A Novel Power Line Communication System for Outdoor ... - IEEE Xplore

2015 IEEE International Symposium on Power Line Communications and Its Applications (ISPLC)

A Novel Power Line Communication System for Outdoor Electric Power Grids Moises V. Ribeiro Fabricio P. V. Campos Guilherme R. Colen Hugo V. Schettino Electrical Engineering Department Federal University of Juiz de Fora Juiz de Fora, Brazil 36036 330 e-mail: [email protected] [email protected] [email protected] [email protected]

Diogo Fernandes Lucas M. Sirimarco Victor Fernandes Smarti9 Ltda Juiz de Fora, Brazil 36036 330 e-mail: [email protected] [email protected] [email protected]

Abstract—This paper outlines a novel power line communication (PLC) system that was specified, designed, and prototyped to address the features of low-voltage and outdoor Brazilian electric power grids when the frequency band between 1.7 and 50 MHz, which complies with the Brazilian regulation for broadband PLC, is considered. The proposed PLC system is constituted by a PLC base station and several PLC modems. Also, it makes use of the clustered orthogonal frequency division multiplexing scheme to divide the total frequency bandwidth into several subchannels (clusters). As a result, the maximum throughput achieved by a PLC modem is a fraction of the maximum throughput that a PLC base station can offer and, as a consequence, the computational complexity as well as hardware resource utilization demanded by a PLC modem is considerably reduced in relation to the PLC base station. Data-rate as high as 24 Mbps can be attained in each subchannel at the application level, finally, but not the least, this PLC system can be useful for smart grid communication and digital inclusion.

I.

I NTRODUCTION

The increasing need for data communication to fulfill digital inclusion and smart grid communication demands are pushing forward the development of novel generation of power line communication (PLC) systems to efficiently make use of the electric power grids for bidirectional information exchange, in addition to energy delivery. The main outputs associated with these drives are the introduction of PLC standards for broadband and narrowband data communications by the IEEE and ITU-T [1], [2] as well as national regulation for broadband PLC in different nations. Also, advanced generation of PLC systems that will take into account multiple input multiple output (MIMO) [3] and cooperative concepts and will exploit the existing diversity between power line and wireless channels [4]–[6] to improve system performance, coverage, and reliability are being worldwide investigated. Due to the fact that the power systems are the most complex artificial systems developed so far by the human beings, their complete understanding and characterization as a communication medium is a challenging issue that need to be addressed worldwide because the dynamics of electric

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Antonio A. M. Picorone CEMIG D S.A. Juiz de Fora, Brazil 36010 040 e-mail: [email protected]

power grids can vary considerably, mainly in relation to: income, population density, power system topology, frequency bandwidth, distance, environment, and type of loads. There are several contributions addressing these issues in developed countries but similar analyzes for in-developing and underdeveloped countries are missing in the literature. Therefore, PLC standards are being introduced in the market based solely on the features of electric power grids of developed countries. This become a considerable problem because the main markets for PLC technologies are in-developing and under-developed countries. As a result, the performance of PLC systems may not offer what is expected because the features of electric power grids in those kinds of countries were not taken into account in specification and design stages of current PLC standards. In order to address this gap, [7] presents a statistical analysis of outdoor electric power grids as data communication medium, highlighting distinctions among power line channels in Brazil, Europe, and USA that deserve attention for developing PLC systems that could be suitable for in-developing and under-developed countries, besides the developed ones. Aiming to address the necessity of PLC technologies for low-income population, this work outlines a broadband PLC system for digital inclusion and smart grids for in-developing country with the following novelties: its specifications are based on electric power grids of a typical in-developing country; it makes use of the clustered orthogonal frequency division multiplexing (clustered-OFDM) scheme [8] for reducing the cost associated with PLC modems; and it takes into account the Brazilian regulation for Broadband PLC. In this regards, we briefly describe the main characteristics of the hardware, physical and link layers. Additionally, we address PLC network operation. Finally, we discuss important aspects related to the legacy of such kind of R&D project as well as some performance results related to this novel PLC technology. The rest of this paper is organized as follows: Section II presents a brief but comprehensive description of the novel PLC system by addressing physical and link layers, manage-


Fig. 2. The prototype of the PLC Base Station installed in a box that is secured in the pole. Fig. 1.

