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All-purpose Multi-channel Aviation Communication System (AMACS)

ICAO ACP WG T2 – 5 October 2007

Presented byLuc Deneufchatel, DSNALarry Johnsson, LFV

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Introduction

Future Communication Study E-TDMA proposed by DSNA XDL4 proposed by LFV

Emerging understanding Spectrum availability and RF environment will dictate

our options Plug in of generic systems (COTS) in aviation

environment is difficult and challenging AMACS

Based on: E-TDMA + XDL4 + experience from other aviation systems + COTS elements

Constraint driven development approach One multichannel narrowband alternative in L-band

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AMACS system overview

Flexible multipurpose communication system Cellular narrowband (100-400 kHz) point-to-point system

intended to operate primarily within the 960-975 MHz frequency allocation designed for flexible deployment Supports different channel bandwidths and bit rates to cope with

various operational needs (high and medium density airspace) Robust physical layer based on GSM/UAT modulation types associated

with strong data coding Efficient handling of QoS with guaranteed transmission delay (based

on the TDMA structured MAC layer) Support of unicast and multicast data communications

taking advantage of VDL Mode 4 broadcast experience Support of air-air point-to-point data communications

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A flexible and scalable solution providing for operational expansion

A configurable channel size to match the foreseen traffic densities of Europe in 2020+ Frequency plan needed to allocate the available spectrum to the

various types of channels (bandwidth and type of service)

An adapted performance for the different QoS classes Frame structure identifies distinct time slots at MAC layer Specific and reserved channel resources for high QoS transmissions

Strong robustness at physical layer level to ensure: Achievement of the highest QoS in terms of latency Predictive behaviour in a typical distorted propagation channel Co-site operation on board aircraft by minimizing susceptibility level

AMACS performance objectives

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Key design drivers Robustness, flexibility, scalability

E-TDMA and XDL4 concepts have been merged Providing an adapted technical solution to data-link communications

needs of 2020+ EMC “constraint driven” development

Based on proven concepts Robust proven GSM physical layer High performance E-TDMA MAC layer VDL Mode 4 broadcast protocols

Designed to handle up to 175 aircraft per cell in high- density airspace Efficient air-initiated cell handover mechanism Uses aircraft knowledge of cell locations and characteristics

(through either EFB loading or CSC channel)

AMACS key facts 1

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AMACS key facts 2

Initial deployment in the lower L-band to support: New ATM point-to-point services requiring high QoS

(support to SESAR or NEXTGEN future concept) Broadcast services provided in segregated channel if

spectrum availability in the lower L-band is sufficient Air-air data communication provided in segregated channels AOC data communications achievable if extra spectrum is

available for dedicated channels

Could be transposed to the VHF band in the long term when it becomes available for new technology More capacity offered to cover all the needs above

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AMACS key facts 3

Airborne co-site interference in the lower L-band is addressed by using: A common synchronization bus between L-band

systems to protect other L-band systems from AMACS transmissions

Other systems are notified of any transmission from AMACS to take the appropriate measure

A strong coding of the channel to provide high robustness for airborne reception

The ratio between the shortest bit duration in a slot and the duration of the spurious burst is approximately 0·5 to 1

This leads to potential interference windows covering no more than two or three consecutive bits

These can be recovered by the various coding mechanisms

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AMACS Presentation

This presentation focuses on an air/ground point-to-point channel supporting the highest bit-rate per cell 400 kHz/520 kbps Foreseen for the en-route high-density area of Western Europe

Minimal configurations can be tailored for the periphery of Western Europe 100 kHz/130 kbps

Intermediate configurations can be tailored for major TMA areas 200 kHz/ 260 kbps

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Typical high bit-rate point to point instantiation of theAMACS system

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Design goals Low Bit Error Rate at low Signal-to-Noise ratio Occupation of least possible bandwidth Good performance in multipath and fading environments Introduce least amount of residual power in the RF

environment Simple and cost effective to implement

MAC considerations 148 octets afforded per slot for data to meet the most critical

services defined in the COCR

Lower layers characteristics 1/2

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Narrowband system based on GSM physical layer Modulation based using Gaussian Minimum Shift Keying (GMSK) Pre-filtering leads to compact waveform (minimal sidelobes) 400 kHz channels Gross Bit rate of 520 kbps C/I of 9dB (including FEC) May allow reuse of some GSM hardware components

Error Correction

Concatenated coding Inner code – Convolutional code with puncturing Interleaver – Block and diagonal interleaving Outer code – Reed-Solomon

Lower layers 2/2

Inner codeOuter code

RS coding InterleavingConvolutive

coding

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Why GMSK modulation rather than CPFSK or GFSK ?

