WO2001050698A1 - Offset correction in a spread spectrum communication system - Google Patents
Offset correction in a spread spectrum communication system Download PDFInfo
- Publication number
- WO2001050698A1 WO2001050698A1 PCT/US2000/022217 US0022217W WO0150698A1 WO 2001050698 A1 WO2001050698 A1 WO 2001050698A1 US 0022217 W US0022217 W US 0022217W WO 0150698 A1 WO0150698 A1 WO 0150698A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- signal
- frequency
- correction
- code
- data rate
- Prior art date
Links
Classifications
-
- 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
- H04B1/7073—Synchronisation aspects
- H04B1/7075—Synchronisation aspects with code phase acquisition
- H04B1/7077—Multi-step acquisition, e.g. multi-dwell, coarse-fine or validation
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03J—TUNING RESONANT CIRCUITS; SELECTING RESONANT CIRCUITS
- H03J7/00—Automatic frequency control; Automatic scanning over a band of frequencies
- H03J7/02—Automatic frequency control
- H03J7/04—Automatic frequency control where the frequency control is accomplished by varying the electrical characteristics of a non-mechanically adjustable element or where the nature of the frequency controlling element is not significant
-
- 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
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
- H04L2027/0024—Carrier regulation at the receiver end
- H04L2027/0026—Correction of carrier offset
- H04L2027/003—Correction of carrier offset at baseband only
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
- H04L2027/0044—Control loops for carrier regulation
- H04L2027/0053—Closed loops
- H04L2027/0057—Closed loops quadrature phase
Definitions
- the present invention relates to a method and system for correcting an offset in a digital system.
- a correction may be required in a digital communication system in which a frequency offset is present.
- Digital communication systems are widely used for transmitting and receiving information. Examples of such digital communication systems include mobile telephone systems, for example the European GSM (Global System for Mobile Communications) system and the American D-AMPS system. Digital communication systems are also being developed for television transmissions.
- GSM Global System for Mobile Communications
- D-AMPS American D-AMPS
- Digital communication systems are also being developed for television transmissions.
- a single telecommunication protocol to be introduced enabling the use of a single mobile telephone anywhere in the world. Whilst such a system is not yet in operation, it is apparent that the basis for such a system is likely to be one which uses a spread spectrum signal, in the form of a code division multiple access (CDMA) system.
- CDMA code division multiple access
- a digital data signal is modulated by a second digital signal, the second signal generally having a significantly higher switching rate.
- Spread spectrum communication systems were originally developed for military and satellite applications but have since been developed for commercial mobile telephone applications. In the United States of America, the Telecommunications Industry Association has introduced the
- a radio receiver converts the received radio signal into an electrical signal. This is then mixed with an oscillator signal to recover the analog spread spectrum signal.
- An analog to digital converter samples the analog signal to obtain a digital signal, which may be anoversampled signal. This digital signal is despread by correlating the signal with the same pseudo-random code used in the generation of the digital spread spectrum signal to recreate the data signal.
- European patent application EP-A-0 762 666 describes a CDMA mobile communication system incorporating means for performing a phase error correction in which a transmitted pilot signal isdespread and used to determine phase correction signals which are then fed back to a voltage controlled oscillator.
- frequency correction is performed by deriving phase correction signals from phase rotation changes of data with no use of pilot signals for frequency control.
- the present invention provides a method of providing frequency correction for a spread spectrum communication receiver, said receiver being arranged todespread a code-spread signal having a first data rate to provide at least one despread data signal having a second, lower data rate, wherein said method comprises the steps of: i) determining a frequency offset by processing successive samples of said despread data signal; ii) generating a correction sequence from said determined frequency offset; and iii) combining said code-spread signal having said first data rate with said correction sequence obtained from said despread data signal having said second, lower data rate to correct the determined frequency offset.
- the method includes a filtering step wherein any noise component in determined frequency offsets is reduced. This filtering may be performed by using a weighted average of previous measurements.
- the determination of the frequency offset may be performed by calculating the mathematical argument of a complex sample of the despread data signal multiplied by the complex conjugate of the immediately preceding sample.
- the correction sequence may be combined with the code-spread signal including a frequency error by the multiplication of a complex correction factor therewith.
- a linear interpolation from the despread signal offset may be beneficially used.
- a spread spectrum communication system comprising a plurality of receivers for receiving transmitted signals, wherein each receiver comprises: an RF signal receiver for generating an analog signal from a received RF signal; an analog to digital converter for converting said analog signal into a digital signal; a digital signal despreader for processing a code-spread signal having a first data rate to obtain a despread digital signal having a second data rate, said second data rate being lower than said first data rate; and a frequency corrector, wherein said frequency corrector comprises a feedback loop including a frequency offset detector for obtaining a measure of a frequency offset from saiddespread digital signal and a frequency correction generator for generating a frequency correction and a combiner for combining said frequency correction with said code- spread signal to correct said frequency offset.
