EP0489794A1 - Ameliorations relatives a un systeme de transmission multiplexe a spectre etendu - Google Patents

Ameliorations relatives a un systeme de transmission multiplexe a spectre etendu

Info

Publication number
EP0489794A1
EP0489794A1 EP90912920A EP90912920A EP0489794A1 EP 0489794 A1 EP0489794 A1 EP 0489794A1 EP 90912920 A EP90912920 A EP 90912920A EP 90912920 A EP90912920 A EP 90912920A EP 0489794 A1 EP0489794 A1 EP 0489794A1
Authority
EP
European Patent Office
Prior art keywords
spread
spectrum
signals
signal
bandwidth
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP90912920A
Other languages
German (de)
English (en)
Other versions
EP0489794A4 (en
Inventor
Michael John Yerbury
Gregory Charles Hurst
Geoffrey David Sizer
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Advanced Systems Research Pty Ltd
Original Assignee
Advanced Systems Research Pty Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Advanced Systems Research Pty Ltd filed Critical Advanced Systems Research Pty Ltd
Publication of EP0489794A1 publication Critical patent/EP0489794A1/fr
Publication of EP0489794A4 publication Critical patent/EP0489794A4/en
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
    • G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
    • G01S5/06—Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J13/00—Code division multiplex systems
    • H04J13/0007—Code type
    • H04J13/0022—PN, e.g. Kronecker
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J13/00—Code division multiplex systems
    • H04J13/10—Code generation

