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1
Defending Our Spectrum:
What Do You Mean When You Say
You’ve “Used” Spectrum
Air Force Flight Test Center
Charles H. Jones, PhD
812TSS/ENTI
I n t e g r i t y - S e r v i c e - E x c e l l e n c e
2
Outline
• Spectrum
– Overviewed
– Example Propagation
– Two Definitions of “Use”
• Legal Allocation
– Licensed and Scheduled
• Dimensional Separation
• On-Demand Buffering
• Misc Comments
• Summary and Conclusion
3
Defending Our Spectrum
• Over the last several years the T&E
community has been asked to provide
spectrum “usage” data
• Part of the response has been based on data
from the Interrange Frequency Deconfliction
System (IFDS)
– This data captures when data was “scheduled”
– We’ve been asked what our “real” usage is
• This presentation discusses why “scheduled”
usage is “real” usage
4
Spectrum
• Radio Frequencies (RF)
– Electromagnetic waves from about 3 Hz to 300
GHz propagating through time and space.
– Commonly called “wireless communication”
• Dimensions of Spectrum
– Three of Space
– Time
– Frequency
– Power
– Polarization
5
First Definition of “Use”
“Spectrum is used if there are
electromagnetic waves propagating
through a particular geographic
location at a particular time.”
6
First Definition of “Use”
“Spectrum is used if there are
electromagnetic waves propagating
through a particular geographic
location at a particular time.”
It is the intent of this presentation to convince
people that this is a naïve definition.
7
Omnidirectional Propagation
• Consider a standard FM Radio
Station with a dipole antenna
– Ideal radiation pattern emits
energy in all horizontal
directions
• Consider a standard airborne
telemetry dual antenna
transmission pattern
– Ideal radiation pattern is
spherical about the moving
platform
9
Non-Ideal Propagation
• The “porcupine” composite antenna shape is
due to difference in amplitude and phase of
the signals being transmitted between the top
and bottom antennas
• The edge of a radio station pattern is
amorphous due to geography and multipath
that interfere with the radial propagation
• Transmission power and receiver sensitivity
affect distance
• The “frequency” is actually the center
frequency in a band of contiguous
frequencies
10
What Spectrum is “Used”?
• Do the non-lobe portions of the antenna
pattern represent spectrum use?
– There is no, or little, electromagnetic energy
propagating through those portions of space.
• At what distance do we say that a radio
station is no longer using the spectrum in its
geographic vicinity?
The point here is that even when doing a simple
spatial analysis, it is not clear what “use” is in
terms of RF transmission.
11
Second Definition of “Use”
“Use” is “Denial to Others”
But there are many legitimate reasons for
denying use to others even when no energy is
being propagated.
Propagating energy through portions of
the spectrum is a primary method of
denying use to others
12
Legal Allocation
• Spectrum is legally allocated for use to
individuals or organizations through a
complex, often political, process.
• The U.N. Treaty created Radio Bureau of the International Telecommunications Union (ITU-R) establishes international radio regulations which are then used as the basis for national level allocations
– Meets at the World Radiocommunication Conference (WRC) every 3 years.
The International Process
13
United
Nations
International
Telecommunications
Union
DoD FAA Non-Federal
Users
etc National
Telecommunications
Delegates
US State
Department
FCC US DOC
Federal Users
Regional Organizations
14
National Level Allocation
• After the ITU-R process is over, then the
individual nations make more detailed
allocations.
The National Process
15
Federal Communications Commission
FCC Commissioners (5)
Congress
Asstnt Sctry of Commerce for Comm and Information
(Administrator, NTIA)
Department of Commerce
National Telecommunications &
Information Administration
Interdepartment Radio Advisory Committee
President
FCC Bureaus
•Non-Federal Users •Commercial •State & local government •NGOs •Private individuals
Participation/ License Applications
Licenses
President appoints
All Federal Users
A
s
s
i
g
n
m
e
n
t
s
Coordination
16
U.S. Spectrum Allocation
• Which leads to the following chart
• (Ovals added to point out some of the Mobile
Telemetry Bands)
18
Assignments and Licenses
• This trickles down to assignments of use of
particular frequencies at particular times by
particular organizations.
– This assignment is what T&E Frequency
Managers do.
– Commercial users get licenses. The spectrum
auctioned off in recent years is via licenses.
• When spectrum is auctioned off and
“repurposed” it changes the words on the
previous chart.
19
No Obligation
• Many licenses and assignments contain no
legal obligation to transmit.
Legal ownership is denial to
others without necessarily
transmitting.
20
Business
• In 2006, SpectrumCo LLC, (Comcast, Time
Warner, Cox, Advance/Newhouse and Sprint
Nextel), won 137 wireless spectrum licenses
for $2.37 billion in the FCC’s auction
– Verizon is trying to buy these for $3.6 billion.
– For 6 years there’s been no transmission.
• TV broadcasters “white space” is another
example of spectrum with no transmission
over a long period of time.