- a receiver for a spread spectrum communication system comprising: an RF signal receiver for generating an analog signal from a received RF signal; an analog to digital converter for converting said analog signal into a digital signal; a digital signal despreader for processing a code-spread signal having a first data rate to obtain a despread digital signal having a second data rate, said second data rate being lower than said first data rate; and a frequency corrector, wherein said frequency corrector comprises a feedback loop including a frequency offset detector for obtaining a measure of a frequency offset from saiddespread digital signal and a frequency correction generator for generating a frequency correction and a combiner for combining said frequency correction with said code- spread signal to correct said frequency offset.
- the spread spectrum communication system may be a code division multiple access system, and in particular a telephone communication system.
- T x and R oscillator frequency offsets and the relative movement of a transmitter and a receiver requires the necessity to perform frequency correction.
- Another example of a communication system in which the invention may be incorporated is a wireless local loop link.
- Fig. 1 shows a schematic diagram of a generalized CDMA communication system
- Fig. 2 shows a schematic representation of a CDMA communication system receiver incorporating frequency correction
- FIG. 3 shows the Fig. 2 frequency correction arrangement in greater detail.
- a CDMA communication system indicated generally by 10.
- the communication system 10 comprises a transmitter 12 and a receiver 14.
- each communication unit will have transmit and receive capabilities.
- a base station will be capable of transmitting data to and receiving data from a plurality of remote units whilst each remote unit will receive messages from and transmit to, in general, only a single base station at any one time.
- a data signal to be transmitted 16 is modulated by a chip code 18 in a modulation circuit 20.
- the data signal to be transmitted is transmitted according to a quaternary phase shift keying (QPSK) scheme, in which in-phase (I) andquadrature (Q) signals are modulated separately.
- QPSK quaternary phase shift keying
- the resulting digital spread spectrum signals are converted into an analog spread spectrum signal by a filter 22 and then mixed with a carrier frequency 24 before transmission over a channel 26.
- a received RF signal has the carrier frequency removed and is then demodulated into I and Q signals. These are sampled and converted into digital signals by an analog to digital converter 28. These digital signals are despread by a correlator 30 in combination with a despreading code 31. Generally, the de- spreading code 31 is identical to the chip code 18. Following despreading the resulting data signals may be processed in an error correction circuit 32.
- the data signal 16 has a data rate which is referred to as the symbol rate and which may typically be 128 kbit/s.
- the chip code 18 might typically have a data rate, known as the chip rate of 4096 chips/s. These two figures indicate a spreading factor of 32, this being the number of chips/bit.
- Figure 2 shows a CDMA communication receiver 50 for incorporation in the system 10 having automatic frequency control which measures an offset at the symbol rate level and implements a frequency correction at the chip rate level. In the following description, it is assumed that quadrature demodulation was used for final down conversion of the analog signal. Hence, all operations in the signal processing chain have to be performed on both in-phase and quadrature components.
- a data modulated and CDMA-code spread complex base-band signal 52 is passed through an analog filter 54 to an analog to digital converter 56.
- the analog to digital converter 56 receives a clock input from a clock 58 at a frequency which is four times that of the chip rate.
- the analog signal is thus sampled at arate which is four times the chip rate, or 16.4 Msamples/s.
- the resulting oversampled signal 60 passes through a conventional noise and pulse shaping square root raised cosine complex filter 62.
- Timing correction is performed by a timing circuitry 64.
- the resulting data stream is thereby synchronized with respect toframe, symbol and chip time at more than the chip rate.
- the next stage is for the data stream to be down-converted by a down-converter 66 to the chip rate to provide one complex sample pair per chip, taken at the optimum sampling instant.
- the data stream is passed to a despreading stage which includes the automatic frequency control of the invention.
- the resulting time-aligned complex samples Z Chip,0ffs (k) before frequency correction have a frequency offset f offS to be corrected. This correction is performed by multiplying Z Ch ⁇ P,0ffs (k) by the complex conjugate of a determined correction envelope Z 0ffS (k).
- despreading is performed by a despreader 68.
- the despreader 68 two parallel code-length tap FIR- structure matched filters (not shown) are employed, one for each of the in-phase andquadrature branches.
- the same real valued CDMA-code vector is applied as filter coefficients to both the in-phase andquadrature branch. Since timing is known, multiply and add operations are executed at the known symbol sampling instants only, i.e. once per symbol for every tap.
