ETSI TR 103 956 V1.1.1 (2018-12)

Satellite Earth Stations and Systems (SES); Technical analysis for the Radio Frequency, Modulation and Coding for Telemetry Command and Ranging (TCR) of Communications Satellites

DTR/SES-00428

General Information

• Status: Published
• Publication Date: 16-Dec-2018
• Technical Committee: SES SCN - Satellite Communications and Navigation
• Current Stage: 12 - Completion
• Due Date: 06-Dec-2018
• Completion Date: 17-Dec-2018

Standards Content (Sample)

TECHNICAL REPORT
Satellite Earth Stations and Systems (SES);
Technical analysis for the Radio Frequency, Modulation and
Coding for Telemetry Command and Ranging (TCR)
of Communications Satellites
2 ETSI TR 103 956 V1.1.1 (2018-12)

Reference
DTR/SES-00428
Keywords
coding, modulation, satellite, telemetry
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3 ETSI TR 103 956 V1.1.1 (2018-12)
Contents
Intellectual Property Rights . 4
Foreword . 4
Modal verbs terminology . 4
Executive summary . 4
Introduction . 5
1 Scope . 6
2 References . 6
2.1 Normative references . 6
2.2 Informative references . 6
3 Definition of terms and abbreviations . 7
3.1 Terms . 7
3.2 Abbreviations . 10
4 Frequency Planning and Operational Scenarios . 11
4.1 Frequency Planning . 11
4.1.1 Frequency Bands . 11
4.1.2 Frequency Flexibility . 11
4.2 Hosted Payloads . 12
4.3 Operation during Launch and Early Orbit . 12
4.4 Operation in Other Orbits . 12
5 Spread Spectrum Modulation . 13
5.1 Extension of PN Code Family . 13
5.2 Symbol Rate . 15
5.3 New MTC3 Mode . 15
5.4 In-phase to Quadrature Power Ratio . 17
5.5 Out-of-Band Emission and Discrete Spurious . 18
5.6 Physical Layer Operations Procedure. 18
6 Non-spread Modulation . 19
6.1 Uplink Phase Modulation . 19
6.2 Miscellaneous . 19
7 Coding and Interleaving . 19
7.1 General . 19
7.2 Uplink . 20
7.2.1 Forward Error Correction . 20
7.2.2 Pseudo-randomization . 21
7.3 Downlink . 22
7.3.1 Forward Error Correction . 22
7.3.2 Pseudo-randomization . 23
8 Conclusion . 23
Annex A: Generation and Validation of the Extended PN Code Library . 24
A.1 Dual Channel Gold Codes . 24
Annex B: Validation of Doppler and Doppler Rate requirements . 27
B.1 Summary of Previous Work . 27
B.2 Additional Analysis . 27
B.3 Conclusion . 29
Annex C: Cryptographic Pseudo-random Codes . 30
History . 32

