Steps towards the Standardization of a more efficient Uplink Protocol and Code

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Steps towards the Standardization of a more
efficient Uplink Protocol and Code

• Greg J. Kazz
• Ed Greenberg
• NASA/JPL

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Problem Statement

• Greater Demand It is anticipated that greater
  uplink data throughput is required for future
  lunar and deep space missions. These trends are
  driven by
• Use of Selective repeat (ARQ) protocols ,e.g.,
  CFDP
• Available Uplink margin while using ARQ
• Standalone antenna replacement with arrays
• On-board applications demand larger uplink
  volumes
• Video applications drive data rates

CFDP CCSDS File Delivery Protocol
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IOAG Requirement

• Need for a standard approach Develop the
  functional and performance requirements for a
  CCSDS telecommand standard accommodating data
  rates up to 10 Mb/s by the year 2007.
• CCSDS Working Group High Rate Uplink WG formed
  to provide an integrated solution across
• RF Modulation
• Channel Coding
• Link Layer Protocols
• IOAG is the Interagency Operations Advisory
  Group composed of ASI, CNES, DLR, ESA, JAXA,
  NASA.

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ARQ Protocols are a potential Uplink Hog

• How do ARQ protocols require more bandwidth?
• Example
• If downlink rate is 30 Mb/sec and file size is
  6Mb, then ground receives 5 files/second.
• CFDP transfers 5 Finish Messages per second (file
  accepted by the ground) to the spacecraft
• Each Finish message is 160 bits long.
• 5 x 160 800 bits/sec
• Assuming nominal 2000 bit/sec uplink, CFDP
  nominal traffic is 40 which is required
  throughout the pass
• Worst case If retransmission necessary, more
  bandwidth needed

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But ARQ can provide more Uplink Margin

• Typical deep space mission operate with minimum 3
  dB uplink margin and require a minimum BER of
  10-5.
• By operating at 10-4 BER, a factor of 10 more
  errors can occur, but selective or preemptive
  retransmission fills in the missing file elements
• When sufficient RT light time permits use
  selective repeat (e.g., CFDP acknowledged mode)
• Else use Preemptive Retransmission (send
  duplicates) for small subsets of critical data

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Other Considerations

• Standalone Antenna Replacement with Arrays
• An array is efficient for supporting multiple
  spacecraft per aperture but is unproven for
  uplink.
• Initial array operations will time share the
  uplink which result in shorter commanding times
  per mission
• On-board Applications require larger volume
  Uplinks
• Software radio reconfiguration (FPGAs)
• Flight Software Uploads
• Video applications require high rates for manned
  missions

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Uplink Constraints Physical Layer

• Current State of the Art (SDST) deep space
  transponders are data rate limited
• Maximum uplink rate 4 Kb/sec
• Receiver Symbol SNR requirements limit the use
  of high performance codes
• Frequency bands and allocated bandwidths must
  meet SFCG guidelines
• Cost of reducing receiver thermal noise is
  significant

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Uplink Constraints Coding Layer

• The chosen code must be relatively easy for the
  flight system to implement
• Provide a mechanism to ensure extremely low
  undetected frame error rates
• Coding gain increases with increasing block size
  increases and decreasing code rate
• To benefit from the coding gain of state of the
  art coding, two things must be achieved in the
  receiver
• More power must allocated to the data and less to
  the carrier. (Set the Mod index)
• Receiver must be capable of operating at much
  lower Symbol Signal to Noise Ratios (SSNR)

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Uplink Constraints Link Layer

• Additional requirements for carrying voice and
  video along with data
• Create a data system that provides varying
  Qualities of Service based upon the data type
  (Low latency voice vs low error tolerant video).
• Potential need to multiplex IP and other types of
  data
• CCSDS link layer provides a rich set of
  multiplexing capability
• IP, SCPS, CCSDS Space packets, Encapsulation
  packets onto the same or different virtual
  channels
• Maximum frame length limits for data latency
  considerations
• Current max frame length is 16,384 bits
  (constrained by 1st header pointer)
• Frame lengths could be extended but current
  proposed block code lengths do not exceed it
• Use of Block Codes on the uplink requires a
  synchronous data link
• Currently the CCSDS Telecommand standard is an
  asynchronous data link

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Uplink Application Profiles
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Acknowledgement

• The authors also acknowledge the contribution of
  Ken Andrews, Dariush Divsalar, Sam Dollinar, Jon
  Hamkins, Bruce Moision and Fabrizio Pollara of
  JPLs Communications Architectures Research
  Section for the coding related aspects of this
  paper.

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Conclusion

• This paper summarizes the issues, constraints,
  and potential benefits associated with developing
  a standard higher rate uplink throughput to space
  missions.
• CCSDS has embarked upon the creation of a High
  Rate Uplink Working Group
• Charter is to develop an integrated RF and
  modulation, channel coding, and link layer
  solution to the high rate uplink needs expressed
  in this paper.