Symbol synchronisation for multicarrier transmission

Abstract: When a symbol timing detection circuit (107) has detected a change in symbol generation timing, it immediately supplies an FFT window signal to an FFT circuit (103), thereby shifting the position of the FFT window. At this time, the output phase of the FFT circuit (103) varies. A window position change detection circuit (108) detects a change in window position. A phase correction circuit (105) controls the output phase of a one-symbol delay circuit (104) in accordance with the change, whereby the output phase of the circuit (104) is made to be identical to the output phase of the FFT circuit (103) supplied to a delay detection circuit (106). The phase of data obtained a one-symbol period earlier is corrected in accordance with the phase of the present FFT output. Thus, normal demodulation can be executed by delay detection.

Description: This invention relates to an OFDM (Orthogonal Frequency Division Multiplex) demodulation apparatus used for, for example, a terrestrial digital broadcast, and more particularly to a control system for controlling the demodulation timing of OFDM symbols. In the field of voice/image signal transmission systems, a large number of digital demodulation systems have recently been developed. In particular, in the terrestrial digital broadcast, attention is paid to an OFDM (Orthogonal Frequency Division Multiplex) demodulation system that shows a high resistance against multipass jamming. Prior art relating to the present invention will be described. An OFDM signal is obtained by assigning data to a plurality of orthogonal carriers to thereby convert a frequency-domain signal to a time-domain signal. One symbol period of the OFDM signal consists of a guard period and an effective symbol period. The guard period is provided for removing the influence of a delay wave due to multipass, and part of a signal in the effective symbol period is copied in the guard period. Accordingly, in a reception apparatus, it is necessary to detect the timing of symbols from a received OFDM signal and to perform demodulation for all effective symbol periods. FIG. 1 shows a conventional OFDM demodulation apparatus for demodulating an OFDM signal in which each carrier is DQPSK-modulated. As shown in FIG. 1, the input OFDM signal is converted into a digital signal by an A/D converter 11, and further into a complex base band signal by an IQ demodulation circuit 12. The output of the demodulation circuit 12 is supplied to a fast Fourier transform (FFT) circuit 13 and also to a symbol timing detection circuit 16. The symbol timing detection circuit 16 detects the timing of the generation of symbols, using the guard periods contained in the OFDM signal, and generates an FFT window signal used for determining the point in time at which the FFT demodulation is started. The FFT window signal is supplied to the FFT circuit 13. The FFT circuit 13 extracts each effective symbol period of the OFDM signal on the basis of the FFT window signal supplied from the symbol timing detection circuit 16, thereby performing FFT processing to convert the time-domain signal into a frequency-domain signal and obtain data carried by each carrier. The output of the FFT circuit 13 is supplied to a one-symbol storage circuit 14 and also to a delay wave detection circuit 15. The delay wave detection circuit 15 detects the delay wave of each carrier, using the outputs of the FFT circuit 13 and the one-symbol storage circuit 14, thereby obtaining demodulated data. As described above, the initial determination of the position of the FFT window is executed by the symbol timing detection circuit 16. After that, to hold the FFT window, a clock phase error is detected using the symbol generation timing detected by the symbol timing detection circuit 16, and the clock phase is controlled by a PLL circuit 17 and a VCXO (voltage control type crystal Oscillation) circuit 18. When the position of the FFT window shifts, the phase of the FFT-demodulated output varies. Accordingly, in, for example, the OFDM demodulation apparatus shown in FIG. 1, a phase difference occurs between the outputs of the FFT circuit 13 and the one-symbol storage circuit 14, and hence normal demodulation cannot be performed. In order to avoid an abrupt phase difference, in the FIG. 1 apparatus, the clock phase is controlled by the PLL to control the position of the FTT window. When the state of reception has abruptly changed while, for example, the apparatus is moved, it is necessary to change the position of the FFT window at a high speed. However, during the control of the clock phase using the PLL, the FFT window position cannot be changed quickly. As described above, in the OFDM demodulation apparatus, the clock phase is controlled by the PLL to suppress variations in FFT window position in order to prevent an abrupt phase change, and accordingly the FFT window position cannot be changed quickly when the state of reception has abruptly changed. An OFDM demodulation apparatus according to the precharacterizing part of claim 1 is known from WO-A-96 19056. The object of the invention is to provide an OFDM demodulation apparatus capable of performing