**SECTION 9**

**SUMMARY OF RESULTS**

**9.1 INTRODUCTION**

Section 9.2 summarizes the results of NTIA's preliminary investigations
(Sections 2 -- 5). These investigations helped refine the scope and
approach of NTIA's analyses and established certain technical
assumptions. Section 9.3 summarizes the results of NTIA's Phase 1
analyses of interference risks (Section 6), measurement procedures
(Section 7) and techniques for prevention and mitigation of interference
(Section 8). Section 9.4 summarizes matters requiring further study.

# 9.2 PRELIMINARY INVESTIGATIONS

**9.2.1 Descriptions of BPL Systems**

NTIA identified three architectures for access BPL networks (Section 2):
(1) BPL systems using different frequencies on medium- and low-voltage
power lines for networking within a neighborhood and extensions to
users' premises, respectively; (2) BPL use of only medium voltage lines
for networking within a neighborhood, with other technologies being used
for network extensions to users' premises; and (3) BPL use of the same
frequencies on medium- and low-voltage power lines for networking in a
neighborhood and extensions to users' premises. Responses of BPL
manufacturers and operators to the FCC's BPL NOI generally indicate that
BPL systems will operate at or near the Part 15 field strength limits in
order to achieve maximum throughput and distance separation between BPL
devices. NTIA addressed simple BPL deployment models in the Phase 1
interference risk analyses (Section 6). Specifically, a single BPL
device and associated power lines were considered for cases of potential
interference to ground-based radio receivers and several co-frequency
BPL devices were assumed to be deployed throughout the area covered by
an aircraft receiver antenna. For future studies, NTIA developed
preliminary BPL deployment models addressing three geographic scales
(Appendix F): a "neighborhood" deployment model useful for analyses of
interference to radio receivers having antennas at heights lower than
power lines; an "antenna coverage area" model useful for consideration
of radio antennas atop buildings and towers and on aircraft; and a
"regional" deployment model for studies of potential interference via
ionospheric signal propagation.

**9.2.2 Studies and Relevant Regulations**

NTIA reviewed studies performed by other parties and applicable FCC and
foreign regulations to ensure that NTIA's studies would address
important interference mechanisms and factors as well as potential means
for effectively accommodating BPL and radio systems (Section 3). NTIA
noted that BPL apparently has been implemented with success in some
countries, while other countries have postponed implementation of BPL
systems until further interference studies are being conducted. Still
others have withdrawn their approval for operation of BPL systems after
experiencing interference problems. Several emission limits have been
adopted or proposed for evaluation on international, national and
regional bases. Most studies have sought to determine whether
interference will occur at the variously proposed limits. In contrast,
NTIA has oriented its study to appropriately manage the risk of
interference to radio systems.

Technical information and analyses submitted in response to the FCC NOI
included several relevant observations. BPL signals unintentionally
radiate from power lines, although there is substantial disagreement as
to the strength of the emissions and their potential for causing
interference to licensed radio services. Analyses indicate that the peak
field strength due to unintentional BPL radiation occurs above the
physical horizon of power lines. Current ad hoc measurement techniques
used in Part 15 compliance tests may significantly underestimate the
peak field strength generated by BPL systems as a result of using a loop
antenna in the near field; performing measurements with an antenna
situated near ground level (*e.g.*, 1 meter); and measuring emissions in
the vicinity of BPL devices without also considering emissions from the
power lines.

**9.2.3 Federal Government Radio Systems and Spectrum Usage**

Frequencies between 1.7 MHz and 80 MHz are allocated to a total of 13
radio services, with the Federal Government using most of these radio
services to satisfy various mandated mission requirements (Section 4).
Federal agencies currently have over 59,000 frequency assignments in
this frequency range. Allocations for the fixed and mobile services
accommodate communications for homeland security, distress and safety,
and other critical functions. These communications occupy over one-half
of the frequency range and NTIA chose them as the focus of this Phase 1
study. Characteristics of fixed and mobile equipment largely group into
uses below 30 MHz and above 30 MHz and the equipment characteristics
show considerable consistency within these two categories.

