Broadband Powerline Communications Networks Design phần 4 potx

72 Broadband Powerline Communications NetworksFrequency Narrowband noise Background noise Figure 3.22 Spectral density model for the generalized background noise and build therefore freq

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72 Broadband Powerline Communications Networks

Frequency

Narrowband noise Background noise

Figure 3.22 Spectral density model for the generalized background noise

and build therefore frequency bundles that are usually approximated by a narrowbandoccupation Therefore, for its modeling, this noise will be seen as a narrowband noise withvery low psd The power density of the colored background noise is time-averaged for themodeling by NCBN(f ) The time-dependence characteristic of this noise can be modeled

independently with the knowledge of the standard deviation; [Beny03] Therefore, the psd

of the generalized background noise can be written under the following form:

where NCBN(f ) is the psd of the colored background noise, NNN(f ) the psd of the

narrowband noise andN k

NN(f ) is the psd of the subcomponent k generated by the interferer

k of the narrowband noise.

For the model of the colored background noise psd, the measurements have shown that aﬁrst-order exponential function is more adequate, as formulated by Eq (3.25); [Beny03]

NCBN(f ) = N0+ N1· e−f f1 (3.25)

with N0 the constant noise density, N1 and f1 are the parameters of the exponentialfunction, and the unit of the psd is dBµV/Hz1/2 Through different investigations andmeasurements of noise in residential and industrial environments, it was possible to ﬁndout approximations for the parameters of this model and the psd of the colored back-ground noise can be described by Eqs (3.26) and (3.27) for residential and industrialenvironments respectively; [Phil00]:

NBN(f )= −35 + 35 · e−f [MHz3,6 ] for residential environments and (3.26)

NBN(f )= −33 + 40 · e−f [MHz8,6 ] for industrial environments (3.27)

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PLC Network Characteristics 73

For the approximation of the narrowband noise interferers, the parametric Gaussianfunction is used, whose main advantages are the few parameters required for specifying themodel Furthermore, the parameters can be individually found out from the measurements,which have shown only a small variance; [Beny03]:

of which an example is shown in Fig 3.23

The aim of these investigations and measurements is to ﬁnd out the statistical acteristics of the noise parameters, such as the probability distribution of the impulseswidth and their interarrival time distribution, representing the time between two succes-sive impulses, Fig 3.24 One approach to model these impulses is a pulse train with pulsewidthtw, pulse amplitudeA, interarrival time taand a generalized pulse functionp(t/tw)

char-with unit amplitude and impulse width tw; [ZimmDo00a]:

Impulses envelope Impulses signal

Figure 3.23 Example of some measured impulses in the time domain in a PLC network

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Figure 3.24 The impulse model used for impulsive noise class modeling

The parameters tw,i , Ai and ta,i of impulse i are random variables, whose statistical

properties are measured and investigated in [ZimmDo00a] The measured impulses haveshown that 90% of their amplitudes are between 100 and 200 mV Only less than 1%exceeds a maximum amplitude of 2 V The measurements of the impulse width tw havealso shown that only about 1% of the measured impulses have a width exceeding 500µsand only 0.2% of them exceeded 1 ms Finally, the interarrival time that separates twosuccessive impulses is below 200 ms for more than 90% of the recorded impulses Othermore detailed measurements show that about 30% of the detected pulses had an interarrivaltime of 10 or 20 ms, which represents the impulsive noise that is synchronous with themains supply frequency, noise type 3 The interarrival times, lying above 200 ms, have

an exponential distribution

3.4.4 Disturbance Modeling

The disturbances can have a big impact on the transmission in PLC networks on differentnetwork layers As this book focuses on the design of the MAC layer, we considerthe disturbance modeling to be used in such investigations In the following section,

we describe a simple on–off disturbance model and a complex disturbance model forapplication in investigations of OFDM-based transmission systems

