Unmanned Aircraft System (UAS) Terrestrial C2 Frequency-Planning Activities in RTCA SC-228

1. 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 * 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 • 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 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

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

8. Examples of Potential Adjacent-Channel Interference (ACI) between Cells VICTIM UA POTENTIALLY INTERFERING UA DESIRED UPLINK VICTIM GS DESIRED DOWNLINK POTENTIALLY INTERFERING 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 • 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?) Necessary Overhead Required Throughput Min. Acceptable Freq. Reuse, 1/K (1/12?) Cell Radius (69 nmi?) Max. UA Ground Speed (850 kn) Symbol Rate (e.g., 87.5 kbaud) Max. GS Distance from Cell Edge (69 nmi?) Required Availability (99.8%) Freq. Stability (1 ppm) Min. UA Altitude at Cell Edge (4000 feet?) Modulation (GMSK, BT = 0.2?) Necessary Multipath/ Rain/Airframe Loss Margin Receiver Mask Transmitter Mask Diversity Assumptions Frequency-Dependent Rejection Curve Necessary Adjacent-Channel Rejection (ACR) UA SWAP Constraints Max. UA Transmitter Power (40 dBm) 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 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: * 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. Strawman C-Band Masks 0 Design assumptions: GMSK (BT = 0.2) 87.5 kbaud 850-knot Doppler shift 1.0-ppm frequency instability Attenuation (dB) Receiver Trans-mitter 70 80 122.5 160.3 39.5 21.9 Offset from Channel Center Frequency (kHz)

14. Frequency-Dependent Rejection (FDR) of87.5-kbaud C-band Transmitter and Receiver Red curve allows for Doppler shift and frequency instability

15. 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

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 NOTE: One or more smaller channel spacings (TBD) are also needed for narrowband signals.

18. Potential Cochannel Interference to and fromVery-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 7 8 NUAS GS VUAS GS 5 3 300 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  spaceand 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 24