GMSK is a modulation known and tried with GSM

The global deployment of GSM implies cheap costs of development for equipment

A cellular system and a waveform adapted to frequency re-use radio networking (C/Icc=9dB and C/Iadj=-9dB)

Allows the best compromise between BER and bit-rate

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Link budget

Hypothesis: Free space propagation Frequency : f =975MHz Propagation distance: d=150 NM =278 Km Antennas Gains: Ge= -3dB Gr= 0dB Reception power: Pr=-100 dBm (to ensure BER=10-3 on a 400 kHz channel)

)(log*20)(log*2044.32][

10][

100MHzKmfdA

The pathloss is computed by the following formula:

dBA 1.1410

WdBmPe 261.44

0AGPGP rree

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Error correcting scheme

310InCCBER 710OutRSBER

Convolutive decoding De-Interleaving RS decoding

OutCCBER InRSBER

];;;;;[ 76

65

54

43

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21CC RS

Interleaving does not affect the BER but improves the distribution of errors OutCCInRS BERBER

Convolutive code are used to remove isolated error

RS code has the effect of removing burst of errors

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Error correcting scheme

The code rateRSCC *75,0

810

620

];;[ 76

65

54CCSo only three rates are practical:

31*231

31tx

RS

- The RS code is the RS(31, x; 5)

CCRS 75,0

87,076

75,0 RSSo only two rates are practical, with t=[1;2] ];[ 31

293127RS

- The convolutive code is the well known punctured (133,171), constraint length 7.

75,0CC

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Error correcting scheme

Four configurations are suitable:1)

75,0* RSCC

);();( 3127

76RSCC

2) );();( 3129

54RSCC

75,0* RSCC

3) );();( 3129

65RSCC

78,0* RSCC

4) );();( 3129

76RSCC

80,0* RSCC

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Error correcting scheme

BER in convolutional code With a convolutional code (5,7) a BER=10-3 at the input gives a

BER=10-5 at the output. In order to mitigate the puncturing, the BER at the output will be

considered equal to 10-4.

BER in RS code:

BER at the input of RS decoder BER at the output of RS decoder BER at the input of RS decoder BER at the output of RS decoder5,0E-03 2,1E-03 5,0E-03 4,8E-041,0E-03 2,5E-05 1,0E-03 1,2E-067,0E-04 9,0E-06 7,0E-04 3,0E-085,0E-04 3,3E-064,5E-04 2,4E-064,0E-04 1,7E-063,5E-04 1,2E-063,0E-05 7,0E-07

RS Code (31,29,5) RS Code (31,27,5)

With a the BER at the output of the RS code (31,27,5), is arround 10-7, so the conditions are met for two of the configurations

410OutCCBER

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Other solution

Using only the RS coder

80,031

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De-Interleaving RS decoding

310InBER OutRSBER

With a RS coder (31;25), the code rate will be:

And the BER :BER at the input of RS decoder BER at the output of RS decoder

2,0E-03 1,1E-061,5E-03 3,0E-071,0E-03 3,9E-08

RS Code (31,25,5)

This solution seems relevant but must be modelled and simulated over an appropriate representative radio channel

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TDMA access scheme with 4 millisecond slots Ramp-up/down times total < 0·1 ms Guard time allowance of 0·9 ms, allows a GS range of 150 NM Usable slot duration 3 ms

Time synchronization to UTC will be required Time information uplinked by the ground station for aircraft use

Basic slot characteristics

Guard tim

e depending on

cell size

individual slot structure

SynchIn bits

signalling

and dataIn bits

CRCIn bits

de

cay

total slot duration 4ms

next slotFEC,R

am

p-u

p

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For point-to-point channels, AMACS will use the MAC layer principles developed for E-TDMA

Channel will have a frame repeating every 2 seconds

Uplink sections - use is configurable (dynamically) by the ground station (GS) Ground reserved area for uplinks and ground-directed signalling

Downlink sections - divided into sub-sections for different Classes Of Service (COS) Each A/C has one exclusive slot for high QoS messages More downlink slots are available on request

