Consultative Committee for Space Data Systems
RECOMMENDATION FOR SPACE DATA SYSTEM STANDARDS
TELEMETRY CHANNEL CODING
CCSDS 101.0-B-3
BLUE BOOK
MAY 1992
TMG 8/92
CCSDS RECOMMENDATION FOR TELEMETRY CHANNEL CODING
AUTHORITY
Issue: Date: Location:
Blue Book, Issue-3 May 1992 CCSDS Meeting, Oberpfaffenhofen, Germany
This Recommendation reflects the consensus technical agreement of the following member Agencies of the Consultative Committee for Space Data Systems (CCSDS): o o o o o o o o British National Space Centre (BNSC)/United Kingdom Canadian Space Agency (CSA)/Canada Centre National d'Etudes Spatiales (CNES)/France Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR)/Germany European Space Agency (ESA)/Europe Instituto de Pesquisas Espaciais (INPE)/Brazil. National Aeronautics and Space Administration (NASA)/USA National Space Development Agency of Japan (NASDA)/Japan
The following observer Agencies also concur with this Recommendation: o o Department of Communications, Communications Research Centre (DOC-CRC)/Canada. Institute of Space and Astronautical Science (ISAS)/Japan
This Recommendation is published and maintained by: CCSDS Secretariat Communications and Data Systems Division (Code-OS) National Aeronautics and Space Administration Washington, DC 20546, USA
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CCSDS RECOMMENDATION FOR TELEMETRY CHANNEL CODING
STATEMENT OF INTENT
The Consultative Committee for Space Data Systems (CCSDS) is an organization officially established by the management of member space Agencies. The Committee meets periodically to address data systems problems that are common to all participants, and to formulate sound technical solutions to these problems. Inasmuch as participation in the CCSDS is completely voluntary, the results of Committee actions are termed RECOMMENDATIONS and are not considered binding on any Agency. This RECOMMENDATION is issued by, and represents the consensus of, the CCSDS Plenary body. Agency endorsement of this RECOMMENDATION is entirely voluntary. Endorsement, however, indicates the following understandings: o Whenever an Agency establishes a CCSDS-related STANDARD, this STANDARD will be in accord with the relevant RECOMMENDATION. Establishing such a STANDARD does not preclude other provisions which an Agency may develop. Whenever an Agency establishes a CCSDS-related STANDARD, the Agency will provide other CCSDS member Agencies with the following information: ---o The STANDARD itself. The anticipated date of initial operational capability. The anticipated duration of operational service.
o
Specific service arrangements shall be made via memoranda of agreement. Neither this RECOMMENDATION nor any ensuing STANDARD is a substitute for a memorandum of agreement.
No later than five years from its date of issuance, this Recommendation will be reviewed by the CCSDS to determine whether it should: (1) remain in effect without change; (2) be changed to reflect the impact of new technologies, new requirements, or new directions; or (3) be retired or cancelled.
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FOREWORD
This document is a technical Recommendation for use in developing telemetry channel coding systems and has been prepared by the Consultative Committee for Space Data Systems (CCSDS). The telemetry channel coding concept described herein is the baseline concept for spacecraft-to-ground data communication within missions that are cross-supported between Agencies of the CCSDS. This Recommendation establishes a common framework and provides a common basis for the coding schemes used on spacecraft telemetry streams. It allows implementing organizations within each Agency to proceed coherently with the development of compatible derived Standards for the flight and ground systems that are within their cognizance. Derived Agency Standards may implement only a subset of the optional features allowed by the Recommendation and may incorporate features not addressed by the Recommendation. Through the process of normal evolution, it is expected that expansion, deletion or modification to this document may occur. This Recommendation is therefore subject to CCSDS document management and change control procedures which are defined in Reference [1].
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DOCUMENT CONTROL
Document/Title
Recommendation for Space Data System Standards: Telemetry Channel Coding, Issue-1.
Date
May 1984
Status and Substantive Changes
Original Issue.
CCSDS 101.0-B-2, Recommendation for Space Data System Standards: Telemetry Channel Coding, Issue-2.
Jan. 1987
1. Supersedes Issue-1. 2. Removes ASM from R-S encoded data space. 3. Specifies marker pattern for ASM. 4. Transfers Annex A ("Rationale") to Green Book.
CCSDS 101.0-B-3, Recommendation for Space Data System Standards: Telemetry Channel Coding, Issue-3.
