|ll|l|||||||||||||||fl|||||||||||||||l||l|||||||||||||||l||||||||||||||||ll
`
`USOlJ6993t1841l 1
`
`(12) United States Patent
`Eberlcin et al.
`
`(10) Patent N0.:
`
`(45) Date of Patent:
`
`US 6,993,084 B1
`Jan. 31, 2006
`
`(54) COARSE FREQUENCY SYNCHRONISATION
`IN MUL’I‘ICARRIER SYSTEMS
`
`5,550,812 A "
`
`8.31996 Philips
`
`370.1203
`
`(75)
`
`Inventors: Ernst Eberleltt, (itossenseebach (DE);
`Snbah Badrl, Erlangen (DE); Stefan
`[.lpp, Erlangcn (DE); Stephan
`Buchholz. Munich (DE); Albert
`Heuberger, Eriangen (DE); Heinz
`Gerhocuscr, Waischenfeld (DE)
`
`(73) Assignee: Fraunhofer-Gesellschnft zur
`Foenderung der nngewandten
`Forschung e.V., Munich (DE)
`
`( " ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(1)) by 0 days.
`
`(21) Appl. No;
`
`nears-:33“
`
`(22)
`
`PCT Filed:
`
`Apr. 14, 1998
`
`(86)
`
`PCT No.:
`
`PCT} BP98I02169
`
`§ 371 (6X1).
`(2), (4) Date:
`
`Nov. 28, 2000
`
`(87)
`
`PCT Pub. No: W099r'53665
`
`PCT Pub. Date: Oct. 21, 1999
`
`(51)
`
`(2006.01)
`
`Int. Cl.
`H041. 27(28
`375,!260
`(52) U.S. Cl.
`375r'l32,
`(58) Field of Classification Search
`3750.30, 135. 26L 326. 379. 395. 376, 324,
`375266, 268. 140. 260, 262. 271; 370.006.
`370517, 2113. 204, 207, 208, 216, 343, 480,
`370:2“), 347, 510, 509, 470, 27‘), 209; 35:“138
`See application file for complete search history.
`
`(56)
`
`References Cited
`U.S. PKI'ENT DOCUMENTS
`
`(Continued)
`FOREIGN PA'I‘EN'I' oocumm‘s
`0631406
`12r1994
`
`EP
`
`(Continued)
`O‘I‘l IER PU BLIC‘A'l’lONS
`
`Moose. “A Technique for Orthogonal Frequency Division
`Multiplexing Frequency Ofllset Correction". iEEE Transac-
`tions on Cormmmicdfimts‘. vol. 42. No. 10. pp. 2908-29”
`(OcL 1994).
`
`(Continued)
`
`
`Primary Examiner Emmanuel Bayard
`(74)Arrorrt({i-', Agent, or Firm—Roylance, Abrams, Berdo &
`Goodman, I..I..F'.
`
`(5?)
`
`ABSTRACT
`
`A method for generating a signal having a frame slructure,
`each frame of the frame structure comprising at least one
`useful symbol. a guard interval aseociated to the at least one
`useful symbol and a reference symbol. comprises the steps
`of performing an amplitude modulation of a bit sequence
`such that
`the envelope ol.’
`the amplitude modulated hit
`sequence defines a reference pattern of the reference symbol
`and inserting the amplitude modulated hit sequence into said
`signal as said reference symbol. A method for frame syn-
`chronization of a signal having such a
`frame structure
`comprises the steps of receiving the signal. down-converting
`the received signal. performing an amplitudedemodulalion
`of the down~eonvertetl signal in order to generate an enve-
`lope. correlating the envelope with a pretletcnninetl refer~
`ence pattern in order to detect a signal reference pattern of
`the reference symbol in the signal, and performing the frame
`synchronization based on the detection of the signal refer-
`ence pattern.
`
`5.191525 A
`
`3.31993 Pommicr ct al.
`
`3Tflr'18
`
`46 Claims, 3 Drawing Sheets
`
`
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 1
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 1
`
`

