2.2. Emerging communication systems
2.2.2. Future aeronautical communication
As mentioned in section 2.1.1, AeroMACS is the new aviation-dedicated transmission technology based on the WiMAX IEEE 802.16e standard. The aim is to support safety and regularity of flight communications at the airport surface. The AeroMACS technology allows Mobile Stations (MSs), such as aircraft or surface vehicles, to communicate with airline operators and airport staff at three different surface zones: RAMP (where the aircraft is at the gate before departure), GROUND (the aircraft is taxing to the runway) and TOWER (until the aircraft takes-off).
Using a WiMAX-based technology standard is profitable for the aviation industry for many reasons. First, the current standardization and deployment processes are fast and cost-effective as opposed to a newly developed standard simply for the sake of airport communications. Moreover, the scientific community has been working on IEEE 802.16 standards for many years. Highly qualified certification agencies, such as the WiMAX Forum, are continuously looking out for interoperability and technical issues related to the standard. The AeroMACS standard is currently a hot topic in data link communications, and many tests are already running for a future deployment. Recently, the RTCA SC-223 and EUROCAE WG-82 have jointly defined an AeroMACS profile that is intended to fulfil performance requirements for the system implementation. The AeroMACS profile is discussed in section 2.2.2.1.2.
2.2.2.1.1. AeroMACS network architecture
Figure 2.1 illustrates the AeroMACS reference network architecture where logical domains, functional entities and Reference Points (RPs) appear. The main functional entities are MSs (i.e. aircraft and surface vehicles), the Access Service Network (ASN) and Connectivity Service Network (CSN) networks. The ASN network represents the boundary for functional interoperability between MSs and AeroMACS connectivity services. It integrates many functions, such as forwarding Authorization, Authentication, and Accounting (AAA) messages between MSs and the Home Network service Provider (H-NSP), or relaying network service messages (e.g. Dynamic Host Configuration Protocol (DHCP) request/response). The CSN network provides connectivity to public networks, such as the Internet.
The logical domains, which are basically a set of functions associated with a single domain, and considered in the network reference architecture, are the Network Access Point (NAP), and the Visited Network Service Provider (V-NSP). The NAP is the physical point used by MSs to access the network. The H-NSP is the AeroMACS service provider, which provides Service Level Agreement (SLA) to the MSs, such as IP connectivity and core network services. These NSPs could be, for instance, Société Internationale de Télécommunications Aéronautiques (SITA), Aeronautical Radio Incorporated (ARINC) or even the airlines depending on the provided service. The V-NSP is visited by the MSs to access the network in a roaming scenario (which usually depends on the roaming agreement made between the MS’s H-NSP and the V-NSP). RPs are the communication end points between functional entities and represent the interfaces that ensure the interoperability between AeroMACS-related components. RPs are described in Table 2.1.
RP Interface Functionality R1 Between MSs and BSs Air interface R2 Between MSs and the CSN IP host configuration
IP mobility
Security (Authentication, Authorization) R3 Between the ASN and the CSN Mobility management
Tunneling
Security (Authentication, Authorization) R4 Between ASNs Mobility management
ASNs interworking
R5 Between CSNs Roaming between V-NSP and the H-NSP Table 2.1. AeroMACS reference point interface description
2.2.2.1.2. AeroMACS profile
AeroMACS can be seen as an instantiation profile of WiMAX, as it has been defined for a specific application scenario, resulting in a subset of selected options from the IEEE 802.16-2009 features with slight modifications in terms of allocation schemes, handover management or frequency range. The ED-222 AeroMACS profile was released by EUROCAE in November 2013 [EUR 13a]. The document was prepared
jointly by EUROCAE WG 82, “Mobile Radio Communication: Airport Surface Radio Link”, and RTCA SC-223, “Aeronautical Mobile Airport Communication System”. ED-222 specifies requirements for the unique adaptions of the current IEEE 802.16-2009 standard to provide AeroMACS communications in airport surface.
