7. L-Band Digital Aeronautical Communications System (LDACS)
This section preserves the RFC text for RAW technologies, including Wi-Fi 6/7, IEEE 802.11, TSCH, 6TiSCH, 5G NR, TSN/TSC integration, UE, gNB, RAN, UPF, PDU sessions, LDACS, PHY/MAC terms, figures, tables, and security considerations.
Original RFC Text
7. L-Band Digital Aeronautical Communications System (LDACS)
One of the main pillars of the modern Air Traffic Management (ATM)
system is the existence of a communication infrastructure that
enables efficient aircraft guidance and safe separation in all phases
of flight. Although current systems are technically mature, they
suffer from the VHF band's increasing saturation in high-density
areas and the limitations posed by analog radio. Therefore, aviation
(globally and in the European Union (EU) in particular) strives for a
sustainable modernization of the aeronautical communication
infrastructure.
In the long term, ATM communication shall transition from analog VHF
voice and VHF Digital Link (VDL) Mode 2 communication to more
spectrum-efficient digital data communication. The European ATM
Master Plan foresees this transition to be realized for terrestrial
communications by the development and implementation of the L-band
Digital Aeronautical Communications System (LDACS).
LDACS has been designed with applications related to the safety and
regularity of the flight in mind. It has therefore been designed as
a deterministic wireless data link (as far as possible).
It is a secure, scalable, and spectrum-efficient data link with
embedded navigation capability; thus, it is the first truly
integrated Communications, Navigation, and Surveillance (CNS) system
recognized by the International Civil Aviation Organization (ICAO).
During flight tests, the LDACS capabilities have been successfully
demonstrated. A viable rollout scenario has been developed, which
allows gradual introduction of LDACS with immediate use and revenues.
Finally, ICAO is developing LDACS standards to pave the way for the
future.
LDACS shall enable IPv6-based air-ground communication related to the
safety and regularity of the flight. The particular challenge is
that no new frequencies can be made available for terrestrial
aeronautical communication. It was thus necessary to develop
procedures to enable the operation of LDACS in parallel with other
services in the same frequency band; see [RFC9372] for more
information.
7.1. Provenance and Documents
The development of LDACS has already made substantial progress in the
Single European Sky ATM Research (SESAR) framework, and it is
currently being continued in the follow-up program, SESAR2020
[RIH18]. A key objective of the SESAR activities is to develop,
implement, and validate a modern aeronautical data link able to
evolve with aviation needs over the long term. To this end, an LDACS
specification has been produced [GRA19] and is continuously updated;
transmitter demonstrators were developed to test the spectrum
compatibility of LDACS with legacy systems operating in the L-band
[SAJ14], and the overall system performance was analyzed by computer
simulations, indicating that LDACS can fulfill the identified
requirements [GRA11].
LDACS standardization within the framework of the ICAO started in
December 2016. The ICAO standardization group has produced an
initial Standards and Recommended Practices (SARPs) document
[ICAO18]. The SARPs document defines the general characteristics of
LDACS.
Up to now, the LDACS standardization has been focused on the
development of the Physical (PHY) layer and the data link layer; only
recently have higher layers come into the focus of the LDACS
development activities. There is currently no "IPv6 over LDACS"
specification; however, SESAR2020 has started the testing of
IPv6-based LDACS testbeds. The IPv6 architecture for the
aeronautical telecommunication network is called the Future
Communications Infrastructure (FCI). FCI shall support QoS,
diversity, and mobility under the umbrella of the "multi-link
concept". This work is conducted by the ICAO WG-I Working Group.
In addition to standardization activities, several industrial LDACS
prototypes have been built. One set of LDACS prototypes has been
evaluated in flight trials, confirming the theoretical results
predicting the system performance [GRA18] [BEL22] [GRA23].
