Towards a new VDL strategy Some key issues & possible way forward

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 needed to provide an efficient general purpose datalink

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

Key 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)

Provide a sustainable migration path (1) Mode S extended squitter and STDMA for ADS-B E-TDMA for AOC, ATSC ADS-B and ASAS-C VDL Mode 2 for AOC and ATSC ACARS for AOC and some ATSC 2000 2005 2010 2015 2020 1 channels possibly available for VDL in Europe 2 or More ?

limit 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...)

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

Comparison with VDL Mode 3 & 4 (*) Percentage of a channel required to meet the QoS requirements (European Scenarios) 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%) 160 NM radius, 570 aircrafts max, 95% satisfied transfer delay, D8PSK modulation Solution 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 (*) 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)

Provide a sustainable migration path (2) VHF band for aeronautical communications (25 kHz channels) data channels today Protected sub-band for 8.33 kHz voice channels 2005 25 kHz channels freed by the migration into the sub-band 2010+ Protected E-TDMA clusters (to minimise the interference problems channels used by adjacent cells should belong to different clusters)

Hybrid ATM concept combining global (re-)planning and local autonomy air-air data exchange (ADS-B + ASAS) local semi-autonomous conflict-solving air-ground ATN links for ATC and tactical re-planning discrete rendezvous points defining a 4D Flight Contract

The foreseen E-TDMA Traffic Mix required downlink throughput required uplink throughput mobile-originated emissions ATSC & AOC ADS-B emissions from ASAS the ground station(s) ATSC & AOC

forward listening to adjacent cells Limit the aircraft equipment cost: the autonomous aircraft equippage upgrade for an air-air ASAS/ADS-B capability consists of 2 additional receivers tuned on downstream cells: forward listening to adjacent cells talking and listening in the current cell

Summary description of some features proposed in the E-TDMA study

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 µs E-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

Statistical 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

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

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 =>TRANSPORT NACK AND RETRANSMISSION ERROR =>LL NACK AND RETRANSMISSION ES IS IS ES

performance 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 service interface of the E-TDMA subnetwork (allowing the ATN routers to establish SVCs according to QoS) SVC2 (QoS3) ground router SVC1 (QoS2) aircraft router SVC3 (QoS1)

Modular error correction an E-TDMA slot would combine only a few different codes defined according to the target Residual Error Rates (RER): 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 example for a small slot: B0 + 3*B1 => 151 data bits + 69 CRC bits CRC overhead = 45 % (worst case based on BER = 10-3) example for a larger slot: B0 + 9*B2 => 1051 data bits + 335 CRC bits CRC overhead = 30 % (worst case based on BER = 10-3)

support different datalink services: globally addressed applications locally addressed applications ATN IP broadcast applications (sub)network Layer broadcast LL sub-layer MAC sub-layer SYNCH sub-layer Physical Layer

QoS monitoring : E-TDMA solution: there is a need to monitor and report all the problems so as to allow the E-TDMA system to self-reconfigure quickly whenever necessary (switching a whole cell to a backup frequency when the current one becomes 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

Backup frequency switching protocol E-TDMA solution: 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

The E-TDMA frame (1) frame (N-1) frame (N) frame (N+1) E-TDMA cycle E-TDMA cycle E-TDMA cycle 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)

The E-TDMA frame (2) data individual slot structure next slot propagation guard time (3.3 µs / km + S3 guard) ramp-up and synchro (1.9 ms) CRC and decay next slot data total slot duration

The E-TDMA frame (3) QoS0 QoS1 QoS2 exclusive primary slot for ADS-B and short urgent messages shared secondary slots for other messages (longer and less frequent ones)

The E-TDMA frame (4) QoS0 QoS1 QoS2 SYNCH SYNCH uplink slots can be left contiguous and the QoS breakdown remains 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 station is at the center of the cell)

E-TDMA QoS categories 1°) exclusive primary slot: QoS0 mean transit time = E-TDMA cycle / 2 max transit time = E-TDMA cycle period = E-TDMA cycle min throughput = slot length / E-TDMA cycle 2°) shared secondary slot: QoSi , i = 1, ... n Ki slots shared between N aircraft, with an Li average percentage load mean transit time = (Li / 100) * (N / 2*Ki) * E-TDMA cycle max transit time = (N / Ki) * E-TDMA cycle min throughput = slot length / (N * E-TDMA cycle / Ki) available throughput = 100 / Li * min throughput period = (N / Ki) * E-TDMA cycle

MAC protocol (1) E-TDMA deterministic slot assignment 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 each slot: 7 4 1 ... 5 2 ... 6 3 1 2 3 4 5 6 7 ... 3 shared secondary slots relaxing the modulo Ki constraint yields less deterministic solutions which are still completely collision-free owing to this distributed queueing system

MAC protocol (2) E-TDMA distributed per-QoS-scheduling solution: reservation flags are set in the primary slots the ground station echoes-back the reservations the reservation order rules are implicit yet unambiguous reservation echo-back b a b a reservation flags in the primary slots shared pool of secondary slots

Handover versus Self-Insertion (1) fully coordinated ground infrastructure: en route cells (continuous tessellated coverage) airport-centered cell

E-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 I'll now switch to station B Hello, this is A/C x A/C x 1 5 on B, you have slot N, 'bye A/C x comes in on slot N 4 3 give me a slot for A/C x 2 station A station B

Handover versus Self-insertion (2) loosely coordinated ground infrastructure: en route cells (continuous coverage) airport-centered cell boundary

Self-insertion protocol (1) successful insertions are echoed-back by the ground station(s) p-persistent CSMA access scheme on a small set of Hello mini-slots to be used by candidate aircraft not handed over by another station p-persistent CSMA/CD and acknowledgment module

without ground stations Handover versus Self-insertion (3) no ground infrastructure: low density cells without ground stations low density multi-station macro-cell coast line

The autonomous mode must adapt the design principles of the E-TDMA concept to operational situations when: no ground infrastructure exists the air traffic density is low the E-TDMA is used only in the air-air local mode (broadcast or addressed)

E-TDMA in the autonomous mode: Statistical Self-Synchronisation (S3) Fixed E-TDMA cycle in each cell Fixed cellular layout Support different datalink services High integrity datalink Deterministic MAC sublayer Self-insertion mechanism Distributed QoS monitoring features that nedd to be adapted to the autonomous mode

The autonomous E-TDMA Traffic Mix required "downlink" throughput ADS-B ASAS-B ASAS-C other air-air services (eg SIGMET-B) Network and QoS Management services

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: 7 4 1 ... 5 2 ... 6 3 1 2 3 4 5 6 7 ... 3 shared secondary slots

Self-insertion in autonomous mode Incoming mobiles broadcast arrival messages into free primary slots (no dedicated insertion slots) Arrival messages are re-emitted by other mobiles across the whole cell (echo-back or handovers by means of ground stations are not available) A rebroadcast counter is decremented, to stop the propagation process after a finite number of hops Collisions between the cycle-simultaneous arrivals are also (re)broadcast by the other mobiles, with a collision rebroadcast counter, allowing to sort out the collisions between non-simultaneous arrivals

Distributed QoS Monitoring The primary slots carry additional System 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)

Propagation scheme for self-insertion the arrival is notified 5 cycles later at the other end of the cell the incoming aircraft broadcasts an arrival message in a number of free slots C=4 C=0 C=3 C=2 C=1 autonomous E-TDMA cell requiring 5 propagation hops

Back-propagation of collisions (1) incoming aircraft loses the slot incoming aircraft loses the slot CC=2 CC=2 CC= 0 CC=1 CC=1 C=4 CC=2 CC=0 CC=1 C=3 C'=4 C'=3 C=2 C'=2 shortest path for collision rebroadcast

Back-propagation of collisions (2) the earliest incoming aircraft retains the slot (the collision rebroadcast does not reach her) the latest incoming aircraft loses the slot CC=0 CC=1 C=4 CC=1 CC=0 CC=0 CC=1 C'=4 C=3 C=2 C=1 C'=3 shortest path for the collision rebroadcast