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| A Note from the Publisher:
We continue to enjoy the feedback from our readers regarding
the first two issues of Rural Signals.
Our team of technical consultants strives to bring you understandable
information regarding current issues facing the wireless telecommunications
industry. In this third issue of Rural
Signals, our team writes about more areas of great interest
to wireless carriers, including antenna co-location and interference
issues, 700 MHz spectrum developments, and potential radiated
power limit increases for wideband technologies.
While tower siting and zoning regulations intensify and the
Nationwide Programmatic Agreement emerges, co-location continues
to grow as a very attractive and convenient alternative. However,
co-location is not without its pitfalls. In this issue of Rural
Signals, Judy Deng explains the perils of intermodulation
interference that can result from co-location, and David Fritz
discusses compliance of antenna facilities with RF exposure
regulations.
At the same time, potential service disruption does not always come from other radio transmissions. Wind turbines are popping up as a great alternative energy source, but also pose an interference threat to some telecom facilities. Malick Sohrab alerts us to the issues and how to avoid a big surprise from a new wind turbine farm.
The Rural Telecommunications Group (RTG)
is at it again, and that's a good thing for rural carriers.
RTG has already scored a big victory for rural carriers in its
joint FCC filing regarding Advanced Wireless Service spectrum
allocation. Len Garavalia reports on the final band plan, and
then provides an in-depth discussion of RTG's latest effort:
a rural-friendly modification to the FCC's 700 MHz spectrum
plan. In what we hope will be a recurring feature to provide
tips and reminders to our readers, Len also offers some timely
reminders about FCC filings in the Lower 700 MHz C-block.
In addition to the 700 MHz bands, developments continue to
abound in the cellular and PCS world. Rural
Signals editor Jim Egyud reports on an FCC rulemaking
proceeding that could permit higher radiated power limits in
the cellular and PCS services, but only for wideband technologies.
The article discusses some options under consideration and the
potential impact for service providers.
As a final item, working at a law firm and examining countless
FCC and FAA applications allows us to see common inadvertent
errors on those applications. One of the most common involves
tower height, and how the overall structure height differs from
the support structure height in the various forms. Rural
Signals offers a little explanation to clarify the difference,
as well as a handy suggestion for keeping it consistent.
Enjoy Rural Signals and, as always, your feedback is priceless.
Bennet & Bennet, PLLC
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Co-location
and Intermodulation Analysis
"An Ounce of Prevention Is
Worth a Pound of Cure" |
|
By Judy Deng
Unexpected interference from the unlikeliest of sources has
been an issue that has befuddled RF engineers and technicians
for years. Many hours, and perhaps significant revenue, are
lost in the process of trying to determine and remove an interference
source, especially if the source is not an obvious system using
the same frequency band or channel. Intermodulation (IM) interference
or "intermod" as it is more commonly known, is often the culprit,
but it is one that can be predicted, if not entirely prevented.
The proliferation of mobile networks and the co-location of
multiple antenna systems on single sites increase the possibility
of harmful RF interference among tenants at the same site. The
factors driving such interference include the number of active
channels at the site, the relative placement of the antennas,
the frequency bands used, the characteristics of the technology,
and the base station equipment.
Interference Defined
An understanding of intermod and its importance goes beyond the cursory knowledge that interference can distort a signal. Broadening the scope, interference may be defined as the effect of unwanted energy due to one or a combination of emissions, radiation, or induction upon the reception of a radio system, manifested by the serious degradation, obstruction, or repeated interruption in communication. Interference may be generated by sources at a shared site as well as by signals arriving from another site. Typically, the site owners and wireless operators have little control over interfering signals generated from a distant system or a neighboring tower, especially if all parties have followed the FCC's frequency coordination procedures and other rules for their respective services. From a practical standpoint, the operators are more capable of concentrating on interference generated among sources at the shared site. However, strong signals generated from a neighboring site can also be considered where possible.
The Usual Suspects
To further complicate matters, RF interference at a shared
site can result from a number of different factors, and is typically
caused by one of the following:
- Transmitter intermodulation;
- Receiver intermodulation;
- Passive intermodulation;
- Receiver desensitization; and
- Out-of-band emissions such as transmitter noise, transmitter
spurious emissions, transmitter harmonics, and receiver spurious
emissions.
Intermodulation occurs when signals of different frequencies
combine to form additional signals at entirely different frequencies,
depending on the points where their waveforms meet. Because
the intermod frequencies result from combining harmonics or
multiples of the source frequencies, we often refer to them
as “intermod products”. One of the most common shared
site interference mechanisms is transmitter intermodulation.
This results from the signals from one or more transmitters
sneaking into the non-linear final output stage circuitry of
another transmitter through its antenna system. (A circuit is
said to be “non-linear” if its output signal level
does not vary in proportion to its input.) The resulting “intermod”
frequency is then emitted from that transmitter’s antenna.