MAC layer organization

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Downlink Classes Of Service (COS)

COS1 High QoS Service Dedicated section of the frame for high-priority short

messages from aircraft Each aircraft within range of the ground station is

allocated its own slot in which it may transmit in every frame (thus every 2 seconds)

COS2 Lower QoS Service A section of the frame for lower priority and/or longer

messages from aircraft Section also allows for re-sends in the same frame

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Shared section

Uplink section

Uplink section

Frame

CoS1 CoS2UP2UP1

Exclusive primaryslots for short, high QoS messages or

RTS messages

Shared slots, reserved or random access:

used for any messages

Second uplink for ACKs, CTS,

reservations

Reserved slots for uplink messages

Downlink section

Framingmessage

Cell insertion

Start of UTCsecond

Frame structure – point-to-point

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Uplink & Cell Insertion Frame Sections

UP1 1st Uplink Section for ground station use For data uplink and ACKs of received data

UP2 2nd Uplink Section for ground station use For CTS/ACK ALL messages For reservation messages reserving space in COS2 For framing message

Cell Insertion Dedicated section for new aircraft to logon to the ground

station when it comes within range

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Flexible frame structure

The flexibility to cope with different numbers of aircraft and traffic demand is built into the frame structure

Lengths of each section of the frame (COS1, COS2, UP1, UP2) can be varied by the ground station In particular the length of the COS1 section follows the

number of logged-on aircraft very closely

Details of the current frame structure and of the frame structure in x frame’s time will be broadcast every frame in a Framing Message

The framing message will also broadcast the length of the Cell Insertion section

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MAC layer characteristics

Frame length of 2 seconds Divided into 500 slots of length 4 ms It is assumed that this size is fixed globally

Slot characteristics Active slot length: 4 ms – (ramp + guard times) = 3 ms Bits per slot: Active slot length × Bit rate = 1,620 bits Bits for CRC/FEC: ~30% of bits per slot = 376 bits (47

octets) Remainder: Bits per slot – CRC = 1244 bits = 155·5 octets ISO flags + reservation header = 3 octets Addresses plus administrative flags (average) = 4·5

octets User data space = 148 octets

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ISO Flag

Ramp-down

Slot structure

4 ms

Ramp-up n

1 octet

Reservation header 3 octets (if required)

User data 148 octets

FEC / CRC 47 octets

Guard time 0·9 ms

m

NOTE: n + m < 0·1 ms

1 octet

Addresses plus flags 4·5 octets (typical)

ISO Flag

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Cellular deployment

Cellular deployment 12 frequency re-use pattern

Worst case (air-air interference) Carrier/Interferer (C/I) calculation dw = R and di = 4R, for cell radius R C/I = Att (interference) – Att (wanted) Propagation model:

Att = (constant) + a.10 log(d) a = 2 (Free space) or more C/I = a.6 dB, Thus C/I ≥ 12 dB

But for GMSK, 9 dB is enough, with GSM FEC rate 260/456 (0.57 ratio), and a very light interleaving

dw=R

di=4R

dw=R

di=4R

dw=R

di=4R

dw=R

di=4R

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AMACS Network Architecture

AMACS infrastructure comprises a number of AMACS Ground Stations which are organized into clusters

Each Ground Station in a cluster will be connected to some concentrator, the Ground Network Interface (GNI)

GNI

GNI

ATN A/G Router

ATN A/G Router

IPv6 Router

IPv6 Router

Cluster 1

Cluster 2

ATN G/G Router

IP Router

ATNApplications

TCP/IPApplications

WAN

GNI

GNI

ATN A/G Router

ATN A/G Router

IPv6 Router

IPv6 Router

Cluster 1

Cluster 2

ATN G/G Router

IP Router

ATNApplications

TCP/IPApplications

WAN

ATN A/G Routers and the IPv6 Routers are ground-based users of the AMACS sub-network service and the airborne ATN and IP routers are mobile users of the AMACS sub-network service

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Airborne Architecture

Avionics for AMACS implementation of ATS, AOC and ADS-B functions

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System operations - Entry

Aircraft entry Section at the beginning of CoS2 dedicated to cell insertion

A/C will already know the GS frequency A/C will listen for 2 seconds to hear the “framing” message This will tell it the GS ICAO address and the cell frame structure A/C will then transmit cell insertion message in the dedicated slots This contains the A/C ICAO address and the GS ICAO address

