Transmission system for OFDM-signals with optimized synchronization

Abstract

The present invention relates to a transmission apparatus for transmitting OFDM-signals comprising modulation means 4 for modulating said signals onto a plurality of subcarriers using a OFDM-modulation method, transformation means 5 for transforming said modulated signals into the time domain, and transmission means for transmitting said signals, whereby in said modulation means every M-th subcarrier is modulated, wherein M is an integer and M≧2. The present invention also relates to a corresponding transmission method for transmitting OFDM-signals. The present invention further relates to a receiving apparatus for receiving OFDM-signals comprising M identical or respectively mirrored wave forms within one OFDM-timeburst, wherein M is an integer and M≧2, comprising receiving means for receiving said OFDM-signals, correlation means 22 for correlating said wave forms to obtain time synchronization, transformation means 23 for transforming said signals into the frequency domain and demodulation means 24 for demodulating said signals. The present invention also relates to a corresponding receiving method for receiving OFDM-signals. The present invention provides a much better time and frequency synchronisation performance based on correlation techniques than conventional OFDM-systems.

What is claimed is: 
     
       1. Transmission method for transmitting OFDM-signals, comprising the steps of
 modulating said signals onto a plurality of subcarriers using a OFDM-modulation method, 
 transforming said modulated signals into the time domain, and 
 transmitting said signals characterized in that in said modulating step every M-th subcarrier is modulated with a signal, wherein M is an integer and M≧2. 
 
     
     
       2. Transmission method according to  claim 1 , characterized in, that the not modulated subcarriers are set to zero. 
     
     
       3. Transmission method according to  claim 1 , characterized in, that M=2 and only subcarriers with even indices are modulated. 
     
     
       4. Transmission method according to  claim 1 , characterized in, that said modulation step comprises the steps of
 generating integer values form 0 to L−1, wherein L is the number of available subcarriers, and 
 modulating every M-th signal onto said subcarriers on the basis of said integer values. 
 
     
     
       5. Transmission method according to  claim 1 , wherein:
 said modulating step includes providing a switch control signal to a switch having a first input and a second input, wherein the first input receives a signal to be modulated onto a subcarrier and the second input receives a zero value signal. 
 
     
     
       6. Transmission apparatus for transmitting OFDM-signals, comprising:
 modulation means ( 4 ) for modulating said signals onto a plurality of subcarriers using a OFDM-modulation method, 
 transformation means ( 5 ) for transforming said modulated signals into the time domain, and 
 transmission means for transmitting said signals characterized in that in said modulation means every M-th subcarrier is modulated, wherein M is an integer and M≧2. 
 
     
     
       7. Transmission apparatus according to  claim 6 , characterized in, that in said modulation means ( 4 ) the not modulated subcarriers are set to zero. 
     
     
       8. Transmission apparatus according to  claim 6 , characterized in, that in said modulation means ( 4 ) M=2 and only subcarriers with even indices are modulated. 
     
     
       9. Transmission apparatus according to  claim 6 , characterized in that said modulation means ( 4 ) comprises means ( 10 ) for generating integer values from 0 to L−1, wherein L is the number of available subcarriers, whereby said modulation means ( 4 ) modulates every M-th signal onto said subcarriers on the basis of said integer values. 
     
     
       10. Transmission-apparatus according to  claim 6 , wherein:
 said modulation means includes a switch having a first input and a second input, wherein the first input receives a signal to be modulated onto a subcarrier and the second input receives a zero value signal. 
 
     
     
       11. Receiving method for receiving OFDM-signals comprising M identical or respectively mirrored wave forms within one OFDM-timeburst, wherein M is an integer and M≧2, comprising the steps of
 receiving said OFDM-signals, 
 correlating said waveforms to obtain time synchronization using M−1 correlators, 
 transforming said signals into the frequency domain, and 
 demodulating said signals. 
 
     
     
       12. Receiving method according to  claim 11 , characterized in, that in said correlation step said wave form parts are correlated on the basis of a delay value L 1 =S/M samples and averaged over L 2 ≦S/M samples, whereby S is the total number of samples in one OFDM-timeburst. 
     
     
       13. Receiving method according to  claim 11 , characterized in, that after said correlation step a peak detection step is carried out to provide time synchronization for said transformation of said signals into the frequency domain. 
     
     
       14. Receiving method according to  claim 11 , characterized in, that after said correlation step a frequency offset detection step is carried out to provide frequency synchronization for said transformation of said signals into the frequency domain. 
     