Both NTIA and FCC have long recognized that certain frequencies or bands
in the radio spectrum require special protection from interference
because of the critical or sensitive functions they support, including
distress and safety, radio astronomy, radionavigation, and others. NTIA
identified forty-one (41) such frequency bands between 1.7 MHz and 80
MHz, totaling approximately 4.2 MHz (5.4% of the total spectrum under
study), that may warrant special protection from interference by
licensed and/or unlicensed transmitters. NTIA will further review the
appropriateness of applying geographic BPL restrictions or other special
BPL provisions to these and other frequencies that warrant special
protection in its Phase 2 study.

**9.2.4 Characterization of BPL Emissions**

Numerous textbooks explain the electromagnetic theory behind wires
serving as transmission lines or antennas. For unshielded wires such as
power lines, the magnitude of radiation is largely affected by the
degree of balance between radio frequency currents in adjacent wires and
the spacing of those wires. Common mode currents (traveling in the same
direction) in parallel wires generally produce mode radiation than
differential currents (traveling in opposite directions) because for
differential currents, the fields generated by each wire tend to cancel
if the wires are closely spaced (*e.g.*, twisted pair used for telephone
lines). Impedance discontinuities can occur on power lines at
transformers, branches and turns, and can produce radiation directly or
cause signal reflections in the power lines that produce standing waves
and associated radiation along the line. The fields generated by radio
frequency currents have different types of spatial distributions in
three successively more distant areas around a radiator: the reactive-
and radiative-near-field and far-field regions. The distances over which
reactive and radiative near-field regions extend increase with the size
of the radiator and frequency. In the far field region, which could
start several kilometers away from a radiating power line, the radiation
patterns are independent of distance and field strength in free space
generally decreases in proportion to increasing distance.

The relevant signal propagation modes in the 1.7 -- 80 MHz frequency
range are ground wave, space wave and sky wave. The ground wave signal
can consist of a direct wave, ground reflected wave and/or a surface
wave, each of which exhibit a different characteristic relationship
between signal loss and distance. The direct wave signal power from a
point source (*i.e.*, very small in relation to wavelength) is inversely
proportional to the square of the distance and when combined with a
strong ground-reflected wave from a radiator several wavelengths above
the ground, the composite signal power is inversely proportional to
distance to the fourth power. The latter high rate of attenuation does
not occur for radiators closer to the ground. A surface wave propagates
close to the ground and exhibits substantially higher rates of
attenuation than the direct wave. Thus, groundwave propagation is
pertinent on BPL signal paths below the power line horizon. Space wave
propagation involves only a direct wave and occurs over elevated signal
paths, *e.g.*, on signal paths above the power line horizon. Sky wave
propagation also occurs above the power line horizon and most
consistently at frequencies between 1.7 MHz and 30 MHz. Skywave signal
paths are represented as rays that are refracted and reflected by the
ionosphere and can extend to distances of thousands of kilometers
depending on the signal elevation angle and frequency as well as
parameters of the ionosphere that exhibit temporal and spatial
variability.

As a part of its study, NTIA modeled an overhead, three-phase Medium
Voltage power line using the NEC software program. The far field
patterns of the electric field indicate that the number of local peaks
in the radiation pattern increase as the ratio of line length to BPL
signal wavelength increases. Varying the source and load impedances have
a minor effect, although the highest radiation was generally associated
with the largest impedance mismatch between source and load. The far
field radiation patterns and radiating near-fields at a height of two
meters both indicate that BPL signal reflections from impedance
discontinuities can generate standing waves that cause radiation from
power lines. Along the direction of the power lines, the peak field
strength in the far field occurs above the horizontal plane containing
the power lines. In the near field, the peak level of the vertical
electric field never occurs at the BPL source; instead, multiple local
peaks occur near and under the power lines. Similarly, the peak
horizontally polarized field in the direction perpendicular to the power
lines never occurs at the BPL source; instead, peaks occur at various
distances away from the BPL source and power lines. Based on the models
considered to date, only in the case of the horizontally polarized
electric field in the direction parallel to the power lines does the
peak field occur at the BPL device. NTIA's modeling showed that
inclusion of a neutral line with three phase medium voltage wiring
tended to increase the overall radiation. Thus, models omitting the
neutral wire tend to predict lower field strength. These modeling
results imply that compliance measurements, taken only around a BPL
device and at heights below the power lines, may significantly
underestimate the peak electric field.