- a data stream Z sym (n) at the symbol rate is obtained.
- Frequency error detection is performed by a detector70
- the detector determines a frequency error and feeds this value to a numerically controlled oscillator 72 which produces a offset correction which is multiplied with the samples to be corrected by a multiplier 74.
- the resultant symbol stream is passed to a carrier phase corrector 76 and thence a coherent demodulator 78. Error correction is provided by a Viterbi convolutional decoder 80. Where appropriate, the resulting processed signal is then processed to arrange the information into individual channels and if these correspond to a voice transmission they may be converted into an analog voice signal.
- the symbol samples may be expressed by the following equation
- the frequency error detector 70 determines f 0ffS (n) from incoming complex signal samplesZ sym (n) using the phase difference ⁇ 0ffS (n) - ⁇ 0ffs (n-l) between two consecutive symbol samples Z sym (n) and Z sym (n-1).
- the frequency offset f 0ffS (n) is obtained by performing the following complex multiplication and argument operation:
- the differential modulation phase ⁇ mod (n) contains one out of a set of m known values and is removed by rotating the constellation point around zero origin so that the resulting vector has an angle value less than half the smallest differential modulation phase. For (D)QPSK modulation this results in
- a second orderlowpass filter 82 is used to reduce noise by averaging consecutive frequency offset estimates.
- the filter coefficients Cj and c 2 determine the noise filter bandwidth above which the spectral noise components are suppressed.
- One filtered frequency offset estimate f 0ffs .n ⁇ ( ⁇ ) per symbol duration T sym is provided to the numerically controlled oscillator 72 indicating the average phase change of the complex data streamZ chip ⁇ 0ffs (k) between the first and the last sample of a symbol caused by oscillator frequency offset.
- the oscillator 72 generates an up- sampled complex correction sequence
- the multiplier 74 performs a complex multiplication, multiplying the incoming erroneous samples Z Ch i P . 0ffs (k) by the complex conjugate of the constructed correction envelope Z offs (k):
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- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Mobile Radio Communication Systems (AREA)
- Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
Abstract
Description
Claims
Priority Applications (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2001550955A JP2003519963A (en) | 2000-01-05 | 2000-08-11 | Offset correction in spread spectrum communication systems. |
EP00955504A EP1245103B1 (en) | 2000-01-05 | 2000-08-11 | Offset correction in a spread spectrum communication system |
DE60041781T DE60041781D1 (en) | 2000-01-05 | 2000-08-11 | OFFSET CORRECTION IN A SPREADING SPECTRUM COMMUNICATION SYTEM |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US09/477,164 | 2000-01-05 | ||
US09/477,164 US6963602B1 (en) | 2000-01-05 | 2000-01-05 | Digital correction method and system |
Publications (1)
Publication Number | Publication Date |
---|---|
WO2001050698A1 true WO2001050698A1 (en) | 2001-07-12 |
Family
ID=23894788
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/US2000/022217 WO2001050698A1 (en) | 2000-01-05 | 2000-08-11 | Offset correction in a spread spectrum communication system |
Country Status (7)
Country | Link |
---|---|
US (1) | US6963602B1 (en) |
EP (1) | EP1245103B1 (en) |
JP (1) | JP2003519963A (en) |
KR (1) | KR100788012B1 (en) |
CN (1) | CN1185839C (en) |
DE (1) | DE60041781D1 (en) |
WO (1) | WO2001050698A1 (en) |
Families Citing this family (9)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
DK1530301T3 (en) | 2000-03-28 | 2010-08-09 | Interdigital Tech Corp | CDMA system using pre-rotation before transmission |
US7292652B2 (en) * | 2003-10-21 | 2007-11-06 | Texas Instruments Incorporated | Symbol detecting with large frequency error |
EP1813071A1 (en) * | 2004-11-16 | 2007-08-01 | Thomson Licensing | Method and apparatus for carrier recovery using multiple sources |
KR100653181B1 (en) * | 2005-12-07 | 2006-12-05 | 한국전자통신연구원 | Non-coherent synchronous direct conversion receive transmit system for estimating frequency offset |
