The present invention relates to methods and apparatus for performing a coarse frequency synchronization compensating for a carrier frequency deviation from an oscillator frequency in a demodulation system. In particular, the present invention relates to such methods and apparatus in a demodulation system for multi-carrier modulation signals, wherein the multi-carrier modulation (MCM) signals have a frame structure comprising at least one useful symbol and a reference symbol.
The present invention is particularly useful in a MCM transmission system using an orthogonal frequency division multiplexing (OFDM) for digital broadcasting.
In a multi carrier transmission system (MCM, OFDM), the effect of a carrier frequency offset is substantially more considerable than in a single carrier transmission system. MCM is more sensitive to phase noise and frequency offset which occurs as amplitude distortion and inter carrier interference (ICI). The inter carrier interference has the effect that the subcarriers are no longer orthogonal in relation to each other. Frequency offsets occur after power on or also later due to frequency deviation of the oscillators used for downconversion into baseband. Typical accuracies for the frequency of a free running oscillator are about ±50 ppm of the carrier frequency. With a carrier frequency in the S-band of 2.34 GHz, for example, there will be a maximum local oscillator (LO) frequency deviation of above 100 kHz (117.25 kHz). The above named effects result in high requirements on the algorithm used for frequency offset correction.
Most prior art algorithms for frequency synchronization divide frequency correction into two stages. In the first stage, a coarse synchronization is performed. In the second stage, a fine correction can be achieved. A frequently used algorithm for coarse synchronization of the carrier frequency uses a synchronization symbol which has a special spectral pattern in the frequency domain. Such a synchronization symbol is, for example, a CAZAC sequence (CAZAC=Constant Amplitude Zero Autocorrelation). Through comparison, i.e. the correlation, of the power spectrum of the received signal with that of the transmitted signal, the frequency carrier offset can be coarsely estimated. These prior art algorithms all work in the frequency domain. Reference is made, for example, to Ferdinand Claben, Heinrich Meyr, “Synchronization Algorithms for an OFDM System for Mobile Communication”, ITG-Fachtagung 130, Codierung für Quelle, Kanal und Übertragung, pp. 105–113, Oct. 26–28, 1994; and Timothy M. Schmidl, Donald C. Cox, “Low-Overhead, Low-Complexity [Burst] Synchronization for OFDM”, in Proceedings of the IEEE International Conference on Communication ICC 1996, pp. 1301–1306 (1996).
For the coarse synchronization of the carrier frequency, Paul H. Moose, “A Technique for Orthogonal Frequency Division Multiplexing Frequency Offset Correction”, IEEE Transaction On Communications, Vol. 42, No. 10, October 1994, suggest increasing the spacing between the subcarriers such that the subcarrier distance is greater than the maximum frequency difference between the received and transmitted carriers. The subcarrier distance is increased by reducing the number of sample values which are transformed by the Fast Fourier Transform. This corresponds to a reduction of the number of sampling values which are transformed by the Fast Fourier Transform.
WO 9800946 A relates to a system for a timing and frequency synchronization of OFDM signals. OFDM training symbols are used to obtain full synchronization in less than two data frames. The OFDM training symbols are placed into the OFDM signal, preferably at least once every frame. The first OFDM training symbol is produced by modulating the even-numbered OFDM sub-carriers whereas the odd-numbered OFDM sub-carriers are suppressed. Thus, the first OFDM training symbol is produced by modulating the even-numbered carriers of this symbol with a first predetermined pseudo noise sequence. This results in a time-domain OFDM symbol that has two identical halves since each of the even-numbered sub-carrier frequencies repeats every half symbol interval. In case a carrier frequency offset is not greater than a sub-carrier bandwidth, the carrier frequency offset can be determined using the phase difference between the two halves of the first OFDM training symbol. In case the carrier frequency offset can be greater than a sub-carrier bandwidth a second OFDM training symbol is used which is formed by using a second predetermined pseudo noise sequence to modulate the even-numbered frequencies of this symbol and by using a third predetermined pseudo noise sequence to modulate the odd-numbered carriers of this symbol. This second OFDM training symbol is used in order to determine an integer part of the carrier frequency offset. This integer part and a positive or negative fractional part determined from the first OFDM training symbol are used for performing the coarse frequency synchronization. In order to determine the integer part of the carrier frequency offset, fast Fourier transforms of the two training symbols are required.
