 7: Average ESD of NYNEX Impulses

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Figure 8a: Temporal variation and ESD of Fair Haven test Impulse #1

Figure 8b: Temporal variation and ESD of Fair Haven test Impulse #2

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Figure 9a: Temporal variation and ESD of Mountain View test Impulse #1

Figure 9b: Temporal variation and ESD of Mountain View test Impulse #2

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Figure 10a: Temporal variation and ESD of Farmingdale test Impulse #1

Figure 10b: Temporal variation and ESD of Farmingdale test Impulse #2 16

Bas van den Heuvel provided spectra from some common Impulse Noise sources [11]. This work showed examples of measured Impulse Noise events from known sources. Figures 11-16 are taken from this document and provide measured examples of the spectra from these sources, [The units indicate “Power Spectral Density.” Even though as has been noted this terminology is not strictly accurate.] Note that the spectra show significant peaks, as high as -120 dBm/Hz (and above) all the way up to 25 MHz.

Figure 11: Spectrum of Impulse Noise event caused by switching off a light-bulb

Figure 12: Spectrum of Impulse Noise event caused by switching on a halogen light

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Figure 13: Spectrum of Impulse Noise event caused by switching on a fluorescent light

Figure 14; Spectrum of Impulse Noise event caused by switching on a vacuum cleaner

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Figure 15: Spectrum of Impulse Noise event caused by an electric drill

Figure 16: Spectrum of Impulse Noise event caused by lightning

Reports have been published concerning the results of a measurement campaign carried out in the Deutsche Bundespost Telekom [12]. Measurements were conducted at seven locations within Germany. Figure 17 is taken from this article. It shows the spectra of both Impulse noise and background noise recorded in the cities of Kassel and Mainz. [The units of the ordinate are in dB(pW/kHz)- again this terminology really relates to a “PSD” and is not strictly accurate.] The

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number “120” must be subtracted from the ordinate in order to convert it to dBm/Hz. With this it can be concluded that these measurements show the spectra concentrated below 2 MHz.

Figure 17: Top figure Impulse Noise and background noise in both Kassel and Mainz. Middle figure Impulse Noise in Kassel. Bottom figure Impulse Noise in Mainz.

Putting all of this admittedly limited material together it reasonable to conclude that Impulse Noise events may well occur over the entire spectrum allocated to DSL technologies- all the way up to 30 MHz. It is not unreasonable to conjecture from this material that the great majority of the sources of Impulse Noise are confined to the “low frequency” region and probably only affect those technologies below 4 MHz e.g. ADSL. Yet, some will affect VDSL2. An “effective” Impulse Noise event, resulting from the superposition of all sources in an operating environment may well affect all DSL allocations up to and including VDSL2. However, this statement can not be taken as an absolute generalization. There may well be many operating environments where the Impulse Noise events are confined to the low frequency region. As was indicated at the beginning of this Section a detailed, well organized and well documented all inclusive experimental measurment program should be carried out in order to have definitive conclusions.

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VI. How has Impulse Noise been modeled and characterized to date? For a really detailed and excellent discussion of this topic the reader is directed to [1]. While this reference is principally oriented to ADSL technology a great deal of the discussion can be generalized to other DSL technologies. The discussion is especially well grounded on experimental measurements reported by France Telecom, BT and Deutsche Telekom. However, it will be well worth here to provide some of the salient points mentioned in this discussion (modified here by way of interpretation): In the time domain Impulse Noise can be considered as a train of individual impulse waveforms. Each individual waveform may be chosen, randomly, from some ensemble of such waveforms. The “amplitude” of the chosen waveform can be chosen, randomly, from some ensemble, of allowed amplitudes. The inter-waveform time duration can be chosen randomly from some ensemble. This general model can be modeled in more detail as a Markov Process. The great majority of impulses in an operating environment have a duration ≤ 250 µsec. The great majority of impulses in the operating environment have amplitudes ≤ 10 mV. Many impulses are short with durations ≤ 100 µsec with small amplitudes. Even when these follow each other in a string of impulses they cause little in the way of service disruptions. There are clearly some very long impulses some being reported as long as 45 msec. However, these have low probability of occurrence. Standards groups have approached the modeling of Impulse Noises by concentrating on the effects of the “tails” of Impulse Noise characteristic densities. In other words, they are not interested in short impulses of small amplitudes which will have little effect on service. Their modeling is directed at large amplitude, long impulses, which have a greater effect on service. From a Power Spectral Density (PSD) point of view the PSD height has been measured as high as -80 dBm/Hz and as low as -120 dBm/Hz. As mentioned in the previous Section work has been carried out by France Telecom at using neural networks to categorize impulse waveforms affecting ADSL performance [5]. This work indicated that an enormous data base of measured impulses could be well categorized into 5 classes. This last point is worth further discussion. It points the way to a methodology for getting a handle on the many, diverse, possibilities of Impulse Noise waveforms affecting all 21

DSL technologies.

