Improving Low Voltage Distribution Line Carrier Communication Systems for Transferring Data by Applying Efficient Modulation Techniques

Transcription

1 Imprving Lw Vltage Distributin Line Carrier Cmmunicatin Systems fr Transferring Data by Applying Efficient Mdulatin Techniques SHAHRAM JAVADI Islamic Azad University Central Tehran Branch Electrical Engineering Department PARASTOO POURANG Islamic Azad University Suth Tehran Branch Cmputer Engineering Department Mshanir Pwer Engineering Cnsultants IRAN IRAN Abstract: Distributin Line Cmmunicatins technlgy allws the usage f electrical pwer supply netwrks fr cmmunicatins purpses and, tday, als bradband cmmunicatin services. The main idea behind DLC is the reductin in peratinal csts and expenditure fr realizatin f new telecmmunicatins netwrks. Using electrical supply netwrks fr telecmmunicatins has als been knwn since the beginning f the twentieth century. Thus high-, medium- and lw-vltage supply netwrks have been used fr internal cmmunicatins f electrical utilities and fr the realizatin f remte measuring and cntrl tasks. Generally, we can divide DLC systems int tw grups: narrwband DLC allwing cmmunicatins services with relatively lw data rates (up t 100 kbps) and ensuring realizatin f varius autmatin and cntrl applicatins as well as a few vice channels, and bradband DLC systems allwing data rates beynd 2Mbps and, accrdingly, realizatin f a number f typical telecmmunicatins services in parallel, such as telephny and internet access. This paper discusses the varius methds f mdulatin techniques fr applicatin f distributin line carrier (DLC) systems n medium vltage lines fr the purpse f energy management and system cntrl. Depending n the attenuatin the range f the technlgy is up t several kilmeters. Keywrds: Pwer Distributin Netwrks, Pwer Line Carrier, Cmmunicatin, Mdulatin Technique. I. Intrductin During the last decades, the usage f telecmmunicatins systems has increased rapidly. Because f a permanent necessity fr new telecmmunicatins services and additinal transmissin capacities, there is als a need fr the develpment f new telecmmunicatins netwrks and transmissin technlgies. Frm the ecnmic pint f view, telecmmunicatins prmise big revenues, mtivating large investments in this area. Therefre, there are a large number f cmmunicatins enterprises that are building up high-speed netwrks, ensuring the realizatin f varius telecmmunicatins services that can be used wrldwide. The direct cnnectin f the custmers/subscribers is realized ver the access netwrks, realizing access f a number f subscribers situated within a radius f several hundreds f meters. Hwever, the csts fr realizatin, installatin and maintenance f the access netwrks are very high. It is usually calculated that abut 50% f all netwrk investments belngs t the access area. On the ther hand, a lnger time is needed fr paying back the invested capital because f the relatively high csts f the access netwrks, calculated per cnnected subscriber. Therefre, the netwrk prviders try t realize the access netwrk with pssibly lw csts. ISSN: ISBN:

2 After the deregulatin f the telecmmunicatins market in a large number f cuntries, the access netwrks are still the prperty f incumbent netwrk prviders (frmer mnplistic telephne cmpanies). Because f this, the new netwrk prviders try t find a slutin t ffer their wn access netwrk. An alternative slutin fr the realizatin f the access netwrks is ffered by the DLC technlgy using the pwer supply grids fr cmmunicatins. Thus, fr the realizatin f the DLC netwrks, there is n need fr the laying f new cmmunicatins cables. Therefre, applicatin f DLC in lw-vltage supply netwrks seems t be a cst-effective slutin fr s-called last mile cmmunicatins netwrks, belnging t the access area. Hwever, pwer supply netwrks are nt designed fr cmmunicatins and they d nt present a favrable transmissin medium. Thus, the DLC transmissin channel is characterized by a large and