WO1995017785A1 - Direct conversion cdma receiver - Google Patents

Direct conversion cdma receiver Download PDF

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Publication number
WO1995017785A1
WO1995017785A1 PCT/GB1994/002770 GB9402770W WO9517785A1 WO 1995017785 A1 WO1995017785 A1 WO 1995017785A1 GB 9402770 W GB9402770 W GB 9402770W WO 9517785 A1 WO9517785 A1 WO 9517785A1
Authority
WO
WIPO (PCT)
Prior art keywords
signal
phase
data signal
cdma
carrier
Prior art date
Application number
PCT/GB1994/002770
Other languages
French (fr)
Inventor
Christoff Karl Pauw
Original Assignee
Csir
Brown, Keith, Edwin, Frank
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 Csir, Brown, Keith, Edwin, Frank filed Critical Csir
Priority to AU12767/95A priority Critical patent/AU1276795A/en
Publication of WO1995017785A1 publication Critical patent/WO1995017785A1/en

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
    • H04L27/22—Demodulator circuits; Receiver circuits
    • H04L27/233—Demodulator circuits; Receiver circuits using non-coherent demodulation
    • H04L27/2332—Demodulator circuits; Receiver circuits using non-coherent demodulation using a non-coherent carrier
    • 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
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J13/00—Code division multiplex systems
    • H04J13/10—Code generation
    • H04J13/102—Combining codes

