  Noise modeling in power line communications has recently drawn the attention
of researchers. However, when characterizing the noise process in narrowband
communications, previous works have only focused on small-scale phenomena
involving fine-grained details. Nevertheless, the communication link's
reliability is also affected by long-term noise phenomena that might affect
transfer rates at higher layers as well. This paper addresses the problem of
long-term noise characterization for narrowband power line communications and
provides a statistical analysis of the long-term trends affecting the noise
levels. We present a statistical description of the noise process in the time
and frequency domains based on real field measurements in the FCC band (10 kHz
- 490 kHz). The collected data comprises more than 1.8 billion samples taken
from three different locations over a time period of approximately 10 days. The
noise samples have been statistically analyzed by considering stationarity,
autocorrelation, and independence. Although our results -- being unprecedented
-- are interesting per se, they improve the noise pattern knowledge, thus
paving the way for the design and implementation of more robust PLC protocols.