May 1992
1. Supersedes Issue-2. 2. Deletes Section 3 ("Convolutional Coding with Interleaving for Tracking and Data Relay Satellite Operations"). 3. Adds R-S interleave depths of I=2,3,4 to existing I=1 and 5. 4. Allows R-S code to be operated in "Standalone Mode" (i.e., not concatenated with the convolutional code). 5. Consolidates codeblock and transfer frame sync specifications (new Section 5). 6. Specifies a standard Pseudo-Randomizer to improve bit synchronization (new Section 6). 7. Corrects several editorial errors.
NOTE:
Substantive technical changes from the previous issue are flagged with change bars in the margin
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CONTENTS
Sections Page
REFERENCES................................................................................ vi 1 INTRODUCTION ............................................................................ 1.1 PURPOSE .............................................................................. 1.2 SCOPE .................................................................................. 1.3 APPLICABILITY...................................................................... 1.4 BIT NUMBERING CONVENTION AND NOMENCLATURE................ 1.5 RATIONALE ........................................................................... 1-1 1-1 1-1 1-2 1-2 1-3
2 3 4
CONVOLUTIONAL CODING ............................................................. 2-1 [Deleted.] REED-SOLOMON CODING ............................................................... 4-1 4.1 INTRODUCTION ..................................................................... 4-1 4.2 SPECIFICATION...................................................................... 4-1 SYNCHRONIZATION ...................................................................... 5-1 PSEUDO-RANDOMIZER ................................................................... 6-1
5 6
Annexes A [Deleted] B TRANSFORMATION BETWEEN BERLEKAMP AND CONVENTIONAL REPRESENTATIONS ........................................ B-1 C EXPANSION OF REED-SOLOMON COEFFICIENTS................................ C-1 D GLOSSARY OF TERMS.................................................................... D-1 Figures 2-1 Convolutional Encoder Block Diagram..................................................... 4-1 Functional Representation of R-S Interleaving............................................ 4-2 Codeblock Partitioning ....................................................................... 6-1 Pseudo-Randomizer Configuration ......................................................... 6-2 Pseudo-Randomizer Logic Diagram ........................................................ B-1 Transformational Equivalence ...............................................................
2-2 4-2 4-4 6-1 6-3 B-2
Table B-1 Equivalence of Representations ............................................................. B-5
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REFERENCES
[1] "Procedures Manual for the Consultative Committee for Space Data Systems", CCSDS A00.0-Y-5.0, Consultative Committee for Space Data Systems, May 1992 or later issue. [Deleted.] "Packet Telemetry", Recommendation CCSDS 102.0-B-2, Consultative Committee for Space Data Systems, January 1987 or later issue. Perlman, M., and Lee, J., Reed-Solomon Encoders - Conventional vs. Berlekamp's Architecture, JPL Publication 82-71, NASA-Jet Propulsion Laboratory, Pasadena, California, December 1, 1982. "Telemetry Concept and Rationale", CCSDS 100.0-G-1, Green Book, Consultative Committee for Space Data Systems, January 1987 or later issue. "Minimum Modulated Symbol Transition Density on the Space-to-Earth Link", Recommendation CCSDS 401 (2.4.9) B-1, Consultative Committee for Space Data Systems, September 1989 or later issue. "Advanced Orbiting Systems: Networks and Data Links," Recommendation CCSDS 701.0-B-1, Consultative Committee for Space Data Systems, October 1989 or later issue.
[2] [3]
[4]
[5]
[6]
[7]
The latest issues of the CCSDS documents may be obtained from the CCSDS Secretariat at the address indicated on page i.
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1
1.1
INTRODUCTION
PURPOSE
The purpose of this document is to establish a common Recommendation for space telemetry channel coding systems to provide cross-support among missions and facilities of member Agencies of the Consultative Committee for Space Data Systems (CCSDS.) In addition, it provides focusing for the development of multi-mission support capabilities within the respective Agencies to eliminate the need for arbitrary, unique capabilities for each mission. Telemetry channel coding is a method of processing data being sent from a source to a destination so that distinct messages are created which are easily distinguishable from one another. This allows reconstruction of the data with low error probability, thus improving the performance of the channel.
