Adaptive Signal Processing for Power Line Communications

This thesis represents a significant part of the research activity conducted during the PhD program in Information Technologies, supported by Selta S.p.A, Cadeo, Italy, focused on the analysis and design of a Power Line Communications (PLC) system. In recent times the PLC technologies have been considered for integration in Smart Grids architectures, as they are used to exploit the existing power line infrastructure for information transmission purposes on low, medium and high voltage lines. The characterization of a reliable PLC system is a current object of research as well as it is the design of modems for communications over the power lines. In this thesis, the focus is on the analysis of a full-duplex PLC modem for communication over high-voltage lines, and, in particular, on the design of the echo canceller device and innovative channel coding schemes. The first part of this thesis investigates the cancellation of the echo that occurs when, in a full-duplex communication system, the same bandwidth is used for bidirectional connection. As a consequence, the transmitted signal is reverberated through the physical environment and results in time-delayed, distorted, and attenuated images, which interfere with the desired received signal. adaptive signal processing, least mean square, LMS, power line communications, weight reset LMS, abrupt channel variations, echo cancellation, fixed-point LMS, impulsive noise, LDPC, channel coding, coded modulations In Chapter 1, the general model of a full-duplex communication system, with particular attention to the echo canceller device, is described. Also, as the PLC environment is characterized by an interference which exhibits an impulsive nature, a review of the model of this interference is provided. The echo canceller filter, which thanks to its capability of self adjustment is a good solution for the echo channel response identification, is described in Chapter 2, where adaptive algorithms for the update of the echo canceller filter are described. A performance analysis is provided for a singlecarrier PLC system. In Chapter 3, the analysis is extended to a multicarrier PLC system, suitable for the communication over frequency-selective and impulsive event-affected PLC channel. The implementation of the adaptive algorithm in the modem Digital Signal Processor (DSP) requires a fixed-point implementation. In Chapter 4, the fixed-point implementation of a Finite Impulse Response (FIR) filter is discussed and possible causes of errors, due to a poor representation accuracy, which arise during the adaptive filter weight update, are described. A performance analysis is carried on for both the singlecarrier and multicarrier systems. The PLC environment is typically affected by occasional (but not rare) abrupt channel variations, due to line impedance mismatches, related, for example, to the periodic maintenance operations on the power lines. The speed of the echo canceller adaptation to the new echo channel affects the communication system reliability, so that in Chapter 5 we investigate the beahavior of the adaptive algorithm in the presence of abrupt channel variations and propose a modification of the classical adaptive algorithm for echo cancellation, which allows a fast recovery from an abrupt variation of the channel conditions. The second part of this thesis investigates channel coding schemes based on the use of Low Density Parity Check (LDPC) codes, for possible implementation in a PLC modem. As we take into account a constraint on the overall encoding and decoding latency, we are interested in evaluating the performance of LDPC codes with low latency, hence with short codeword length. For this reason, we use the LDPC codes adopted in IEEE 802.16e standard (WiMAX which support short codewords. Their structure is defined on the use of the so-called exponent matrix to specify the code parity check matrix, so that this class of LDPC codes allows limited memory consumption for the storage of the parity check matrix in the DSP. In Chapter 6, the structure of the WiMAX LDPC codes is reviewed, different coding schemes are proposed and their performance, also in the PLC scenario, is analyzed.