Coded Modulation for Power Line Communications

We discuss the application of coded modulation for power-line communications. We combine M-ary FSK with diversity and coding to make the transmission robust against permanent frequency disturbances and impulse noise. We give a particular example of the coding/modulation scheme that is in agreement with the existing CENELEC norms. The scheme can be considered as a form of coded Frequency Hopping and is thus extendable to any frequency range. keywords: modulation; power-line communications; coding. INTRODUCTION Power Line Communications (PLC) can be seen as one of the possible solutions to the “last dirty mile” problem for communication providers. However, there are several obstacles: 1) standards; 2) channel characteristics like attenuation, permanent frequency disturbances and impulsive noise; 3) network conditions. This paper focuses on the low frequency range below 150 kHz, where the CENELEC norms apply. According to the CENELEC norms, EN 50065.1, part 6.3.2, the maximum allowed peak voltage for narrow band transmitter output voltages at 9 kHz equals 5 V, exponentially decreasing to 1 V at 95 kHz; and for broad-band transmitters equals 5 V = 134 dB (μV). As a consequence, the transmitters are output voltage limited and bandwidth limited. Here, for narrow band signals the maximum signal power spectral density (PSD) within a bandwidth of 5 kHz is 20 dB larger than the signal PSD at the edges of the particular frequency band, i.e. a 20-dB bandwidth of less than 5 kHz in width. Broadband signals are defined as signals with a 20-dB bandwidth of more than 5 kHz in width. The candidate modulation schemes with a constant envelope signal modulation such as binary-FSK and M-ary FSK are in agreement with to the CENELEC norms. A successful binary modulation scheme called Spread-FSK can be found in Schaub, [1]. This scheme is particularly suited for channels with non white interferers and/or channels with different attenuations at the two signaling frequencies. Another interesting multi carrier modulation scheme is Orthogonal Frequency Division Modulation ( OFDM, see [7] ). OFDM based systems are being developed for high speed transmission in frequency bands above 100 kHZ. Special algorithms are developed to reduce the maximum signal amplitude of the modulator output, see [8]. Both schemes can be combined with block or convolutional error control coding. Recent developments in modem design for PLC can be found in [9]. Channel characteristics regarding attenuation and noise have been reported in [2] and [3]. Television sets or computer terminals generate narrow band noise and thus, this type of noise is permanent over a long period of time and of great importance in power line communication systems. Impulse noise has been reported in [3]. From [3] it can be concluded that impulses have a duration of typically less than 100 μsec. However, more information about the statistical behavior of the impulse duration and inter-arrival times is needed. Measurements in networks indicate that the inter-arrival times are independent and .1 to 1 second apart. A modulation /coding scheme that incorporates frequencyand time diversity can be expected to be robust against both types of disturbances. The main goal of the paper is to show that a combination of M-ary FSK modulation and coding can provide for a constant envelope modulation signal, frequency spreading to avoid bad parts of the frequency spectrum, and redundancy to facilitate correction of frequency disturbances and impulse noise simultaneously. M-ary FSK has the advantage of a constant envelope signal modulation and a demodulation in a coherent as well as a noncoherent way, leading to low complexity transceivers. In an M-ary FSK modulation scheme, symbols are modulated as one of the sinusoidal waves described by