Figure 2.1. AeroMACS network reference model
A special Aviation Working Group (AWG) has been incorporated within the WiMAX forum, with the aim of guaranteeing that equipment vendors, manufacturers and the aviation industry are able to support the AeroMACS development. The AeroMACS profile document shows that the WiMAX Release 1.0 Version 0.9 (WMF-T23-001-R010v09) [WIM 08] can be used to operate in an aviation-oriented profile without any technical obstacles.
Besides, the profile has been updated due to outcomes of the SESAR WP 15.2.7.
AeroMACS operational applications
According to the AeroMACS profile, user applications have been classified into five different functional domain categories:
– ATM and ATC;
– Aeronautical Information Services (AISs) and MET (Meteorological Data);
– Aircraft owners and operators;
– Airport authorities;
– Airport infrastructures.
Applications belonging to these functional domains are currently under investigation. For instance, Digital Notice to Airmen (D-NOTAM), which is an application used by government agencies to alert pilots of hazards in the NAS, has been identified as a strong candidate for transport over AeroMACS. Also, applications transported through legacy systems, such as ACARS, are also candidates for implementation over AeroMACS; examples are Pre-Departure Clearance (PDC) and Digital Automatic Terminal Information System (D-ATIS). An exhaustive list of final applications to be used over AeroMACS is currently defined under FAA and EUROCONTROL responsibilities.
AeroMACS protocol reference model
Figure 2.2 illustrates the protocol stack model of AeroMACS. The Medium Access Control (MAC) layer is composed of three different sublayers. The role of the service-specific Convergence Sublayer (CS) is to map external network data received from higher layers through the CS Service Access Point (SAP) into MAC Service Data Units (SDUs) and transfer it to the MAC Common Part Sublayer (CPS) through the MAC SAP.
The management of SDUs includes classification and mapping to the relevant MAC Service Flow Identifier (SFID) and Connection Identifier (CID). Multiple CS specifications can be provided in order to support interfacing with various protocols at higher layers, such as IPv4 and IPv6.
The MAC CPS sublayer provides the main MAC functionalities, meaning system access, bandwidth allocation, connection establishment and connection maintenance. It receives classified data (according to their CIDs) from the various CSs through the MAC SAP. The MAC CPS is also responsible for applying QoS to the transmission and scheduling of data over the PHY layer. Unlike the IEEE 802.11 standard (WiFi) where security was not a built-in feature, a complete security framework defined at the MAC
CPS through a specific sublayer. Several security services are addressed such as user authentication, authorization, key establishment and data confidentiality. Finally, data and PHY control messages are transferred between the MAC CPS and PHY layer via the PHY SAP.
Figure 2.2. AeroMACS protocol reference model
AeroMACS physical layer
Compared to the IEEE 802.16-2009 standard, which can operate in several bandwidths in order to support the data throughput and capacity, the AeroMACS profile mentions a potential operation band between 5,000 and 5,150 MHz in channels of a 5 MHz bandwidth with a Fast Fourier Transform (FFT) sizes of 512 points. Two lateral frequency bands are left unused; thus, there are 29 potential channels with nominal center frequencies ranging from 5,005 to 5,145 MHz. The last central frequency (i.e.
5,145 MHz) is selected as the reference frequency. These central frequencies are the preferred ones for AeroMACS operations. However, in order to avoid interferences with other systems operating in the same band, such as the future AMT system provided by Airbus, the AeroMACS mobile equipment
will be able to operate at center frequencies with a 250 KHz offset from these preferred frequencies.
In order to protect against interferences with adjacent bands, AeroMACS unique PHY profile includes network synchronization settings and transmitter output spectral mask. Current International Telecommunications Union (ITU) allocations allow for AM(R)S operations in bands including the 5,091–5,150 MHz band. The spectrum mask requirement, which has been taken from Title 47 Code of Federal Regulations (CFR) part 90.210 [FCC 14], is based on the emission mask “M” for all power levels authorized for the AeroMACS service. Figure 2.3 shows a typical spectral mask taken from the latest AeroMACS Minimum Operational Performance Standards (MOPSs) [EUR 13a]. In general, frequency masks drop from a few dB below the maximum level at the edge of the band in use to a fixed level of typically 50–60 dB below the maximum level one channel width away from the edge of the band.
