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
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
  • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
  • H04B7/08—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station
  • H04B7/0837—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station using pre-detection combining
  • H04B7/0842—Weighted combining
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
• • 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
  • H04L27/2662—Symbol 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
  • H04L27/2673—Details of algorithms characterised by synchronisation parameters
  • H04L27/2676—Blind, i.e. without using known symbols
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
  • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
  • H04B7/08—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station
  • H04B7/0837—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station using pre-detection combining
  • H04B7/0842—Weighted combining
  • H04B7/0848—Joint weighting
  • H04B7/0857—Joint weighting using maximum ratio combining techniques, e.g. signal-to- interference ratio [SIR], received signal strenght indication [RSS]
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
  • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
  • H04B7/08—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station
  • H04B7/0837—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station using pre-detection combining
  • H04B7/0842—Weighted combining
  • H04B7/086—Weighted combining using weights depending on external parameters, e.g. direction of arrival [DOA], predetermined weights or beamforming

Definitions

• the present invention relates to an apparatus for estimating a time offset and a frequency offset in an Orthogonal Frequency Division Multiplexing (OFDM) communication system, and a method thereof; and, more particularly, to an apparatus for estimating a time offset and a frequency offset that can estimate a time offset and a frequency offset in an environment having a low signal-to-noise ratio (SNR) and multi-path channels by combining diversities of antennas in an OFDM communication system, and a method thereof.
• OFDM Orthogonal Frequency Division Multiplexing
• An Orthogonal Frequency Division Multiplexing (OFDM) system includes an Orthogonal Frequency Division Multiple Access (OFDMA) system and an Orthogonal Frequency and Code Division Multiplexing (OFCDM) system.
• OFDM Orthogonal Frequency Division Multiplexing
• the OFDMA method which is an effective digital signal transmission method based on limited bandwidth and it is proposed by Chang. Recently, the OFDMA method is applied to a digital signal transmission system such as a digital audio broadcasting or a digital video broadcasting in Europe, an Asymmetric Digital Subscriber Line (ADSL), and a wireless Local Area Network (WLAN).
• a digital signal transmission system such as a digital audio broadcasting or a digital video broadcasting in Europe, an Asymmetric Digital Subscriber Line (ADSL), and a wireless Local Area Network (WLAN).
• ADSL Asymmetric Digital Subscriber Line
• WLAN wireless Local Area Network
• the OFDMA method has advantages that it is strong to an inter-symbol interference (ISI) discussed for high- efficiency of frequency usage and high-speed communication and that it makes a selective fading of frequency look like a non-selective fading.
• ISI inter-symbol interference
• the OFDM access method has a serious disadvantage that inconformity of oscillators between a transmitter and a receiver, and sensitiveness of carrier frequency error by a Doppler frequency variation in the receiver.
• the carrier frequency error breaks orthogonality between sub carriers in the OFDM system so that the sub carriers are influenced by an inter-carrier interference (ICI) from the other sub carriers. Therefore, a small amount of a carrier frequency error causes bad performance of the OFDM system.
• ICI inter-carrier interference
• Conventional methods for solving the above problems include a data-dependent method and a blind method.
• the data-dependent method using two pilot symbols for time synchronization and estimating an integer and a fractional part of a frequency offset to estimate frequency error is faster and more reliable than the blind method.
• data rate and power efficiency of the data-dependent method are decreased due to the use of two pilot symbols.
• the blind method synchronizes frequencies based on a cyclic prefix (CP) inserted for decreasing the ISI, data rate or power efficiency is not decreased.
• CP cyclic prefix
• the CP and maximum likelihood (ML), which is known as the best estimation method of frequency synchronization, are used to estimate and compensate for the frequency offset.
• the blind method uses a characteristic that there is only a phase difference between a CP and a copied CP when time is synchronized, and the others except the phase difference are same.
• performance of the blind method presents fine performance at a high signal-to-noise ratio (SNR) of an Additive White Gaussian Noise (AWGN) channel but poor performance in an environment of a low SNR and a multi-path channel.
• SNR signal-to-noise ratio
• AWGN Additive White Gaussian Noise
• an object of the present invention devised to resolve the above problems is to provide an apparatus for estimating a time offset and a frequency offset that can estimate a time offset and a frequency offset in a communication environment of a low signal-to- noise ratio (SNR) and multi-path channels by combining diversities of antennas in an Orthogonal Frequency Division Multiplexing (OFDM) communication system, and a method thereof.