VII. What Impulse noises have been used for Standard’s based testing? This section provides a summary of what Impulse Noises have been used for testing in the various communications Standards that deal with DSL technology. ETSI

1. ETSI TS 101 270-1 V1.2.1 (1999-09) The noise shall consist of burst of Additive White Gaussian Noise injected onto the line with sufficient power to ensure effective erasure of the data for the period of the burst, i.e. the bit error ratio during the burst should be approximately 0,5. The noise burst shall be applied regularly at a repetition rate of at least 1 Hz. “The maximum delay of 20 msec being capable of sustaining error free performance when the path is subject to a noise burst of up to 500 µs. Optionally, it is permitted to operate with a maximum delay of up to 10 msec when subject to a noise burst of duration up to 250 µs.” 2. ETSI TS 101 388 V1.3.1 (2002-05) The noise shall consist of bursts of Additive White Gaussian Noise (AWGN) injected onto the line with sufficient power to ensure effective erasure of the data for the period of the burst, i.e. the bit error ratio during the burst should be approximately 0.5. The duration of the burst shall be no longer than 5 µs, and it shall be applied to the line at a frequency of once per second. 3. ETSI TS 101 524 V1.2.1 (2003-03) The impulsive noise is for further study. ITU

1. G.991.2 The impulse noise waveform V (t) (hereafter called the “test impulse”) is defined as:

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 K t 3 / 4  V (t )   0  K t  3 / 4 

t  0  t  0 t  0 

where t is time given in units of seconds and K is a constant defined numerically in provides the following table for the he Standard provides the following table for the Impulse Noise peak-to-peak voltage requirement K 1.775  10-6

VP-P of the test impulse sampled at 2Msamples/s 320 mV

Figure 18 below gives a time domain representation of the Test Pulse Sampled. 200

150

Amplitude (mV)

100

50

0

-50

-100

-150

-200 -50

-40

-30

-20

-10

0

10

20

Time (s)

Figure 18: G.991.2 Impulse Noise

2. G. 992.1 Impulse 1 and Impulse 2 as defined in G.test (G.996.1)

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30

40

50

These are shown in Figure 19 and Figure 20 below. It should be noted that they are based upon the Impulses shown in Figure 8a and Figure 8b. These were first adopted as test Impulses in the Standard ANSI T1.413. These Impulses are defined more precisely in the tables in this Standard- which have been omitted here for brevity.

30

Amplitude (mV)

20 10 0 –10 –20 –30

0.125

0.130

0.135

0.140

0.145

Time (ms)

Figure 19:

0.150 T1532220-99

Test Impulse 1

40 30

Amplitude (mV)

20 10 0 –10 –20 –30 –40

0.125

0.130

0.135 Time (ms)

0.140

0.145

0.150 T1532230-99

Figure 20: Test Impulse 2

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2. G.992.3 For Impulse Noise events refers to DSL Forum TR-048 [09] section 8.8.

3. G.993.1 The noise shall consist of bursts of Additive White Gaussian Noise (AWGN) injected onto the line with sufficient power to ensure effective erasure of the data for the period of the burst, i.e., the bit error ratio during the burst shall be approximately 0.5 (assuming FEC is not applied). The noise burst shall be applied regularly at a repetition rate of at least 1 Hz. The duration of the burst is variable; at least values of 10, 50, 100, 250, and 500 s shall be supported. The AWGN shall be generated with crest-factor of 5 and flat PSD up to 12 MHz.

DSL Forum/Broadband Forum

1. TR-48: Same as G.992.1

2. WT-062 Same as G.992.1 3. TR-100

3.1 Isolated Noise Burst An AWGN noise burst. This Impulse Noise event is known familiarly by the acronym “SHINE.” This refers to Single High (level) Impulse Noise Event. At most one test noise burst shall be applied within a 2 minute interval. The burst length changes by the loop length (according to Table 7-7) of the Standard. The variation is provided below: Table 2: Isolated Noise Burst Characteristics 26 AWG Loop Length (kft) 3 5 15

Impulse Noise Burst Length (msec) 4000 1000 100

3.2 Repetitive High level Impulse Noise (REIN) REIN is a “Burst of pseudo random AWGN” of 100μs duration at a level of –90dBm/Hz differential mode. The repetition rate is 100 Hz. The Standard refers to a “REIN – PSD” so this terminology will be used here. It is given by: 25

REIN- PSD (f) = -90 dBm/Hz, f ˂ 2.2 x 106 Hz. = max (-90 -40 x log10 (f/2.2 x 106), -150) dBm/Hz, f ≥ 2.2 x 106 Hz.

4. TR-114

4.1 Repetitive High level Impulse Noise (REIN) REIN uses a burst of pseudo random noise of 100μs duration whose differential signal power spectral density is described by equation REIN- PSD (f) = -90 dBm/Hz, f ˂ 2.2 x 106 Hz. = max (-90 -40 x log10 (f/2.2 x 106), -150) dBm/Hz, f ≥ 2.2 x 106 Hz. The repetition rate is regionally dependent and is 100 Hz for profile BA8c testing.

4.2 Prolonged Electrical Impulse Noise (PEIN) Model PEIN uses “Bursts of pseudo random AWGN” of varying duration, 1.2, 2.4 and 3.6ms with probabilities of 0.647, 0.229 and 0.124 respectively. The PSD/levels of the noise bursts in the differential mode SHALL be drawn from the distribution provided by Table 3. Table 3: PEIN Levels

PEIN PSD X dBm/Hz -86 -88 -90 -92 -94 -96 -98 -100

p(X) 0.0044 0.0133 0.0222 0.0311 0.0400 0.0489 0.0578 0.00667

PEIN PSD X dBm/Hz -102 -104 -106 -108 -110 -112 -114

p(X) 0.0756 0.0844 0.0933 0.1022 0.1111 0.1200 0.1289

The inter-arrival times of PEIN bursts are chosen between 4 s and 1094 s, with the discrete probability of choosing an inter-arrival time of x seconds being proportional to 1/x, the median

26

inter-arrival time will be 61 s and the average inter-arrival time will be roughly 190 seconds. This equates to a probability of p(x) = 1/ (x * ln(273.5)).