`

`US 6,993,084 Bl
`Page 2
`
`US. PATEN'I‘ DOCU MEN'I‘S
`
`2:499? Sckiclal.
`5.602.835 A "
`5.31997 Sandbet‘g at a].
`5,631,610 A °
`
`5.646335 A " W199?
`Ishikawa el al.
`5,657,313 A ’
`811997 Takahashi e1 al.
`
`5.694.389 A "
`12’19‘}? Scki et al.
`
`5,771,224 A "
`6:19.98 Scki Cl
`:31.
`
`5.832.387 A " 11i1998 Bac cl :1].
`6.009.073 A * 121990 Kaneko
`
`6.092.122 A " WM Lin 9! a].
`6.151296 A ‘
`lliZDOfl Vijayan eta].
`..
`6.173.916 BI "
`“200] Suzuki
`6.175,55(J BI "
`”2130] van Nee ..........
`6359333 Bl “
`3.9002 Aslrtnis el al.
`6,363,115 Bl ‘
`3i2002 Schcircr ct al.
`6,687,315 32‘
`212004 chvill el al.
`
`370206
`3321170
`3702207
`390%?”
`370.1203
`371M206
`455f522
`37mm
`70992?
`370,908
`3751295
`3711906
`375F260
`382/112
`
`. ..............375,341
`
`FOREIGN PATENT DOCUMENTS
`
`EI’
`J P
`JP
`JP
`JP
`W0
`W0
`
`0722235
`62502932
`08251135
`9116465
`034464
`36092 23
`93111946
`
`NI‘J‘RS
`1 I i [987
`9il996
`511997
`BMW?
`12.31986
`[”998
`
`OTHER PUBLICATIONS
`
`Keller and Hanzo; “Orthogonal Frequency Division Mulli-
`plcx synchronisation Techniques for Wirclcos Local Area
`Networks“. Hz‘EE International Symposium on Pomona},
`indoor and Mobiie Radio Corrmmm‘cmt'ons. pp. 963-967
`(Oct. 1996).
`
`Palacherla, “DSl’-,rrP Routine Computes Magnitude", EDN
`Efecrrt'crri Design News, vol. 34, No. 22, pp. 225-226 (Ocl.
`1989).
`Warner and Lcung. “OFDMW Frame Synchronization for
`Mobile Radio Data Communication". JEEE Tmnsacrr'ons
`On Vehicular Tec'hrroiogy, vol. 42, pp. 3(E-313 (Aug. 1993).
`Classen and Meyr, "Synchronization Algorilhms for an
`OFDM System for Mobile Communication", Condicntng
`ffir Qucllc. Renal and Uherlragung: lTG-Fachberichl, pp.
`105-114 (Oct. 1994).
`”low-Overhead. Low-Complexity
`Schmidl
`and Cox.
`[Burst] Synchronizalion for OFDM". Proc. iEEE 1m. Corrfi
`on Common, pp. 1301-1306 (1996).
`ver de Beck, Sandell.
`lsaksson and Bfirjesson, “Low-
`Complex Frame Synchronization in OFDM Systems". Proc.
`of fire ICUI’C (1995).
`Lambretlc.
`l-lorstmannshofl' and Melr. "Techniques for
`Frame Synchronization on Unknown Frequency Sclctive
`Channels", Proc. Vehic. Tedmaiogv Conference (199?).
`21011 and Wu, "COFDM: An Overview“, IEEE lrrmwcrions
`on Broadcasting. vol. 41. No. 1, pp. 108 (Mar. 1995).
`l'or
`Adams
`and Brady.
`”Magnitude Approximations
`Microprocessor Implementation". IEEE Micro, vol. 3, No.
`5, PP. 27-31 (Oct. 1983).
`Lambretle, Spcth and Meyr, “()FDM Burst Frequency
`Synchronization by Single Carrier Training Data", [EEK
`Cacao. Letters, vol. 1, No. 2, pp. 46-48 (mar. 1997).
`
`‘ cited by examiner
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 2
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 2
`
`

`

`US. Patent
`
`Jan. 31, 2006
`
`Sheet 1 of3
`
`US 6,993,084 B1
`
`14
`
`12
`
`1O
`/
`
`m M symbol
`
`16
`
`10
`
`: —I-l-I- flit—j
`
`
`
`1 FRAME
`
`FIG.1
`
`154
`
` Framefrirnirlg
`
`framing
`
`coarse Frequency
`Synchronizaticm
`with AM sequence
`
`
`
`156
`
`MGM
`demodulator
`
`150
`
`152
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 3
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 3
`
`