1.2
SCOPE
Several space telemetry channel coding schemes are described in this document. The characteristics of the codes are specified only to the extent necessary to ensure interoperability and cross-support. The specification does not attempt to quantify the relative coding gain or the merits of each approach discussed, nor the design requirements for encoders or decoders. Some performance information is included in Reference [5]. This Recommendation does not require that coding be used on all cross-supported missions. However, for those planning to use coding, the recommended codes to be used are those described in this document. The rate 1/2 convolutional code recommended for cross-support is described in Section 2, "Convolutional Coding". Depending on performance requirements, this code alone may be satisfactory. For telecommunication channels which are bandwidth-constrained and cannot tolerate the increase in bandwidth required by the convolutional code in Section 2, the Reed-Solomon Code specified in Section 4 has the advantage of smaller bandwidth expansion and has the capability to indicate the presence of uncorrectable errors. Where a greater coding gain is needed than can be provided by the convolutional code or ReedSolomon code alone, a concatenation of the convolutional code as the inner code with the Reed-Solomon code as the outer code may be used for improved performance. The recommended method for frame (or codeblock) synchronization is described in Section 5. To improve bit transition density as an aid to bit synchronization, a recommended method of pseudo-randomizing data to be sent over the telemetry channel is described in Section 6.
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Annex B provides a discussion of the transformation between the Berlekamp and conventional Reed-Solomon symbol representations; Annex C provides a table showing the expansion of the Reed-Solomon coefficients; and Annex D is a glossary of coding terminology used in this document.
1.3
APPLICABILITY
This Recommendation applies to telemetry channel coding applications of space missions anticipating cross-support among CCSDS member Agencies at the coding layer. In addition, it serves as a guideline for the development of compatible internal Agency Standards in this field, based on good engineering practice. In addition to being applicable to conventional Packet Telemetry systems [3], the codes in this recommendation are applicable to the forward and return links of Advanced Orbiting Systems (AOS) [7]. For coding purposes, the terms "Transfer Frame" and "Reed-Solomon Codeblock" as used in this recommendation are understood to be equivalent to the AOS terms "Virtual Channel Data Unit" (VCDU), and "Coded Virtual Channel Data Unit" (CVCDU), respectively.
1.4
BIT NUMBERING CONVENTION AND NOMENCLATURE
In this document, the following convention is used to identify each bit in a forward-justified N-bit field. The first bit in the field to be transmitted (i.e., the most left justified when drawing a figure) is defined to be "Bit 0"; the following bit is defined to be "Bit 1" and so on up to "Bit N-1". When the field is used to express a binary value (such as a counter), the Most Significant Bit (MSB) shall be the first transmitted bit of the field, i.e., "Bit 0".
BIT 0 BIT N-1
N-BIT DATA FIELD
FIRST BIT TRANSMITTED = MSB
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In accordance with modern data communications practice, spacecraft data fields are often grouped into 8-bit "words" which conform to the above convention. Throughout this Recommendation, the following nomenclature is used to describe this grouping:
8-BIT WORD = "OCTET"
1.5
RATIONALE
The CCSDS believes it is important to document the rationale underlying the standards chosen, so that future evaluations of proposed changes or improvements will not lose sight of previous decisions. The concept and rationale for Telemetry Channel Coding may be found in Reference [5].
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2
CONVOLUTIONAL CODING
The basic code selected for cross-support is a rate 1/2, constraint-length 7, transparent convolutional code which is well suited for channels with predominantly Gaussian noise. The convolutional decoder is a maximum-likelihood (Viterbi) decoder. If the decoder's correction capability is exceeded, undetected burst errors may appear in the output. The code may be used alone, as described in this section, or in conjunction with enhancements described in the following sections. This recommendation is a non-systematic code and a specific decoding procedure, with the following characteristics:1 (1) Nomenclature: Convolutional code with maximum-likelihood (Viterbi) decoding 1/2 bit per symbol 7 bits G1 = 1111001; G2 = 1011011
(2) (3) (4) (5) (6)
Code rate: Constraint length: Connection vectors: Phase relationship: Symbol inversion:
G1 is associated with first Symbol On output path of G2
An encoder block diagram is shown in Figure 2-1. It is recommended that soft bit decisions with at least 3-bit quantization be used whenever constraints (such as location of decoder) permit.
1
When suppressed-carrier modulation systems are used, NRZ-M or NRZ-L may be used as a modulating waveform. If the user contemplates conversion of his modulating waveform from NRZ-L to NRZ-M, such conversion should be performed on-board at the input to the convolutional encoder. Correspondingly, the conversion on the ground from NRZ-M to NRZ-L should be performed at the output of the convolutional decoder. This avoids unnecessary link performance loss. CAUTION: When a fixed pattern (the fixed part of the convolutionally encoded Attached Sync Marker) in the symbol stream is used to provide node synchronization for the Viterbi decoder, care must be taken to account for any modification of the pattern due to the modulating waveform conversion.