Figure 2.3. AeroMACS spectral mask according to the MOPS [EUR 13b]
AeroMACS is operational with Doppler velocities of up to 50 nautical miles per hour. Its PHY layer is based on Orthogonal Frequency Division Multiple Access (OFDMA), which is a multicarrier technique offering
Normalized frequency
Relative power spectral density [dB]
mitigation capabilities for frequency-dependent distortions. OFDMA also simplifies the equalization in a multipath fading environment, which is quite useful for the AeroMACS system due to the airport propagation channel characteristics. Physical resources are distributed among users according to the Partially Used Subcarriers (PUSCs) scheme, which assigns data regions to subchannels following a pseudo-random permutation. In order to provide more efficient support of asymmetric traffic, AeroMACS will use time- division multiplexing with a fixed frame length equal to 5 ms.
The profile supports multiple modulation and coding schemes, enabling effective adaptation to the channel parameters and users needs. Examples of supported modulation schemes are Quadrature Phase Shift Keying QPSK, 16 Quadrature Amplitude Modulation (QAM) and 64 QAM (which is optional in the uplink). Convolutional codes and convolutional turbo codes are also supported with coding rates of ½, 23 and ¾. According to the propagation conditions of the communication channel, the appropriate modulation and coding schemes are selected.
AeroMACS MAC layer provides access to the channel using a joint Time Division Multiple Access (TDMA) and OFDMA duplexing schemes. In each time slot, a group of contiguous subcarriers is assigned to each user. A slot is composed of 48 data subcarriers, but it differs from UpLink (UL) and DownLink (DL). In DL, a slot is composed of two clusters containing 14 subcarriers per symbol over two symbols with data and pilot allocation (48 data and eight pilot subcarriers per slot). In UL, a slot is composed of six tiles; each tile spans three symbols and four subcarriers, and contains eight data and four pilot subcarriers. A Cyclic Prefix (CP) of 1/8 of the symbol time is adopted to avoid intersymbol interference.
Figure 2.4 shows the AeroMACS frame with an adaptive DL/UL subframe width.
Figure 2.4. AeroMACS frame with an adaptive DL/UL subframe width
Table 2.2 summarizes the AeroMACS PHY parameters as described in the AeroMACS profile and MOPS documents.
Parameters Description Multiple access scheme OFDMA
Frequency band 5,000–5,150 MHz Reference frequency 5,145 MHz
Bandwidth 5 MHz
FTT size 512
Sampling factor 28/25
Base station antenna gain 13 dBi including 2 dBi cable loss Mobile station antenna gain 6 dBi
Sampling frequency 11.2 MHz Subcarrier spacing 10.94 KHz
Frame time 5 ms
Modulation schemes QPSK, 16QAM, 64QAM Coding schemes Convolutional, Turbo
Table 2.2. AeroMACS system parameters
AeroMACS MAC layer
As for IEEE 802.16-2009, AeroMACS provides several options for fragmenting and reassembling MAC SDUs. The profile mentions variable and fixed lengths for MAC SDUs. Each MAC PDU contains a fixed 6 bytes MAC header, a variable payload and a 4 bytes Cyclic Redundancy Checksum (CRC). The MAC header contains the connection CID with which the PDU is mapped. Automatic Repeat Request (ARQ) is required in AeroMACS, as it is used to synchronize data flows between sending and receiving entities throughout an Acknowledgment (ACK) mechanism. The AeroMACS standard relies on four different types of ARQ, namely cumulative ARQ, selective-reject and two combinations of cumulative ARQ and selective-reject. ARQ messages are a 4–12 bytes in size and contain both the CID and Block Sequence Number (BSN).
