Patent EA-002611-B1 (EA)
Title: Frame structure and frequency synchronization in multicarrier systems
Inventor: EBERLEIN ERNST
IPC: H04J1/00
Filed: 1998-04-14
Abstract: 1. A method for generating a signal having a frame structure, each frame of said frame structure comprising at least one useful symbol (12), a guard interval (14) associated to said at least one useful symbol (12) and a reference symbol (16), said method comprising the step of: performing an amplitude modulation of a bit sequence such that the envelope of the amplitude modulated bit sequence defines a reference pattern of said reference symbol (16); and introducing said an amplitude modulation of a bit sequence in a signal as a reference symbol. 2. The method according to claim 1, wherein said signal is an orthogonal frequency division multiplexed signal. 3. The method according to claim 1 or 2, wherein said amplitude modulation is performed such that a mean amplitude of said reference symbol (16) substantially corresponds to a mean amplitude of the remaining signal. 4. A method for generating a multi-carrier modulated signal having a frame structure, each frame of said frame structure comprising at least one useful symbol (12), a guard interval (14) associated to said at least one useful symbol (12) and a reference symbol (16), said method comprising the steps of: providing a bitstream (104); mapping bits of said bitstream (104) to carriers in order to provide a sequence of spectra (108); performing an inverse Fourier transform (110) in order to provide multi-carrier modulated symbols (112); associating (114) a guard interval to each multi-carrier modulated symbol; generating said reference symbol (16) by performing an amplitude modulation of a bit sequence such that the envelope of the amplitude modulated bit sequence defines a reference pattern of said reference symbol; and associating (116) said reference symbol (16) to a predetermined number of multi-carrier modulated symbols and associated guard intervals in order to define said frame. 5. The method according to claim 4, wherein said multi-carrier modulated signal is an orthogonal frequency division multiplex signal. 6. The method according to claim 4, wherein said amplitude modulation is performed such that a mean amplitude of said reference symbol (16) substantially corresponds to a mean amplitude of the remaining multi-carrier modulated signal. 7. The method according to claim 1, wherein said bit sequence is a pseudo random bit sequence having good autocorrelation characteristics. 8. The method according to claim 1, wherein a number of useful symbols (12) in each frame is defined depending on channel properties of a channel (122) through which the signal or the multi-carrier modulated signal is transmitted. 9. A method for frame synchronization of a signal having a frame structure, each frame of said frame structure comprising at least one useful symbol (12), a guard interval (14) associated with said at least one useful symbol (12) and a reference symbol (16), said method comprising the steps of: receiving said signal; down-converting said received signal;  performing (164) an amplitude-demodulation of said down-converted signal in order to generate an envelope; correlating (166) said envelope with a predetermined reference pattern in order to detect a signal reference pattern of said reference symbol (16) in said signal; and performing said frame synchronization based on the detection of said signal reference pattern. 10. The method according to claim 9, further comprising the step of performing a fast automatic gain control (162) of said received down-converted signal prior to the step of performing said amplitude-demodulation (164). 11. The method according to claim 9, wherein the step of performing (164) said amplitude-demodulation comprises the step of calculating an amplitude of said signal using the alphamax+betamin-method. 12. The method according to claim 9, further comprising the steps of sampling respective amplitudes of said received down-converted signal and comparing said sampled amplitudes with a predetermined threshold in order to generate a bit sequence in order to perform said amplitude demodulation. 13. The method according to claim 12, wherein the step of sampling respective amplitudes of said received down-converted signal further comprises the step of performing an over-sampling of said received down-converted signal. 14. The method according to claim 9, further comprising the step of applying a result of the frame synchronization for a frame in said signal to at least one subsequent frame in said signal. 