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FEDERAL COMMUNICATIONS COMMISSION DA 00- 2664
Before the FEDERAL COMMUNICATIONS COMMISSION
Washington, D. C. 20554
)
In the Matter of )
)
Petition of Spacenet, Inc. for a )
Declaratory Ruling that Section 25.134 )
of the Commission's Rules Permits )
VSAT Remote Stations in the Fixed )
Satellite Service to Use Network Access ) RM- 9864
Schemes that Allow Statistically Infrequent )
Overlapping Transmissions of Short )
Duration, or, in the Alternative, )
For Rulemaking to Amend that Section )
)
ORDER
Adopted: December 6, 2000 Released: December 7, 2000
By the Chief, International Bureau:
I. INTRODUCTION
1. By this Order, we deny a petition for declaratory ruling filed by Spacenet, Inc.
(Spacenet). Spacenet requests a declaratory ruling that the Commission's rules allow networks of
very small aperture terminals (VSAT networks) "to employ access schemes that entail
statistically infrequent overlapping transmissions of remote earth stations." 1 We recognize,
however, that, due to confusion about the rule at issue, some operators may already be using these
access techniques. We grant all these operators a waiver on our own motion, so that they may
continue to serve their customers. Consequently, by today's actions, we eliminate uncertainty
over the interpretation of the Commission's VSAT rules. We also preserve the status quo until
the Commission can complete a forthcoming rulemaking proceeding to consider fully the issues
raised by Spacenet.
II. BACKGROUND
2. The Commission's rules permit parties to obtain a single or "blanket" license for a
large number of technically identical small aperture antenna earth stations operating in the Ku-band. 2
These systems consist of one or more hub stations transmitting via a satellite to
technically identical remote small aperture antennas. These systems are referred to as very small
1 Spacenet Petition at 1.
2 For purposes of this Order, the "Ku- band" denotes the 11. 7- 12. 2 and 14. 0. 14. 5 GHz
frequency bands.
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aperture antenna terminal (VSAT) networks. 3 VSAT networks employ different techniques to
prevent or limit the transmissions to and from their remote earth stations from interfering with
each other and with adjacent satellite networks. To this end, most VSAT networks originally
used frequency division multiple access (FDMA), which assigns different portions of the Ku-band
frequencies to different remote earth stations. 4 Subsequently, VSAT system operators
developed techniques that enabled some remote earth stations to share frequency bands. One of
these techniques is called time division multiple access (TDMA), in which each remote earth
station is assigned a different time to transmit and receive information. Another, code division
multiple access (CDMA), prevents interference between remote earth stations by assigning a
different digital code to different earth stations.
3. The access technique that is the subject of Spacenet's petition is called the "slotted
Aloha" technique. 5 In this technique, the hub earth station synchronizes all remote VSAT
stations so that they transmit only in discrete time slots, typically tens of milliseconds in
duration. 6 Unlike the TDMA techniques, however, Aloha permits two or more remote earth
stations to transmit simultaneously. Aloha relies on statistical probability calculations to limit the
number and duration of simultaneous transmissions. When two or more remote earth stations in
Spacenet's VSAT network transmit simultaneously, those transmissions can "collide," and the
resulting power level caused by these collisions exceeds the level specified in the Commission's
rules during the time period of simultaneous transmission, although for no more than tens of
milliseconds. 7 Increasing the power levels of a transmission increases the possibility that it will
cause unacceptable interference to other satellite systems. According to Spacenet, however,
because the collisions in its VSAT network are infrequent and of short duration, they do not cause
unacceptable interference to adjacent satellite systems. 8
4. Many of the requirements applicable to VSAT networks appear in Section 25.134 of
the Commission's rules. 9 Section 25.134( a) states that, to qualify for routine licensing, VSAT
3 Routine Licensing of Large Networks of Small Antenna Earth Stations Operating in the
12/ 14 GHz Frequency Bands, 51 Fed. Reg. 15067 (Apr. 22, 1986) (1986 VSAT Order); Routine Licensing
of Earth Stations in the 6 GHz and 14 GHz Bands Using Antennas Less that 9 Meters and 5 Meters in
Diameter, Respectively, for Both Full Transponder and Narrowband Transmissions, Declaratory Order, 2
FCC Rcd 2149 (Com. Car. Bur. 1987) (1987 VSAT Order). See also 47 C. F. R. §25. 134.
