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ACP WG-F/31 WP08
Unmanned Aircraft System (UAS)
Terrestrial C2 Frequency-Planning
Activities in RTCA SC-228
Frank Box, Alexe Leu, and Leo Globus
22 September 2014
1
C-Band Terrestrial Frequency Planning
•
•
•
•
•
•
Characteristics of Strawman C2 System
Coexistence Rules for Terrestrial C2 Links
Channelization Planning
Very-High-Altitude UAS
Coexistence with UAS C2 SATCOM
Appendix: Sample Link Budgets
2
System Design Constraints
• Available frequency bands:
– L-band (960–1164 MHz)
– C-band (5030–5091 MHz)
• Maximum UA transmitter power per band for basic
service: 10 watts
• Required availability (per band) = 99.8%
• Maximum UA groundspeed = 850 knots
• Frequency instability: 1.0 ppm or better
• Transmitter mask: GMSK (BT = 0.2) or comparable
• Time-division duplexing
– Synchronized among all users
3
Link Throughput Requirements
Service Class
1
2
3
4









Services Provided:
Basic Telecommand and Telemetry
ATC voice and ATS data relay
Navaid and Detect-and-Avoid Data

Video* and Airborne Wx Radar Data
Required Throughput (kbps):
Uplink (automatic UA operation)
1.242
6.091
6.230
6.230
Downlink (automatic UA operation)
1.272
6.131
11.163
308.933
Uplink (manual UA operation)
4.593
9.442
10.108
10.108
Downlink (manual UA operation)
7.595
12.454
18.391
316.161
* These video links (for takeoff, landing, taxiing) would each carry 217 kbps plus overhead.
A need for a single nationwide emergency video channel that would use 435 kbps (plus
overhead) has also recently been identified but is not considered in the above table.
4
Strawman System Configurations
Configuration
Information Rate
(kbps)
Symbol Rate
(kbaud)
Standalone telecommand uplink or basic
telemetry downlink
14.80
87.5
Medium-throughput downlink
35.28
150
Networked TDMA uplink with 4 slots
28.48
200
Networked TDMA uplink with 8 slots
56.96
400
Networked TDMA uplink with 12 slots
85.44
600
High-throughput (video-capable) downlink
237.52
750
Networked TDMA uplink with 16 slots
113.92
800
Networked TDMA uplink with 20 slots
142.40
1000
System design is under review to improve spectral efficiency by:
• Providing additional configurations with smaller information rates
• Finding ways to reduce symbol rates for all configurations
5
3-D Cellular Frequency Plan
1/12
frequency
reuse
Highest
altitude tier
(50 kft)
1/3
frequency
reuse
(better)
Lowest
altitude tier
(surface)
INTERMEDIATE TIERS NOT SHOWN
• “Cells” are airspace volumes
• Frequency list for each cell, assignable as needed when UA in cell
– Nationwide plan to be developed
• Ground stations (standalone/gapfiller) can be anywhere in a cell
6
Low-Altitude Coverage and Gapfillers
Coverage
down to
4000’
• Likely cell radius  69 nmi
Gapfiller
Down to
1000’
Down to
ground
• In most of cell, low-altitude
UA need “gapfiller” GSs
CENTRAL
100’
TOWER
Gapfiller
• Central ground station
(GS) cannot provide
coverage down to ground
throughout cell
Gapfiller
Gapfiller
• When gapfillers are far
enough apart, they may
be able to share
frequencies in same cell
Gapfiller
CELL BOUNDARY
69 NAUTICAL MILES
7
Examples of Potential Adjacent-Channel
Interference (ACI) between Cells
VICTIM
UA
DESIRED
UPLINK
POTENTIALLY
INTERFERING
GS
VICTIM
GS
DESIRED
GS
•
•
•
•
•
Uplink-to-uplink ACI scenario
Desired GS and interferer on (first)
adjacent channels
Victim UA at edge of its cell
Both UA have omni antennas
Potential interferer must limit power
radiated toward cell boundary
Adequate adjacent-channel rejection
(ACR) also needed to prevent ACI
POTENTIALLY
INTERFERING
UA
DESIRED DOWNLINK
Downlink-to-downlink ACI worst case
• Desired UA and interferer are:
– On first adjacent channels
– Roughly equidistant from victim GS
– Both in victim GS’s main beam
•
•
Both UA have omni antennas
Here, ACR may be victim GS’s only
protection against ACI
8
Intersite Coexistence Rules (1 of 2)
• Interference prevention between cells
– Power flux density (PFD) limits, in dBm/m2, at cell
boundaries
• Interference prevention within cells
– Single-transmitter radiation limits
– EIRP limits
– Frequency-sharing rules for “gapfiller” and standalone
ground stations
• PFD, radiated-power, and EIRP limits will:
– Be different for uplinks and downlinks
– Depend on channel symbol rate (kbaud)
9
Intersite Coexistence Rules (2 of 2)
• Free-space PFD at cell edge shouldn’t exceed what
the potentially interfering link would need for good
availability if its own receiver were there
• In some scenarios, only protection against ACI is to
ensure that ACR is large enough to provide link
margin needed to allow for multipath, etc.