normal demodulation even when the FFT window position has changed at a high speed. To attain the object, there is provided an OFDM demodulation apparatus which receives an OFDM (Orthogonal Frequency Division Multiplex) signal, detects the timing of the demodulation of OFDM symbols, creates a demodulation timing signal, demodulates the OFDM symbols on the basis of the demodulation timing signal, stores the demodulated OFDM output in storage means for the period of a predetermined number of symbols, detects a change in the demodulation timing signal in the period of the predetermined number of symbols, and corrects the output phase of the storage means on the basis of the detection result, thereby demodulating data, as well as the demodulated OFDM signal, by performing, for example, delay detection. Since in the above-described structure, the output phase of the storage means is corrected so as to be identical to that of the demodulated OFDM signal when the timing of the generation of the demodulation timing signal has changed, normal demodulation processing can be performed later. This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features. The invention can be more fully under stood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a block diagram illustrating an example of a structure of a conventional OFDM demodulation apparatus; FIG. 2 is a block diagram illustrating the structure of an OFDM demodulation apparatus according to a first embodiment of the invention; FIG. 3 is a block diagram showing the specific structure of a phase correction circuit incorporated in the first embodiment; FIG. 4 is a block diagram illustrating the structure of an OFDM demodulation apparatus according to a second embodiment of the invention; FIG. 5 is a view useful in explaining an example of a format employed in an OFDM signal; and FIG. 6 is a block diagram illustrating the structure of an OFDM demodulation apparatus according to a third embodiment of the invention. The embodiments of the invention will be described in detail with reference to FIGS. 2 to 6. (First Embodiment) FIG. 2 is a block diagram showing the structure of an OFDM demodulation apparatus, according to a first embodiment of the invention, for receiving an OFDM signal having each carrier DQPSK-modulated. As shown, the input OFDM signal is converted into a digital signal by an A/D converter 101 and then into a complex base band signal by an IQ demodulation circuit 102. The output of the IQ demodulation circuit 102 is supplied to an FFT circuit 103 and also to a symbol timing detection circuit 107. The symbol timing detection circuit 107 detects the timing of the generation of symbols, using guard periods contained in the OFDM signal, and generates an FFT window signal. The FFT window signal is supplied to the FFT circuit 103 and also to a window position change detection circuit 108. The window position change detection circuit 108 detects a change Δn in FFT window position between symbols. The FFT circuit 103 extracts each effective symbol period of the OFDM signal on the basis of the FFT window signal supplied from the symbol timing detection circuit 107, thereby performing FFT processing to convert the time-domain signal into a frequency-domain signal and obtain data carried by each carrier. The output of the FFT circuit 103 is supplied to a one-symbol storage circuit 104 and also to a phase correction circuit 105. The phase correction circuit 105 calculates a phase change in each carrier based on the change Δn in FFT window position, thereby correcting the output phase of the one-symbol storage circuit 104. FIG. 3 is a block diagram showing the specific structure of the phase correction circuit 105. In FIG. 3, a carrier number generation circuit 201 generates a carrier number k in synchronism with the FFT window, and informs a phase error generation circuit 202 of the carrier number k . The phase error generation circuit 202 calculates a phase error (k) in accordance with the following formula, using the carrier number k and the window position change Δn supplied from the window position change detection circuit 108. Δn < 0 is satisfied when the FTT window position has shifted forward as compared with the previous detection. (k) = -2π Δn/N N: Number of FFT points k: Carrier number, -N/2 ≦ k < N/2 The phase error (k) of each carrier is supplied to a complex multiplier 205 via a cos circuit 203 and a sin circuit 204. The complex multiplier 205 corrects the phase of each carrier by complex multiplication of I and Q signals output from the one-symbol delay circuit 104 and the phase error in the complex form output from the cos circuit 203 and the sin circuit 204. The phase correction circuit used in each embodiment of the invention is realized by the structure shown in FIG. 3. In FIG. 2, the outputs of the FFT circuit 103 and the phase correction circuit 105 are supplied to a delay detection circuit 106, where delay detection is performed and demodulated data is output. Further, the output of the delay detection circuit 106 is supplied to a clock AFC circuit 109, where clock frequency synchronization is performed by controlling the oscillation frequency of the delay detection circuit 106. In