Unlike the IEEE 802.16-2009 standard, which offers several ARQ ACK types, the AeroMACS profile does not support all of them. Only type 1 and 2 are supported and even mandated by the standard:
– ACK type 1 is relevant to cumulative ACK entry. It uses the BSN to acknowledge all the received ARQ blocks cumulatively and has a fixed 4 bytes size;
– ACK type 2 is relevant to cumulative/selective ACK entry, which is a combination between ARQ ACK type 0 (not supported by AeroMACS) and type 1.
A scheduling service is available in order to provide bandwidth to MSs for UL transmissions or possibilities to request bandwidth. BSs perform UL requests and grant scheduling using a specific scheduling type and its associated QoS parameters. Thus, each BS is able to anticipate the throughput and latency needs for the UL traffic and provide polls/grants at the appropriate time. The supported scheduling services are:
– Unsolicited Grand Service (UGS), relevant to real-time applications with fixed bit rates;
– real-time Polling Service (rtPS), intended to real-time applications with variable bit rates;
– non-real-time Polling Service (nrtPS), for applications requiring a guaranteed data bit rate and is tolerant to delays;
– Best Effort (BE), served to applications that do not have any particular QoS requirements.
2.2.2.2. L-DACS
L-DACS is intended to provide communication links with a medium range corresponding to the en-route and TMA domain (typically 200 nm).
The principle is to rely on a cellular network operating at L-band, i.e. around 1 GHz. L-band is a compromise between radio resource availability and communication range. The main concern, however, is that these bands are already crowded, and a sole coprimary allocation has been obtained from the ITU in 2007. L-DACS must thus be designed so that the signals do not interfere with other services, mainly navigation and surveillance systems. In order to overcome this constraint, two proposals are under investigation, denoted L-DACS1 and L-DACS2. L-DACS1 relies on OFDM similarly to
WiMAX and Long-Term Evolution (LTE) mobile networks. L-DACS2 relies on GMSK as GSM mobile networks. SESAR project P15.2.4 (Future Mobile Data Link system definition) investigates the two proposed L-DACS options.
2.2.2.2.1. L-DACS1 physical layer and system architecture
L-DACS1 system essentially reproduces the characteristics of the system 802.16 (WiMax) [EUR 09a]. The system is designed to provide air–ground connections (A/G) as well as interaircraft links (A/A). The presentation here focuses on A/G system architecture.
L-DACS specification uses the following convention: the forward link (FL) is the link from ground station (GS) to aircraft; the reverse link (RL) is the link from aircraft to ground station. The forward link FL uses OFDM access, well suited for continuous transmission of a Time Division Multiplex (TDM) data flow. The return link RL uses OFDMA/TDMA with dynamic capacity allocation by the base station. A fairly similar pattern is also used by fourth generation cellular networks (LTE).
OFDM relies on a distribution of information on several subcarriers. The main benefit derives from the lower symbol rate on each subcarrier compared with single-carrier transmission, which is particularly favorable in the case of mobile radio channels where the channel is frequency selective, and multipaths are present. In the case of L-DACS1, the number of subcarriers is equal to 50; each subcarrier is spaced by 9.765625 kHz. The symbol duration is fixed at 0.12 ms. The modulation on each subcarrier may be QPSK, 16-QAM or 64-QAM. Associated channel coding rate ranges between ½ and ¾. Achievable data rates on the forward link FL vary between 300 and 1,400 kbit/s.
The RL, unlike the FL, must implement multiple access. The physical channel basis is a set of 50 subcarriers defined with a 63 MHz offset relative to the FL. The choice has been made to apply OFDMA/TDMA multiple access: in the same time interval, two terminals may transmit simultaneously, each occupying 25 subcarriers. A time division is carried out, that is to say that the blocks of symbols received by the base station may belong to different aircrafts. There is, therefore, a combination of frequency division retaining the principle of the OFDM transmission and time multiplexing.