     
       15. Receiving apparatus for receiving OFDM-signals comprising M identical or respectively mirrored wave forms within one OFDM-timeburst, wherein M is an integer and M≧2, comprising:
 receiving means for receiving said OFDM-signals, 
 correlating means ( 28 ,  29 ,  30 ,  31 ) correlating said waveforms to obtain time synchronization, wherein said correlation means includes M−1 correlators, 
 synchronization, transformation means for transforming said signals into the frequency domain, and 
 demodulating said signals. 
 
     
     
       16. Receiving apparatus according to  claim 15 , characterized in, that in said correlation means ( 28 ,  29 ,  30 ,  31 ) said identical wave forms are correlated on the basis of a delay value L 1 =S/M and averaged over L 2 ≦S/M samples, whereby S is the total number of samples in one OFDM-timeburst. 
     
     
       17. Receiving apparatus according to  claim 15 , characterized in, that after said correlation means ( 28 ,  29 ,  30 ,  31 ) a peak detection means ( 46 ) is provided for providing time synchronization for said transformation of said signals into the frequency domain. 
     
     
       18. Receiving apparatus according to  claim 15 , characterized in, that after said correlation means ( 28 ,  29 ,  30 ,  31 ) a frequency offset detection means ( 47 ) is provided for providing frequency synchronization for said transformation of said signals into the frequency domain. 
     
     
       19. Transmission system for transmitting OFDM-signals, comprising:
 a transmission apparatus including modulation means for modulating said signals onto a plurality of subcarriers by OFDM-modulation, transformation means for transforming said modulated signals into the time domain, and transmission means for transmitting said signals characterized in that in said modulation means every M-th subcarrier is modulated, wherein M is an integer greater than or equal to 2; and 
 a receiving apparatus for receiving said OFDM-signals having M identical or respectively mirrored waveforms within one OFDM-timeburst, including receiving means for receiving said OFDM-signals, correlation means for correlating said waveforms to obtain time synchronization, transformation means for transforming said signals into the frequency domain, and demodulation means for demodulating said transformed signals. 
 
     
     
       20. A method of transmitting predetermined values on a subset of OFDM subcarriers, comprising steps of:
 associating a first subset of a sequence of N subcarriers with the predetermined values, and associating a second subset of the sequence of N subcarriers with a 0 value;   respectively modulating said first subset of N subcarriers with the predetermined values, and not modulating the second subset of the sequence of N subcarriers so as to form a modulated subcarrier sequence with modulated separated by unmodulated subcarriers;   transforming said modulated subcarrier sequence into a time domain to create a time domain waveform, and   transmitting said time domain waveform, wherein   a repeating pattern of said modulated subcarrier sequence includes a subcarrier from the first subset followed by 3 unmodulated subcarriers from the second subset of N subcarriers, and   said repeating pattern continues for at least 5 cycles, including at least 5 subcarriers from said first subset and 15 subcarriers from said second subset.   
     
     
       21. The method of claim 20, wherein:
 said repeating pattern continues for at least 6 cycles, including at least 6 subcarriers from said first subset and 18 subcarriers from said second subset.   
     
     
       22. The method of claim 20, wherein:
 said predetermined values are non-zero, and have a common magnitude.   
     
     
       23. The method of claim 20, wherein:
 said time domain waveform includes 4 substantially identical or mirrored waveforms within one OFDM timeburst.   
     
     
       24. The method of claim 20, wherein said information includes synchronization information. 
     
     
       25. A method of transmitting predetermined values on a subset of OFDM subcarriers, comprising steps of:
 associating a first subset of a sequence of N subcarriers with the predetermined values, and associating a second subset of the sequence of N subcarriers with a 0 value;   modulating said first subset of N subcarriers with the predetermined values, and not modulating the second subset of the sequence of N subcarriers so as to create a modulated subcarrier sequence;   time domain transforming said modulated subcarrier sequence to create a time domain waveform, and   transmitting said time domain waveform, wherein   respective of said N subcarriers are separated by 3 unmodulated subcarriers from the second subset of N subcarriers in a repeating pattern, the repeating pattern including one of the N subcarriers followed sequentially by 3 of the second subset of subcarriers, and   said repeating pattern continues for at least 5 cycles such that the modulated subcarrier sequence includes at least 5 subcarriers from said first subset and 15 subcarriers from said second subset.   
     
     
       26. The method of claim 25, wherein:
 said repeating pattern continues for at least 6 cycles, including at least 6 subcarriers from said first subset and 18 subcarriers from said second subset.   
     