other words, when in the OFDM demodulation apparatus constructed as above, the symbol timing detection circuit 107 has detected a change in the timing of the generation of symbols, it immediately outputs the FFT window signal to the FFT circuit 103, thereby displacing the FFT window. Although at this time, a phase change occurs in the output of the FFT circuit 103, a displacement of the window position is detected by the window position change detection circuit 108, and the output phase of the one-symbol delay circuit 104 is controlled in accordance with the detected displacement so as to be identical to the output phase of the FFT circuit 103 supplied to the delay detection circuit 106. Since the window position change detection circuit 108 and the phase correction circuit 105 correct, when the FFT window position has shifted, the data phase corresponding to the last symbol in accordance with the phase of the present FFT output, correct demodulation can be executed using delay detection. Moreover, since a change in the FFT window position can be corrected, no PLL circuit but only the AFC circuit is needed for synchronization reproduction, unlike the conventional case. (Second Embodiment) FIG. 4 is a block diagram illustrating an OFDM demodulation apparatus according to a second embodiment. This apparatus receives an OFDM signal in which continuous pilot signals (CP) and separate pilot signals (SP), as well as data signals, are assigned to predetermined portions of a symbol as shown in FIG. 5. In FIG. 4, the input OFDM signal is converted into a digital signal by an A/D converter 301 and then into a complex base band signal by an IQ demodulation circuit 302. The output of the IQ demodulation circuit 302 is supplied to an FFT circuit 303 and also to a symbol timing detection circuit 307. The symbol timing detection circuit 307 detects the timing of the generation of symbols, using guard periods contained in the OFDM signal, thereby generating an FFT window signal. The FFT window signal is supplied to the FFT circuit 303 and also to a window position change detection circuit 308, where a change Δn in FFT window position between symbols. The FFT circuit 303 extracts each effective symbol period of the OFDM signal on the basis of the FFT window signal supplied from the symbol timing detection circuit 307, thereby performing FFT processing to convert the time-domain signal into a frequency-domain signal and obtain data carried by each carrier. The output of the FFT circuit 303 is supplied to a CP storage circuit 304, where it is temporarily stored and delayed by one symbol. The output of the CP storage circuit 304 is supplied to a phase correction circuit 305. The phase correction circuit 305 calculates a phase change in each carrier based on the change Δn in FFT window position which is supplied from the window position change detection circuit 308, thereby correcting the output phase of the CP storage circuit 304. The outputs of the FFT circuit 303 and the phase correction circuit 305 are supplied to an error detection circuit 306, where a phase difference between the symbols of the CP carrier is detected. The output of the error detection circuit 306 is supplied to a clock AFC circuit 309, where clock frequency synchronization is performed by controlling the oscillation frequency of a VCXO circuit 310. As described above, even when the FFT window position has shifted, the phase of CP data corresponding to the last symbol is corrected by the window position change detection circuit 308 and the phase correction circuit 305 in accordance with the phase of the present FFT output, with the result that normal error detection is possible and hence the accuracy of the AFC processing can be maintained. The output of the FFT circuit 303 is also supplied to an SP storage circuit 311. The SP storage circuit 311 holds an SP signal contained in the output of the FFT circuit 303 in synchronism with the generation of each SP signal as shown in FIG. 5. The output of the SP storage circuit 311 is supplied to a phase correction circuit 312. The phase correction circuit 312 calculates a phase change in each carrier based on the change Δn in FFT window position, thereby correcting the output phase of the SP storage circuit 311 except for presently received SP data. At this time, the corrected SP data is stored by the SP storage circuit 311 and used in the next SP data processing. The output of the phase correction circuit 312 is supplied to a frequency-domain interpolation circuit 313, where interpolation of the SP data is executed. The output of the frequency-domain interpolation circuit 313 is supplied to an equalization circuit 314, where the output of the FFT circuit 303 is equalized to obtain demodulated data. As described above, even when the FFT window position has shifted, the previous SP data is corrected by the window position change detection circuit 398 and the phase correction circuit 312 in accordance with the phase of the present FFT output, whereby normal equalization error detection can be executed. (Third Embodiment) FIG. 6 is a block diagram illustrating an OFDM demodulation apparatus according to a third embodiment of the invention. This apparatus receives