`

`US. Patent
`
`Jan. 31, 2006
`
`Sheet 2 of 3
`
`US 6,993,084 B1
`
`02‘
`
`«9.
`
`of.
`
`Hummuww--unique-mmF0%
`
`
`
`1.Katha—t“;
`
`335.:0.mEEmcgd.8:222
`a:3
`
`H2525
`Ewan
`
`5.53
`
`o S
`
`52523
`
`20.2
`
`82-88?...
`
`cognates
`msEmE
`
`mg
`
`_§OEE
`
`oEE.
`
`
`
`HE::o=mu_:Ezo§m
`
`..w>_ooom202
`
`NOE
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 4
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 4
`
`

`

`US. Patent
`
`Jan. 31, 2006
`
`Sheet 3 of3
`
`US 6,993,084 B1
`
`136.“! 38
`
`
`
`FIG.4
`
`FIG.5
`
`ahfidr)
`
`+_._.._._.__._..—_—.—-—.———-————-——-+-
`max. about 60 user:
`
`lime
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 5
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 5
`
`

`

`US 6,993,084 B1
`
`1
`COARSE FREQUENCY SYNCHRONISATION
`1N MULTICARRIER SYSTEMS
`
`FIELD OF THE INVENTION
`
`The present invention relates to methods and apparatus
`for generating a signal having a frame structure. wherein
`each frame of the frame structure is composed of useful
`symbols. a guard interval associated to each useful symbol
`and one reference symbol. in addition, the present invention
`relates to methods and apparatus for frame synchronization
`of signals having the above structure.
`The present invention is particularly useful in a MC‘M
`transmission system (MCM2Multi-carrier modulation)
`using an orthogonal
`frequency division multiplexing
`(OFDM) for digital broadcasting.
`
`BACKGROUND Ol’ THE INVENTION
`
`In a MCM (OFIJM) transmission system the binary
`information is represented in the form of a complex spec-
`trum, i.e. a distinct number of complex subcarrier symbols
`in the frequency domain. In the modulator a bitstream is
`represented by a sequence of spectra. Using an inverse
`Fourier-transform (IFFI') a MCM time domain signal
`is
`produced from this sequence of spectra.
`In case of a transmission of this described MCM signal
`via a multipath channel with memory. intersymbol interfer-
`ence (15]) occurs due to multipath dispersion. To avoid lSl
`a guard interval of fixed length is added between adjacent
`MCM symbols in time. The guard interval
`is chosen as
`cyclic prefix. This means that the last part of a time domain
`MCM symbol is placed in front of the symbol to get a
`periodic extension. If the fixed length of the chosen guard
`interval is. greater than the maximum multipath delay. lSl
`will not occur.
`
`In the receiver the information which is in the frequency
`and time domain (MCM) has to be recovered from the MCM
`time domain signal. This is performed in two steps. Firstly.
`optimally locating the FFT window, thus eliminating the
`guard intewai in front of each MCM time domain symbol.
`Secondly. performing a Fourier Transform of the sequence
`of useful time samples thus obtained.
`As a result a sequence of spectral symbols is thus recov-
`ered. Each of the symbols contains a distinct number of
`information carrying subcarrier symbols. Out of these, the
`information bits are recovered using the inverse process of
`the modulator.
`
`Performing the above described method, the following
`problem occurs in the receiver. The exact position of the
`guard interval and hence the position of the original useful
`pans of the time domain MCM symbols is generally
`unknown. Extraction of the guard interval and the subse-
`quent PET-transform of the resulting useful part of the time
`signal is not possible without additional information. To
`provide this additional information, a known (single carrier)
`sequence in the form of a (time domain) reference symbol is
`inserted into the time signal. With the knowledge about the
`positions of the reference symbols in the received signal. the
`exact positions ofthe guard intervals and thus the interesting
`information carrying time samples are known.
`The periodical insertion of the reference symbol results in
`a frame structure of the MCM signal. This frame structure of
