Analysis of the use of CSK for future GNSS signals (797922)
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Analysis of the use of CSK for Future GNSS SignalsAxel Javier Garcia Peña, Daniel Salós, Olivier Julien, Lionel Ries, ThomasGrelierTo cite this version:Axel Javier Garcia Peña, Daniel Salós, Olivier Julien, Lionel Ries, Thomas Grelier. Analysisof the use of CSK for Future GNSS Signals.
ION GNSS 2013, 26th International TechnicalMeeting of The Satellite Division of the Institute of Navigation, Sep 2013, Nashville, UnitedStates. ION, pp 1461-1479, 2013. <hal-00936952>HAL Id: hal-00936952https://hal-enac.archives-ouvertes.fr/hal-00936952Submitted on 7 Feb 2014HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestinée au dépôt et à la diffusion de documentsscientifiques de niveau recherche, publiés ou non,émanant des établissements d’enseignement et derecherche français ou étrangers, des laboratoirespublics ou privés.Analysis of the use of CSK for future GNSSSignalsAxel Garcia-Pena, Daniel Salos, Olivier Julien, ENAC (Ecole Nationale de l’Aviation Civile)Lionel Ries, Thomas Grelier, CNES (Centre national d'études spatiales)BIOGRAPHYAxel Garcia is a researcher/lecturer with the SIGnalprocessing and NAVigation (SIGNAV) research group ofthe TELECOM lab of ENAC (French Civil AviationUniversity), Toulouse, France.
His research interests areGNSS navigation message demodulation, optimizationand design, GNSS receiver design and GNSS satellitepayload. He received his double engineer degree in 2006in digital communications from SUPAERO and UPC, andhis PhD in 2010 from the Department of Mathematics,Computer Science and Telecommunications of the INPT(Polytechnic National Institute of Toulouse), France.Daniel Salós graduated as a telecommunicationsengineer in 2006 from the University of Saragossa, Spain,and received his PhD in 2012 from the University ofToulouse, France. Since 2008, he has been with theSIGnal processing and NAVigation (SIGNAV) researchgroup of the TELECOM lab of ENAC (French CivilAviation University), France. His field of interest is signalprocessing applied to satellite navigation systems.Olivier Julien is a researcher/lecturer with the SIGnalprocessing and NAVigation (SIGNAV) research group ofthe TELECOM lab of ENAC (French Civil AviationUniversity), Toulouse, France.
His research interests areGNSS receiver design, GNSS multipath and interferencemitigation, and interoperability. He received his engineerdegree in 2001 in digital communications from ENACand his PhD in 2005 from the Department of GeomaticsEngineering of the University of Calgary, Canada.Lionel RIES is head of the localization / navigationsignal department in CNES, the French Space Agency.The department activities deal with signal processing,receivers and payload regarding localization andnavigation systems including GNSS (Galileo, GNSSspace receivers), Search & Rescue by satellite(SARSAT,MEOSAR), and Argos.
He is also coordinatingfor CNES, research activities for future location/navigation signals, user segment equipment and payloads.Thomas Grelier has been a radionavigation engineer atCNES since 2004. His research activities focus on GNSSsignal processing and design, development of GNSSspace receivers and radiofrequency metrology sensor forsatellite formation flying. He graduated from the Frenchengineering school Supelec in 2003 and received a M.S.in electrical and computer engineering from Georgia Tech(USA) in 2004.ABSTRACTThis paper presents an extended analysis on theimplementation of a Code shift Keying (CSK) or CodeCyclic Shift Keying (CCSK) modulation on a GNSSsignal: an orthogonal M-ary modulation specificallydesigned to increase the bandwidth efficiency of directsequence spread spectrum (DS-SS) signals.
This paperprovides a brief description of a CSK modulator as wellas the description of two possible demodulators: a bank ofcorrelators and a FT-based demodulator which simplifiesthe receiver complexity. The advantages and drawbacksof using a CSK modulation instead of a BPSK modulationin a GNSS signal are discussed.Four different pairs “channel codes – decodingmethods” are presented as suitable candidates to beimplemented by a CSK modulation. For a binary channel,the classical sequential decoding is presented altogetherwith two iterative methods, Horizontal DimensionMultistage Decoding (HDMD) and Bit-interleaved codedModulation – Iterative Decoding (BICM-ID).