an OFDM signal which contains continuous pilot signals (CP) and separate pilot signals (SP) as shown in FIG. 5. In FIG. 6, similar structural elements to those in FIG. 4 are denoted by corresponding reference numerals, and are not described. As shown in FIG. 6, the output of an FFT circuit 303 is supplied to an SP storage circuit 501, which stores two timed continuous SP signals in accordance with the timing of the generation of SP signals as shown in FIG. 5. The output of the SP storage circuit 501 is supplied to a phase correction circuit 502. The phase correction circuit 502 calculates a phase change in each carrier based on a change Δn in FFT window position supplied from a window position change detection circuit 308, thereby correcting the phase of SP data output from the SP storage circuit 501 except for presently received SP data. At this time, the corrected SP data is stored by the SP storage circuit 501 and used in the next SP data processing. The output of the phase correction circuit 502 is supplied to a time-domain interpolation circuit 503, where interpolation of the present SP data is executed using the two timed continuous SP data items. The output of the time-domain interpolation circuit 503 is supplied to a frequency-domain interpolation circuit 504, where frequency-domain interpolation of the SP data is performed. The output of the frequency-domain interpolation circuit 504 is supplied to an equalization circuit 508. The output of the FFT circuit 303 is also supplied to a three-symbol delay circuit 505, where it is delayed by three symbols. The output of the three-symbol delay circuit 505 is supplied to a phase correction circuit 506. The phase correction circuit 506 corrects the output phase of the three-symbol delay circuit 505 on the basis of a window position change which occurs in the period of three symbols and is calculated by a three-symbol adder circuit 507, to which the output of the window position change detection circuit 308 is supplied. The output of the phase correction circuit 506 is supplied to the equalization circuit 508, where it is equalized using an error signal output from the frequency-domain interpolation circuit 504, thereby obtaining demodulated data. Even when, in the above-described structure, the FFT window position has shifted, the previous SP data is corrected by the window position change detection circuit 308 and the phase correction circuit 502 in accordance with the phase of the present FFT output, while an OFDM symbol, which is obtained three symbols earlier, is corrected by the three-symbol adder circuit 507 and the phase correction circuit 506 in accordance with the phase of the present FFT output. As a result, normal equalization processing is possible. As described above, the invention can provide an OFDM demodulation apparatus which can perform normal signal processing by correcting data stored in a storage, even when the FFT window has shifted, and can make the FFT window follow at a high speed in accordance with a reception state, thereby performing normal demodulation processing.

Claims: An OFDM demodulation apparatus for demodulating an OFDM, Orthogonal Frequency Division Multiplex, signal obtained by assigning data to a plurality of orthogonal carriers to thereby convert a frequency-domain signal into a time-domain signal, the OFDM signal having a one-symbol period consisting of an effective symbol period and a guard period obtained by copying part of a signal contained in the effective symbol period, said apparatus comprising: synchronization reproducing means (101, 102, 109, 110, 301, 302, 309, 310) for receiving the OFDM signal and executing synchronization reproduction of the OFDM signal; demodulation timing signal generation means (107, 307) for receiving the OFDM signal obtained after the synchronization reproduction by the synchronization reproducing means, detecting timing of switching of OFDM symbols from one to another, and generating a signal indicating the timing, as a demodulation timing signal; OFDM demodulation means (103, 303) for extracting an effective symbol on the basis of the demodulation timing signal output from the demodulation timing signal generating means (107, 307) to demodulate it; storage means (104, 304, 311, 501, 505) for storing a demodulated output of the OFDM demodulation means (103, 303) for a predetermined symbol period; demodulation timing change detecting means (108, 308, 507) for detecting a change in the predetermined symbol period for the output of the demodulation timing signal generating means (107, 307); correction means (105, 305, 312, 502, 506) for correcting a phase of an output of the storage means (104, 304, 311, 501, 505) on the basis of an output of the demodulation timing change detecting means (108, 308, 507); and signal processing means (106, 306, 313, 314, 503, 504, 508) for performing signal processing for data demodulation or synchronization reproduction, using the output of the OFDM demodulation means (103, 303) and an output of the correction means (105, 305, 312, 502, 506), characterized in that when timing of generation of demodulation timing signals by the demodulation timing signal generating means (107, 307) has changed, the