NTIA performed measurements at three different BPL deployment sites in
order to characterize the BPL fundamental emissions. Measurements
indicate that the BPL electric field does not generally decay
monotonically with distance from the BPL source as the measurement
antenna was positioned near to and moving along the length of the power
line. As the measurement antenna was moved away from the BPL energized
power line, the radiated power decreased with increasing distance, but
the decrease was not always monotonic and a number of local peaks were
observed at some locations. In some cases, the BPL signal decayed with
distance away from the power line at a rate slower than would be
predicted by space wave loss from a point source. At one measurement
location where a large number of BPL devices were deployed on multiple
three-phase and single-phase MV power lines, appreciable BPL signal
levels (*i.e.*, at least 5 dB higher than ambient noise) were observed
beyond 500 meters from the nearest BPL energized power lines. Finally,
NTIA's measurements show that the radiated power from the BPL energized
power lines was consistently higher when the measurement antenna was
placed at a greater height (*e.g.*, 10 meter vs. 2 meter). These results
indicate a need to refine the Part 15 compliance measurement guidelines
to ensure that the peak field strength of any unintentional BPL
emissions is measured.

**9.3 PHASE 1 ANALYSES**

**9.3.1 Evaluation of Potential Interference Risks**

NTIA evaluated interference risks using NEC models for four
representative types of federal radio stations operating in the fixed
and mobile services (Section 6): a land vehicular radio; shipborne
radio; a fixed or mobile-base station with roof top antenna; and an
aircraft radio in flight. These risks were gauged from the size of
geographic areas in which BPL emissions would reduce the ratio of
desired radio signal power to ambient noise power by amounts associated
with moderate and high probabilities of interference (*i.e.*, 3 dB and
10 dB reductions in (S/N), respectively). Predicted nationwide,
Springtime, median ambient noise power levels were assumed and analyses
were performed for frequencies of 4 MHz, 15 MHz, 25 MHz and 40 MHz.
Three-phase power lines were modeled as straight American Wire Gauge
(AWG) 4/0 copper wires, 340 meters in length, and horizontally spaced by
60 centimeters. No neutral line was included in the model in order to
reduce NEC execution time; this benefit was in trade for underestimation
of field strength by a few dB (Section 5.4.3). The three phase lines
were assumed to be 8.5 meters above ground having typical electrical
characteristics. The BPL device was assumed to present a source
impedance of 150 Ω, coupled on an outer power line, halfway between the
ends of the lines. The lines were terminated with 50 Ω loads to emulate
an impedance discontinuity (*e.g.*, transformer) and on-going power
lines with additional loads; however, emissions beyond the ends of the
lines were not considered because field strength levels may be
non-typical and radio receivers would more typically be located adjacent
to power lines. The BPL device output was adjusted to produce emissions
at the limits of Part 15 for unintentional radiators (Class B above 30
MHz), as generally determined by compliance measurement practices extant
with the exception that measurement distances were applied with respect
to the BPL device and power lines rather than only the BPL device. This
exception generally results in compliance at BPL output power levels
lower than output levels that yield compliance when distances are
measured from the BPL device. For all of these analyses, the frequencies
at which the lowest and highest reductions in S/N occur may change for
different power line configurations.

The results for the vehicular mobile receiver predict that the received
BPL signal power near the Earth surface falls off rapidly with distance
from the lines. For the two frequencies at which the highest BPL signal
power levels were received (15 MHz and 25 MHz), signal power from one
co-frequency BPL system (one device) equaled noise power (3 dB reduction
in S/N) at 50% of the locations within 70 and 75 meters of the power
lines. At these same frequencies, BPL signals reduced S/N by 10 dB at
50% of locations within 25 and 30 meters of the power lines. The
distances within which these thresholds were exceeded at 50% of
locations were modestly smaller at a third frequency (4 MHz) and much
smaller at the fourth frequency (40 MHz). In all land vehicular cases
considered, reductions in S/N were less than 3 dB and 10 dB beyond 125
meters and 55 meters, respectively.

