WO2004014005A1 - Extraction de la phase d'un echantillon de signal a multiplexage par repartition orthogonal de la frequence - Google Patents

Extraction de la phase d'un echantillon de signal a multiplexage par repartition orthogonal de la frequence Download PDF

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Publication number
WO2004014005A1
WO2004014005A1 PCT/US2002/024263 US0224263W WO2004014005A1 WO 2004014005 A1 WO2004014005 A1 WO 2004014005A1 US 0224263 W US0224263 W US 0224263W WO 2004014005 A1 WO2004014005 A1 WO 2004014005A1
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WO
WIPO (PCT)
Prior art keywords
complex number
circuitry
phase
complex
squared
Prior art date
Application number
PCT/US2002/024263
Other languages
English (en)
Inventor
Maxim Borisovich Belotserkovsky
Vincent Demoulin
Louis Robert Litwin, Jr.
Original Assignee
Thomson Licensing S.A.
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 Thomson Licensing S.A. filed Critical Thomson Licensing S.A.
Priority to AU2002322823A priority Critical patent/AU2002322823A1/en
Priority to PCT/US2002/024263 priority patent/WO2004014005A1/fr
Priority to CNA028296907A priority patent/CN1669256A/zh
Priority to EP02756842A priority patent/EP1532759A4/fr
Priority to JP2004525924A priority patent/JP2005535224A/ja
Priority to US10/523,450 priority patent/US7760616B2/en
Publication of WO2004014005A1 publication Critical patent/WO2004014005A1/fr

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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
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2657—Carrier synchronisation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2668—Details of algorithms
    • H04L27/2681—Details of algorithms characterised by constraints
    • H04L27/2684—Complexity
    • 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
    • H04L2027/0063—Elements of loops
    • H04L2027/0067—Phase error detectors