15. A method for frame synchronization of a multi-carrier modulated signal having frame structure, each frame of said frame structure comprising at least one useful symbol (12), a guard interval (14) associated to said at least one useful symbol (12) and a reference symbol (16), said method comprising the steps of: receiving said multi-carrier modulated signal; down-converting said received multi-carrier modulated signal; performing (164) an amplitude-demodulation of said down-converted multi-carrier modulated signal in order to generate an envelope; correlating (166) said envelope with a predetermined reference pattern in order to detect a signal reference pattern of said reference symbol in said multi-carrier modulated signal; performing said frame synchronization based on the detection of said signal reference pattern; extracting (136/138) said reference symbol (16) and said at least one guard interval (14) from said down-converted received multi-carrier modulated signal based on said frame synchronization; performing (140) a Fourier transform in order to provide a sequence of spectra from said at least one useful symbol; de-mapping (142) said sequence of spectra in order to provide a bitstream. 16. The method according to claim 15, further comprising the step of performing (162) a fast automatic gain control of said received down-converted multi-carrier modulated signal prior to the step of performing said amplitude-demodulation. 17. The method according to claim 15, wherein the step of performing (164) said amplitude-demodulation comprises the step of calculating an amplitude of said multi-carrier modulated signal using the alphamax+ betamin-method. 18. The method according to claim 15, further comprising the steps of sampling respective amplitudes of said received down-converted multi-carrier modulated signal and comparing said sampled amplitudes with a predetermined threshold in order to generate a bit sequence in order to perform said amplitude demodulation. 19. The method according to claim 18, wherein the step of sampling respective amplitudes of said received down-converted multi-carrier modulated signal further comprises the step of performing an over-sampling of said received down-converted multi-carrier modulated signal. 20. The method according to one of claims 15, further comprising the step of applying a result of the frame synchronization for a frame in said signal to at least one subsequent frame in said multi-carrier modulated signal. 21. The method according to one of claims 9, further comprising the step of detecting a location of said signal reference pattern based on an occurrence of a maximum of a correlation signal when correlating said envelope with said predetermined reference pattern. 22. The method according to claim 21, further comprising the steps of: weighting a plurality of maxima of said correlation signal such that a maximum occurring first is weighted stronger than any subsequently occurring maximum; and detecting said location of said signal reference pattern based on the greatest one of said weighted maxima. 23. The method according to claim 22, further comprising the step of: disabling the step of performing said frame synchronization for a predetermined period of time after having switched-on a receiver (130) performing said method for frame synchronization. 24. An apparatus for generating a signal having a frame structure, each frame of said frame structure comprising at least one useful symbol (12), a guard interval (14) associated to said at least one useful symbol (12) and a reference symbol (14), said apparatus comprising: an amplitude modulator for performing an amplitude modulation of a bit sequence such that the envelope of the amplitude modulated bit sequence defines a reference pattern of said reference symbol (16) and  means for introducing an amplitude modulation of a bit sequence in a signal as a reference symbol. 25. The apparatus according to claim 24, wherein said signal is an orthogonal frequency division multiplexed signal. 26. The apparatus according to claim 24, wherein a mean amplitude of said reference symbol (16) substantially corresponds to a mean amplitude of the remaining signal. 27. An apparatus for generating a multi-carrier modulated signal having a frame structure, each frame of said frame structure comprising at least one useful symbol (12), a guard interval (14) associated to said at least one useful symbol (12) and a reference symbol (16), said apparatus comprising: means (102) for providing a bitstream (104); means (106) for mapping bits of said bitstream (104) to carriers in order to provide a sequence of spectra (108); means (110) for performing an inverse Fourier transform in order to provide multi-carrier modulated symbols (112); means (114) for associating a guard interval to each multi-carrier modulated symbol; means for generating said reference symbol (16) comprising an amplitude modulator for performing an amplitude modulation of a bit sequence such that the envelope of the amplitude modulated bit sequence defines a reference pattern of said reference symbol (16); means (116) for associating said reference symbol (16) to a predetermined number of multi-carrier modulated symbols (12) and associated guard intervals (14) in order to define said frame and means for introducing an amplitude modulation of a bit sequence in a signal as a reference symbol. 28. The apparatus according to claim 27, wherein said multi carrier modulated signal is an orthog