If the intermod product falls within the frequency band of a
nearby receiver and the signal level is of sufficient amplitude
(i.e., strength), it can degrade the performance of
the receiver.
In a similar manner, receiver intermodulation occurs when two
or more transmitter signals enter and mix in a receiver’s
RF amplifier or mixer stage when the receiver is operating in
a non-linear stage, and the resulting intermod products then
appear at the receiver’s demodulator input.
Passive intermodulation is the result of transmitter signals
mixing in other non-linear “junctions”. These junctions
are usually metallic, such as rusty bolts on a tower and dissimilar
metallic junctions. This type of intermodulation can be reduced
by minimizing the number of loose metallic joints within system
components such as antennas, cables, and connectors, and in
the external environment on towers and wire fences. Needless
to say, passive intermodulation can be very tricky to isolate
and often requires some keen observance in the field.
Receiver desensitization occurs when an undesired signal from
a nearby adjacent channel transmitter is sufficiently close
to a receiver's operating frequency. The signal may sneak through
the RF selectivity of the receiver, degrade the noise floor,
and reduce the performance of the receiver. A high-power transmitter
can be operating several megahertz away from the receiver frequency,
and/or the transmitter’s antenna can be located several
thousand feet from the receiver's antenna, and still cause "de-sense"
interference.
Transmitter noise interference occurs because a transmitter
radiates energy on its operating frequency as well as on frequencies
above and below the assigned frequency. The energy that is radiated
above and below the assigned frequency is known as sideband
noise energy and extends for several megahertz on either side
of the operating frequency. This undesired noise energy could
fall within the passband of a nearby receiver, even if the receiver's
operating frequency is several megahertz away. The transmitter
noise appears as "on-channel" noise interference and
cannot be filtered out at the receiver. It is therefore on the
receiver's operating frequency and competes with the desired
signal, which, in effect, degrades the operational performance.
An Ounce of Prevention
The old saying continues to be true: “An ounce of prevention
is worth a pound of cure.” Any of the forms of interference
discussed here can cause an insidious performance reduction,
which can baffle even the most seasoned system operators and
field engineers. Therefore, whenever a wireless facility, whether
a transmitter or receiver, is added to a site, a co-location
intermod analysis should be performed to predict the potential
of the new system to cause or suffer from harmful interference
involving other facilities at the site. The analysis calculates
all possible IM product frequencies that could potentially interfere
with receivers at the shared site based on each receiver’s
individual bandwidth. It then calculates predicted interference
levels and determines whether or not harmful interference might
occur at the input of each receiver. The analysis should take
into account each device’s power output, modulation bandwidth,
antenna configuration, and other RF components that are present
in each system.
In addition, Transmitter Noise and Receiver Desensitization
interference should be modeled and studied. These problems usually
only arise when transmitters and receivers are operating on
adjacent frequencies. Additionally, the analysis determines
how much isolation, if any, is required to prevent receiver
performance degradation by intermodulation frequencies, receiver
desensitization and transmitter noise interference.
There are a number of factors that can assist in the prevention
of IM interference, and these depend on where the IM products
are generated. The key to their suppression lies in identifying
where the signals are mixed. Given the many different types
of interference discussed here, along with the high number of
transmitters, receivers, and antennas that might share a radio
site, preventive studies can be quite iterative, and must be
extensively detailed. However, compared to the difficulty of
finding an interfering needle in the radio spectrum haystack
during a nighttime maintenance window, the predictive analysis
is well worth the effort.
******
Bennet & Bennet, PLLC’s staff of technical consultants
is experienced at performing the analyses discussed herein.
If you should have any questions regarding co-location interference
issues or have a site that might need analysis, please do not
hesitate to contact Judy
Deng.
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Evaluating
Compliance With RF Safety Requirements
A Discussion of OET Bulletin 65 |
|
By David Fritz
Since the passage of the National Environmental Policy Act
of 1969 (NEPA), the FCC has been charged with considering whether
transmitting facilities operating under its jurisdiction have
significant impact on the environment. With this requirement,
the FCC (with the aid of the Environmental Protection Agency,
Food and Drug Administration, National Institute for Occupational
Safety and Health, and the Occupational Safety and Health Administration)