     
       27. The method of claim 25, wherein:
 said predetermined values are non-zero, and have a common magnitude.   
     
     
       28. The method of claim 25, wherein:
 said time domain waveform includes 4 substantially identical or mirrored waveforms within one OFDM timeburst.   
     
     
       29. The method of claim 25, wherein said information includes synchronization information. 
     
     
       30. A transmission method for transmitting information from an OFDM transmitter, comprising the steps of:
 modulating said information onto a plurality of OFDM subcarriers;   transforming said modulated information into the time domain; and   transmitting said information, wherein   said modulating step includes modulating a first subcarrier with a first value of said information, but not modulating M−1 successive subcarriers, followed by modulating a second subcarrier with a second value of said information, but not modulating M−1 successive subcarriers,   M being an integer that is 2 or larger.   
     
     
       31. Transmission method for transmitting OFDM synchronization signals, comprising the steps of
 modulating synchronization values onto a subcarrier sequence using a OFDM-modulation method,   transforming said modulated subcarrier sequence into the time domain in order to obtain a time domain OFDM synchronization signal, and   transmitting said time domain OFDM synchronization signal,   wherein in said modulating step every M-th subcarrier is modulated with said synchronization values, M being an integer and M≧2.   
     
     
       32. Transmission method for transmitting OFDM synchronization signals, comprising the steps of
 modulating synchronization values onto subcarriers a subcarrier sequence using a OFDM-modulation method, said subcarrier sequence comprising at least a first and a second subset of subcarriers, wherein every M-th subcarrier of said first subset is modulated with said synchronization values, M being an integer and M≧2, and subcarriers in between modulated subcarriers of said first subset are unmodulated,   transforming said subcarrier sequence into the time domain in order to obtain a time domain synchronization signal, and   transmitting said time domain synchronization signal.   
     
     
       33. Transmission method according to claim 32, wherein at least 5 subcarriers of said first subset are modulated with synchronization information, and 3 consecutive subcarriers in between said modulated subcarriers remain unmodulated, and wherein said second subset comprises at least 3 consecutive subcarriers. 
     
     
       34. Transmission method for transmitting OFDM-signals and synchronization signals, comprising the steps of:
 modulating said OFDM signals onto a plurality of available subcarriers using an OFDM-modulation method,   transforming said modulated signals into the time domain, and   transmitting said transformed signals;   wherein in said modulating step said synchronization signals are modulated on every 4th subcarrier of said plurality of available subcarriers and the other subcarriers of said plurality of available subcarriers are not modulated.   
     
     
       35. Transmission method for transmitting OFDM synchronization signals by using an OFDM transmission scheme, comprising the steps of:
 modulating said OFDM synchronization signals onto every 4th subcarrier of a plurality of available subcarriers being assigned to the OFDM transmission scheme in the frequency domain;   transforming a frequency domain sequence of said modulated synchronization signals into the time domain to generate a time domain sequence, and   transmitting said time domain sequence.   
     
     
       36. Transmission method for transmitting OFDM data and synchronization signals, comprising the steps of:
 modulating said OFDM data onto a plurality of subcarriers using an OFDM-modulation method, wherein said synchronization signals are modulated on every M-th subcarrier of said plurality of subcarriers in the frequency domain so as to generate a frequency domain sequence comprising modulated and un-modulated subcarriers, wherein M is an integer and M≧2;   generating a frequency domain representation comprising said frequency domain sequence and zero amplitude sequences being arranged at the front and end of said frequency domain sequence;   transforming said frequency domain representation into a time domain representation by using Inverse Discrete Fourier Transformation, wherein the number of samples of said time domain representation is equal to the sum of the number of symbols of said frequency domain sequence and the number of symbols of said zero amplitude sequences; and   transmitting said time domain representation.   
     
     
       37. Transmission method for transmitting OFDM synchronization signals by using OFDM transmission scheme, comprising the steps of:
 generating a frequency domain sequence by modulating said OFDM synchronization signals on every M-th subcarrier of a plurality of subcarriers used in said OFDM transmission scheme, wherein M is an integer and M≧2, and wherein said frequency domain sequence comprises modulated subcarriers having nonzero values and un-modulated subcarriers having a zero value;   providing a frequency domain representation comprising said frequency domain sequence and second frequency domain sequences having zero amplitude symbols, wherein said second frequency domain sequences are arranged at the front and end of said frequency domain sequence;   transforming said frequency domain representation into a time domain representation by using Inverse Discrete Fourier Transformation, wherein the number of samples of said time domain representation is equal to the total number of symbols in said frequency domain sequence and second frequency domain sequences; and   transmitting said time domain representation.   
     