frequency-dependent attenuatin, changing impedance and fading as well as unfavrable nise cnditins. Varius nise surces, acting frm the supply netwrk, due t different electric devices cnnected t the netwrk, and frm the netwrk envirnment, can negatively influence a DLC system, causing disturbances in an errr-free data transmissin. On the ther hand, t prvide higher data rates, DLC netwrks have t perate in a frequency spectrum f up t 30 MHz, which is als used by varius radi services. Unfrtunately, a DLC netwrk acts as an antenna prducing electrmagnetic radiatin in its envirnment and disturbs ther services perating in the same frequency range. II. Mdulatin Techniques Mdulatin is the prcess f varying a peridic wavefrm, i.e. a tne, in rder t use that signal t cnvey a message. 1) Analg mdulatin methds In analg mdulatin, the mdulatin is applied cntinuusly in respnse t the analg infrmatin signal. A lw-frequency message signal may be carried by an AM r FM radi wave. Cmmn analg mdulatin techniques are: Amplitude mdulatin (AM) Duble-sideband mdulatin (DSB) Single-sideband mdulatin (SSB, r SSB-AM), Vestigial sideband mdulatin (VSB, r VSB-AM) Quadrature amplitude mdulatin (QAM) Angle mdulatin Frequency mdulatin (FM) Phase mdulatin (PM) 2) Digital mdulatin methds In digital mdulatin, an analg carrier signal is mdulated by a digital bit stream. Digital mdulatin methds can be cnsidered as digital-t-analg cnversin, and the crrespnding demdulatin r detectin as analg-t-digital cnversin. The changes in the carrier signal are chsen frm a finite number f M alternative symbls. The mst cmmn digital mdulatin techniques are: Phase-shift keying (PSK): Frequency-shift keying (FSK): Amplitude-shift keying (ASK) n-ff keying (OOK), the mst cmmn ASK frm Quadrature amplitude mdulatin (QAM) - a cmbinatin f PSK and ASK: Cntinuus phase mdulatin (CPM) methds: Orthgnal frequency divisin multiplexing (OFDM) mdulatin: Wavelet mdulatin Trellis cded mdulatin (TCM), als knwn as trellis mdulatin 3) Spread-spectrum mdulatin: These techniques are methds by which energy generated in a particular bandwidth is ISSN: ISBN:

3 deliberately spread in the frequency dmain, resulting in a signal with a wider bandwidth. These techniques are used fr a variety f reasns, including the establishment f secure cmmunicatins, increasing resistance t natural interference and jamming, and t prevent detectin. Frequency-hpping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), timehpping spread spectrum (THSS), chirp spread spectrum (CSS), and cmbinatins f these techniques are frms f spread spectrum. Each f these techniques emplys pseudrandm number sequences, created using pseudrandm number generatrs t determine and cntrl the spreading pattern f the signal acrss the alltted bandwidth. Ultra-wideband (UWB) is anther mdulatin technique that accmplishes the same purpse, based n transmitting shrt duratin pulses. Wireless Ethernet standard IEEE uses either FHSS r DSSS in its radi interface. T reduce the negative impact f distributin line transmissin medium, DLC systems have t apply efficient mdulatin, such as spread spectrum and Orthgnal Frequency Divisin Multiplexing (OFDM). Tw imprtant tpics have been studied t imprve the DLC perfrmance: The first is related t the line cuplers fr lw and high vltage level lines [1], [2]. The secnd is related t the mdulatin techniques used fr data transmissin. The chice f the mdulatin technique fr a given cmmunicatins system strngly depends n the nature and the characteristics f the medium n which it has t perate. The pwer line channel presents hstile prperties fr cmmunicatins signal transmissin, such as nise, multi-path, strng channel selectivity. Besides the lw realizatin csts, the mdulatin t be applied fr a DLC system must als vercme these channel impairments. Fr example, the mdulatin, t be a candidate fr implementatin in DLC system, must be able t vercme the nnlinear channel characteristics. This channel nnlinearity wuld make