investigates radio frequency (RF) exposure and has developed
regulations to govern the monitoring and compliance of facilities
emitting RF energy. The first FCC-adopted guidelines were issued
in 1985 and revised in 1996 to include limits for Maximum Permissible
Exposure (MPE) in terms of electrical fields, magnetic fields,
and power densities for frequency bands between 300 kHz and
100 GHz.
The Guidelines
For FCC licensees, the key technical document in evaluating
and measuring RF compliance is the Office of Engineering Technology
(OET) Bulletin 65. The FCC guidelines break the MPE limits into
categories of “Occupational/controlled” and “General
population/uncontrolled.” The two categories reflect the
difference between persons who have specific control over the
RF environment and those who do not. The FCC groups the MPE
limits by frequency and specifies them in terms of electric
field strength (V/m), magnetic field strength (Amps/m), and
power density (mW/cm²). The FCC bases the limits on a specific
absorption threshold rate of four Watts per kilogram, which
applies to the power density spatially averaged over the whole
body. Also, the FCC guidelines allow averaging of MPE limits
over specific periods of time. For the “Occupational/controlled”
category, this time interval is six minutes, and for the “General
population/uncontrolled” group, 30 minutes. Using the
power density level of exposure, the MPE limits, and the MPE
averaging time limits, one can calculate the allowable time
for exposure. Maximum MPE limits for both categories across
the frequency ranges can be found in Bulletin 65.
Typically, licensees are required
to perform an initial environmental evaluation and prepare an
Environmental Assessment if the initial review of the transmitting
operation or device exceeds the limits outlined in OET Bulletin
65. Some transmitting facilities are termed “categorically
excluded” from the initial evaluation based on specific
criteria because the facilities have very little potential to
exceed the exposure limits. However, these “categorically
excluded” facilities are not excluded from compliance
with the MPE limits, but are only excluded from requiring the
initial evaluations to demonstrate compliance. The exclusion
criteria are based on deployment factors such as type of service,
antenna height, type of supporting structure, and total effective
operating power. Bulletin 65 covers different radio service
types and should be referenced to determine if a transmitting
facility is subject to routine environmental evaluation.
To evaluate compliance, Bulletin 65 offers a set of equations
that provide a prediction method to be used as an evaluation
tool for all transmitting facilities. The power density for
any transmitter, with the exception of FM and TV broadcast,
can be calculated using Equation (1) for a truly worst-case
prediction, assuming 100% reflection of incoming radiation on
the surface or point of interest.
Equation (1): S = 1.64*ERP/(πR²)
where: S
= Power density (mW/cm²)
ERP
= Effective Radiated Power (mW)
R
= Distance to center of radiation of antenna (cm)
*Cleavland, R.F.; Sylvar, D.M.; Ulcek, J.L.:”Bulletin
65 Edition 97-01”, FCC, Washington, DC, page 20, equation
6.
For FM and TV broadcast antennas, Bulletin 65 provides Equation
(2), based on a modified ground reflection approximation developed
by the Environmental Protection Agency.
Equation (2): S = 1.05*ERP/(πR²)
where: S
= Power density (mW/cm²)
ERP
= Effective Radiated Power (mW)
R
= Distance to center of radiation of antenna (cm)
*Cleavland, R.F.; Sylvar, D.M.; Ulcek, J.L.:”Bulletin
65 Edition 97-01”, FCC, Washington, DC, page 21, equation
8.
Equations (1) and (2) reflect a conservative approximation
of power density. For a more accurate evaluation in the case
of a directional antenna, the antenna’s gain at the angle
to the surface or point of interest can be factored into the
calculation. In general, Equations (1) and (2) are good predictors
of power density in the “far-field,” but may significantly
over-predict the RF environment at distances very close to an
antenna. For these cases and some other special circumstances,
Bulletin 65 outlines additional models and prediction methods
that can be used for detailed evaluations.
For studies involving multiple transmitters and complex environments,
each transmitter’s power density contribution is calculated
as a percentage of its own MPE limit and then each percentage
is summed to determine if the point of interest exceeds the
MPE limits (i.e. if the combined percentages exceed 100%). In
these cases, compliance with the limits is a shared responsibility
of all the licensees. When an evaluation is being made, it is
important that all significant contributors used in the study,
even those that may fit the “categorically excluded”
criteria, are included.
Due to the complexity of some transmitting
facilities, theoretical prediction methods cannot be used, and
actual RF exposure measurements will be required to demonstrate
compliance. For this, Bulletin 65 provides guidelines
and information on testing as well as details on controlling
exposure to RF fields.
******
With the FCC’s recent focus on NEPA, it is important
for wireless providers to remember that any pre-construction
review should include an assessment on RF safety to ensure compliance
with exposure limits. For more information on RF safety issues
and detailed studies into RF exposure, please contact David
Fritz.
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| A Source for Potential Interference: Wind Turbines? |
| By Malick Sohrab
As a result of increasing energy costs, extensive reliance
on fossil fuels, and the need for renewable energy sources,
modern wind turbine structures are emerging across the North
American landscape. While they might remind us of the windmills
of past times, the modern versions are transducer systems that
typically generate electricity via tower-mounted generators
connected to large, wind-driven rotor blades. The energy source,
of course, requires these turbines to be located in areas where
wind is in abundance. These installations could range from a
single generator-tower to hundreds of them over a specific area
or “windmill farm”.
