For many years,
we have sought to exploit the ubiquitous electrical
network as a means for high speed data communications. As we turn to
increasingly hostile media to meet mankind's veracious technological
appetite for communication, we must continue to develop ever more
ingenious technological tools. The existing power line infrastructure is
growing in importance as a commercially viable, low cost option and
communication researchers are well motivated in this field.
Throughout the world, close
collaboration with industry is helping to deepen our understanding of
the digital power line environment. We began this work in 1995 at
in parallel with
in Germany, by modelling channel characteristics, exploring
different modulation schemes and novel modem technology.
At what rates can we use the 220 V
power lines for communications?
We estimated the capacity to be about 1 Mbit/sec for the CENELEC band
9-94 kHz. The power-line communication channel is one of the practical
channels where Shannon theory partly gives an answer to the question
about channel capacity. From measurements it followed that the logarithm
of the noise power spectral density is a linear decreasing function of
the frequency. We could apply the "water-filling" arguments to show that
the channel can be used at a data rate of about 1 Mbit/sec. on a
bandwidth of 100 kHz. In practice it means that most of the power should
go to the higher frequency region. We presented these results on the
"2nd International Symposium on Power-Line Communications and its
Applications" in Japan.
What kind of modulation and coding is suitable for the 220V channel?
Modulation schemes with a constant envelope signal modulation such as
binary-FSK and M-ary FSK are in agreement with the CENELEC norms, EN
50065.1, part 6.3.2.
In our research at that moment we focused on the low frequency range
below 150 kHz. In this region, there are several channel
characteristics like attenuation, permanent frequency disturbances and
impulsive noise that need special attention.
The main goal was 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 time
spreading to facilitate correction of frequency disturbances and impulse
noise simultaneously. A transmission scheme combining 4-FSK modulation
with diversity and coding can make the transmission over power lines
robust against permanent frequency disturbances and impulse noise.
We suggest simple non-coherent detection and derive the corresponding
error correcting capabilities of the scheme. Since we use 4-FSK, the
scheme is in agreement with the existing CENELEC norms. The scheme can
be considered as a form of coded Frequency Hopping