International Civil Aviation Organization
ACP-WGF27/WP-19
2012-09-11
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
27th MEETING OF WORKING GROUP F
Montreal, Canada 17 – 26 September 2012
Agenda Item 6:
Any other business
UAS CNPC Frequency Plan for the Band from 5030 MHz to 5091 MHz
(Presented by Warren J. Wilson)
SUMMARY
This paper provides an overview of a potential band plan that will allow the
frequency band from 5030 MHz to 5091 MHz to be used to support both
terrestrial and satellite communications links for command and control of
unmanned aircraft systems.
ACTION
It is proposed that the working group take this information into account during
its deliberations.
1.
INTRODUCTION
This paper provides an example of a possible frequency plan to support control and non-payload
communications (CNPC) for unmanned aircraft systems (UAS) in the frequency band from 5030 MHz to
5091 MHz (called C-band herein). This band supports the Microwave Landing System (MLS).
Currently, MLS is not widely deployed, so CNPC can coexist with it in this band on a noninterfering
basis.
The material in this paper relies heavily on information that is available in two International
Telecommunication Union (Radiocommunication Sector) reports – ITU-R M.2171 [1] and ITU-R
M.2205 [2]. The first document summarizes the total information loading requirements for various
categories of UAS. The second document describes some specific examples of communications systems
that have the potential to support the anticipated communications requirements.
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The examples comprise a satellite beyond-line-of-sight (BLOS) system operating in C-band and a
terrestrial line-of-sight (LOS) system operating in both C-band and the band from 960 MHz to 1164
MHz. (Further detailed information on the candidate architecture can be found in reference [3].) This
paper focuses on the satellite system and the C-band aspects of the terrestrial system.
2.
DISCUSSION
In this section the satellite C-band frequency plan and the terrestrial C-band plan are discussed separately.
The plans are based to a large extent of the information found in references [2] and [3]. The plans have
been slightly modified in order to coordinate their usage of the band with each other and with MLS. Note
that the MLS is organized into 300 kHz channels so that, for example, channel 500 corresponds to 5031
MHz and channel 698 corresponds to 5090.4 MHz (see reference [4]). In general, the relationship
between the MLS channel number (N) and frequency (F) (in MHz) is as follows:
F = 4881 + 0.3 N
It is assumed in this paper that the MLS frequencies so defined are the center frequencies of their
respective channels.
2.1 Satellite System Frequency Plan
Details of the satellite system design can be found in Annex 2 of reference [2]. Only a brief summary is
provided here. The geometry of the links is shown in Figure 1. The complete path from the pilot to the
satellite to the UA (consisting of paths 1 and 2) is designated as the forward link, and the complete path
from the UA to the satellite to the pilot (consisting of paths 3 and 4) is the reverse link. In order to
eliminate potential cosite interference issues on board the UA, access to forward and reverse links is half
duplex. In other words, each UA can receive on the forward link half the time and transmit on the reverse
link the other half. Efficiency is not necessarily reduced due to the half-duplex approach since a nearby
UA can receive and transmit on the same frequencies in the reverse time slot order.
Figure 1. Satellite Link Geometry
Due to an asymmetry in the loading requirements for the forward and reverse links, the bandwidth
allocated to reverse links is four times the bandwidth allocated to forward links. (Although the
description of the satellite system in reference [2] does not show how the specific bandwidths are derived;
their ratio seems reasonable given the ratio of forward and reverse link requirements.) Thus, 46 300-kHz
channels are split into two to provide 92 150-kHz reverse “path 3” channels, and 11.5 300-kHz channels
are split into eighths to provide 92 37.5-kHz forward “path 1” channels. These channels are received by
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the satellite in the band from 5073 MHz to 5090.25 MHz. The satellite then down-converts these
receptions by 42 MHz so that it transmits 92 150-kHz reverse “path 4” channels and 92 37.5-kHz forward
“path 2” channels – occupying the band from 5031 MHz to 5048.5 MHz. This arrangement is shown
pictorially in Figure 2. The number in each rectangle corresponds to the link shown in Figure 1. Clearly,
there is a 24.75-MHz band from 5048.25 MHz to 5073 MHz that is not used by the satellite system; that
band is used by the terrestrial C-band system described below.