ETSI
4 ETSI TR 103 956 V1.1.1 (2018-12)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (https://ipr.etsi.org/).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Trademarks
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ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
Foreword
This Technical Report (TR) has been produced by ETSI Technical Committee Satellite Earth Stations and Systems
(SES).
Modal verbs terminology
In the present document "should", "should not", "may", "need not", "will", "will not", "can" and "cannot" are to be
interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
Executive summary
The present document provides the rationale for the revision of the ETSI TCR Standard [i.1].
The need for such revision appeared mainly as a consequence of the evolution of satellites, with new operational
frequency bands and configurations like mega-constellations; the progress on spread spectrum technology; the quest for
more flexibility in frequency planning and operations on geostationary telecommunication satellite fleets; and novel
demands concerning the accommodation of hosted payloads and their segregated operations in those satellites.
Therefore, the existing standard was revised in the following areas: frequency plan, operational phases, hosted payload
management application, mega-constellation application, spread spectrum modulation, phase and frequency modulation
and finally, coding and interleaving.
The revision has borrowed from the experience acquired by suppliers, operators and space agencies as well as from
standards produced in other relevant standardization fora like the European Cooperation for Space Standardization
(ECSS) or the Consultative Committee for Space Data Systems (CCSDS).
In summary, the present document provides a sufficient justification of the revision with pointers to annexes and
relevant references for those readers seeking further detail.
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5 ETSI TR 103 956 V1.1.1 (2018-12)
Introduction
The European Telecommunication Standards Institute (ETSI) established a telemetry, command and ranging (TCR)
standard ETSI EN 301 926 [i.2] in 2002.
In recent years telecommunication satellite operators have shown a renewed interest in spread spectrum systems. Their
improved performance under interference over the classical frequency modulation (FM) or phase modulation (PM)
could ease operations. In mission phases like orbit drift the TCR can be subject to interference while crossing the
equatorial orbital arc. With the emergence of electric orbit raising strategies, this phase has actually become much
longer making resistance to interference even more relevant.
In addition, hosted payloads are emerging as an attractive business proposition for telecommunication satellite
operators. The capability to have direct telecommand (TC) and telemetry (TM) communications with segregated
radiofrequency (RF) carriers and avionics could off-load to some extent hosted payload operations from satellite
operations. In addition, it could limit the interface impact between the satellite and the hosted payload respective ground
and space segments.
Meanwhile the technology of transponders/receivers as well as ground modems have evolved to support spread
spectrum modulation as well as frequency flexibility on FM/PM. For instance, the ability to acquire and track a spread
spectrum signal under high dynamics is no longer considered an issue, in contrast to the times when the first version of
the ETSI TCR standard was published. Such capability could simplify satellite TT&C sub-systems by eliminating the
need for dual-mode transponders (FM/PM and spread spectrum).
Moreover, mega-constellations for telecommunication missions are currently being developed. To accommodate a very
large number of new TCR carriers on existing bands, spread spectrum modulation could offer an efficient solution.
In consideration of all the above, ETSI initiated a work item to revise the standard in 2015. The goal was to match the
revised standard with the current and expected capabilities of transponders and ground modems for future
telecommunication missions, not only geostationary. ETSI EN 301 926 [i.1] revision has been published in 2017. The
present document, therefore, provides a description and justification of this revision.
Readers are encouraged to take into account that the present document builds upon and complements ETSI
TR 101 956 [i.3]. ETSI EN 301 926 (V1.3.1) [i.1] has not questioned the existing modulation trade-offs carried out for
the definition of the first issue of the standard.
Furthermore, it does not question the concept of Collocated Equivalent Capacity (CEC). However, it is recognized that
the addition of channel coding and interleaving will impact CEC by allowing to enhance capacity with respect to the
first version of the standard.
Following this introduction, the present document is organized as follows.
Clause 1 outlines the scope of the standard revision.
Clause 2 provides relevant informative references that can assist readers seeking a more detailed understanding of some
modifications.
Clause 3 recalls the terms and abbreviations employed throughout the document.
Clause 4 discusses the modifications impacting frequency planning and operational scenarios.
Clause 5 provides a detailed discussion of the key modifications affecting spread spectrum modulation like the
extension of the Pseudo-noise code family and others.
Clause 6 addresses key modifications affecting non-spread modulations.
Clause 7 introduces coding and interleaving options added to the standard.
Clause 8 gives a conclusion.
Finally, annexes A and B complement the main body of the document addressing detailed aspects, annex C provides
information for future possible work.