4 Satellite systems used this access technique even before the Commission developed its
VSAT policies. See Satellite Business Systems, 62 FCC 2d 997, 1083 (para. 247) (1977).
5 With the "unslotted Aloha" technique, remote earth stations in the VSAT network can
transmit randomly at any time, meaning that the transmissions are not synchronized in time or duration.
The "unslotted Aloha" technique is distinguishable from the "slotted Aloha" technique, in which remote
earth stations transmit in specific time slots, which means that the transmissions are synchronized but not
coordinated. In other words, the remote earth stations transmitting in a given time slot can transmit
regardless of whether there are other earth stations transmitting in the same time slot. G. Maral, VSAT
Networks at 144- 45 (John Wiley and Sons, ed. 1995).
6 Spacenet Petition at 6.
7 Spacenet maintains that the duration of an inbound transmission is typically between 15
and 50 milliseconds. Spacenet Petition at 8.
8 Spacenet Petition at 7.
9 47 C. F. R. § 25. 134.
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remote earth stations must operate with an input power density no higher than -14 dBW/ 4 kHz. 10
Section 25.134( b) states that VSAT network license applicants planning to use a power spectral
density higher than -14 dBW/ 4 kHz must submit additional information in their applications,
including an Adjacent Satellite Interference Analysis (ASIA), to demonstrate that use of the
higher power level will not cause unacceptable interference into adjacent satellite systems. 11 In
addition, Section 25.134( c) requires operators of VSAT networks to coordinate with operators of
subsequently launched satellites, and to lower the VSAT power levels to those specified in
Section 25.134( a) if a coordination arrangement cannot be reached. 12
5. On April 5, 2000, Spacenet filed a petition for declaratory ruling or rulemaking
requesting that the power levels generated by the Aloha access technique during transmission
collisions do not violate Section 25.134 of the Commission's rules. Spacenet's petition was placed
on public notice on April 28, 2000. Hughes Network Systems (Hughes) and PanAmSat
Corporation (PanAmSat) filed comments, and ALOHA Networks, Inc. (ALOHA Networks) filed
electronic comments. Spacenet and Hughes filed replies. 13
III. DISCUSSION
A. Aloha Access Technique
6. As Spacenet describes the slotted Aloha technique, the VSAT network operator first
derives the proportion of time slots occupied by exactly one transmission. 14 Then, the VSAT
network operator uses statistical methods to determine the probability of two or more
transmissions from remote terminals "colliding" at various peak network loads. 15 By this method,
the VSAT network operator can adjust the number of transmissions to limit the number of
anticipated "collisions." This methodology is discussed in detail in Appendix A to this Order.
7. Spacenet maintains that VSAT operators have used multiple access techniques like
Aloha for years. 16 Spacenet requests us to declare that the slotted Aloha technique is consistent
with Section 25.134 of the Commissions' rules, provided that:
(i) each station individually satisfies the power density limits of
Section 25.134( a);
10 47 C. F. R. § 25. 134( a).
11 47 C. F. R. § 25. 134( b).
12 47 C. F. R. § 25. 134( c).
13 In addition, Spacenet filed an ex parte statement on September 15, 2000.
14 This proportion is derived by the following formula: S = G* e (- G) , where S is the
utilization or throughput per slot, G is the load or average transmissions per slot, and e is Euler's number, a
constant equal to approximately 2. 73. Spacenet Petition at 6.
