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Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers

OCM Workshop & Meeting CCSDS Meeting, Darmstadt, May 2012 Speaker: Tomaso de Cola. Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers. Outline. Introduction Why optical communications may be challenging for CCSDS protocols? Available Reliability Options LTP BP CFDP

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Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers

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  1. OCM Workshop & Meeting CCSDS Meeting, Darmstadt, May 2012 Speaker: Tomaso de Cola Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers

  2. Outline • Introduction • Why optical communications may be challenging for CCSDS protocols? • Available Reliability Options • LTP • BP • CFDP • Advanced Options • Erasure Coding • Hybrid ARQ (with erasure codes)

  3. Why optical communications may be challenging for CCSDS protocols? • Optical Freespace Channel suffers from AWGN and Fading • Causes for Fading: * Pointing- / Tracking-Error due to PAT-mechanism and signal roundtrip* Beam-Wander due to AIRT (uplink only)* Intensity-Scintillations (caused by AIRT)  leads to Rx Power Fading * Wavefront-Distortions (caused by AIRT)  Heterodyning Efficiency / Mode-Distortion (effects depend on modulation format and receiver- technology) AIRT: Atmospheric Index-of-Refraction Turbulence

  4. Why optical communications may be challenging for CCSDS protocols? • Optical deep-space links require large telescopes: diameters  1 m. • Links must cope with atmospheric perturbations. • Links at large zenith angles suffer from strong perturbations due to turbulence (random refractive-index variations). • Link Availability limited by cloud coverage. • Larger telescope apertures lead to weaker and slower scintillation • The use of adaptive optics (AO) to reduce the receiver's field of view modifies the channel model. • If the AO system is not well dimensioned in terms of sufficient resolution and sufficient bandwidth, additional fades appear. • For example a higher bandwidth is required during strong-wind periods.

  5. Why optical communications may be challenging for CCSDS protocols? • Fade-durations: 1ms..100ms, scenario-dependent • typ. Datarates: >Gbps (LEO-downlink) n*100Mbps (Moon-Exploration) n*10Mbps (Mars-Link) bits lost during 1 fade: 1Mbit .. 1Gbit

  6. Optical Satellite Downlink to Oberpfaffenhofen • Satellite: OICETS (other name Kirari) from JAXA • Orbit: circular at the altitude of 610 km with inclination of 97.8 deg • Communication wavelength: 848 nm • Tx-Power: 100 mWmean • Communication data-rate: • 49.3724 MBit/s • NRZ PRBS 215-1 • Modulation scheme: on-off keying (OOK) • Optical Ground Station in Oberpfaffenhofen (Germany) • Optical Ground Station antenna / telescope diameter: 40 cm • Receiver type: PIN silicon photo detector

  7. Statistics on the Mean Fade Duration

  8. Statistics on the Fade DurationAutocorrelation Function of the RX Signal Power

  9. Statistics on the Fade DurationTime Series • Received signal power fluctuations ofthe normalized signal • Fade == PRx < 1Fades ~ 2 ÷ 6 ms

  10. Optical Downlink Aircraft to Oberpfaffenhofen • Project “VABENE”, “Verkehrsmanagement bei Großereignissen und Katastrophen” • Optical cameras recording aerial photographs with a geometric resolution up to 15 cm. • RADAR sensors applied during the night and in case of bad weather. • Both optical and RADAR data is sent in near real time to a ground station. • Technology: optical link established from a (mobile) aircraft to a (fixed) ground station • Data rate: ~60 Mbit/s • Wavelength: 1550 nm

  11. Optical Downlink Aircraft to Oberpfaffenhofen

  12. Optical Downlink Aircraft to OberpfaffenhofenDistance= 40 km, Antenna aperture = 5 cm

  13. Optical Downlink Aircraft to OberpfaffenhofenDistance= 40 km, Antenna aperture = 5 cm • Scinillation Index of Rx Power: 0.626157 • Standard Deviation: 0.780 % • Probability of 3 db Fades: 7.296 % • Mean Duration of 3 dB Fades: 2.515e-004 s

  14. Optical Downlink Aircraft to OberpfaffenhofenDistance= 40 km, Antenna aperture = 50 cm

  15. Optical Downlink Aircraft to OberpfaffenhofenDistance= 40 km, Antenna aperture = 50 cm • Scintillation Index of Rx Power: 0.193090 • Standard Deviation: 0.440 % • Probability of 3 db Fades: 10.319 % • Probability of 6 db Fades: 0.589 % • Probability of 10 db Fades: 0.002 % • Mean Duration of 3 dB Fades: 1.266e-003 s • Mean Duration of 6 dB Fades: 7.735e-004 s • Mean Duration of 10 dB Fades: 3.356e-004 s