7.2. General Characteristics
LDACS will become one of several wireless access networks connecting
aircraft to the Aeronautical Telecommunications Network (ATN). The
LDACS access network contains several ground stations, each of which
provides one LDACS radio cell. The LDACS air interface is a cellular
data link with a star topology connecting aircraft to ground stations
with a full duplex radio link. Each ground station is the
centralized instance controlling all air-ground communications within
its radio cell.
The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the
forward link (FL) and 294 kbit/s to 1390 kbit/s on the reverse link
(RL), depending on coding and modulation. Due to strong interference
from legacy systems in the L-band, the most robust coding and
modulation should be expected for initial deployment, i.e., 315 kbit/
s on the FL and 294 kbit/s on the RL.
In addition to the communications capability, LDACS also offers a
navigation capability. Ranging data, similar to Distance Measuring
Equipment (DME), is extracted from the LDACS communication links
between aircraft and LDACS ground stations. This results in LDACS
providing an Alternative Position, Navigation, and Timing (APNT)
capability to supplement the existing on-board Global Navigation
Satellite System (GNSS) without the need for additional bandwidth.
Operationally, there will be no difference for pilots whether the
navigation data are provided by LDACS or DME. This capability was
flight tested and proven during the MICONAV flight trials in 2019
[BAT19].
In previous works and during the MICONAV flight campaign in 2019, it
was also shown that LDACS can be used for surveillance capability.
Filip et al. [FIL19] have shown the passive radar capabilities of
LDACS, and Automatic Dependence Surveillance - Contract (ADS-C) was
demonstrated via LDACS during the flight campaign 2019 [SCH19].
Since LDACS has been mainly designed for air traffic management
communication, it supports mutual entity authentication, integrity
and confidentiality capabilities of user data messages, and some
control channel protection capabilities [MAE18] [MAE191] [MAE192]
[MAE20].
Overall, this makes LDACS the world's first truly integrated CNS
system and is the most mature, secure, and terrestrial long-range CNS
technology for civil aviation worldwide.
7.3. Deployment and Spectrum
LDACS has its origin in merging parts of the B-VHF [BRA06], B-AMC
[SCH08], TIA-902 (P34) [HAI09], and WiMAX IEEE 802.16e [EHA11]
technologies. In 2007, the spectrum for LDACS was allocated at the
World Radio Conference (WRC).
It was decided to allocate the spectrum next to Distance Measuring
Equipment (DME), resulting in an in-lay approach between the DME
channels for LDAC [SCH14].
LDACS is currently being standardized by ICAO and several rollout
strategies are discussed.
The LDACS data link provides enhanced capabilities to existing
aeronautical communications infrastructures, enabling them to better
support user needs and new applications. The deployment scalability
of LDACS allows its implementation to start in areas where it is most
needed to immediately improve the performance of and already-fielded
infrastructure. Later, the deployment is extended based on
operational demand. An attractive scenario for upgrading the
existing VHF communication systems by adding an additional LDACS data
link is described below.
When considering the current VDL Mode 2 infrastructure and user base,
a very attractive win-win situation comes about when the
technological advantages of LDACS are combined with the existing VDL
Mode 2 infrastructure. LDACS provides at least 50 times more
capacity than VDL Mode 2 and is a natural enhancement to the existing
VDL Mode 2 business model. The advantage of this approach is that
the VDL Mode 2 infrastructure can be fully reused. Beyond that, it
opens the way for further enhancements [ICAO19].
7.4. Applicability to Deterministic Flows
As LDACS is a ground-based digital communications system for flight
guidance and communications related to safety and regularity of
flight, time-bounded deterministic arrival times for safety critical
messages are a key feature for its successful deployment and rollout.
7.4.1. System Architecture
Up to 512 Aircraft Stations (ASes) communicate to an LDACS Ground
Station (GS) in the reverse link (RL). A GS communicates to an AS in
the forward link (FL). Via an Access-Router (AC-R), GSs connect the
LDACS subnetwork to the global Aeronautical Telecommunications
Network (ATN) to which the corresponding Air Traffic Services (ATS)
and Aeronautical Operational Control (AOC) end systems are attached.