Typical scenario to deploy the novel PLC system.

ment software, and hardware. Additionally, some R&D outputs related to the novel PLC system are briefly discussed in Section III. Section IV discusses performance results obtained with the novel PLC system. Finally, concluding remarks are outlined in Section V. II.

T HE N OVEL PLC S YSTEM

Fig. 1 shows the use of the novel PLC systems in a typical low-voltage electric power grids. As we can see, there are one PLC base station (BS) and several PLC modems that communicate with each other. In this system, the BS works as a bridge by providing 10/100/1000 Mbps ethernet interface for connecting the modems belonging to the PLC access network with another data network (optic, wireless, or cable) for internet access. The PLC BS and PLC modems have a PLC coupling interface that injects and extracts signals of the power cables in the frequency band between 1.7 and 50 MHz. The BS is responsible for allocating the PLC modems in the clusters. This decision is made based on the signal to noise ratio (SNR) of all clusters as well as the current allocation of modems in order to guarantee a fair use of network resources. Additionally, the PLC BS is responsible for managing the operation of PLC access network by controlling the connection and disconnection of PLC modems as well as their dynamic resource allocations and scheduling. The hardware of PLC BS is more complex than that one of the PLC modem. Also, the BS can be installed on a pole holding or not the medium-to-low voltage transformer and support up to 180 PLC modems. Fig. 2 shows the prototype of the PLC BS together with personal computer and a uninterruptible power supply in a typical field tests in Juiz de Fora city, Brazil. The PLC modem has a serial interface for connecting the meter equipment and a 10/100 Mbps ethernet interface that can be used to establish a wireless local area network. Also, the PLC modem can only operate in one subchannel (cluster) and due to the specification of the PLC system it can achieve datarate up to 93 Mbps in the physical layer. Based on the fact that the number of clusters are equal to five, the PLC BS delivers a data-rate up to 465 Mbps at its physical layer and its hardware

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Fig. 3. The prototype of the PLC modem installed at the entrance premise of a costumer and its connection to the smart meter.

is more complex than that one of the PLC modem. Fig. 3 shows the prototype of PLC modem connected to a smart meter in a field test. Based on a measurement campaign, we noted that the PLC modem connection to power cable have to be made before the power meter equipment because it considerably attenuates the PLC signal. A. Physical Layer The novel PLC system is mainly based on clusteredOFDM scheme. This PLC system have five clusters, each cluster occupies a bandwidth equal to 10 MHz and runs an independent hermitian symmetric-OFDM (HS-OFDM) scheme [8]. The clusters #1 to #5 have an 0 to 10 MHz, 10 to 20 MHz, 20 to 30 MHz, 30 to 40 MHz and 40 to 50 MHz frequency bands, respectively. In practice, the cluster #1 occupies frequency band between 1.7 and 10 MHz due to regulatory constraints. The BS can simultaneously and independently use all subchannels (clusters) to communication with all PLC modems. A PLC modem can make use of only one cluster for data communication. At the physical layer of the transmitter, it is implemented a scrambler, interleaver, channel coding (Reed-Solomon concatenated with convolutional codes to provide different cod-


ing rates), and digital modulation. The digital modulation includes a binary phase shift keying and square quadrature amplitude modulations with cardinality equal to M = 2b , b = 1, 2, 4, 8, 10, 12. Moreover, the Gray code is used for constellation construction and a bitloading technique is implemented to allocate integer number of bits in each sub-carrier. The used HS-OFDM scheme is shown in Fig. 4(a). The output of the digital modulation block has an HS-OFDM symbol Xp ∈ CN ×1 , in which p denotes the pth cluster, and N ∈ Z+ is the number of sub-carriers of the HS-OFDM scheme operating in the cluster. Xp is mapped by the block M and, soon after, it is transformed by the normalized inverse discrete fourier transform (IDFT). The cyclic prefix is appended to the vector at the output of IDFT (xp ∈ C2N ×1 ). The resultant signal is up-sampled by a factor U = 6 and filtered by a band-pass, low-pass or high-pass filter (hp [m]) that chooses one of the cluster for data communication. Finally, the signal sp [m] is sent to the digital-to-analog converter. Due to the use of the clustered-OFDM scheme, only one channel for digitalto-analog and analog-to-digital conversions are employed for passband or baseband data communication. At the physical layer of the receiver (see in Fig. 4(b)), the received signal, after the analog-to-digital conversion, by the q th user allocated in the pth cluster, is obtained and represented by rpq [m]. This signal is filtered by a band-pass, low-pass or high-pass filter (hp [m]), (which is equal to that one used by the transmitter) and, soon after, it is down-sampled by a factor U . In the following, the cyclic prefix is removed. After that, the normalized DFT, frequency domain equalization (FEQ) and de-mapping functions are, in this order, applied. ˆ pq , is an estimate of the transmitted HS-OFDM The vector X symbol that will be submitted to digital demodulation. Note that the receiver also performs clock offset error estimation and correction, symbol synchronization, carrier frequency error estimation, channel estimation, noise variance estimation, digital demodulation, channel decoding, inverses of interleaver and of scrambler. Xp IDF T

