1. Frank Box, Alexe Leu, and Leo Globus 22 September 2014
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

2. 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

3. 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

4. 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
Downlink (automatic UA operation)
1.272
6.131
11.163
Uplink (manual UA operation)
4.593
9.442
10.108
Downlink (manual UA operation)
7.595
12.454
18.391
* 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.

5. Strawman System Configurations
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

6. 3-D Cellular Frequency Plan
Highest altitude tier (50 kft)
1/12 frequency reuse
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

7. Low-Altitude Coverage and Gapfillers
CENTRAL 100’ TOWER
Coverage down to 4000’
Down to 1000’
Down to ground
CELL BOUNDARY
Gapfiller
69 NAUTICAL MILES
Likely cell radius  69 nmi
Central ground station (GS) cannot provide coverage down to ground throughout cell
In most of cell, low-altitude UA need “gapfiller” GSs
When gapfillers are far enough apart, they may be able to share frequencies in same cell 7

8. POTENTIALLY INTERFERING GS POTENTIALLY INTERFERING UA
Examples of Potential Adjacent-Channel Interference (ACI) between Cells
DESIRED GS
POTENTIALLY INTERFERING GS
DESIRED UPLINK
VICTIM UA
DESIRED DOWNLINK
VICTIM GS
POTENTIALLY INTERFERING UA
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
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

9. 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)

10. 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

11. Channelization Planning Decision Tree
Channels Needed per Cell (20?)
Max. Radio-Horizon Distance (261 nmi?)
Max. Cell Altitude (45,000 feet?) A
Required Throughput
Necessary Overhead
Min. Acceptable Freq. Reuse, 1/K (1/12?)
Cell Radius (69 nmi?)
Symbol Rate (e.g., 87.5 kbaud)
Max. UA Ground Speed (850 kn)
Required Availability
(99.8%)
Max. GS Distance from Cell Edge (69 nmi?)
Min. UA Altitude at Cell Edge (4000 feet?)
Modulation (GMSK,
BT = 0.2?)
Necessary Multipath/ Rain/Airframe Loss Margin
Transmitter Mask
Receiver Mask
Freq. Stability (1 ppm)
Diversity Assumptions
Frequency-Dependent Rejection Curve
Necessary Adjacent-Channel Rejection (ACR)
Max. UA Transmitter Power (40 dBm)
UA SWAP Constraints
Channel Spacing for Given Symbol Rate
Necessary Ground-Antenna Gain
(L-band: 19 dBi? C-band: 38 dBi?)
Number of Channels Available
Necessary Ground-Antenna Aperture
(L-band: 1 m2? C:-band: 3 m2?) A

12. 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
Worst-case 99.8%-availability link margin (dB) needed for multipath/rain/airframe losses
29.6*
33.6*
Required Eb/N0 (dB)
2.5
Implementation margin (dB)
1.0
Allowance (dB) for interference = noise
3.0
Total (minimum required ACR in dB)
36*
40*
* 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.

13. Offset from Channel Center Frequency (kHz)
Strawman C-Band Masks
39.5 70
160.3
122.5
21.9 80
Attenuation (dB)
Offset from Channel Center Frequency (kHz)
Design assumptions:
GMSK (BT = 0.2)
87.5 kbaud
850-knot Doppler shift
1.0-ppm frequency instability
Receiver
Trans-mitter

14. Frequency-Dependent Rejection (FDR) of 87
Frequency-Dependent Rejection (FDR) of 87.5-kbaud C-band Transmitter and Receiver
Red curve allows for Doppler shift and frequency instability

15. Channelization Goals Spectral efficiency ACI prevention Simplicity
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

16. 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

17. Possible C-band Channelization Plans
Info Rate (kbps)
Symbol Rate (kbaud)
Simple Plan (Harmonized with MLS Channelization Plan)
More Spectrally-Efficient Plan
(Would Support More UAS)
Spacing (kHz)
ACR (dB)
Channels in 60 MHz
14.80
87.5
150 44
400
35.28
300 69
200
250 58
240
28.48 55 39
56.96
600 63
100
500 48
120
85.44
900 66
750 51 80
237.52
1200 68 50
1000 57 60
113.92
800 67 52
142.40
1500 40
1250 53
NOTE: One or more smaller channel spacings (TBD) are also needed for narrowband signals.

18. “Footprint” of highest-altitude tier of cells
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
Potential Interference Path 65
kft
NUA
“Footprint” of highest-altitude tier of cells 12 10 7 8 7
VUAS GS
NUAS GS 3 5
300 nmi ( 4.35 cell radii)

19. 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

20. Coexistence between Terrestrial and SATCOM UAS C2 Links (1 of 2)
Note: This slide and the next summarize ACP WGF28/WP13(rev1), “5-GHz 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

21. 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

22. 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

23. Appendix: Sample C-Band Uplink Budgets

24. Sample C-Band Uplink Budgets
Parameter
Symbol
Units
Values
Notes
Frequency f
MHz
5060
Pex = Pt + Gt - Lct
Aircraft altitude, AGL Ha ft
18000
4000
Pe = Pex - Lpt
Ground-antenna height Ht
100
Dpf = Pe - 20 log d - 10 log (4p*1852^2) = Pe - 20 log d
Path distance d
nmi 71
Lf = 20 log f + 20 log d
Symbol rate Rs
kbaud
87.5
Prm = Pe - Lf + Gr - Lcr
Transmitter power Pt
dBm
40.0
Eb/N0 = 2.5 dB for GMSK with BT = 0.2
Maximum transmitting-antenna gain Gt
dBi
38.0
Bn assumed equal to Rs
Transmitter cable loss
Lct dB
1.0
N = Nt + 10 log Bn + Fn
Maximum EIRP
Pex
77.0
Dual airborne-antenna diversity assumed, but not ground diversity
Transmitting-antenna pointing loss
Lpt
2.0
La obtained from SC203-CC016 data for 2.4-GHz signal
EIRP toward receiver Pe
75.0
Va estimated by assuming airframe loss increases from S- to C-band
Free-space value of received signal PFD
Dpf
dBm/m2
-38.4
Lx based on ITU-R Recs. P , P , P.838, P.676-6, and P.840-3
Free-space path loss Lf
148.9
Mc = sqrt ((La + Va)^2 + Lx^2 + Mb^2) -- approximation in lieu of convolution
Mean receiving-antenna gain Gr
M = Mm + Mc + Mi + Ma
Receiver cable loss
Lcr
Prq = Eb/N0 + N + M
Mean received signal power
Prm
-73.9
Mx = Prm - Prq
Required signal-to-noise ratio per bit
Eb/N0
2.5
Thermal-noise spectral power density Nt
dBm/kHz
-144.0
Noise bandwidth of receiver Bn
kHz
Receiver noise figure Fn
4.0
Total receiver noise N
-120.6
Implementation margin Mm
Airframe loss value for CDF = 0.002 La
20.0
Est. variation from 2.4-GHz airframe loss Va
Excess path-loss value for CDF = 0.002 Lx
16.4
24.7
RFI multipath boost for CDF = 0.002 Mb
6.0
Combined airframe/path/RFI margin Mc
28.1
33.6
Allowance for interference = noise Mi
3.0
Aviation safety margin Ma
Total required margin for 99.8% avail. M
38.1
43.6
Required signal power
Prq
-80.0
-74.5
Excess margin Mx
6.1
0.6 24