Why should this be of concern?
For entities that utilize microwave links for critical network
backhaul or other communication needs, these towers could become
disruptive if not properly planned prior to their implementation
by the wind power company. With the wind turbines using tall
towers and rotating blades that extend even higher, these structures
could become potential obstructions to microwave paths, if not
placed properly. Moreover, the potential obstruction area is
actually widened to a circular area governed by the radius of
the rotating blades. Should any portion of this potential obstruction
enter the first Fresnel zone of a microwave path, the probability
of interference along the path increases, sometimes dramatically.
With the potential for more than one of these turbines going
up on a single wind farm, the possibility of obstruction becomes
larger. Microwave paths subject to such obstruction could suddenly
experience effects resulting anywhere from a reduction in reliability
to ultimate failure. Engineering the wind turbine placement
via prior coordination with existing microwave path operators,
and therefore away from the critical Fresnel zone, is the key
to not having this problem become a surprise. As one such recent
project involved 110 wind towers spanning approximately 29 square
miles, diligence is crucial.
Changes to the radio environment may not be limited to just
microwave paths. Adding to the concern for wireless operators,
there are instances where wind turbines have been installed
in close proximity to cell sites and other wireless facilities.
Given that generators are typically a notorious source for thermal
noise, wireless providers should be concerned about the impact
of having several large generators installed in close proximity
to a radio site.
Must wind turbine companies notify existing telecom operators?
Wireless service providers may wonder why such wind turbine
farms might suddenly show up within their existing paths and/or
service areas, and pose a problem to their network. The FCC
rules, in particular those governing the provision of wireless
services, only provide specific legal and technical parameters
for antenna structures. In general, placement of “man
made” structures such as wind turbines are governed under
local, state, and federal land use statutes, which are beyond
the scope of this article. Therefore, although there does not
appear to be an FCC rule that would require wind turbine companies
to coordinate their projects with existing telecom providers
in the area, such coordination might still be required by local,
state, and federal land use statutes where applicable in some
areas.
Fortunately, our experience shows that some wind turbine companies
may go the extra mile in finding and verifying licensed microwave
paths in the area prior to implementing their projects. However,
licensed microwave path data is public information. Data for
microwave paths using the FCC’s “unlicensed”
spectrum, which requires no coordination or licensing, is not.
The wind turbine companies would have little or no knowledge
of these paths unless they happen to stumble upon them while
verifying licensed path data or see the path facilities in the
field. There is no known central database of unlicensed path
or communication facility data that these companies could search.
What are telecom operators to do?
Given the potential impact on their networks, we recommend
that telecom providers remain alert and actively participate
with the local government in their areas in order to facilitate
local regulations and learn of proposed wind farm development.
Telecom providers should also actively monitor their networks
and service areas for the signs of wind turbine installations
or unexpected network problems, and communicate with the wind
turbine company directly if the company has not already contacted
them. The turbines should be analyzed with respect to any existing
and proposed communication facilities in the area, to which
the turbines may pose a risk. If a potential problem is foreseen
by that analysis, the telecom provider should discuss the situation
with the company and request it to move the turbine in question.
In our experience, turbine coordinators have been cooperative
and responsive to such requests prior to construction.
******
While wind turbines can provide an outstanding alternative
energy source, their coordination with telecom providers is
critical. If you would like to learn more about this subject
or have concerns about the potential impact of a specific wind
turbine facility, please do not hesitate to contact Malick
Sohrab.
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Final Advanced Wireless Services Band Plan
FCC Adopts Proposal That Should Benefit Rural Carriers |
| By Len Garavalia
As we reported in our inaugural edition of Rural
Signals, T-Mobile USA, Inc. ("T-Mobile") and
the Rural Telecommunications Group, Inc. (RTG)
petitioned the FCC to revise its spectrum plan for Advanced
Wireless Services ("AWS") in the 1.7 GHz and 2.1 GHz
bands (see T-Mobile
and RTG Join Forces to Bring AWS to Rural America).
T-Mobile and RTG recommended that the FCC reconfigure the AWS
frequency allocation to create paired spectrum for use within
Metropolitan Statistical Areas and Rural Service Areas (MSAs
and RSAs).
The dedication of 20 MHz of paired spectrum for MSAs/RSAs was
proposed so that rural licensees would have a competitive share
of the spectrum, comparable to the licensees in the other blocks.
