High Data Throughput
Recommended Standard
NASA Presentation to CCSDS Optical
Communications Working Group
11 November 2014
11/11/2014
1
Space Relay Architecture
User Segment
Relay Segment
Space
Relay
B/A
Space
Relay
B/A
Space
User A
Airborne
User A
A
Ground
User A
Ground
Relay
Ground
Network
Primary near term interest is in relay architecture for
conveying data from user to terrestrial ground network
11/11/2014
2
Scalable, Extensible Architecture
• Architecture should support efficient high-rate
data transfer to/from space, air, and ground users
• Signaling should support functions such as
channel state monitoring and switching/routing
at the GEO terminal
• Data rate should be variable to support a wide
variety of users and missions
– Near term goal: few Mbps to several Gbps
• Data rate should be scalable to support future
high-rate (>10 Gbps) users
11/11/2014
3
Power Efficient System Design
• Bandwidth expansion (e.g. from modulation
and/or coding) should be used efficiently to
improve link performance
– Reduces SWaP and cost of terminals
– Allows future scaling to higher data rates
• Space-relay and/or Direct-to-Earth architectures
enable use of capacity-approaching coding
techniques, since high-complexity decoding may
be performed at ground terminal
11/11/2014
4
Fiber Telecom Wavelength
• Leverage commercial terrestrial telecom
investments at 1.55 µm
– Large global vendor base offering wide selection of
high performance components
– Current trend toward higher levels of photonic and
electronic integration and more sophisticated optical
signal processing for telecom applications expected to
benefit space applications
– Enables future data rate scaling using WDM and other
telecom approaches
• Vast majority of U.S. lasercom investments and
demonstrations are at 1.55 µm
11/11/2014
5
High Rate Signaling Overview
Input from
Link Layer
(e.g. Ethernet or
CCSDS frames)
Q-Repeat
Encapsulation
(HDLC)
Physical Layer
Framing
FEC
(DVB-S2)
Channel
Interleaving
(optional)
Randomizer
Modulation
To amplifier or
telescope
11/11/2014
6
Encapsulation
• Frames at terminal input are encapsulated using HDLC
• Rationale:
– Provides transparent interface between bursty/asynchronous source
frames and synchronous modem
– Supports many input frame protocols (e.g. Ethernet, CCSDS, etc.)
– Industry standard, based on RFC 1662
01111110
Flag
Address
00000011
Control
0011111011111111
Protocol
HDLC Payload
FCS
01111110
Flag
8 bits
8 bits
8 bits
16 bits
Variable
32 bits
8 bits
11/11/2014
Inter-frame Fill
or next Address
7
Forward Error Correction
• Links primarily utilize ½-rate DVB-S2 code
– BCH outer code + LDPC inner code
• Other DVB-S2 code rates may be used with coordination between
user and ground relay
• Rationale:
– Industry standard, based on ETSI EN 302 307
– Excellent power efficiency
32,208
source bits
11/11/2014
BCH
32,400
code bits
LDPC
64,800
code bits
8
Channel Interleaving
• Channel interleaver is used for all links going through the Earth
atmosphere
• Convolutional bit interleaver with data-rate dependent parameters
• Rationale:
– Mitigates effects of atmospheric fading cannel
– Convolultional interleaver requires ½ memory of equivalent
performance block interleaver
11/11/2014
9
Q-Repeat
• For lower data rate modes (<51 Mbps), individual (interleaved)
codewords are repeated Q times during transmission
• Rationale:
– Enables low data rate links
– Limits dead time from burst-mode DPSK
Interleaved
Codeword
(64800b)
Q-Repeat
Interleaved Interleaved
Codeword Codeword
(64800b)
(64800b)
Interleaved
Codeword
(64800b)
Codeword repeated Q times
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10
Physical Layer Framing
• 1024-bit header is appended to each (interleaved) codeword at
physical layer
– Unique ID for synchronization, content specification, channel state
monitoring
– Remaining bits may be used for channel state information, frame
sequence counters, physical layer control, etc.
• Header contents may be encoded separately from payload data
with a code that may be processed at space relay
• Rationale:
– Enables physical layer synchronization
– Enables multiplexing and switching at physical layer in relay nodes
Unique Word
(384b)
Channel State,
FSN, etc. (640b)
11/11/20141024-bit
Header
Interleaved Codeword (64800b)
11
Randomizer
• Based on LFSR, 1 + x + x3 + x12 + x16, initialized to 0xFFFF at
beginning of frame
• Unique word portion of frame is not randomized
• Rationale:
– Reduces likelihood of long runs of 1’s or 0’s in transmitted sequence
11/11/2014
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Modulation
• Differential phase shift keying at slot rate of 2.88 GHz
• Data transmitted in bursts of 176 bits. Deadtime between
bursts varied to change data rate
• Rationale:
– DPSK provides good power efficiency with low-complexity incoherent
receiver
– Burst mode is compatible with average-power limited transmitters
– Enables low-complexity multi-rate transceivers
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Modulation
pp
Max Rate
pp
pp
hh11 hh22
pp
…
…
pp
pp
hhNN bb11
pp
bb22
pp
b…3
pp
bbM4
pp
bb15
pp
bb26
pp
…
…
pp
pp
bbMN hh11
pp
= Optical Pulse
ed
= Empty Bit (Off-Time)
hhin
= Header Bit
bbin
= (Encoded/Interleaved) Channel Bit
1024 header bits 64800 channel bits
p
p p p
h1 … hN e
176 header
bits
…
e
p
…
e
Dx176 empty
bits
p p p
h1 … hN e
176header
bits
…
e
p
…
e
p p p
h1 … hN e
176 header
bits
Dx176 empty
bits
Max Rate
D 1
…
e
p
…
e
Dx176 empty
bits
…
p p p
h1 … hN e
176header
bits
p
…
e
e
p p p
h1 … hN e
176header
bits
Dx176 empty
bits
…
e
p
…
e
Dx176 empty
bits
p
h1 …
p
p
p
hN b1
144 header
bits
p
…
p
bN
e