the demdulatr very cmplex and very expensive, if nt impssible, fr data rates abve 10 Mbps with single-carrier mdulatin. Therefre, the DLC mdulatin must vercme this prblem withut the need fr a highly cmplicated equalizatin. Impedance mismatch n pwer lines results in ech signal causing delay spread, cnsisting in anther challenge fr the mdulatin technique, which must vercme this multi-path. The chsen mdulatin must ffer a high flexibility in using and/r aviding sme given frequencies if these are strngly disturbed r are allcated t anther service and therefre frbidden t be used fr DLC signals. In this paper we have fcused n tw mdulatin techniques that have shwn gd perfrmances in ther difficult envirnment and were therefre adpted fr different systems with wide deplyment. First, the Orthgnal Frequency Divisin Multiplexing (OFDM), which has been adpted fr the Eurpean Digital Audi Bradcasting (DAB), the Digital Subscriber Line (DSL) technlgy, and s n. Secnd, the spread-spectrum mdulatin, which is widely used in wireless applicatins, ffering an adequate mdulatin t be applied with a wide range f the multiple access schemes. III. Orthgnal Frequency Divisin Multiplexing Orthgnal Frequency Divisin Multiplexing is a special frm f Multi-Carrier Mdulatin (MCM) with densely spaced sub carriers and verlapping spectra, as shwn by the OFDM symbl representatin in the frequency dmain in figure 1. Figure 1. OFDM symbl presentatin in the frequency dmain Cmpared t mdulatin methds such as Binary Phase Shift Keying (BPSK) r Quadrature Phase Shift Keying (QPSK), OFDM transmits symbls that have relatively lng time duratin, but a narrw bandwidth. Usually, ISSN: ISBN:

4 OFDM systems are designed s that each subcarrier is narrw enugh t experience frequency-flat fading. This als allws the sub carriers t remain rthgnal when the signal is transmitted ver a frequency-selective but timeinvariant channel. The primary advantage f OFDM ver singlecarrier schemes is its ability t cpe with severe channel cnditins. Fr example, attenuatin f high frequencies in a lng cpper wire, narrwband interference and frequency-selective fading due t multipath withut cmplex equalizatin filters. Channel equalizatin is simplified because OFDM may be viewed as using many slwly-mdulated narrwband signals rather than ne rapidly-mdulated wideband signal. The lw symbl rate makes the use f a guard interval between symbls affrdable, making it pssible t handle timespreading and eliminate inter-symbl interference (ISI). OFDM is used by pwer line devices t extend Ethernet cnnectins t ther rms in a hme thrugh its pwer wiring. Adaptive mdulatin is particularly imprtant with such a nisy channel as electrical wiring. An OFDM carrier signal is the sum f a number f rthgnal sub-carriers, with baseband data n each sub-carrier being independently mdulated cmmnly using sme type f quadrature amplitude mdulatin (QAM) r phase-shift keying (PSK). This cmpsite baseband signal is typically used t mdulate a main RF carrier. A typical OFDM transmitter-receiver is shwn belw in figures 2 and 3. Figure 2. OFDM Transmitter Figure 3. OFDM Receiver The OFDM transmissin scheme has the fllwing key advantages[3]: Makes efficient use f the spectrum by allwing verlap By dividing the channel int narrwband flat fading subchannels, OFDM is mre resistant t frequency selective fading than single carrier systems are. Eliminates ISI thrugh use f a cyclic prefix. Using adequate channel cding and interleaving ne can recver symbls lst due t the frequency selectivity f the channel. Channel equalizatin becmes simpler than by using adaptive equalizatin techniques with single carrier systems. It is pssible t use maximum likelihd decding with reasnable cmplexity, as discussed in OFDM is cmputatinally efficient by using FFT techniques t implement the mdulatin and demdulatin functins. In cnjunctin with differential mdulatin there is n need t implement a channel estimatr. Is less sensitive t sample