It is an object of the present invention to provide methods and apparatus for performing a coarse frequency synchronization even in the case of frequency offsets that correspond to a multiple of the subcarrier distance in a MCM signal. In accordance with a first aspect, the present invention provides a method of performing a coarse frequency synchronization compensating for a carrier frequency deviation from an oscillator frequency in a demodulation system capable of demodulating a signal having a frame structure, said frame structure comprising at least one useful symbol and a reference symbol, said reference symbol being an amplitude-modulated bit sequence, the method comprising the steps of:
In accordance with a second aspect, the present invention provides a method of performing a coarse frequency synchronization compensating for a carrier frequency deviation from an oscillator frequency in a demodulation system capable of demodulating a signal having a frame structure, the frame structure comprising at least one useful symbol and a reference symbol, the reference symbol being an amplitude-modulated bit sequence which comprises two identical sequences, the method comprising the steps of:
In accordance with a third aspect, the present invention provides an apparatus for performing a coarse frequency synchronization compensating for a carrier frequency deviation from an oscillator frequency, for a demodulation system capable of demodulating a signal having a frame structure, the frame structure comprising at least one useful symbol and a reference symbol, the reference symbol being an amplitude-modulated bit sequence, the apparatus comprising:
In accordance with a fourth aspect, the present invention provides an apparatus for performing a coarse frequency synchronization compensating for a carrier frequency deviation from an oscillator frequency, for a demodulation system capable of demodulating a signal having a frame structure, the frame structure comprising at least one useful symbol and a reference symbol, the reference symbol being an amplitude-modulated bit sequence which comprises two identical sequences, the apparatus comprising:
The present invention provides a new scheme for a coarse frequency synchronization, in particular in MCM systems. The present invention is particularly useful in systems which use a differential coding and mapping along the frequency axis. In accordance with the present invention, the algorithm for the coarse frequency synchronization is based on a reference symbol which is formed by an amplitude-modulated sequence. The length of this amplitude-modulated sequence symbol may be less than that of the useful symbol. The algorithm in accordance with the present invention can be used in the time domain or the frequency domain. In order to determine a frequency offset, a correlation of the received MCM symbol with a predetermined reference pattern is performed in accordance with a first embodiment of the present invention. In accordance with a second embodiment of the present invention, the reference symbol comprises at least two identical amplitude-modulated sequences, wherein a frequency offset is determined based on a correlation between demodulated portions corresponding to these identical sequences. It is preferred to select the mean amplitude of the reference symbol identically to the mean amplitude of the rest of the signal, i.e. to select all of the samples of the demodulated amplitude-modulated sequence in the middle of their amplitude range. Care has to be taken that the time constant of an automatic gain control (AGC) is selected to be long enough that the strong signal part of the reference symbol does not excessively influence the automatic gain control signal. Otherwise, the signal occurring after the amplitude-modulated sequence would be strongly attenuated.
According to preferred embodiments of the present invention, the amplitude-modulated sequence of the reference symbol is chosen to be a pseudo random bit sequence (PRBS) since such a sequence has good autocorrelation properties with a distinct correlation maximum in a correlation signal which should be as wide as possible.
In accordance with preferred embodiments of the present invention, the coarse frequency synchronization can be performed using the amplitude-modulated sequence after a frame synchronization of a MCM signal has been accomplished. The inventive algorithm works both in the time and the frequency domains. Frequency offsets as high as ±10 times the subcarrier spacing can be corrected.
In the following, preferred embodiments of the present invention will be explained in detail on the basis of the drawings enclosed, in which:
Before discussing the present invention in detail, the mode of operation of a MCM transmission system is described referring to
Referring to
A data source 102 provides a serial bitstream 104 to the MCM transmitter. The incoming serial bitstream 104 is applied to a bit-carrier mapper 106 which produces a sequence of spectra 108 from the incoming serial bitstream 104. An inverse fast Fourier transform (IFFT) 110 is performed on the sequence of spectra 108 in order to produce a MCM time domain signal 112. The MCM time domain signal forms the useful MCM symbol of the MCM time signal. To avoid intersymbol interference (ISI) caused by multipath distortion, a unit 114 is provided for inserting a guard interval of fixed length between adjacent MCM symbols in time. In accordance with a preferred embodiment of the present invention, the last part of the useful MCM symbol is used as the guard interval by placing same in front of the useful symbol. The resulting MCM symbol is shown at 115 in
In order to obtain the final frame structure shown in
In accordance with the present invention, the reference symbol is an amplitude modulated bit sequence. Thus, an amplitude modulation of a bit sequence is performed such that the envelope of the amplitude modulated bit sequence defines a reference pattern of the reference symbol. This reference pattern defined by the envelope of the amplitude modulated bit sequence has to be detected when receiving the MCM signal at a MCM receiver. In a preferred embodiment of the present invention, a pseudo random bit sequence having good autocorrelation properties is used as the bit sequence for the amplitude modulation.
