(12)
`
`United States Patent
`Eberlein et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,061,997 Bl
`Jun. 13, 2006
`
`US007061997B1
`
`(54)
`
`(75)
`
`(73)
`
`METHOD AND APPARATUS FOR FINE
`FREQUENCY SYNCHRONIZATION IN
`MULTI-CARRIER DEMODULATION
`SYSTEMS
`
`Inventors: Ernst Eberlein, Grossenseebach (DE):
`Sabah Badri, Erlangen (DE); Stefan
`Lipp, Erlangen (DE); Stephan
`Buchholz, Munich (DE); Albert
`Heuberger. Erlangen (DE); Heinz
`Gerhaeuser, Waischenfeld (DE)
`Assignee: Fraunhofer-Gesellschaft zur
`Foerderung der angewandten
`Forschung e.V., Munich (DE)
`
`Notice:
`
`Subject to any disclaimer, the termof this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21)
`
`Appl. No.:
`
`09/673,270
`
`(22)
`
`PCTFiled:
`
`Apr. 14, 1998
`
`(86)
`
`PCTNo.:
`
`PCT/EP98/02184
`
`§ 371 (c)(1),
`(2), (4) Date:
`
`Nov. 29, 2000
`
`PCTPub. No.: WO99/53667
`
`PCTPub. Date: Oct. 21, 1999
`
`Int. Cl.
`(2006.01)
`HO3D 3/22
`(2006.01)
`HOI 11/00
`WS CU. srccecccsvecsasesssessvossscserseaene SPMOSE, STO2IO
`Field of Classification Search.
`................. 375/332.
`375/200; 370/203, 200, 207, 480, 210
`See applicationfile for complete search history.
`
`References Cited
`
`(87)
`
`(31)
`
`(52)
`(58)
`
`(56)
`
`U.S. PATENT DOCUMENTS
`4.347483 A *
`8/1982 Flasza etal.
`............... 331/12
`§,267,273 A *
`11/1993 Dartois et al.
`........0.. 375/955
`
`§,345,440 A *
`
`9/1994 Gledhill et al.
`
`............. 370/210
`
`(Continued)
`
`EP
`
`FOREIGN PATENT DOCUMENTS
`0822682
`2/1998
`
`(Continued)
`
`OTHER PUBLICATIONS
`
`Keller and Hanzo; “Orthogonal Frequency Division Multi-
`plex Synchronisation Techniques for Wireless Local Area
`Networks”, JEEE International Symposium on Personal,
`Indoor and Mobile Radio Communications, pp. 963-967
`(Oct, 1996).
`
`(Continued)
`
`Primary Examiner—Stephen Chin
`Assistant Examiner—Cicely Ware
`(74) Attorney, Agent, or Firm—Roylance,Abrams,Berdo &
`Goodman, L.L.P.
`
`(57)
`
`ABSTRACT
`
`A method and an apparatus relating to a fine frequency
`synchronization compensating for a carrier frequency devia-
`tion from anoscillator frequency in a multi-carrier demodu-
`lation system ofthe type capable ofcarrying out adiffer-
`ential phase decoding of multi-carrier modulated signals, the
`signals comprising a plurality of symbols, each symbol
`being defined by phase differences between simultaneous
`carriers having different
`frequencies. A phase difference
`between phases of the samecarrier in different symbols is
`determined. Thereafter, a frequency offset is determined by
`eliminating phase shift uncertainties related to the transmit-
`ted information from the phase difference making use of a
`M-PSK decision device. Finally, a feedback correction of
`the carrier frequency deviation is performed based on the
`determined frequencyoffset. Alternatively, an averaged fre-
`quency offset can be determined by averaging determined
`frequency offsets of a plurality of carriers. Then, the feed-
`back correction of the frequency deviation is performed
`based on the averaged frequencyoffset.
`
`7 Claims, 13 Drawing Sheets
`
`
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 1
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 1
`
`