3.4.4.1 On –Off Model

In Sec 3.4.2, it is shown that the generalized background noise is stationary over seconds,minutes or even hours It is also concluded that periodic impulses, synchronous to themean frequency (noise type 4) have a short duration and low psd On the other hand,the short-term variance in the powerline noise environment is mostly introduced by theasynchronous impulsive noise (type 5) Those impulses can reach a duration of up toseveral milliseconds and a higher psd

Suitable methods for forward error correction and interleaving (Sec 4.3) can deal withdisturbances caused by the impulsive noise However, a certain error probability remains,

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Toff

Ton

Figure 3.25 On – Off disturbance model

which results in erroneous data transmission and the resulting retransmission of the aged data units Incorrect data transmission has a big inﬂuence on the performance ofMAC and higher network layers Therefore, an on–off disturbance model is developed

dam-to represent the inﬂuence of the asynchronous impulsive noise on the data transmission.The noise impulses can make a transmission channel for a certain time period After theimpulse disappears, the affected transmission channel is again available Under this kind

of noise, the disturbances in a PLC transmission channel can be represented by an on–offmodel with two states; Tonand Toff (Fig 3.25) [HrasHa00]

Toff state represents the duration of an impulse making the channel unavailable for thetime of its duration.Tonis the time without disturbances (absence of disturbance impulses)when the channel is considered available Both duration of the disturbance impulsesand their interarrival time can be represented by two random variables that are negativeexponentially distributed, according to the behavior of the noise impulses [ZimmDo00,ZimmDo00a, Zimm00]

3.4.4.2 Complex Disturbance Models for OFDM-based Systems

In the consideration above, an on–off error model is deﬁned describing the availability of

a transmission channel However, if a disturbance impulse occurs, it can affect a variablenumber of OFDM subcarrier frequencies depending on its characteristics, spectral power,origin, and so on Therefore, the disturbances have to be modeled not only in the timedomain (duration and interarrival time of impulses) but also in the frequency domain,specifying how many and which subcarriers are affected by a disturbance impulse.Furthermore, in the simple on–off disturbance model, an OFDM subcarrier can be only

in two hard deﬁned states: On – available for the transmission, or Off – not available Onthe other hand, an OFDM system can apply bit loading (Sec 4.2.1) to provide variable datarates of a subcarrier according to its quality, which depends on the noise behavior on thesubcarrier frequency To model an OFDM system using bit loading, the on–off disturbancemodel is extended to include several states between “channel is Off” (transmission notpossible) and “channel is On” (full data rate is possible) as is presented in Tab 3.7.The states between “Off” and “On” represent the situations when a subcarrier is affected

by the disturbance impulse, but is still able to transmit the data In such cases, the based systems are able to reduce the data rate over affected subcarriers and to make the

OFDM-Table 3.7 Subcarrier data rates in a multistate error model – an example

Subcarrier status On On−1 On−2 On−3 On−4 On−5 On−6 On−7 Off

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transmission possible Therefore, the multistate error model make sense if an based PLC system is investigated As is mentioned above, the length of typical PLC accessnetworks is up to several hundreds meters Thus, we can expect that the distrubances candifferently affect particular network segments; for example, depending on the position ofnoise source, protection of powerline grids in different network sections, and so on Inthis case, a PLC network is under the inﬂuence of so-called selective distrubances, wherethe network stations are differently affected by particular disturbances, which primarlydepend on their position in the network Such distrubances are represented by selectivedisturbance models It can be concluded that the distrubances can act selectively in twodifferent ways, frequency and space/position dependent

OFDM-3.4.4.3 Model Parameters

For the speciﬁcation of the parameters representing general disturbance characteristics inPLC access networks, measurements of the disturbance behaviors have to be carried out innumerous networks operating in various environments: rural and urban areas, business andindustrial areas, PLC networks designed with various technologies (e.g different types

of cables), and so on Local conditions and realizations of PLC networks can be verydifferent from each other and the achieved measurement results can strongly vary fromnetwork to network Therefore, there is not only a need for the general characterization

of the disturbance behavior but also for the characterization of each individual PLCaccess network