– Ground-antenna diversity (if affordable) would reduce
ACR requirements
– C2 channel spacings must be set large enough to
ensure adequate ACR
• Although ACR is main threat, cochannel PFD limits
also needed for very-high-altitude UA with very long
radio lines of sight
10
Channelization Planning Decision Tree
A
Required
Throughput
Necessary
Overhead
Symbol Rate
(e.g., 87.5 kbaud)
Modulation
(GMSK,
BT = 0.2?)
Transmitter
Mask
Channels Needed
per Cell (20?)
Min. Acceptable Freq.
Reuse, 1/K (1/12?)
Max. UA Ground
Speed (850 kn)
Required Availability
(99.8%)
Freq.
Stability
Receiver
Mask
(1 ppm)
Frequency-Dependent
Rejection Curve
Max. Radio-Horizon
Distance (261 nmi?)
Channel Spacing for Given Symbol Rate
(45,000 feet?)
Cell Radius (69 nmi?)
Max. GS Distance from
Cell Edge (69 nmi?)
Necessary Multipath/
Rain/Airframe Loss
Margin
Necessary AdjacentChannel Rejection (ACR)
Max. Cell Altitude
Min. UA Altitude at
Cell Edge (4000 feet?)
Diversity Assumptions
Max. UA Transmitter
Power (40 dBm)
UA SWAP
Constraints
Necessary Ground-Antenna Gain
(L-band: 19 dBi? C-band: 38 dBi?)
Number of Channels Available
Necessary Ground-Antenna Aperture
A
(L-band: 1 m2? C:-band: 3 m2?)
11
How Much ACR Is Necessary?
• Ensure, through GS power/pointing/location restrictions, that at cell
boundaries (the worst case) free-space interference power flux
density (PFD) will not exceed free-space signal PFD
• Design link budgets to allow received interference power (after
filtering) to equal receiver noise power (INR = 0 dB)
– Sample C-band link budgets shown in Appendix A
• Then the minimum ACR sufficient to allow 99.8% availability in the
presence of potential ACI from an adjoining cell can be calculated as:
Parameter
L-band
C-band
29.6*
33.6*
Required Eb/N0 (dB)
2.5
2.5
Implementation margin (dB)
1.0
1.0
Allowance (dB) for interference = noise
3.0
3.0
Total (minimum required ACR in dB)
36*
40*
Worst-case 99.8%-availability link margin (dB)
needed for multipath/rain/airframe losses
* Assumes dual airborne-antenna diversity but no ground diversity. Using dual or triple
ground diversity could reduce necessary link margins and ACR values by 9–14 dB.
12
Strawman C-Band Masks
Design assumptions:
GMSK (BT = 0.2)
87.5 kbaud
850-knot Doppler shift
1.0-ppm frequency instability
Attenuation (dB)
0
Transmitter
Receiver
70
80
21.9 39.5
122.5
160.3
Offset from Channel Center Frequency (kHz)
13
Frequency-Dependent Rejection (FDR) of
87.5-kbaud C-band Transmitter and Receiver
70
60
FDR (dB)
50
40
30
20
10
0
0
50
100
150
Frequency Offset (kHz)
200
250
Red curve allows for Doppler shift and frequency instability
14
Channelization Goals
• Spectral efficiency
– Small channel spacings ( large number of channels)
• ACI prevention
– Spacings large enough to provide adequate ACR
• Simplicity
– Every channel spacing should be integer multiple of smallest
spacing in band
– Round numbers preferred
• Harmonization
– Consistency with channel spacings of other systems in band
• MLS (300 kHz)
• UAS C2 SATCOM (300 kHz?)