`

`US 7,061,997 B1
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`Zou and Wu, “COFDM: An Overview”, JEEE Transactions
`
`$,694389 A *
`5.771.224 A
`6.192.068 BL*
`6,219,333 BL*®
`6,226,337 BL*
`
`......ccecee. 370/208
`12/1997 Seki et al.
`6/1998 Seki et al. oo... 370/206
`2/2001 Fattouche et al.
`.......... 375/130
`4/2001 ARM w...cceeeseccseeeeeees 370/203
`5/2001 Klank et al.
`.........006. 375/367
`
`
`
`JP
`JP
`wo
`wo
`
`POREIGN PATENT DOCUMENTS
`8265293
`10/1996
`104199]
`2/1998
`9205646
`4/1992
`9800946
`1/1998
`
`OTHER PUBLICATIONS
`
`Moose, “A Technique for Orthogonal Frequency Division
`Multiplexing Frequency Offset Correction”, /EEE Transac-
`tions on Communications, vol. 42, No. 10, pp. 2908-2914
`(Oct. 1994),
`Classen and Meyr, “Synchronization Algorithms for an
`OFDM System for Mobile Communication”, Condierung
`fiir Quelle, Kanal and Wbertragung: ITG-Fachbericht, pp.
`105-114 (Oct. 1994).
`“Low-Overhead, Low-Complexity
`Schmidl
`and Cox,
`[Burst] Synchronization for OFDM”, Proce. IEEEInt. Conf
`on Commun., pp. 1301-1306 (1996).
`
`on Broadcasting, vol. 41, No. 1, pp. 1-8 (Mar. 1995).
`Palacherla, “DSP-:P Routine Computes Magnitude”, EDN
`Electrical Design News, vol. 34, No. 22, pp. 225-226 (Oct.
`1989).
`Adams and Brady, “Magnitude Approxmations for Micro-
`processor Implementation”, /EEE Micro, vol. 3, No. 5, pp.
`27-31 (Oct. 1983).
`Luise and Regiannini, “Carrier Frequency Acquisition and
`Tracking for OFDM Systems”, /EEE Transactions on Com-
`munications, vol. 44, No. 11, pp. 1590-1598 (Nov. 1996).
`Tuisel and Kammeyer, “Carrier-Recovery for Multicarrier-
`Transmission Over Mobile Radio Channels”, /nt. Conf: on
`Acoustics, Speech and Signal Processing (ICASSP 92), San
`Francisco, Band 4, pp. 677-680 (1992).
`Moose, “Differentially Coded Multi-Frequency Modulation
`for
`Digital
`Communications”,
`Signal
`Processing
`V—Theories and Applications, Proceedings of EUSIPCO-
`90
`Fifth European
`Signal
`Processing Conference,
`Barcelona, Spain, vol. III, pp. 1807-1810 (Sep. 1990).
`
`* cited by examiner
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 2
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 2
`
`

`

`Jun. 13, 2006
`
`Sheet 1 of 13
`
`US 7,061,997 BI
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`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 3
`
`
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`

`

`U.S. Patent
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`Jun. 13, 2006
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`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 4
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`

`

`U.S. Patent
`
`Jun. 13, 2006
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`Sheet 3 of 13
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`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 5
`
`

`

`U.S. Patent
`
`Jun. 13, 2006
`
`Sheet 4 of 13
`
`US 7,061,997 B1
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`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 6
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 6
`
`

`

`U.S. Patent
`
`Jun. 13, 2006
`
`Sheet 5 of 13
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`US 7,061,997 B1
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`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 7
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 7
`
`

`

`U.S. Patent
`
`Jun. 13, 2006
`
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`US 7,061,997 B1
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`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 8
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`
`
`

`

`U.S. Patent
`
`Jun. 13, 2006
`
`Sheet 7 of 13
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`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 9
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`

`

`Jun. 13, 2006
`
`Sheet 8 of 13
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`US 7,061,997 B1
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`U.S. Patent
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`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 10
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`

`

`U.S. Patent
`
`Jun. 13, 2006
`
`Sheet 9 of 13
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`US 7,061,997 B1
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`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 11
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 11
`
`

`

`U.S. Patent
`
`Jun. 13, 2006
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`Sheet 10 of 13
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`US 7,061,997 B1
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`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 12
`
`

`

`U.S. Patent
`
`Jun. 13, 2006
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`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 13
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`

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`U.S. Patent
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`Jun. 13, 2006
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`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 14
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`U.S. Patent
`
`Jun. 13, 2006
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`
`