15 Spacenet Petition at 5- 7.
16 Spacenet Petition at 8- 10. See also PanAmSat Comments at 2.
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(ii) the probability P of k stations transmitting, for k greater than
1, is below the limit defined by P[ k] = (0. 38/ k!)* e (- 0.38) , and
(iii) the maximum duration of any single collision is less than
100 milliseconds. 17
In the alternative, Spacenet requests a rulemaking to add these requirements to Section 25.134. 18
8. PanAmSat agrees with Spacenet that the Commission should clarify or amend its rules
to permit random access techniques, provided that the probability of collisions is within a defined
range. 19 PanAmSat argues further that the Commission's VSAT rules have become outdated, and
that the Commission should increase the power level limits to reflect technological advances. 20
Hughes questions whether any action is required to clarify Section 25.134, but agrees that, if
clarification is necessary, a declaratory ruling is better than a rulemaking proceeding. 21 Hughes
argues, however, that the second requirement in Spacenet's proposed rule is unnecessarily
detailed, and could unreasonably restrict the flexibility of VSAT operators to adapt their networks
to future technological developments. 22 Hughes proposes the following requirement as an
alternative: "( ii) the total average power radiated toward the target satellite by all the remote
earth stations in the network, using an averaging period of one second, is less than that of a single
remote earth station transmitting continuously." 23 Spacenet supports PanAmSat's proposal to
relax the power level standards in a future proceeding, but opposes delaying action on its petition
while considering PanAmSat's proposal. 24 Finally, Spacenet expresses support for Hughes's
proposed rule as an alternative to its own proposal. 25
9. We decline to adopt any declaratory ruling at this time because both Spacenet's and
Hughes's proposals raise issues that require further analysis. Sections 25.134( a) and (b) establish
finite limits for individual earth station antenna input power densities. 26 When signals from two or
more earth stations within a VSAT network collide, the resulting power level exceeds the limits
specified in Sections 25.134( a) and (b), and so the Aloha method, and variants such as slotted
Aloha, cannot be construed to comply with the letter of Section 25. 134 if each terminal transmits
at the maximum power density limit. This result is discussed in more detail in Appendix A.
17 Spacenet Petition at 9.
18 Spacenet Petition at 11.
19 PanAmSat Comments at 1- 2.
20 PanAmSat Comments at 2- 3.
21 Hughes Comments at 2- 4.
22 Hughes Comments at 3- 4.
23 Hughes Comments at 4- 5. See also ALOHA Networks Comments (recommending
amending Section 25. 134 to permit VSAT networks to exceed the power limits by no more than one second
per hour when the network is operated at peak load for one hour).
24 Spacenet Reply at 5.
25 Spacenet Ex Parte Statement at 1.
26 47 C. F. R. § 25. 134( a), (b).
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10. We also decline to adopt a declaratory ruling at this time because it would not
adequately address cases where the bandwidth used by the earth station( s) causing interference to
other earth stations is wider than the bandwidth used by the victim earth station( s) of the
interference. Assuming the transmission bandwidth for the interfering earth stations is a few
hundred kHz, the potential interference resulting from co- channel carrier collisions to a victim
earth station operating over a wider bandwidth (e. g., greater than ten times the interfering
bandwidth) may corrupt only a fraction of the victim earth station’s transmission data. The
victim’s corrupted data would likely be recovered by the error correction techniques employed by
the victim satellite system. Thus, this issue is not as problematic as might be thought. However,
if the victim earth station( s) is operating in a narrower bandwidth or approximately the same as
the interfering earth station, the victim earth station’s transmission data would be significantly
degraded, possibly beyond recovery. The narrow- bandwidth problem can be compounded as
satellite- based voice/ data traffic increases.
11. Instead of addressing the issues raised by the Aloha access technique in this
proceeding, the Commission will address these issues in a forthcoming rulemaking. In that
proceeding, the Commission will also examine the current power limit rules, as PanAmSat
requests. 27
12. Nevertheless, based on our review of Spacenet's calculations, and the record we have
developed on Spacenet's random access technique, we find that Spacenet has shown persuasively
that its random access technique does not, at this time, cause harmful interference to other
satellite systems. Accordingly, we find that Spacenet has shown good cause for a waiver of
Sections 25.134( a) and (b) for purposes of continuing to use its Aloha random access technique.
Further, because of confusion and to prevent disruption in service, we grant all VSAT operators a
waiver, on our own motion, for purposes of continuing to use any multiple access techniques
VSAT operators are using at the time this Order becomes effective. The waiver we grant here
will remain in effect only until the upcoming rulemaking on this and other related issues takes
effect, and does not prejudge any decision in that rulemaking proceeding. These VSAT
operations will then be subject to implementation of the final rulemaking decision.
