SOFREAVIA ATM Department Presentation to RTCA/SC 172, 15 april 98, & AMCP-WG/C2-WP26 7-11 May 2001 # 1 The E-TDMA Concept Towards a new VDL strategy Some

SOFREAVIA ATM DepartmentPresentation to RTCA/SC 172, 15 april 98, & AMCP-WG/C2-WP26 7-11 May 2001 # 1

The E-TDMA Concept

Towards a new VDL strategy

Some key issues& possible

way forward


The E-TDMA Concept Why a new VDL strategy ?no requirement for integrated voice-data

the main datalink activity today is AOC

the development of ATSC will be slow

performance requirements are still unclear

efficiency requires a data integration strategy

the technology-driven approach is flawed

Neither VDL Mode 3 nor VDL Mode 4 are adequate.A more flexible second generation system is neededto provide an efficient general purpose datalink


The E-TDMA Concept

The E-TDMA design approach:Identify requirements and

constraints for designing a general purpose VDL TDMA

derive a stable list of design drivers

Propose generic solutions that can be tuned to a variety of operational conditions and quantitative QoS

requirements

Discuss them with CAAs and manufacturers


The E-TDMA ConceptKey issues for a general purpose VDL TDMA(1) provide a sustainable migration path(take into account the initially limited number of available channels)

(2) limit the cost of aircraft equipment(stick to no more than 3 multi-purpose VDR, one for the voicelink,one for the datalink, and one as backup for either voicelink or datalink, and avoiding an awkward multiplication of datalink emittors)

(3) support end-to-end safety certification (offer a deterministic behaviour and traceable QoS specifications, AND avoid common failure modes with other CNS equipments)

(4) support a variety of datalink services (incl. addressed Air-Ground, addressed Air-Air and broadcast)

(5) support different ground infrastructures(support a variable air-traffic density and connectivity of ground stations)


The E-TDMA Concept

Provide a sustainable migration path (1)

channels possibly available for VDL in Europe

2000 2005 2010 2015 2020

1

2 or More ?

ACARS for AOC and some ATSC

VDL Mode 2 for AOC and ATSC

Mode S extended squitter and STDMA for ADS-B

E-TDMA for AOC, ATSC ADS-B and ASAS-C


The E-TDMA Conceptlimit the cost of aircraft equipments:• adopt a GSM-like cellular deployment model, to emit

a single datalink channel at any location in airspace

support safety-oriented certification:• rely on a built-in Statistical Self-Synchronisation• design a stable cellular layout after constraints set by air traffic density, and the required throughput and transfer delays

provide QoS management mechanisms at every service interface• design the medium access control policy to guarantee if necessary the transit delay performance for time-bounded ATSC transactions (ADS, CPDLC, ASAS...)


The E-TDMA Concept cells tailored to

operations: air traffic density deployed applications en-route, TMA, airport

cellular layout description: loaded as pre-flight information periodically broadcast on a GSC

handover protocol: aircraft-initiated (based on the

cellular layout and own position) inter-connected ground stations

self-insertion mechanism: for popping-up aircraft as a backup or alternative to

handover


The E-TDMA ConceptComparison with VDL Mode 3 & 4 (*)

(*) source: Dassault Electronique with Alcatel Bell, National Avionics and IAA Final Report of the TREATY 8 study funded by CEC DG 13 (delivered in april 98)

Percentage of a channel required to meet the QoS requirements

Solution 1 Solution 2 Solution 3

TMA Short-Term 52.6 % (40.1 %) 54.1% (41.6%) 32.1% (26.1%)

TMA Medium-Term 176.6% (120.4%) 181.3% (125%) 66.7% (54.6%)

En-Route Short-Term 66.4% 69.3% 51.3%

En-Route Medium-Term 306.1% (287.4%) 315.5% (296.8%) 301.8% (239.1%)

(European Scenarios)

160 NM radius, 570 aircrafts max, 95% satisfied transfer delay, D8PSK modulationSolution 1 represents the ground-centralised VDL Mode 3T (data-only mode)Solution 2 represents a modified VDL Mode 4 (10 seconds frame instead of 1 mn)Solution 3 represents a variation of E-TDMA without guaranteed access delay,parenthesised figures were obtained after downgrading a certain QoS category


The E-TDMA ConceptProvide a sustainable migration path (2)