↑U

CP

hp [m]

(a)

rpq [m]

hp [m]

↓U

CP −1

DF T

F EQ

M−1

ˆpq X

(b)

Fig. 4.

max RPLC =

(N − Nguard ) bmax , Tsimb

(2)

where Nguard = 64 is the number os guard sub-carrier and max bmax = 12. Then, RPLC = 93, 34 Mbps. The total data-rate max that a PLC BS can deliver is RBS = P RPLC = 466, 67 Mbps, where P = 5 is the total number os clusters. B. Link Layer The designed PLC link layer is responsible for encapsulating the data delivered by the upper layer (network); frame synchronization; data-rate control; quality of service (QoS) monitoring; queue management and packet scheduling; and connection, authentication, disconnection and resource allocation of PLC modems. The medium access is made periodically through the use of a time division multiplex access (TDMA) protocol and the BS is in change of this. The PLC frames at the physical layer are composed of 32 downlink symbols (downlink subframe), where the transmission direction is from PLC BS to PLC modems, and 16 uplink symbols (uplink subframe), where the transmission direction is from PLC modem to PLC BS. An association of 512 frames constitute a superframe. The superframe is divided into three parts: •

Ranging: it is intended for the transmission of register messages.

•

Data: it is designed to transmit encapsulated upper layer data (data frames).

•

Network Adaptation: it is designed to update the parameters of registered PLC modems.

The ranging part consists of the first ten frames of each superframe and is used for the registration of new users. PLC modems wanting to register should choose the frame and the cluster in the ranging region to send a request to the PLC BS. This message is called ranging request. Next, the PLC BS sends the response message (ranging response) with a confirmation in case of success or a registration rejection to PLC modems. The downlink and uplink subframes of data region is illustrated in Fig. 5).

sp [m] M

os sub-carriers, LCP = 128 is the length of cyclic prefix and fs = 120 MHz is the sample frequency. As a result, Tsimb = 57, 6 µs. Then, the maximum data-rate that a PLC modem can deliver in the physical layer is

The block diagram of a clustered-OFDM scheme.

Due to the estimated coherence time discussed in [7], we assume that the PLC channel is time invariant during a time period that corresponds to 10 HS-OFDM symbols. The HSOFDM symbol period is given by (2N + LCP )U , (1) fs where U = 6 is the up-sampling factor, N = 512 is the number Tsimb =

230

All subframes have preamble, pilot and noise estimation symbols. The preamble symbol aims to synchronize the transmitter with the receiver, while the pilot symbols are applied to estimate the frequency response of the PLC channel. During noise estimation period, both PLC BS and PLC modems are in ”silence” mode to ensure that noise variances in all sub-carriers are reliable and unbiased estimated. Furthermore, the downlink (DL) and uplink (UL) maps are created by the PLC BS for controlling the medium access. This fields identify the PLC modem that will receive and transmit data on the DL and UL subframes. Also, they inform the parameters for the configuration of the physical layer of the PLC modem to guarantee that both PLC BS and PLC modem communicate with each other at the physical layer.


previously analyzed with respect to performance, cost, and specially for fulfilling physical and link layers specifications.

Fig. 5.

The structure of a frame.