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G2
+
+
+
+
2 INPUT 0 1 2 3 4 5 6 1 S1 OUTPUT
+
+
+
+
G1
NOTES: 1. = SINGLE BIT DELAY.
2.
FOR EVERY INPUT BIT, TWO SYMBOLS ARE GENERATED BY COMPLETION OF A CYCLE FOR S1: POSITION 1, POSITION 2. S1 IS IN THE POSITION SHOWN (1) FOR THE FIRST SYMBOL ASSOCIATED WITH AN INCOMING BIT.
3.
4.
+
= MODULO-2 ADDER.
5.
= INVERTER.
Figure 2-1: Convolutional Encoder Block Diagram
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3
[Deleted.]
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4
4.1
REED-SOLOMON CODING
INTRODUCTION
The Reed-Solomon code defined in this section is a powerful burst error correcting code. In addition, the code chosen has an extremely low undetected error rate. This means that the decoder can reliably indicate whether it can make the proper corrections or not. To achieve this reliability, proper codeblock synchronization is mandatory. The Reed-Solomon code may be used alone, and as such it provides an excellent forward error correction capability in a burst-noise channel. However, should the Reed-Solomon code alone not provide sufficient coding gain, it may be concatenated with the convolutional code defined in Section 2. Used this way, the Reed-Solomon code is the outer code, while the convolutional code is the inner code. 4.2 SPECIFICATION
The parameters of the selected Reed-Solomon (R-S) code are as follows: (1) (2) (3) J = 8 bits per R-S symbol. E = 16 R-S symbol error correction capability within a Reed-Solomon codeword. General characteristics of Reed-Solomon codes: (a) (b) (c) J, E, and I (the depth of interleaving) are independent parameters. n = 2J-1 = 255 symbols per R-S codeword. 2E is the number of R-S symbols among n symbols of an R-S codeword representing checks. k = n-2E is the number of R-S symbols among n R-S symbols of an R-S codeword representing information.
(d)
(4)
Field generator polynomial: F(x) = x8 + x7 + x2 + x + 1 over GF(2).
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(5)
Code generator polynomial:
143 32
g(x) =
Π (x - α11j) j = 112
=
∑0Gixi i=
over GF(28), where F(α) = 0. It should be recognized that α11 is a primitive element in GF(28) and that F(x) and g(x) characterize a (255,223) Reed-Solomon code. (6) (7) The selected code is a systematic code. This results in a systematic codeblock. Symbol Interleaving The allowable values of interleaving depth are I=1, 2, 3, 4, and 5. I=1 is equivalent to the absence of interleaving. The interleaving depth shall normally be fixed on a physical channel for a mission. Symbol interleaving is accomplished in a manner functionally described with the aid of Figure 4-1. (It should be noted that this functional description does not necessarily correspond to the physical implementation of an encoder.)
R-S ENCODER #1 S1 IN • • • S2 OUT
R-S ENCODER #I
Figure 4-1:
Functional Representation of R-S Interleaving
Data bits to be encoded into a single Reed-Solomon Codeblock enter at the port labeled "IN". Switches S1 and S2 are synchronized together and advance from encoder to encoder in the sequence 1,2, ... I, 1 ... , spending one R-S symbol time (8 bits) in each position. One codeblock will be formed from 223I R-S symbols entering "IN". In this functional representation, a space of 32I R-S symbols in duration is required between each entering set of 223I R-S information symbols.