VHF band for aeronautical communications (25 kHz channels)

today

2005

2010+

Protected sub-band for 8.33 kHz voice channels

25 kHz channels freed by the migration into the sub-band

Protected E-TDMA clusters (to minimise the interference problemschannels used by adjacent cells should belong to different clusters)

datachannels


The E-TDMA Concept

local semi-autonomousconflict-solving

discrete rendezvous pointsdefining a 4D Flight Contract

air-air data exchange (ADS-B + ASAS)

air-ground ATN links for ATC and tactical re-

planning

Hybrid ATM concept combining global (re-)planning and local autonomy


The E-TDMA Concept

The foreseen E-TDMA Traffic Mix

mobile-originated emissions ATSC & AOCADS-B emissions from

ASAS the ground station(s) ATSC & AOC

required downlink throughputrequired uplink throughput


The E-TDMA Concept

talking andlistening in the

current cell

Limit the aircraft equipment cost:the autonomous aircraft equippage upgrade for an air-airASAS/ADS-B capability consists of 2 additional receiverstuned on downstream cells:

forward listening to adjacent cells


The E-TDMA Concept

Summary description of some features

proposed in the E-TDMA study


The E-TDMA Concept

Statistical Self-Synchronisation (S3)• a robust, low-cost synchronisation is a crucial issue• UTC accuracy for applications must be 1 s• D8PSK VDR must not drift by more than 50 µsE-TDMA solution:• no constraint on the VDR (just a quartz clock)• no external master clock (ground or space-based)• global coordination among all stations not needed•can use imprecise position information (RNP level)• low overhead (a few percents of the capacity)• confined to a distinctive synchronisation sublayer


The E-TDMA ConceptStatistical Self-Synchronisation (S3)

Oh I'm late, I should speed up

Fine, I can slow down a bit

• Each station detects if it is "late" or "early" by monitoring the emissions of the other ones (coarse position information can be used to improve the correlation of delays)• It shifts its time back or forth by a small quantum when certain guard time thresholds are no longer respected• Some extra guard time is needed for the resynchronisation


The E-TDMA Concept

High integrity MAC sublayer

• expected Physical Bit Error Rate: 10-3

• existence of error bursts (due to fading)• required Residual Message Error Rate: 10-7

• limit cost of real-time error processing

E-TDMA solutions:• interleaving for scattering error bursts• a small number of combinable BCH and RS modules• target Undetected Error Rate: 10-5 to 10-6

• additional CRC at LLC layer with target RER < 10-7


The E-TDMA Concept

support end-to-end safety certification:

include a strong error detection/correction within the MAC sublayer to minimise losses

of end-to-end integrity and repetition delays

ERROR =>LL NACK AND RETRANSMISSION

IS IS ES ES

ERROR =>TRANSPORT NACK AND RETRANSMISSION


The E-TDMA Conceptperformance certification in an ATN context:

offer an ISO 8208 service interface + QoS params(low overhead in local reference mode for ATN)

support QoS selection parameters at the serviceinterface of the E-TDMA subnetwork (allowing theATN routers to establish SVCs according to QoS)

groundrouter

aircraftrouter

SVC1 (QoS2)

SVC2 (QoS3)

SVC3 (QoS1)


The E-TDMA ConceptModular error correction

header blocks B0:RER = 3.10-8

BCH (31, 16)

small slots B1:RER = 3.10-6

BCH (63, 45)

large slots B2:RER = 10-6

RS (31, 23)5 bits symbols

an E-TDMA slot would combine only a few different codesdefined according to the target Residual Error Rates (RER):

example for a small slot: B0 + 3*B1 => 151 data bits + 69 CRC bitsCRC overhead = 45 % (worst case based on BER = 10-3)

example for a larger slot: B0 + 9*B2 => 1051 data bits + 335 CRC bitsCRC overhead = 30 % (worst case based on BER = 10-3)


The E-TDMA Concept

support different datalink services:

Physical Layer

MAC sub-layer

(sub)network Layer

broadcastapplications

locally addressedapplications

LL sub-layerbroadcast

ATN IP

SYNCH sub-layer

globally addressedapplications


The E-TDMA ConceptQoS monitoring :

there is a need to monitor and report all the problems so as to allowthe E-TDMA system to self-reconfigure quickly whenever necessary(switching a whole cell to a backup frequency when the current onebecomes too disturbed to be relied on for safe ATM operation)