• SNR signal-to- noise ratio
• OFDM Orthogonal Frequency Division Multiplexing
• an apparatus for estimating a time offset and a frequency offset based on signals inputted through at least two antennas including: a signal generating unit for giving weights to each of the signals received through the antennas and generating an input signal; a delay unit for delaying the input signal by a predetermined index; a first adding unit for adding a square value of the delayed signal and a square value of the input signal; a first summating unit for summating a predetermined number of the output value of the first adding unit and multiplying the summed value by a predetermined value, which is decided based on the weights, signal power and noise power, to thereby acquire a first summation value; a second adding unit for adding the input signal and a conjugated signal obtained by changing the sign of an imaginary part of the complex delayed signal; a second summating unit for summating a predetermined number of the output value of the second adding unit to thereby acquire a second summation value; a time offset
• a method for estimating a time offset and a frequency offset based on signals inputted through at least two antennas including the steps of: a) giving weights to the signals received through the antennas and generating an input signal; b) delaying the input signal by a predetermined index and adding a square value of the delayed signal and a square value of the input signal; c) summating a predetermined number of the output value of the step b) and multiplying the summed value and a predetermined value, which is decided based on the weights, signal power and noise power; d) adding the input signal and a conjugated delayed signal, which is obtained by changing the sign of an imaginary part of the complex delayed signal; e) summating a predetermined number of the output value of the step d) ; f) detecting the maximum value of the time offset by subtracting the output value of the step c) from the absolute value of the the output value of the step e); and g) calculating
• the present invention provides excellent performances of estimating a time offset and a frequency offset by using a weight in an environment having a low signal-to-noise ratio (SNR) and multi-path channels in an OFDM communication system.
• SNR signal-to-noise ratio
• Fig. 1 is a block diagram illustrating a general Orthogonal Frequency Division Multiplexing (OFDM) system
• Fig. 2 is a diagram illustrating a general symbol structure of the OFDM system
• Fig. 3 is a diagram illustrating an OFDM system having two antennas in accordance with an embodiment of the present invention.
• Fig. 4 is a block diagram illustrating an apparatus for estimating a time offset and a frequency offset in accordance with an embodiment of the present invention.
• OFDM Orthogonal Frequency Division Multiplexing
• Fig. 1 is a block diagram illustrating a general OFDM system.
• Fig. 1 shows data transmission between a transmitting antenna and a receiving antenna.
• a transmission signal is transmitted to a receiving block through an inverse discrete Fourier transformer (IDFT) 11 and a parallel-to- serial converter 12, the receiving block receives the transmission signal through a serial-to-parallel converter 13 and a discrete Fourier transformer (DFT) 14 reversely.
• IDFT inverse discrete Fourier transformer
• DFT discrete Fourier transformer
• S 0 , Si, S 2 to S L _i are cyclic prefixes (CP).
• CP cyclic prefixes
• the OFDM system illustrated in Fig. 1 uses N carriers and L CPs in order to transmit one OFDM symbol.
• the Maximum likelihood (ML) estimation of the OFDM system will be described referring to Fig. 2.
• an I sector is a CP in the i th OFDM symbol and an I' sector is a data sample which is duplicated into the I sector in the i th OFDM symbol. Since samples are received randomly except I and I' , each received sample is independent from each other. Since I and I' are the same samples, there is a correlation between I and I' .
• a probability density function of a received signal r is obtained when a time offset ( ⁇ ) and a frequency offset ( ⁇ ) are predetermined, and the time offset ( ⁇ ) and the frequency offset ( ⁇ ) are estimated by acquiring ⁇ and ⁇ values that maximize the log-likelihood function of the probability density function. Since, the above estimating method of the time offset and the frequency offset, i.e., the maximum likelihood (ML) estimation, is widely known, detailed description on it will be omitted.
• Fig. 3 is a diagram illustrating an OFDM system having two antennas to help technical understanding prior to description of a time and frequency offset estimation method in accordance with an embodiment of the present invention.
• An OFDM diversity receiver includes two antennas, two decoders for converting analog signals received through each antenna into digital signals and decoding the digital signals into original signals by performing fast Fourier transformation, two equalizers for compensating for distortion of the decoded signals occurring in transmission, and a combiner for combining the distortion-compensated signals acquired in the two equalizers.
• a first antenna receives an ri signal
• a second antenna receives an r 2 signal.
• the two antennas are departed from each other by over ⁇ /2 so that the two reception signals are independent from each other.
• the two signals ri and r 2 are multiplied by two weights pi and p 2 based on an antenna combining method, respectively.
• ML estimation is induced based on the weighted signals, inputted through a plurality of antennas to acquire a diversity gain.
• the received signal r is expressed as following Eq. 1.
• ri(k) and r 2 (k) are the k th received signal in the observation duration 2N+L from the first antenna and the second antenna, respectively.
• An integrated k th signal r(k) is obtained by multiplying weights and each received signals.
• samples can be divided into those existing in I and those not existing in I.
• I and I' which is apart by N from I, and other signals are transmitted randomly because samples existing in I are duplicated CPs of samples in I' .
• Correlations between received signals in the observation duration k which is from 1 to 2N+L are represented as following Eq. 2. That is, when k is included in I, autocorrelation characteristics are obtained due to correlation between I and I' . In addition, when k is out of the in I and I' , there is no correlation relationship.
