1

High Data Throughput Recommended Standard

NASA Presentation to CCSDS Optical Communications Working Group

11 November 2014

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Space Relay ArchitectureUser Segment

A

B/A

A

A

A

B/A

Relay Segment

SpaceUser

AirborneUser

GroundUser

SpaceRelay

SpaceRelay

GroundRelay

GroundNetwork

Primary near term interest is in relay architecture for conveying data from user to terrestrial ground network

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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

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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

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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

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High Rate Signaling Overview

Encapsulation (HDLC)

FEC (DVB-S2)

Channel Interleaving (optional)

Physical Layer Framing RandomizerQ-Repeat

Input from Link Layer(e.g. Ethernet orCCSDS frames)

To amplifier ortelescope

Modulation

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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

0011111011111111

16 bits

Protocol HDLC Payload

Variable

FCS

32 bits

Flag

8 bits

01111110Address

8 bits

Control

8 bits

00000011Flag

8 bits

01111110 Inter-frame Fill or next Address

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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

BCH LDPC

64,800 code bits

32,400code bits

32,208 source bits

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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

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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-RepeatInterleaved Codeword (64800b)

Interleaved Codeword (64800b)

Interleaved Codeword (64800b)

Codeword repeated Q times

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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) Interleaved Codeword (64800b)Channel State,

FSN, etc. (640b)

1024-bit Header11/11/2014

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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

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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

p p p p … p p p p p … p

h1

p

176 channelbits

Dx176 emptybits

64768 channel bits (368 bursts of 176 channel bits)

e e … e

… p

Dx176 emptybits

h1

p

…

p

hN

p p

176 header bits

e e … e

…

Dx176 emptybits

p p p p

176header bits

…

Dx176 emptybits

p p p

144 headerbits

b1 b2

p

…

p

bN

p p

176 channelbits

1024 header bits

1024 header bits

h1

p

h2

p

…

p

hN

p

b1

p

b2

p

…

p

bM

p p

b1

b2

p

…

p

bM

p p

h1

pp p p p p p p p p p p p p p

64800 channel bits

pp = Optical Pulsehihn = Header Bit b

ibn = (Encoded/Interleaved) Channel Bit

de = Empty Bit (Off-Time)

p

h1 … hN h1 … hNe e … e

e e … eb1 b2 … bN e e … eb1 b2 … bN

h1

p

…

p

hN

p p

176 header bits

e e … e

…

Dx176 emptybits

p p p p

176header bits

…

Dx176 emptybits

h1 … hN e e … e

1D

RateMax

RateMax h1 hN…h2 b1 b2 … bNb3 b4 b5 b6 h1

p p p p

176header bits

…

Dx176 emptybits

h1 … hN e e … e

p p p p p …

e e … eb1 b2 … bN

176 channelbits

Dx176 emptybits

b1 … bN

p pp p

32 channelbits

64800 channel bits

32 channel bits (from previous burst)

e

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Summary of Data Rate Modes

Mode Name User Data Rate (Mbps)

Q Repetition

Interleaver N

Interleaver B (bits)

U-1244 1244 (Max) 1 162 96,000

U-622 622 (Max/2) 1 162 48,000

U-311 311 (Max/4) 1 162 24,000

U-155 155.5 (Max/8) 1 162 11,200

U-51.8 51.8 (Max/24) 1 162 4,000

U-16 16 2 162 1216

U-8 8 4 162 608

U-4 4 8 162 288

U-2 2 16 162 128

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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

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Relay Link ExampleApplication

Transport

Network

Link

LAN HDLC

FEC/ILV/ Q-Repeat

Physical Link

User Platform

User Terminal

Phy Frame

Relay Platform

Relay Terminal 1

Phy FrameRelay Terminal 2

Physical Link

HDLC

Ground RelayNetwork

Link

Ground Network

Switch

Optical relay serves as a transparent link-layer bridge between User Platform and Ground Relay

Phy Frame

Random

Modulation

De-Random

De-Mod

Random

Modulation

FEC/DeILV/ De-Q-Rep.

Phy Frame

De-Random

De-Mod

Transparent Bridge

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