`a MCM signal is shown in FIG. 1. One frame of the MCM
`signal is composed of a plurality of MCM symbols 10. Each
`MCM symbol 10 is formed by an useful symbol 12 and a
`
`2
`
`guard interval 14 associated therewith. As shown in FIG. 1,
`each frame comprises one reference symbol 16.
`A functioning synchronization in the receiver. i.c. frame,
`frequency. phase. guard interval synchronization is neces-
`sary for the subsequent MCM demodulation. Consequently.
`the first and most important task of the base band processing
`in the receiver is to find and synchronize to the reference
`symbol.
`
`DESCRIPTION OF THE PRIOR ART
`
`Most prior art methods for frame synchronization have
`been developed for single carrier transmission over the
`AWGN channel (AWGN=Additive White Gaussian Noise).
`These prior art methods based on correlation are, without
`major changes, not applicable for transmission over multi-
`path fading channels with large frequency offsets or MCM
`transmission systems that use. for example. an orthogonal
`frequency division multiplexing.
`For MCM transmission systems particular frame synchro-
`nization methods have been developed.
`Warner. W. D., Leung C.: OFDMKFM Frame Synchroni-
`zation for Mobile Radio Data Communication, IEEE Trans.
`On Vehicular Technology. vol. VI'—42, August 1993, pp. 302
`to 313, teaches the insertion of reference symbols in the
`form of tones in parallel with the data into the MCM symbol.
`The reference symbols occupy several carriers of the MCM
`signal.
`in the receiver.
`the synchronization carriers are
`extracted in the frequency domain. after a l'il-T transform
`(Fl-‘I‘=fast Fourier transform) using a correlation detector. In
`the presence of large frequency olIsets,
`this algorithm
`becomes very complex because several correlators must be
`implemented in parallel.
`A further prior art
`technique is to insert a periodic
`reference symbol
`into the modulated MCM signal. This
`re ferenoe symbol is a CAZAC.‘ sequence (CAZAC=Constant
`Amplitude Zero Automrrelation). Such techniques are
`taught by: Classen, F.. Meyr. H.: Synchronization algorithms
`for an OFDM system Wine. Technology Conference, 1997;
`Schmidl. “I‘. M. Cox. D. C: Low-Overhead, Low-Complex-
`ity [Burst] Synchronization for OFI)M Transmission, Prac.
`JEEE Int. Cort}: on Calumet, 1996. In such systems, the
`receiver‘s processor looks for a periodic repetition. For these
`algorithms coarse frequency synchronization has to be
`achieved prior to or at
`least simultaneously with frame
`synchronization.
`Van de Beck. .1, Sandell, M., Isaksson. M, Borjesson, 1).:
`Low-Complex Frame Synchronization in UFDM Systems,
`Proc. oftlte ICUPC. 1995, avoid the insertion of additional
`reference symbols or pilot carriers and use instead the
`periodicity in the MCM signal which is inherent in the guard
`interval and the associated cyclical extension. This method
`is suitable only for slowly varying fading channels and small
`frequency ofl'sets.
`U.S. Pat. No. 5,191,576 relates to a method [or the
`diffusion of digital data designed to be received notably by
`mobile receivers moving in an urban environment. In this
`method.
`the header of each frame of a broadcast signal
`having a frame stmcture has a first empty synchronization
`symbol and a second unmodulated wobbled signal forming
`a two-stage analog synchronization system. The recovery of
`the synchronization signal is achieved in an analog way,
`without prior extraction of a clock signal at the binary level.
`EP 063 l 406 A relates to data signals. COFDM signals. for
`example, and to methods and apparatus for diffusing said
`signals. The COFDM signals comprises a sequence of
`symbols. each symbol having an useful portion and a guard
`
`l."
`
`ll)
`
`30
`
`35
`
`4D
`
`45
`
`5|]
`
`'Jt‘1:
`
`fill
`
`65
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 6
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 6
`
`

