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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,

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Presentation on theme: "1 All-purpose Multi-channel Aviation Communication System ( AMACS) ICAO ACP WG T 2 – 5 October 2007 Presented by Luc Deneufchatel, DSNA Larry Johnsson,"— Presentation transcript:

1 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 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 3 AMACS system overview Flexible multipurpose communication system Cellular narrowband ( kHz) point-to-point system intended to operate primarily within the 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 4 A flexible and scalable solution providing for operational expansion A configurable channel size to match the foreseen traffic densities of Europe in 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 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 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 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 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 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 9 Typical high bit-rate point to point instantiation of the AMACS system

10 10 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 Lower layers characteristics 1/2

11 11 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 Lower layers 2/2 Inner codeOuter code RScodingInterleaving Convolutive coding

12 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/I cc =9dB and C/I adj =-9dB) Allows the best compromise between BER and bit-rate

13 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 14 Error correcting scheme Convolutive decodingDe-InterleavingRS 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 15 Error correcting scheme The code rate 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] - The convolutive code is the well known punctured (133,171), constraint length 7.

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

17 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 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 18 Other solution Using only the RS coder De-InterleavingRS 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 19 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 Basic slot characteristics Guard time depending on cell size individual slot structure Synch In bits signalling and data In bits CRC In bits decay total slot duration 4ms next slot FEC, Ramp-up

20 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 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 22 Shared section Uplink section Frame CoS1CoS2UP2UP1 Exclusive primary slots for short, high QoS messages or RTS messages Shared slots, reserved or random access : used for any messages Second uplink for ACKs, CTS, reservations Reserved slots for uplink messages Downlink section Framing message Cell insertion Start of UTC second Frame structure – point-to-point

23 23 Uplink & Cell Insertion Frame Sections UP1 1 st Uplink Section for ground station use For data uplink and ACKs of received data UP2 2 nd 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 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 frames time will be broadcast every frame in a Framing Message The framing message will also broadcast the length of the Cell Insertion section

25 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 26 ISO Flag Ramp-down Slot structure 4 ms Ramp-up n 1 octet Reservation header 3 octets (if required) User data 148 octets FEC / CRC 47 octets Guard time 0·9 ms m NOTE: n + m < 0·1 ms 1 octet Addresses plus flags 4·5 octets (typical) ISO Flag

27 27 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 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 29 Airborne Architecture Avionics for AMACS implementation of ATS, AOC and ADS-B functions

30 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 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 stations full ICAO address UTC time, Frame section sizes UP2 is also used for transmitting the combined ACK/CTS message to all aircraft

32 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 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 its 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/Cs 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 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 35 Point-to-point channel Defined AMACS messages Binary codes for AMACS message types: 6 bits is not used Message type Binary code Message type Binary code CoS1 Downlink Uplink CoS1 Keep-alive Block reservation CoS2 Downlink Framing message CoS2 RA short CTS CoS2 RA long ACK CoS2 RA RTS ACK/CTS ALL Cell exit Cell insertion

36 36 Example Message structure Cell insertion A/C Tx 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 CELL_INS message type GS ICAO address 27 A/C ICAO address 27 Authentication (32) 109 bits 8 ISO flag Message type 6 Version number 2 Destination ground station Address length flag 1 Binary 00 Binary Binary 0 for local addresses Binary 1 for 27-bit ICAO addresses Binary Size not fixed Message identifier 6 1 to 64 ( to )

37 37 AMACS summary Flexible multipurpose L-band communication system Cellular, narrowband system Channel bandwidths ( 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 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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