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Due to the action of S1, each encoder accepts 223 of these symbols, each symbol spaced I symbols apart (in the original stream). These 223 symbols are passed directly to the output of each encoder. The synchronized action of S2 reassembles the symbols at the port labeled "OUT" in the same way as they entered at "IN". Following this, each encoder outputs its 32 check symbols, one symbol at a time, as it is sampled in sequence by S2. If, for I=5, the original symbol stream is
1 1 5 1 1 2 5 2 1 223 5
d ... d d ... d ... d
... d
223
[32 x 5 spaces]
then the output is the same sequence with the [32 x 5 spaces] filled by the [32 x 5] check symbols as shown below:
1 5 1 1 5
p ... p
1
... p
32
... p
32
where
i i i 223 i i 32
d d ... d
1 2
p ... p
1
is the R-S codeword produced by the ith encoder. If q virtual fill symbols are used in each codeword, then replace 223 by (223 - q) in the above discussion. With this method of interleaving, the original kI consecutive information symbols that entered the encoder appear unchanged at the output of the encoder with 2EI R-S check symbols appended. (8) Maximum Codeblock Length The maximum codeblock length, in R-S symbols, is given by: Lmax = nI = (2J - 1)I = 255I
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(9)
Shortened Codeblock Length 1 A shortened codeblock length may be used to accommodate frame lengths smaller than the maximum. However, since the Reed-Solomon code is a block code, the decoder must always operate on a full block basis. To achieve a full codeblock, "virtual fill" must be added to make up the difference between the shortened block and the maximum codeblock length. The characteristics and limitations of virtual fill are covered in paragraph (10). Since the virtual fill is not transmitted, both encoder and decoder must be set to insert it with the proper length for the encoding and decoding processes to be carried out properly. When an encoder (initially cleared at the start of a block) receives kI-Q symbols representing information (where Q, representing fill, is a multiple of I, and is less than kI), 2EI check symbols are computed over kI symbols, of which the leading Q symbols are treated as all-zero symbols. A (nI-Q, kI-Q) shortened codeblock results where the leading Q symbols (all zeros) are neither entered into the encoder nor transmitted.
(10) Partitioning and Virtual Fill The codeblock is partitioned as shown in Figure 4-2.
ATTACHED SYNC MARKER SYNC • • • SYNC • • • TRANSMITTED CODEBLOCK TRANSFER FRAME (UNCODED) R-S CHECK SYMBOLS
SYNC
•
•
VIRTUAL FILL (OPTIONAL)
TRANSFER FRAME (UNCODED) LOGICAL CODEBLOCK
R-S CHECK SYMBOLS
Figure 4-2:
Codeblock Partitioning
1 It
should be noted that shortening the transmitted codeblock length in this way changes the overall performance to a degree dependent on the amount of virtual fill used. Since it incorporates no virtual fill, the maximum Packet Telemetry transfer frame length recommended in Reference [3] allows full performance. In addition, as virtual fill in a codeblock is increased (at a specific bit rate), the number of codeblocks per unit time that the decoder must handle increases. Therefore, care should be taken so that the maximum operating speed of the decoder (codeblocks per unit time) is not exceeded.
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The Reed-Solomon Check Symbols consist of the trailing 2EI symbols (2EIJ bits) of the codeblock. (As an example, for I=5 this is always 1280 bits.) The Telemetry Transfer Frame is defined by the CCSDS Recommendation for Packet Telemetry (Reference [3]). Whether used with R-S coding or not, it has a maximum length of 8920 bits, not including the 32-bit Attached Sync Marker. The Attached Sync Marker is a 32-bit pattern specified in Section 5 as an aid to synchronization, and precedes the Telemetry Transfer Frame (if R-S coding is NOT used) or the Transmitted Codeblock (if R-S coding IS used). Frame synchronizers should, therefore, be set to expect a marker at every Telemetry Transfer Frame + 32 bits (if not R-S coded,) or at every Transmitted Codeblock + 32 bits. The Transmitted Codeblock consists of the Telemetry Transfer Frame (without the 32-bit sync marker) and R-S check symbols. It is the received data entity physically fed into the R-S decoder. (As an example, using I=5 and no virtual fill, the length of the transmitted codeblock will be 10,200 bits; if virtual fill is used, it will be incrementally shorter, depending on the amount used.) The Logical Codeblock is the logical data entity operated upon by the R-S decoder. It can have a different length than the transmitted codeblock because it accounts for the amount of virtual fill that was introduced. (As an example, for I=5 the logical codeblock always appears to have exactly 10,200 bits in length.) Virtual fill is used to logically complete the codeblock and is not transmitted. If used, virtual fill shall: (a) (b) (c) (d) Consist of all zeros. Not be transmitted. Not change in length during a tracking pass. Be inserted only at the beginning of the codeblock (i.e., after the attached sync marker but before the beginning of the transmitted codeblock). Be inserted only in integer multiples of 8I bits.
(e)
(11) Dual basis symbol representation and ordering for transmission Each 8-bit Reed-Solomon symbol is an element of the finite field GF(256). Since GF(256) is a vector space of dimension 8 over the binary field GF(2), the actual 8bit representation of a symbol is a function of the particular basis that is chosen.