`

`US 6,993,084 Bl
`
`3
`interval. 1th symbols of a COFDM signal are provided as
`synchronization symbols. One of the two symbols is a zero
`symbol, whereas the other thereof is a synchronization
`symbol which is formed by an unmodulated multiplex of the
`carrier frequencies having a constant envelope. Beside the
`two symbols as synchronization symbols, it is taught in IEP
`0631406 A to modulate the pilot frequency of the data signal
`with a reference signal which carries the synchronization
`information. This reference signal modulated on the pilot
`frequency of the data signal can be used by a MABLR
`demodulator.
`
`5
`
`10
`
`W0 98i0tt94o A relates to a system for a timing and
`frequency synchronization of 0130M signals. ‘l‘wo UPDM
`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 Ol-‘DM sub-carriers whereas
`the odd-numbered OFDM sub-carriers are suppressed. Thus.
`in accordance with W0 98l0094o A, the first OFDM training
`symbol is produced by modulating the even-numbered car-
`riers of this symbol with a first predetermined PN sequence.
`Moose: "A technique for orthogonal frequency division
`multiplexing frequency offset correction", IEEE 'I'RANS-
`ACTIONS ON COMMUNICA'I'IONS, Vo. 42, No.
`IO,
`October 1994, pages 2908 to 2914, teaches methods for
`correcting frequency oflscts in OFDM digital communica-
`tions. The methods involve repetition of a data symbol and
`comparison of the phases ofeach of the carriers between the
`successive symbols. The phase shift of each of the carriers
`between the repeated symbols is due to the frequency tifisct
`since the modulation phase values are not changed in the
`repeated symbols.
`Keller; Hanzo: “Orthogonal frequency division multiplex
`synchronization techniques for wireless local area net-
`works". IEEE INTERNATIONAL SYMPOSIUM ON PIER-
`SONAL. INDOOR AND MOBILE RADIO COMMUNI-
`CATIONS, Oct. 15, 1996. pages 963 to 967, teach frequency
`acquisition, frequency tracking. symbol synchronization and
`frame synchronization techniques. Regarding the frame syn-
`chronization. it is taught to use a reference symbol which
`consists of repetitive copies of a synchronization pattern of
`pseudo-random samples. The frame synchronization is
`achieved by autocorrelation techniques using the periodic
`synchronization scgmean such that for the synchronization
`algorithms proposed no a priori knowledge of the synchro-
`nization sequences is required.
`The methods for frame synchronization available up to
`date require either prior achieved frequency synchronization
`or become very complex when the signal in the receiver is
`corrupted by a large frequency offset.
`If there is a frequency offset in the receiver. as can easily
`be the case when a receiver is powered-on and the frequency
`synchronization loop is not yet locked, problems will occur.
`When performing a simple correlation there will only be
`noise at the output of the correlator. i.e. no maximum can be
`found if the frequency offset exceeds a certain bound. The
`size of the frequency offset depends on the length (time) of
`the correlation to be performed, i.e. the longer it takes. the
`smaller the allowed frequency offset becomes. In general,
`frequency ofl'set increases implementation complexity.
`Frequency offsets occur after power-on or later due to
`frequency deviation of the oscillators used for down-con-
`version to basehand. Typical accuracies for the frequency of
`a free running local oscillator (LO) are at :50 ppm of the
`carrier frequency. With a carrier frequency in S-band (e.g.