Definitions

  • the present invention relates generally to• improvements in spread-spectrum transmission systems and in a preferred embodiment the invention is applied to a vehicle location and tracking system.
  • a number of information bearing channels can share the same medium and approximately the same frequency band and yet be separated at the receiving end with satisfactory interchannel isolation if suitable pseudo-noise (PN) codes are used asynchronously to direct-sequence modulate the channel carriers at a high rate relative to the data rate. This has the effect of spreading the spectrum of the transmitted energy.
  • PN pseudo-noise
  • the information in each channel is extracted by cross-correlating the incoming composite stream with the code associated with the desired channel.
  • the clock rates and the epochs of the in-coming and locally-generated codes match, the spread-spectrum energy is collapsed to the relatively narrow, data bandwidth for that channel whilst all the other channel spectra remain spread.
  • This method enables a particular medium (eg a coaxial-cable transmission line) to carry a large number of channels, separation being achieved at the receiving end by code-division multiple access (CDMA) .
  • CDMA code-division multiple access
  • the performance of the scheme in terms of signal- to-noise ratio depends on the relative orthogonality of the codes; that is, on their cross-correlation properties.
  • a unique feature is the smooth degradation of signal-to-noise ratio as more users come into the system compared to the sudden loss of performance which occurs in a conventional frequency division multiple access (FDMA) system once the channel capacity is exceeded.
  • FDMA frequency division multiple access
  • process gain is given as:
  • the process gain is infinite because b— •> 0.
  • the zero-data example might be a ranging system where it is necessary only to locate the code epoch and, knowing the propagation delay, the range to the transmitter may be calculated; range ambiguity can be avoided by making the code repetition period much greater than the propagation delay.
  • the process gain can be very large, but not infinite, and is limited mainly by the extent of the loss of coherence of the carrier at the receiver relative to the receiver local oscillator. If the 'coherence time' of the received carrier is 7- then b ⁇ " ⁇ ' and process gain can be increased only by spreading the spectrum of the transmitted signal still further. This can be done by increasing the chip-rate (code clock rate) of the PN code up to a limit set by the electronics or by the ability of the transmission medium to support the spread-spectrum bandwidth.
  • Fig. 1 it may be seen that in a spread- spectrum location and tracking system, the vehicle 10 or object to be located emits a continual direct sequence spread-spectrum radio signal 11. This transmission is received at a number of well-spaced receiving stations 12 in the coverage area and the differences in the times of arrival of the signals at these receivers are measured. Inverse hyperbolic navigation techniques then may be used to compute the position of the transmitter at the central computer 13 which then sends this information to an operator terminal.
  • the coherence time of the carrier depends roughly inversely on the width of the frequency-modulation spectrum so that this scattering sets a lower limit to b, the post-correlation bandwidth.
  • the radio- frequency spread-spectrum bandwidth cannot be made arbitrarily wide because of limitations on the coherence bandwidth caused by different fading in different parts of the spectrum.
  • a rough estimate of the available process gain using urban mobile transmitters may be obtained from published data.
  • the minimum coherence time is about 5 ms and the coherence bandwidth is around 1 MHz giving an available process gain of approximately 37 dB. This figure gives a measure of the level of signal enhancement, over broadband spectrally- continuous noise and interference, achievable by receiver processing.
  • each transmitter to be located and tracked has (M-l) interferers.
  • M-l the cross-correlation properties of the codes of the wanted and unwanted signals will determine the extent of the interference.
  • the cross-correlation between any pair of codes generated using n-bit shift registers is bounded by
  • n the better the wanted signal can be distinguished from the unwanted ones.
  • N the longer the sequence length (N) the better.
  • N T R f c . . . ( 2 )
  • both T- K, and fc have practical upper limits set by the coherence time and coherence bandwidth respectively so there is a practical upper limit set on the choice of N.
  • N For the particular example quoted above we have N ⁇ - 5000. With this value of N we have n ' ⁇ 12 and hence R 32 giving a maximum 'process gain' of about 15 dB.
  • CDMA falls well short when its performance is compared to the available process gain (over an interference continuum) of 37 dB.
  • long PN codes have spectral lines very close together and short PN codes have widely-separated lines.
  • a long code may be modelled to have a continuous power spectrum but with a short code the discrete lines must be considered, particularly as they affect the process gain which varies in discrete steps according to the number of spectral lines falling into the passband of the post-correlation filter.
  • the usefulness of a vehicle-tracking or locating system is enhanced in proportion to the number of vehicles which can be located or tracked at the same time.
  • a high, realisable, process gain is needed in such a spread-spectrum multi-vehicle tracking system because of the necessity of isolating each received transmission from the others; a requirement which is exacerbated by the 'near-far problem' .
  • the resulting spread-spectrum signals comprise a multiplicity of discrete spectral lines located either side of the carrier frequency and equally spaced from one another and the carrier.
  • This invention exploits the quasi-discrete nature of the mobile transmitters' spectra and employs an improvement to the novel form of frequency division multiple access (FDMA) disclosed in PCT Application No. PCT/AU87/00020 to effect this isolation.
  • FDMA frequency division multiple access
  • the present invention consists in a spread-spectrum transmission system wherein one or more spread-spectrum signals share a frequency band with other services, wherein the other services occupying the frequency band are arranged in discrete channels separated by guard bands, and each spread-spectrum signal has an information bandwidth which is significantly less than its spectral line or band spacing, each signal being produced by modulating a carrier with a pseudo-noise code and the spread-spectrum signal having a centre or carrier frequency and modulation frequency which are selected to cause its spectral lines or bands to occupy the guard bands separating the other service channels, the guard bands being greater than the spread spectrum information bandwidth.
  • a plurality of spread-spectrum signals occupy the frequency band and the centre or carrier frequencies of the spread-spectrum signals are spaced by an increment selected to cause the spectral lines of the respective transmitted signals to be interleaved with each other and with the channels of the other services in the band.
  • the invention is applicable to all spread-spectrum transmission systems where the information bandwidth is much less than the spectral line or band spacing of the transmitted spectrum.
  • the utility of spread-spectrum systems in which the information bandwidth is essentially zero, such as systems using spread-spectrum signals for ranging purposes, is particularly enhanced.
  • a receiver for a spread-spectrum multiplexed transmission system and a spread-spectrum vehicle tracking system are also provided.
  • Fig. 1 generally illustrates a vehicle tracking system in which the spread-spectrum multiplexed transmission system of the present invention might be used;
  • Fig. 2 graphically illustrates the frequency domain representation of a spread-spectrum signal;
  • Fig. 3 graphically illustrates the frequency domain representation of a spread-spectrum signal occupying the same frequency band as a conventional radio communications system
  • Fig. 4 is a block diagram of a spread-spectrum transmitter for use in a vehicle tracking system using the present invention
  • Fig. 5 is a block diagram of a remote site receiver installation for use in a vehicle tracking system using the present invention.
  • Fig. 6 is a block diagram of the installation of Fig. 5 showing the receiver arrangement in greater detail. DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • the present invention exploits the quasi-discrete nature of the transmitters' spectra and employs an improved form of Frequency Division Multiple Access (FDMA) . It was recognised in PCT Patent Application No. PCT/AU87/00020 that a novel form of FDMA could be employed with spread-spectrum signals to achieve a saving of bandwidth by interleaving spread-spectrum signals. It is now recognised that even greater efficiencies can be achieved by interleaving not only spread-spectrum signals, but also other communications channels which tend to be equally spaced across a band of the spectrum. Examples of this are broadcast radio and cellular telephone systems which generally have a number of transmitters using a number of channels which are substantially equally spaced across the band.