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One basis for GF(256) over GF(2) is the set {1, α1, α2, ..., α7}. This means that any element of GF(256) has a representation of the form u7α7 + u6α6 + ... + u1α1 + u0α0 where each ui is either a zero or a one. Another basis over GF(2) is the set (1, ß1, ß2, . . . ß7) where ß = α 117. To this basis there exists a so-called "dual basis" ( 0 , 1 , . . . 7). It has the property that Tr ( iβj) =
{
1, if i = j 0, otherwise
for each j = 0, 1, . . . 7. The function Tr(z), called the "trace", is defined by
Tr (z) =
∑
k=0
7
z2
k
for each element z of GF(256). Each Reed-Solomon symbol can also be represented as z0 0 + z1 1 + ... + z7 7 where each zi is either a zero or a one. The representation used in this recommendation is the dual basis eight-bit string z0, z1, ..., z7, transmitted in that order (i.e., with z0 first). The relationship between the two representations is given by the two equations 1 0 0 0 1 1 0 1 1 1 1 0 1 1 1 1 1 1 1 0 1 1 0 0 [z 0 , . . . , z 7 ] = [ u 7 , . . . , u 0 ] 1 0 0 0 0 1 1 0 1 1 1 1 1 0 1 0 1 0 0 1 1 0 0 1 1 0 1 0 1 1 1 1 0 1 1 1 1 0 1 1 and 1 0 0 1 1 0 1 1 1 1 0 1 1 1 0 1 0 0 1 1 1 1 1 0 0 0 0 1 1 1 0 0 0 0 1 1 0 1 1 1 1 0 1 1 0 0 1 1 0 1 1 0 0 0 0 0 1 0 0 1 0 1 0 0
May 1992
[u7 , . . . , u 0 ] = [ z0 , . . . , z 7 ]
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Further information relating the dual basis (Berlekamp) and conventional representations is given in Annex B. Also included is a recommended scheme for permitting the symbols generated in a conventional encoder to be transformed to meet the symbol representation required by this document. (12) Synchronization Codeblock synchronization of the Reed-Solomon decoder is achieved by synchronization of the Attached Sync Marker associated with each codeblock. (See Section 5.) (13) Ambiguity Resolution The ambiguity between true and complemented data must be resolved so that only true data is provided to the Reed-Solomon decoder. Data in NRZ-L form is normally resolved using the 32-bit Attached Sync Marker, while NRZ-M data is self-resolving.
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5
5.1
SYNCHRONIZATION
INTRODUCTION
Frame or Codeblock synchronization is necessary for proper decoding of the Reed-Solomon Codeblock and subsequent processing of the Transfer Frames. Furthermore, it is necessary for synchronization of the pseudo-random generator, if used (See Section 6). It is also useful in assisting the node synchronization process of the Viterbi decoder for the convolutional code. Synchronization of the Codeblock (or Transfer Frame, if the telemetry channel is not ReedSolomon coded) is achieved by using a stream of fixed-length Codeblocks (or Transfer Frames) with an Attached Sync Marker (ASM) between them. Synchronization is acquired on the receiving end by recognizing the specific bit pattern of the ASM in the telemetry channel data stream; synchronization is then customarily confirmed by making further checks. 5.2 ASM BIT PATTERN
The ASM for the telemetry channel data stream shall consist of a 32-bit (4-octet) marker with a pattern as follows:
0001 1010 1100 1111 1111 1100 0001 1101
FIRST TRANSMITTED BIT (Bit 0)
LAST TRANSMITTED BIT (Bit 31)
The pattern is represented in hexadecimal notation as: 1ACFFC1D
5.3
LOCATION OF ASM
The ASM is attached to (i.e., shall immediately precede) the Reed-Solomon Codeblock (or the Transfer Frame if the telemetry channel is not Reed-Solomon coded). The ASM for one Codeblock (or Transfer Frame) shall immediately follow the end of the preceding Codeblock (or Transfer Frame); i.e., there shall be no intervening bits (data or fill) preceding the ASM.
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5.4
RELATIONSHIP OF ASM TO R-S CODEBLOCK
Since the ASM is NOT a part of the encoded data space of the Reed-Solomon Codeblock, it is not presented to the input of the Reed-Solomon encoder or decoder. This prevents the encoder from routinely regenerating a second, identical marker in the check symbol field under certain repeating data-dependent conditions (e.g., a test pattern of 01010101010 ... among others) which could cause synchronization difficulties at the receiving end. The relationship among the ASM, Reed-Solomon Codeblock, Transfer Frame and virtual fill is illustrated in Figure 4-2.