3.3 Single High Level Impulse Noise Event (SHINE) The SHINE noise burst uses an AWGN noise burst with amplitude of -86 dBm/Hz from 138 kHz up to 7.0 MHz. The out-of-band noise SHALL NOT be higher than -86 dBm/Hz. The SHINE noise is applied after the 60 seconds settling time. 4. TR-115 15 impulses spaced at least 1 second apart. Each impulse SHALL be a “burst of pseudorandom AWGN” of 200us duration at a level of -90dBm/Hz differential mode.

VIII. Mitigating Impulse Noise in Present, Implemented, DSL technologies Again for a really detailed and excellent discussion of this topic the reader is directed to the [1]. This is indeed a very complex subject and a totally detailed treatment of it is well beyond the intent of this present work. Nonetheless, it will be worthwhile to give a very brief and simplified overview of the current techniques employed and/or being considered to mitigate the effect of Impulse Noise in DSL connections. To begin with it must be understood that while Impulse Noise events can cause significant impairment for any type of communication system they particularly impact DMT modulation which is used in ADSL, ADSL2Plus and VDSL2.This is due to the fact that in DMT data is transmitted in blocks. Consequently, the impact of an Impulse Noise event is not limited to the on-set time of a particular Impulse or limited to its duration. Even a short Impulse Noise event may corrupt an entire DMT symbol and adjacent symbols. Commonly, it is assumed that an Impulse Noise event will corrupt an integer number of DMT symbols- this is called “spreading.” At the outset it must be noted that Impulse Noise cannot be combated by just a straightforward increase in the “Signal-to-Noise” ratio. When an Impulse Noise event occurs it so “overwhelms” the modulation waveform. The practicality of combating it by increasing signal power is out of the question. Current techniques for present, implemented, ADSL, ADSL2 Plus and VDSL2 technologies are generally based upon coding, either Forward Error Correction (FEC), Error Detection or a combination of both. Looked at from another direction these current techniques can be partitioned into three categories; 1) Erasure, 2) Interleaving, 3) Retransmission. There may be many techniques in each of these categories. 27

. Before continuing, it must be noted that there are other mitigation approaches. “Gating Techniques”- which are independent of coding, seek to limit the impact of impulse noise on performance. There are also techniques where an estimate is made of Impulse Noise component and then canceled limiting its impact on the input to the demodulator. An excellent example of this approach is the work reported by K. Kerpez [13]. It is not clear though whether these approaches have ever been implemented in actual practice in the field. With the “Erasure” category an Impulse Noise event is considered as providing an “erasure” of all of the data bits transmitted during the time of the event. An “Erasure” is simply a formal expression of “minimal knowledge” of the output of the DMT symbol demodulator during the Impulse Noise event. The “minimal knowledge” is really limited to knowing which input symbols yield “no demodulator output.” Techniques in the Erasure category essentially parse the stream of DMT symbol to be transmitted into segments with each segment containing some fixed number of DMT symbols. Each segment provides the input to a data symbol of Reed Solomon (RS) code word. The RS code is specifically designed with redundancy to correct those of its symbols which are caught as erasures- due to the Impulse Noise event. In truth, there are other algebraic coding techniques besides the RS which can be used. However, RS coding and decoding has been developed and well established over decades and has an advantage in limited complexity and being easy to implement. However, it must be pointed out that Erasure techniques inherently require some a priori assumptions concerning the duration of the Impulse Noise events. This is required in order to design the segmentation so that duration of the Impulse Noise event can be matched to the duration of an RS code symbol- so that it can be corrected. Because of this requirement these techniques cannot be characterized as being “Robust.” If the a priori assumptions concerning the duration are wrong then these techniques may well fail to meet the goals. Techniques in the “Interleaving” category really originate in work done in the early 1960s by researchers such as G. D. Forney (MIT/Codex) and D. Falconer (MIT/Bell Telephone Laboratories). To understand these types of techniques it is best to view the stream of DMT symbols as the stream of data bits which they represent down to the binary level. FEC works best when the transmission error experienced by these bits is isolated. The error event then can be considered as independent from bit to bit. Of course, this is absolutely not the case when an Impulse Noise event occurs. Then all bits in a time sequence will be “hit together.” There will be no isolation of errors. Interleaving techniques remedy this essentially forcing the isolation of errors. By way of example consider the data field that would normally be used as input to a DMT symbol. Suppose there are “B” bits in this data field. In the most extreme form of Interleaving each of these B bits would be encoded into some FEC code word say of length “C” binary code symbols. This is often referred to as the “Outer Code.” The C binary symbols of this Outer Code word would then be separated and transmitted in C separate DMT symbols. Each individual DMT symbol still does have FEC within it- which is separate- often called the “Inner Code.” This Inner Code is based upon Trellis coding. Under “normal circumstance” with no Impulse Noise events the Inner Code is sufficient for error correction. Now if a specific DMT symbol is hit by an Impulse Noise event then indeed 28

