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Power Line Communications (PLC) technologies explore the use of electric power lines (LV, MV or even HV) as a medium that enables the bidirectional data exchange. They have been used for decades in the utility industry for remote metering and load control applications [57]. In recent years, smart grid activities and advances in building and home automation have brought a lot of attention to PLC technologies as an alternative to unwanted or impractical wiring to setup a communications network.

From a technological point of view PLC implementations can be divided into narrow and broadband and they typically target different network segments within smart grids. The narrowband uses transmis-sion frequencies up to a few hundreds of kHz (e.g., 3~500 kHz) whereas broadband operates in the MHz band (e.g., 2~30 MHz or higher).

However the objective might not always be a matter of applying advanced modulation and/or different coding techniques to achieve higher data rates. Power lines are inherently exposed to time-frequency varying noise, unmatched loads and interference from electrical or communications devices. The fact that in electric distribution networks the power lines can vary from grounded to overhead lines introduces even further challenges in using this type of medium to propagate data. On the other hand national legislation limits the transmission power and accepted frequency bands. For these reasons a large number of PLC technologies have been developed each one addressing different applications and thus targeting different throughput values, frequency bands and channel access mechanisms. The implementations of PLC are supported by standards that range from open to proprietary, including some cross solutions.

In the narrowband version, PLC is already used in some applications for power systems to ensure connectivity and data exchange within the electric grid. The technology has also been used in domestic environments in home automation applications or as support for general purpose LANs. This narrowband technology offers modest data rates, up to a few dozen kbps, and it is considered mainly for smart metering applications. However, the adverse characteristics of the PLC medium may introduce severe limitations even for simple applications, particularly in dense urban areas with hundreds of potential communicating nodes attached to the same communication channel.

The recently approved Broadband PLC (BPL) standards promise high throughput in the order of tens or even hundreds of Mbps and enhanced control and coding mechanisms are potential candidates to be considered and evaluated for the electric grids, as the technology becomes increasingly mature.

Given the difficulties conveying data in PLC network, wireless technologies have started to emerge as feasible alternatives, which is reflected in a number of initiatives led by standardization bodies, utilities and manufacturers, along with demonstration projects and pilot trials in electric grids. Nonetheless, wireless solutions present themselves their own challenges, but the ability to extend the coverage area of a communications infrastructure is one of the main attributes to be explored within smart grid scenarios.

Besides, PLC technologies are becoming more advanced and promise to deliver data rates up to 1Gbps [58].

2.8.1.1 Narrowband PLC

Narrowband PLC (NB-PLC) currently includes two versions: low and high speed. Narrowband high speed is also sometimes referred to as medium speed, to distinguish from “true” high speed data rate usually associated with the BPL definition. Besides the allowed frequency ranges, there is also a maximum allowable transmission power in each range that must be respected, according to the legal dispositions of each country. The development and dissemination of NB-PLC is promoted by Standard Development Organizations (SDO) as well as by non-SDO consortia. The narrowband region can be further divided into legally allowed frequency bands depending on specific continent or country definitions:

ˆ In Europe CENELEC has standardized and authorized the use and range of frequency bands between 3 to 148.5 kHz, as described in EN50065-1. Bands are further subdivided into what is generally known as “CENELEC bands”²:

– Band A: 3-95 kHz. Only utilities are allowed to use this band;

2The total frequency range of 3 and 148.5 kHz is available for utilities whereas for end-user applications the 95 to 148.5 kHz is available

– Band B: 95-125 kHz. All may use this band.

– Band C: 125-140 kHz. All may use this band when using CSMA;

– Band D: 140-148.5 kHz. All may use this band.

ˆ In USA, the FCC has established the use of band ranges from 10 to 490 kHz;

ˆ In Japan, the ARIB defined band ranges from 10 to 450 kHz [59].

The first generation of NB-PLC made use of single or double carrier transmission schemes with simple modulation schemes such as Phase Shift Keying (PSK) and Frequency Shift Keying (FSK) to achieve a few kbps usually targeting remote metering applications.