`2.34 GHz) there will be a maximum LO frequency deviation
`
`4
`
`of above 100 kHz (11?.25 kllz). A deviation ttf this mag»
`nitudc puts high demands on the above methods.
`In the case of multipath impaired transmission channel, a
`correlation method yields several correlation maxima in
`addition to the distinct maximum for an AWGN channel.
`
`The best possible frame header position. i.e. the reference
`symbol, has to be selected to cope with this number of
`maxima.
`In multipath channels,
`frame synchronization
`methods with correlations can not be used without major
`changes. Moreover, it is not possible to use data demodu-
`lated from the MCM system. because the demodulation is
`based on the knowledge of the position of the guard interval
`and the useful part of the MCM symbol.
`
`SUMMARY OF THE INVENTION
`
`It is an object ofthe present invention to provide a method
`and an apparatus for generating a signal having a frame
`structure that allow a frame synchronization after the signals
`have been transmitted even in the ease of a carrier frequency
`offset or in the case ofa transmission via a multipath fading
`channel
`
`It is a further object of the present invention to provide a
`method and an apparatus for frame synchronization of a
`signal having a frame structure even in the case of a carrier
`frequency oflset.
`In accordance with a first aspect, the present invention
`provides a method for generating a signal having a frame
`structure, each frame of the frame structure comprising at
`least one useful symbol. a guard interval associated to the at
`least one useful symbol and a reference symbol. the method
`comprising the steps of performing an amplitude modulation
`of a bit sequence, the envelope of the amplitude modulated
`bit sequence defining the reference pattern of the reference
`symbol and inserting the amplitude modulated bit sequence
`into said signal as said reference symbol.
`In accordance with a second aspect. the present invention
`provides a method for generating a multi-carrier modulated
`signal having a frame structure. each frame of the frame
`structure comprising at
`least one useful symbol, a guard
`interval associated to the at least one useful symbol and a
`reference symbol. the method comprising the steps of:
`providing a bitstrean't;
`mapping bits of the bitstream to carriers in order to
`provide a sequence of spectra;
`performing an inverse Fourier transform in order to
`provide multi-carrier modulated symbols;
`associating a guard interval to each mulli-carrier modu-
`lated symbol:
`generating the reference symbol by performing an ampli-
`tude modulation of a hit sequence, the envelope of the
`amplitude modulated bit sequence defining the refer-
`ence pattern of the reference symbol;
`associating the reference symbol to a predetermined num-
`ber of multi-carrier modulated symboLs and associated
`guard intervals in order to define the frame; and
`inserting said amplitude modulated bit sequence into said
`signal as said reference symbol.
`In accordance with a third aspect, the present invention
`provides a method for frame synchronization of a signal
`having a frame structure, each frame of the frame structure
`comprising at
`least one useful symbol. a guard interval
`assmiated with the at least one useful symbol and a refer-
`ence symbol. the method comprising the steps of:
`
`30
`
`35
`
`4|)
`
`45
`
`5|]
`
`'JI‘1:
`
`fill
`
`65
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 7
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 7
`
`