In addition, since Broadband Radio Service channels 1 and 2
encumber the F-block frequencies from 2150-2162 MHz, RTG proposed
an alternate frequency block. This is important because, while
there are other unencumbered licenses proposed, only one MSA/RSA
license block was proposed. If the MSA/RSA license were to remain
in the F-block, the upper pairing would remain unavailable for
rural broadband deployment.
Thanks in large part to the RTG/T-Mobile proposal, we are pleased
to report that the FCC adopted the following band plan for the
AWS Service:
Block |
MHz |
Pairings |
Area |
Licenses |
A |
20 |
1710-1720 paired with 2110-2120 MHz |
MSA/RSA |
734 |
B |
20 |
1720-1730 paired with 2120-2130 MHz |
EA |
176 |
C |
10 |
1730-1735 paired with 2130-2135 MHz |
EA |
176 |
D |
10 |
1735-1740 paired with 2135-2140 MHz |
REAG |
12 |
E |
10 |
1740-1745 paired with 2140-2145 MHz |
REAG |
12 |
F |
20 |
1745-1755 paired with 2145-2155 MHz |
REAG |
12 |
For more information about the AWS plan and its potential for
wireless service providers, or if you should have any questions,
please contact Len
Garavalia.
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| RTG
Proposes 700 MHz Band Plans With Rural Needs In Mind |
| By Len Garavalia
In support of other industry associations, Rural Telecommunications
Group, Inc. (RTG)
has again taken the initiative to preserve the ability of rural
broadband service providers to obtain spectrum in chunks that
are manageable and affordable. The FCC is being pressed to license
the Upper and Lower 700 MHz frequency blocks with geographic
boundaries that follow MSAs and RSAs, in which licensees are
currently deploying systems using the Lower 700 MHz C-Block
channels.
RTG has made similar arguments to those made in its successful
fight for a rural-friendly AWS spectrum plan [see Final
Advanced Wireless Services Band Plan in this issue
of Rural Signals] in support of
MSA/RSA market segments:
- Maximum flexibility in both the Upper and Lower 700 MHz
bands for licensees;
- Need for MSA/RSA markets in the Upper Blocks (alongside
public safety allocations) since the Upper and Lower 700 MHz
bands may develop at different rates and may be used to provide
different services;
- Ensuring participation in the Upper 700 MHz band by small
and rural companies;
- Encouraging the deployment of services utilizing this spectrum
in rural areas; and
- Fostering the development of interoperable equipment in
both bands, which will facilitate efficiencies, including
economies of scale.
Technically, additional 700 MHz spectrum licensed on an MSA/RSA
basis will allow licensees to deploy bandwidth-intensive services
such as high-speed Internet. Rural broadband providers that are
deploying services using the paired 6 MHz C-Block will need additional
bandwidth for throughput capacity necessary for growth and to
accommodate diverse technologies. As the Lower 700 MHz C-Block
licensees deploy their networks and the demand for broadband over
these networks grows, the useable throughput available over paired
6 MHz spectrum will be exhausted. Making the
most of technology still requires spectrum
While the advances in technology seem swift, some Lower 700
MHz licensees will move cautiously to deploy technology that
can be easily integrated with their existing infrastructure.
In its book titled Bringing Home the Bits¹, the Committee
on Broadband Last Mile Technology stated that there will be
a “continuing incremental investment in existing infrastructure.
Because of investor expectations for short-term return on investment,
incumbents will continue to make use of existing equipment and
plant and the incumbents' deployment will be based on incremental
upgrades.” This is especially true for some licensees
that are currently deploying Lower 700 MHz C-Block equipment
based on the DOCSIS standard, which can be integrated with the
existing cable television infrastructure. While this 700 MHz
equipment is relatively inexpensive as an add-on to the existing
cable network, its RF section is based on TDMA technology, and
it utilizes the spectrum resource by applying a frequency division-multiplexing
(“FDD”) scheme that separates the available frequency
bandwidth into sub-channels of smaller bandwidth, for use on
the uplink and downlink (i.e., 2-way) communication channels
to the subscribers. As a result, careful frequency planning
is necessary, and frequency re-use (and available bandwidth)
is dependent on the channel bandwidth selection and the available
spectrum within the paired 6 MHz C-Block allocation.
The Committee on Broadband Last Mile Technology stated that
there will be a “Continued exploitation of technology
skills. Companies possessing particular expertise will exploit
opportunities where these skills give them an advantage. For
example, designing, launching, and operating a satellite system
all require know-how very different from that required to upgrade
a cable or telephone system.” A 700 MHz equipment
manufacturer that currently provides Lower 700 MHz networks
utilizes the paired 6 MHz of C-Block spectrum by applying time
division duplexing (TDD) in its FDMA solution, which combines
uplink and downlink communication on the same frequency channel
by managing the communications over time. This manufacturer
has re-banded its current product offering to fit the 700 MHz
market. As opposed to FDD, where the spectrum must be divided
into channels dedicated for uplink and downlink communications,
the available 700 MHz spectrum is used by both links in TDD.