The results for the fixed service (or mobile base station) receiver
predict that the received BPL signal power falls off less rapidly with
distance from the power lines than occurred for the land vehicle case.
For the two frequencies at which the highest BPL signal power levels
were received, signal power from one co-frequency BPL system (one
device) equaled noise power (3 dB reduction in S/N) at 50% of the
locations within 310 and 400 meters of the power lines. At these same
frequencies, BPL signals reduced S/N by 10 dB at 50% of locations within
175 and 230 meters of the power lines. In all cases, reductions in S/N
were less than 3 dB and 10 dB beyond 770 meters and 450 meters,
respectively.

The results for the shipborne receiver predict that the received BPL
signal power falls off rapidly with distance from the power lines, but
less rapidly than for the land vehicle case. For the two frequencies at
which the highest BPL signal power levels were received, signal power
from one co-frequency BPL system (one device) equaled noise power (3 dB
reduction in S/N) at 50% of the locations within 100 meters of the power
lines. At these same frequencies, BPL signals reduced S/N by 10 dB at
50% of locations within 55 meters of the power lines. In all cases,
reductions in S/N were less than 3 dB and 10 dB beyond 135 meters and 85
meters, respectively.

For the aircraft receiver, aggregate interference effects were
considered for simultaneously active, co-frequency BPL systems deployed
at a density of one per square kilometer over an area having a 10
kilometers radius. The power lines were assumed to be randomly oriented
and an average of the power line far-field gain levels were used in each
direction under consideration. Aircraft were assumed to be operating at
altitudes of 6 to 12 km at locations ranging from 0 to 50 kilometers
from the center of the BPL deployment area. Results showed that
aggregate interference levels to the aircraft could exceed average
ambient RF noise levels at two frequencies (15 MHz and 25 MHz), at
distances ranging from 33 kilometers (6 kilometers altitude) to over 50
kilometers (altitudes between 6 and 12 kilometers). The S/N reduction
exceeded 10 dB at only one frequency, at 6 kilometers altitude within 12
kilometers of the center of the BPL deployment area. At the two
frequencies where the assumed BPL systems produced the lowest
interfering signal power levels (*i.e.*, 4 MHz and 40 MHz), S/N
reductions peaked at about 0.8 dB and 0.3 dB directly over the center of
the BPL deployment area. Higher or lower densities of active
co-frequency BPL units would raise or lower the predicted interference
levels in direct proportion to the unit density.

**9.3.2 Risk Reduction Through Compliance Measurement Procedures**

The Phase 1 analyses assumed that for outdoor overhead power lines,
compliance measurements were performed using a one-meter high
measurement antenna (Section 7). This ad hoc measurement approach does
not demonstrate compliance with the field strength limits because, as
shown by NTIA's measurements and models (Section 5), peak field strength
levels are not necessarily centered at the BPL device and do not occur
at a height of 1-meter above the ground. Moreover, all of the receiving
antennas assumed in the Phase 1 analyses were located at least 2 meters
above the ground. Other potential sources of measurement underestimation
of BPL field strength include: the measurement distance and
extrapolation factor; frequency-selective radiation effects; estimation
of electric fields using a loop antenna; and selection of representative
BPL installations for testing. Solutions to most of these measurement
challenges are at hand within existing Part 15 measurement guidelines.

In light of the above considerations and the high perceived interference
risks, NTIA recommends that the FCC not relax field strength limits for
BPL systems and that measurement procedures be refined and clarified to
better ensure compliance. These recommendations should be effected as
quickly as possible in order to better protect radio communications.
Specifically, NTIA recommends the following BPL compliance measurement
provisions.