     
       38. Transmission method for transmitting OFDM data and synchronization signals, comprising the steps of:
 modulating said OFDM data onto a plurality of subcarriers using an OFDM-modulation method, wherein said synchronization signals are modulated on every M-th subcarrier of said plurality of subcarriers in the frequency domain so as to generate a frequency domain sequence wherein M is an integer and M≧2;   transforming said frequency domain sequence into a time domain sequence by using Inverse Discrete Fourier Transformation,   cyclically extending said time domain sequence in time domain so that the length of the extended time domain sequence is longer than the one OFDM timeburst period; and   transmitting said extended time domain sequence.   
     
     
       39. Transmission method for transmitting OFDM synchronization signals by using an OFDM transmission scheme, comprising the steps of:
 generating a frequency domain sequence by modulating said OFDM synchronization signals on every M-th subcarrier of a plurality of subcarriers used in said OFDM transmission scheme, wherein M is an integer and M≧2;   transforming said frequency domain sequence into a time domain sequence by using Inverse Discrete Fourier Transformation,   cyclically extending said time domain sequence in time domain so that the number of samples of the extended time domain sequence is larger than the number of samples of said time domain sequence; and   transmitting said extended time domain sequence.   
     
     
       40. Transmission method for transmitting OFDM data and synchronization signals, comprising the steps of:
 modulating said OFDM data onto a plurality of subcarriers using an OFDM-modulation method, wherein said synchronization signals are modulated on every M-th subcarrier of said plurality of subcarriers in the frequency domain so as to generate a frequency domain sequence, wherein M is an integer and M≧2;   transforming said frequency domain sequence into a time domain sequence by using a scheme of Inverse Discrete Fourier Transformation so that said time domain sequence comprises IN-phase M-th identical waveforms and QUAD-phase M-th identical waveforms, wherein each of said QUAD-phase waveforms is different from the corresponding each of said IN-phase waveforms in the time domain; and   transmitting said time domain sequence.   
     
     
       41. Transmission method for transmitting OFDM synchronization signals by using OFDM transmission scheme, comprising the steps of:
 generating a frequency domain sequence by modulating said OFDM synchronization signals on every M-th subcarrier of a plurality of subcarriers used in said OFDM transmission scheme, wherein M is an integer and M≧2;   transforming said frequency domain sequence into a time domain sequence by using a scheme of Inverse Discrete Fourier Transformation so that said time domain sequence comprises IN-phase M-th identical waveforms and QUAD-phase M-th identical waveforms, wherein each of said QUAD-phase waveforms is different from the corresponding each of said IN-phase waveforms in the time domain; and   transmitting said time domain sequence.   
     
     
       42. Transmission method for transmitting OFDM data and synchronization signals, comprising the steps of:
 modulating said OFDM data onto a plurality of subcarriers using an OFDM-modulation method, wherein said synchronization signals are modulated on every M-th subcarrier of said plurality of subcarriers in the frequency domain so as to generate a frequency domain sequence, wherein M is an integer and M≧2;   transforming said frequency domain sequence into a time domain sequence by using a scheme of Inverse Discrete Fourier Transformation so that said time domain sequence comprises M-th identical waveforms, wherein each of said M-th identical waveforms is correlated in the receiver device to detect the synchronization timing information; and   transmitting said time domain sequence.   
     
     
       43. A transmission method for transmitting OFDM signals from an OFDM transmitter, the method comprising:
 modulating said OFDM signals onto a first plurality of subcarriers using an OFDM modulation method, wherein every 4 th  subcarrier of a second plurality of subcarriers corresponds to the first plurality of subcarriers;   transforming said first plurality of modulated subcarriers into the time domain, and   transmitting said time domain transformed modulated subcarriers.   
     
     
       44. The transmission method according to claim 43, wherein subcarriers of said second plurality of subcarriers that do not correspond to said first plurality of subcarriers are not modulated. 
     
     
       45. A transmission apparatus configured to transmit OFDM signals, the transmission apparatus comprising:
 a modulation unit configured to modulate said OFDM signals onto a first plurality of subcarriers using an OFDM modulation method, wherein every 4 th  subcarrier of a second plurality of subcarriers corresponds to the first plurality of subcarriers;   an inverse discrete Fourier transform unit configured to transform said first plurality of modulated subcarriers into the time domain; and   a transmitter configured to transmit said time domain transformed modulated subcarriers.   
     
     
       46. The transmission apparatus according to claim 45, wherein subcarriers of said second plurality of subcarriers that do not correspond to said first plurality of subcarriers are not modulated.