`

`US 7,061,997 B1
`
`1
`METHOD AND APPARATUS FOR FINE
`FREQUENCY SYNCHRONIZATION IN
`MULTI-CARRIER DEMODULATION
`SYSTEMS
`
`This application is a 371 of PCT/EP98/02184 Apr. 14,
`1998.
`
`FIELD OF THE INVENTION
`
`The present invention relates to methods and apparatus
`for performing a fine frequency synchronization in mullti-
`carrier demodulation systems, and in particular to methods
`and apparatus for performing a fine frequency synchroniza-
`tion compensating for a carrier frequency deviation from an
`oscillator frequency in a multi-carrier demodulation system
`of the type capable of carrying out a differential phase
`decoding of multi-carrier modulated signals, wherein the
`signals comprise a plurality of symbols, each symbol being
`defined by phase differences between simultaneous carriers
`having different frequencies.
`
`BACKGROUND OFTHE INVENTION
`
`In a multi carrier transmission system (MCM, OFDM),
`the effect of a carrier frequency offset is substantially more
`considerable than in a single carrier transmission system.
`MCM is more sensitive to phase noise and frequency offset
`which occurs as amplitude distortion and inter carrier inter-
`ference (IC]). Theinter carrier interference has the effect that
`the subcarriers are no longer orthogonalin relation to each
`other. Frequency offsets occur after power on oralso later
`due to frequency deviation of the oscillators used for down-
`conversion into baseband. Typical accuracies for the fre-
`quency ofa free running oscillator are about +50 ppm of the
`carrier frequency. With a carrier frequency in the S-band of
`2.34 Ghz, for example, there will be a maximum local
`oscillator (LO)
`frequency deviation of above 100 kHz
`(117.25 kHz). The above named effects result
`in high
`requirements on the algorithm used for frequency offset
`correction.
`
`DESCRIPTION OF PRIOR ART
`
`Most prior art algorithms for frequency synchronization
`divide frequency correction into two stages. In the first
`stage, a coarse synchronization is performed. In the second
`stage, a fine correction can be achieved. A frequently used
`algorithm for coarse synchronization of the carrier fre-
`quency uses a synchronization symbol] which has a special
`spectral pattern in the frequency domain. Such a synchro-
`nization symbol
`is,
`for example, a CAZAC sequence
`(CAZAC=Constant Amplitude
`zero Autocorrelation).
`Through comparison,
`i.e.
`the correlation, of the power
`spectrum of the received signal with that of the transmitted
`signal,
`the frequency carrier offset can be coarsely esti-
`mated, These prior art algorithmsall work in the frequency
`domain. Reference is made,
`for example,
`to Ferdinand
`Clapen, Heinrich Meyr, “Synchronization Algorithms for an
`OFDM System for Mobile Communication”, [TG-Fachta-
`gung 130, Codierung fiir Quelle, Kanal und Ubertragung.
`pp. 105-113, Oct. 26-28, 1994; and Timothy M. Schmid,
`Donald C. Cox, “Low-overhead, Low-Complexity [Burst]
`synchronization for OFDM”,
`in Proceedings of the IEEE
`International conference on communication ICC 1996, pp.
`1301-1306 (1996).
`
`10
`
`20
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`For the coarse synchronization of the carrier frequency,
`Paul H. Moose, “A Technique for orthogonal Frequency
`Division Multiplexing Frequency offset Correction”, IEEE
`Transaction on communications, Vol. 42, No. 10, October
`1994, suggest increasing the spacing between the subcarriers
`such that the subcarrier distance is greater than the maxi-
`mum frequency difference between the received and trans-
`mitted carriers. The subcarrier distance is increased by
`reducing the number of sample values which are trans-
`formed by the Fast Fourier Transform. This corresponds to
`a reduction of the number of sampling values which are
`transformed by the Fast Fourier Transform.
`WO9205646 A relates to methods for the reception of
`orthogonal frequency division multiplexed signals compris-
`ing data which are preferably differentially coded in the
`direction of the time axis. Phase drift of the demodulated
`samples from one block to the next is used to indicate the
`degree of local oscillator frequency error. Phase drift
`is
`assessed by multiplying complex values by the complex
`conjugate ofan earlier sample demodulated from the same
`OFDMcarrier and using the resulting measure to steer the
`local oscillator frequency via a frequency locked loop.
`
`SUMMARY OF THE INVENTION
`
`It is an object ofthe present invention to provide methods
`and apparatus for performing a fine frequency synchroniza-
`tion which allow a fine frequency synchronization compen-
`salting for a carrier frequency deviation from an oscillator
`frequency in a MCM transmission system which makes use
`of MCM signals in which informationis differential phase
`encoded between simultaneous sub-carriers having different
`frequencies.
`In accordance with a first aspect, the present invention
`provides a method of performing afine frequency synchro-
`nization compensating for a carrier frequency deviation
`from anoscillator frequencyin a multi-carrier demodulation
`system of the type capable of carrying out a differential
`phase decoding of multi-carrier modulated signals, the sig-
`nals comprising a plurality of symbols, each symbol being
`defined by phase differences between simultaneouscarriers
`having different
`frequencies,
`the method comprising the
`steps of:
`determining a phase difference between phases of the same
`carrier in different symbols;
`determining a frequency offset by eliminating phase shift
`uncertainties related to the transmitted information from
`the phase difference making use of a M-PSK decision
`device: and
`
`performing a feedback correction of the carrier frequency
`deviation based on the determined frequency offset.
`In accordance with a second aspect, the present invention
`provides a method ofperforming a fine frequency synchro-
`nization compensating for a carrier frequency deviation
`from anoscillator frequency in a multi-carrier demodulation
`system ofthe type capable of carrying out a differential
`phase decoding of multi-carrier modulated signals, the sig-
`nals comprising a plurality of symbols, each symbol being
`defined by phase differences between simultaneous carriers
`having different
`frequencies,
`the method comprising the
`steps of:
`determining respective phases ofthe same carrier in differ-
`ent symbols;
`eliminating phase shift uncertainties related to the transmit-
`ted information from the phases to determine respective
`phase deviations making use ofa M-PSK decision device;
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 16
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 16
`
`

