1.1 Historical Overview and Motivation 1. INTRODUCTION

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1. INTRODUCTION

1.1 Historical Overview and Motivation

The first reported applications of Power-line communication (PLC), at a very early stage of its development, were remote voltage monitoring in telegraph systems and remote voltage meter reading. The use of PLC technology has been experiencing significant attention lately due to a combination of factors such as the development of improved signal modulation schemes, the liberalization of telecommunications and the birth and growth of the internet and the increasing demand for communication networks for home and office automation, computer communications, security monitoring and several other applications. Nowadays Power line communication has received growing attention as a proven and competitive broadband technology. The interest in Power line communication today spans several important application: indoor wired local area Networks (LANs) for residential and business premises, broadband Internet access, smart grid applications (advanced metering control, distributed energy generation etc.), ground vehicles, railway systems, aerospace systems and maritime environment. The major advantages of Power Line Communication is the reduction in operational costs and expenditure for realization of new telecommunication networks. Moreover current generation of PLC offers broadband services at transmission rate equivalent to other access technologies. The use of dedicated modulation techniques, like Orthogonal Frequency Division Multiplexing technique (OFDM), allows for a broadband transmission and for this reason modern PLC systems are identified as Broadband Power Line Communication (BPL). Modern

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powerline modems offer a throughput rate of up to 200 Mbps on the physical layer. This number should be considered with caution as it doesn’t guarantee the maximum throughput to the final application on any platform.

1.2 Technical Challenges

Due to the fact that power lines were not conceived for communication purposes but to deliver electrical energy (what involve signals of much lower frequency and higher power), the use of this medium presents several technical challenges. The power line channel is a harsh and noisy transmission medium very difficult to model, frequency selective and affected by coloured background noise and by periodic and a-periodic impulsive noise. Additionally the structure of the mains grid exhibits remarkable differences from country to country and even within a country. Even in a specific power grid the conditions change according to the network topology (often unknown), the type of wires installed, the connected loads (of heterogeneous nature) and on the selected transmission path (Le. sockets where the transmitter and receiver are plugged). The channel transfer function may vary abruptly when the topology changes. Furthermore, having a priori knowledge on the cable parameter, an estimate of the power line channel, a selected transmission path, response exhibits a time varying behaviour with a twofold nature. The first one is the well-known long-term variation caused by the connection and disconnection of electrical appliances to the grid [4]. The second one is a short-time variation, synchronous to the mains, whose origin is the dependence of the impedance presented by electrical devices on the mains voltage [3],[4],[5]. Additionally challenges are due to the fact that Power line cables are often unshielded and thus become a source of electromagnetic interference (EMI).

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1.3 Thesis layout

This thesis presents a theoretical and experimental analysis on the application of power line communication. This work, address the challenges related to the following topics:

 Development of high speed power line communication (PLC) systems in a cruise ship;

 Estimation of changes in the transmission channel in order to have the maximum possible data transfer speed;

 Classification and detection of noise in the power line grid channel;

We have investigated the possibility of using the power network of a ship as a PLC channel [1]. An onboard measurement campaign has been conducted on a portion of the power grid of a cruise ship, which is undergoing a major revamping available for the courtesy of the owner. The measurement campaign results have shown the behavior of different channels as a function of the power grid topology, together with the performance during some transmission tests. The measurements have been performed by the use of portable personal computers connected to National Instruments acquisition data cards (digitizer), whose characteristics are 200 Msample/s, and the input and output resistances are set to 50 Ω (standard for such instruments). The results obtained give an indication of the behavior of the whole power grid of the ship as PLC channel.