Two non-SDO second generation NB-PLC technologies are G3 and PRIME, which were developed having in mind smart metering communications scenarios. Both use an Orthogonal Frequency Division Multiplexing (OFDM) modulation technique, but subtle physical layer details make the two technologies differ a little [60]. As a general rule, PRIME achieves higher data rates while G3 has a more powerful error correcting algorithm in order to achieve improved reliability [60]. At higher layers the two technologies are similar. The maximum data rates within CENELEC band A are 33 kbps for G3 and 128 kbps in the PRIME case. The G3 standard is maintained by the G3-PLC³ Alliance while PRIME is maintained by the PRIME Alliance⁴; both are available as open industry-standards.

Despite the initial motivation of G3 and PRIME, EV communications is also considered as a potential application to be supported by these technologies. In fact, one of the main advantages of narrowband PLC communications is the possibility of dedicated utility frequency band. For this reason companies have implemented point-to-point variants of G3 and PRIME, which are designed for the communication between two nodes, as in the case of EV and the EVSE charging infrastructure and the EVSE and the distribution grid. In the G3 case, it is possible to extend beyond the CENELEC A band to achieve higher data rates.

Since neither G3 nor PRIME were specifically developed for EV communications and the fact that they represent non-SDO standardized implementations, several communications technologies are considered for ISO/IEC 15118-3, which include G3, PRIME and other solutions rendering some uncertainty towards the PLC variant used in EV applications [22].

The International Telecommunication Union (ITU) defined G.hnem project to address home network-ing for energy management usnetwork-ing high speed OFDM NB-PLC. One objective of the ITU Telecommunica-tion Sector (ITU-T) in this project was the development of a unified next generaTelecommunica-tion narrowband PLC.

It integrates some of the features present in G3 and PRIME, which were complemented with coherent reception, enhanced protection against power line impulsive noise, multiple bands for worldwide compat-ibility, adaptive medium access rules and support for multiple network protocols [61]. Within this project scope, ITU-T targeted at applications such as AMI (residential or business) and EV charging. Recom-mendations G.9955 and G.9956 are part of G.hnem and define respectively the physical and data link layers. The physical layer uses CENELEC and FCC bands with up to 16-QAM subcarrier modulation with data rates up to 1 Mbps. Forward error correction schemes are used to improve robustness against

3“G3-PLC Alliance” - http://www.g3-plc.com/

4“PoweRline Intelligent Metering Evolution” - http://www.prime-alliance.org/

noise. The defined medium access method is a prioritized CSMA/CA. Automotive support is provided allowing operation over main and pilot wires [62].

An emergent standard for narrowband PLC is P1901.2, which is being developed by IEEE since 2009⁵. Defined as a low frequency OFDM-based narrowband power line standard for smart grid applications, it is set to use frequencies below 500 kHz and data rates up to 500 kbps, supporting both indoor and outdoor communications. In the outdoors context this standard targets the use of MV and LV electric distribution networks for both urban and long-distance rural feeders. It defines a communications medium for WAN and FAN segments, ensuring connectivity between the electric grid and the customer, through the smart meter. In indoors environments the standard is regarded as an alternative for HAN implementations.

The potential applications targeted by P1901.2 are grid to utility metering devices communications, EV to charging station, and HAN related communications aspects, along with other candidate applications.

One particular aspect of this standard seems to be the coexistence philosophy, as defined by NIST PAP 15, which is being adopted by IEEE. It aims at providing the required mechanisms to allow this technology to coexist with PRIME and G3, somewhat similar to the earlier described approach of ITU-T regarding G.hnem (G.9955 and G.9956).

2.8.1.2 Broadband PLC

The cradle of broadband PLC was the domestic environment as a technological alternative to enable Internet services to end users through existing power lines at home. This should not be confused with the provision of Internet services using PLC in the last-mile access network, as an alternative to copper (e.g., DSL) or optical fiber. Although it is often associated with a replacement to in-building Ethernet networks, the use of BPL has been implemented using different technologies and approaches. Recently it has been considered as another candidate for communications outside the building environment, namely for the last-mile segment.