`

`US 6,993,084 Bl
`
`5
`
`receiving the signal;
`down-converting the received signal;
`performing an amplitude-demodulation of the down-con-
`verted signal in order to generate an envelope;
`correlating the envelope with a predetermined reference
`pattern in order to detect the signal reference pattern of
`the reference symbol in the signal; and
`performing the frame synchronization based on the detec-
`tion of the signal reference pattern.
`In accordance with a fourth aspect. the present invention
`provides a method for frame synchronization of a multi-
`carrier modulated signal having frame structure. each frame
`of the frame structure comprising at least one useful symbol,
`a guard interval associated to the at least one useful symbol
`and a reference symbol. the method comprising the steps of:
`receiving the multi-earrier modulated signal;
`down-converting the received multi-carrier modulated
`signal;
`performing an amplitude-demodulation of the down-con-
`verted multi-carrier modulated signal in order to gen-
`erate an envelope;
`correlating the envelope with a predetermined reference
`pattern in order to detect the signal reference pattern of
`the reference symbol
`in the multi-carrier modulated
`signal;
`performing the frame synchronization based on the detec-
`tion of the signal reference pattern:
`extracting the reference symbol and the at least one guard
`interval from the down-converted received multi-car—
`ricr modulated signal based on the frame synchroniza-
`tion;
`performing a Fourier transform in order to provide a
`sequence of spectra from the at least one useful symbol;
`de-mapping the sequence of spectra in order to provide a
`bitstrearn.
`
`in accordance with a fifth aspect, the present invention
`provides an apparatus for generating a signal having a frame
`structure. each frame of the frame structure comprising at
`least one useful symbol, a guard interval associated to the at
`least one Useful symbol and a reference symbol, the appa-
`ratus comprising an amplitude modulator for performing an
`amplitude modulation of a bit sequence. the envelope of the
`amplitude modulated bit sequence defining the reference
`pattern of the reference symbol: and
`means for inserting the amplitude modulated bit sequence
`into said signal as said reference symbol.
`in accordance with a sixth aspect, the present invention
`provides an apparatus for generating a multi-carrier modu-
`lated signal having a frame structure. each frame of the
`frame structure comprising at
`least one useful symbol, a
`guard interval associated to the at least one useful symbol
`and a reference symbol, the apparatus comprising:
`means for providing a bitstream;
`means for mapping bits of the hitstt‘eam to carriers in
`order to provide a sequence of spectra;
`means for performing an inverse Fourier transform in
`order to provide multi-carn'er modulated symbols;
`means for associating a guard interval
`to each multi-
`carrier modulated symbol;
`means for generating the reference symbol by an ampli-
`tude modulator for performing an amplitude modula-
`tion of a bit sequence, the envelope of the amplitude
`modulated bit sequence defining the reference pattern
`of the reference symbol;
`
`6
`means for associating the reference symbol to a prede-
`termined number of multi-carrier modulated symbols
`and associated guard intervals in order to define the
`frame; and
`means for inserting the amplitude modulated bit sequence
`into said signal as said reference symbol.
`In accordance with a seventh aspect, the present invention
`provides an apparatus for frame synchronization of a signal
`having a frame structure, each frame of the frame structure
`comprising at
`least one useful symbol, a guard interval
`associated to the at least one useful symbol and a reference
`symbol. the apparatus comprising:
`receiving means for receiving the signal;
`a down~converter for down-converting the received sig»
`nal;
`an amplitude-demodulator for performing an amplitude
`demodulation of the down-conVerted signal in order to
`generate an envelope;
`a correlator for correlating the envelope with a predeter-
`mined reference pattern in order to detect the signal
`reference pattern of the reference symbol in the signal;
`and
`
`means for performing the frame synchronization band on
`the detection of the signal reference pattern.
`In accordance with a eighth aspect, the present invention
`provides an apparatus for frame synchronization of a multi-
`carrier modulated signal having a
`frame structure, each
`frame of the frame structure comprising at least one useful
`symbol, a guard interval associated to the at least one useful
`symbol and a reference symbol, the apparatus comprising:
`a receiver for receiving the multi-carrier modulated sig-
`nal;
`for down-converting the received
`a down-converter
`mulIi-carrier modulated signal:
`an amplitude-demodulator for performing an amplitude-
`demodulation of
`the down-converted multi-carrier
`modulated signal in order to generate an envelope;
`a correlator for correlating the envelope with a predeter-
`mined reference pattern in order to detect the signal
`reference pattern of the reference symbol in the multi-
`carrier modulated signal;
`means for performing the frame synchronization based on
`the detection of the signal reference pattern;
`means for extracting the reference symbol and the at least
`one guard interval from the down-converted received
`multi-carricr modulated signal based on the frame
`synchronization in order to generate the at
`least one
`useful symbol;
`means for performing a Fourier transform in order to
`provide a sequence of spectra from the at
`least one
`useful symbol; and
`means for tie-mapping the sequence of spectra in order to
`provide a biLstream.
`The present invention provides a novel structure of the
`reference symbol along with a method to determine the
`position of the re ferenoe symbol anti thus the start of a frame
`in a signal having a frame structure as shown for example in
`FIG. 1.
`
`finding frame
`The invention relates to a method for
`headers independently of other synchronization information
`and thus for positioning the FFT windows correctly. This
`includes the extraction of a guard interval. The method is
`based on the detection of a known reference symbol of the
`frame header in the reception signal. e.g.
`in the digital
`complex baseband. The new frame synchronization will be
`performed as the first synchronization task.
`
`if)
`
`30
`
`35
`
`4|)
`
`45
`
`5|]
`
`'JI‘1:
`
`an
`
`65
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 8
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 8
`
`