After the ranging part, in one of the data frames, the UL map is replaced by the periodic ranging map reporting the parameters and which PLC modem can transmit the periodic messages. The periodic ranging message is always sent by each PLC modem in the first data frame in each superframe. PLC modem makes use of periodic ranging message to send configuration parameters of physical layer (e.g., coding rate and bit loading information) to PLC BS.

Fig. 6.

Block diagram of the digital circuit for both BS and PLC modem.

Fig. 7.

Block diagram of the front-end for both BS and PLC modem.

The periodic ranging message is also used to notify PLC BS about the current status of each user. If PLC modem interrupts the data transmission, it does not send the periodic ranging message in the expected frame. In this case, the PLC BS disconnects the PLC modem, preventing the superframe to be allocated to inactive PLC modems. C. Management Software Some parameters of the link layer are controlled through a management software running at the PLC BS. The software is an embedded web server that is responsible for managing the PLC network. The changes of parameters are made using a web page. The functionalities of the web server are authentication of PLC modems, data-rate control, connection statistics, server network addressing and server access authentication. D. Hardware The block diagrams for the digital and analog hardwares for both PLC BS and PLC modem equipment are shown in Fig. 6 and Fig. 7, respectively. Note that the digital signal processor part, which is based on field programmable gate array (FPGA) device, is separated from the analog one (front-end). The communication between both of these parts is established by using a high speed mezzanine card (HSMC) interface. The PLC BS and the PLC modem have their analog and digital circuits supplied by different and independently power sources in order to minimize electromagnet interferences between them. The main component of PLC BS and PLC modem equipment is the FPGA device. The chosen FPGA devices for both PLC BS and PLC modem belong to the Arria II GX [9] and Cyclone IV [10] families, respectively. Such families have been

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Regarding the hardware, the main difference between the PLC BS and PLC modem refers to the type of external memories that they access. The PLC BS makes use of double data rate three with memory (DDR3-RAM). Arria II GX FPGA component can access DDR3-RAM at a clock speed as high as 320 MHz [11], which enables effective data-rates over 500 Mbps. As a result, the data-rate demands of the PLC BS is fulfilled. Due to its low data-rate, the PLC modem makes use of a double data rate two synchronous dynamic random access memory. The circuit boards of both PLC BS and PLC modem are constituted by serial interfaces (RS-232 and ethernet), E2PROM, configuration memory, flash memory, DDR3 (PLC BS) and DDR2 (PLC modem), SDRAM (PLC modem), power supplies, digital and analog, and two HSMC interfaces to communicate with the front-end and to expand the system in the future. The front-end is composed of analog to digital converter (ADC) [12], digital to analog converter (DAC) [13], automatic gain control (AGC) [14]; PLC coupler, analog filter, and HSMC connector to allow the connection of ADC and


TABLE I.

P ERSONNEL INVOLVED IN THE R&D P ROJECT Type of member Postdoctoral Researcher Ph.D. student M.Sc. student Professional Undergraduate Student

Number 6 8 12 7 35

DAC components with FPGA device. The HSMC connector also allows the transfer of control signals for the purpose of controlling the AGC. III.

S OME I NTERESTING O UTPUTS

The development of the novel PLC system enabled the constitution of a research group focused on the PLC field at the Federal University of Juiz de Fora and a laboratory facility, which is called Laboratory for Data Communication (LCom), occupying 80 squared meters for R&D activities related to PLC. Vector network analyzers, impedance analyzers, vector signal generators and signal analyzers, amplifiers, ultra wideband signal generator, impedance analyzer high-speed data acquisition and generation board, data flow network analyzers (from layer one up to layer seven) and atomic oscillators are typical instruments found at the LCom for R&D activities related to PLC. The novel PLC system was developed during three years and involved 68 people, see details of personnel in Tab. I. The main outputs associated with the R&D project carried out to develop the novel PLC system are as follows: a methodology and measurement setup to characterize electric power grids as communication medium [8]; a measurement campaign and statistical characterization of low-voltage in the Brazilian outdoor electric power grids [7]; hardware prototypes of PLC BS and PLC modem based on FPGA device; a prototype of a front-end that is capable of injecting and extracting PLC signals in the frequency band between 1.7 and 50 MHz; software for managing, operating and controlling the PLC access network constituted by PLC modems and PLC BS; new specifications and designs of physical and link layers that maximize the performance of PLC systems based on clusteredOFDM scheme; and, finally, a platform to implement, test and analyze novelties that can improve PLC system performance. IV.

