Allpurpose Multichannel Aviation Communication System AMACS

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
AMACS performance objectives

• 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

5
AMACS key facts 1

• 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)

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 05 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
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

Inner code
Outer 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/Icc9dB and
  C/Iadj-9dB)
• Allows the best compromise between BER and
  bit-rate

13
Link budget

• Hypothesis
• Free space propagation
• Frequency f 975MHz
• Propagation distance d150 NM 278 Km
• Antennas Gains Ge -3dB Gr 0dB
• Reception power Pr-100 dBm (to ensure BER10-3
  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 t12
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 BER10-3 at
  the input gives a BER10-5 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 (3125), 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
Basic slot characteristics

• TDMA access scheme with 4 millisecond slots
• Ramp-up/down times total lt 01 ms
• Guard time allowance of 09 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
MAC layer organization

• 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

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
CoS1
CoS2
UP2
UP1
Cell insertion
Framing message
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
Shared section
Uplink section
Uplink section
Downlink 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 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
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 1555 octets
• ISO flags reservation header 3 octets
• Addresses plus administrative flags (average)
  45 octets
• User data space 148 octets

26
Slot structure
Ramp-up
n
1 octet
ISO Flag
Addresses plus flags
45 octets (typical)
User data
148 octets
4 ms
Reservation header
3 octets (if required)
FEC / CRC
47 octets
ISO Flag
1 octet
Guard time
09 ms
Ramp-down
m
NOTE n m lt 01 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 stations 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 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
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 11 1111)
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