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All-purpose Multi-channel Aviation Communication System ( AMACS)

All-purpose Multi-channel Aviation Communication System ( AMACS). ICAO ACP WG T 2 – 5 October 2007 Presented by Luc Deneufchatel, DSNA Larry Johnsson, LFV. Introduction. Future Communication Study E-TDMA proposed by DSNA XDL4 proposed by LFV Emerging understanding

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All-purpose Multi-channel Aviation Communication System ( AMACS)

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  1. All-purpose Multi-channel Aviation Communication System (AMACS) ICAO ACP WG T 2 – 5 October 2007 Presented by Luc Deneufchatel, DSNA Larry Johnsson, LFV

  2. Introduction • Future Communication Study • E-TDMA proposed by DSNA • XDL4 proposed by LFV • Emerging understanding • Spectrum availability and RF environment will dictate our options • Plug in of generic systems (COTS) in aviation environment is difficult and challenging • AMACS • Based on: E-TDMA + XDL4 + experience from other aviation systems + COTS elements • Constraint driven development approach • One multichannel narrowband alternative in L-band

  3. AMACS system overview • Flexible multipurpose communication system • Cellular narrowband (100-400 kHz) point-to-point system intended to operate primarily within the 960-975 MHz frequency allocation designed for flexible deployment • Supports different channel bandwidths and bit rates to cope with various operational needs (high and medium density airspace) • Robust physical layer based on GSM/UAT modulation types associated with strong data coding • Efficient handling of QoS with guaranteed transmission delay (based on the TDMA structured MAC layer) • Support of unicast and multicast data communications taking advantage of VDL Mode 4 broadcast experience • Support of air-air point-to-point data communications

  4. A flexible and scalable solution providing for operational expansion A configurable channel size to match the foreseen traffic densities of Europe in 2020+ Frequency plan needed to allocate the available spectrum to the various types of channels (bandwidth and type of service) An adapted performance for the different QoS classes Frame structure identifies distinct time slots at MAC layer Specific and reserved channel resources for high QoS transmissions Strong robustness at physical layer level to ensure: Achievement of the highest QoS in terms of latency Predictive behaviour in a typical distorted propagation channel Co-site operation on board aircraft by minimizing susceptibility level AMACS performance objectives

  5. Key design drivers Robustness, flexibility, scalability E-TDMA and XDL4 concepts have been merged Providing an adapted technical solution to data-link communications needs of 2020+ EMC “constraint driven” development Based on proven concepts Robust proven GSM physical layer High performance E-TDMA MAC layer VDL Mode 4 broadcast protocols Designed to handle up to 175 aircraft per cell in high- density airspace Efficient air-initiated cell handover mechanism Uses aircraft knowledge of cell locations and characteristics (through either EFB loading or CSC channel) AMACS key facts 1

  6. AMACS key facts 2 • Initial deployment in the lower L-band to support: • New ATM point-to-point services requiring high QoS (support to SESAR or NEXTGEN future concept) • Broadcast services provided in segregated channel if spectrum availability in the lower L-band is sufficient • Air-air data communication provided in segregated channels • AOC data communications achievable if extra spectrum is available for dedicated channels • Could be transposed to the VHF band in the long term when it becomes available for new technology • More capacity offered to cover all the needs above

  7. AMACS key facts 3 • Airborne co-site interference in the lower L-band is addressed by using: • A common synchronization bus between L-band systems to protect other L-band systems from AMACS transmissions • Other systems are notified of any transmission from AMACS to take the appropriate measure • A strong coding of the channel to provide high robustness for airborne reception • The ratio between the shortest bit duration in a slot and the duration of the spurious burst is approximately 0·5 to 1 • This leads to potential interference windows covering no more than two or three consecutive bits • These can be recovered by the various coding mechanisms