ETSI
6 ETSI TR 103 956 V1.1.1 (2018-12)
1 Scope
The present document provides the rationale for the revision of the ETSI TCR Standard ETSI EN 301 926 [i.1] in the
following areas:
• frequency plan;
• operational phases;
• hosted payload management application;
• mega-constellation application;
• spread spectrum modulation;
• phase and frequency modulation; and
• coding and interleaving.
2 References
2.1 Normative references
Normative references are not applicable in the present document.
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] ETSI EN 301 926 (V1.3.1) (10-2017): "Satellite Earth Stations and Systems (SES); Radio
Frequency and Modulation Standard for Telemetry, Command and Ranging (TCR) of
Communications Satellites".
[i.2] ETSI EN 301 926 (V1.2.1) (06-2002): "Satellite Earth Stations and Systems (SES); Radio
Frequency and Modulation Standard for Telemetry, Command and Ranging (TCR) of
Geostationary Communications Satellites".
[i.3] ETSI TR 101 956: "Satellite Earth Stations and Systems (SES); Technical analysis of Spread
Spectrum Solutions for Telemetry Command and Ranging (TCR) of Geostationary
Communications Satellites".
[i.4] CCSDS 231.0-B-x: "TC Synchronization and Channel Coding".
[i.5] CCSDS 131.0-B-x: "TM Synchronization and Channel Coding".
NOTE: CCSDS standards always include the issue number on their numbering system; the parameter 'x' on
references [i.4] and [i.5] is understood as the highest published number and therefore latest issue of the
standard.
[i.6] IEEE Transactions on Information Theory: "Optimal Binary Sequences for Spread Spectrum
Multiplexing", R. Gold, vol. IT-13, no. 1, pp. 619-621, 1967.
ETSI
7 ETSI TR 103 956 V1.1.1 (2018-12)
[i.7] NASA Publication Contract NAS 5-22546: "TDRSS Telecommunication System PN Code
Analysis final Report Addendum", R. Gold, Sept. 1977.
[i.8] Space Network Interoperability Group: "Space Network Interoperable PN Code Libraries",
Revision 1, Sept. 1998.
[i.9] CCSDS 230.1-G-x: "TC Synchronization and Channel Coding - Summary of Concept and
Rationale".
[i.10] CCSDS 130.1-G-x: "TM Synchronization and Channel Coding - Summary of Concept and
Rationale".
NOTE: CCSDS reports always include the issue number on their numbering system; the parameter 'x' on
references [i.9] and [i.10] is understood as the highest published number and therefore latest issue of the
standard.
[i.11] R. L. Miller, L. J. Deutsch, and S. A. Butman: "On the Error Statistics of Viterbi Decoding and the
Performance of Concatenated Codes". JPL Publication 81-9. Pasadena, California: JPL,
September 1, 1981.
[i.12] L. Deutsch, F. Pollara, and L. Swanson: "Effects of NRZ-M Modulation on Convolutional Codes
Performance". TDA Progress Report 42-77, January-March 1984 (May 15, 1984): 33-40.
[i.13] Space Network Users' Guide (SNUG). Revision 10. 450-SNUG. Greenbelt, Maryland: NASA
Goddard Space Flight Center, August 2012.
[i.14] I. Aguilar Sánchez et al.: "The Navigation and Communication Systems for the Automated
Transfer Vehicle", proceedings of the IEEE 49th Vehicular Technology Conference, Vol. 2,
pp. 1187-1192, 1999.
[i.15] G. Lesthievent et al.: "Concatenating the convolutional (7,1/2) code with the BCH in TED mode
with CRC for improved TC link in the CNES Myriad satellites family", Paper SLS-NGU-10-
CNES01, CCSDS Next Generation Uplink Working Group, London (UK), October 2010.
[i.16] CCSDS 231.1-O-1: "Short Block Length LDPC Codes for TC Synchronization and Channel
Coding".
[i.17] ECSS-E-ST-50-05C Rev. 2: "Space Engineering - Radio frequency and modulation", European
Cooperation for Space Standardization, 4 October 2011.
[i.18] NIST: "Advanced Encryption Standard (AES)", Federal Information Processing Standard
Publication 197, United States, November 26, 2001.
[i.19] NIST Special Publication 800-38A: "Recommendation for Block Cipher Modes of Operation:
Methods and Techniques", United States, December 2001.
3 Definition of terms and abbreviations
3.1 Terms
For the purposes of the present document, the following terms apply:
binary channel: binary communications channel (BPSK has 1 channel, QPSK has 2 channels)
channel symbol rate: rate of binary elements, considered on a single wire, after FEC coding and channel allocation
NOTE: See Figures 2, 3 and 4. This applies only to multi-channel modulations, thus to spread spectrum QPSK
modes and not to PM/FM modes.
Co-located Equivalent Capacity (CEC): number of collocated satellites that can be controlled with a perfect power
balanced link between the ground and the satellite
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8 ETSI TR 103 956 V1.1.1 (2018-12)
Code Division Multiple Access (CDMA): technique for spread-spectrum multiple-access digital communications that
creates channels through the use of unique code sequences
Command Link Transmission Unit (CLTU): telecommand protocol data structure providing synchronization for the
codeblock and delimiting the beginning of user data
NOTE: See [i.4], section 4 for further details.
data rate: total number of uncoded data bits per second after packet and frame encoding
NOTE: See Figures 1 to 4. This is the data rate used in link budgets in ETSI TR 101 956 [i.3].
Direct Sequence Spread Spectrum (DSSS): form of modulation where a combination of data to be transmitted and a
known code sequence (chip sequence) is used to directly modulate a carrier, e.g. by phase shift keying
symbol rate: rate of binary elements, considered on a single wire, after FEC coding
NOTE: See Figures 1 to 4.
MTC1 / MTM1
Ranging
Tones
RF
Carrier
PM or FM
Modulation
Symbol Rate
Subcarrier
Data Waveform BPSK
Channel Coding
Source Formating Modulation
Scope of this Document
Channel Coding
Bit Rate
Pseudo- Differential Convolutional
Block Code
Start Sequence
(optional) Randomizer Coder Coder
ASM
BCH, R-S, LDPC
(optional) (optional) (optional)