timing ffsets than single carrier systems are. Prvides gd prtectin against cchannel interference and impulsive parasitic nise. In terms f drawbacks OFDM has the fllwing characteristics: The OFDM signal has a nise like amplitude with a very large dynamic range; therefre it requires RF pwer amplifiers with a high peak t average pwer rati. It is mre sensitive t carrier frequency ffset and drift than single carrier systems are due t leakage f the DFT. IV. Spread-Spectrum Mdulatin Spread spectrum is a type f mdulatin that spreads data t be transmitted acrss the entire available frequency band, in excess f the minimum bandwidth required t send the infrmatin. ISSN: ISBN:

5 Spread spectrum riginates frm military needs and finds mst applicatins in secure cmmunicatins envirnments; such is the case in the DLC envirnments. Its typical applicatins are the crdless telephnes, wireless LANs, DLC systems and cable replacement systems such as Bluetth. In sme cases, there is n central cntrl ver the radi resurces, and the systems have t perate even in the presence f strng interferences frm ther cmmunicatin systems and ther electrical and electrnic devices. In this case, the jamming is nt intentinal, but the electrmagnetic interferences may be strng enugh t disturb the cmmunicatin f the nnspread spectrum systems perating in the same spectrum. The principle f the spread spectrum is illustrated in figure 4, where the riginal infrmatin signal, having a bandwidth B and duratin T S, is cnverted thrugh a pseud-nise signal int a signal with a spectrum ccupatin W, with W>>B. Fr military applicatins, the S F is between 100 t 1000, and in the UMTS/W- CDMA system the SF lies between 4 and 256. This parameter is als knwn as spreading gain r prcessing gain and is defined by Eq. (1). W G = = W. T S (1) B an averaging type system where the reductin f interference takes place because the interference can be averaged ver a large time interval. The FH and TH systems are avidance systems. 1) Direct Sequence Spread Spectrum Direct Sequence Spread Spectrum (DSSS) is the mst applied frm f the spread spectrum in several cmmunicatins systems. T spread the spectrum f the transmitted infrmatin signal, the DSSS mdulates the data signal by a high rate pseudrandm sequence f phase mdulated pulses befre mixing the signal up t the carrier frequency f the transmissin system which is shwn in figure 5. Figure 5. Synptic scheme f a DSSS transmitter 2) Frequency Hpping Spread Spectrum In a Frequency Hpping Spread-Spectrum system (FHSS) the signal frequency is cnstant fr specified time duratin, referred t as a time chip Tc. The transmissin frequencies are then changed peridically which is shwn in figures 6 and 7. Figure 4. Principle f bandwidth spreading in DSSS Amng the several advantages f spreadspectrum technlgies, ne can mentin the inherent transmissin security, resistance t interference frm ther systems, redundancy, and resistance t multipath and fading effects. The cmmn speed spread-spectrum techniques are Direct Sequence (DS), Frequency Hpping (FH), Time Hpping (TH), and the Multi-Carrier (MC). Of curse, it is als pssible t mix these spreadspectrum techniques t frm hybrids that have the advantages f different techniques. We fcus in this paragraph nly n DS and HF. The DS is Figure 6. Transmitter fr FHSS Figure 7. Receiver fr FHSS ISSN: ISBN:

6 3) Cmparisn f DSSS and FHSS The cmparisn can be achieved accrding t different evaluatin parameters, such as the spectral density reductin, interference susceptibility, capacity, and s n. Furthermre, the chice f the suitable scheme accrding t the system needs is based n parameters that are linear r inversely dependent n each ther. Bth DSSS and FHSS reduce the average pwer spectral density f a signal. The way they d it is fundamentally different and has serius cnsequences fr ther users. Fr an ptimal system realizatin, the bjectives are t reduce bth transmitted pwer and pwer spectral density, t