  8. AMACS Presentation • This presentation focuses on an air/ground point-to-point channel supporting the highest bit-rate per cell • 400 kHz/520 kbps • Foreseen for the en-route high-density area of Western Europe • Minimal configurations can be tailored for the periphery of Western Europe • 100 kHz/130 kbps • Intermediate configurations can be tailored for major TMA areas • 200 kHz/ 260 kbps

  9. Typical high bit-rate point to point instantiation of theAMACS system

  10. Lower layers characteristics 1/2 • Design goals • Low Bit Error Rate at low Signal-to-Noise ratio • Occupation of least possible bandwidth • Good performance in multipath and fading environments • Introduce least amount of residual power in the RF environment • Simple and cost effective to implement • MAC considerations • 148 octets afforded per slot for data to meet the most critical services defined in the COCR

  11. Convolutive RS coding Interleaving coding Lower layers 2/2 • Narrowband system based on GSM physical layer • Modulation based using Gaussian Minimum Shift Keying (GMSK) • Pre-filtering leads to compact waveform (minimal sidelobes) • 400 kHz channels • Gross Bit rate of 520 kbps • C/I of 9dB (including FEC) • May allow reuse of some GSM hardware components • Error Correction • Concatenated coding • Inner code – Convolutional code with puncturing • Interleaver – Block and diagonal interleaving • Outer code – Reed-Solomon Outer code Inner code

  12. Why GMSK modulation rather than CPFSK or GFSK ? • GMSK is a modulation known and tried with GSM • The global deployment of GSM implies cheap costs of development for equipment • A cellular system and a waveform adapted to frequency re-use radio networking (C/Icc=9dBand C/Iadj=-9dB) • Allows the best compromise between BER and bit-rate

  13. Link budget • Hypothesis: • Free space propagation • Frequency : f =975MHz • Propagation distance: d=150 NM =278 Km • Antennas Gains: Ge= -3dB Gr= 0dB • Reception power: Pr=-100 dBm (to ensure BER=10-3 on a 400 kHz channel) The pathloss is computed by the following formula:

  14. Error correcting scheme Convolutive decoding De-Interleaving RS decoding Interleaving does not affect the BER but improves the distribution of errors Convolutive code are used to remove isolated error RS code has the effect of removing burst of errors

  15. Error correcting scheme • The code rate - The convolutive code is the well known punctured (133,171), constraint length 7. So only three rates are practical: - The RS code is the RS(31, x; 5) So only two rates are practical, with t=[1;2]

  16. Error correcting scheme • Four configurations are suitable: 1) 2) 3) 4)

  17. Error correcting scheme • BER in convolutional code • With a convolutional code (5,7) a BER=10-3 at the input gives a BER=10-5 at the output. • In order to mitigate the puncturing, the BER at the output will be considered equal to 10-4. • BER in RS code: With a the BER at the output of the RS code (31,27,5), is arround 10-7, so the conditions are met for two of the configurations

  18. Other solution • Using only the RS coder De-Interleaving RS decoding With a RS coder (31;25), the code rate will be: And the BER : This solution seems relevant but must be modelled and simulated over an appropriate representative radio channel

  19. individual slot structure FEC, signalling Synch In bits next slot CRC In bits decay and data In bits Ramp-up Guard time depending on cell size total slot duration 4ms Basic slot characteristics • TDMA access scheme with 4 millisecond slots • Ramp-up/down times total < 0·1 ms • Guard time allowance of 0·9 ms, allows a GS range of 150 NM • Usable slot duration 3 ms • Time synchronization to UTC will be required • Time information uplinked by the ground station for aircraft use

  20. For point-to-point channels, AMACS will use the MAC layer principles developed for E-TDMA Channel will have a frame repeating every 2 seconds Uplink sections - use is configurable (dynamically) by the ground station (GS) Ground reserved area for uplinks and ground-directed signalling Downlink sections - divided into sub-sections for different Classes Of Service (COS) Each A/C has one exclusive slot for high QoS messages More downlink slots are available on request MAC layer organization