CN101232489B (en) * | 2006-10-05 | 2013-03-13 | 马维尔国际贸易有限公司 | Difference-related baseband demodulalation system and method |
KR100932626B1 (en) * | 2007-12-13 | 2009-12-17 | 한국전자통신연구원 | Hybrid Sequence Spreader with Direct Sequence Spread Spectrum |
CN102916721A (en) * | 2011-08-04 | 2013-02-06 | 华为技术有限公司 | Frequency deviation acquiring apparatus and method |
US9673842B2 (en) * | 2012-04-25 | 2017-06-06 | Qualcomm Incorporated | Combining multiple desired signals into a single baseband signal |
US10879952B2 (en) * | 2018-04-18 | 2020-12-29 | Huawei Technologies Co., Ltd. | Apparatus and receiver for performing synchronization in analog spread spectrum systems |
Citations (4)
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US5361276A (en) | 1993-09-13 | 1994-11-01 | At&T Bell Laboratories | All digital maximum likelihood based spread spectrum receiver |
EP0762666A2 (en) | 1995-09-11 | 1997-03-12 | Hitachi, Ltd. | Code division multiple access mobile communication system |
US5764630A (en) * | 1996-03-25 | 1998-06-09 | Stanford Telecommunications, Inc. | Forward link carrier recovery in an OCDMA spread spectrum communication system without a pilot tone |
WO2000065797A1 (en) * | 1999-04-23 | 2000-11-02 | Qualcomm Incorporated | Method and apparatus for frequency offset correction |
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US5579338A (en) * | 1992-06-29 | 1996-11-26 | Mitsubishi Denki Kabushiki Kaisha | Spread spectrum receiver using partial correlations |
JP3128997B2 (en) * | 1992-10-09 | 2001-01-29 | ソニー株式会社 | Receiver and method for selecting multipath signal |
JP2605615B2 (en) * | 1993-12-30 | 1997-04-30 | 日本電気株式会社 | Spread spectrum receiver |
US5654955A (en) * | 1994-12-15 | 1997-08-05 | Stanford Telecommunications, Inc. | Network entry channel for CDMA systems |
US5854813A (en) * | 1994-12-29 | 1998-12-29 | Motorola, Inc. | Multiple access up converter/modulator and method |
JP2705613B2 (en) * | 1995-01-31 | 1998-01-28 | 日本電気株式会社 | Frequency offset correction device |
JPH10190526A (en) * | 1996-12-26 | 1998-07-21 | Sony Corp | Receiving device, receiving method and terminal equipment of radio system |
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US5940450A (en) * | 1997-02-28 | 1999-08-17 | Hitachi America, Ltd. | Carrier recovery method and apparatus |
KR100377257B1 (en) * | 1998-04-14 | 2003-03-26 | 프라운호퍼-게젤샤프트 츄어 푀르더룽 데어 안게반텐 포르슝에.파우. | Method and apparatus for fine frequency synchronization in multi-carrier demodulation systems |
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US6621857B1 (en) * | 1999-12-31 | 2003-09-16 | Thomson Licensing S.A. | Carrier tracking loop for direct sequence spread spectrum systems |
-
2000
- 2000-01-05 US US09/477,164 patent/US6963602B1/en not_active Expired - Fee Related
- 2000-08-11 EP EP00955504A patent/EP1245103B1/en not_active Expired - Lifetime
- 2000-08-11 JP JP2001550955A patent/JP2003519963A/en active Pending
- 2000-08-11 DE DE60041781T patent/DE60041781D1/en not_active Expired - Lifetime
- 2000-08-11 WO PCT/US2000/022217 patent/WO2001050698A1/en active Application Filing
- 2000-08-11 CN CNB008180369A patent/CN1185839C/en not_active Expired - Fee Related
- 2000-08-11 KR KR1020027008760A patent/KR100788012B1/en not_active IP Right Cessation
Patent Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
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US5361276A (en) | 1993-09-13 | 1994-11-01 | At&T Bell Laboratories | All digital maximum likelihood based spread spectrum receiver |
EP0762666A2 (en) | 1995-09-11 | 1997-03-12 | Hitachi, Ltd. | Code division multiple access mobile communication system |
US5764630A (en) * | 1996-03-25 | 1998-06-09 | Stanford Telecommunications, Inc. | Forward link carrier recovery in an OCDMA spread spectrum communication system without a pilot tone |
WO2000065797A1 (en) * | 1999-04-23 | 2000-11-02 | Qualcomm Incorporated | Method and apparatus for frequency offset correction |
Also Published As
Publication number | Publication date |
---|---|
EP1245103A1 (en) | 2002-10-02 |
US6963602B1 (en) | 2005-11-08 |
KR100788012B1 (en) | 2007-12-21 |
KR20020067601A (en) | 2002-08-22 |
DE60041781D1 (en) | 2009-04-23 |
EP1245103B1 (en) | 2009-03-11 |
JP2003519963A (en) | 2003-06-24 |
CN1185839C (en) | 2005-01-19 |
CN1415156A (en) | 2003-04-30 |
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