Business decisions can lead to denial to
others without necessarily transmitting.
21
Scheduling
• At the bottom of this legal process, T&E
Frequency Managers schedule spectrum by
assigning contiguous blocks of spectrum to
particular test programs.
• These assignments can be thought of as
rectangles in the time/frequency grid.
• Due to logistical reasons it is not always
possible to transmit during the entire time
scheduled.
Scheduling is denial to others.
22
Time/Frequency Grid
2200
2205
2210
2215
2220
2225
2230
2235
2240
2245 Scheduled Mission
2250
2255
2260
2265
2270
2275
2280
2285
2290
0:0
0
1:0
0
2:0
0
3:0
0
4:0
0
5:0
0
6:0
0
7:0
0
8:0
0
9:0
0
10:0
0
11:0
0
12:0
0
13:0
0
Time (Hours)
Fre
qu
en
cy (
MH
z)
X
Y
23
Dimensional Separation
• To avoid interference, transmission signals
are separated in one or more dimensions.
– This evolves from the physics of transmission
techniques
• This separation includes purposely not
transmitting in portions of the spectrum
between the separated signals.
– In some cases there may be RF “leakage” or
spurious transmissions into the separation
zones, which is why the zones are created
24
PCM/FM Separation
-70
-60
-50
-40
-30
-20
-10
0
1435 1455 1475 1495 1515 1535
dBc
Frequency (MHz)
25
TDMA Separation
• Time Division Multiple Access (TDMA)
provides time slots dedicated to particular
users.
– Because of timing inaccuracies, guard bands
(small time slices) are created between the
assigned time slots
– iNET will be using TDMA
User
1
G
B
User
2
G
B
User
1
G
B
User
2
…
26
Dimensions
• Spatial separation is near universal
• Frequency separation is common
– There are no even decimals on the FM dial
• Scheduled telemetry separates in space,
frequency, and time
• Separation via polarization is rare, but is used
• Spread spectrum techniques, such as CDMA
used by cell phone companies, separate in
the dimension of power
Use of separation zones is denial to others.
27
Spread Spectrum and CDMA
• In a CDMA system the same frequency can be
used in every cell, because channelization is
done using pseudo-random codes
– The power is spread out over more frequencies
than necessary
– This allows multiple people to use the same
frequency at the same time and location
• This could be considered Power Division
Multiple Access since each user is allowed a
certain amount of power on a common
frequency
28
CDMA and Pseudorandom Codes
• XOR Data Signal and Pseudorandom Code to
Transmission Signal.
• XOR Transmitted Signal and Pseudorandom
Code to get the Data Signal.
29
On-Demand Buffering
• Cell phones use spectrum on an on-demand
basis, coming and going on individual whim.
– CDMA lends itself to this type of access
– WiFi and garage door openers are also
examples
• There must be a constant surplus of spectrum
in order to accommodate on-demand service
• Cell phone frequencies are used almost
continuously but rarely, if ever, at full capacity
– There is transmission over frequencies but not
to a maximal power limit
30
T&E and Cell Phones
A Comparison
• Scheduled spectrum for T&E is a form of on-
demand buffering.
– A best effort is made to schedule exactly when
spectrum will be used
– The logistical difficulties of coordinating all
assets for a test make it virtually impossible to
be transmitting exactly when scheduled
T&E buffers in the dimension of time
Cell Phones buffer in the dimension of power
On-demand buffering is denial to others.
31
Efficiency
• Every method and application of RF
transmission can be questioned as to their
efficiency.
– 4G is more efficient than 3G
– SOQPSK is more efficient than PCM/FM
– Probably even SpectrumCo would not claim
they made efficient use of their spectrum
Inefficient transmission is denial to others.
32
Other Examples
• The National Radio Quiet Zone around Green
Bank National Observatory (NTIA Manual
8.3.9).
– Any transmission in any frequency in this area
interferes with Radio Astronomy
• Some national borders have frequency buffer
zones for cross border non-interference
– There is one along the U.S. – Mexican border
• Radar is not a continuous transmission
• Spectrum can be fragmented due to
difficulties of scheduling
33
Standardization Effort
• The Range Commander’s Council (RCC)
Frequency Manager’s Group (FMG) has
established a formal Task to develop a
standard on frequency management metrics.
• Part of this will include formally defining
spectrum “use” within the context of T&E.
34
Summary
• Just because there is no electromagnetic
energy flowing through time and space, does
not mean that that spectrum is not being
“used”
– Legal authority to use
– Impacts of business decisions
– Dimensional separation
– On-demand buffering
– Other reasons
35
Conclusion
• In order to define and quantify spectrum
“use” it is necessary to look at many factors
– Functional requirements
– Physics and technology used
– Business incentives
– Legal realities
• A one-size-fits-all definition is not practical
• It seems reasonable to start with
then develop application specific refinements
“Use” is “Denial to Others”