Again, based upon channel bandwidth requirements, a TDD implementation
gives the impression that more channels would be available.
However, based upon proprietary channel spacing and intra-system
interference specifications, the maximum usable channels are
limited and careful frequency planning is still necessary.
As the Committee on Broadband Last Mile Technology also stated,
there will be “Varying levels of technology maturity.
Before wide-scale deployment, technologies must undergo an extensive
development process to reduce the costs of components, installation,
and management. More mature technologies will see wider deployment
at the same time that less mature technologies are being developed
in test markets.” With the rising support for the
adoption of standards utilizing OFDMA technologies combined
with the current deployments on competing technologies, market
borders between co-channel licensees will require spectrum management
techniques similar to those presently implemented in the 824-894
MHz cellular industry. Specifically, along numerous 824-894
MHz cellular market borders, spectrum clearing is required to
mitigate interference between competing co-channel technologies.
The need for spectrum clearing effectively reduces the availability
of bandwidth for all carriers, sometimes by 50% depending on
the adjacent markets’ technologies. Assuming that CDMA
and GSM technologies eventually enter the 700 MHz market (even
with migration to a common W-CDMA, UMTS or HSDPA platform),
spectrum clearing and diminished bandwidth and throughput capacity
will be experienced.
RTG’s 700 MHz Spectrum Proposal
Therefore, additional spectrum for the Lower 700 MHz C-Block
licensees is an absolute necessity to support effective deployment
of broadband service to the rural community. To that end, the
attached band plans as proposed by RTG would revise the Commission’s
spectrum allocations to result in an equitable distribution:
- 84 MHz Total Spectrum Available, broken down as follows:
- 6 MHz of Spectrum allotted to MEAs (Major Economic Areas,
seen here)
(No change from FCC plan.)
- 34 MHz of Spectrum allotted to MSAs/RSAs (seen here)
(RTG’s proposal adds 22 MHz.)
- 44 MHz of Spectrum allotted to EAGs (Economic Area Groupings,
seen here)
(RTG’s proposal subtracts 22 MHz.)
For a graphical depiction of the proposed spectrum plans, please
see the attached diagrams of RTG's
Lower 700 MHz Band Plan and RTG's
Upper 700 MHz Band Plan. These plans would appear to once
again strike a balance among the needs of the various stakeholders
and the need for sufficient spectrum and bandwidth in rural
areas.
******
We will update you on the Commission’s decision and response
in future editions of Rural Signals.
For more information about the 700 MHz band plans and technologies,
please contact Len
Garavalia.
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Rural
Signal: A note about 700 MHz FCC Filings
Lower 700 MHz C-Block: FCC Rule Part 27 requires compliance! |
| It is important to understand
that all Lower 700 MHz Band C-Block licensees must evaluate
TV/DTV channels 53, 54, 55, 58, 59 and 60 for potential interference
to incumbent stations. Even though a proposed base station transmitter
site may be located outside of the Grade B contour of these
stations, there are still specific geographic separation requirements
that must be maintained for interference protection. For detailed
discussions of the Grade B contour and geographic separation,
please see our Spring
2005 issue of Rural Signals.
The FCC’s co-channel and adjacent-channel separation standards
will apply simultaneously to both base station and customer
premise transmitting equipment if a Time Division Duplex technology
is proposed on either channel 54 or 59. The separation standards
will also apply separately for channel 54 (customer premise
equipment) and channel 59 (base station transmitter) if a Frequency
Division Duplex technology is planned.
Please contact Len
Garavalia or anyone at Bennet
& Bennet for an evaluation specific to your licensed
Rural or Metropolitan Service Areas.
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FCC Considers PCS Power Level Increases
Further Notice of Proposed Rulemaking Could Also Affect Cellular and AWS |
| By Jim Egyud
The FCC has issued a Further Notice of
Proposed Rulemaking (FNPRM) that could lead it to increase
the radiated power levels permitted from PCS, cellular, and
AWS licensees using wideband (e.g., CDMA) technologies.
The FNPRM came on the heels of the FCC’s biennial review
proceeding, during which the subject of wideband EIRP (Equivalent
Isotropic Radiated Power) in the PCS rules took on a life of
its own in a flurry of compelling arguments that were raised
by major industry players, including CTIA and Motorola. In the
ensuing Report and Order, the FCC removed the limits
that its rules had previously placed on PCS transmitter output,
leaving limits only on the radiated power at the antenna level,
as in EIRP. The FNPRM seeks to address what commenters perceive
as a disparity in the rules that appears to constrain coverage
more for wideband technologies than for narrowband technologies,
such as GSM.