Definitions

  • THIS invention relates to a method of transmitting a CDMA signal and to a method of extracting data from the transmitted signal.
  • the invention extends to transmitter and receiver apparatus for carrying out the method.
  • CDMA Code division multiple access
  • a method of transmitting a CDMA signal includes the steps of:
  • the first and second codes are selected for their suitability for distinguishing the in-phase and quadrature-phase components.
  • the data signal may be mixed with a third code prior to being mixed with the first and second codes.
  • the third code is selected for its properties as a multiple- access code.
  • a method of extracting a data signal from a CDMA signal transmitted by the method defined above includes the steps of:
  • the replica of the carrier is preferably an approximation of the carrier of the received signal.
  • the in-phase and quadrature components of the baseband signal are preferably extracted by mixing the baseband signal with first and second orthogonal or near-orthogonal codes for the in-phase and quadrature components thereof.
  • a transmitter for transmitting a CDMA signal comprises: first and second means for mixing a data signal with first and second codes to generate first and second coded signals;
  • first and second modulators for modulating in-phase and quadrature carriers with the first and second coded signals
  • summation means for summing the modulated carriers to generate an output signal for transmission.
  • the transmitter may include a third mixer for mixing the data signal with a third code prior to mixing thereof with the first and second codes.
  • a receiver for extracting a data signal from a CDMA signal transmitted by the transmitter defined above comprises:
  • a carrier reference oscillator for generating an approximation of the carrier of the transmitted signal
  • a mixer for mixing a received CDMA signal with the approximation of the carrier
  • an anti-aliasing filter for filtering the output of the mixer
  • an analogue to digital converter for generating a digital data signal from the output of the anti-aliasing filter
  • FIG. 1 is a simplified schematic diagram of a conventional CDMA receiver employing digital signal processing
  • FIG. 2 is a simplified schematic diagram of a CDMA receiver according to the invention.
  • Figure 3 is a diagrammatic illustration showing the generation of a CDMA signal according to the method of the invention.
  • Figure 4 is a block schematic diagram illustrating the processing of the received CDMA signal according to the invention.
  • Figure 5 is a diagram, similar to that of Figure 3, illustrating an alternative modulation scheme according to the invention
  • Figure 6 is a block schematic diagram similar to that of Figure 4, showing the processing of a received signal modulated according to the scheme of Figure 5;
  • Figure 7 is an explanatory phasor diagram
  • Figure 8 is a schematic illustration of a set of suitable codes for use in carrying out the method of invention.
  • FIG 1 shows a conventional CDMA receiver which uses digital signal processing (DSP).
  • DSP digital signal processing
  • This system requires two carrier references or carrier replicas which are derived from a carrier reference oscillator using a signal splitter and phase shifter.
  • the carrier replicas are fed to respective mixers, the outputs of which are fed to separate anti-aliasing filters and analogue to digital convertors, the outputs of which are fed to a digital signal processor.
  • the carrier reference is generated by an uncontrolled oscillator, which implies that the oscillator frequency will be only approximately correct and its phase relative to the received signal will be time-varying. The correction for this time-varying phase difference is made in the digital signal processor.
  • FIG. 7 illustrates what happens if there is only one channel, say the in-phase channel.
  • the mixer performs a multiplying operation, and its output is given by
  • a disadvantage of the prior art receiver illustrated in Figure 1 is that two carrier references or carrier replicas are required, together with two mixers, two anti-aliasing filters and two analogue to digital convertors, increasing the cost of the receiver. According to the invention, this problem is avoided by transmitting simultaneously two differently coded signals on each of the in-phase and the quadrature-phase channels. As a result, alignment of the carrier reference is not a problem, since the in-phase and the quadrature phase signals are both present, and knowing which one is present is achieved by detecting the two different codes.
  • a CDMA receiver according to the invention is shown, which comprises an antenna 10, a low noise front end amplifier 12 and a filter 14.
  • the output of the filter 14 is fed to a single mixer 16 together with a carrier replica or carrier reference signal from a carrier reference oscillator 18.
  • the output of the mixer 16 is fed to an anti ⁇ aliasing filter 20 and to an analogue to digital convenor 22 before being fed to a digital signal processor circuit 24.
  • the circuit of Figure 2 requires only a single carrier reference, a single mixer, a single anti-aliasing filter and a single analogue to digital convertor, resulting in significant cost saving compared with the convention circuit of Figure 1. This is due not only to the reduced parts count, but also because there are no longer two signal paths that need to be aligned with one another in phase and in amplitude. On the other hand, a greater amount of digital signal processing is required to recover the data in the received signal, but the additional cost involved here is less than the saving mentioned above. In addition, the DSP part of the system tends to be more reliable over its lifetime than the analogue parts that it replaces.
  • Figure 3 shows how a data signal is modulated onto two different codes C-. and before being modulated with respective in-phase and quadrature phase carriers which are summed to produce a composite signal for transmission.
  • Figure 4 shows how the received signal is processed in the digital signal processor circuit 24 of Figure 2.
  • the upper output of the circuit in Figure 4 is the data in the CDMA signal, while the signal at the lower output is used for estimating the carrier phase.
  • the difference between the actual phase offset ⁇ and the estimated phase offset ⁇ will be small and d(t) cos( ⁇ - ⁇ ) « d(t), allowing the values of the data to be estimated.
  • the residual phase error is easy to find, since the two outputs represent Cartesian co-ordinates which can be transformed to polar co-ordinates to find the signal amplitude and the residual phase error.
  • the residual phase error can be used in a number of well known ways to correct the carrier phase estimate.
  • Each multiplier together with the summation device at the end of the circuit forms the co ⁇ elator for the code fed into that multiplier.
  • the summation device is a digital integrate-and-dump circuit. It is cleared at the beginning of a data interval and its output is sampled at the end of each data interval.
  • Figures 5 and 6 show a further refinement of the system, using a third code C j .
  • the data signal is modulated onto the third code prior to being modulated onto the first and second codes.
  • the addition of the third code allows the requirements for a good multiple access code to be separated from the requirements for codes suitable for separating the in-phase and the quadrature-phase channels from one another.
  • a suitable set of codes for implementing the above described system is shown schematically in Figure 8.
  • the operation of the invention has been described on the basis of a receiver utilising digital signal processing. However, the invention could also be implemented using conventional analogue and digital hardware.
  • the above description is based on a direct conversion receiver.
  • the invention could equally well be applied to a receiver employing an intermediate frequency stage. In such a case, the references in the text above to carriers or carrier frequencies would relate to the intermediate carrier or carrier frequency.

Abstract

The invention provides a method of transmitting a CDMA signal and of extracting a data signal from the transmitted CDMA signal, as well as transmitter and receiver apparatus for carrying out the method. According to the method, a data signal is mixed with first and second codes to generate separate coded signals, which are modulated with in-phase and quadrature carriers and then summed to generate an output signal for transmission. The received signal is mixed with a replica of the carrier in a single mixer and then processed to extract in-phase and quadrature components of the resultant baseband signal. The method and apparatus of the invention allow a CDMA system to be implemented at relatively low cost.