Figure 1: Functional stages of transmit chain for FM/PM modulation (MTC1/MTM1)
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9 ETSI TR 103 956 V1.1.1 (2018-12)

Figure 2: Functional stages of transmit chain for spread spectrum modulation MTC2

Figure 3: Functional stages of transmission chain for spread spectrum modulation MTC
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10 ETSI TR 103 956 V1.1.1 (2018-12)
MTM2 / MTM3
Chip Rate I Ch. PN Code
I Channel
Waveform
BPSK
Formating
Modulation
Symbol Rate
RF
Carrier
Data Channel Coding
Channel Symbol Rate
Source (*)
(*) Refer to
MTC1 / MTM1
Q Channel
Waveform
BPSK
Formating
Modulation
Chip Rate Q Ch. PN Code
Scope of this Document
Figure 4: Functional stages of transmission chain for spread spectrum modulation MTM2/MTM3
3.2 Abbreviations
For the purposes of the present document, the following abbreviations apply:
AES Advanced Encryption Standard
ATV Automated Transfer Vehicle
BCH Bose-Chaudhuri-Hocquenghem
BER Bit Error Rate
BPSK Binary Phase Shift Keying
CCSDS Consultative Committee for Space Data Systems
CDMA Code Division Multiple Access
CEC Co-located Equivalent Capacity
CLTU Command Link Transmission Unit
CMM Carrier Modulation Modes
CNES Centre National d'Etudes Spatiales
CRC Cyclic Redundancy Check
CW Continuous Wave
DC Direct Current
DSSS Direct Sequence Spread Spectrum
ECSS European Cooperation for Space Standardization
FEC Forward Error Correction
FM Frequency Modulation
GSID Ground-to-Satellite Interface Specification
GSO Geo-Stationary Orbit
GTO Geostationary Transfer Orbit
I In-phase
LDPC Low Density Parity Check
LEO Low Earth Orbit
LEOP Launch and Early Orbit Phase
MAI Multiple Access Interference
MTC1 TeleCommand Mode 1
MTC2 TeleCommand Mode 2
MTC3 TeleCommand Mode 3
MTM1 TeleMetry Mode 1
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11 ETSI TR 103 956 V1.1.1 (2018-12)
MTM2 TeleMetry Mode 2
MTM3 TeleMetry Mode 3
NASA National Aeronautics and Space Administration (USA)
NIST National Institute of Standards and Technology
NRZ Non-Return Zero
NRZ-L Non Return to Zero-Level
NRZ-M Non Return to Zero-Mark
NTIA National Telecommunications Industry Association
PDF Probability Density Function
PED Phase Error Detector
PLL Phase Locked Loop
PLOP Physical Layer Operating Procedures
PM Phase Modulation
PN Pseudo Noise
Q Quadrature
QPSK Quaternary Phase Shift Keying
RF Radio Frequency
RS Reed-Solomon
SEC Single Error Correction
SER Symbol Error Rate
SNR Signal to Noise Ratio
SSTO Single-Stage-To-Orbit
TC TeleCommand
TCR Telemetry, Command and Ranging
TDRSS Tracking and Data Relay Satellite System (NASA)
TED Triple Error Detection
TM TeleMetry
TT&C Telemetry, Tracking and Command
VLSI Very Large Scale Integration
4 Frequency Planning and Operational Scenarios
4.1 Frequency Planning
4.1.1 Frequency Bands
a) C-band: 5 850 MHz to 6 725 MHz uplink, 3 400 MHz to 4 200 MHz downlink;
b) Ku-band: 12 750 MHz to 14 800 MHz and 17 300 MHz to 18 100 MHz uplink, 10 700 MHz to 12 750 MHz
downlink;
c) Commercial Ka-band: 27 500 MHz to 30 000 MHz uplink, 17 700 MHz to 20 700 MHz downlink.
It should be noted that these bands are due to prevailing regulations, not physics. Possible usage of the TCR techniques
considered in the present document and ETSI EN 301 926 [i.1] in adjacent bands between 1 GHz and 44 GHz may be
envisaged.
4.1.2 Frequency Flexibility
Modern command receivers and telemetry transmitters often utilize fractional N phase-locked loop (PLL) synthesizers
for frequency generation. It is possible to generate different output frequencies from a single input reference frequency
using this technology. The frequency resolution of such synthesizers is very high, in the order of a few Hertz, which is