Conversations Lacking Power?
In the consensus interpretation of the current FCC rules, EIRP
limits in the PCS band apply to each emission (i.e.,
channel carrier) from a cell site, regardless of the emission’s
bandwidth. Thus, the current EIRP limit of 1640 Watts applies
both to a 1.25 MHz CDMA carrier and to a 200 kHz GSM channel.
Since GSM uses a form of Time Division Multiple Access (TDMA)
with up to eight voice conversations on the same channel, each
conversation is afforded the same maximum EIRP. In the case
of CDMA, multiple traffic channels (voice paths) and control
channels are spread across the carrier, with each using a smaller
portion of the allotted power. At the same carrier EIRP limit,
it therefore stands to reason that an individual CDMA traffic
channel travels with less power than a time slot in a GSM channel,
or a TDMA channel for that matter. CTIA and the other proponents
argue that this places CDMA licensees at a disadvantage because
individual cell sites have less of a coverage footprint than
an otherwise equivalent narrowband site.
While it is not an apples-to-apples comparison, the commenters
make several valid points. It is no secret that a conversation
on a CDMA carrier with many users will not carry as far as one
on a carrier with few users, although this is also attributable
to rise in the noise floor on which CDMA is dependent. It is
also known that CDMA engineers tend to plan their systems with
the pilot and traffic channels taking small percentages of the
full CDMA carrier power, often less than 10% in the case of
a traffic channel. However, CDMA enjoys a number of offsetting
factors that narrowband technologies do not. For example, CDMA
receiver sensitivity is bolstered by multi-server gain and spread-spectrum
processing gain, so that CDMA receivers can decipher signals
at lower levels than their GSM and TDMA brethren. At any rate,
the FCC has noted the perceived disparity and is seeking to
promote its initiative of competitive neutrality among different
commercial wireless services and technologies.
Let the Bandwidth Be Your Guide
As a result, the FNPRM discusses several options for increasing
wideband EIRP limits. CTIA submitted perhaps the most interesting
proposal, whereby EIRP limits would increase on a per-MHz basis
for all bandwidths of more than 500 kHz. So as not to take anything
away from narrowband technologies, the existing EIRP limits
would not be affected at lower bandwidths. However, the FCC
indicated that it would consider extending the “per MHz”
approach down through the lower bandwidths if it did not lower
an existing limit. Also, looking at the imposing implications
of the total power that could be permitted to a wideband or
ultra-wideband CDMA carrier under CTIA’s proposal, the
FCC stated that it would strongly consider a cap on the total
carrier power at or above certain bandwidths.
The FCC also suggested a couple of “step” approaches,
also based on carrier bandwidth. One such suggestion broke the
continuum of bandwidths into two sections: those greater than
1 MHz, and those less than 1 MHz. A specific EIRP limit would
be determined for each side of the 1 MHz bandwidth demarcation.
The second suggestion would set a different fixed EIRP limit
for each of the commonly used bandwidths in the wireless radio
services, such as 200 kHz (GSM), 1.25 MHz (CDMA), or 5 MHz (wCDMA),
or for sets of ranges that include those bandwidths.
In much the same manner as its recent Rural Report and
Order, the various EIRP limit options discussed by the
FCC would generally permit greater power from sites in counties
defined as “rural”, based on the county’s
population, than from non-rural sites. However, FCC EIRP limitations
based on such factors as antenna height, location within designated
quiet zones, and total site EMF emission would still apply.
Antenna height-based reductions would trigger at a threshold
of 300 meters Above Average Terrain.
Too Much of a Good Thing?
The FCC made it quite clear that it was concerned about aggregate
power levels that could occur under some of the proposals for
wideband and ultra-wideband carriers, citing the primary need
to prevent inter- and intra-system interference. If any of the
proposals or variants thereof were to be adopted, the FCC would
most likely not permit EIRP limits to increase infinitely by
setting an EIRP cap at or above a bandwidth to be determined.
As a practical matter, with or without an FCC cap, several factors
might serve to keep EIRP levels closer to the current reality.
Particularly in the CDMA world, service providers will have
to consider such factors as the noise floor, cell footprint
management, and link balance (i.e., the handset’s talkback
range). We are not aware of any FCC proposals to permit increased
power from handsets. Moreover, at rural “high sites”
using taller towers, the attenuation (power absorption) caused
by lengthy waveguide runs could pose a few problems. If the
antenna height itself does not trigger a regulatory clamp, the
service provider will have to consider the radio output and
antennas needed to achieve the higher EIRP.