Wide frequency bands, typically between 2~30MHz, are generally available worldwide for all purposes, except in Japan, where it is not allowed to use PLC in this frequency range. Technically, the upper limit to wide band communications in power lines is dictated by the minimum communication distance and the use of TV broadcast signals above 80 MHz. Some wide band solutions use a frequency range up to 60 MHz. Evidently, a wider band allows a higher number of OFDM subcarriers to be used and thus yields higher theoretical data throughput, although this also depends on other factors like the modulation scheme used in each subcarrier. For wideband PLC a high number of subcarriers can be used, depending on the standard, when compared to the narrowband implementation where typically around 36 subcarriers are used in the CENELEC bands [60].

A particular implementation of BPL is HomePlug, which was designed for the domestic environment.

It was developed by a non-SDO industrial consortia, the HomePlug Alliance, which is responsible for the development of MAC and PHY layers and of different standard versions. In 2001 HomePlug 1.0 was made available using Differential Binary Phase Shift Keying (DBPSK) and Differential Quadrature Phase Shift Keying (DQPSK) modulations and Forward Error Correction (FEC) mechanisms to achieve data rates near 14 Mbps [63]. The variant HomePlug AV, released in 2005 and aiming at high quality multi-stream data over power lines, uses flexible modulation schemes from BPSK to 1024-QAM [64].

5“IEEE P1901.2” http://standards.ieee.org/develop/project/1901.2.html

Using FEC schemes along with Robust OFDM (ROBO) or adaptive bit loading techniques, it enables data rates ranging from 10 Mbps up to 200 Mbps at the physical layer. HomePlug Green PHY (GP) released in 2010, is the most recent variant for in-home smart grid and smart energy applications [65].

The HomePlug GP is basically a scaled down version of the HomePlug AV within the context of domestic SG communications where a high data throughput is not the main objective. Instead it focuses on ensuring reliable communications, with good coverage. Hence, HomePlug GP does not support adaptive bit loading using only QPSK as ROBO modulation scheme to ensure high reliability, whilst achieving 10 Mbps [60]. The simplifications introduced in GP makes it lightweight in terms of processing, memory and power consumption requirements, when compared to other HomePlug variants.

Given that GP and AV versions of HomePlug use the same frequency band, they will have to share the available time-on-wire. The CSMA scheme is used in both and given the adverse effect in the medium access conflict resolution, only 7% of time-on-wire is allowed to HomePlug GP devices to ensure inter-operable scenarios. This allows smart grid oriented applications to use the GP version within an already deployed customer HAN based on HomePlug. The EV is considered by HomePlug manufacturers as an important entity to incorporate in domestic networks, namely those based on PLC technologies, with the GP variant targeting also EV communications.

An SDO-based implementation of BPL can be found on IEEE 1901 standard, which defines the MAC and PHY layers for high-speed communications over power lines with date rates up to hundreds of Mbps.

The standard defines two MAC layers, targeting separately in-home and access networks (over LV and MV distribution lines) with different requirements and potential applications [66]. It also defines two possible implementations of the PHY layer distinguished by the used modulation scheme: one based on FFT OFDM (FFT-PHY) and another based on Wavelet OFDM (Wavelet-PHY). These two implementations are not compatible and manufacturers can implement only one of them or both. The FFT-PHY can use up to 1974 carriers from 1.8 to 50 MHz with different subcarrier modulation ranging from BPSK up to the optional 4096-QAM. The wavelet-PHY uses 512 subcarriers between 1.8 and 28 MHz using M-PAM modulation schemes, up to 32-PAM. Robust signaling schemes and FEC mechanisms are used to ensure resilience over the transmission medium [67]. Some of the approaches used in IEEE 1901 are expected to be adopted in the future narrowband version, the IEEE 1901.2.

In 2010, ITU-T has also defined a broadband PLC standard for home networking, designated G.hn, with the purpose of supporting smart grid applications such as AMI and energy management including EV. It was designed to be used for robust in-home or last-mile communications, comprising the definition of a physical layer, in G.9960, and data link layer, in G.9961. At the PHY layer an OFDM implementation is defined to be used in two frequency bands. The first band ranges from 2 to 100 MHz allowing up to 1 Gbps data rate. The second band is defined between 2 and 25 MHz and uses a low complexity profile enabling a data rate between 5 and 50 Mbps. The standard defines robust transmission schemes, FEC and repetition encoding to tackle the power line communications medium [68].