3.5 Summary

The low-voltage networks have complex topologies that can differ strongly from onenetwork to another This difference comes from the fact that they have parameters whosevalues can be varied, such as the users density, the users activity, the connected appliances,and so on Generally, it can be concluded that low-voltage power supply networks, alsoincluding in-home part of the network, have a physical tree topology However, on thelogical level, a PLC access network can be considered a bus network, representing ashared transmission medium Because PLC networks perform on shared medium, there

is the need for medium access management policy This task is taken by a base station,which control the access to the medium over the whole or only a part of the consideredPLC network The base station is also the point over which access to the WAN is possible.Additional PLC devices, such as repeaters and/or gateways can also be implemented.Low-voltage networks were designed only for energy distribution to households and awide range of devices and appliances are either switched on or off at any location and atany time This variation in the network charge leads to strong fluctuation of the mediumimpedance These impedance fluctuations and discontinuity lead to multipath behavior ofthe PLC channel, making its utilization for the information transmission more delicate.Beside these channel impairments, the noise present in the PLC environment makes thereception of error-free communication signal more difficult The noise in PLC networks

is diverse and is described as the superposition of ﬁve additive noise types, that are

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categorized into two main classes – on the one hand is the background noise, whichremains stationary over long time intervals, and on the other is the impulsive noise,which consists of the principle obstacle for a free data transmission, because of its relativehigh intensity This impulsive noise results in error bursts, whose duration can exceedthe limit to be detected and corrected usually by used error correcting codes Therefore,the impulsive noise in PLC networks has to be represented in appropriate disturbancemodels

EMC is the ﬁrst requirement to be met by any device, before it enters the marketand even before it enters the wide production phase However, this remains the mainchallenge that the PLC community is facing Several services use one or multiple parts

of the spectrum 0–30 MHz that is targeted by the PLC system This makes the set ofpossible EM victims of PLC devices larger In spite of it, standardization activities aregoing on and trying to reach international ﬂexible standards for the electrical ﬁeld strengthlimits, like those imposed by the FCC Part 15

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Realization of PLC Access

Systems

As considered in Chapter 3, PLC access networks are characterized by given topology

of low-voltage supply networks, unfavorable transmission conditions over power grids,problem of electromagnetic compatibility and resulting low data rates and sensitivity todisturbances from the network itself and from the network environment To solve theseproblems and to be able to ensure data transmission over power grids, achieving certaindata rates necessary for realization of the broadband access, various transmission mecha-nisms and protocols can be applied As mentioned in Sec 2.3.3, PLC access systems arerealized by several network elements Basically, the communication within a PLC accessnetwork takes place between a base station and a number of PLC modems, connectingPLC subscribers and their communications devices In this chapter, we present realization

of PLC access systems including their transmission and protocol architecture implementedwithin the network elements, as well as telecommunications services which are applied

to broadband PLC networks

4.1 Architecture of the PLC Systems

Exchange of information between distant communicating partners seems to be very plex The communications devices used can differ from each other, and the informationﬂow between them can be carried out over multiple networks, which can apply dif-ferent transmission technologies To understand the complex communications structures,the entire communications process has been universally standardized and organized inindividual hierarchical communications layers [Walke99] The hierarchical model exactlyspeciﬁes tasks of each communications layer as well as interfaces between them, ensuring

com-an easier speciﬁcation com-and stcom-andardization of communications protocols

Nowadays, the ISO/OSI Reference Model (International Standardization

Organiza-tion/Open Systems Interconnection, Fig 4.1) is mainly used for description of various

communications systems It consists of seven layers, each of them carrying a preciselydefined function (or several functions) Every higher layer represents a new level ofabstraction compared to the layer below it The first network layer specifies data trans-mission on a so-called physical network layer (transmission medium), and every higher