keep them frm interfering with ther users in the band. DSSS spreads its energy by phase-chpping the signal s that it is cntinuus nly fr brief time intervals (r chip). Therefre, instead f having all the transmitted energy cncentrated in the data bandwidth, it is spread ut ver the spreading bandwidth. The ttal pwer is the same, but the spectral density is lwer. Of curse, mre channels are interfered with than befre, but at a much lwer level. Furthermre, if the spread signal cmes in under the nise level f mst ther users, it will nt be nticed. Traditinal FHSS signals lwer nly their average pwer spectral density hpping ver many channels. But during ne hp, a FHSS signal appears t be a narrw band signal, with a higher pwer spectral density. The interference susceptibility is anther imprtant parameter which allws the system t perate prperly. In DSSS receivers, the despreading peratin cnsists in multiplying the received signal by a lcal replica f the spreading cde. This crrelates with the desired signal t push it back t the data bandwidth, while spreading all ther nn-crrelating signals. After the de-spread signal is filtered t the data bandwidth, mst f the nise is utside this new narrwer bandwidth and is rejected. This helps nly with all types f narrwband and uncrrelated interference, and it has n advantage fr wideband interference since spread nise is still nise and the percentage that falls within the data bandwidth is unchanged. The FHSS signal is agile and des nt spend much time n any ne frequency. When it hits a frequency that has t much interference, the desired signal is lst. In a packet switched system, this results in a retransmissin, usually ver a clearer channel. In a fast enugh FHSS system, the prtin f lst signal may be recvered by using a FEC. Other parameters and cmparisns f the DSSS and FHSS and als ther methds are listed in Table 1, frm which it becmes clear that the DSSS shws mre advantages than the FHSS systems. A cmpared graph between DSSS and FHSS methds is shwn in figures 8 and 9. Figure 8. Direct Sequence Figure 9. Frequency Hpping V. Chice f Mdulatin Scheme fr DLC Systems Several investigatins have been carried ut t find suitable OFDM implementatins fr DLC netwrks. In rder t avid hard degradatin f OFDM signal ver the transmissin channel, which is caused by the frequency-selective fading, a methd fr sub-carriers pwer cntrl ISSN: ISBN:

7 cnsists f cntrlling the transmissin pwer f each subcarrier f OFDM signal in rder t maximize the average SNR f each subcarrier f the received signal. This cntrlling is s flexible that the ttal transmitted pwer is nt increased. Further imprvement f such cntrlling is pssible by spreading the parallel sub-streams at the utput f the serial-t-parallel cnverter utput. An OFDM system which subdivides the riginal infrmatin int three parallel data grups, where each grup is mapped either accrding t BPSK r QPSK and cded accrding t Reed Slmn cde r cnvlutin cde, is als investigated in [KuriHa03]. Perfrmances f OFDM system were als investigated under different nise scenaris, especially under the impulsive nise, which is cnsidered the dminating nise in PLC envirnment. Spread-spectrum mdulatin techniques, with direct sequence r frequency hpping, were investigated t be implemented in DLC physical layer. An iterative detectin algrithm fr M- ary spread-spectrum system ver a nisy channel is investigated and this shws a remarkable imprvement f the detectin perfrmance fr M-ary systems. Hwever, the main drawback f the spread-spectrum technique is the relative lwer realizable bit rate, in cmparisn with OFDM systems. This makes any decisin abut the mdulatin t be adpted fr a DLC system mre difficult. By deciding fr a given mdulatin, the system designer must knw which perfrmances have the higher pririty fr him and which nes have less imprtance. Besides the high realizable bit rates, the OFDM systems als shw a high rbustness against the channel distrtins, a flexibility