clearly more than needed for communication satellite transceivers.
For practical purposes, a 100 kHz resolution is recommended. This is based on experience from commercial programs,
which already use frequency flexibility and also the fact that the resolution does not need to be higher than the downlink
frequency stability requirement of ±5 ppm [i.1], clause 5.1.2.
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12 ETSI TR 103 956 V1.1.1 (2018-12)
4.2 Hosted Payloads
Hosted payloads are emerging as an attractive business proposition for telecommunication satellite operators. When
additional "Hosted" payloads are embarked on a given "Host" satellite, they may require telemetry and command
functionalities beyond what the host's heritage hardware is designed for. The traditional concept for the command and
telemetry of hosted payloads relies on a tight share of the classical TCR and spacecraft platform avionics resources by
means of data multiplexing mechanisms and the common spacecraft computer and applications software.
An alternative design solution consists on adding dedicated TCR hardware to the hosted payload. Thus, monitoring and
control can be carried out independently of the host, decoupled from host avionics and data handling. The resulting
reduction of interface control documentation between host and hosted payload is an additional advantage. It is a use
case considered as natural for minimally intrusive and spectrally robust TCR systems. Since the hosted payload TCR
use case is in effect a separate payload that just happens to be on the same structure as the host, the same TCR standards
apply as for the host.
In addition, such architecture can off-load to some extent hosted payload operations from satellite operations. Thus,
higher levels of operational autonomy can be reached and offered to hosted payload operators in contrast to the classical
operations concept.
4.3 Operation during Launch and Early Orbit
After successful launch and separation all satellites have a Low Earth Orbit Phase (LEOP) while propulsive manoeuvres
are performed to get the spacecraft to their intended final orbit. TCR is also needed in this phase.
The main TCR consequences of LEOP are due to geometry. The spacecraft is moving differently (and generally faster)
than in the final orbit, the link ranges may be different and, because one may want to do certain manoeuvres at specific
parts of the LEOP orbits, time criticality means that the technical TCR solutions used are expected to be robust and
reliable.
Except when operating in a GSO, the satellite has a relative velocity to the earth-fixed ground station. This relative
velocity (radial velocity) results in a Doppler shift of all frequency components (carrier, subcarrier, chip rate, symbol
rate, ranging tone). The maximum Doppler shift has been specified as 22 ppm in ETSI EN 301 926 [i.2], clause 6.1, and
this value has been maintained in ETSI EN 301 926 [i.1], clause 5.1.
As a satellite passes by an earth station (worst case is directly over) the Doppler frequency shift changes from positive
to negative, passing through zero at closest approach, and thus resulting in a Doppler Rate. The Doppler Rate is
maximal when a satellite passes directly over a ground station, and increases as the altitude decreases. The maximum
Doppler Rate has been specified as 1,7 ppm/s in ETSI EN 301 926 [i.2], clause 6.1, and this value has been maintained
in ETSI EN 301 926 [i.1], clause 5.1.
These limits of 22 ppm for the Doppler shift and 1.7 ppm/s for the Doppler Rate were determined for a Geosynchronous
Transfer Orbit and justified in ETSI EN 301 926 [i.2], annex A.