Description

DIRECT CONVERSION CDMA RECEIVER
BACKGROUND OF THE INVENTION
THIS invention relates to a method of transmitting a CDMA signal and to a method of extracting data from the transmitted signal. The invention extends to transmitter and receiver apparatus for carrying out the method.
Code division multiple access (CDMA) systems are presently receiving much attention from manufacturers of wireless communications systems. Examples of such systems are wireless local area networks for computer systems and wireless telephone systems such as cordless phones, cellular phones, and conventional telephones that use radio transmission instead of copper cables in the local loop between the local exchange and the telephone instrument.
SUMMARY OF THE INVENTION
According to the invention a method of transmitting a CDMA signal includes the steps of:
mixing a data signal with first and second codes to generate first and second coded signals;
modulating in-phase and quadrature carriers with the first and second coded signals; and
summing the modulated carriers to generate an output signal for transmission. Preferably, the first and second codes are selected for their suitability for distinguishing the in-phase and quadrature-phase components.
The data signal may be mixed with a third code prior to being mixed with the first and second codes.
Preferably, the third code is selected for its properties as a multiple- access code.
Further according to the invention, a method of extracting a data signal from a CDMA signal transmitted by the method defined above includes the steps of:
receiving the transmitted CDMA signal;
mixing the received signal with a replica of the carrier in a single mixer; and
processing the resultant baseband signal to extract in- phase and quadrature components thereof.
The replica of the carrier is preferably an approximation of the carrier of the received signal.
The in-phase and quadrature components of the baseband signal are preferably extracted by mixing the baseband signal with first and second orthogonal or near-orthogonal codes for the in-phase and quadrature components thereof.
Still further according to the invention a transmitter for transmitting a CDMA signal comprises: first and second means for mixing a data signal with first and second codes to generate first and second coded signals;
first and second modulators for modulating in-phase and quadrature carriers with the first and second coded signals; and
summation means for summing the modulated carriers to generate an output signal for transmission.
The transmitter may include a third mixer for mixing the data signal with a third code prior to mixing thereof with the first and second codes.
Still further according to the invention a receiver for extracting a data signal from a CDMA signal transmitted by the transmitter defined above comprises:
a carrier reference oscillator for generating an approximation of the carrier of the transmitted signal;
a mixer for mixing a received CDMA signal with the approximation of the carrier;
an anti-aliasing filter for filtering the output of the mixer;
an analogue to digital converter for generating a digital data signal from the output of the anti-aliasing filter; and
digital signal processing means for recovering the data signal from the digital data signal. BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a simplified schematic diagram of a conventional CDMA receiver employing digital signal processing;
Figure 2 is a simplified schematic diagram of a CDMA receiver according to the invention;
Figure 3 is a diagrammatic illustration showing the generation of a CDMA signal according to the method of the invention;
Figure 4 is a block schematic diagram illustrating the processing of the received CDMA signal according to the invention;
Figure 5 is a diagram, similar to that of Figure 3, illustrating an alternative modulation scheme according to the invention;
Figure 6 is a block schematic diagram similar to that of Figure 4, showing the processing of a received signal modulated according to the scheme of Figure 5;
Figure 7 is an explanatory phasor diagram; and
Figure 8 is a schematic illustration of a set of suitable codes for use in carrying out the method of invention.
DESCRIPTION OF EMBODIMENTS
Figure 1 shows a conventional CDMA receiver which uses digital signal processing (DSP). This system requires two carrier references or carrier replicas which are derived from a carrier reference oscillator using a signal splitter and phase shifter. The carrier replicas are fed to respective mixers, the outputs of which are fed to separate anti-aliasing filters and analogue to digital convertors, the outputs of which are fed to a digital signal processor. The carrier reference is generated by an uncontrolled oscillator, which implies that the oscillator frequency will be only approximately correct and its phase relative to the received signal will be time-varying. The correction for this time-varying phase difference is made in the digital signal processor.
The diagram of Figure 7 illustrates what happens if there is only one channel, say the in-phase channel. The mixer performs a multiplying operation, and its output is given by