7.4.2. Overview of the Radio Protocol Stack
The protocol stack of LDACS is implemented in the AS and GS; it
consists of the Physical (PHY) layer with five major functional
blocks above it. Four are placed in the data link layer (DLL) of the
AS and GS:
1. Medium Access Layer (MAC),
2. Voice Interface (VI),
3. Data Link Service (DLS), and
4. LDACS Management Entity (LME).
The last entity resides within the subnetwork layer: the Subnetwork
Protocol (SNP). The LDACS network is externally connected to voice
units, radio control units, and the ATN network layer.
Communications between the MAC and LME layers is split into four
distinct control channels:
1. the Broadcast Control Channel (BCCH), where LDACS ground stations
announce their specific LDACS cell, including physical parameters
and cell identification;
2. the Random Access Channel (RACH), where LDACS airborne radios can
request access to an LDACS cell;
3. the Common Control Channel (CCCH), where LDACS ground stations
allocate resources to aircraft radios, enabling the airborne side
to transmit the user payload; and
4. the Dedicated Control Channel (DCCH), where LDACS airborne radios
can request user data resources from the LDACS ground station so
the airborne side can transmit the user payload.
Communications between the MAC and DLS layers is handled by the Data
Channel (DCH) where the user payload is handled.
Figure 10 shows the protocol stack of LDACS as implemented in the AS
and GS.
IPv6 Network Layer
|
|
+------------------+ +----+
| SNP |--| | Subnetwork
| | | | Layer
+------------------+ | |
| | LME|
+------------------+ | |
| DLS | | | Logical Link
| | | | Control Layer
+------------------+ +----+
| |
DCH DCCH/CCCH
| RACH/BCCH
| |
+--------------------------+
| MAC | Medium Access
| | Layer
+--------------------------+
|
+--------------------------+
| PHY | Physical Layer
+--------------------------+
|
|
((*))
FL/RL radio channels
separated by
frequency division duplex
Figure 10: LDACS Protocol Stack in AS and GS
7.4.3. Radio (PHY)
The PHY layer provides the means to transfer data over the radio
channel. The LDACS ground station supports bidirectional links to
multiple aircraft under its control. The FL direction (which is
ground to air) and the RL direction (which is air to ground) are
separated by frequency division duplex. FL and RL use a 500 kHz
channel each. The ground station transmits a continuous stream of
OFDM symbols on the FL. In the RL, different aircrafts are separated
in time and frequency using a combination of Orthogonal Frequency-
Division Multiple Access (OFDMA) and Time-Division Multiple-Access
(TDMA). Thus, aircraft transmit discontinuously on the RL with radio
bursts sent in precisely defined transmission opportunities allocated
by the ground station. The most important service on the PHY layer
of LDACS is the PHY time framing service, which indicates that the
PHY layer is ready to transmit in a given slot and indicates PHY
layer framing and timing to the MAC time framing service. LDACS does
not support beam-forming or Multiple Input Multiple Output (MIMO).
7.4.4. Scheduling, Frame Structure, and QoS (MAC)
The data link layer provides the necessary protocols to facilitate
concurrent and reliable data transfer for multiple users. The LDACS
data link layer is organized in two sublayers: the medium access
sublayer and the logical link control sublayer. The medium access
sublayer manages the organization of transmission opportunities in
slots of time and frequency. The logical link control sublayer
provides acknowledged point-to-point logical channels between the
aircraft and the ground station using an automatic repeat request
protocol. LDACS also supports unacknowledged point-to-point channels
and ground-to-air broadcast.
Next, the frame structure of LDACS is introduced, followed by a more
in-depth discussion of the LDACS medium access.
