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
  • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
  • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
  • G01S19/13—Receivers
  • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
  • G01S19/30—Acquisition or tracking or demodulation of signals transmitted by the system code related
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/709—Correlator structure
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
  • H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
  • H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
  • H04B2201/70715—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation with application-specific features

Definitions

• the invention relates to a receiver for spread spectrum direct sequence code division multiple access signals (DS-CDMA).
• DS-CDMA spread spectrum direct sequence code division multiple access signals
• the invention particularly, but not exclusively, relates to such receivers for receiving GPS signals.
• GPS is intended to include not only the US Global Positioning System but also the Russian Global Satellite System (GLONASS) and any other equivalent systems which may be established in the future.
• GLONASS Russian Global Satellite System
• the US GPS system is a time of arrival positioning system which uses a nominal constellation of 24 low earth orbit satellites to provide a position fix anywhere on the earth's surface. The satellites broadcast their position and timing information using a direct sequence spread spectrum signal. Currently two frequencies are used in the full system but for low cost civilian use the carrier is at 1575 MHz .
• the satellites' orbits are designed such that provided that there is a clear view of the sky at least five satellites will be in view anywhere at any time.
• the minimum number of satellites required to be in view for a full position fix is four. These are needed to resolve the three unknown spacial dimensions and the ambiguity between the receiver clock and satellite clocks.
• the GPS signal is a spread spectrum DS-CDMA signal. In order to retrieve the data from this signal it must be correlated with a copy of the PRN code that was used to spread the signal on transmission. This code is known, but the frequency at which the code is being received and the exact timing of the code are not known.
• the standard technique for finding the GPS signal is to perform a two dimensional search through the carrier frequency and code phase spaces. This involves choosing a possible frequency and shifting the code through the one thousand and twenty three possible phases while monitoring for a correlation product. If one is not found the code phase shifting must be repeated for a slightly different frequency until a correlation is located. The total process may take many seconds to complete depending on how close to the actual values of frequency and code phase the original choice was.
• Various algorithms are used to determine the best order and pattern in which to search.
• GPS receivers normally comprise a number of channels for example eight or twelve which can acquire different satellites in parallel.
• the time taken to acquire signals from several satellites will not be significantly longer than that requires to acquire the signal from one satellite.
• this technique could be expanded to reduce the time necessary to acquire the signal from a single satellite. For example it is already been said that having chosen a possible frequency the code is stepped through the one thousand and twenty three possible locations while monitoring for correlation product.
• US Patent No. 5600670 discloses a GPS receiver in which the channels of the GPS receiver system are dynamically allocated during acquisition mode to hierarchically chain a slave channel module to a master channel.
• Each channel circuit includes two correlators, where each correlator receives a digitized received GPS signal and a delayed local PRN reference code signal to provide an output signal to an accumulator.
• the system provides two correlator channels where each channel includes two correlators for a total of four correlators.
• a PRN reference code signal is progressively delayed to provide a sequence of progressively delayed PRN reference code signals, where one of the sequence of progressively delayed PRN reference code signals are applied to each of the correlators.
• Each correlator channel also includes a digital mixer which receives a digitized GPS IF signal and a digital carrier reference signal and which provides the digitized received GPS signal.
• a digital mixer which receives a digitized GPS IF signal and a digital carrier reference signal and which provides the digitized received GPS signal.
• the invention provides a receiver for direct sequence spread spectrum code division multiple access (DS-CDMA) signals, said receiver comprising at least one correlation channel; wherein the correlation channel includes means for generating a plurality of versions of the spreading code mutually offset by an integer of the chip period, means for summing the plurality of spreading codes, and means for correlating the received signal with the summed spreading codes.