• ⁇ s 2 is an average power of transmission signals
• ⁇ N 2 is an average power of noise
• ⁇ is a frequency offset
• a log-likelihood function of the time offset ( ⁇ ) and the frequency offset ( ⁇ ) for estimating a maximum likelihood (ML) is a logarithm of a probability density function for 2N+L observation signals.
• the log- likelihood function is expressed as following Eq. 3.
• r is a received signal vector including received signals acquired in 2N+L observation durations; the received signal vector r is represented as [r(l)... r(2N+L)] T ; and f is the probability density function.
• ⁇ (m) denotes a sum of L continuous correlation values between received signals apart by N; ⁇ (m) is an energy of received signals; and p 2 is a magnitude of a correlation coefficient between r(k) and r(k+N).
• the ⁇ (m) , ⁇ (m) and p 2 are expressed as following Eq. 5.
• the ML is estimated by acquiring a time offset value and a frequency offset value maximizing the log- likelihood function as shown in the following Eq. 6.
• the antenna diversity gain is obtained from the reception signal r(k). Moreover, referring to Eq. 7, since the correlation coefficient p 2 is larger than that of using one antenna due to the antenna diversity gain, performance of the apparatus for estimating time offset and frequency offset is more excellent than the conventional apparatus.
• Fig. 4 is a block diagram illustrating an apparatus for estimating a time offset and a frequency offset in accordance with an embodiment of the present . invention .
• the time offset is estimated based on the signals received through two antennas and then the frequency offset is estimated.
• pi and p 2 are different based on an antenna combining method. In case of an identical gain combining method, pi and p 2 are equal to 1. In case of a selection combining method, one having a large signal-to-noise ratio (SNR) is 1 and the other is 0. In case of a maximum ratio combining (MRC) method, pi and p 2 determined such that a sum of two received signals is the maximum SNR.
• SNR signal-to-noise ratio
• the apparatus for estimating the time offset and the frequency offset includes a signal generator 100, a delay unit 101, a first square value calculator 102, a second square value calculator 103, a first adder 104, a first summator 105, a conjugator 106, a second adder 107, a second summator 108, a subtractor 109, a time offset detector 110 and a frequency offset calculator 111.
• the signal generator 100 receives two signals transmitted through two antennas, gives weights to the two signals, and integrates the two weighted signals to thereby generate an input signal.
• the delay unit 101 delays the input signal with a predetermined index Z .
• the first square value calculator 102 calculates a first square value of the delayed signal.
• the second square value calculator 103 calculates a second square value of the input signal.
• the first adder 104 adds up the first square value and the second square value to thereby generate a first adding value.
• the first summator 105 summates the predetermined number of the first adding value and multiplies the summated value and a predetermined value which is decided based on the weights, signal power and noise power.
• the conjugator 106 conjugates the delayed signal which changes the sign of imaginary part of the complex delayed signal.
• the second adder 107 adds up the input signal and the delayed signal to thereby generate a second adding value.
• the second summator 108 summates the predetermined number of the second adding value.
• the subtractor 109 subtracts the output value from the first summator 105 from the output value from the second summator 108.
• the time offset detector 110 detects the maximum time offset of the output values from the subtractor 109.
• the frequency offset calculator 111 calculates a radian degree of the absolute value of the output value from the second summator 108 and calculates a frequency offset based on the detected time offset in the time offset detector 110.
• One of the outstanding features of the time offset and the frequency offset estimating apparatus is adding a square value of the delayed input signal and a square value of the input signal to thereby produce an added value, summating a predetermined number of the added value, and estimating the time offset based on a predetermined value acquired based on weights given to the two received signals, signal power and noise power.
• the method for estimating the time offset and the frequency offset in the present invention will be described referring to Fig. 4.
• an input signal is generated by giving weights to signals received through two antennas. Then, the input signal is delayed by a predetermined index Z . A first square value, which is a square value of the delayed signal, is calculated; and a second square value, which is a square value of the input signal, is calculated. Then, the first square value and the second square value are added to thereby generate a first adding value .
• the predetermined number of the first adding values are summated, the summated value and a predetermined value p 2 /2, which is acquired based on the weights, signal power and noise power, are multiplied to thereby calculate a first summation value.
• the sign of imaginary part of the complex delayed signal is changed and the input signal and the conjugated delayed signal are added to thereby generate a second adding value.
• the predetermined number of the second adding values are summated to thereby calculate a second summation value.
• the absolute value of the second summation value is obtained.
• the maximum time offset is acquired by- subtracting the first summation value from the absolute value of the second summation value.
• the frequency offset is estimated based on the time offset. That is, radian degree of the absolute value of the second summation value is obtained and the frequency offset is calculated based on the detected time offset.

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• 2005
  • 2005-12-09 KR KR1020050121133A patent/KR100770008B1/ko not_active IP Right Cessation
• 2006
  • 2006-12-11 EP EP06824083A patent/EP1958409A1/en not_active Withdrawn
  • 2006-12-11 WO PCT/KR2006/005377 patent/WO2007067018A1/en active Application Filing

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