d(t) c(t) cos ωct cos(ωct - θ)
= Vid(t) c(t) cos θ + double frequency term
If θ is momentarily equal to ± π/2 then cos θ = 0. When this occurs, no signal at all will be received, and errors will occur. The receiver of Figure 1 would, in this case, disregard the in-phase signal and use the quadrature signal instead. In practice, the receiver automatically monitors the two signals and combines them to obtain a relatively constant output signal.
A disadvantage of the prior art receiver illustrated in Figure 1 is that two carrier references or carrier replicas are required, together with two mixers, two anti-aliasing filters and two analogue to digital convertors, increasing the cost of the receiver. According to the invention, this problem is avoided by transmitting simultaneously two differently coded signals on each of the in-phase and the quadrature-phase channels. As a result, alignment of the carrier reference is not a problem, since the in-phase and the quadrature phase signals are both present, and knowing which one is present is achieved by detecting the two different codes.
Referring now to Figure 2, a CDMA receiver according to the invention is shown, which comprises an antenna 10, a low noise front end amplifier 12 and a filter 14. The output of the filter 14 is fed to a single mixer 16 together with a carrier replica or carrier reference signal from a carrier reference oscillator 18. The output of the mixer 16 is fed to an anti¬ aliasing filter 20 and to an analogue to digital convenor 22 before being fed to a digital signal processor circuit 24.
It can be seen that the circuit of Figure 2 requires only a single carrier reference, a single mixer, a single anti-aliasing filter and a single analogue to digital convertor, resulting in significant cost saving compared with the convention circuit of Figure 1. This is due not only to the reduced parts count, but also because there are no longer two signal paths that need to be aligned with one another in phase and in amplitude. On the other hand, a greater amount of digital signal processing is required to recover the data in the received signal, but the additional cost involved here is less than the saving mentioned above. In addition, the DSP part of the system tends to be more reliable over its lifetime than the analogue parts that it replaces.
Figure 3 shows how a data signal is modulated onto two different codes C-. and before being modulated with respective in-phase and quadrature phase carriers which are summed to produce a composite signal for transmission.
Figure 4 shows how the received signal is processed in the digital signal processor circuit 24 of Figure 2. The upper output of the circuit in Figure 4 is the data in the CDMA signal, while the signal at the lower output is used for estimating the carrier phase. These two outputs are no different from what would have been produced by the conventional receiver of Figure 1, and further processing to extract the data and to track the carrier phase is the same.
After the digital processor circuit 24 has locked onto the carrier offset frequency, the difference between the actual phase offset θ and the estimated phase offset θ will be small and d(t) cos(θ - θ) « d(t), allowing the values of the data to be estimated.
The residual phase error is easy to find, since the two outputs represent Cartesian co-ordinates which can be transformed to polar co-ordinates to find the signal amplitude and the residual phase error. The residual phase error can be used in a number of well known ways to correct the carrier phase estimate. The outputs are generated as follows, as is usual in CDMA receivers: multiplication of the coded signals with the respective codes effectively removes the codes ( 1 x 1 = 1 and -1 x -1 = 1). Each multiplier together with the summation device at the end of the circuit forms the coπelator for the code fed into that multiplier. The summation device is a digital integrate-and-dump circuit. It is cleared at the beginning of a data interval and its output is sampled at the end of each data interval.
Figures 5 and 6 show a further refinement of the system, using a third code Cj. As shown in Figure 5, the data signal is modulated onto the third code prior to being modulated onto the first and second codes. The addition of the third code allows the requirements for a good multiple access code to be separated from the requirements for codes suitable for separating the in-phase and the quadrature-phase channels from one another. A suitable set of codes for implementing the above described system is shown schematically in Figure 8. The operation of the invention has been described on the basis of a receiver utilising digital signal processing. However, the invention could also be implemented using conventional analogue and digital hardware. In addition, the above description is based on a direct conversion receiver. The invention could equally well be applied to a receiver employing an intermediate frequency stage. In such a case, the references in the text above to carriers or carrier frequencies would relate to the intermediate carrier or carrier frequency.