• DS-CDMA direct sequence spread spectrum code division multiple access
• the invention further provides a receiver for direct sequence spread spectrum code division multiple access (DS-CDMA) signals, said receiver comprising at least one correlation channel; wherein the correlation channel includes means for generating a plurality of delayed versions of the received signal, each delayed version being delayed by a integer multiple of the chip period, means for summing the plurality of delayed versions of the received signal, and means for correlating the summed received signals with the spreading codes.
• DS-CDMA direct sequence spread spectrum code division multiple access
• the number of versions of the spreading code or the number of delayed versions of the received signal may be dependent on the actual or predicted strength of the received signal.
• the signal to noise ratio will depend on the position of the satellite that is transmitting the signal to be acquired and may differ by up to 7dB between strong and weak signals.
• Basic search algorithms previously used make no distinction between the strong and weak signals and in order to acquire weak signals they use a longer time than is necessary to acquire strong signals.
• By attempting a correlation between a greater number of delayed versions of the signal and the spreading code or between a greater number of offset spreading codes and the signal when the signal to noise ration is high less time is used for acquiring the strong signal than is necessary for acquiring a weaker signal.
• the invention further provides a GPS receiver comprising a receiver as claimed in any preceding claim, said receiver having a plurality of reception channels for locking on to signals from a plurality of satellites, and means for calculating the position of the receiver by processing the signals received from the satellites.
• the invention has particular application in GPS receivers where the time to acquire the position (which requires signals to be obtained from at least four satellites; assuming that no other position information is available) is a parameter which is frequently of great importance to the user. Whilst in most cases the signals will be acquired from the satellites in parallel it is necessary to obtain all four signals before the position calculation can be made. Further when acquiring a satellite for the first time any performance gain in sweeping the code space more quickly will be further multiplied by the number of frequencies which have to be searched.
• FIG. 1 shows in block schematic form a GPS receiver in which the invention may be embodied
• Figure 2 shows in block schematic form an embodiment of a correlator channel incorporating the invention
• Figure 3 shows an embodiment of a delay and summing circuit suitable for incorporation in the correlator channel of Figure 2.
• FIG. 1 shows in block schematic form a typical GPS receiver which comprises an aerial 1 which is connected to the input of a low noise amplifier 2 to amplify the incoming signal.
• This signal is passed through a band pass filter 3 which has its pass band centered on the frequency of 1575 MHz.
• the output of the band pass filter is fed to a first input of a mixer 4 whose second input is fed from a local oscillator 5.
• the resultant output is fed to a first intermediate frequency band pass filter 6 which has each pass band centered on a frequency of 42 MHz.
• the output of the band pass filter 6 fed to a first input of a second mixer 7 whose second input is fed by the local oscillator 5 through a frequency divider 8.
• the output of the mixer 7 is fed through a low pass filter 9 whose output is connected to the input of an analogue to digital converter 10.
• the output of the analogue to digital converter 10 is fed to a bank of correlators 1 1.
• a microprocessor 12 controls the operation of the correlators and receives output signals from them. The microprocessor 12 also calculates from the data received from the correlators the position of the receiver.
• the signal appears at the aerial 1 as a very weak wide band signal. This is first amplified using the low noise amplifier 2. The 1575 MHz signal is then mixed down to a more manageable intermediate frequency. At this point it is still an analogue spread spectrum signal. It is then converted into a digital signal by the analogue to digital converter ten for application to the bank of correlators 1 1. Generally one correlator channel is required for each satellite to be simultaneously tracked, although time multiplexing of channels is possible. The acquisition and tracking of the correlation is controlled by the microprocessor 12 which will also perform the position calculation. The bank of correlators 1 1 and microprocessor 12 form the baseband part of the system which performs several functions. The first is to correlate with the DS-CDMA signals from the satellite vehicles.
• the invention is particularly concerned with reducing the time taken for the code search.
• FIG. 2 shows in block schematic form a correlator channel.
• an input 20 which receives the signal produced by the analogue to digital converter ten is connected to first inputs of first and second mixer circuits 21 and 22.