all of the binary symbols in this DMT symbol will be lost. The Inner Code basically fails in error correction. However, the Outer Code can still do error correction. The remainder of its binary code symbols have been spread to other DMT symbols which may not have experienced the Impulse Noise event- they may be transmitted past its duration. In this manner, error correction can be carried out during an Impulse Noise event. A comparison of the Interleaving techniques and the Erasure techniques will be worthwhile at this point. There are more detailed design issues to deal with in using Interleaving techniques as compared to Erasure techniques. To begin with the implementation of the Interleaving techniques is more complex than the RS coding-decoding of the Erasure techniques. This originates with the Interleaving techniques really having no a priori information as to which bits are in error. The Interleaving techniques must identify the bit error locations and then correct them. The Erasure techniques have “somewhat less to do.” They “know” which bits are in error e.g. the CRC failed and they can concentrate directly on correction. For the same level of FEC the throughput penalty on the Interleaving techniques for the redundant code bits required is more than the burden on the Erasure techniques. For “t” extra code bits the Interleaving techniques can only correct “t/2” errors while the Erasure techniques can correct “t” errors. Both the Erasure techniques and the Interleaving techniques have to implement the encoding and decoding. However, the Interleaving techniques have the extra burden of implementing the actual interleaving. This is additional processing. This is generally accomplished by using a bank of shift registers. The bits to be interleaved are shifted in serially. The interleaved bits are shifted out in parallel. For DSL this bank of shift registers is arranged in a particular architecture referred to as a convolutional interleaver. The time duration over which interleaving is carried out has to be considered carefully. On the one hand, the greater the duration the greater protection from an Impulse Noise event. On the other hand, the greater the duration the more latency in delivering data and certain broadband communication offerings may not be able to withstand long latencies. The degree to which interleaving provides protection against an Impulse Noise event is usually referred to as the INP Level. The different INP Levels correspond to the duration over which the interleaving is accomplished as measured in DMT symbols- each of which is 250 µsec long. INP Levels generally range from 1 to 5. Above Level 5 the latency may make this category of techniques impractical. With the Interleaving techniques and the Erasure techniques the level of protection against an Impulse Noise event is configured in the modems using a series of Management Information Base (MIB) parameters. With these the Impulse length for which protection is desired must be set. The maximum delay must be set. Interleaving techniques along with Erasure techniques have a number of weaknesses. Throughput is reduced because of the presence of the coding overhead- the redundancy. This 29

throughput reduction is present then regardless of whether or not an Impulse Noise event has actually occurred. The longer the Impulse Noise event for which protection is required, the higher the coding overhead and the greater the reduction in throughput. For long impulses under harsh delay requirements this overhead may climb to as high as 50%. Significant delay, non-negligible latency of ≈ 8 msec, is also introduced. The Interleaving techniques and Erasure techniques inherently require some a priori assumptions concerning the duration of the Impulse Noise events and what is more- their periodicity. This is required in order to design the depth of interleaving so that the Outer Code can correct its binary symbols caught in the Impulse Noise event. Along with this the depth of interleaving cannot be so great as to allow the “spread symbols” to be “caught” in the next Impulse Noise event in the case of periodic Impulse Noise. Because of this requirement for a priori knowledge Interleaving techniques, as with Erasure techniques cannot be characterized as being “Robust.” If the a priori assumptions concerning the duration are wrong then these techniques will fail in their goal. This point needs to be stressed. Because if the a priori assumptions are incorrect then instead of mitigating the effect of Impulse Noise interleaving could possibly make the situation worse- actually spreading the errors out rather than correcting them. Retransmission techniques are just becoming of interest at the present time with respect to DSL technology. It is currently addressed in the ITU-T Recommendation Standard Retransmission System G.998.4 (formerly G.inp). Using Retransmission this defines improved Impulse Noise protection for VDSL2, ADSL2 and ADSL2 Plus loops. This Recommendation is revisited and discussed in greater detail in Section X. Retransmission techniques can be traced to telegraphy in the 19th century- where a telegraph operator might wait in keying a message for an acknowledgement that the previous message sent along the line was decoded correctly. Techniques in this category are even present in Ethernet implementations in the form of Collision Sensing and Collision Detection. More generally, these techniques have traditionally come under the label of Automatic Repeat reQuest (ARQ). They do not require FEC but they do require encoding to allow error detection. Furthermore, they require essentially a two way channel. In the terminology of G.998.4 this is a Retransmission Request Channel (RRC). This is certainly not a problem with DSL technologies but could well be a problem in other communication technologies. With all ARQ techniques the data stream being transmitted is parsed into what we will call here, for the benefit of this simplified discussion “ARQ Frames.” Each ARQ Frame will be composed of a certain number of the payloads of DMT symbols. There may be other overhead associated with synchronization, FEC, sequencing etc. However, most importantly there are redundancy bits, specifically a “parity tail” computed from all of the other bits. This parity tail allows errors to be detected upon receipt of the ARQ Frame. Certainly, there will be such errors if the ARQ Frame is caught in an Impulse Noise event. If the ARQ Frame is received without errors then an ACKnowledgement (ACK) Message is sent back to the originating transmission side. If the ARQ Frame is received with errors then a Negative AcKnowledgment (NAK) Message is sent back to the originating transmission side. Usually, these messages are encapsulated in data fields of ARQ Frames going in this backward direction. In any case, this is why you need a two way channel for its 30