E-TDMA solution:• the mobile stations broadcast event reports on any serious incident like the non reception of emissions from the ground station(s) • the ground station(s) manage counters and averagers to determine the duration (number of cycles), the extension (number of users) and the intensity (number of slots) of the perturbation, according to its own monitoring activity and the reports sent by the mobiles• the ground station(s) and/or the mobiles use decision thresholds in a sliding observation window to trigger a change of frequency


The E-TDMA ConceptBackup frequency switching protocol

• the cellular frequency plan (including backup frequencies) is a priori known by everybody (eg broadcast on the GSC)

• warm restart: the ground station uses the uplink part of the cycle to broadcast on the new frequency the list of all the mobiles, that confirm their presence in their primary slot, and so in one cycle a normal situation is re-established (normal case)

• cold restart: if the situation seems abnormal (e.g. collisions occur on primary slots) the ground station(s) invite(s) the mobiles to use the insertion-and-echoback protocol (with more Hello mini-slots offered than in the standard situation) in order to come back to a coherent state in a few cycles

E-TDMA solution:


The E-TDMA Concept

The E-TDMA frame (1)

frame (N-1) frame (N) frame (N+1)

the frame duration must not be larger than: either the ADS-B period, or the minimax access time to be 100% guaranteed

• E-TDMA cycle between 2 and 10 seconds (depending on local requirements)

E-TDMA cycle E-TDMA cycle E-TDMA cycle


The E-TDMA ConceptThe E-TDMA frame (2)

propagation guard time(3.3 µs / km+ S3 guard)

individual slot structure

ramp-upand

synchro(1.9 ms) data

CRC and

decay

total slot duration

next slot


The E-TDMA ConceptThe E-TDMA frame (3)

shared secondary slotsfor other messages

(longer and less frequent ones)

exclusive primary slotfor ADS-B and

short urgent messages

QoS0 QoS1 QoS2


The E-TDMA Concept

The E-TDMA frame (4)QoS0 QoS1 QoS2

uplink slots can be left contiguous and the QoS breakdownremains virtual (dynamically managed by the ground station)intermediate resynchronisation beacons may be needed(depending on the drift performance of VDR clocks)the initial guard time may be halved (since the ground stationis at the center of the cell)

SYNCH SYNCH


The E-TDMA Concept

E-TDMA QoS categories

mean transit time = E-TDMA cycle / 2max transit time = E-TDMA cycleperiod = E-TDMA cycle min throughput = slot length / E-TDMA cycle

1°) exclusive primary slot: QoS0

2°) shared secondary slot: QoSi , i = 1, ... nKi slots shared between N aircraft, with an Li average percentage load

mean transit time = (Li / 100) * (N / 2*Ki) * E-TDMA cyclemax transit time = (N / Ki) * E-TDMA cyclemin throughput = slot length / (N * E-TDMA cycle / Ki)available throughput = 100 / Li * min throughputperiod = (N / Ki) * E-TDMA cycle


The E-TDMA ConceptMAC protocol (1)

• every secondary slot of QoSi is shared between all the aircraft that have the same primary slot number modulo Ki, Ki being the maximum number of secondary slots available for QoSi.• when the maximum number of aircraft N is reached, at most N/Ki aircraft may queue up for each slot:

1 2 3 4 5 6 7 ...

741

...52

...63

3 shared secondary slots

E-TDMA deterministic slot assignment scheme:

• relaxing the modulo Ki constraint yields less deterministic solutions which are still completely collision-free owing to this distributed queueing system


The E-TDMA Concept

MAC protocol (2)

• reservation flags are set in the primary slots• the ground station echoes-back the reservations• the reservation order rules are implicit yet unambiguous