• Not feasible to achieve every goal in same plan
– Tradeoffs necessary; no perfect plan
15
Channelization Principles
• Flexibility
– Each C-band C2 radio may have full repertoire of channel spacings
throughout its tuning range
• No part of tuning range to be permanently tied to a single channel spacing
• Channels of same size should be grouped together
– Helps protect wide channels against ACI from narrow ones
• Partitions between channel groups should be movable
– Since relative utilization of symbol rates is unpredictable and will
evolve over time
• C-band needs wider channel spacings than L-band
– Greater Doppler shifts and frequency instability
16
Possible C-band Channelization Plans
Info
Rate
(kbps)
Symbol
Rate
(kbaud)
14.80
87.5
Simple Plan (Harmonized with
MLS Channelization Plan)
More Spectrally-Efficient Plan
(Would Support More UAS)
Spacing
(kHz)
ACR
(dB)
Channels
in 60 MHz
Spacing
(kHz)
ACR
(dB)
Channels
in 60 MHz
150
44
400
150
44
400
35.28
150
300
69
200
250
58
240
28.48
200
300
55
200
250
39
240
56.96
400
600
63
100
500
48
120
85.44
600
900
66
66
750
51
80
237.52
750
1200
68
50
1000
57
60
113.92
800
1200
67
50
1000
52
60
142.40
1000
1500
67
40
1250
53
48
NOTE: One or more smaller channel spacings (TBD) are also needed for narrowband signals.
17
Potential Cochannel Interference to and from
Very-High-Altitude UA (VUA)
Scenario:
• VUA stays 65 kft above its GS (above highest C2 cell)
• VUAS uses frequencies allocated to highest-tier cell beneath it
• “Not-very-high-altitude UA” (NUA) uses same frequency as VUA
• Since VUA > 50 kft AGL, K=12 cell plan allows ground/air RLOS to
6 “cochannel” cells (only one shown in picture)
• Cochannel RFI (CCI) threatens VUAS uplink & NUAS downlink
(VUAS downlink & NUAS uplink protected by earth curvature)
VUA
65
kft
NUA
“Footprint” of highest-altitude tier of cells
12
7
10
8
VUAS GS
5
3
300 nmi ( 4.35 cell radii)
7
NUAS GS
18
Key Findings of VUAS Analysis
• CCI to and from very-high-altitude UAS (VUAS)
can be prevented by:
– Assigning to each VUAS a frequency that has been
allocated to the highest-tier cell beneath it
– Appropriately reducing VUAS uplink
and downlink transmitter powers
– Using highly directional VUAS GS antennas
• To protect VUAS against downlink ACI,
operational procedures may be needed to keep
not-very-high-altitude UA (NUA) from staying too
close to VUAS GS in its main beam for too long
19
Coexistence between Terrestrial and
SATCOM UAS C2 Links (1 of 2)
Note: This slide and the next summarize ACP WGF28/WP13(rev1), “5GHz Band-Planning Considerations for UAS CNPC Links,” March 2013
• WRC-12 decided 5030–5091 MHz band can be shared
by AMS(R)S and AM(R)S C2 links
• Unless AM(R)S or AMS(R)S is absent in a given region,
putting AM(R)S in center of band and AMS(R)S at high
and low ends would have these advantages:
– If AMS(R)S uses frequency-division duplexing, it needs to
maximize frequency separation between Earth  space and
space  Earth segments, because of filter-design constraints
– Radio Regulations footnote 5.443C limits AM(R)S EIRP density
to –75 dBW/MHz in the 5010–5030 MHz band, so large
separation between that band and the AM(R)S segment would
be useful