  16. Communication Reliability for Challenging ScenariosPreliminary Recommendations • Optical communication can be hampered by a number of factors which are environment-dependent. • It can happen that such impairments cannot be completely takled by the physical layer channel coding, thus resulting in frame erasures (random or correlated) • The upper layer should react to these information reasures, by applying the most appropriate recovery strategy depending on: • Mission peculiarities (propagation delay, data-rate, ...) • Frame error rate, fade duration • Two main couteractions: • Automatic Retransmission Query (ARQ) (current practice in CCSDS protocols) • Erasure Codes (being promoted for future extensions of CCSDS protocols)

  17. ARQ SolutionsOverview • ARQ affected by: • Long round trip delay in deep space missions: lengthy retransmission cycles very low throughput figures; • Feedback channel not always available: degradation of performance because of either retransmission timeout expiration of overly delayed retransmission very low throughput figures; • Limited on-board storage: long runs of retransmissions could be not guaranteed • ARQ solutions currently implemented in CCSDS: • CCSDS File Delivery Protocol (CFDP) • Bundle Protocol (BP) • LickLider Transmission Protocol (LTP)

  18. ARQ SolutionsLicklider Transmission Protocol (LTP) • LTP PDUs can contain red and/or green parts. • Transmission reliability of red blocks in ensured by ARQ, relying on Negative Acknowledgment (NAK) • Green blocks do not have any reliabilty guarantee • ARQ strategy based on immediate and deferred schemes used also in CFDP • A maximum number of retries is allowed. Once this limit is reached, a failed transmission signal is sent to the upper layers.

  19. ARQ SolutionsBundle Protocol(BP) • BP allows reliable data communication, by exploiting custody transfer option: • Nodes with sufficient storage capacity are elected custodials • They are responsible for successfully forwarding bundles to the next custodial or the final destination • ARQ implementation, relying on positive acknowledgment (ACK) • For each received bundle, a recipt confirmation ACK is sent back • In case of bundle loss, retransmission phase is triggered at the forwarder upon timeout expiration • A maximum number of retries allowed. Once this limit is reached, a fail signal is sent back or upwards the application layer • Suspend/resume option available within the Bundle Protocol: • Upon transmission fail signal received by the underlying layer, the transfer can be suspended • Upon a-priori knowledge of the available-contact duration, transmission can be suspended and later resumed

  20. ARQ SolutionsCCSDS File Delivery Protocol(CFDP) • CFDP can work in either in unacknowledged or acknowledged mode. • In the latter, communication reliability is ensured by ARQ scheme, relying on Negative Acknowledgment (NAK): • Immediate • Deferred • Prompted • Asynchronous • Upon missed PDU detection: • NAK is issued according to the above algorithms • NAK reception triggers retranmission of missed PDUs • Completion of recovery phase is ruled by NAK-timers, whose expirance forces the transmission of new NAKs • A maximum number of retries is allowed. Once this limit is reached, the file transfer is aborted.

  21. Erasure CodesOverview • Erasure Codes are a sort of packet-level coding, according to which redundancy packets are generated out of source packets: • Powerful LDPC-based codes can effectively contrast packet erasures: accurate design of codes can give performance really close to the Shannon bound; • Possibility to face long bursts of erasures: reduction of number of retransmissions; • Some packet losses could be un-recovered: non-zero information loss; • Complexity of encoding/decoding engines.

  22. Erasure CodesConcept

  23. Erasure CodesImplementation • The packet-level encoder generated m redundancy packets out of k source packets • Upon physical layer decoding, only a subset of the transmitted packets can be forwarded to the upper layer • The packet-level decoder is able to recover the original set of source packets if a sufficient number of packets is received

  24. Hybrid ARQ • Joint use of erasure codes and ARQ can be profitable to take advantage on the benefits of both techniques • LTP red block rtx + erasure codes • BP custodial transfer-ARQ + erasure codes • CFDP NAK-ARQ + erasure codes • The most performant combination of techniques is strictly ruled by: • Space mission configuration (deep space, near Earth, data rate, ...) • Implementation complexity

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