implementation. If a NAK is received at the originating side then the ARQ Frame in error is re-transmitted. There are many different versions of Retransmission techniques. In the simplest version the originating side waits for either an ACK or NAK for a given ARQ Frame before transmitting the next one. In the most complex version the transmission of ARQ Frames from the originating side is continuous. However, if a NAK is received then the ARQ Frame in error is retransmitted –being fitted into the continuous stream. Besides requiring a two way channel all Retransmission techniques require the originating side to store a copy of the ARQ Frame until it has determined through the ACK/NAK process that the ARQ Frame was received without error. The Receiver is responsible for buffering any out-of –sequence ARQ frames until the retransmitted frame is correctly received. The Transmitter is responsible for storing any ARQ frames that have not been acknowledged as correct by the Receiver. This storage requirement probably represents the greatest complexity associated with this category of techniques. It may well be the vast reduction of the price of storage and computation makes such techniques to have a complexity advantage over the previous ones discussed. The approach to Retransmission defined in G.998.4 is based upon using a fundamental block element called the Data Transfer Unit (DTU). This is effectively the ARQ frame. Each DTU contains an integer multiple of RS code words. In the case of a Retransmission the RS code words are not interleaved prior to encapsulation in a DTU. Prior to being transmitted, each DTU is placed in a Retransmission queue for potential Retransmission at a later time. Retransmission techniques do have some drawbacks. First among these is “jitter.” There is the inherent jitter in the data rate. This is due to the interrupts present whenever a Retransmission occurs. Secondly, these techniques take no account of “the importance of information data in error.” Information bits received in error which may well be discardedwithout penalty- because they are “unimportant on the Application Level are retransmitted nevertheless. These techniques effectively have a variable “received information data rate” as compared to the constant rate associated with - the Interleaving techniques and Erasure techniques. These could cause buffering problems in higher layers of the overall protocol structure. Finally, it must be pointed out that Retransmission only gives the illusion of being continuously variable. In actuality there is a requirement to set the total delay allowed or equivalently the retransmission buffer size, which then limits the number of correctable DMT symbols. There is a way around this. Retransmission could alternatively, lower the speed on a continual basis but this would wreak havoc with services like IPTV. At present, it is because of these drawbacks that such services as IPTV instead often (but not exclusively) prefer Constant Bit Rate (CBR) techniques or capped Variable Bit Rate (VBR) with occasional retransmissions generated from the TCP IP level. But, most Importantly, Retransmission techniques have several major advantages over Erasure and Interleaving techniques. No special type of erasure coding is required. There is latency. But, it is only realized as a burden when an actual Impulse Noise event occurs. This is markedly different than the Interleaving techniques where this burden is present all the 31

time. To the point, overhead for the correction of ARQ frames, due to Impulse Noise induced errors, is only realized when an Impulse Noise event actually occurs. Finally, Retransmission techniques have a Robustness which, as has been pointed out, is absent in the other techniques. There is little in the way of a priori knowledge of the duration or periodicity characteristics of the Impulse Noise required in order to implement these techniques. To be sure care does have to be exercised in the design of the ARQ Frame. On the one hand, they should be long enough so that the overhead represented by the parity tail, ACK/NAK and possible sequencing fields associated with ARQ, is not too much of burden. On the other hand, they should not be so long as to have retransmissions caused by the occasional, but rare, errors dues to ordinary, low level, Additive White Gaussian Noise (AWGN). Furthermore, sufficient memory must be reserved for retransmissions. It needs to be pointed out that in the design of a DSL loop the communications engineer is not limited to using just one of these techniques. Instead, they can be layered together and greater robustness in protection can be obtained in this manner. Furthermore, while FEC redundancy and interleaving depth, INP protection, cannot be changed “on the fly” they can be changed during reprofiling on a slow periodic basis. This would allow a “tailoring” of the FEC and interleaving to match changes in the loop Impulse Noise characteristics. Of course, this comes at the price of greater complexity and decreased throughput. As was mentioned above, in a “Long Impulse” environment the various protection techniques could possibly reduce throughput by as much as 50%. Finally, it must be understood that even if these techniques do not achieve fully the task of mitigating the error effects of Impulse Noise, higher level protocols such as TCP IP may provide the needed additional protection to get the desired level of performance.

IX. The Problem of Impulse Noise in Vectoring The leading edge DSL Technology at the present time is Vectoring. This is not the place for a complete and detailed discussion of Vectoring. Suffice it to say that Vectoring is focused on significantly increasing the throughput/capacity of copper local loops by carrying out cancellation of crosstalk, specifically FEXT, between loops within a binder (Intra Binder) and also between loops in different binders. Figure 21 illustrates the situation which Vectoring addresses. This figure shows 4 binders constituting a local loop cable employed for implementing DSL service-although in principle there can be more than 4 binders in a cable. It shows the possibility of FEXT between 2 loops within the same binder and between binders.

32

Figure 21: Binder geometry which Vectoring addresses

The interested reader is referred to “DSL Advances” by Starr, Sorbarra, Cioffi and Silverman and also the ITU Standard G.993.5 and the current Broadband Forum working text WT-249. While generally applicable to all DSL technologies, at present, the development of Vectoring is focused on increasing the throughputs of VDSL2. Briefly, Vectoring operates on a set of loops termed the Vectored Group. Vectoring, requires the construction Cross Correlation Matrix, R, corresponding to all of the loops in the Vectored Group. Once this is constructed its inverse, W, is obtained. In Down Stream operation the DSLAM modulator outputs from all ports in the Vectored Group are processed using W so that when FEXT hits them during transmission there is an effective cancellation of crosstalk- a pretransmission distortion type of processing. In Up Stream operation the DSLAM demodulator outputs from all ports in the Vectored Group are also processed using W so as to cancel the FEXT crosstalk. The cancellation of the crosstalk significantly raises the “Signal to Noise” ratio and with it throughput/capacity. The set of DSLAM modulator outputs/DSLAM demodulator inputs is mathematically represented as a vector. In terms of the present discussion the important thing to understand about Vectoring is that through its cancellation of FEXT it essentially eliminates crosstalk as the dominant deleterious effect limiting DSL performance. There are two processing phases in the implementation of Vectoring. For purposes of this simplified discussion they will be referred to as “Discovery Phase” and “Operational Phase.” In the Discovery Phase signals are interchanged between the CPE end and the CO end which allow the vectoring processing at the CO end “to learn” the Cross Correlation Matrix, R. From this it can learn W and effect the FEXT cancellation. The Discovery Phase is effectively a 33