Annex B provides the results of an analysis in different types of transfer orbits or highly elliptical orbits (GTO, Super
Synchronous Transfer Orbit, Molniya orbit).
This analysis confirms the validity of the specified limits for all of these orbits under the specified limitation of a true
anomaly between 40° and 320°.
4.4 Operation in Other Orbits
The opportunity to open the standard to future telecommunication missions (e.g. mega-constellations) operating at other
orbits than the geo-stationary (GSO) was one of the key goals of the revision effort and has been taken into account to
some extent. In contrast to classical GSO telecommunication missions, emerging non-GSO systems often have unique
architectures. At first glance standardization appears to be at odds with the motivation behind the designers and
operators of such systems, which seems to be ad-hoc design and operation.
Furthermore, without some in-depth knowledge of those systems and their operations concept it is considered
impractical to attempt standardization.
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13 ETSI TR 103 956 V1.1.1 (2018-12)
In consideration of those arguments, ETSI EN 301 926 [i.1] to address this area has taken a pragmatic approach. It has
considered two main common characteristics of these novel systems: orbits other than GSO and a very large numbers of
satellites (mega-constellations).
The first characteristic is covered with the revision of signal frequency dynamics to support LEOP.
Annex B provides the results of an analysis in different types of Low Earth Orbit (LEO) circular orbits at altitudes as
low as 781 km. This analysis confirms the validity of the 22 ppm limit for the Doppler shift and 1,7 ppm/s limit for the
Doppler Rate for these orbits. The analysis has not been conducted for LEO circular orbits with an altitude lower than
780 km.
Thus, transponders/transceivers/receivers compliant with the revised ETSI TCR [ref] should be able to accommodate
the expected frequency dynamics of those systems.
For the second characteristic, it is considered attractive but also crucial for these systems to provide a sufficiently large
number of unique spread spectrum pseudo-noise (PN) codes so that they can share efficiently and effectively the
available spectrum with code division multiple access (CDMA) technique.
5 Spread Spectrum Modulation
5.1 Extension of PN Code Family
In order to fulfil the expected future needs of telecommunication satellite fleets as well as missions embarking hosted
payloads, the introduction of more flexibility on spread spectrum modulation was desired. The standard PN code family
found on the initial version of the standard defines only 85 different codes. It was considered necessary to extend the
current PN code family in order to be able to support both larger and smaller CDMA systems. Therefore, both shorter
and longer PN sequence lengths have been accommodated.
In contrast to the existing code family, where different types of codes have be defined, all codes of the extended family
are defined as dual channel Gold codes. Such codes are currently used only for MTM3 mode. Figure 5 shows the
schematic diagram of the generic dual channel (In-phase/Quadrature) Gold code generator. The feedback taps of
Register A and C are the same. The feedback taps of register B and register A (or C) should form a preferred pair
according to R. Gold's rule [i.6]. Register B is always initialized with 00…01. The initial settings of Register A and C
determine the individual codewords from the code defined by the preferred pair of feedback taps.
ETSI
14 ETSI TR 103 956 V1.1.1 (2018-12)