  • FDMA Frequency Division Multiple Access
  • centre and modulation frequencies of one or more spread-spectrum signals can be made to occupy the inter-channel guard bands of the other communications signals thereby providing a new service, in a frequency band which is already in use, without detrimental effect to the existing services in that band.
  • FIG. 2 shows details of the spectrum emitted by a transmitter using a maximal PN code of length N to direct- sequence bi-phase modulate a carrier on a frequency f .
  • this signal is emitted from a mobile vehicle in an urban area it undergoes Rayleigh scattering and Doppler frequency-shift as the radio waves propagate by a multitude of paths to the receiver.
  • Each line in the spectrum of the received signal exhibits random frequency modulation (as.described above) with most of the energy of the line being contained within a bandwidth of twice the maximum Doppler frequency-shift.
  • the final local oscillator can be direct- sequence modulated with the same PN code as used in the transmitter, the local epoch of the code being adjusted until it matches that of the incoming code.
  • the energy contained in all the spectral lines of the received signal is concentrated essentially into the bandwidth 2 f centered on the final intermediate frequency.
  • the spectrum is collapsed or 'despread' and the process gain is achieved. From the foregoing it is clear that the bandwidth of the final IF must be wide enough to accommodate the collapsed spectral energy. Allowing for an uncertainty + Sf in the carrier frequency of the transmitter, the final IF bandwidth should not be less than 2( ⁇ f + ⁇ £f).
  • the radiated spread-spectrum signal from each transmitter occupies a relatively wide bandwidth B (typically of the order of 1MHz).
  • B typically of the order of 1MHz.
  • M transmitters are operating simultaneously, as in a multi-vehicle tracking system, the use of FDMA would suggest a bandwidth requirement of at least M x B for the system as a whole.
  • the radio-frequency spectrum is viewed as a scarce resource much in demand. Consequently, the use of a bandwidth M x B is likely to be considered extravagent.
  • the present invention offers an acceptable answer to these objections without the degradation in process gain associated with CDMA.
  • the present invention uses the fact that although the bandwidth of the signal received from each transmitter is very wide, it has a quasi-discrete line spectrum arranged symmetrically about the carrier. If the spacing between the 'lines' is made large compared to the frequency band each one occupies and if the same spacing is used for all transmitters, it is possible to interleave these 'lines' (or bands) with conventional communications channels. Several of these spread-spectrum channels may be located in the inter-channel guard bands of the conventional communications channels by interleaving the spread-spectrum signals with each other. This is achieved by offsetting the centre frequencies of all the transmitters by relatively small amounts in the following way:
  • a typical, conventional, narrowband communications system will have available a multiplicity of discrete channels each of which occupies bandwidth F , , evenly-spaced within the band with separation of centre
  • the unoccupied region between channels, or guard band is
  • a single or multi-channel spread-spectrum system is implemented, where the band of frequencies occupied by the spread-spectrum system is shared with conventional services, but the spectral lines of the spread-spectrum signals lie within the guard bands between channels of the conventional system. Interference with the conventional radio system channels by the spread-spectrum signals can be maintained within acceptable limits by limiting the power in the spectral lines of the spread-spectrum signals to a level comparable with that allowed from the adjacent narrowband channel. The process gain available by the use of spread-spectrum techniques can enable interference to the spread-spectrum system by the conventional system to be maintained within acceptable limits.
  • the potential number of simultaneous transmissions be M for a system comprising only spread-spectrum signal transmissions. If all of the transmitters have centre-frequency offsets which are multiples of f p P/M where P is a non-zero integer such that P and M have no common factors then the i channel has a centre frequency offset
  • the spectral lines of the spread-spectrum system can be made to fall within the guard bands between adjacent channels of the conventional system. This is depicted in Fig. 3 where item 15 denotes a channel of a conventional communications system and item 16 denotes a spectral line of a spread-spectrum signal. It will be appreciated that in such a system the actual number of channels available
  • SUBSTITUTE S ⁇ EET will be significantly less than M because of the bandwidth requirements of the narrowband communications channels.
  • Equation 3 can be developed as follows: Let the selected spacing between spectral 'lines ' of the composite spread-spectrum signal be f_ , the code repetition frequency be f R and the channel spacing of the conventional communication system be- F , with a channel occupied bandwidth F .. Firstly, we select f R to be an integer multiple of
  • R will be small.
  • M represents the maximum number of spread-spectrum channels, given the code repetition frequency f and practical limitations on f_ , and assuming there are no communications channels with which to share the spectrum.
  • Q the maximum available number of spread-spectrum channels to be interleaved as Q, where
  • Equation (6) is to be interpreted as meaning that Q is selected to be the largest integer value satisfying the - 13 -
  • Nominal centre frequency of spread-spectrum transmissions ( ⁇ Q ) 470 MHz
  • the transmitter is shown schematically in Fig. 4.
  • crystal-controlled oscillator and divider 21 provide a clock for the pseudo- noise code generator 22 and a reference to which the voltage-controlled radio-frequenc oscillator 23 is locked via a programmable divider 24 an phase comparator 25.
  • the output from the pseudo-noise generator is applied to the modulator 26- which b i-phase modulates (0 or 7T ) the RF carrier. This modulated wave i s amplified in the output amplifier 27 and radiated from the antenna 28.
  • FIG. 5 A block diagram of the receiving electronics of a Q -channel system is shown in Fig. 5.
  • the timing at all remote sites is synchronised to a received t i ming signal radiated from a fixed location.
  • This timin transmission is received preferably by means of a high-gain antenna 31 such as a Yagi connected to the radi frequency (RF) section 32 of the timing receiver.
  • the intermediate frequency (IF) stages of both the timing reference and main receivers are housed in the same unit 33 where the local clock is synchronised by control signals from the channel 0 timing reference receiver 37 which also supplies a reference epoch signal 50 (see Fig 6) for distribution through this common unit to the Q mai receiver channels 34.
  • Fig. 6 shows in more detail how the Q individual transmissions are acquired and their epochs are tracked.
  • the first wideband amplifier 41 At an intermediate frequency F. This amplifier has a bandwidth wide enough to pass all the spread-spectrum signals from the mobile transmitters. These amplified signals are split equally and passed to Q identical first mixers 42 each of which is fed by a different local oscillator 43. The frequencies of the local oscillators
  • the outputs of the first mixers are amplified in second wideband amplifiers 44 and applied to second mixers 45 where the PN code generated in code generators 47 operates on the local oscillator ports.
  • the epochs in each channel are independently varied in response to epoch control signals 48 produced by microcontrollers contained in the detector and microcontroller blocks 46.
  • Each epoch is adjusted until it matches that of the incoming signal for that channel. When this occurs, the spectrum of this signal collapses to the relatively narrow band of frequencies (determined by the Doppler frequency-shift and transmitter crystal-oscillator uncertainty as discussed above) all centred on F .
  • This narrowband signal appears at the output of the second mixer 45 and passes through the narrowband filter 49 to the detector and microcontroller block 46 which detects the signal and maintains a match between the incoming and locally- generated code epoch by appropriate advance/retard adjustment of the locally-generated code.
  • the time difference between the epoch of the code in a tracking channel and the timing reference 50 is measured in the detector and microcontroller block 46 and the time measured for each channel is passed to the remote site computer 38 and finally via the modem 39 and land line 14 of Fig. 5 to the central computer 13 shown in Fig. 1.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • General Physics & Mathematics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Mobile Radio Communication Systems (AREA)