“training period.” In the Operational Phase “the learned” W is used as part of the post demodulation process to cancel the FEXT. In considering the problem of Impulse Noise in Vectoring it will be best to consider these phases separately. In the Discovery Phase if Impulse Noise events are present they may very well cause the “estimate” of R that is “learned” to have large errors. In this situation when the “noisy” version of W is used FEXT may not be effectively cancelled. In fact, the throughput may possibly deteriorate rather than improve. The gravity of this situation must be emphasized. The presence of Impulse Noise during the Discovery Phase may well lead to an unstable almost chaotic situation. The algorithms and procedures used to estimate R may not converge and if they do converge they may be very inaccurate. Basically, in this situation the structure of Vectoring is being built on a foundation of “quick sand and could collapse.” What should be done to mitigate this possibility? The previous techniques discussed above for combating the effects of Impulse Noise were Erasure, Interleaving and Retransmission. These all operate on “the bit level.” The measurement of R is operating on the actual signal level. These previous techniques do not seem to help directly here. What is needed is work to be done at developing effective techniques for dealing with this issue. There certainly are possibilities but they need to be explored, analyzed and tested. An obvious possibility is spreading the Discovery Phase out over time, repeating it multiple times, and taking some average to obtain the estimate of R. In other words, averaging out the effect of the Impulse Noise. This is in the “philosophy of Interleaving” and has the burden of latency. But it may be possible to limit the latency. Along these same lines there is the possibility of implementing some type of cleverly designed Retransmission scheme, but with a tolerable latency. This needs to be investigated. Giving another possibility, it may be that Impulse Noise events are correlated on all loops in the vectored group. In such a situation the effect of the Impulse Noise may appear as a bias—actually a noise sample at the output of the demodulator which is constant for the duration of the Impulse Noise event. It may be possible to deal with this bias by having its presence, detected, estimated, and subtracted out. The point is work needs to done in limiting the vulnerability of the Discovery Phase to Impulse Noise or else the maximal benefits of Vectoring cannot be attained. In the Operational Phase the 3 techniques of Erasure, Interleaving and Retransmission can be applied. However, the question cannot really be closed with this. In the upstream direction W does have the effect of “amplifying” the alien crosstalk and other “foreign noises” which are not part of the cancellation process. This is very similar to what happens in adaptive equalization when it cancels InterSymbol Interference (ISI) at the expense of amplifying background noise. As such, it will “amplify” the effect of an Impulse Noise event. Whether or not this is of a level to be significant is open to question. It should certainly be a subject in testing of Vectoring.

X.

Dealing with Impulse Noise in a Post Vectored Environment

As one leading DSL technology leader has mentioned to the author, “Vectoring ‘drains the swamp’ of crosstalk.” Where does that leave DSL performance? It leaves it being limited by the 34

remaining interference sources, Additive White Gaussian Noise (AWGN), Radio Frequency Interference (RFI) and Impulse Noise. Other than the normal FEC which is currently being carried out there is little that can be done to limit the effect of AWGN- and in any case its level is generally low to begin with. RFI is generally highly correlated on all loops in a binder. As such, it is amenable to limitation if not outright cancellation through the differential signaling employed. This then leaves the Impulse Noises as the dominant source of performance limitation in an environment where Vectoring has been successful- the Post Vectoring environment. The conclusion from this is quite clear. In order for DSL technology to be able to take the next great leap forward after Vectoring more efficient approaches will be required for mitigating the deleterious effects of Impulse Noise. It is worthwhile to provide, at this point, a discussion of what work should be done along these lines. This discussion must begin with an observation of what is the driver in stimulating the need for throughput improvements beyond that achievable by Vectoring. While all triple play services are of interest it is quite clearly the need to accommodate greater video traffic whether it is for IPTV, HDTV, interactive video, gaming or a plethora of yet to be defined video services. Quality of Service (QoS) –essentially high bit rate, low error rate and low latency- for video services are much more stringent than for data services of digitized voice. Consequently, video services will require greater protection against Impulse Noise events than the other services. The present delivery of video services through DSL technology and other broadband technologies leaves a great deal to be desired from a Quality of Experience (QoE) point of view. QoE is usually defined as the measure of end-to-end performance at the services level from the user’s perspective. It is an indication of customer satisfaction with a Service Provider’s network. Pixelization, also called Checkerboarding, Freeze Frames, Dropped Frames and Video artifacts are all common problems affecting video delivery QoE. Why do these problems occur? They occur because, quality video is intolerant of packet loss and requires a packet loss of 10-6 delivered to the video decoder contained in the Set Top Box (STB) or other video decoding device. Bit errors due to Impulse Noise- even with the current mitigation techniques described before-on DSL loops actually cause significant video packet loss. The resultant packet loss is probably a couple of orders of magnitude below the desired threshold for high quality video delivery. As such, errors due to Impulse noise limit the deployment of high-quality video to a percentage of available access loops in order to meet the packet-loss requirements. As we have pointed out in the previous discussion the present principal methods for limiting the effects of Impulse Noise all do this at the burden of increasing the latency. The Erasure and Interleaving techniques have a burden of a fixed and significant latency whether or not an Impulse Noise event is even present. One of the driving factors in the development of Retransmission through the ITU-T Recommendation Standard Retransmission System G.998.4 (formerly G.inp) was to assure the correct reception of data that is affected by Impulse noise while gaining the benefit of the latency burden only when an Impulse Noise event takes place. ITU-T Recommendation Standard Retransmission System G.998.4 seeks to provide an improved 35