Figure 5: Generic I/Q Gold code generator
The codes are uniquely defined by the shift register length and the feedback taps of register B and register A (or C).
Different links in a CDMA system use different codewords of the same code for spreading. These codewords are
defined by the initial settings of register A and C (while the initial setting of register B is 00…01 for all codewords).
Gold Codes can be found for arbitrary shift register lengths . Strictly speaking, Gold Codes are only defined for such
which are no integer multiple of 4. R. Gold [i.6] showed that, for all other , preferred pairs of maximum length
⁄
sequences can be found with 3 level cross-correlation. The 3 levels are −1 and −1 ± 2 , if even, or −1 and
⁄
−1 ± 2 , if odd. Further he described a simple algebraic procedure how to find these preferred pairs. If is an
integer multiple of 4, then it is at least possible to find pairs of maximum length sequences with 5 level cross-
⁄⁄
correlation. The levels are −1, −1 ± 2 , and −1 ± 2 . That means, the highest level is equal to that for
even but no integer multiple of 4. These sequences can be considered as equivalent to Gold sequences and can be
applied in the same way. To our best knowledge no proof is known for that statement, nor a simple algebraic algorithm
to find these sequences is known. Suitable sequences can however be found by brute force testing arbitrary pairs. The

number of different Gold sequences for a given preferred pair can be shown to be 2 +1 for all .
The codes are grouped into two sets which are called "short" codes or "long" codes. Short codes are defined for
=9,…,12; long codes are defined for =15,…,24. Usually, short codes and long codes are required for each link.
The short codes used for TC and TM may have different length. If the long codes are used for ranging, then same length
is mandatory. To ease the initial synchronization of the long code used for TC, its length should be an integer multiple
of the short code length. This can be achieved by truncating the length of the long code. Let be the length of the short
code. Then the long code generator consists of shift registers with length +, which are periodically reset to their

initial values after every 22 −1 shift.
The dual channel structure of the extended PN code family particularly fits into the needs of the new MTC3 mode.
However they can also be applied to the MTC2 mode by using only a single channel.
The longer a code, the more it resembles a noise like signal. Noise like spreading is desirable because de-spreading with
noise like sequences turns all types of interferers into white noise, even if the interfering signal is deterministic as for
example CW. This means that the link performance only depends on the SNR but not on the particular type of
interference. If short Gold codes are used, interference is more deterministic. The average interference is the same as for
noise like sequences, but a particular interference can differ significantly from this average. The probability that a
particular interference is by a given amount larger than the average decreases with increasing Gold code length.
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The price to be paid is that acquisition time increases with code length. The number of different code phases to be tested
for acquisition is equal to the code length and hence increases exponentially with the shift register length .
5.2 Symbol Rate
A customary way to apply BPSK modulation to a DSSS carrier is to multiply the NRZ data sequence with the NRZ chip
sequence prior to chip shaping. Two cases can be distinguished here, synchronous and asynchronous modulation.
Synchronous modulation means that the bit rate is restricted to values for which the chip rate is exactly an integer
multiple of the bit rate. Asynchronous modulation imposes no such restrictions on the data rate and thus is required to
obtain high system flexibility. However, asynchronous modulation implies that different bit periods may have different
lengths. An example is shown in Figure 6, where the chip rate is 6,5 times the data rate. It can be seen that the first data
bit covers 7 chips, while the second bit only covers 6 chips.