phase of the output of the storage means (104, 304, 311, 501, 505) is corrected in accordance with a change in the timing of generation of demodulation timing signals. An OFDM demodulation apparatus according to claim 1, characterized by means for subjecting each carrier contained in the OFDM signal to differential modulation, and wherein the storage means (104) are adapted to store an OFDM symbol output from the OFDM demodulation means (103) for a one-symbol period; the demodulation timing change detecting means (108) are adapted to detect a change in the timing of generation of demodulation timing signals; the correction means (105) are adapted to correct a phase of a last OFDM symbol output from the storage means (104); and the signal processing means (106) are adapted to perform delay detection using the output of the OFDM demodulation means (103) and an output of the correction means (105). An OFDM demodulation apparatus according to claim 1, characterized in that the synchronization reproducing means (101, 109, 110) are adapted to perform synchronization reproduction of the OFDM signal on the basis of the output of the OFDM demodulation means (103). An OFDM demodulation apparatus according to claim 1, characterized in that the OFDM signal contains a pilot carrier that is not modulated or is subjected to phase modulation; the storage means (304) are adapted to store pilot data contained in the output of the OFDM demodulation means (303) for a one-symbol period; the demodulation timing change detecting means (308) are adapted to detect a change in the timing of generation of demodulation timing signals in the one-symbol period; the correction means (305) are adapted to correct a phase of last pilot data output from the storage means (304); the signal processing means (306) are adapted to detect a phase difference between symbols of the pilot carrier, using pilot data contained in the output of the OFDM demodulation means (103) and an output of the correction means (305); and the synchronization reproducing means (301, 309, 310) are adapted to perform synchronization reproduction on the basis of the phase difference obtained by the signal processing means (306). An OFDM demodulation apparatus according to claim 1, characterized in that the OFDM signal contains non-modulated symbols for data demodulation transmitted at symbol intervals of n; the storage means (311) are adapted to store non-modulated symbols contained in the output of the OFDM demodulation means (303) for the one-symbol period; the demodulation timing change detecting means (308) are adapted to detect a change in the timing of generation of demodulation timing signals in the one-symbol period; the correction means (312) are adapted to correct a phase of a last non-modulated symbol output from the storage means (311), and to update a delay amount of the storage means (311) in accordance with the corrected phase; and the signal processing means (313, 314) are adapted to equalizes the output of the OFDM demodulation means (303) using an output of the correction means (312). An OFDM demodulation apparatus according to claim 1, characterized in that the OFDM signal contains non-modulated symbols for data demodulation transmitted at symbol intervals of n; the storage means (501, 505) include first and second storage sections (501, 505); the demodulation timing change detecting means (308, 507) include first and second demodulation timing change detecting sections (308, 507); the correction means (502, 506) include first and second correcting sections (502, 506); the first storage section (501) is adapted to store non-modulated symbols contained in an output of the OFDM demodulation means (303) in a two-symbol period; the first demodulation timing change detecting section (308) is adapted to detect a demodulation timing change in the one-symbol period; the first correcting section (502) is adapted to correct a phase of each non-modulated symbol contained in the two-symbol period and output from the first storage section (501), except for a presently received non-modulated symbol, thereby replacing contents of the first storage section (501) with the corrected data; the second storage section (505) is adapted to store OFDM symbols contained in the output of the OFDM demodulation means (303) in an (n-1) symbol period; the second demodulation timing change detecting section (507) is adapted to detect a demodulation timing change in the (n-1) symbol period; the second correcting section (506) is adapted to correct a phase of an OFDM symbol that is obtained the (n-1) symbol period earlier and output from the second storage section (505); and the signal processing means (503, 504, 508) are adapted to estimate an error signal for equalization, using the non-modulated symbols transmitted at the symbol intervals of n and supplied from the first correcting section (502), thereby equalizing an output of the second correction section (506).

Inventors: SEKI TAKASHI, TAGA NOBORU, SEKI, TAKASHI, TAGA, NOBORU, Seki, Takashi, c/o K. K. Toshiba, Taga, Noboru, c/o K. K. Toshiba
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1999
Main classification: H04L27
IPC codes: H03L7, H04L7, H04J11, H04L27