For a Q-arychannel, a Reed-Solomon channel code is proposed withthe typical Berlekamp-Forney decoding algorithm.Afterwards, this paper presents the methodology usedto construct CSK signals which pursue two differentobjectives, to keep the same useful bit rate as a referenceBPSK signal and to increase the useful bit rate withrespect to a BPSK signal but maintaining the samesymbol rate. This methodology includes the calculationand comparison of signal demodulation performances, thegeneration of CSK symbols allowing the desired bit rateand the determination of the codeword durations.
Themethodology has been applied to the different pairs“channel codes – decoding” methods in order to comparethem and proposals for real signals have been made.Finally, this paper has analyzed the impact ofprocessing a CSK modulated signal on a GNSS receiverwith respect to a BPSK signal. This analysis includes theincrease of complexity of the demodulator block and thepossible performance degradation of the acquisition and,the carrier and code delay tracking.I. INTRODUCTIONGNSS signals are designed (in order to fulfill thespecial needs of a GNSS system) to provide the receiverwithprecisesynchronizationorpseudo-rangemeasurements and to broadcast limit amount of essentialinformation such as the satellites ephemeris, clock errorcorrection, etc.
The combination of these two elementsallows a GNSS system to provide the user with its PVT(position, velocity, time) [1][2].The historical design choice for the GNSS systemsynchronization part consists in implementing directsequence-spread spectrum (DS-SS) characterized by avery narrow autocorrelation function. Additionally, theintroduction of several almost orthogonal directsequences, one for each satellite, was used to implementthe simultaneous access of the original GPS systemconstellation satellites; this technique is known as CodeDivision Multiple Access (CDMA) [3]. Therefore, the useof direct sequences in a GNSS system has become a keyelement and an inherent part of the signal design.The historical design choice of the GNSS signalcommunication part is the implementation of a BPSKmodulation (be aware that a BOC modulation is a BPSKmodulation from the demodulation point of view) [3].This choice was made in order to allow the easyimplementation of the synchronization part: directsequences.
Moreover, the low bandwidth efficiency of aBPSK modulation (number of bits/second/Hz) [3] did notpresent any limitation to the signal design: the low powerof the received signal added to the limited requiredinformation imposed a low bit rate.However, nowadays this choice of hybrid signalstructure can be adapted due to the introduction of a newdataless (pilot) channel on all the new civil GNSS signalsas well as the extension of the GNSS user communitywith high expectations in terms of new services andpositioningcapabilitiesinmorechallengingenvironments. On one hand, the pilot channel introductionto a GNSS signal and the possibility for the receiver togenerate pseudo-range measurements from this channelimplies that the data channel is no longer necessarilyrestricted by the GNSS system special hybridcharacteristics [2].
Therefore, the data channel can belooked at as a more traditional communication channel.One example is the LEX signal of the Japanese QZSSsystem [5]. Another example could be the GPS L1Csignal: 75% power allocation to the pilot channel [6]could lead to receivers discarding the data channel forsynchronization purpose.On the other hand, nowadays new applications and newservices such as precise positioning, safety-of-life, etc.,demand a much higher data rate (currently obtained viaother systems) [7][8]. Moreover, a higher data rate canimprove the signal demodulation performance by, forinstance, means of increasing the transmitted informationtemporal diversity: more repetitions of the ephemeris dataallow the receiver to obtain the information more quicklyor to accumulate the information for a lowerdemodulation C/N0 threshold.The main limitation of using a BPSK data modulationto increase the signal data rate is the signal design choiceof employing direct sequences (necessary for CDMA andprecise pseudo-range measurements).
The PRN code islimited by the data symbol duration which must decreasein order to increase the data rate and thus either thechipping rate or the PRN code must also be modified.In this paper, the modulation known as Code ShiftKeying (CSK) [9][10][11][12], specially designed toincrease the transmission rate of a spread spectrum signal[9], is inspected.In this paper, first, the CSK modulation and itsfundamentals are defined.
Second, the advantages anddisadvantages of using a CSK modulation on a GNSSsignals are presented. Third, the different pairs channelcodes–decoding methods implemented for a CSKmodulation are described. Fourth, the objectives and themethodology used to design a CSK signal are given.Fifth, real numeric propositions of CSK signals are made.Sixth, the impact of a CSK modulated signal on a GNSSreceiver is analyzed. Seventh, the conclusions are given.II.CODE SHIFTTECHNIQUEKEYINGMODULATIONThe main characteristics of a CSK modulation aregiven in the following subsections.II.A. CSK DefinitionThe CSK modulation technique is a DS–SS signalingmethod which overcomes the spreading gain versus datarate limitations [9].The CSK is a form of orthogonal M-ary signaling overa communication channel [10] since M orthogonalsignaling waveforms are used in order to transmit U =log2(M) bits.
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