Definitions

  • the present invention relates to processing orthogonal frequency division multiplexed (OFDM) signals.
  • a wireless LAN is a flexible data communications system implemented as an alternative or extension to a wired LAN within a building or campus. Using electromagnetic waves, WLANs transmit and receive data over the air, minimizing the need for wired connections. Thus, WLANs combine data connectivity with user mobility, and, through simplified configuration, enable movable LANs.
  • Some industries that have benefited from the productivity gains of using portable terminals (e.g., notebook computers) to transmit and receive real-time information are the digital home networking, health-care, retail, manufacturing, and warehousing industries.
  • Manufacturers of WLANs have a range of transmission technologies to choose from when designing a WLAN.
  • Some exemplary technologies are multicarrier systems, spread spectrum systems, narrowband systems, and infrared systems. Although each system has its own benefits and detriments, one particular type of multicarrier transmission system, orthogonal frequency division multiplexing (OFDM), has proven to be exceptionally useful for WLAN communications.
  • OFDM orthogonal frequency division multiplexing
  • OFDM is a robust technique for efficiently transmitting data over a channel.
  • the technique uses a plurality of sub-carrier frequencies (sub- carriers) within a channel bandwidth to transmit data. These sub-carriers are arranged for optimal bandwidth efficiency compared to conventional frequency division multiplexing (FDM) which can waste portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI).
  • FDM frequency division multiplexing
  • ICI inter-carrier interference
  • the transmission of data through a channel via OFDM signals also provides several other advantages over more conventional transmission techniques. Some of these advantages are a tolerance to multipath delay spread and frequency selective fading, efficient spectrum usage, simplified sub-channel equalization, and good interference properties.
  • a complex number is a frequency domain subcarrier value.
  • the ability to determine the phase of complex numbers representing an input signal is useful for many purposes, such as synchronizing the received data signal to ensure the maximum amount of integrity in the received data compared to the transmitted data.
  • determining the phase of a complex number would require first finding the tangent of the phase of the complex number:
  • an inverse tangent look-up table could be used to determine the phase angle of the complex number corresponding to the input signal. Depending on the accuracy required, such an inverse tangent look-up table could be very large and expensive to implement in hardware.
  • a method and apparatus capable of determining the phase of a complex number corresponding to an input signal without accessing an inverse tangent look-up table is desirable.
  • the disclosed embodiments relate to exploiting circuitry that exists in a typical Orthogonal Frequency Division Multiplexing (OFDM) receiver to find the phase of a complex number corresponding to an input signal without implementing additional costly circuitry or employing a relatively slow inverse tangent look-up table.
  • the magnitude of a complex number corresponding to an input signal is normalized, which has the effect of extracting the exponent of the complex portion of the sample.
  • the exponent may be passed through a closed loop that includes a numerically controlled oscillator (NCO).
  • NCO numerically controlled oscillator
  • the input value to the closed loop may stay constant for a predetermined number of clock cycles sufficiently long to allow the loop to converge for any phase of input sample. After convergence, the output of the NCO will be a signal that is proportional to the phase of the complex number corresponding to the input signal. Additional mathematical processing may be needed to convert the NCO output to the desired phase value.
  • FIG. 1 is a block diagram of an exemplary OFDM receiver
  • FIG. 2 is a diagram illustrating the placement of a training sequence, user data, and pilot signals within an OFDM symbol frame;
  • FIG. 3 is a block diagram of a circuit for extracting the phase of a complex number corresponding to an input signal
  • FIG. 4 is an alternative embodiment of a circuit for normalizing a complex number corresponding to an input signal
  • FIG. 5 is a process flow diagram illustrating the operation of an exemplary embodiment of the present invention.
  • the first element of a typical OFDM receiver 10 is an RF receiver 12.
  • the RF receiver 12 includes an antenna 14, a low noise amplifier (LNA) 16, an RF band pass filter 18, an automatic gain control (AGC) circuit 20, an RF mixer 22, an RF carrier frequency local oscillator 24, and an IF band pass filter 26.
  • LNA low noise amplifier
  • AGC automatic gain control
  • the RF receiver 12 couples in the RF
  • the RF receiver 12 downconverts the RF OFDM-modulated carrier to obtain a received IF OFDM signal.
  • the frequency difference between the receiver carrier and the transmitter carrier contributes to the carrier frequency offset, delta f c .
  • This received IF OFDM signal is coupled to a mixer 28 and a mixer 30 to be mixed with an in-phase IF signal and a 90° phase-shifted (quadrature) IF signal, respectively, to produce in-phase and quadrature OFDM signals, respectively.
  • the in-phase IF signal that feeds into the mixer 28 is produced by an IF local oscillator 32.
  • the 90° phase-shifted IF signal that feeds into mixer 30 is derived from the in-phase IF signal of the IF local oscillator 32 by passing the in-phase IF signal through a 90° phase shifter 34 before providing it to the mixer 30.
  • the in-phase and quadrature OFDM signals then pass into analog- to-digital converters (ADCs) 36 and 38, respectively, where they are digitized at a sampling rate f Ck _. as determined by a clock circuit 40.
  • the ADCs 36 and 38 produce digital samples that form an in-phase and a quadrature discrete-time OFDM signal, respectively.
  • the unfiltered in-phase and quadrature discrete-time OFDM signals from the ADCs 36 and 38 then pass through digital low-pass filters 42 and 44, respectively.
  • the output of the low pass digital filters 42 and 44 are filtered in-phase and quadrature samples, respectively, of the received OFDM signal.
  • These in-phase and quadrature (real-valued and imaginary-valued) samples of the received OFDM signal are then delivered to an FFT 46.
  • the analog-to-digital conversion is done before the IF mixing process.
  • the mixing process involves the use of digital mixers and a digital frequency synthesizer.
  • the digital-to- analog conversion is performed after the filtering.
  • the FFT 46 performs the Fast Fourier Transform (FFT) of the received OFDM signal in order to recover the sequences of frequency- domain sub-symbols that were used to modulate the sub-carriers during each OFDM symbol interval.
  • the FFT 46 then delivers these sequences of sub-symbols to a decoder 48.