The choice of length and repetition rate of the reference symbol depends on the properties of the channel through which the MCM signal is transmitted, e.g. the coherence time of the channel. In addition, the repetition rate and the length of the reference symbol, in other words the number of useful symbols in each frame, depends on the receiver requirements concerning mean time for initial synchronization and mean time for resynchronization after synchronization loss due to a channel fade.
The resulting MCM signal having the structure shown at 118 in
Following, the mode of operation of a MCM receiver 130 is shortly described referring to
The down-converted MCM signal is provided to a symbol frame/carrier frequency synchronization unit 134.
A first object of the symbol frame/carrier frequency synchronization unit is to perform a frame synchronization on the basis of the amplitude-modulated reference symbol. This frame synchronization is performed on the basis of a correlation between the amplitude-demodulated reference symbol and a predetermined reference pattern stored in the MCM receiver.
A second object of the symbol frame/carrier frequency synchronization unit is to perform a coarse frequency synchronization of the MCM signal. To this end, the symbol frame/carrier frequency synchronization unit 134 serves as a coarse frequency synchronization unit for determining a coarse frequency offset of the carrier frequency caused, for example, by a difference of the frequencies between the local oscillator of the transmitter and the local oscillator of the receiver. The determined frequency is used in order to perform a coarse frequency correction. The mode of operation of the coarse frequency synchronization unit is described in detail referring to
As described above, the frame synchronization unit 134 determines the location of the reference symbol in the MCM signal. Based on the determination of the frame synchronization unit 134, a reference symbol extracting unit 136 extracts the framing information, i.e. the reference symbol, from the MCM signal coming from the receiver front end 132. After the extraction of the reference symbol, the MCM signal is applied to a guard interval removal unit 138. The result of the signal processing performed hereherto in the MCM receiver are the useful MCM symbols.
The useful MCM symbols output from the guard interval removal unit 138 are provided to a fast Fourier transform unit 140 in order to provide a sequence of spectra from the useful symbols. Thereafter, the sequence of spectra is provided to a carrier-bit mapper 142 in which the serial bitstream is recovered. This serial bitstream is provided to a data sink 144.
Following, the mode of operation of the coarse frequency synchronization unit will be described in detail referring to
As described above, the frame/timing synchronization unit uses the amplitude-modulated sequence in the received signal in order to extract the framing information from the MCM signal and further to remove the guard intervals therefrom. After the frame/timing synchronization unit 202 it follows a coarse frequency synchronization unit 204 which estimates a coarse frequency offset based on the amplitude-modulated sequence of the reference symbol of the MCM signal. In the coarse frequency synchronization unit 204, a frequency offset of the carrier frequency with respect to the oscillator frequency in the MCM receiver is determined in order to perform a frequency offset correction in a block 206. This frequency offset correction in block 206 is performed by a complex multiplication. The output of the frequency offset correction block 206 is applied to the MCM demodulator 208 formed by the Fast Fourier Transform unit 140 and the carrier-bit mapper 142 shown in
In order to perform the inventive coarse frequency synchronization, in either case, an amplitude-demodulation has to be performed on a preprocessed MCM signal. The preprocessing may be, for example, the down-conversion and the analog/digital conversion of the MCM signal. The result of the amplitude-demodulation of the preprocessed MCM signal is an envelope representing the amplitude of the MCM signal.
For the amplitude demodulation, a simple alphamax+betamin− method can be used. This method is described for example in Palacherla A.: DSP-μP Routine Computes Magnitude, EDN, Oct. 26, 1989; and Adams, W. T., and Bradley, J.: Magnitude Approximations for Microprocessor Implementation, IEEE Micro, Vol. 3, No. 5, October 1983.
It is clear that amplitude determining methods different from the described alphamax+ betamin− method can be used. For simplification, it is possible to reduce the amplitude calculation to a detection as to whether the current amplitude is above or below the average amplitude. The output signal then consists of a −1/+1 sequence which can be used to determine a coarse frequency offset by performing a correlation. This correlation can easily be performed using a simple integrated circuit (IC).