E-TDMA distributed per-QoS-scheduling solution:

reservation flagsin the primary slots

shared pool of secondary slots

reservation echo-backa b a

b


The E-TDMA ConceptHandover versus Self-Insertion (1)fully coordinated ground infrastructure:

en route cells(continuous tessellated coverage)

airport-centered cell


The E-TDMA ConceptE-TDMA air-initiated ground-coordinated handover:• the aircraft knows approximately her position (published RNP) and

the cellular structure and she initiates the handover automatically• if the RNP capability is lost, the handover must be initiated manually

station A station B

I'll now switch to station B 1

2give me a slotfor A/C x

A/C x

3

A/C x comesin on slot N

4

on B, you haveslot N, 'bye

Hello, this is A/C x 5


The E-TDMA ConceptHandover versus Self-insertion (2)loosely coordinated ground infrastructure:

en route cells(continuous coverage)

airport-centered cell

boundary


The E-TDMA ConceptSelf-insertion protocol (1)

p-persistent CSMA access schemeon a small set of Hello mini-slotsto be used by candidate aircraft

not handed over by another station

successful insertions are echoed-back by the ground station(s)

p-persistent CSMA/CD and acknowledgment module


The E-TDMA ConceptHandover versus Self-insertion (3)

no ground infrastructure:

coast line

low densitymulti-stationmacro-cell

low density cellswithout ground stations


The E-TDMA Concept

The autonomous mode mustadapt the design principlesof the E-TDMA concept tooperational situations when:

no ground infrastructure exists

the air traffic density is low

the E-TDMA is used only in the air-airlocal mode (broadcast or addressed)


The E-TDMA Concept

Statistical Self-Synchronisation (S3)Fixed E-TDMA cycle in each cellFixed cellular layoutSupport different datalink servicesHigh integrity datalinkDeterministic MAC sublayerSelf-insertion mechanismDistributed QoS monitoring

E-TDMA in the autonomous mode:

features that nedd to be adapted to the autonomous mode


The E-TDMA Concept

The autonomous E-TDMA Traffic Mix

ADS-BASAS-BASAS-C

other air-air services (eg SIGMET-B) Network and QoS Management

services

required "downlink" throughput


The E-TDMA Concept

The distributed round robin scheme• every secondary slot of QoSi is shared between all the aircraft that have the same primary slot number modulo Ki, Ki being the maximum number of secondary slots available for QoSi

• when the maximum number of aircraft N is reached, at most N/Ki aircraft may queue up for a slot:

1 2 3 4 5 6 7 ...

741

...52

...63

3 shared secondary slots


The E-TDMA ConceptSelf-insertion in autonomous mode

Incoming mobiles broadcast arrival messages into free primary slots (no dedicated insertion slots)

Arrival messages are re-emitted by other mobilesacross the whole cell (echo-back or handovers bymeans of ground stations are not available)

A rebroadcast counter is decremented, to stop thepropagation process after a finite number of hops

Collisions between the cycle-simultaneous arrivalsare also (re)broadcast by the other mobiles, with acollision rebroadcast counter, allowing to sort outthe collisions between non-simultaneous arrivals


The E-TDMA Concept

Distributed QoS MonitoringThe primary slots carry additionalSystem Management information:

a slot occupancy bitmap (as consolidated by the mobile)

fields for broadcasting short messages that describe some special anomalous events, as detected by the receiver part: unexpected loss of air-air connectivity with another mobile, erroneous data transmission, severe signal jamming...

every mobile monitors its environment, and it may trigger QoS alarms (broadcast to the other mobiles, and also sent to the cockpit) when some threshold is crossed (eg error rate)


The E-TDMA ConceptPropagation scheme for self-insertion

autonomous E-TDMA cell requiring 5 propagation hops

the incomingaircraft

broadcastsan arrivalmessage

in a number of free slots

C=4

C=3C=2 C=1

C=0

the arrival is notified 5 cycles later at the otherend of the cell


The E-TDMA Concept

Back-propagation of collisions (1)

C=4

C=3C=2 C'=2

CC=2

CC=1CC= 0

CC=2

incomingaircraftloses

the slot

C'=4C'=3

CC=1CC=0

incomingaircraftloses

the slot

CC=1CC=2

shortest path for collision rebroadcast


The E-TDMA Concept

shortest path for the collision rebroadcast

the earliestincomingaircraftretains the slot

(the collisionrebroadcast

does notreach her)

C=4

C=3C=2 C=1

Back-propagation of collisions (2)

the latestincomingaircraftloses

the slot

C'=4

C'=3

CC=1CC=0CC=1CC=0

CC=0CC=1

Documents

SOFREAVIA ATM Department Presentation to RTCA/SC 172, 15 april 98, & AMCP-WG/C2-WP26 7-11 May 2001 # 1 The E-TDMA Concept Towards a new VDL strategy Some