`

`3
`determining a frequency offset by determining a phase
`difference between the phase deviations; and
`performing a feedback correction ofsaid carrier frequency
`deviation based on the determined frequencyoffset.
`In accordance with a third aspect, the present invention
`provides an apparatus for performing a fine frequency
`synchronization compensating for a carrier frequency devia-
`tion from an oscillator
`frequency,
`for a multi-carrier
`demodulation system ofthe type capable ofcarrying out a
`differential phase decoding of multi-carrier modulated sig-
`nals, the signals comprising a plurality of symbols, each
`symbol being defined by phase differences between simul-
`taneous carriers having different frequencies, the apparatus
`comprising:
`meansfor determining a phase difference between phases of
`the samecarrier in different symbols:
`M-PSK decision device for determining a frequencyoffset
`by eliminating phase shift uncertainties related to the
`transmitted information from the phase difference: and
`meansfor performing a feedback correction ofthe frequency
`deviation based on the determined frequency offset.
`In accordance with a fourth aspect, the present invention
`provides an apparatus for performing a fine frequency
`synchronization compensating for a carrier frequency devia-
`tion from an oscillator
`frequency,
`for a multi-carrier
`demodulation system of the type capable of carrying out a
`differential phase decoding of multi-carrier modulated sig-
`nals, said signals comprising a plurality of symbols, each
`symbol being defined by phase differences between simul-
`taneous carriers having different frequencies, the apparatus ~
`comprising:
`means for determining respective phases ofthe same carrier
`in different symbols;
`M-PSK decision device for eliminating phase shift uncer-
`tainties related to the transmitted information from the
`phases to determine respective phase deviations;
`means for determining a frequency offset by determining a
`phase difference between the phase deviations; and
`meansfor performing a feedback correction ofthe frequency
`deviation based on the determined frequency offset.
`The present invention relates to methods and apparatus
`for performing a fine frequency synchronization compen-
`sating for a carrier frequency deviation from an oscillator
`frequency. This fine frequency synchronization is preferably
`performed after completion of a coarse frequency synchro-
`nization, such that the frequency offsets after the coarse
`frequency synchronization are smaller than half the sub-
`carrier distance in the MCM signal. Since the frequency
`offsets which are to be corrected by the inventive fine :
`frequency synchronization methods and apparatus, a correc-
`tion ofthe frequency offsets by using a phase rotation with
`differential decoding and de-mapping in the time axis can be
`used. The frequency offsets are detected by determining the
`frequency differences between time contiguous sub-carrier -
`symbols along the time axis. The frequency error is calcu-
`lated by measuring the rotation of the I1-Q Cartesian coor-
`dinates of each sub-carrier and, in preferred embodiments,
`averaging them overall n sub-carriers of a MCM symbol.
`Firstly, the phase ambiguity or uncertainty is eliminated
`by using a M-PSK decision device and correlating the output
`ofthe decision device with the input signal for a respective
`sub-carrier symbol. Thus, the phase offset for a sub-carrier
`symbol is determined and can be used for restructuring the
`frequency error in form ofa feed-backward structure. Alter-
`natively, the phase offsets of the sub-carrier symbols of one
`MCM symbol can be averaged overall of the active carriers
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`20
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`US 7,061,997 B1
`
`4
`ofa MCM symbol, wherein the averaged phase offset is used
`to restructure the frequency error.
`In accordance with the present invention, the determina-
`tion ofthe frequency offset is performed in the frequency
`domain. The feedback correction in accordance with the
`
`inventive fine frequency synchronization is performed in the
`time domain. To this end, a differential decoderin the time
`domain is provided in order to detect frequency offsets of
`sub-carriers on the basis of the phases of timely successive
`sub-carrier symbols of different MCM symbols.
`
`)
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In the following, preferred embodiments ofthe present
`invention will be explained in detail on the basis of the
`drawings enclosed, in which:
`FIG. 1 shows a schematic overview of a MCM transmis-
`sion system to which the present application can be applied;
`FIGS, 2A and 2B show schematic views representing a
`scheme for differential mapping in the time axis and a
`schemefor differential mapping in the frequency axis;
`FIG. 3 showsa functional block diagram for performing
`a differential mapping in the frequency axis:
`FIG. 4 shows a representation of time variation of all
`sub-carriers in MCM symbols;
`FIG, 5 shows a QPSK-constellation for each sub-carrier
`with a frequency offset;
`FIG. 6 shows a general block diagram illustrating the
`position of the inventive fine frequency synchronization
`device in a MCM receiver;
`FIG. 7 shows a block diagramof the fine frequency error
`detector shownin FIG. 6:
`
`FIG. 8 shows a block diagram of a MCM receiver
`comprising a coarse frequency synchronization unit and a
`fine frequency synchronization unit;
`FIG. 9 shows a block diagram of a unit for performing a
`coarse frequency synchronization;
`FIG. 10 shows a schematic view of a reference symbol
`used for performing a coarse frequency synchronization;
`FIG. 11 shows a schematic view of a typical MCM signal
`having a frame structure;
`FIG. 12 shows scatter diagrams of the output of an
`(differential de-mapper of an MCM receiverfor illustrating
`the effect of an echo phase offset correction;
`FIG, 13 shows a schematic block diagramforillustrating
`the position and the functionality of an echo phase offset
`correction unit;
`FIG, 14 shows a schematic block diagram of a preferred
`form ofan echo phase offset correction device; and
`FIG. 15 shows schematic views forillustrating a projec-
`tion performed by another echo phase offset correction
`algorithm.
`
`DETAILED DESCRIPTION OF THE
`EMBODIMENTS
`
`the
`invention in detail,
`Before discussing the present
`mode of operation of a MCM transmission system is
`described referring to FIG. 1.
`Referring to FIG. 1, at 100 a MCM transmitter is shown
`that substantially corresponds to a prior art MCM transmit-
`ter. A description of such a MCMtransmitter can be found,
`for example, in William Y. Zou, Yiyan Wu, “COFDM: AN
`OVERVIEW”, IEEE Transactions on Broadcasting, vol. 41,
`No. 1, March 1995,
`A data source 102 provides a serial bitstream 104 to the
`MCM transmitter. The incoming serial bitstream 104 is
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 17
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 17
`
`