> \(a\) Consistent with §15.31(f), (h), (j) and (k), BPL measurements
> should address the BPL devices and power lines to which they are
> connected. Measurement reports submitted by contractors hired by BPL
> proponents to test compliance of trial BPL systems with Part 15 field
> strength limits showed that measurements were performed on radials
> emanating from a power line pole to which a BPL access device was
> mounted.
>
> \(b\) BPL systems should be tested i*n situ* using the maximum
> potential frequency reuse in accordance with §15.31(h) and (i).
>
> \(c\) Measurement antenna heights should address all directions of BPL
> signal radiation toward potential local radio antennas. NTIA\'s work
> to date indicates that a measurement antenna height of the order of
> the power line height may properly protect radio receivers having
> antennas at rooftop heights. In any case, measurements must identify
> the peak level of electric field strength consistent with
> §15.31(f)(5).
>
> \(d\) A ten (10) meter measurement distance should be used uniformly
> with respect to the BPL devices and power lines to which they are
> connected. A uniform measurement distance will greatly simplify
> compliance measurements.
>
> \(e\) A modified distance extrapolation factor should be applied for
> BPL systems that reflect realistic decay in field strength with
> increasing distance. The extrapolation factors assumed in Part 15
> appear to be unrealistic for BPL systems (40 dB/decade and 20
> dB/decade below and above 30 MHz, respectively (§§15.31(f)(1) and
> (2)). Further study is needed to determine the appropriate
> extrapolation factors.
>
> \(f\) Radiated emissions must be measured with the BPL devices
> operating at all frequencies at which they are capable of operating.
> This will require sequential tuning and measurements in each abutting
> frequency band within the tuning range of the BPL devices. Measurement
> with the BPL devices tuned to each possible operating frequency is
> required for consistency with §15.31(g).
>
> \(g\) Measurements below 30 MHz should be made with either a
> calibrated rod antenna (direct measurement of electric field) or a
> loop antenna in connection with adjustment factors that properly
> account for the ratio of BPL near-field electric and magnetic field
> strengths for vertical, horizontal-parallel, and
> horizontal-perpendicular polarization. NTIA\'s work to date indicates
> that in the near-field of BPL emissions, this ratio may differ
> significantly from the 377 Ω far-field value assumed in Part 15 for
> other devices.
>
> \(h\) Consistent with §15.31(d), power lines used for *in situ*
> testing of BPL devices should be carefully selected to be
> representative of deployments that produce the highest levels of field
> strength. Further study is needed of the power line features that
> should be included.
>
> \(i\) In the course of measurements, if it is determined that BPL
> device output power must be reduced in order to obtain compliance with
> field strength limits, the measurements preceding this discovery
> should be included in the measurement report and measurements should
> be repeated with the lower required output power. As required under
> §15.15(b), the equipment to be marketed should be constructed to
> prevent operation at field strength levels exceeding the limiting
> values.

**9.3.3 Techniques for Prevention and Mitigation of Interference**

NTIA identified a number of currently employed techniques and other
potential means to reduce the interference risks or facilitate
mitigation of interference problems (Section 8):

***Minimize Power Level*.** The single most effective method for
reducing the potential for interference may be to reduce BPL device
output power. Consistent with §15.15(c), BPL system operators are
encouraged to use the least power needed to carry out power line
communications. The use of adaptive transmitter power control could be
used to ensure that the furthest subscriber in the line has an adequate
but not excessive conducted signal level.

***Avoidance of Locally Used Frequencies*.** Shifting or notching BPL
signal frequencies to avoid interference to local radio receivers may be
an effective interference prevention or mitigation technique. More
advanced methods would include agile or adaptive filtering in real time,
which may be very effective in reducing interference to simplex-mode
communications originating in the local environment. These adaptive
techniques are not expected to be effective in reducing interference to
duplex-mode communications or simplex communications originating outside
the local area, where the associated radio transmitter may be tens,
hundreds, or even thousands of miles away. NTIA further recommends
consideration of excluding BPL use of certain narrow frequency bands,
but further study is needed to determine whether these exclusions can be
specified on a geographical basis. Generally, BPL systems should not
operate in certain frequency bands in order to protect distress, alarm,
urgency or safety communications in accordance with ITU Radio
Regulations (RR No. 4.22).

***Differential-mode Signal Injection*.** Use of differential-mode
injection of the RF signal onto two parallel power lines could
potentially reduce radiated BPL emissions in a manner similar to
unshielded twin-lead transmission lines used in communications systems.
The generally-unbalanced nature of power line pairs will limit the
effectiveness of this technique.