Also at issue is whether or not the FCC will apply the power
limits to peak or average radiated power of the transmitted
signal. With today’s various modulation schemes, peak
power of the modulated waveform envelope (i.e., the amplitude)
can substantially exceed its average power, if only for brief
periods. Nonetheless, this also raises a red flag of potential
interference in the FCC’s eyes, and the FCC has therefore
requested comment on whether limits should apply to peak or
average power. Interference bursts will be much more difficult
to regulate and detect in the field if the FCC were to apply
EIRP limits to average power.
For the Good of the Many
Even so, the proposed rules appear promising for wireless providers
using wideband technologies or planning migrations to wideband
technologies, such as GSM to UMTS, particularly in rural areas.
While the FNPRM focused on EIRP limit methods for PCS, it also
indicated that it would apply any such rule changes in a similar
manner to AWS and cellular services. We would therefore expect
little or no resistance from most existing narrowband users,
as evidenced by the participation of CTIA in the proceeding.
Rural GSM, AMPS, and TDMA providers that do not plan near-term
migrations to wideband air interfaces might have cause for concern
from a competitive standpoint.
******
Subscribers to Bennet & Bennet, PLLC’s memo service
have received a summary of the biennial review R&O, which
also contained a summary of the FNPRM. If you would like to
receive a copy of this memo and learn more about our subscription
service, or if you should have any questions about the possible
power limit rule changes, please do not hesitate to contact
Jim Egyud.
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Clarifying
Overall Structure Height
FAA vs. ASR vs. FCC Form 601 |
| By Rural Signals
staff
Since the release of the FCC’s Antenna
Structure Registration (ASR) rules, some confusion has existed
over reporting the overall structure height and supporting structure
height. This confusion also shows up occasionally on FAA Notifications
of Proposed Construction (FAA Form 7460-1) and becomes
an issue during the application process, site audits, or due
diligence procedures.
To help set the matter straight, the heights shown in red in
the figure found here
should be known when seeking authorization from the FAA and
FCC. For example, when proposing a 250-foot tower (supporting
structure), the overall height most often will exceed 250 feet
as a result of various devices mounted atop the tower. Therefore,
the overall height is defined as the height of the support structure
plus any surmounted (i.e.,
top-mounted) appurtenances. These appurtenances could be antennas,
FAA obstruction lighting, lightning rods, and even safety climbing
devices. The same overall height should be reported on the 7460-1,
ASR, and where required on other FCC applications, such as Form
601 (i.e. applications for cellular, 700 MHz, point-to-point
microwave, etc.). Most applications will also ask for the supporting
structure height.
A Method of Consistency and Common Sense
For consistent reporting among the FAA and FCC forms, note
that the uppermost tip of the lightning rod acts as the highest
point of most structures. The lightning rod should be installed
regardless of other appurtenances (along with an appropriately
sized copper conductor downlead) for the initial defense against
lightning-induced high-voltage surges. In the absence of an
unusual FAA restriction on a tower site, the height to the tip
of the lightning rod typically remains the tallest point of
the structure throughout the deployment process, regardless
of antenna changes. In addition, if top-mounted antennas need
to be added later, as long as those antennas are shorter than
the lightning rod, no additional FAA or ASR modifications should
be necessary unless the FAA would require additional notification
of a new frequency band.
Existing towers should not be overlooked in the FAA and
FCC ASR processes. Constructing, co-locating on, or modifying
an existing tower requires adherence to the FCC rules that,
in turn, require compliance with the Nationwide Programmatic
Agreement (NPA) that went into effect on March 7, 2005. Information
on this and other concerns regarding Tower and Antenna Siting
will be addressed in following issues of Rural
Signals.
******
In the meantime, if you have any questions related
to antenna structure height, please contact any of our technical
staff. Bennet &
Bennet, PLLC would be glad to work with your organization
on a recommended plan for addressing FAA, ASR, NPA and grounding
issues for proposed tower sites and modifications.
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If you have come across Rural Signals on-line and do not already receive our free quarterly e-mailed version, simply e-mail the Editor, Jim Egyud, by clicking here. Thank you for your interest.
Questions??? Call Rural
Signals Editor Jim Egyud [(202) 371-1500], and refer to
Vol. 1, No. 3.
About Rural Signals
Rural Signals is a quarterly publication of Bennet & Bennet, PLLC's technical consulting service division. Rural Signals is delivered by e-mail four times a year and features technical discussions on current spectrum related happenings affecting rural America. For subscription information or to inquire about specific rural spectrum issues, please call/fax/e-mail Rural Signals Editor Jim Egyud at 202-371-1500 or 202-371-1558 (fax).
While it is our intention to provide valuable information to readers of Rural Signals, the transmission of this newsletter does not create an attorney-client relationship. You should not act upon any information contained in Rural Signals or at www.bennetlaw.com without first seeking the advice of an attorney.
Copyright 2005 Bennet & Bennet, PLLC
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