Abstract

L'invention concerne un système de transmission à spectre étendu dans lequel un ou plusieurs signaux à spectre étendu occupent la même bande de fréquences qu'un groupe de canaux de communication classiques espacés régulièrement, l'espacement en lignes spectrales des signaux à spectre étendu étant sélectionné pour correspondre à l'espacement intercanaux du groupe de canaux de communications, de sorte que les composantes spectrales des signaux à spectre étendus tombent dans les bandes de garde intercanaux des canaux de communications. Si l'on utilise plus d'un canal à spectre étendu, ils sont multiplexés par division des fréquences par décalage des fréquences centrales ou porteuses des signaux à spectres étendus par une fraction de l'espacement en lignes spectrales des signaux. Les signaux sont produits par modulation d'une porteuse à l'aide d'un signal de code de pseudo-bruit (PB). On procède au démultiplexage par production du même code PB dans un générateur de codes (47), et par mélange du code PB avec le signal reçu dans un mélangeur (45). L'époque du code produit par le générateur (47) est ensuite avancée ou retardée en réponse à un signal de commande d'époque (48) émis par un contrôleur (46). On passe le signal mélangé dans un filtre à bande étroite (49) afin de sélectionner le signal de la bande de base lequel est ensuite transmis au contrôleur (46) afin de permettre l'émission du signal de commande d'époque (48). Lorsqu'on l'applique à un système de poursuite de véhicules, on utilise la relation entre l'époque du code de bruit et du signal de référence (50) afin d'indiquer la temporisation de la propagation relative, une fois obtenue la corrélation avec le signal reçu. On peut utiliser les informations de temporisation, lorsqu'on les combine avec des informations provenant de récepteurs se trouvant à d'autres emplacements, afin de calculer la position de l'émetteur.
EP19900912920 1987-01-27 1990-08-31 Improvements in a spread-spectrum multiplexed transmission system Withdrawn EP0489794A4 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US26585887A 1987-01-27 1987-01-27
AU6082/89 1989-09-01
AUPJ608289 1989-09-01