With regard to second topic , the current research motivation is based on the fact that the strong variation in time of the transmission channel can greatly reduce the efficiency of the system because some frequency bands used for the transmission can be randomly unavailable. The primary motivation behind the estimation of changes in the channel is related to the problem of tracking the time variation of frequency response. It is necessary to be able to move some subcarriers from one frequency channel to another in order to maintain the same transmission speed

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[15]. Classical linear approaches to channel estimation and equalization reveal their limits in PLC, the use of soft computing techniques will play a very important role in the development of intelligent systems for estimation and equalization that are computationally efficient, profoundly precise, highly predictable, reliable, and robust. These methods are able to maximize the transmission speed without the use of the "pilot" signals, but working directly on the transmitted useful signals. In particular three novel algorithms are developed for blind channel estimation and equalization under an OFDM transmission. The proposed method has the SOM as a particular case and is a general decision directed estimation method that reveals to be very effective in the presence of time-varying channels. The performance of the proposed estimation method [2] for power-line channels is evidenced by simulating the transmission over a number of different PLC channels obtained considering different realistic home grid. We have simulated the transmission of random data by using a OFDM modulation with a number of 512 subcarriers. Each subcarrier is modulated with a gray-coded 16-QAM symbol constellation and the transmitted power is uniformly distributed between subcarriers. We simulate different conditions of signal to noise ratio by varying the total transmitted power and considering a fixed noise condition. We can reasonably assume that the frequency response is constant during the duration of one OFDM frame. A novel decision directed estimation algorithm for OFDM systems has been introduced. This method generalizes a previous proposed estimation scheme based on a SOM neural network [14], introducing the possibility to implement the dynamic of a filter, arbitrarily designed, inside the SOM. This leads to a better estimation and tracking of a time variant frequency response. The use of a filter with a second order dynamic reveals a better BER in the simulated transmissions over a realistic home grid that exhibits a slow time variation of the frequency response. If high speed rates must be obtained with PLC in cyclostationary channels, the proposed method has shown high potentials to reach the maximum allowable transmission speed in these channels.

With regard to the third topic, it is important to mention that the channel is time and frequency variant, and is characterized also by being location dependent, according to the network topology, which is often unknown. In principle, channel

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attenuation depends on the physical structure of the cables and of the loads. While the frequency dependence can be mathematically modeled the load variation cannot be analytically determined since the loads are plugged and unplugged several times a day; furthermore, the load variations are quite significant, since they can range from a few ohms up to the order of the Kohms. For this reason, it is necessary to construct a channel model (or validate a model based on analytical approaches) by several and accurate measures performed on existing components including both the long- and short-time variation of the channel. The long-time variation is due to the electrical appliances switching on and off. A channel model for the broadband characterization of indoor power lines in presence of time variation of the loads has been presented. In this way, we achieve a characterization of the long-term variation of channel (with regard to load variation) in a very short CPU time. By means of soft computing technique an environmental monitoring system able to estimate where and when the loads are plugged and unplugged to the power line grid is presented. These method are able to estimate and predict the long-term variation of the channel without the knowledge of the changes happening on the power grid channel, but working directly on the received useful signals. The early detection and isolation of long-term variations in power line channel is indispensable for the on-line broadband characterization of power lines. In particular a new model for external response of the load connected is presented. Every device connected to the power grid represents a load to the network, whose model includes, besides impedance values, noise sources associated with them. These frequency- and time-variant values have been obtained directly from measurements of typical electrical loads. Some of them have constant impedance values but others exist that present random variation due to on and off switching of the loads. This load variation is equivalent to the insertion of some additional source (disturbances) represented by their parameter at each node. After evaluating transfer function of the new power grid by means of soft computing technique an environmental monitoring system able to estimate the loads are plugged and unplugged to the power line grid is presented. These method working on the loads variation is able to estimate and predict the long-term variation of the channel without the knowledge of the changes

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happening on the power grid channel, but working directly on the received useful signals. The technique proposed in this thesis is based on an estimation of load variation in transmission line grid model. The approach presented is supervised clustering method based on least square support vector machine applicable to identification and localization of long term variation noise sources. The presented soft computing techniques have been developed within the area of digital communication, nevertheless they posses general applicability.