32 channel
bits
1024 header bits

p
p
b1
p
b2
p
…
…
p
bN
176 channel
bits
e
e
p
…
e
Dx176 empty
bits
p
b1
p
b2
p
…
…
p
bN
176 channel
bits
e
e
…
p
e
Dx176 empty
bits
p
b1
p
b2
p
…
…
p
bN
e

e
p
…
e
p
b1
p
b2
p
…
…
p
bN
176 channel
bits
e
e
p
…
e
p
h1
Dx176 empty
bits
64768 channel bits (368 bursts of 176 channel bits)
32 channel bits (from previous burst)
64800 channel bits
11/11/2014
14
Summary of Data Rate Modes
11/11/2014
Mode Name
User Data Rate
(Mbps)
Q
Repetition
Interleaver
N
Interleaver
B (bits)
U-1244
U-622
U-311
U-155
U-51.8
U-16
U-8
U-4
U-2
1244 (Max)
622 (Max/2)
311 (Max/4)
155.5 (Max/8)
51.8 (Max/24)
16
8
4
2
1
1
1
1
1
2
4
8
16
162
162
162
162
162
162
162
162
162
96,000
48,000
24,000
11,200
4,000
1216
608
288
128
15
Wavelength
• Separate transmit, receive, acquisition wavelengths
• Selected from ITU-T G.694.1, v.2.0 (2012-02-13)
– 50-Ghz spacing, fixed grid
– Optical C-band (1530-1565 nm)
• A/B terminal specification determines transmit and receive
wavelengths in system architecture
• Rationale:
– Allows leveraging of current and future telecom industry technology
developments
– Large potential vendor base
– Enables eventual data throughput scaling using industry-standard
wavelength division multiplexing
11/11/2014
16
Relay Link Example
User Platform
Application
Optical relay serves as a transparent link-layer
bridge between User Platform and Ground Relay
Transport
Ground
Relay
Network
Network
Transparent
Bridge
Link
Ground
Network
Link
User Terminal
LAN
HDLC
FEC/ILV/
Q-Repeat
11/11/2014
HDLC
Relay Platform
Switch
FEC/DeILV/
De-Q-Rep.
Relay Terminal 1
Relay Terminal 2
Phy Frame
Phy Frame
Phy Frame
Phy Frame
Random
De-Random
Random
De-Random
Modulation
De-Mod
Modulation
De-Mod
Physical Link
Physical Link
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