In addition, an oversampling of the signal received at the RF front end can be performed. For example, the received signal can be expressed with two times oversampling.
In accordance with a first embodiment of the present invention, a carrier frequency offset of the MCM signal from an oscillator frequency in the MCM receiver is determined by correlating the envelope obtained by performing the amplitude-demodulation as described above with a predetermined reference pattern.
In case there is no frequency offset, the received reference symbol r(k) will be:
wherein n(k) designates “additive Gaussian noise” and SAM denotes the AM sequence which has been sent. In order to simplify the calculation the additive Gaussian noise can be neglected. It follows:
In case a constant frequency offset ?f is present, the received signal will be:
Information regarding the frequency offset is derived from the correlation of the received signal r(k) with the AM sequence SAM which is known in the receiver:
Thus, the frequency offset is:
Since the argument of ‥SAM(k)|2 is zero the frequency offset is:
In accordance with a second embodiment of the coarse frequency synchronization algorithm in accordance with the present invention, a reference symbol comprising at least two identical sequences 300 as shown in
As shown in
Following, a mathematical derivation of the determination of a carrier frequency deviation is presented. In accordance with
If no frequency offset is present, the following equation 8 will be met by the received signal:
r(k) designates the values of the identical sequences. k is an index from one to L/2 for the respective samples.
If there is a frequency offset of, for example, ?f, the received signal is:
{tilde over (r)} (k) designates sample values of the received portion which are based on the identical sequences. Information regarding the frequency offset is derived from the correlation of the received signal {tilde over (r)} (k+L/2) with the received signal {tilde over (r)} (k). This correlation is given by the following equation:
{tilde over (r)}* designates the complex conjugate of the sample values of the portion mentioned above.
Thus, the frequency offset is
Since the argument of |r(k)|2 equals zero, the frequency off-set becomes
Thus, it is clear that in both embodiments, described above, the frequency position of the maximum of the resulting out-put of the correlation determines the estimated value of the offset carrier. Furthermore, as it is also shown in
An apparatus for performing the coarse frequency synchronization using a reference symbol having two identical sections of the length of L/2 each which has been described above is shown in
Also shown in
The output of the correlator 406 is connected to an operation unit 408 for performing an argument operation on the output signal of the correlator 406. The output of the operation unit 408 is connected to a multiplier 410 which multiplies the output by ½π(L/2)TMCM). A further operation unit 412 for performing a e−j(πΔfT
In case of a channel with strong reflections, for example due to a high building density, the correlations described above might be insufficient for obtaining a suitable coarse frequency synchronization. Therefore, in accordance with a third embodiment of the present invention, corresponding values of the two portions which are correlated in accordance with a second embodiment, can be weighting with corresponding values of stored predetermined reference patterns corresponding to said two identical sequences of the reference symbol. This weighting can maximize the probability of correctly determining the frequency offset. The mathematical description of this weighting is as follows:
SAM designates the amplitude-modulated sequence which is known in the receiver, and S*AM designates the complex conjugate thereof.
If the above correlations are calculated in the frequency domain, the amount of
is used rather than the argument. This amount is maximized as a function of a frequency correction. The position of the maximum determines the estimation of the frequency deviation. As mentioned above, the correction is performed in a feed forward structure.
A block diagram of an apparatus for performing the coarse frequency synchronization in accordance with the third embodiment of the present invention is shown in
A multiplier 428 is provided which multiplies the output of the correlator 406 by the output of the correlator 426. The output of the multiplier 428 is connected to an argument operation unit 408. The output of the multiplier is applied to an argument operation unit 408, a multiplier 410 and an operation unit 412 in sequence. The mode of operation of these units corresponds to that of the corresponding units which are shown in
A alternative structure of an apparatus for performing the coarse frequency synchronization in accordance with the third embodiment of the present invention in the frequency domain is shown in
In case of performing the coarse frequency synchronization in the frequency domain it is possible to make use of the existing FFT at the beginning of the detection for the coarse frequency synchronization rather than providing an additional fast Fourier transform unit.
Following the course frequency synchronization described above, a fine frequency synchronization can be performed in case such a fine frequency synchronization is useful.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP98/02170 | 4/14/1998 | WO | 00 | 11/29/2000 |
Publishing Document | Publishing Date | Country | Kind |
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WO99/53666 | 10/21/1999 | WO | A |
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8251135 | Sep 1996 | JP |
9800946 | Jan 1998 | WO |