• the second input of the mixers 21 and 22 are fed with the outputs of a carrier numerically controlled oscillator (NCO) 23.
• the two outputs of the numerically controlled oscillator 23 are in phase quadrature with each other.
• the output of the mixers 21 and 22 are fed to first inputs of further mixers 24 and 25.
• a code numerically controlled oscillator (NCO) 26 again produces an output which is fed to the second inputs of the mixers 24 and 25.
• the outputs of the mixers 24 and 25 are fed to respective integrate and dump registers 27 and 28 whose outputs are fed as data outputs to the microprocessor 12.
• a control output from the microprocessor 12 is fed to the code and carrier NCOs 23 and 26, to control their output frequencies.
• the element 29 comprises multiple delay and summing units and Figure 3 shows such a unit suitable for use as the element 29 in the position between the input 20 and the two mixers 21 and 22.
• the unit has an input 30 which is connected to the signal input 20 and an ouput 31 which is connected to the first inputs of the mixers 21 and 22.
• the input 30 is connected to a first input of a summing circuit 33 and to the inputs of a number of delay circuits 34-1 , 34-2 to 34-N.
• the outputs of the delay circuits 34-1 to 34-N are connected to further inputs of the summing circuit 33 and the output of the summing circuit 33 is connected to the output 31.
• the GPS signal has been converted to a digitised baseband signal. At this point it is a pseudo-random signal from which data can only be retrieved by correlation with the original spreading code.
• the delay and summing circuit 29 therefore, generates a new signal prior to correlation by summing copies of the original signal which have been delayed by integer multiples of the bit period of the PRN code, also known as a chip period. On despreading a correlation will be formed if the despreading code matches with any one of the delayed components. Thus several code positions can be searched simultaneously.
• the delay between the components which are summed can be a single chip period or a multiple of the chip period.
• a single chip period delay would result in block of code space being simultaneously correlated allowing a faster code sweep speed.
• a multiple delay would result in separated areas of code space being searched simultaneously.
• the element 39 which takes the output of the code NCO 26 and produces from it multiple offset versions of the spreading code summed together could be placed between the code NCO 26 and the mixers 24 and 25.
• the second implementation is to produce a composite despreading code formed by summing copies of the original despreading code with delayed copies of itself.
• the despreading code is formed using a shift register. Therefore the composite code can be produced by tapping various points of the shift register rather than by adding delay elements. This may enable a reduced hardware requirement.
• the digital baseband GPS signal would then be despread using the composite despreading code. The ultimate result is the same as for the first implementation.
• the number of components to be summed may be selectable. This would allow fast searches for known strong signals and slower searches for unknown or weak signals where the reduction in signal to noise ratio would more seriously effect the number of false correlations and the possibility of failing to detect true correlations and may result in a longer time to acquisition than if fewer or no multiple components were used to attempt correlation.
• the invention has a more general application to receivers for DS-CDMA signals and will give the same advantages with respect to acquisition of the transmitted signal.
• the invention may be applied in general communications receivers and the claims appended hereto are intended to include within their scope such receivers unless specifically restricted to GPS receivers.
• the baseband signal processing has been carried out in the digital domain this is not essential, but may be advantageous when separate ICs are used for the front end and the baseband signal processing, and appropriate analogue circuitry could be used in each correlator channel, in which case the ADC 10 would not be required.

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• Engineering & Computer Science (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Computer Networks & Wireless Communication (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Position Fixing By Use Of Radio Waves (AREA)
• Mobile Radio Communication Systems (AREA)

Applications Claiming Priority (3)

Publications (1)

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Family Applications (1)

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• 1998
  • 1998-11-28 GB GBGB9826043.3A patent/GB9826043D0/en not_active Ceased
• 1999
  • 1999-11-16 EP EP99960990A patent/EP1051637A1/en not_active Withdrawn
  • 1999-11-16 JP JP2000585687A patent/JP2002531965A/ja active Pending
  • 1999-11-16 WO PCT/EP1999/008934 patent/WO2000033102A1/en not_active Application Discontinuation

Non-Patent Citations (1)

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