  • the decoder 48 recovers the transmitted data bits from the sequences of frequency-domain sub-symbols that are delivered to it from the FFT 46. This recovery is performed by decoding the frequency- domain sub-symbols to obtain a stream of data bits which should ideally match the stream of data bits that were fed into the OFDM transmitter.
  • This decoding process can include soft Viterbi decoding and/or Reed- Solomon decoding, for example, to recover the data from the block and/or convolutionally encoded sub-symbols.
  • the symbol frame 50 includes a training sequence or symbol 52 containing known transmission values for each subcarrier in the OFDM symbol, and a predetermined number of a cyclic prefix 54 and user data 56 pairs.
  • a training sequence or symbol 52 containing known transmission values for each subcarrier in the OFDM symbol, and a predetermined number of a cyclic prefix 54 and user data 56 pairs.
  • the proposed ETSI-BRAN HIPERLAN/2 (Europe) and IEEE 802.11a (USA) wireless LAN standards herein incorporated by reference, assign 64 known values or subsymbols (i.e., 52 non-zero values and 12 zero values) to selected training symbols of a training sequence (e.g., "training symbol C" of the proposed ETSI standard and "long OFDM training symbol" of the proposed IEEE standard).
  • the user data 56 has a predetermined number of pilots 58, also containing known transmission values, embedded on predetermined subcarriers.
  • the proposed ETSI and IEEE standards have four pilots located at bins or subcarriers ⁇ 7 and ⁇ 21.
  • FIG. 3 is a block diagram of a circuit for extracting the phase of a complex number corresponding to an input signal.
  • the complex number may be generally represented as follows:
  • the complex number representation must be manipulated mathematically to determine the angle phi.
  • the first step in this mathematical manipulation is to extract the exponent from the complex representation of the sample. Extraction of the exponent may be accomplished by normalizing the magnitude of the sample to one. Normalizing the magnitude of the complex number ensures a constant gain closed-loop operation, which in turn assures loop convergence over a fixed number of clock cycles. The step of normalizing the magnitude of the complex number allows sampling of the loop output after a predetermined number of clocks (i.e. after the loop is known to have converged), thus eliminating the need to implement a relatively expensive and potentially unreliable loop-lock indicator.
  • is squared by a squaring circuit 62, which typically exists in most OFDM receivers (i.e. it does not have to be specially included).
  • a squaring circuit 64 squares the entire value of the complex number (both the real and imaginary components). If no additional squaring circuit is present in the OFDM receiver in which the invention is implemented, the squaring circuit 62 may be used to square the complex number as well as to determine the square of the magnitude of the complex number.
  • An inversion circuit 66 (also typically present in most OFDM receivers) is used to invert the output of the squaring circuit 62. The output of the squaring circuit 64 and the inversion circuit 66 are combined by multiplier 68.
  • the output of the multiplier 68 equates to the normalized value of the square of the complex number, which may be expressed as e 2j*phl .
  • the second step in determining the phase angle of the complex number comprises sending the output of the multiplier 68 through a closed loop.
  • the second order carrier tracking loop which is typically implemented in an OFDM receiver (i.e. does not have to be specifically added), is useful for this purpose.
  • the output of the multiplier 68 must be provided to the tracking loop (beginning with a derotator 70) for a predetermined number of clock cycles to ensure stabilization of the loop.
  • the exact number of clock cycles needed to ensure loop stabilization in a given OFDM receiver configuration may be readily determined by those of ordinary skill in the art.
  • the determination of the number of clock cycles required for loop stabilization is not a crucial aspect of the present invention.
  • the output from the multiplier 68 is fed into the derotator 70 and the output of the derotator 70 is passed on to a phase detector 72.
  • the output of the phase detector 72 is passed on to an integral gain amplifier 74 and a proportional gain amplifier 76.
  • the output of the integral gain amplifier 74 is fed into an integrator 78.
  • the output of the integrator 78 is combined with the output of the proportional gain amplifier 76 by a summing circuit 80.
  • the output of the summing circuit 80 is fed into a numerically controlled oscillator (NCO) 82, which feeds into a sine/cosine look-up table 84.
  • NCO numerically controlled oscillator
  • the output of the sine/cosine look-up table 84 is provided as feedback to the derotator 70.
  • the output of the NCO 82 is equal to twice the phase of the complex number corresponding to the input signal. Division of the output of the NCO 82 by two with a divider circuit 84 results in a signal 86 that is equivalent to the phase angle phi of the complex number.
  • the present invention determines the phase angle of a complex number corresponding to an input signal without the need of a cumbersome and costly inverse tangent look-up table.
  • FIG. 4 is an alternative embodiment of a circuit for normalizing a complex number. In the embodiment shown in FIG. 4, the complex number 60 is inverted by an inversion circuit 90. The complex conjugate of the inverse of the sample is determined by a complex conjugation circuit 92.
  • FIG. 5 is a process flow diagram illustrating the operation of an exemplary embodiment of the present invention.
  • the overall process is referred to by the reference numeral 100.
  • the process begins at 102.
  • a complex number corresponding to an input signal is obtained. This complex number may be representative of a portion of an OFDM signal that has been received by an OFDM receiver.
  • the complex number is normalized.
  • Normalization may be performed using any known method, including hardware methods, software methods and combined hardware and software methods. Examples of circuits that may be used for normalization of a complex number are shown and described with reference to FIG. 3 (the circuitry that feeds into the derotator 70) and FIG. 4. As described above, normalization of a complex number corresponding to an input signal ensures that loop convergence can be guaranteed within a predetermined number of receiver clock cycles.
  • the normalized complex number is fed into closed loop at 108.
  • An example of such a loop is the carrier tracking loop typically present in OFDM receivers.
  • the input to the closed loop is maintained for a predetermined number of clock cycles.
  • the purpose of waiting for a predetermined number of clock cycles is to allow loop convergence around a value that is proportional to the phase of the complex number.
  • the loop output is divided by 2 to yield the phase of the complex.
  • the method of the present invention eliminates the need to implement a costly and time consuming inverse tangent lookup table to determine the phase of a complex number corresponding to an input signal.
  • the process of FIG. 5 ends.

