Unmanned Aircraft System (UAS) Terrestrial C2 Frequency-Planning

Activities in RTCA SC-228

Frank Box, Alexe Leu, and Leo Globus

22 September 2014

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ACP WG-F/31 WP08

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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

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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

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Link Throughput RequirementsService 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.

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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

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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

Low-Altitude Coverage and Gapfillers

• 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

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CENTRAL 100’

TOWER

Coverage down to

4000’

Down to 1000’

Down to ground

CELL BOUNDARY

Gapfiller

Gapfiller

Gapfiller

Gapfiller

Gapfiller

69 NAUTICAL MILES

Examples of Potential Adjacent-Channel Interference (ACI) between Cells

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DESIRED GS

POTENTIALLY INTERFERING

GS

DESIRED UPLINK

VICTIM UA

DESIRED DOWNLINKVICTIM

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

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)

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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

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Channelization Planning Decision Tree

Required Throughput

Necessary Overhead

Symbol Rate (e.g., 87.5 kbaud)

Transmitter Mask

Modulation (GMSK,

BT = 0.2?)

Max. UA Ground Speed (850 kn)

Receiver Mask

Frequency-Dependent Rejection Curve

Channel Spacing for Given Symbol Rate

Necessary Adjacent-Channel Rejection (ACR)

Necessary Multipath/ Rain/Airframe Loss

Margin

Required Availability(99.8%)

Max. GS Distance from Cell Edge (69 nmi?)

Cell Radius (69 nmi?)

Max. Radio-Horizon Distance (261 nmi?)

Max. Cell Altitude (45,000 feet?)

Min. Acceptable Freq. Reuse, 1/K (1/12?)

UA SWAP Constraints

Max. UA Transmitter Power (40 dBm)

Necessary Ground-Antenna Gain(L-band: 19 dBi? C-band: 38 dBi?)

Necessary Ground-Antenna Aperture

(L-band: 1 m2? C:-band: 3 m2?)

Min. UA Altitude at Cell Edge (4000 feet?)

Number of Channels Available

Channels Needed per Cell (20?)

A

A

Freq. Stability (1 ppm)

Diversity Assumptions

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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:

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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 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*

* 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.

Strawman C-Band Masks

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0

39.5

70

160.3122.521.9

80

Att

enu

atio

n (

dB

)

Offset from Channel Center Frequency (kHz)

ReceiverTrans-mitter

Design assumptions:GMSK (BT = 0.2)87.5 kbaud850-knot Doppler shift1.0-ppm frequency instability

Frequency-Dependent Rejection (FDR) of87.5-kbaud C-band Transmitter and Receiver

0 50 100 150 200 2500

10

20

30

40

50

60

70

Frequency Offset (kHz)

FD

R (

dB)

Red curve allows for Doppler shift and frequency instability

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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

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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

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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

Spacing (kHz) ACR

(dB)

Channels in 60 MHz

14.80 87.5 150 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

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

Potential Cochannel Interference to and fromVery-High-Altitude UA (VUA)

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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

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Potential Interference Path

NUA

300 nmi ( 4.35 cell radii)

NUAS GS7

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3

8

5VUAS GS

“Footprint” of highest-altitude tier of cells

65kft

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

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Coexistence between Terrestrial and SATCOM UAS C2 Links (1 of 2)

• 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

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Note: This slide and the next summarize ACP WGF28/WP13(rev1), “5-GHz Band-Planning Considerations for UAS CNPC Links,” March 2013

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

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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

procedures22

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Appendix:

Sample C-Band Uplink Budgets

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Parameter Symbol Units Values Notes

Frequency 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

Path distance d nmi 71 71 Lf = 20 log f + 20 log d + 37.8

Symbol rate Rs kbaud 87.5 87.5 Prm = Pe - Lf + Gr - Lcr

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 Dual airborne-antenna diversity assumed, but not ground diversity

Transmitting-antenna pointing loss Lpt dB 2.0 2.0 La obtained from SC203-CC016 data for 2.4-GHz signal

EIRP toward receiver Pe dBm 75.0 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 -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

Receiver cable loss Lcr dB 2.0 2.0 Prq = Eb/N0 + N + M

Mean received signal power Prm dBm -73.9 -73.9 Mx = Prm - Prq

Required signal-to-noise ratio per bit Eb/N0 dB 2.5 2.5

Thermal-noise spectral power density Nt dBm/kHz -144.0 -144.0

Noise bandwidth of receiver Bn kHz 87.5 87.5

Receiver noise figure Fn dB 4.0 4.0Total receiver noise N dBm -120.6 -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 Ma dB 6.0 6.0Total required margin for 99.8% avail. M dB 38.1 43.6

Required signal power Prq dBm -80.0 -74.5

Excess margin Mx dB 6.1 0.6

Sample C-Band Uplink Budgets

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