N UMERICAL R ESULTS

The novel PLC system was tested in the LCom at the Federal University of Juiz de Fora. The waveforms of consecutive HS-OFDM symbols in the time domain, which were generated by the transmitter, are shown in Fig. 8. The performance analysis in terms of data-rate in the five clusters was carried out with three distinct tests. The first test refers to the maximum data-rate on the output of the physical layer when 4096-QAM symbols are transmitted by all sub-carriers. The maximum data-rate of all clusters is the same because we assume that PLC channel is ideal. The attained data-rate is presented in Tab. II and we can see that the maximum data-rate has reduced from 466.67 Mbps (see (2)) to 384.6 Mbps at the PLC BS and from 93.34 Mbps to 76.92 Mbps at the PLC modem because part of the frequency bandwidth must be used for channel

232

Fig. 8. Waveforms of consecutive HS-OFDM symbols at the output of PLC coupling circuit.

estimation, noise variance estimation, symbol synchronization, and medium access protocol. The second test was carried out to obtain the maximum data-rate that can be attained at the application layer when 4096-QAM symbols are transmitted by all sub-carriers. This test were performed and analyzed by using the Spirent SPT3U EDM100-2B equipment because it is capable of generating data, and analyzing transmitted packets at the application level. The attained data-rate is presented in Tab. III in which we can note that the maximum data-rate at the application layer is approximately 1/3 of the maximum data-rate at the physical layer. Finally, Tab. IV presents the achieved data-rate at the application level per user when the novel PLC system was installed in an outdoor and low voltage electric power grid of a condominium composed of 1215 houses in the city of Juiz de Fora, Brazil. The software Colasoft Capsa was used to transmit, receive, and analyze data packets through the Ethernet port of the PLC BS and the PLC modem. We noted that PLC modems could not establish a connection when the distance between them was longer than 300 meters. According to Tab. IV, data-rates obtained in the field test is enough for smart grid communication as well as digital inclusion in indeveloping and under-developed countries. TABLE II.

M AXIMUM D ATA -R ATES (M BPS ) AT THE PHYSICAL LAYER WHEN ALL SUB - CARRIERS MODULATES 4096-QAM SYMBOLS . Data rate Downlink Uplink Total

Number of clusters 1 5 51.28 256.4 25.64 128.2 76.92 384.6

TABLE III.

D ATA - RATES (M BPS ) AT THE APPLICATION LEVEL WHEN ALL SUB - CARRIERS MODULATES 4096-QAM SYMBOLS

Data rate Downlink Uplink Total

#1 17.71 7.10 24.81

#2 17.74 7.09 24.83

Cluster #3 #4 17.77 17.78 7.11 7.07 24.88 24.85

#5 16,53 0.91 17.44

Total 87.53 29.28 116.81


TABLE IV.

M AXIMUM DATA - RATES (M BPS ) AT THE APPLICATION LAYER OBTAINED IN A FIELD TEST ( ADAPTIVE MODULATION )(M BPS ) Data Rate Downlink Uplink Total

#1 1.88 0.73 2.61

V.

#2 1.88 0.73 2.61

Cluster #3 #4 3.60 2.41 1.55 0.92 5.15 3.33

#5 1.1 0.13 1.23

[13] [14]

Total 10.87 4.06 14.93

C ONCLUSION

This work described a novel broadband PLC system to constitute a access network in low-voltage and outdoor electric power grids. The proposed PLC system makes use of clusteredOFDM scheme in order to design a PLC modem that operates in a subchannel and demands lower hardware resource utilization than PLC BS. The novel PLC system introduces novel physical and link layers in order to maximize the performance of the clustered-OFDM scheme. Additionally, we presented an FPGA-based prototypes (PLC BS and PLC modem) of the novel PLC system and briefly described the software for managing and operating it. The reported results show that the novel PLC system can be useful to address smart grid and digital inclusion in-developing and under-developed countries. ACKNOWLEDGMENT The authors would like to thank FINEP, FUNTTEL, CNPq, CAPES, FAPEMIG, P&D ANEEL, CEMIG, Smarti9, and INERGE for their financial support. R EFERENCES [1]

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