`

`US 6,993,084 B1
`
`7
`Synchronization to the reference symbol. i.e. the frame
`header
`is the first step to initiate radio reception. The
`reference symbol
`is structured to accomplish this. The
`information contained in the reference symbol must there-
`fore be independent of other synchronization parameters.
`e.g. frequency offset. For this reason, in accordance with the
`present invention, the form ofthe reference symbol selected
`is an amplitude modulated sequence {AM sequence) in the
`complex bamband. Thus. the information contained in the
`reference symbol is only that given in the amplitude and not
`that in the phase. Note that the phase information will be
`corrupted by a possible frequency offset.
`In preferred
`embodiments of the present invention. the AM information
`is constructed from a bit sequence with special features. The
`informaLion sequence is selected in a way which makes it
`easy and secure to find it in the time domain. A bit sequence
`with good autocorrelat ion properties is chosen. Good auto-
`correlation properties means a distinct correlation maximum
`in a correlation signal which should be as white as possible.
`A pseudo random bit sequence (PRES) having good
`autocorrelation properties meets the above requirements.
`Using the envelope of the signal to carry bit information
`offers additional flexibility. First it has to be decided which
`envelope values should correspond to the binary values of (l
`and 1. The parameters are mean amplitude and modulation
`rate. Attention should be paid to selecting the mean ampli-
`tude of the reference symbol (performance) identically to the
`mean amplitude of the rest of the frame. This is due to the
`amplitude normalization (AGO; AGC=Automatic Gain
`Control) performed in the receiver. It is also possible to
`select the mean amplitude of the reference symbol higher
`than the mean signal amplitude, but then care has to be taken
`that the time constant of the AGC (Ilsensitivity) is selected
`high enough to secure that the strong (boosted) signal of the
`reference symbol does not inlluencc the AGC control signal
`and thus attenuate the signal following the reference symbol.
`Another degree of freedom can be characterized as modu-
`lation degree d. This parameter is responsible for the infor-
`mation density of the modulating signal mod(t) formed out
`of the binary sequence binft) as follows: mod(t)=bin(ti’d).
`This modulation degree can be chosen as free parameter
`fixed by an integer or real relation to the sampling rate. It is
`appropriate to choose the modulation degree d as an integer
`value because of the discrete values of the binary sequence:
`
`d - t: mod (n1) - bin (ml
`form even
`d n 1!: mod (mt = bin (null
`[hr m odd
`- bin_int (1W3)
`:9.
`x0.
`:3,
`fl.
`for
`rn
`-
`(1 - 3‘. mod (In) - bin (n13)
`
`
`int (111:3:bin. else
`
`The signal values bin_int(mfd) are computed from the
`binary sequence bin{m) by ideal interpolation (between the
`discrete integer values m) with the factor ofd. This is similar
`to an ideal sampling rate expansion (with sin(x)!x interpo-
`lation). but the sampling rate remains. only less bits of the
`binary sequence bin(m) correspond to the resulting interpo-
`lated sequence mod(m). This parameter at
`indicates the
`discrete time.
`
`is
`With increasing m the modulating signal mod(t)
`expanded in time relative to the basic binary sequence, this
`results in a bandwidth compression of the resulting AM
`spectrum with regard to the basic binary sequence. A time
`expansion by a factor 2 results in a bandwidth compression
`by the same factor 2. In addition to the bandwidth compres-
`
`8
`sion, a further advantage ofa higher modulation degree d is
`a reduced complexity of the search method in the receiver
`due to the fact that only each dth sample has a corresponding
`binary value. Choosing the factor d=l is not preferred since
`this would result in aliasing due to disregard of the sampling
`theorem. For this reason, in a preferred embodiment of the
`present invention d is chosen to be 2.
`The choice of length and repetition rate of the reference
`symbol
`is. on the one hand, dominated by the channel
`properties, e.g. the channel’s coherence time. On the other
`hand the choice depends on the receiver requirements con-
`eerning mean time for initial synchronittation and mean time
`for resynchronization after synchronization loss due to a
`channel fade.
`
`In the receiver, the first step after the down-conversion of
`the received signal is to perform an amplitude-demodulation
`of the down-converted signal in order to generate an enve-
`lope. i.e.
`in order to determine the amplitude of the signal.
`This envelope is correlated with a replica reference pattern
`in order to detect the signal reference pattern of the reference
`symbol in the signal. In the case of a AWGN channel, the
`result of this correlation will be a white noise signal with
`Zero mean value and with a clearly visible (positive) maxi-
`mum. In the case of a multipath channel, several maxima
`will Occur in the correlation signal computed by this corre-
`lation. In the former case.
`the location of the reference
`symbol
`is determined based on the signal maximum.
`whereas in the latter case a weighting procedure is per-
`formed in order to find out the maximum corresponding to
`the location of the reference symbol.
`Thus, the present invention shows how to find a reference
`symbol by a detection method which is simple. Furthermore,
`the present invention can be used for one-carrier or multi-
`carrier systems. The present invention is particularly useful
`in multi-carricr modulation systems using an orthogonal
`frequency division multiplexing. for example in the field of
`digital broadcasting. The synchronization methods accord-
`ing to the present invention are independent of other syn-
`chronization steps. Since the informat