General Terrestrial Data Link - LDACS

Under EUROCONTROL/FAA Action Plan 17 (AP17) activities, various candidate technologies were considered and evaluated for their supporting of the future aeronautical requirements. While operations in the VHF band would be preferred, this band is heavily congested, therefore, the L band was identified as the next best candidate band primarily due to propagation characteristics. Therefore the various technologies for the general continental system were considered to operate in the L band.

However, it was found that none of the considered technologies could be fully recommended primarily due to concerns about the operational compatibility (spectrum interference) with existing systems in the L band. Nevertheless, the assessment of the candidate technologies led to the identification of desirable technology features that could be used as a basis for the development of an L-band data link solution that would be spectrally compatible/

Considering these features and the most promising candidates, two options for the L-band Digital Aeronautical Communication System (LDACS) were identified. These options need further consideration before final selection of a single data link technology. The first option for LDACS is a frequency division duplex (FDD) configuration utilizing OFDM modulation techniques, reservation based access control and advanced network protocols. This solution is a derivative of the B-AMC and TIA-902 (P34) technologies. The second LDACS option is a time division duplex (TDD) configuration utilizing a binary modulation derivative of the implemented UAT system (CPFSK family) and of existing commercial (e.g. GSM) systems and custom protocols for lower layers providing high quality-of-service management capability. This solution is a derivative of the LDL and AMACS technologies. The following table depicts the key characteristics of the two options.

Options Access Scheme Modulation Type Origins
L-DACS1 FDD OFDM B-AMC, TIA 902 (P34)
L-DACS2 TDD CPFK/GMSK Type LDL, AMACS

The LDACS1 option represents the state of the art in the commercial developments employing modern modulation techniques and may lead to utilisation/adaptation of commercial products and standards. The LDACS2 option capitalises on experience from aviation specific systems and standards such as the VDL3, VDL4 and UAT.

In addition to the air/ground capability, some of the assessed technologies could also support additional features such as air/air (point to point and/or broadcast) communications and digital voice. However the support of these capabilities needs further investigation. The L band data link investigations were primarily based on simulations and analytical investigations. Therefore there is the need to validate the theoretical findings and confirm expected performances using real equipment.

AP17 proposed follow-on activities to further characterize the proposed LDACS options, validate their performance, and lead to a single technology recommendation for the L band. AP17recommended to complete the L band data link investigations (with emphasis in proving the spectrum compatibility with other systems) aiming at a final decision by 2009/2010, to enable system availability for operational use by 2020. AP17 identified the following activities for the L band data link:

  • Complete the investigation of compatibility of prototyped LDACS components with existing systems in the L-band particularly with regard to the onboard co-site interference and agree on the overall design characteristics;
  • Evaluate and validate the performance of the proposed solution in the relevant environments through trials and test bed development; and
     
  • Considering the design trade-offs, propose the appropriate LDACS solution for input to a global aeronautical standardisation activity.

Similarly, the SESAR Definition Phase recommended expediting the development and validation of the L band selected technology by developing initial prototypes to support feasibility assessment. Furthermore, it recommended making final technology selection in coordination with other regions by 2010, to allow the development of the technical specifications for inclusion in ICAO SARPs and Manuals.

Therefore, in order to complete the selection of the LDACS, it is required to:

  • Develop detailed specifications for LDACS1 and LDACS2
     
  • Develop and test the LDACS1 and LDACS2 prototypes
     
  • Assess the overall performance of LDACS1 and LDACS2 systems.

When doing the testing of the LDACS prototypes, it is important that the spectrum compatibility investigations are made in a consistent way for both options in order to ensure a fair assessment of the two options. Therefore, it is important to develop common interference scenarios to be investigated and the definition of acceptability criteria for each scenario. The figure below illustrates the required activities that will lead to the selection of the LDACS system with an estimated time line association. The first box (in blue) depicts the current Eurocontrol activities that will provide input to the follow on SESAR JU activities (2nd box, in yellow) which will be responsible for prototyping, testing and validation. The third box (in green) deals with the selection of the most appropriate system which needs to be considered in a global framework involving ICAO.

Deliverables from previous studies are available in the L-Band Continental System library.

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