## 3 Review of PLC Systems
### Evolution of PLC Technology and Systems  
The origins of PLC technology date at least back to 1918 in Japan, as [24] states in relation with the first test and commercial operation of PLC for voice telephony over power lines. PLC ideas were even generated earlier at the end of the 19 th century, with certain patents claiming voice and data transport over any type of wired media including power lines. Few years later, the first experiments were carried out with this flourishing technology in the USA, Europe and Asia [24], in such a way that by 1927 PLC had been widely adopted [25]. The first implementations of PLC were intended for long distance applications in HV power lines (high power transmissions around 50 W and tens of kilometers covered using frequencies from 50 kHz to 300 kHz [25]), where regular telephone lines were performing worse than PLC due to the weakness of the poles used, and the interferences among power and telephony lines that often occurred (economic aspects may also be mentioned). These initial voice communications evolved to include low-speed data transmission, including the evolution to highly efficient digital communications for operational services [26] and the recent knowledge transfer from the BB PLC for access and in-home progresses [27,28]. However, PLC applications for meter reading (1897 as per [29], and 1903 as per [30]) were even earlier than voice applications. After the massive adoption by electricity utilities of long distance PLC transmission over HV lines, it was in 1930 [31], and more consistently in the 1950s when a group of companies [32] developed a PLC system for central load management, with the objective to control load peaks in the grid. These systems are the PLC systems known as \"ripple control\", and apart from the fact that some of them still survive in our days, they have the privilege to have continued the non-voice application uses of PLC technology, thus being the predecessors of Smart Metering and Smart Grid solutions. Reference [9] offers a sound and complete classification of PLC systems, from the technology point of view. It focuses on the most evident aspects of the technology, such as the frequency band, the bandwidth, the data rate and the communications range achieved. Thus PLC systems can be classified as follows:* Ultra-Narrowband (UNB) PLC. These systems work in either Ultra Low Frequency (0.3-3 kHz) or in the Super Low Frequency (30-300 Hz). \"Ripple control\" systems can be considered a historical example of this group of PLC technologies, even if these ripple control systems were one-way communications. These UNB systems convey very low data rates (roughly 100 bps) at tens or even one hundred kilometers. * Narrowband (NB) PLC. These systems use the frequency band from 3 kHz to 500 kHz that include the European Committee for Electrotechnical Standardization (CENELEC) A band (Europe, 3-148.5 kHz), the Federal Communications Commission (FCC) band (USA, 10-490 kHz), ARIB band (Japan, 10-450 kHz) and Chinese band (3-500 kHz). These technologies have a range that depending on the power lines can reach from hundreds of meters to some kilometers. NB PLC technologies can be further classified as:
* Low Data Rate (LDR) NB PLC. Single carrier technologies capable transmitting a few kbps;
* High Data Rate (HDR) NB PLC. Multicarrier technologies capable transmitting hundreds of kbps. * BB PLC. These systems operate in bands from 1.8 MHz to 250 MHz, covering distances from hundreds of meters to kilometers, and providing data rates from several Mbps to hundreds of Mbps. Different applications have been associated to different types of systems through history. Two sections of the evolution can be distinguished, focusing on the NB or BB nature of the system:  
* Narrowband PLC systems evolution (UNB and NB):
* Ripple control systems were the first example of 'demand side management' in the middle of the 20 th century.