20
Coexistence between Terrestrial and
SATCOM UAS C2 Links (2 of 2)
• If band is partitioned between AM(R)S and AMS(R)S,
boundaries between segments should be movable
– Protects against having to make premature, binding
decisions on relative terrestrial and SATCOM allocations
• Boundary adjustments would be made infrequently based on
capacity demand patterns
– Allows for the possibility that some regions might use only
one of the two types of 5-GHz link (terrestrial or SATCOM,
but not both)
– Allows common wideband RF filter (over the entire 5030–
5091 MHz band) that would be:
• Simpler to implement than narrowband filters
• Usable by hybrid terrestrial/SATCOM terminals
21
Next Steps
•
•
•
•
•
•
•
Refine terrestrial C2 system design
Firm up necessary data and symbol rates
Redesign masks and recompute FDR curves
Develop firm list of channel spacings for each band
Recommend specific channel placements
Develop nationwide channel plan
Develop dynamic frequency-assignment
procedures
22
Appendix:
Sample C-Band Uplink Budgets
23
Sample C-Band Uplink Budgets
Parameter
Frequency
Symbol
Units
Values
Notes
f
MHz
5060
5060
Pex = Pt + Gt - Lct
Aircraft altitude, AGL
Ha
ft
18000
4000
Pe = Pex - Lpt
Ground-antenna height
Ht
ft
100
100
Dpf = Pe - 20 log d - 10 log (4p*1852^2) = Pe - 20 log d - 76.345
Lf = 20 log f + 20 log d + 37.8
Path distance
d
nmi
71
71
Symbol rate
Rs
kbaud
87.5
87.5
Transmitter power
Pt
dBm
40.0
40.0
Eb/N0 = 2.5 dB for GMSK with BT = 0.2
Maximum transmitting-antenna gain
Gt
dBi
38.0
38.0
Bn assumed equal to Rs
Transmitter cable loss
Lct
dB
1.0
1.0
N = Nt + 10 log Bn + Fn
Maximum EIRP
Pex
dBm
77.0
77.0
Transmitting-antenna pointing loss
Lpt
dB
2.0
2.0
Prm = Pe - Lf + Gr - Lcr
Dual airborne-antenna diversity assumed, but not ground diversity
La obtained from SC203-CC016 data for 2.4-GHz signal
EIRP toward receiver
Pe
dBm
75.0
75.0
Free-space value of received signal PFD
Dpf
dBm/m2
-38.4
-38.4
Lx based on ITU-R Recs. P.530-11, P. 618-9, P.838, P.676-6, and P.840-3
Free-space path loss
Lf
dB
148.9
148.9
Mc = sqrt ((La + Va)^2 + Lx^2 + Mb^2) -- approximation in lieu of convolution
Mean receiving-antenna gain
Gr
dBi
2.0
2.0
M = Mm + Mc + Mi + Ma
Prq = Eb/N0 + N + M
Receiver cable loss
Lcr
dB
Mean received signal power
Prm
dBm
Eb/N0
dB
Required signal-to-noise ratio per bit
2.0
2.0
-73.9
-73.9
2.5
2.5
dBm/kHz -144.0
-144.0
Thermal-noise spectral power density
Nt
Noise bandwidth of receiver
Bn
kHz
87.5
87.5
Receiver noise figure
Total receiver noise
Fn
N
dB
dBm
4.0
-120.6
4.0
-120.6
Implementation margin
Mm
dB
1.0
1.0
Airframe loss value for CDF = 0.002
La
dB
20.0
20.0
Est. variation from 2.4-GHz airframe loss
Va
dB
2.0
2.0
Excess path-loss value for CDF = 0.002
Lx
dB
16.4
24.7
RFI multipath boost for CDF = 0.002
Mb
dB
6.0
6.0
Combined airframe/path/RFI margin
Mc
dB
28.1
33.6
Allowance for interference = noise
Mi
dB
3.0
3.0
Aviation safety margin
Total required margin for 99.8% avail.
Ma
M
dB
dB
6.0
38.1
6.0
43.6
Required signal power
Prq
dBm
-80.0
-74.5
Excess margin
Mx
dB
6.1
0.6
Va estimated by assuming airframe loss increases from S- to C-band
Mx = Prm - Prq
24
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