5.5
ASM FOR EMBEDDED DATA STREAM
A different ASM pattern may be required where another data stream (e.g., a stream of transfer frames played back from a tape recorder in the forward direction) is inserted into the data field of the Transfer Frame of the main stream appearing on the telemetry channel. The ASM for the embedded data stream, to differentiate it from the main stream marker, shall consist of a 32-bit (4-Octet) marker with a pattern as follows:
0011 0101 0010 1110 1111 1000 0101 0011
FIRST TRANSMITTED BIT (Bit 0)
LAST TRANSMITTED BIT (Bit 31)
The pattern is represented in hexadecimal notation as: 352EF853
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6
6.1
PSEUDO-RANDOMIZER
INTRODUCTION In order to maintain bit (or symbol) synchronization with the received telemetry signal, every ground data capture system requires that the incoming signal have a minimum bit transition density (See Ref. [6]). If a sufficient bit transition density is not ensured for the channel by other methods (e.g., by use of certain modulation techniques or one of the recommended convolutional codes) then the Pseudo-Randomizer defined in this section is required. Its use is optional otherwise. The presence or absence of Pseudo-Randomization is fixed for a physical channel and is managed (i.e., its presence or absence is not signalled in the telemetry but must be known a-priori by the ground system).
6.2
PSEUDO-RANDOMIZER DESCRIPTION The method for ensuring sufficient transitions is to exclusive-OR each bit of the Codeblock or Transfer Frame with a standard pseudo-random sequence. On the receiving end, the same sequence is exclusive-ORed with the received Codeblock or Transfer Frame to remove the randomized pattern and restore the original data. If the Pseudo-Randomizer is used, on the sending end it is applied to the Codeblock or Transfer Frame but before convolutional encoding. On the receiving end, it is applied to derandomize the data after convolutional decoding and codeblock synchronization but before Reed-Solomon decoding, if used. The configuration at the sending end is shown in Figure 6-1.
CODEBLOCK OR TRANSFER FRAME
ATTACHED SYNC MARKER
Randomized output to modulator or convolutional encoder (if used)
PSEUDO-RANDOM SEQUENCE GENERATOR
Figure 6-1: Pseudo-Randomizer Configuration
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6.3
SYNCHRONIZATION AND APPLICATION OF PSEUDORANDOMIZER The Attached Sync Marker (ASM) is already optimally configured for synchronization purposes and it is therefore used for synchronizing the Pseudo-Randomizer. The pseudo-random sequence is applied starting with the first bit of the Codeblock or Transfer Frame. On the sending end, the Codeblock or Transfer Frame is randomized by exclusive-ORing the first bit of the Codeblock or Transfer Frame with the first bit of the pseudo-random sequence, followed by the second bit of the Codeblock or Transfer Frame with the second bit of the pseudo-random sequence, and so on. On the receiving end, the original Codeblock or Transfer Frame is reconstructed using the same pseudo-random sequence. After locating the ASM in the received data stream, the pseudo-random sequence is exclusive-ORed with the data bits immediately following the ASM. The pseudo-random sequence is applied by exclusive-ORing the first bit following the ASM with the first bit of the pseudo-random sequence, followed by the second bit of the data stream with the second bit of the pseudo-random sequence, and so on. The pseudo-random sequence shall NOT be exclusive-ORed with the ASM.
6.4
SEQUENCE SPECIFICATION The pseudo-random sequence shall be generated using the following polynomial: h(x) = x8 + x7 + x5 + x3 + 1 This sequence begins at the first bit of the Codeblock or Transfer Frame and repeats after 255 bits, continuing repeatedly until the end of the Codeblock or Transfer Frame. The sequence generator is re-initialized to an all-ones state during each ASM period. The first 40 bits of the pseudo-random sequence from the generator are shown below; the left-most bit is the first bit of the sequence to be exclusive-ORed with the first bit of the Codeblock or Transfer Frame; the second bit of the sequence is exclusive-ORed with the second bit of the Codeblock or Transfer Frame, and so on. 1111 1111 0100 1000 0000 1110 1100 0000 1001 1010 ....
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6.5
LOGIC DIAGRAM Figure 6-2 represents a possible generator for the specified sequence.
DATA IN (Codeblock or Transfer Frame)
+
DATA OUT (Randomized Codeblock or Transfer Frame)
+
+
+
X8
X7
X6
X5
X4
x3
X2
X1
Pseudo-random sequence
Initialize to an "all ones" state for each Codeblock or Transfer Frame during ASM period
+
= Modulo-2 adder (Exclusive-OR)
= Single Bit Delay
Figure 6-2: Pseudo-Randomizer Logic Diagram
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ANNEX B TRANSFORMATION BETWEEN BERLEKAMP AND CONVENTIONAL REPRESENTATIONS
(THIS ANNEX IS NOT PART OF THE RECOMMENDATION)
Purpose: This Annex provides information to assist users of the Reed-Solomon code in this Recommendation to transform between the Berlekamp (dual basis) and Conventional representations. In addition, it shows where transformations are made to allow a conventional encoder to produce the dual basis representation on which the Recommendation is based.
