Comments on BPL or PLC.
24 February 2004
* INTRODUCTION *
My name is John Matz. I have been an amateur radio operator
for 40 years, currently Amateur Extra Class, current callsign
KB9II. I have a BSEE and MSEE from Northwestern University.
I am a Professional Engineer. I worked as an antenna designer
for 4 years, for Motorola Research for 4 years, in Motorola
Microwave for 9 years, in Motorola Cellular Infrastucture for
10 years. I am now an independent consultant.
* BPL COMMENTS *
Part 15 limits:
If it's an unintentional radiator, are there any limits below
30 MHz at all? Does it have to meet the intentional radiator
limits?
If it's an intentional radiator, 30 uV/m at 30 meters below
30 MHz. But in what bandwidth? Above 1000 MHz the resolution
bandwidth is given as 1 MHz minimum. But I saw nothing below
1000 MHz.
*** You must specify a bandwidth !!!***
Below 30 MHz, Part 15's limit is 30 uV/m @ 30 meters. This
corresponds to about -80 dBm/sq meter at 30 meters. At
14 MHz and lower, atmospherics are conservatively estimated
at 34 dB above the thermal noise floor. The 30m and 20m bands
can be much quieter. The atmospheric noise using a dipole
receive antenna in a 10 kHz BW shortwave receiver is about
-100 dBm equivalent at the antenna (NEP). That's about 2 uV
on 50 ohms. Using Part 15 limits measured in a 1 MHz BW, the
interference is about -65 dBm in 1 MHz, or about -85 dBm in
10 kHz, or about 15 dB above typical atmospherics. If the
Part 15 limit is measured in 10 KHz BW, noise could be 35 dB
above atmospherics. Part 15's 30 uV/m, if measured in 1 KHz
BW, would give interference at 45 dB over atmospherics.
*** You must specify a bandwidth !!!***
At 30 MHz, the atmospherics will drop down to maybe 4 dB above
thermal noise, so any BPL noise at Part 15 limits becomes
really obvious at 40 dB to 70 dB over receiver noise.
Just a comment, light dimmers, brushes, etc. will cause
60/120 Hz modulation of the noise. This will be very annoying
on AM detection for SW BC, etc.
I think people have forgotten that 30 dB is a factor of 1000
and 60 dB is a factor of 1,000,000. That's big !!
* MORE BPL COMMENTS *
The BPL NPRM Appendix C:
The test procedure calls out a loop antenna below 30 MHz,
oriented in a vertical plane and rotated about a vertical axis.
If I read this right, this does NOT measure any horizontally
polarized radiation.
Above 30 MHz, a biconical dipole, parallel to the wire, rotated
V and H, is specified. If I read this right, it does not measure
any Er. This Er doesn't propagate, but it's still noise to be
picked up locally.
* ADAPTIVE INTERFERENCE COMPENSATION *
one of the proposed interference mitigators is adaptive
compensation. I am not sure what this will be, but in OFDM
as proposed, it may simply mean not using data carriers on
a frequency where the unit can detect activity. In a DSSS
system, notches may be used to remove offending frequencies.
The adequacy of these countermeasures is questionable if many
frequencies are in use in the system and/or a transmission
has not occurred near the frequency in question (SWL's).
Notches of 30 dB help but don't eliminate a 60 dB degradation.
* GENERAL*
Part 15, as it stands today, is very inadequate to control
noise and interference. The measurement bandwidth is not
specified. The standards are the same at 1.705 MHz and at
30 MHz. That's a factor of 17 in frequency, a factor of 300
smaller in dipole antenna capture area (25 dB), and most
importantly, a reduction in atmospheric band noise of 50 dB
from a simple vertical or dipole. Also antennas at 30 MHz
are often 10 dB gain, that is, they have 10 times the capture
area of a dipole. That means that 30 uV/m in 1 MHz varies
from -55 dBm in 10 kHz with an atmospheric noise of -80 dBm
(25 dB degradation) to -70 dBm from a small beam pointed at the
power wires with an atmospheric noise level of -130 dBm (60 dB
degradation).
The only reason Part 15 sort of works is that spurious emissions
used to be unmodulated and narrowband carriers. That's why
bandwidth was not even specified. A problem would only even
exist for a small extent in frequency. In the case of BPL, heavy
data modulation fills the whole spectrum with interferers. It
will be bad over whole bands at a time.
People have asked will it have long distance effects since these
bands support ionospheric propogation. Well at 10 MHz, free space
path loss in 300 kilometers is about 80 dB more than the loss in
the first 30 meters, so the contribution of a short section of
power line should be very small. Having 1000 of the noisy sections
would raise the noise 30 dB, still OK.
* MODULATION *
Apparently the modulation techniques are proprietary, so we must
assume something. If the data rate per user is 1 MBPS and we
could have ten downloads simultaneously, the aggregate data rate
is 10 MBPS. Since the system is relatively robust, we will assume
a simple modulation scheme, so the modem output occupies 10 MHz
of spectrum in the range 2 to 80 MHz. Let's also assume that two
systems are in service so 20 MHz is used, with guard bands, this
is 30 MHz. This is about half the available band. It is possible
to "move around" a bit, especially with an OFDM-based modem, but
since almost all the spectrum is allocated, the modem is going to
step on someone. Of course, propagation being variable, there may
be preferred frequencies to avoid at certain times of day and season
and solar cycle. If the system is lightly loaded so there is some
"elbow room", it might be acceptable.
* RECOMMENDATIONS *
If the BPL network just makes Part 15, the interference potential
is huge.
The keys to operating this system without interference is frequency
agility of the transmitters/injectors and the willingness of the FCC
to call this a Part 15 service that must not interfere with licensed
stations and operations. They must adjust or shut down. The
problem is that what will be considered more important, one
interference problem or 500 paying Internet connections dropped.
In the past, the FCC seemed to do what had to be done. Now ...
I don't know. The FCC may just buckle under public pressure.
Anyway, Part 15 needs to be re-vamped, tightened, and clarified
to be really usable to control interference in the 21st century.
The measurement bandwidth should be specified. Similarly, the
spurious specification for transmitters of -13 dBm should have
a measurement bandwidth associated with it. Also, -13 dBm is
really not tight enough at VHF and UHF frequencies to control
interference. Japanese and European goals are often 20 dB tighter
to be accepted by regulatory agencies.
Also the measurement techniques of NPRM Appendix C need to be
revised. Not all cases are even measured, such as horizontal
polarization below 30 MHz.
John Matz KB9II
24 Feb 2004