# *Filters and Signal Terminations*. The use of filters on the power lines that would absorb, rather than reflect, RF signals at impedance discontinuities or termination points beyond the last subscriber on the line could reduce unnecessary RF emissions from BPL energized power lines. Further, the use of absorbing filters on LV lines to prevent RF signals from entering the premises of non-subscribers may mitigate certain interference problems.

***Implementation of a "One Active Device per Frequency and Area"
Rule.*** Several implementations of BPL systems use a technique whereby
only one device in a local "cell" is active on the same frequency at any
one time. Such techniques would reduce or eliminate the chance of any
potential local, ground level aggregate BPL interference effects.
However, in order to increase BPL network capacity or decrease network
latency in a given area, it may be desirable to operate independent,
co-frequency BPL devices on two or three phases of the same run of
three-phase power lines. In any case, compliance measurements are to
address radiated field strength due to all BPL devices operating
co-frequency within the BPL network in accordance with §15.31(k).

***Judicious Signal Carrier Choice*.** Due to the frequency selectivity
potentially established by various physical and electrical
characteristics of a given section of power line, it is conceptually
possible to identify frequency segments within the range 1.7-80 MHz that
would allow higher levels of injected signal yet at the same time
exhibiting lower radiation levels.

***Maintenance of a Single Point of Control*.** To facilitate rapid
resolution of actual cases of interference without third-party
intervention, a single point of control should be employed for each BPL
service area and a BPL point of contact should be designated to address
cases of suspected interference and resolving actual interference.

***Web-based Access to Radio License Information*.** Knowledge of what
licensed radio systems may be located in the local environment of a BPL
system could assist BPL operators in selecting frequency, power levels,
and other technical parameters that minimize interference. NTIA will
further investigate which elements, if any, of the federal frequency
assignment data base might be made available via a web-based mechanism.
The FCC assignment data base already is publicly available.

***BPL Installation and Equipment Registration*.** By registering their
current and planned BPL deployment details in a central, publicly
accessible data base, BPL operators will have equipped local radio users
with information they need to alert the BPL operator of potential
interference problems. The database also could assist radio operators in
diagnosing cases of suspected interference. NTIA will further study and
recommend the BPL deployment parameters that should be included in the
registration database.

**9.4 TOPICS FOR FURTHER STUDY**

> \(a\) The appropriate measurement antenna height and need for a
> height-adjustment factor should be determined with a goal of
> identifying the minimum set of measurements that will ensure
> identification of peak BPL emissions in important directions of
> radiation.
>
> \(b\) Measurement distance extrapolation factors reflecting the
> realistic decay of BPL field strength with increasing distance should
> be determined.
>
> \(c\) To enable suitable estimation of electric field strength using a
> loop antenna below 30 MHz, the appropriate ratio of electric to
> magnetic field strength should be determined for the recommended ten
> (10) meter measurement distance and measurement antenna heights.
>
> \(d\) Quasi-peak to rms conversion factors should be further
> investigated for BPL systems. This will ensure that the levels due to
> a radiated BPL signal and noise can be specified in consistent terms
> for analysis purposes.
>
> []{.mark}
>
> \(e\) Aggregation of emissions from BPL systems via ionospheric
> propagation and the associated BPL deployment models require further
> study. This is of concern in the long-term insofar as skyward
> emissions from many hundreds of BPL systems deployed over a large
> region might produce significant composite interfering signal levels
> at a very distant receiver.
>
> \(f\) The local interference risk reductions obtained from the
> proposed compliance measurement guidelines (Section 9.3.2 and item
> (a), above) should be determined to ensure that BPL systems will
> neither be unnecessarily constrained or pose unacceptably high
> interference risks.
>
> \(g\) Possibilities for issuing specific guidance on local Federal
> Government and other frequency usage should be explored in order to
> enable interference to be prevented. For example, special current
> versions of NTIA and FCC frequency assignment databases might be made
> available via a web site.
>
> \(h\) Potential new requirements should be identified for more
> frequent testing of Federal Government radio systems used for backup
> or emergency purposes in the vicinity of BPL systems.