Impact of Optical Links Fades on
CCSDS-DTN Upper Protocol Layers
OCM Workshop & Meeting
CCSDS Meeting, Darmstadt, May 2012
Speaker: Tomaso de Cola
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)
Slide 2
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
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 receivertechnology)
AIRT: Atmospheric Index-of-Refraction Turbulence
Slide 3
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
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.
Slide 4
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
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
Slide 5
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
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
Slide 6
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Statistics on the Mean Fade Duration
0.007
Mean Fade Duration [s]
0.006
Mean Fade Duration (s)
0.005
0.004
0.003
0.002
0.001
0
4.5003
10.6314
15.0914
22.847
28.7572
56.442
Slide 7
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Elevation, deg
Statistics on the Fade Duration
Autocorrelation Function of the RX Signal Power
Slide 8
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Statistics on the Fade Duration
Time Series
Normalized Signal @ 5°
2.5
2
PRx
Received signal power fluctuations of
the normalized signal
Fade == PRx < 1
Fades ~ 2 ÷ 6 ms
1.5
1
0.5
0
2
4
6
Normalized Signal @ 46°
1.1
8
10
Time (ms)
12
14
16
18
Normalized Signal @ 25°
1.7
1.6
1.05
1.5
1
1.4
1.3
0.9
PRx
PRx
0.95
1.2
1.1
0.85
1
0.8
0.9
0.75
0.7
0
0.8
2
4
6
8
10
Time (ms)
12
14
16
18
0.7
0
2
4
6
8
10
Time (ms)
12
14
16
18
20
Slide 9
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
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
Slide 10
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Optical Downlink Aircraft to Oberpfaffenhofen
Slide 11
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Optical Downlink Aircraft to Oberpfaffenhofen
Distance= 40 km, Antenna aperture = 5 cm
Slide 12
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Optical Downlink Aircraft to Oberpfaffenhofen
Distance= 40 km, Antenna aperture = 5 cm
Scinillation Index of Rx Power:
Standard Deviation:
Probability of 3 db Fades:
Mean Duration of 3 dB Fades:
0.626157
0.780 %
7.296 %
2.515e-004 s
Slide 13
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Optical Downlink Aircraft to Oberpfaffenhofen
Distance= 40 km, Antenna aperture = 50 cm
Slide 14
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Optical Downlink Aircraft to Oberpfaffenhofen
Distance= 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
Slide 15
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Slide 16
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Communication Reliability for Challenging Scenarios
Preliminary 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)
Slide 17
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
ARQ Solutions
Overview
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)
Slide 18
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
ARQ Solutions
Licklider 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.
Slide 19
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
ARQ Solutions
Bundle 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
Slide 20
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
ARQ Solutions
CCSDS 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.
Slide 21
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Erasure Codes
Overview
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.
Slide 22
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Erasure Codes
Concept
Encoding:
generation of
redundancy
packets
Transmission of the encoded frames
Spacecraft
Packets extracted and lost packets
recovered by a packet erasure decoder
Control Center
s
ame Center
r
F
r
rol
sfe
Tran the Cont
o
red t
r
e
f
s
tran
Ground Station
Decoding and frame validation
Implemented in the Ground
Station
• “Good” frames
• “Bad” frames
Slide 23
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
Erasure Codes
Implementation
Coded Block
Source PK 1
Source PK 2
Source PK 1
Packet-Level
Encoder
Source PK k
Source PK 2
...
...
Source PK k
Redundant PK 1
FEC decoding,
decaps., frame
validation
(CRC)
CRC addition,
PHY layer
encapsulation/
FEC
Redundant PK 2
...
Physical
Channel
Redundant PK m
Source PK 1
Source PK 2
Packet-Level
Decoder
RX
side
...
Source PK k
Redundant PK 1
Redundant PK 2
Source PK 1
...
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
Source File
Redundant PK m
Source PK 2
...
Source PK k
Slide 24
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers
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
Slide 25
Impact of Optical Links Fades on CCSDS-DTN Upper Protocol Layers