3.45 M
2
24.75 MHz
13.8 MHz
3.45 M
Terrestrial
4
1
13.8 MHz
3
5090.25 MHz
5031 MHz
Figure 2. Satellite Link Frequency Plan
2.2 Terrestrial System Frequency Plan
The geometry of the terrestrial system is shown in Figure 3. The proposed communications architecture
outlined in references [2] and [3] is based on a combination of time division duplex (TDD) and frequency
division multiple access (FDMA). As in the satellite architecture each UA alternates between time slots
for reception and transmission. To accommodate the asymmetry in loading requirements, the slots
supporting the forward links (path 1) are shorter than those supporting the reverse links (path 2). In this
terrestrial architecture all forward link transmissions are aligned in one set of slots and all reverse link
transmissions are aligned in a completely separate set of slots. The purpose of this arrangement is to
eliminate all interference between forward and reverse links.
UA
1
2
Pilot
Figure 3. Terrestrial Link Geometry
The terrestrial system is assumed to be cellular, with frequency sets being reused in a 12-cell pattern. The
cell size is chosen so that there are (on average) 10 UAS in each cell (based on population estimates
found in reference [1]). In order to take into account statistical fluctuations in the number of users, each
cell is provisioned to support up to 20 users. Since the architecture is based on FDMA, this means that
each cell uses 20 separate frequency channels.
One final feature that affects the terrestrial frequency plan is that the architecture supports a limited
number of UA that downlink airborne weather-radar and/or video information. To provide these
capabilities the 20 channels in each of 12 cell types comprise the following:

12 channels, each with 37.5 kHz bandwidth, to support basic command and control information
transmission (12 x 12 x 0.0375 MHz = 5.4 MHz)
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

4 channels, each with 75 kHz bandwidth, to support basic and weather radar information
transmission (12 x 4 x 0.075 MHz = 3.6 MHz)
4 channels, each with 300 kHz bandwidth, to support basic, weather and video information
transmission (12 x 4 x 0.3 MHz = 14.4 MHz)
The fact that the bandwidth of the terrestrial basic channels (37.5 kHz) is the same as the bandwidth of the
reverse channels in the satellite design is purely coincidental. Also, note that the actual information
throughputs are not proportional to these bandwidths and depend on a number of factors including
modulation type, time-slot size, error coding, other overheads etc.
This terrestrial frequency plan is shown pictorially in Figure 4.
Guard
5.4 MHz
3.6 MHz
14.4 MHz
5072.55 MHz
5048.55 MHz
Basic
Basic + Weather
Basic + Weather + Video
Figure 4. Terrestrial Link Frequency Plan
2.3 Overall Frequency Plan in Terms of MLS Channels
The overall disposition of frequencies is tabulated in Table 1, using an extended version of the MLS
channel numbering system. If the MLS channel numbered N is split in two, the lower and upper halves
are designated NA and NB, respectively. Similarly, if the channel is split into fourths, the designations are
Na, Nb, Nc and Nd. Finally, if the channel is split into eighths, the resulting channels are N 1 through N8.
The third and fourth columns indicate whether the channels are part of the satellite or terrestrial system
and (in each case) how the channels fit into either architecture. The letter “G” in the right hand column
means that the listed frequencies are used as guard bands. The size of the guard bands between the
different modulation types has been liberally set to be 300 kHz. This is just an educated estimate of what
is required. A more refined determination would require more details regarding the transmitter spectral
masks, receiver selectivities, modulation types etc. that have not been defined. In any case, the total
amount of guard listed in Table 1 is 1.5 MHz, which is a relatively small fraction of the total bandwidth in
question, 61 MHz. Thus, the results of this paper would not be greatly modified if the guard requirements
changed somewhat.