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`US 7,061,997 B1
`
`5
`applied to a bit-carrier mapper 106 which produces a
`sequence ofspectra 108 from the incomingserial bitstream
`104. An inverse fast Fourier transform (IFFT) 110 is per-
`formed on the sequence ofspectra 108 in order to produce
`a MCM time domain signal 112. The MCM time domain
`signal forms the useful MCM symbol of the MCM time
`signal. To avoid intersymbol interference (ISI) caused by
`multipath distortion, a unit 114 is provided for inserting a
`guard interval offixed length between adjacent MCM sym-
`bols in time. In accordance with a preferred embodiment of
`the present
`invention,
`the last part of the useful MCM
`symbol is used as the guardinterval by placing samein front
`ofthe useful symbol. The resulting MCM symbol is shown
`at 115 in FIG. 1 and corresponds to a MCM symbol! 160
`depicted in FIG. 11.
`FIG. 11 showsthe construction of a typical MCM signal
`having a framestructure. One frame of the MCM timesignal
`is composed ofa plurality of MCM symbols 160. Each
`MCM symbol 160 is formed by an useful symbol 162 and
`a guard interval 164 associated therewith. AS shown in FIG,
`11, each frame comprises one reference symbol 166. The
`present invention can advantageously be used with such a
`MCM signal, however, such a signal structure being not
`necessary for performingthe present invention as long as the
`transmitted signal comprises a useful portion and at least one 2
`reference symbol.
`In orderto obtain the final frame structure shownin FIG.
`li, a unit 116 for adding a reference symbol
`for each
`predetermined number of MCM symbols is provided.
`In accordance with the present invention, the reference ;
`symbol is an amplitude modulated bit sequence. Thus, an
`amplitude modulation ofa bit sequence is performed such
`that the envelope of the amplitude modulated bit sequence
`defines a reference pattern of the reference symbol. This
`reference pattern defined by the envelope ofthe amplitude 3:
`modulated bit sequence has to be detected when receiving
`the MCM signal at a MCM receiver. In a preferred embodi-
`mentof the present invention, a pseudo random bit sequence
`having good autocorrelation properties is used as the bit
`sequence that is amplitude modulated.
`The choice of length and repetition rate of the reference
`symbol depends on the properties of the channel through
`which the MCM signal is transmitted, e.g. the coherence
`time of the channel. In addition, the repetition rate and the
`length ofthe reference symbol, in other words the number
`of useful symbols in each frame, depends on the receiver
`requirements concerning mean time forinitial synchroniza-
`tion and mean time for resynchronization after synchroni-
`zation loss due to a channel fade.
`The resulting MCM signal having the structure shownat :
`118 in FIG.
`1
`is applied to the transmitter front end 120.
`Roughly speaking,at the transmitter front end 120, a digital/
`analog conversion and an up-converting of the MCM signal
`is performed. Thereafter, the MCM signal
`is transmitted
`through a channel 122.
`Following, the mode of operation of a MCMreceiver 130
`is shortly described referring to FIG. 1.The MCM signalis
`received at the receiver front end 132. In the receiver front
`end 132, the MCM signal is down-converted and, further-
`more, an analog/digital conversion of the down-converted
`signal is performed.
`The down-converted MCM signal is provided to a symbol
`frame/carrier frequency synchronization unit 134.
`A first object of the symbol
`frame/carrier frequency
`synchronization unit 134 is to perform a frame synchroni-
`zation on the basis of the amplitude-modulated reference
`symbol. This frame synchronization is performed on the
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`6
`basis of a correlation between the amplitude-demodulated