method for Impulse Noise Protection (INP) based upon the use of Retransmission. The introduction of the Standard Retransmission System G.998.4 has been such an important step forward in protection against Impulse Noise events. Some additional detailed description on its operation will be worthwhile. Retransmission is a core element in ITU-T Recommendation G.998.4. The Retransmission procedures of G.998.4 are implemented in either the Physical Media Specific-Transmission Convergence (PMS-TC) or in the Transport Protocol SpecificTransmission Convergence (TPS-TC) layers of the DSL reference model. This allows for management of the retransmission queues in a manner which is transparent to the modems. A retransmission procedure is provided for both ATM and packet based (PTM) transmission protocols. As mentioned before in Section VIII, the ARQ frame is called the Data Transfer Unit (DTU). A DTU contains an integer number of protocol units. A protocol unit is either a 53-byte ATM cell or a 65-byt PTM fragments. The DTU must also contain an integer multiple of RS code words. The DTU also contains a number of overhead fields needed for the smooth operation of the retransmission process. This includes: •

Sequence Identifier (SI) – 8 bits- to allow transmitter and receiver to identify specific DTUs

•

Time Stamp (TS) -8 bits- which records the first time the DTU is transmitted. This is needed to monitor delay requirement compliance

•

Padding Bytes (V) – to allow flexibility in constructing the DTU and satisfying various framing constraints

DTUs may be of 4 types depending upon whether or not they contain a Cyclic Redundancy Check (CRC) code. Each DMT symbol transmitted contains 24 bits that carry the Retransmission Request Channel (RRC). The RRC transfers information regarding the status of received DTUs and allows the other side to determine DTUs that require to be acknowledged and DTUs to be retransmitted. The RRC data is error correcting coding protected using a version of the Golay code. For VDSL2 Retransmission procedures are allowed in both upstream and downstream directions. For ADSL2 (G.992.3) and ADSL2 Plus (G.992.5) Retransmission procedures are only defined in the downstream direction for error detection. A procedure called the DTU framer extracts data to form DTUs according to the DTU framing type. DTUs are placed in a Retransmission Queue after being transferred for further processingscrambling, further RS coding, combining with other DSL overhead traffic [Indicator Bits (IB), Network Timing Reference (NTR)], modulation and transmission. 36

ITU-T Recommendation G.998.4. goes along way to providing reliable delivery of video services in the presence of Impulse Noise events. Furthermore, compared to the other techniques the latency is much improved. Yet, this may not be enough in the Post Vectoring environment. The delivery of video already suffers latency from other sources. By way of example there is Internet Group Management Protocol (IGMP) signaling delay, between the STB and the router. There is the MPEG decoding delay needed to acquire program-specific information (PSI) frames in order to determine the desired TV channel. There is the I-frame acquisition delay which relates to techniques used to reduce the amount of bandwidth required for digital video transmission. There is the Conditional-Access-System (CAS) key acquisition delay which relates to encryption of digital services. When the latency of the current Impulse Noise mitigation approaches is added to these “video service” delays the result is most likely a total latency which significantly impacts the QoE in a degrading manner. As has been pointed out Impulse Noise events cause “packet loss” in video services such as IPTV and adversely affect the QoE. This subject has been discussed in further detail in [14]. This report explores a range of potential solutions and makes recommendations regarding their applicability to IPTV. What then is needed in the Post Vectoring environment? What are needed are newer and more powerful Impulse Noise mitigation techniques which correct for errors but with less of a latency burden. It will be worthwhile to close by mentioning, on the concept level, several candidates to investigate along these lines: • More Powerful Erasure Codes The use of RS codes with “erased” symbols could be improved. The demodulator driving the decoder could employ a more complex “soft decision” scheme- in other words multiple classes of erasures based upon some measurements of the “signal to noise ratio.” Furthermore, codes with more favorable distance properties than the RS should be considered. This could result in codes of equal or greater error correction capability but which are shorter, with shorter symbolsmore powerful codes with less latency attached. • Time-Frequency Hopping This has long been employed in multipath abatement techniques and also in jam resistant communications. It combines the “time hopping” associated with the Interleaving category with “frequency hopping.” It is based upon the fact that in some operating environments and maybe the great majority of operating environments the spectrum of many of the Impulse Noise types (most probably) do not spread across the full 30 MHz of the VDSL2 band. This was pointed out in Section V. Rather, in many operating environments it is (most likely) confined to a small fraction located at the lower end e.g. below 4 MHz. With this technique interleaving of the Outer Code will not only be carried out across time but also across frequency. An Outer Code symbol may well then be caught in an Impulse Noise burst but be separated by spectrally and be capable of error correction- not erased. This may result in less interleaving depth for the same level of error protection—and consequently less latency.