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Table 1. Proposed Satellite and Terrestrial Channel Usage
MLS Channel Numbers
5001 to 5004
5005 to 5118
512A to 557B
558
5591 to 5768
577
578a to 589d
590
591 to 638
639
6401 to 6404
6405 to 6518
652A to 697B
698
Frequency Subband
(MHz)
5030.85 – 5031.00
5031.00 – 5034.45
5034.45 – 5048.25
5048.25 – 5048.55
5048.55 – 5053.95
5053.95 – 5054.25
5054.25 – 5057.85
5057.85 – 5058.15
5058.15 – 5072.55
5072.55 – 5072.85
5072.85 – 5073.00
5073.00 – 5076.45
5076.45 – 5090.25
5090.25 – 5090.55
BLOS
Link
LOS Link
Bandwidth
Guard
G
2
4
G
37.5 kHz
G
75.0 kHz
G
300 kHz
G
G
1
3
G
Note that the total bandwidth allotted to the satellite system in the C-band is 34.5 MHz (from 5031 MHz
to 5048.25 MHz and from 5073 MHz to 5090.25 MHz). It should be pointed out that the bandwidth
discussed in reference [2] is somewhat higher (about 40 MHz), but that analysis did not address the need
to share the band with a terrestrial CNPC system. The total bandwidth for the terrestrial system is 24
MHz (from 5048.55 MHz to 5072.55 MHz – including 0.6 MHz for guard bands). The terrestrial system
was estimated in reference [1] to require a total bandwidth of about 34 MHz. The remaining 10 MHz may
be available in the lower portion of the 960 MHz to 1164 MHz band. The estimated bandwidth
requirement for the satellite system was 56 MHz. The location of the remaining satellite bandwidth is
still to be determined.
2.4 Design Flexibility
The frequency plan embodied in Table 1 is a specific, preliminary example; the ultimate frequency plan
might result in a different split between satellite and terrestrial bandwidth. Such an adjustment can be
easily accommodated. For instance, if the bandwidth requirement for the satellite system is reduced, the
size of the terrestrial allotment in the center of Figure 2 can be increased. That central bandwidth can be
used by the terrestrial system to provide more communications capacity. The extra capacity could be
used in a number of ways including (but not limited to) providing more channels per cell and/or providing
more high-bandwidth channels per cell.
3.
ACTION BY THE MEETING
This paper provides an overview of a potential band plan that will allow the frequency band from 5030
MHz to 5091 MHz to be used to support both terrestrial and satellite communications links for command
and control of unmanned aircraft systems. In order to avoid delays to development of either link, the
above band plan is proposed.
It should be emphasized that the architectures discussed herein are examples of systems that might
support CNPC, so they serve as useful artifacts for discussing, in a concrete way, issues such as frequency
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planning. Nevertheless, this paper shows that for the chosen architectures the 5030–5091 MHz frequency
band can simultaneously support about 70% of the requirements of the terrestrial system and about 60%
of the satellite bandwidth requirements. The remainder of the estimated terrestrial and satellite
requirements will have to be satisfied in one or more other bands.
It is proposed that the working group take this information into account during its deliberations.
4.
REFERENCES
[1] International Telecommunication Union, “Characteristics of unmanned aircraft systems and spectrum
requirements to support their safe operation in non-segregated airspace,” ITU-R Report M.2171,
December 2009.
[2] International Telecommunication Union, “Results of studies of the AM(R)S allocation in the band 960
– 1 164 MHz and of the AMS(R)S allocation in the band 5 030 – 5 091 MHz to support control and nonpayload communications links for unmanned aircraft systems,“ ITU-R Report M.2205, November 2010.
[3] Wilson, Warren J., “Strawman Design for Terrestrial Unmanned Aircraft Control Links,” International
Communications, Navigation and Surveillance Conference (ICNS 2011), Herndon VA, May 10 – 12,
2011.
[4] Federal Aviation Administration, “Spectrum Management Regulations and Procedures Manual,” FAA
Order 6050.32B, 17 November 2005, App. 3.