Figure 2.5 shows the protocol stack defined for L-DACS1. This protocol stack exhibits a fairly standard design with an access sublayer MAC above the PHY and a logical link control (LLC) sublayer. The LLC sublayer comprises two blocks corresponding to the data (Data Link Service: DLS) and voice service (Voice Interface: VI). Link Management Entity (LME) block realizes the management of the radio capacity, including the implementation of a dynamic waveform adaptation (Adaptive Coding and Modulation: ACM) and the handover management.
Figure 2.5. L-DACS1 protocol stack
Logical channels are summarized below in Table 2.2.
Logical channel Acronym Direction Main use
User plane
DCH Data Channel Both Data transfer
VCH Voice Channel Both Voice
communication
Signaling plane
BCCH Broadcast Control Channel
Forward Signaling from GS to all aircrafts RACH Random Access Channel Return Network entry and
capacity request DCCH Dedicated Control
Channel
Return Signaling from one aircrafts to GS CCCH Common Control
Channel
Forward Signaling from GS to one or a group of aircrafts
Table 2.3. L-DACS1 logical channels
It is then possible to introduce the frame structures used on the FL and RL directions.
Figure 2.6. L-DACS1 frame structure
Several points are worth noting. First, the time scales of the FLs and RLs are synchronous, it being understood that Figure 2.6 shows these time scales at the ground station (FL emission and RL reception). Superframe defining in particular the period of occurrence of the broadcast signal has a duration of 240 ms. It is then subdivided into 58.32 ms multiframes. The borders between data and signaling fields are not fixed but vary depending on the particular waveform used for data transmission. It should also be noted that aircraft access to the return link on the basis of resource allocation by the ground station. For network entry and capacity requests after idle periods, two random access slots are defined at the beginning of each superframe.
The main issue to be solved for the implementation of the L-DACS1 system is the frequency allocation. The bandwidth occupied by the FL and RL signals is 500 kHz. However, L-DACS systems are by nature cellular systems; thus it is necessary to allocate this band around several frequencies so that two adjacent ground stations do not transmit on the same channel. An
“inlay” solution is to have these frequency bands located in the 1 MHz gaps between adjacent Distance Measuring Equipment (DME) channels in the L-band. Experiments will have to be conducted in order to ensure that interference from the inlay system toward the legacy systems and interference from the legacy systems toward the inlay system remain in acceptable domain. The “non-inlay” alternative is to allocate specific bandwidth to L-DACS1.
2.2.2.2.2. L-DACS2 physical layer and system architecture
L-DACS2 uses a waveform directly derived from that of the mobile telephone networks of second-generation GSM. We thus find the GMSK used in a very similar band as GSM900: 960.5–975 MHz. Each channel occupies 200 kHz with a roll-off of 1.3; the throughput per carrier is equal to 270 833 kbit/s (same values as GSM). However, the access method is quite different with access TDD where transmissions in the forward and return directions are in time. Thus, only one carrier is needed for one given cell as minimum allocation.
The L-DACS2 frame structure is shown in Figure 2.7.
Figure 2.7. L-DACS2 frame structure
Frontiers between time blocks can be altered by the ground station according to the network load. A time interval is reserved for random access that will be used by aircraft for network entry (LoG2). Forward time intervals are used by the ground station GS to broadcast signaling and send data. Multiple accesses for aircraft are based on a Request-To-Send (RTS) – Clear-To-Send (CTS) process. GS allocates one slot per aircraft in the CoS1 time interval. This slot will be used by the aircraft in order to request capacity (RTS). GS answers to RTS by CTS: one aircraft is granted a time interval in the CoS2 block and can thus transmit data. As a result, latency for transmission is greater than 1 s when considering the 95 percentile. The advantage of this access method is to provide a deterministic transmission delay.
2.2.2.2.3. Comparison of the two L-DACS proposals
A performance comparison of the two systems should be conducted on the basis of specification prototypes issued by EUROCONTROL [EUR 09c, EUR 09d]. The performance objectives are the same for both systems,