B. Probability Distribution
13. ALOHA Networks maintains that Spacenet used the wrong probability formula for
slotted Aloha networks. ALOHA Networks projects that the number of collisions is three times
greater than Spacenet's estimate. According to ALOHA Networks, the probability formula
Spacenet used applies only to unslotted networks. 28 Spacenet maintains that ALOHA Networks
27 PanAmSat Comments at 2- 3.
28 Aloha Network Comments at 1- 2. Hughes maintains that the Aloha Network's approach
has "technical validity," but argues that the language of its proposed rule is more flexible, and so more
workable in practice. Hughes Reply at 1- 3. Specifically, ALOHA Network criticizes Spacenet for using a
Poisson probability distribution rather than a binomial distribution. The binomial distribution, one of the
more common statistical distributions, is designed for cases like determining the probability of flipping a
coin and getting heads a certain number of times in a row. More generally, for each trial, there are only
two possible outcomes, the probability of each outcome is known, and the probability does not vary from
trial to trial. The Poisson distribution is designed for cases where the probability of an event occurring is
very low, but the number of opportunities for such occurrences are very high. Examples are estimating the
number of people who will contract a rare disease in a population of several million, or the number of
atoms that will undergo radioactive decay in a large amount of uranium.
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is mistaken in asserting that its probability function underestimates the number of collisions. 29
Spacenet also maintains that, if its system had the potential to cause unacceptable interference,
then space station operators would have opposed its petition. 30 Finally, Spacenet maintains that
ALOHA Networks' probability equation would limit the number of collisions more than
necessary to prevent interference, and so would render its slotted Aloha system commercially
nonviable. 31
14. Because we are not granting Spacenet's petition for declaratory ruling, we need not
address the probability formula for projecting the number of collisions here. Rather, the
Commission will examine probability formulas, if necessary, in the context of its rulemaking.
IV. ORDERING CLAUSES
15. Accordingly, IT IS ORDERED, pursuant to Section 1.2 of the Commission's rules,
47 C. F. R. § 1. 2, that the petition for declaratory ruling filed by Spacenet, Inc. on April 5, 2000, IS
DENIED.
16. IT IS FURTHER ORDERED, pursuant to Section 1. 3 of the Commission's rules, 47
C. F. R. § 1.3, that current operators of VSAT networks ARE GRANTED a waiver of Section
25.134 of the Commission's rules, 47 C. F. R. § 25. 134, for purposes of continuing to use any
multiple access techniques being used at the time this Order becomes effective, pending a
Commission rulemaking addressing the use of such techniques.
17. This Order is issued pursuant to Section 0.261 of the Commission's rules on
delegated authority, 47 C. F. R. § 0. 261, and is effective upon release.
FEDERAL COMMUNICATIONS COMMISSION
Donald Abelson
Chief, International Bureau
29 Spacenet Reply at 6.
30 Spacenet Reply at 6- 7.
31 Spacenet Reply at 7.
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APPENDIX A
Technical Analysis of Spacenet's Aloha Random Access Scheme
I. Spacenet's Proposed Rule
Spacenet proposed adding a new paragraph (e) to Section 25.134. Spacenet's proposed
rule is as follows:
1. Each station individually satisfies the antenna input power density limit of
the rule;
2. The probability P of k stations simultaneously transmitting, for k being
greater than 1, is below the limit defined by
P{ k} = (0. 38 k ) * exp (- 0. 38) / k!)
where the channel load G is set for 0.38 as a numerical
constant; and
3. The maximum duration of any individual collision is less than 100
milliseconds.
Spacenet’s proposed Section 25.134( e)( 1) merely restates an existing requirement. Proposed
Section 25.134( e)( 3) does not appear to be controversial based on the record in this proceeding.
Accordingly, we will direct our attention to proposed Section 25.134( e)( 2).
Proposed Section 25.134( e)( 2) raises two questions: (1) What is the probability that a
"collision" of transmissions from two or more earth stations will occur? and (2) How much do
power levels increase during a collision? We address these two issues below.
II. Probability of Collisions
Spacenet based its probability determination on a Poisson probability distribution, such
that, for the standard Aloha approach,
(1) S = G* e (- 2G)
where S is throughput, G is channel loading, and e is a constant equal to approximately 2.73. The
Poisson probability distribution is similar for the slotted Aloha approach
(2) S = G* e (- G)
We replace the "- 2G" term with "- G" because the collision- vulnerable period in the slotted- Aloha
approach is half of that in the standard Aloha approach.