Abstract

Les modes de réalisation de la présente invention concernent l'utilisation de circuits existants dans un récepteur fonctionnant normalement en multiplexage par répartition orthogonal de la fréquence ou 'OFDM' (Orthogonal Frequency Division Multiplexing) pour trouver la phase d'un nombre complexe correspondant à un signal d'entrée, sans mise en oeuvre de circuits supplémentaires dispendieux ou sans employer de table de consultation de tangente inverse relativement lente. En l'occurrence, on normalise (106) la grandeur du nombre complexe, et on la traite au moyen d'une boucle fermée de façon produire une sortie (112) proportionnelle à la phase du nombre complexe.
PCT/US2002/024263 2002-07-31 2002-07-31 Extraction de la phase d'un echantillon de signal a multiplexage par repartition orthogonal de la frequence WO2004014005A1 (fr)

Priority Applications (6)

Application Number Priority Date Filing Date Title
AU2002322823A AU2002322823A1 (en) 2002-07-31 2002-07-31 Extracting the phase of an ofdm signal sample
PCT/US2002/024263 WO2004014005A1 (fr) 2002-07-31 2002-07-31 Extraction de la phase d'un echantillon de signal a multiplexage par repartition orthogonal de la frequence
CNA028296907A CN1669256A (zh) 2002-07-31 2002-07-31 提取正交频分多路复用信号样本的相位
EP02756842A EP1532759A4 (fr) 2002-07-31 2002-07-31 Extraction de la phase d'un echantillon de signal a multiplexage par repartition orthogonal de la frequence
JP2004525924A JP2005535224A (ja) 2002-07-31 2002-07-31 Ｏｆｄｍ信号サンプルの位相検出
US10/523,450 US7760616B2 (en) 2002-07-31 2002-07-31 Extracting the phase of an OFDM signal sample

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/US2002/024263 WO2004014005A1 (fr) 2002-07-31 2002-07-31 Extraction de la phase d'un echantillon de signal a multiplexage par repartition orthogonal de la frequence

Publications (1)

Publication Number Publication Date
WO2004014005A1 true WO2004014005A1 (fr) 2004-02-12

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EP (1) EP1532759A4 (fr)
JP (1) JP2005535224A (fr)
CN (1) CN1669256A (fr)
AU (1) AU2002322823A1 (fr)
WO (1) WO2004014005A1 (fr)

Citations (5)

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Publication number Priority date Publication date Assignee Title
US5255290A (en) * 1992-08-21 1993-10-19 Teknekron Communications System, Inc. Method and apparatus for combined frequency offset and timing offset estimation
US5287067A (en) 1991-10-07 1994-02-15 Nippon Telegraph And Telephone Corporation Method and apparatus for demodulation with adaptive phase control in quasi-coherent detection
US5581582A (en) * 1994-03-25 1996-12-03 Samsung Electronics Co., Ltd. Automatic frequency control method and apparatus therefor
GB2313270A (en) 1996-05-17 1997-11-19 Mitsubishi Electric Corp Digital Broadcasting Receiver
WO1998040991A1 (fr) 1997-03-12 1998-09-17 Interdigital Technology Corporation Boucle a verrouillage de phase a largeur de bande reglable en continu

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR100265735B1 (ko) * 1997-11-25 2000-09-15 윤종용 Fft윈도우위치복원과샘플링클럭제어가연동되는ofdm수신장치및그방법
JP3688113B2 (ja) * 1998-04-23 2005-08-24 三菱電機株式会社 デジタル音声放送受信機

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5287067A (en) 1991-10-07 1994-02-15 Nippon Telegraph And Telephone Corporation Method and apparatus for demodulation with adaptive phase control in quasi-coherent detection
US5255290A (en) * 1992-08-21 1993-10-19 Teknekron Communications System, Inc. Method and apparatus for combined frequency offset and timing offset estimation
US5581582A (en) * 1994-03-25 1996-12-03 Samsung Electronics Co., Ltd. Automatic frequency control method and apparatus therefor
GB2313270A (en) 1996-05-17 1997-11-19 Mitsubishi Electric Corp Digital Broadcasting Receiver
WO1998040991A1 (fr) 1997-03-12 1998-09-17 Interdigital Technology Corporation Boucle a verrouillage de phase a largeur de bande reglable en continu
US6055231A (en) * 1997-03-12 2000-04-25 Interdigital Technology Corporation Continuously adjusted-bandwidth discrete-time phase-locked loop

Non-Patent Citations (1)

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Title
See also references of EP1532759A4 *

Also Published As

Publication number Publication date
CN1669256A (zh) 2005-09-14
EP1532759A4 (fr) 2010-07-14
EP1532759A1 (fr) 2005-05-25
AU2002322823A1 (en) 2004-02-23
JP2005535224A (ja) 2005-11-17

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