Publications (2)

Publication Number Publication Date
EP0489794A1 true EP0489794A1 (fr) 1992-06-17
EP0489794A4 EP0489794A4 (en) 1993-06-09

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EP19900912920 Withdrawn EP0489794A4 (en) 1987-01-27 1990-08-31 Improvements in a spread-spectrum multiplexed transmission system

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EP (1) EP0489794A4 (fr)
CA (1) CA2064898A1 (fr)
WO (1) WO1991003892A1 (fr)

Families Citing this family (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2250154B (en) * 1990-11-21 1994-10-26 Roke Manor Research Apparatus and method for locating an object
GB2267627B (en) * 1992-05-27 1996-01-03 Roke Manor Research Improvements in or relating to radio communication systems
FR2692421B1 (fr) * 1992-06-10 1994-08-26 Setics Dispositifs de codage et décodage pour transmission en sous-bandes de fréquence.
JP3280141B2 (ja) * 1993-04-30 2002-04-30 キヤノン株式会社 スペクトラム拡散受信装置
FI94819C (fi) * 1993-06-10 1995-10-25 Nokia Telecommunications Oy Tiedonsiirtomenetelmä ja CDMA/FDMA-radiojärjestelmä
US7024331B2 (en) 2000-11-15 2006-04-04 Scientific Generics Limited Tag tracking
CA2429178A1 (fr) 2000-11-15 2002-05-23 Racetrace Inc. Localisation d'etiquette
DE60335188D1 (de) 2003-10-16 2011-01-13 Saab Ab Funksignalspositionierung
CN108761434B (zh) * 2018-03-27 2022-06-28 中国电子科技集团公司第二十九研究所 一种基于cdma下行信号的伪距测量方法及系统

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JPS5538777A (en) * 1978-09-11 1980-03-18 Nippon Telegr & Teleph Corp <Ntt> Spectrum diffusion communication system
EP0250105A1 (fr) * 1986-05-22 1987-12-23 Signal Processors Limited Procéde et dispositif de radionavigation

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JPS592458A (ja) * 1982-06-28 1984-01-09 Hitachi Ltd スペクトラム拡散通信システム
JPS6193746A (ja) * 1984-10-12 1986-05-12 Sony Corp スペクトラム拡散通信復調装置
EP0292487B2 (fr) * 1986-02-04 2000-05-24 British Aerospace Australia Limited Systeme de transmission multiplexee a spectre elargi
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US4841544A (en) * 1987-05-14 1989-06-20 The Charles Stark Draper Laboratory, Inc. Digital direct sequence spread spectrum receiver
JPH01151843A (ja) * 1987-12-09 1989-06-14 Nec Corp スペクトラム拡散復調装置
GB2220824A (en) * 1988-07-13 1990-01-17 Philips Electronic Associated Transmission system for sending two signals simultaneously on the same communications channel
US4912722A (en) * 1988-09-20 1990-03-27 At&T Bell Laboratories Self-synchronous spread spectrum transmitter/receiver

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Publication number Priority date Publication date Assignee Title
JPS5538777A (en) * 1978-09-11 1980-03-18 Nippon Telegr & Teleph Corp <Ntt> Spectrum diffusion communication system
EP0250105A1 (fr) * 1986-05-22 1987-12-23 Signal Processors Limited Procéde et dispositif de radionavigation

Non-Patent Citations (2)

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Title
PATENT ABSTRACTS OF JAPAN vol. 4, no. 71 (E-12)(553) 24 May 1980 & JP-A-55 38 777 ( NIPPON DENSHIN DENWA KOSHA ) *
See also references of WO9103892A1 *

Also Published As

Publication number Publication date
CA2064898A1 (fr) 1991-03-02
EP0489794A4 (en) 1993-06-09
WO1991003892A1 (fr) 1991-03-21

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