  21. Downlink Classes Of Service (COS) • COS1 • High QoS Service • Dedicated section of the frame for high-priority short messages from aircraft • Each aircraft within range of the ground station is allocated its own slot in which it may transmit in every frame (thus every 2 seconds) • COS2 • Lower QoS Service • A section of the frame for lower priority and/or longer messages from aircraft • Section also allows for re-sends in the same frame

  22. Frame structure – point-to-point Frame Start of UTC second UP1 CoS1 UP2 CoS2 Cell insertion Framing message Reserved slots for uplink messages Exclusive primary slots for short, high QoS messages or RTS messages Second uplink for ACKs, CTS, reservations Shared slots, reserved or random access: used for any messages Uplink section Downlink section Uplink section Shared section

  23. Uplink & Cell Insertion Frame Sections • UP1 • 1st Uplink Section for ground station use • For data uplink and ACKs of received data • UP2 • 2nd Uplink Section for ground station use • For CTS/ACK ALL messages • For reservation messages reserving space in COS2 • For framing message • Cell Insertion • Dedicated section for new aircraft to logon to the ground station when it comes within range

  24. Flexible frame structure • The flexibility to cope with different numbers of aircraft and traffic demand is built into the frame structure • Lengths of each section of the frame (COS1, COS2, UP1, UP2) can be varied by the ground station • In particular the length of the COS1 section follows the number of logged-on aircraft very closely • Details of the current frame structure and of the frame structure in x frame’s time will be broadcast every frame in a Framing Message • The framing message will also broadcast the length of the Cell Insertion section

  25. MAC layer characteristics • Frame length of 2 seconds • Divided into 500 slots of length 4 ms • It is assumed that this size is fixed globally • Slot characteristics • Active slot length: 4 ms – (ramp + guard times) = 3 ms • Bits per slot: Active slot length × Bit rate = 1,620 bits • Bits for CRC/FEC: ~30% of bits per slot = 376 bits (47 octets) • Remainder: Bits per slot – CRC = 1244 bits = 155·5 octets • ISO flags + reservation header = 3 octets • Addresses plus administrative flags (average) = 4·5 octets • User data space = 148 octets

  26. Slot structure Ramp-up n ISO Flag 1 octet Addresses plus flags 4·5 octets (typical) User data 148 octets 4 ms Reservation header 3 octets (if required) FEC / CRC 47 octets ISO Flag 1 octet Guard time 0·9 ms Ramp-down m NOTE: n + m < 0·1 ms

  27. Cellular deployment • Cellular deployment • 12 frequency re-use pattern • Worst case (air-air interference) • Carrier/Interferer (C/I) calculation • dw = R and di = 4R, for cell radius R • C/I = Att (interference) – Att (wanted) • Propagation model: • Att = (constant) + a.10 log(d) • a = 2 (Free space) or more • C/I = a.6 dB, • Thus C/I ≥ 12 dB • But for GMSK, 9 dB is enough, with GSM FEC rate 260/456 (0.57 ratio), and a very light interleaving

  28. AMACS Network Architecture • AMACS infrastructure comprises a number of AMACS Ground Stations which are organized into clusters • Each Ground Station in a cluster will be connected to some concentrator, the Ground Network Interface (GNI) ATN A/G Routers and the IPv6 Routers are ground-based users of the AMACS sub-network service and the airborne ATN and IP routers are mobile users of the AMACS sub-network service

  29. Airborne Architecture • Avionics for AMACS implementation of ATS, AOC and ADS-B functions

  30. System operations - Entry • Aircraft entry • Section at the beginning of CoS2 dedicated to cell insertion • A/C will already know the GS frequency • A/C will listen for 2 seconds to hear the “framing” message • This will tell it the GS ICAO address and the cell frame structure • A/C will then transmit cell insertion message in the dedicated slots • This contains the A/C ICAO address and the GS ICAO address • GS will reply in UP1 • Containing GS ICAO address, A/C ICAO address, new local 9-bit A/C address, GS 7-bit local address, allocated slot number • Local addresses are used to avoid ICAO 27-bit addresses occupying large amounts of space in transmissions