Broadband Powerline Communications Networks H Hrasnica, A Haidine, and R Lehnert

 2004 John Wiley & Sons, Ltd ISBN: 0-470-85741-2

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Data link Network

Transmission medium (e.g power grid for PLC)

MAC LLC

Transport layers

Figure 4.1 The ISO/OSI reference model

layer speciﬁes processes nearer to communications applications (end user device) TheOSI reference model is well described in the available literature, for example, [Tane98].Therefore, we just give a brief description of functions speciﬁed in the reference model

so as to be able to deﬁne PLC speciﬁc network layers

• Layer 1 – Physical Layer – considers transmission of bits over a communications medium,including electrical and mechanical characteristics of a transmission medium, synchro-nization, signal coding, modulation, and so on

• Layer 2 – Data Link – is divided into two sublayers (e.g [John90]):

– MAC – Medium Access Control (lower sublayer) – speciﬁes access protocols– LLC – Logical Link Control (upper sublayer) – considers error detection and cor-rection, and data ﬂow control

• Layer 3 – Network Layer – is responsible for the set-up and termination of networkconnections, as well as routing

• Layer 4 – Transport Layer – considers end-to-end data transport including tation of transmitted messages, data ﬂow control, error handling, data security, and

• Layer 7 – Application Layer – provides interface to the end user

Network layers 5–7 are nearer to the end user and to a running communications

applica-tion Therefore, these network layers are very often characterized as Application Network

Layers (or Application-oriented Layers) [Kade91] As against the application layers,

net-work layers 1–4 are responsible for the transmission over a netnet-work, and accordingly,

they are called Transport Layers (Fig 4.1), or Transport-oriented Layers.

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Realization of PLC Access Systems 81

As mentioned above, the transport layer (layer 4) takes care of end-to-end connectionsand, accordingly, is implemented within end communication devices (e.g TCP in standardcomputer equipment) On the other hand, network layers 1–3 fulﬁll tasks related to thedata transmission over different communications networks and network sections (subnet-works) In accordance with this, these layers are implemented within various network

elements, such as switching nodes, routers, and so on, and are called Network Dependent

Layers (or Network Layers) Thus, the transport layer (layer 4) represents an interface

between the network layers and the totally network-independent application layers 5–7

A PLC access network consists of a base station and a number of subscribers usingPLC modems The modems provide, usually, various user interfaces to be able to con-nect different communications devices (Fig 4.2) Thus, an user interface can provide anEthernet interface connecting a personal computer On the other hand, a PLC modem isconnected to the powerline transmission medium providing a PLC speciﬁc interface Thecommunication between the PLC transmission medium and the user interface is carriedout on the third network layer Information received on the physical layer form the pow-erline network is delivered through MAC and LLC sublayers to the network layer, which

is organized according to a speciﬁed standard (e.g IP) ensuring communications betweenPLC and Ethernet (or any other) data interfaces The information received by the datainterface of the communications device is forwarded to the application network layers.The base station connects a PLC access network and its powerline transmission medium to

a communications distribution network, and with it to the backbone network (Sec 2.3.4).Accordingly, it provides a PLC speciﬁc interface and a corresponding interface to thecommunications technology used in the distribution network Generally, the data exchangebetween a PLC network and a distribution network is carried out on the third network layer,such as between the PLC interface in the modem and the user interface

In accordance with the consideration presented above, it can be recognized that bothbase stations and PLC modems provide a speciﬁc interface for their connection to thepowerline transmission medium (Fig 4.2) On the other hand, the interfaces for the con-nection to the distribution and backbone networks, as well as to various communicationsdevices, are realized according to communications technologies applied in the backboneand in the end devices, which are speciﬁed in the corresponding telecommunicationsstandards The interconnection between PLC and other communications technologies iscarried out on the third network layer, which is also standardized

Network LLC

PHY

MAC

PHY MAC LLC

Net Tran.