`reference symbol and a predetermined reference pattern
`stored in the MCM receiver.
`A second object of the symbol frame/carrier frequency
`synchronization unit is to perform a coarse frequency syn-
`chronization of the MCM signal. To this end, the symbol
`frame/carrier frequency synchronization unit 134 serves as a
`coarse frequency synchronization unit
`for determining a
`coarse frequency offset ofthe carrier frequence caused, for
`) example, by a difference of the frequencies between the
`local oscillator of the transmitter and the local oscillator of
`the receiver. The determined frequency is used in order to
`performa coarse frequency correction. The mode ofopera-
`tion of the coarse frequency synchronization unit
`is
`described in detail referring to FIGS. 9 and 10 hereinafter.
`As described above, the frame synchronization unit 134
`determines the location ofthe reference symbol in the MCM
`symbol. Based on the determination of the frame synchro-
`nization unit 134, a reference symbol extracting unit 136
`extracts the framing information,i.e. the reference symbol,
`from the MCM symbol coming from the receiver front end
`132. After the extraction of the reference symbol, the MCM
`signal is applied to a guard interval removal unit 138. The
`result of the signal processing performed hereherto in the
`MCM receiver are the useful MCM symbols.
`The useful MCM symbols output from the guard interval
`removal unit 138 are provided to a fast Fourier transform
`unit 140 in order to provide a sequence ofspectra from the
`useful symbols. Thereafter, the sequence of spectra is pro-
`vided to a carrier-bit mapper 142 in which the serial bit-
`stream is recovered. This serial bitstream is provided to a
`data sink 144.
`two modes for
`Next, referring to FIGS. 2A and 2B,
`differential mapping are described.
`In FIG. 2A, a first
`method of differential mapping alongthe time axis is shown.
`AS can be seen from FIG. 2A, a MCM symbolconsists of
`K subcarriers. The sub-carriers comprise different frequen-
`cies and are, in a preferred embodiment, equally spaced in
`the frequency axis direction. When using differential map-
`ping along the time axis, one or more bits are encoded into
`phase and/or amplitude shifts between two sub-carriers of
`the same center frequency in adjacent MCM symbols. The
`arrows depicted between the sub-carrier symbols correspond
`to information encoded in amplitude and/or phase shifts
`between two sub-carrier symbols.
`Asecond methodofdifferential mapping is shownin FIG,
`2B, The present invention is adapted for MCM transmission
`system using the mapping scheme shown in FIG. 2B. This
`mapping schemeis based on a differential mapping inside
`one MCM symbol along the frequency axis. A number of
`MCM symbols 200 are shown in FIG. 2B. Each MCM
`symbol] 200 comprises a number of sub-carrier symbols 202.
`The arrows 204 in FIG, 2Billustrate information encoded
`between two sub-carrier symbols 202. As can be seen from
`the arrows 204, this mapping scheme is based on a differ-
`ential mapping within one MCM symbol along the fre-
`quency axis direction.
`In the embodiment shown in FIG, 2B,the first sub-carrier
`(k=0)
`in an MCM symbol 200 is used as a reference
`sub-carrier 206 (shaded) such that information is encoded
`between the reference sub-carrier and the first active carrier
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`208. The other information of a MCM symbol 200 is
`encoded between active carriers, respectively.
`Thus, for every MCM symbol an absolute phase reference
`exists.
`In accordance with FIG. 2B, this absolute phase
`reference is supplied by a reference symbol inserted into
`every MCM symbol (k=0). The reference symbol can either
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 18
`
`Petitioner Sirius XM Radio Inc. - Ex. 1007, p. 18
`
`