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Referring to Figure B-1, it can be seen that information symbols I entering and check symbols C emanating from the Berlekamp R-S encoder are interpreted as [z0 , z1 , ... , z7 ] where the components zi are coefficients of i, respectively: z0 0 + z1 1 + ... + z7 7 Information symbols I' entering and check symbols C' emanating from the conventional R-S encoder are interpreted as [u 7 , u6 , ... , u0 ] where the components uj are coefficients of αj, respectively: u7α7 + u6α6 + ... + u0 A pre- and post-transformation is required when employing a conventional R-S encoder. I
BERLEKAMP R-S ENCODER
C C
Tα
-1
Tα I' C'
CONVENTIONAL R-S ENCODER
Figure B-1:
Transformational Equivalence
Conventional and Berlekamp types of (255,223) Reed-Solomon encoders are assumed to have the same self-reciprocal generator polynomial whose coefficients appear in paragraph 4.2 (4) and (5). The representation of symbols associated with the conventional encoder is the polynomials in "α" appearing in Table B-1, below. Corresponding to each polynomial in "α" is the representation in the dual basis of symbols associated with the Berlekamp type encoder.
Given
αi = u7α7 + u6α6 + ... + u0
where 0 ≤ i k.
CHANNEL SYMBOL: The unit of output of the innermost encoder.
CODEBLOCK: A codeblock of an (n,k) block code is a sequence of n channel symbols which were produced as a unit by encoding a sequence of k information symbols, and will be decoded as a unit.
CODE RATE: The average ratio of the number of binary digits at the input of an encoder to the number of binary digits at its output.
CODEWORD: In a block code, one of the sequences in the range of the one-to-one transformation (see Block Encoding).
CONCATENATION: The use of two or more codes to process data sequentially with the output of one encoder used as the input of the next.
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CONNECTION VECTOR: In convolutional coding, a device used to specify one of the parity checks to be performed on the shift register in the encoder. For a binary constraint length k convolutional code, a connection vector is a k-bit binary number. A "one" in position i (counted from the left) indicates that the output of the ith stage of the shift register is to be used in computing that parity check.
CONSTRAINT LENGTH: In convolutional coding, the number of consecutive input bits that are needed to determine the value of the output symbols at any time.
CONVOLUTIONAL CODE: As used in this document, a code in which a number of output symbols are produced for each input information bit. Each output symbol is a linear combination of the current input bit as well as some or all of the previous k-1 bits where k is the constraint length of the code.
GF(n): The Galois Field consisting of exactly "n" elements.
INNER CODE: In a concatenated coding system, the last encoding algorithm that is applied to the data stream. The data stream here consists of the codewords generated by the outer decoder.
MODULATING WAVEFORM: A way of representing data bits ("1" and "0") by a particular waveform.
NRZ-L: A modulating waveform in which a data "one" is represented by one of two levels, and a data "zero" is represented by the other level.
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NRZ-M: A modulating waveform in which a data "one" is represented by a change in level and a data "zero" is represented by no change in level.
OUTER CODE: In a concatenated coding system, the first encoding algorithm that is applied to the data stream.
REED-SOLOMON (R-S) SYMBOL: A set of J bits that represents an element in GF(2J), the code alphabet of a J-bit Reed-Solomon code.
SYSTEMATIC CODE: A code in which the input information sequence appears in unaltered form as part of the output codeword.
TRANSPARENT CODE: A code that has the property that complementing the input of the encoder or decoder results in complementing the output.
VIRTUAL FILL: In a systematic block code, a codeword can be divided into an information part and a parity (check) part. Suppose that the information part is N symbols long (a symbol is defined here to be an element of the code's alphabet) and that the parity part is M symbols long. A "shortened" code is created by taking only S (S < N) information symbols as input, appending a fixed string of length N-S and then encoding in the normal way. This fixed string is called "fill". Since the fill is a predetermined sequence of symbols, it need not be transmitted over the channel. Instead, the decoder appends the same fill sequence before decoding. In this case, the fill is called "Virtual Fill".
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