MAC PHY PLC modem

PLC network layers

Figure 4.2 PLC speciﬁc network layers

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The PLC speciﬁc interface includes ﬁrst two network layers: physical layer and MACand LLC sublayers of the second network layer PLC physical layer is organized according

to the speciﬁc features of the powerline transmission medium and is described in Sec 4.2.Owing to the inconvenient noise scenario in PLC networks (Sec 3.4), various mechanismsfor error handling, as a part of the LLC sublayer, are an important issue and they areconsidered in Sec 4.3 A description of PLC services and their classiﬁcation are presented

in Sec 4.4 Because of the fact that the emphasis of this book is set on the MAC sublayer,PLC MAC layer and its protocols are separately considered in Chapter 5 and Chapter 6

4.2 Modulation Techniques for PLC Systems

The choice of the modulation technique for a given communications system stronglydepends on the nature and the characteristics of the medium on which it has to operate.The powerline channel presents hostile properties for communications signal transmission,such as noise, multipath, strong channel selectivity Besides the low realization costs, themodulation to be applied for a PLC system must also overcome these channel impairments.For example, the modulation, to be a candidate for implementation in PLC system, must beable to overcome the nonlinear channel characteristics This channel nonlinearity wouldmake the demodulator very complex and very expensive, if not impossible, for datarates above 10 Mbps with single-carrier modulation Therefore, the PLC modulation mustovercome this problem without the need for a highly complicated equalization Impedancemismatch on power lines results in echo signal causing delay spread, consisting in anotherchallenge for the modulation technique, which must overcome this multipath The chosenmodulation must offer a high ﬂexibility in using and/or avoiding some given frequencies

if these are strongly disturbed or are allocated to another service and therefore forbidden

to be used for PLC signals

Recent investigations have focused on two modulation techniques that have showngood performances in other difﬁcult environment and were therefore adopted for differentsystems with wide deployment First, the Orthogonal Frequency Division Multiplexing(OFDM), which has been adopted for the European Digital Audio Broadcasting (DAB),the Digital Subscriber Line (DSL) technology, and so on Second, the spread-spectrummodulation, which is widely used in wireless applications, offering an adequate modula-tion to be applied with a wide range of the multiple access schemes

In this section, we explain the principles of each modulation technique and their matical background Then, some practical realizations of the demodulator (or transmitter)and its corresponding demodulator (or receiver) are proposed for each modulation Finally,

mathe-a compmathe-arison between these cmathe-andidmathe-ates is discussed, showing the mathe-advmathe-antmathe-ages mathe-and drmathe-aw-backs of each one of them This comparison could make it possible to make a decisionabout the choice of the modulation technique to be adopted for PLC systems, allowing

draw-to meet some performances that can be required from the network, such as the high bitrate, the level of electromagnetic disturbances, or bit error rate, and so on

4.2.1 Orthogonal Frequency Division Multiplexing

4.2.1.1 Modulation Principles

MultiCarrier Modulation (MCM) is the principle of transmitting data by dividing thestream into several parallel bit streams, each of which has a much lower bit rate, and by

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Frequency

B

Figure 4.3 OFDM symbol presentation in the frequency domain

using several carriers, called also subcarriers, to modulate these substreams The basis of

a MCM modulation is illustrated in Fig 4.5 The ﬁrst systems using MCM were military