GS will reply in UP1 Containing GS ICAO address, A/C ICAO address, new local 9-bit A/C

address, GS 7-bit local address, allocated slot number Local addresses are used to avoid ICAO 27-bit addresses occupying

large amounts of space in transmissions

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System operations - Uplink

The GS will transmit data to the A/C in UP1 If correctly received –

Each A/C will send an ACK as part of its CoS1 transmission

If not correctly received – The A/C will send a NACK as part of its CoS1 transmission GS will re-send data in UP2, with an ACK slot reserved in CoS2 A/C will send an ACK or NACK) in the allocated CoS2 slot

GS transmits framing message at start of UP2, containing – The ground station’s full ICAO address UTC time, Frame section sizes

UP2 is also used for transmitting the combined ACK/CTS message to all aircraft

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System operations - Downlink

Each A/C has an allocated CoS1 slot for downlink Regular transmission of short data messages If the data size is too large, an RTS is transmitted in CoS1

(This is a request for a longer CoS2 slot) When an A/C has no data, it transmits a keep-alive message

If CoS1 transmission is correctly received – The GS responds in the combined ACK/CTS message in UP2

If not correctly received – The A/C will re-transmit in CoS2, using random access The GS can reply with a dedicated ACK in CoS2

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System operations – Hand-off

Hand-off procedures A/C will know the locations of ground stations When nearing the edge of a cell, A/C will contact the next GS The A/C will indicate to current GS that it’s exiting the cell

If this process completes correctly handover will be quick (1 slot) Otherwise the link will time-out

GS will de-allocate CoS1 slot after a correct hand-off If contact is broken before hand-off process is complete, the A/C’s

CoS1 slot will remain reserved for a pre-set period This will prevent a disruption of communications caused by

premature slot re-allocation after a short-term signal loss

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Broadcast channel

Superframe characteristics 15,000 slots in one 60 s superframe 4 ms slot length

Same MAC structure as VDL Mode 4 Random access using the VDL Mode 4 reservation protocols Dedicated ground-reserved block at start of each superframe Increased basic message size, more convenient for ADS-B

Most VDL Mode 4 broadcast protocols will be used Modified for single channel and AMACS frame structure No point-to-point transmissions permitted

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Point-to-point channelDefined AMACS messages

Binary codes for AMACS message types: 6 bits 00 0000 is not used

Message typeBinary code

Message typeBinary code

CoS1 Downlink 00 0001 Uplink 00 1000

CoS1 Keep-alive 00 0010 Block reservation 00 1001

CoS2 Downlink 00 0011 Framing message 00 1010

CoS2 RA short 00 0100 CTS 00 1011

CoS2 RA long 00 0101 ACK 00 1100

CoS2 RA RTS 00 0110 ACK/CTS ALL 00 1101

Cell exit 00 0111 Cell insertion 00 1110

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Example Message structureCell insertion

A/C Tx

A/C will listen for framing message to identify the cell insertion slotsGS reply to cell insertion message will be transmitted in the next UP1

CELL_INS message type

GS ICAO address 27

A/C ICAO address 27

Authentication (32)

109 bits

8ISO flag

Message type 6

Version number 2

Destination ground station

Address length flag 1

Binary 00

Binary 0111 1110

Binary 0 for local addressesBinary 1 for 27-bit ICAO addresses

Binary 00 1110

Size not fixed

Message identifier 6 1 to 64 (00 0001 to 11 1111)

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AMACS summary

Flexible multipurpose L-band communication system Cellular, narrowband system Channel bandwidths (100 - 400 kHz bandwidth) and bit

rates adaptable according to operational needs Robust physical layer based on GSM/UAT modulation

types Efficient handling of QoS with guaranteed transmission

delay Support of air-ground point-to-point data

communications and air-air, using multiple channels Support of multicast/broadcast data communications

taking advantage of experience of existing systems

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AMACS Status

The high level design of AMACS is now finalised At Physical and MAC layer levels Complete definitions of frame, slot, and message structures Error correction coding definition completed

Initial channel structure, cellular deployment and network architecture specified

All MAC message types defined Definition of services provided Protocols and system operation defined for both point-to-

point and broadcast communication On going activities at DSNA regarding the airborne co-site

compatibility (DME and Mode S) including laboratory test with GA DME

Further activities to refine the design and assess more accurately the performances are necessary