The probability of a collision of transmissions is P{ k}, where k is the number of stations
transmitting simultaneously. According to the equation in Spacenet's proposed Section
25.134( e)( 2), setting the channel loading G equal to 0.38, the following results:
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Table 1: Probability of Collisions
Number of Earth Stations
Transmitting
Simultaneously
Probability of Collision
involving N transmissions
P( N)
Probability of Collision in
Percentage Terms
0
1 0. 260 26%
2 0. 049 4.9%
3 0. 00625 0.625%
4 0. 00059 0.059%
5 0. 00005 0.005%
The probability of collision for six or more stations is remote and so we can ignore it.
III. Excessive Power Emissions
In a multiple access scheme such as slotted ALOHA, a carrier collision occurs when two
or more stations are transmitting simultaneously on the same frequency. If we assume that each
transmitter in the network is operating at the maximum authorized power level, then a single
transmitter would not cause any increase over the maximum authorized power level. In other
words, there would be a 0 dB increase in potential interference. A two- transmission collision,
however, would double the potential interference (i. e., cause a 3. 0 dB increase in power), and a
three- transmission collision would triple the potential interference (i. e., cause a 4. 77 dB increase
in power). The increase in power level is
(3) 10* log( N)
where N is the number of colliding transmissions. Clearly, the potential for interference to
adjacent satellite operations increases whenever a collision takes place.
Table 2: Power Increases Caused by Collisions
Number of Colliding Transmissions Power Increase in dB
1 0
2 3. 0
3 4. 7
4 6. 0
5 7. 0
Spacenet argues that we should discount high power levels for the low probability of
occurrence. Specifically, Spacenet argues that the Net Power Increase (NPI) should be –5. 85 dB
for a single- station transmission, -10. 09 dB for a two- station collision, and so on. Based on this,
Spacenet claims that Over- Time Net Power Increase (ONPI) from two simultaneously
transmitting remote stations is 4. 20 dB below the emission of a single station transmitting
constantly at the maximum authorized power under the current Rule. By using ONPI, Spacenet
demonstrates the inbound carrier collisions over time do not generate excessive emission in
violation of the Commission’s Rules.
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Table 3: Power Increases Caused by Collisions, Spacenet's Approach
Number of Earth
Stations
Transmitting
Simultaneously
Probability P( N) Power Increase:
10log( N) [dB]
Net Power Increase
Adjusted for
Probability of
Occurrence:
10log( P( N)) +
10log( N) [dB]
1 0. 260 0 -5.85
2 0. 049 3.0 -10.09
3 0. 006 4.7 -17.52
4 0. 001 6.0 -24.00
By summing all the net power increases adjusted for probability of occurrence, we obtain
(4) ONPI = 10log[ 10 -5. 85/ 10 + 10 -10.09/ 10 + 10 -17.52/ 10 + 10 -24.00/ 10 ]
~ -4. 2 dB
We can draw two relevant observations from Spacenet's proposal. First, we recognize
that Spacenet's showing assumes that the described events are taking place randomly under a
Poisson distribution and the network traffic is being held to a 38% load, which yields about 26%
throughput (or efficiency). Based on these assumptions, we can obtain a set of probabilities of
carrier collisions and draw certain conclusions. For example, at 38% load, the probability of a
three- carrier collision is 0.6%, which we are willing to dismiss as de minimis. However, if the
load is doubled to about 75%, the probability of a three- carrier collision becomes 3.3%, which
may be difficult to dismiss as de minimis. Therefore, in Spacenet's approach, Spacenet must be
able to maintain the 38% channel load at all times so that the number of undesired carrier
collisions can be controlled within the network.
Second, Spacenet claims that interference is not an issue because the ONPI is 4. 2 dB less
than the total power emission from a single continuously transmitting earth station. We do not
agree. This by itself is not sufficient to comply with the Commission's rules. Specifically, we are
concerned that the power overruns due to collisions could lead to excessive emissions that may
cause harmful interference to neighboring satellites in the orbit, (even though the probability of
occurrence is low and the total time of occurrence are brief) if there is a significant deviation
from the parameters represented in Spacenet’s Petition. During the very instance of a carrier
collision, short burst of excessive emissions are generated, and the resulting power level will
likely violate the Commission's rules if each network station is operating at maximum permitted
power level. The Commission rule places a limit on power density, rather than a threshold that
may be exceeded. Consequently, the Commission has responsibility to ensure that these
excessive emissions will not interfere with the operation of neighboring satellites in orbit even for
brief instances.
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