in aviding the strngly affected channels and an ptimal bandwidth utilizatin by the usage f the slightly disturbed channels thrugh the bit-lading prcedure. The main advantage f the spread spectrum is its electrmagnetic cmpatibility, by the radiatin f weak electrmagnetic fields in the envirnment. DSSS FHSS Table 1) Cmparisn f the advantages and drawbacks f varius methds Advantages Disadvantages Reduced with prcessing gain Cntinuus spread f the transmitted signal pwer gives minimum interference Cntinuus and bradband transmissin Narrwband interference in the same channel is reduced by the prcessing gain N timing cnstraints in high data rates Self-synchrnizatin Simple frequency planning Gd interference rejectin Very lw access delay Reduced with prcessing gain Hpping makes transmissin n usable channels pssible Simple analg limiter/discriminatr receiver Simple frequency planning Gd interference rejectin Lw-pwer, lw-cst radis In higher data rates If a statin is jammed, it is jammed until the jammer ges away Cmplex baseband prcessing Medium bit rates (up t 11Mbps) Only the average pwer f the transmitted signal is spread, and this gives less interference reductin Discntinuus and narrwband Transmissin Narrwband interference in the same interference is nt reduced In higher data rates, If a channel is jammed, the next available transmissin time n a clear channel may be Tc duratin away Many channels need t search fr synchrnizatin Lw bit rates OFDM Mitigates multipath High cmplexity and deplyment csts Guard bands reduce efficiency Frequency ffsets require accurate AFC Synchrnizatin is difficult Single- Carrier QPSK / QAM High bit rates, prven technlgy Efficient, dynamic capacity allcatin fr bursty surces. High peak-t-average pwer rati requires PA back-ff Susceptible t multipath interference (needs an equalizer) Susceptible t interference (needs interference avidance technique) ISSN: ISBN:

8 VI. Cnclusin In this paper we investigate n several different methds fr mdulatin technique used in data transfer ver pwer lines. As it is shwn, because f nature f envirnment in electrical netwrk, it is useful t use digital mdulatin techniques such as DSSS, FHSS r OFDM. Cnclusin the prper chice f direct sequence r frequency hpping as a spread spectrum technique depends n the actual envirnment in which the system will be deplyed. If there are narrwband interferers f mderate level, then a DSSS system that will cmpletely reject them may be designable. Shuld there be any large interfering signals, then a DSSS link may cmpletely fail while FHSS is likely t cntinue perating, even thugh the interference is nt cmpletely rejected. VII. Acknwledgment The authrs thank Islamic AZAD University fr whle helps and supprts. [6] A. Gusma, R. Dinis, J. Cnceica, N. Esteves Cmparisn f Tw Mdulatin Chices fr Bradband Wireless Cmmunicatins IEEE Vehicular Tech. Cnference 2000 (VTC 2000), Tky, Japan (Vl.2, pg ) [7] H. Zu, H.J. Kim, S. Kim, B. Daneshrad, R. Wesel, W. Magine-Smith Equalized GMSK, Equalized QPSK and OFDM, a Cmparative Study fr High-Speed Wireless Indr Data Cmmunicatins, IEEE Vehicular Tech. Cnference 1999 (VTC 99) [8] R. C. Dixn, Spread spectrum Systems with Cmmercial Applicatins, 3ed, Jhn Wiley & Sns, New Yrk, 1994 [9] M. K. Simn, J. K. Omura, R. A. Schltz, B. K. Levitt, Spread Spectrum Cmmunicatins Handbk, McGraw-Hill, New Yrk, 1994 References [1] K. C. Abraham and S. Ry, "A Nvel High-Speed PLC Cmmunicatin Mdem," IEEE Trans. Pwer Delivery, vl. 7, N. 4, pp , Octber [2] K. S. Murthy, Decupling Netwrks fr Prmting Pwer line Carrier Systems, IEEE Trans. Pwer Delivery, vl. 10, N. 2, pp , April [3] Standard IEEE a [4] A. Blle, O. Erikssn, A. Nascimbene Cmpetitive Bradband Access via Micrwave Technlgy Ericssn Review, N.4, 1998, pg [5] R. Hashlzner, C. Drewes, J.S. Hammerschmidt The Effects f Multiple Access Schemes n Equalizatin fr Bradband WLL Systems IEEE Vehicular Tech. Cnference 1998 (VTC 98), Ottawa, Canada ISSN: ISBN: