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

2. 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

3. 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

4. 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)

5. 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 1
channels possibly available for VDL in Europe
2 or More ?

6. 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...)

7. 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

8. 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)

9. 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)

10. 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

11. 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

12. 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

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

14. 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

15. 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

16. 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

17. 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

18. 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)

19. 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 =
BCH (31, 16)
small slots B1:
RER =
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 CRC bits
CRC overhead = 30 % (worst case based on BER = 10-3)

20. 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

21. 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

22. 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

23. 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)

24. 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

25. 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)

26. 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)

27. 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

28. 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
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

29. 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

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

31. 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

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

33. 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

34. 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

35. 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)

36. 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

37. 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

38. 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
3 shared secondary slots

39. 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

40. 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)

41. 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

42. 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

43. 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