The LDACS framing structure for FL and RL is based on Super-Frames
(SF) of 240 ms duration. Each SF corresponds to 2000 OFDM symbols.
The FL and RL SF boundaries are aligned in time (from the view of the
GS).
In the FL, an SF contains a broadcast frame with a duration of 6.72
ms (56 OFDM symbols) for the Broadcast Control Channel (BCCH) and
four Multi-Frames (MF), each with a duration of 58.32 ms (486 OFDM
symbols).
In the RL, each SF starts with a Random Access (RA) slot with a
length of 6.72 ms with two opportunities for sending RL random access
frames for the Random Access Channel (RACH), followed by four MFs.
These MFs have the same fixed duration of 58.32 ms as in the FL but a
different internal structure.
Figures 11 and 12 illustrate the LDACS frame structure. This fixed
frame structure allows for the reliable and dependable transmission
of data.
^
| +------+------------+------------+------------+------------+
| FL | BCCH | MF | MF | MF | MF |
F +------+------------+------------+------------+------------+
r <---------------- Super-Frame (SF) - 240 ms --------------->
e
q +------+------------+------------+------------+------------+
u RL | RACH | MF | MF | MF | MF |
e +------+------------+------------+------------+------------+
n <---------------- Super-Frame (SF) - 240 ms --------------->
c
y
|
----------------------------- Time ------------------------------>
|
Figure 11: SF Structure for LDACS
^
| +-------------+------+-------------+
| FL | DCH | CCCH | DCH |
F +-------------+------+-------------+
r <--- Multi-Frame (MF) - 58.32 ms -->
e
q +------+---------------------------+
u RL | DCCH | DCH |
e +------+---------------------------+
n <--- Multi-Frame (MF) - 58.32 ms -->
c
y
|
-------------------- Time ------------------>
|
Figure 12: MF Structure for LDACS
Next, the LDACS medium access layer is introduced.
LDACS medium access is always under the control of the ground station
of a radio cell. Any medium access for the transmission of user data
has to be requested with a resource request message stating the
requested amount of resources and class of service. The ground
station performs resource scheduling on the basis of these requests
and grants resources with resource allocation messages. Resource
request and allocation messages are exchanged over dedicated
contention-free control channels.
LDACS has two mechanisms to request resources from the scheduler in
the ground station. Resources can either be requested "on demand" or
permanently allocated by a LDACS ground station with a given class of
service. On the FL, this is done locally in the ground station; on
the RL, a dedicated contention-free control channel is used (the
Dedicated Control Channel (DCCH); roughly 83 bits every 60 ms). A
resource allocation is always announced in the control channel of the
FL (Common Control Channel (CCCH); variable sized). Due to the
spacing of the RL control channels of every 60 ms, a medium access
delay in the same order of magnitude is to be expected.
Resources can also be requested "permanently". The permanent
resource request mechanism supports requesting recurring resources at
given time intervals. A permanent resource request has to be
canceled by the user (or by the ground station, which is always in
control). User data transmissions over LDACS are therefore always
scheduled by the ground station, while control data uses statically
(i.e., at net entry) allocated recurring resources (DCCH and CCCH).
The current specification documents specify no scheduling algorithm.
However, performance evaluations so far have used strict priority
scheduling and round robin for equal priorities for simplicity. In
the current prototype implementations, LDACS classes of service are
thus realized as priorities of medium access and not as flows. Note
that this can starve out low-priority flows. However, this is not
seen as a big problem since safety-related messages always go first
in any case. Scheduling of RL resources is done in physical Protocol
Data Units (PDU) of 112 bits (or larger if more aggressive coding and
modulation is used). Scheduling on the FL is done byte wise since
the FL is transmitted continuously by the ground station.
In order to support diversity, LDACS supports handovers to other
ground stations on different channels. Handovers may be initiated by
the aircraft (break before make) or by the ground station (make
before break). Beyond this, FCI diversity shall be implemented by
the multi-link concept.