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• Selective Retransmission Retransmission techniques already go a long way to limiting latency. The impact of latency only occurs when there is an actual Impulse Noise event causing a burst of errors. With Selective Retransmission, latency, on average, is further reduced. The error detection is combined with a form of Soft Decision demodulation. A metric is developed indicating the level of error deterioration in the received ARQ Frame. The lowest metric would correspond to a complete erasure of the ARQ Frame- equivalent to it being transmitted over a binary channel with the probability of a bit error = 0.5 (effectively a bit erasure). The highest metric would correspond to an ARQ Frame which is beyond the error correction capability within but where “reasonable demodulator signal to noise ratios” are indicated. A threshold is established. Retransmission is only requested for those ARQ frames below the threshold. For those above the threshold further error correction is executed by comparing to the previous ARQ Frame accepted- taking advantage of the inherent redundancy of video. • Selective Protection The impact of Impulse Noise on video depends upon which type of encoded video data information is hit by an Impulse. For example, if an I-Frame, which carries significant reference information of the digitized image, is hit by Impulse Noise then it may be lost and cause significant freeze in the picture. On the other hand, if an Impulse hits one of the other MPEG frame types, B-frame or P-frame, which are based on predicted relational video data then the impact on QoE may not be noticed. It may be that the above techniques should only be applied to the critical frame types e.g. the I-frame, and not the other types thus limiting the latency. Admittedly, this touches on a type of “cross protocol layer” optimization and currently this is usually “shunned” in practice. However, it may be more practical to implement in the future. • Exploiting Correlation In The Cancellation of Impulse Noise There are often situations where the Impulse Noise is correlated across the loops in the binder. In particular, this may be prevalent at the CPE end. For example, automatically opening a garage door may generate correlated Impulse Noise to the residence in question and to neighbors on either side. Some work has been done along the lines of developing techniques for cancelling correlated noises within the processing associated with Vectoring. These techniques have the attractive feature of having negligible latency burden. Whatever delay is introduced is not on the transmission level but rather on the post demodulation Vector processing level. In particular, mention is made of an interesting Patent Application [15. These types of techniques usually require an unused local loop in the binder. Hence, there is an obvious cost tradeoff regarding the increase in throughput on the used loops where the correlated Impulse Noise in cancelled and the lack of throughput on the purposefully unused loop. Also, these techniques require some a priori knowledge about which loops have correlated Impulse Noise on them. Obviously, you do not want to sacrifice throughput on an unused loop unless there really is correlated Impulse Noise to be cancelled in the Vectored group. As a result, at present, the practicality of these techniques is open to question. However, they offer an interesting future possibility for further Impulse Noise abatement. Hopefully, development of these techniques will continue and the issues of practicality will be resolved. 38

• Use Fountain Codes – Rateless Erasure Codes This is a class of erasure codes which have the interesting property that the original “k” information source symbols can be recovered from “k1” coding symbols where k is just slightly larger than k1. The term “fountain” or “rateless” refers to the fact that these codes do not exhibit a fixed code rate. These types of codes could possibly make the Retransmission techniques more efficient. With this type of code it appears that the ARQ frame architecture – as described in VIII- can be abandoned. A new more efficient architecture is used. LT codes were the first practical realization of Fountain codes followed by Raptor and Online codes. Use of these codes could possibly reduce latency. But, it must be pointed out that they have the “disadvantage” of a variable bit rate. Further investigation and development is warranted.

REFERENCES 1. ” DSL Advances,” Ch. 3 by Starr, Sorbarra, Cioffi and Silverman, Prentice Hall 2003. 2. “A Power Line Communication Tutorial- Challenges and Technologies” by Phil Sutterlin and Walter Downey, Echelon Corporation, available on the web. 3. Dohrenburg Systems report available on the web. 4. “Radio-physical methods of analysis for thunderstorm field perturbations,” E.A. Mareev, V. V. Klimenko, Yu. V. Shlyugaev, M.V. Shatalina, D.I. Iudin, Institute of Applied Physics of the Russian Academy of Sciences. 5. “Impulse Noise Classification by a Non-Supervised Training Method,” Rolland, Bardouil, Clerot and Collobert, France Telecom Research and Development Report, December 21, 2000. 6. T1E1.4/90-179, “Study of the Feasibility and Advisability of Digital Subscriber Lines Operating at Rates Substantially in Excess of the Basic Access Rate,” Kenneth J. Kerpez, September 26, 1990. 7. T1E1.4/93-034, “Interfaces Relating to Carrier Connection of Asymmetric Digital Subscriber Line (ADSL) Equipment,” Kenneth J. Kerpez, March 10, 1993. 8. T1E1.4/91-165, ““Study of the Feasibility and Advisability of Digital Subscriber Lines Operating at Rates Substantially in Excess of the Basic Access Rate,” Kenneth J. Kerpez, November 12, 1991 9. T1E1.3/86-144, “NYNEX Corp., ‘Characteristics of Impulse Noise on Selected NYNEX metropolitan loops,’ ECSA Contribution.” 10. T1E1.4/92-227, “Analysis of Loop and Inside Wire Background Noise Measured at Two New Jersey Residential Locations,” C. Valenti, K. Kerpez and B. Blake, December 1, 1992. 11. “Examples of impulse noise events from known sources,” bbf2009.1102.00, Bas van den Heuvel, November 2009.

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12. “A Wideband Impulsive Noise Survey in the German Telephone Network: Statistical Description and Modeling,” Werner Henkel and Thomas Keßler, AEU-International Journal of Electronics and Communications, Vol.48, No.6, (1994). 13. “Minimum Mean Squared Error Impulse Noise Estimation and Cancellation, K. Kerpez, IEEE Transactions On Signal Processing, Vol. 43, No. 7, July 1995. 14. “IPTV Packet Loss Issue Report”- ATIS-0800005, December 2006. 15. Alien Interference Removal In Vectored DSL, Nicholas Sands and Kevin Fisher, No. 20110142111, 06/16/2011.”

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