`

`US 7,061,997 B1
`
`7
`have a constant phase for all MCM symbols or a phase that
`varies from MCM symbol to MCM symbol. A varying phase
`can be obtained by replicating the phase from the last
`subcarrier of the MCM symbol preceding in time.
`In FIG. 3 a preferred embodiment of a device for per-
`forming a differential mapping along the frequency axis is
`shown. Referring to FIG. 3, assembly of MCM symbols in
`the frequency domain using differential mapping along the
`frequency axis according to the present
`invention is
`described.
`
`10
`
`FIG. 3 shows the assembly of one MCM symbol with the
`following parameters:
`NFFTdesignates the number of complex coefficients of the
`discrete Fourier transform, numberofsubcarriers respec-
`lively.
`K designates the number of active carriers. The reference
`carrier is not included in the count for K.
`According to FIG. 3, a quadrature phase shift keying
`(QPSK) is used for mapping the bitstream onto the complex
`symbols. However, other M-ary mapping schemes (MPSK) -
`like 2-PSK, 8-PSK, 16-QAM, 16-APSK, 64-APSK etc. are
`possible.
`Furthermore, for ease offiltering and minimization of
`aliasing effects some subcarriers are not used for encoding
`information in the device shown in FIG. 3. These subearri-
`ers, which are set
`to zero, constitute the so-called guard
`bands on the upper and lower edges of the MCM signal
`spectrum. At the input of the mapping device shown in FIG.
`3, complex signal pairs b0[k], b1[k] of an input bitstreamare
`received. K complex signal pairs are assembled in order to *
`form one MCM symbol. The signal pairs are encoded into
`the K differential phase shifts phi[k] needed for assembly of
`one MCM symbol. In this embodiment, mapping from Bits
`to the 0, 90, 180 and 270 degrees phase shifts is performed
`using Gray Mapping in a quadrature phase shift keying °°
`device 220.
`
`Gray mapping is usedto preventthat differential detection
`phaseerrors smaller than 135 degrees cause double bit errors
`at the receiver.
`
`40
`
`Differential phase encoding ofthe K phases is performed
`in a differential phase encoder 222. At this stage of process-
`ing, the K phases phi[k] generated by the QPSK Gray
`mapperare differentially encoded. In principal, a feedback
`loop 224 calculates a cumulative sumoverall K phases. As
`starting point forthe first computation (k=O)the phase of the
`reference carrier 226 is used. A switch 228 is provided in
`order to provide either the absolute phase ofthe reference
`subcarrier 226 or the phase information encoded onto the
`preceding(i.e. z"', where z~' denotes the unit delay opera-
`tor)