HF radio links in the 1960s Orthogonal Frequency Division Multiplexing is a specialform of MCM with densely spaced subcarriers and overlapping spectra, as shown by theOFDM symbol representation in the frequency domain in Fig 4.3 To allow an error-freereception of OFDM signals, the subcarriers’ waveforms are chosen to be orthogonal toeach other Compared to modulation methods such as Binary Phase Shift Keying (BPSK)

or Quadrature Phase Shift Keying (QPSK), OFDM transmits symbols that have relativelylong time duration, but a narrow bandwidth In the case of a symbol duration which isless than or equal to the maximum delay spread, as is the case with the other modulations,the received signal consists of overlapping versions of these transmitted symbols or Inter-Symbol Interference (ISI) Usually, OFDM systems are designed so that each subcarrier

is narrow enough to experience frequency-ﬂat fading This also allows the subcarriers

to remain orthogonal when the signal is transmitted over a frequency-selective but invariant channel If an OFDM modulated signal is transmitted over such a channel, eachsubcarrier undergoes a different attenuation By coding the data substreams, errors whichare most likely to occur on severely attenuated subcarriers are detected and normallycorrected in the receiver by the mean of forward error correcting codes

time-In spite of its robustness against frequency selectivity, which is seen as an advantage

of OFDM, any time-varying character of the channel is known to pose limits to thesystem performance Time variations are known to deteriorate the orthogonality of thesubcarriers; [Cimi85] In this case, the Inter-Carrier Interference (ICI) appears becausethe signal components of a subcarrier interfere with those of the neighboring subcarriers

By transmitting information onN subcarriers, the symbol duration of an OFDM signal

isN times longer than the symbol duration of an equivalent single-carrier signal

Accord-ingly, ISI effects introduced by linear time dispersive channels are minimized However,

to eliminate the ISI completely, a guard time is inserted with a duration longer than theduration of the impulse response of the channel Moreover, to eliminate ICI, the guardtime is cyclically extended It is to be noted that, in the presence of linear time disper-sive channels, an appropriate guard time avoids ISI but not ICI, unless it is cyclicallyextended [Rodr02] For this reason a guard time withT duration is added to the OFDM

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TCP: cyclic prefix duration

T : OFDM symbol duration

Duplication

Figure 4.4 Adding the cyclic preﬁx by duplicating the ﬁrst part of the original symbol

symbol, and in order to build a kind of periodicity around this OFDM symbol the tent of this guard time is duplicated from the ﬁrst part of the symbol, as represented inFig 4.4 In this case, the guard time becomes the cyclic preﬁx (CP)

con-The insertion of the appropriate cyclically extended guard time eliminates ISI and ICI

in a linear dispersive channel; however, this introduces also a loss in the signal-to-noiseratio (SNR) and an increase of needed bandwidth; [Rodr02] The SNR loss is given by

4.2.1.2 Generation of OFDM Signals

The generation of the OFDM symbols is based on two principles First, the data stream issubdivided into a given number of substreams, where each one has to be modulated over aseparate carrier signal, called subcarrier The resulting modulated signals have to be thenmultiplexed before their transmission Second, by allowing the modulating subcarriers

to be separated by the inverse of the signaling symbol duration, independent separation

of the frequency multiplexed subcarriers is possible This ensures that the spectra ofindividual subcarriers are zeros at other subcarrier frequencies, as illustrated in Fig 4.3,consisting of the fundamental concept of the orthogonality and the OFDM realization.Figure 4.5 shows the basic OFDM system [Cimi85] The data stream is subdivided into

= 1/fs, where fs is the desired symbolrate N serial elements modulate N subcarrier frequencies which are then frequency

division multiplexed The symbol interval has now been increased to

robustness to the delay spread caused by the channel Each one of two adjacent subcarrierfrequencies are then spaced by the interval formulated by Eq (4.3)

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Serial to parallel conversion

Ψ 0 (t )

Ψ k (t )

Ψ N −1(t )

Figure 4.5 Basic OFDM transmitter

This ensures that the subcarrier frequencies are separated by multiples of 1/T so that the

subcarriers are orthogonal over a symbol duration in the absence of distortions It is to

be noted that T in this phase is the OFDM symbol duration to which the cyclic period

Tcp is not yet added

According to the basic OFDM realization, the transmitted signals(t) can be expressed by

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