  31. System operations - Uplink • The GS will transmit data to the A/C in UP1 • If correctly received – • Each A/C will send an ACK as part of its CoS1 transmission • If not correctly received – • The A/C will send a NACK as part of its CoS1 transmission • GS will re-send data in UP2, with an ACK slot reserved in CoS2 • A/C will send an ACK or NACK) in the allocated CoS2 slot • GS transmits framing message at start of UP2, containing – • The ground station’s full ICAO address • UTC time, Frame section sizes • UP2 is also used for transmitting the combined ACK/CTS message to all aircraft

  32. System operations - Downlink • Each A/C has an allocated CoS1 slot for downlink • Regular transmission of short data messages • If the data size is too large, an RTS is transmitted in CoS1 (This is a request for a longer CoS2 slot) • When an A/C has no data, it transmits a keep-alive message • If CoS1 transmission is correctly received – • The GS responds in the combined ACK/CTS message in UP2 • If not correctly received – • The A/C will re-transmit in CoS2, using random access • The GS can reply with a dedicated ACK in CoS2

  33. System operations – Hand-off • Hand-off procedures • A/C will know the locations of ground stations • When nearing the edge of a cell, A/C will contact the next GS • The A/C will indicate to current GS that it’s exiting the cell • If this process completes correctly handover will be quick (1 slot) • Otherwise the link will time-out • GS will de-allocate CoS1 slot after a correct hand-off • If contact is broken before hand-off process is complete, the A/C’s CoS1 slot will remain reserved for a pre-set period • This will prevent a disruption of communications caused by premature slot re-allocation after a short-term signal loss

  34. Broadcast channel • Superframe characteristics • 15,000 slots in one 60 s superframe • 4 ms slot length • Same MAC structure as VDL Mode 4 • Random access using the VDL Mode 4 reservation protocols • Dedicated ground-reserved block at start of each superframe • Increased basic message size, more convenient for ADS-B • Most VDL Mode 4 broadcast protocols will be used • Modified for single channel and AMACS frame structure • No point-to-point transmissions permitted

  35. Point-to-point channelDefined AMACS messages • Binary codes for AMACS message types: 6 bits • 00 0000 is not used

  36. Example Message structureCell insertion CELL_INS message type A/C Tx ISO flag Binary 0111 1110 8 Version number Binary 00 2 Binary 0 for local addresses Binary 1 for 27-bit ICAO addresses Address length flag 1 109 bits A/C ICAO address 27 Message type Binary 00 1110 6 Message identifier 1 to 64 (00 0001 to 111111) 6 GS ICAO address Destination ground station 27 Authentication (32) Size not fixed A/C will listen for framing message to identify the cell insertion slots GS reply to cell insertion message will be transmitted in the next UP1

  37. AMACS summary • Flexible multipurpose L-band communication system • Cellular, narrowband system • Channel bandwidths (100 - 400 kHz bandwidth) and bit rates adaptable according to operational needs • Robust physical layer based on GSM/UAT modulation types • Efficient handling of QoS with guaranteed transmission delay • Support of air-ground point-to-point data communications and air-air, using multiple channels • Support of multicast/broadcast data communications taking advantage of experience of existing systems

  38. AMACS Status • The high level design of AMACS is now finalised • At Physical and MAC layer levels • Complete definitions of frame, slot, and message structures • Error correction coding definition completed • Initial channel structure, cellular deployment and network architecture specified • All MAC message types defined • Definition of services provided • Protocols and system operation defined for both point-to-point and broadcast communication • On going activities at DSNA regarding the airborne co-site compatibility (DME and Mode S) including laboratory test with GA DME • Further activities to refine the design and assess more accurately the performances are necessary

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