Figure 6: Modulating an NRZ chip sequence with an NRZ data sequence
⁄
This can have an impact on jitter and on bit clock recovery. Let the ratio of chip rate and bit rate be + , where is
the greatest integer <⁄ and and are (relatively prime) integers. Then out of bit periods are +1 chips

long and − are chips long. Consider the case =1. Then −1 successive bit periods will have length ,
followed by only 1 bit period of length +1. If is very large, then the clock recovery will adapt to the regular chips
long bit pattern. The intermediate +1-length bit period then introduces a temporary clock shift of exactly one chip
period. Since larger clock offsets are not possible, this can be considered as the worst case. The clock shift first means a
⁄ ⁄
reduction of the useful detection amplitude equal to 1 and second intersymbol-interference with amplitude 1. For
=10, the first is equivalent to a useful signal power reduction of 0,9 dB; the second means a noise floor at −20 dB.
This shows that asynchronous BPSK modulation enables arbitrary data rates at the expense of some SNR loss. This loss
increases with increasing data rate and can be considered as acceptable for data rates below 10 % of the chip rate.
5.3 New MTC3 Mode
The new MTC3 mode is different from the MTC2 mode in that long spread codes are used for spreading the TC uplink
signal during tracking. Long spread codes can be considered as noise-like while short codes are treated as deterministic
signals. The important difference between random signals and deterministic signal here is, that cross-correlation of
random signals changes permanently, while a certain cross-correlation value of deterministic signals can be stable over
long times. Therefore, if noise-like spreading is applied to asynchronous CDMA systems, the mutual interference only
depends on the average cross-correlation of these signals; if deterministic signals are used instead, the maximum cross-
correlation is considered. Thus, the main advantages of random spreading are:
1) Link performance calculation is greatly simplified.
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2) CDMA capacity may be significantly improved: if deterministic signals are used, worst case interference is
taken into account, which may be significantly larger than average interference of random signals.
The simple idea behind the new MTC3 mode is that, since there is already a long code to be synchronized for ranging,
this code could also be used to spread the TC signal. Figure 7 shows the MTC3 mode spreading and modulation
scheme.
Figure 7: MTC3 mode spreading and modulation scheme
During acquisition a short Gold code is used on In-phase (I) channel and a long Gold code on the Quadrature (Q)
channel. The Q-channel power usually is lower than the I-channel power. In Figure 7, a 10:1 ratio is proposed but other
ratios may be used as well (see clause 5.4). So far, this is the same as in MTC2 mode. Therefore, acquisition of first the
short code and subsequently the long code is the same as in MTC2 mode.
After having acquired the long code on Q-channel, the short code on I-channel is replaced by another long code which
is different from the one on Q-channel but epoch-synchronized to it. Therefore no further acquisition is necessary. At
the same time the I/Q power ratio is changed to 1. Data modulation is BPSK by synchronously modulating I and Q
channel with the same data.
Figure 8 shows the block diagram of the carrier recovery in MTC3 mode. The received signal is de-spread with the
local short code in the upper branch as well as with the local long code in the lower branch. The de-spread signals are
subsequently integrated and dumped (I&D). During acquisition, the short code is transmitted; therefore only switch S
is closed so that only the upper signal is fed to the phase error detector (PED).

Figure 8: Carrier recovery in MTC3 mode
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Since the receiver does not know when the short code is replaced by the long code, after acquisition switch S is closed,
too, and the sum of the signals of both branches is forwarded to the PED. Since actually only the short code or the long
code is transmitted, only one of the branches delivers a useful signal while the other produces just noise. This reduces
the link performance by 3 dB, which is acceptable if no data is transmitted in this phase. This is recognized by the
modification of the carrier modulation mode 2 (CMM-2) of the physical layer operation procedure (PLOP) as shown in
Figure 7 and described in [i.1], clause 6.2.
Which code actually is transmitted can be detected by simply observing the output power of the I&Ds. During
acquisition the output of the short code I&D should be much larger than that of the long code I&D. After code change
the ratio of both changes. As soon as the output of the long code I&D is larger, switch S can be opened.
The MTC3 mode preferably uses a dual channel (I/Q) long Gold code for tracking. The Q part of this code is also
transmitted during acquisition. In addition a single channel short Gold code is necessary for acquisition. For this either
channel of the dual channel generator after Figure 6 can be used. In MTC2 mode a maximal length code is transmitted
on the Q-channel for ranging purposes. As shown in Figure 9, the MTC3 tracking codes could also be derived from the
ranging code of the standa
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