Claims

CLAIMS:
1. A method of transmitting a CDMA signal including the steps of:
mixing a data signal with first and second codes to generate first and second coded signals;
modulating in-phase and quadrature carriers with the first and second coded signals; and
Slimming the modulated carriers to generate an output signal for transmission.
2. A method according to claim 1 wherein the first and second codes are selected for their suitability for distinguishing the in- phase and quadrature-phase components.
3. A method according to claim 1 or claim 2 wherein the data signal is mixed with a third code prior to being mixed with the first and second codes.
4. A method according to claim 3 wherein the third code is selected for its properties as a multiple-access code.
5. A method of transmitting a CDMA signal substantially as herein described with reference to Figure 3 or Figure 5 of the accompanying drawings.
6. A method of extracting a data signal from a CDMA signal transmitted by the method of any one of claim 1 to 5 including the steps of: receiving the transmitted CDMA signal;
mixing the received signal with a replica of the carrier in a single mixer; and
processing the resultant baseband signal to extract in- phase and quadrature components thereof.
7. A method according to claim 6 wherein the replica of the carrier is an approximation of the carrier of the received signal.
8. A method according to claim 6 or claim 7 wherein the in-phase and quadrature components of the baseband signal are extracted by mixing the baseband signal with first and second orthogonal or near-orthogonal codes for the in-phase and quadrature components thereof.
9. A method of extracting a data signal from a transmitted CDMA signal substantially as herein described with reference to Figure 4 or Figure 6 of the accompanying drawings.
10. A transmitter for transmitting a CDMA signal comprising:
first and second mixer for mixing a data signal with first and second codes to generate first and second coded signals;
first and second modulators for modulating in-phase and quadrature carriers with the first and second coded signals; and summation means for summing the modulated carriers to generate an output signal for transmission.
11. A transmitter according to claim 10 including a third mixer for mixing the data signal with a third code prior to mixing thereof with the first and second codes.
12. A transmitter for transmitting a CDMA signal substantially as herein described with reference to Figure 3 or Figure 5 of the accompanying drawings.
13. A receiver for extracting a data signal from a CDMA signal transmitted by the transmitter of any one of claims 9 to 12 comprising:
a carrier reference oscillator for generating an approximation of the carrier of the transmitted signal;
a mixer for mixing a received CDMA signal with the approximation of the carrier;
an anti-aliasing filter for filtering the output of the mixer;
an analogue to digital converter for generating a digital data signal from the output of the anti-aliasing filter; and
digital signal processing means for recovering the data signal from the digital data signal.
14. A receiver for extracting a data signal from a CDMA signal substantially as herein described with refernece to Figure 2 and Figure 4 or Figure 6 of the accompanying drawings.
PCT/GB1994/002770 1993-12-20 1994-12-20 Direct conversion cdma receiver WO1995017785A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU12767/95A AU1276795A (en) 1993-12-20 1994-12-20 Direct conversion cdma receiver

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
ZA93/9524 1993-12-20
ZA939524 1993-12-20

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WO (1) WO1995017785A1 (en)
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US5742595A (en) * 1995-06-02 1998-04-21 Dsc Communications Corporation Processing CDMA signals
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US6061365A (en) * 1995-06-02 2000-05-09 Airspan Communications Corporation Control message transmission in telecommunications systems
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Cited By (15)

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Publication number Priority date Publication date Assignee Title
US5809093A (en) * 1995-06-02 1998-09-15 Dsc Communications Corporation Apparatus and method of frame aligning information in a wireless telecommunications system
US5828339A (en) * 1995-06-02 1998-10-27 Dsc Communications Corporation Integrated directional antenna
US5742595A (en) * 1995-06-02 1998-04-21 Dsc Communications Corporation Processing CDMA signals
US5745496A (en) * 1995-06-02 1998-04-28 Dsc Communications Corporation Apparatus and method of establishing a downlink communication path in a wireless telecommunications system
US5761429A (en) * 1995-06-02 1998-06-02 Dsc Communications Corporation Network controller for monitoring the status of a network
US5786770A (en) * 1995-06-02 1998-07-28 Dsc Communications Corporation Message handling in a telecommunications network
US5696766A (en) * 1995-06-02 1997-12-09 Dsc Communications Corporation Apparatus and method of synchronizing a transmitter in a subscriber terminal of a wireless telecommunications system
US5815798A (en) * 1995-06-02 1998-09-29 Dsc Communications Corporation Apparatus and method of controlling transmitting power in a subscriber terminal of a wireless telecommunications system
GB2301753A (en) * 1995-06-02 1996-12-11 Dsc Communications Processing CDMA signals
US6061365A (en) * 1995-06-02 2000-05-09 Airspan Communications Corporation Control message transmission in telecommunications systems
US5923668A (en) * 1995-06-02 1999-07-13 Airspan Communications Corporation Apparatus and method of establishing a downlink communication path in a wireless telecommunications system
US5915216A (en) * 1995-06-02 1999-06-22 Dsc Communications Corporation Apparatus and method of transmitting and receiving information in a wireless telecommunications system
WO1998053563A3 (en) * 1997-05-22 1999-05-14 Spectraplex Inc Method and system for communicating digital data by simultaneously code division multiple accessing a plurality of channels
WO1998053563A2 (en) * 1997-05-22 1998-11-26 Spectraplex, Inc. Method and system for communicating digital data by simultaneously code division multiple accessing a plurality of channels
EP2242176A3 (en) * 2001-11-27 2014-03-19 Qualcomm Incorporated Direct conversion of narrow-band RF signals

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ZA9410128B (en) 1995-08-25

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