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`
`UStJtJé‘JI’I 1084111
`
`(13) United States Patent
`Eberiein et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,931,084 BI
`Aug. 16, 2005
`
`(54)
`
`(75)
`
`DIFFERENTIAL CODING AND CARRIER
`RECOVERY FOR MUL1‘ICARRIER
`SYSTEMS
`
`InVonlors: Ernst Eben-lain, Grossenseehach (DIE);
`Suhah Badrl. Erlangen (DE); Stefan
`Llpp. Erlangen {DE}; Stephan
`Buehhnlz, Munich (DE); Albert
`Heuberger, Erlangen (DE); Helm.
`Perhneuser. Waischenfeld (DE);
`Robert Fischer. Erlangen (DE)
`
`(73)
`
`Msignee: qunhofer-Gesel Isehnft zur
`Fnerdenlng der angewnndten
`Forschung e.V.. Munich (DE)
`
`('l
`
`Notice:
`
`Subject to any disclaimer. the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(1)) by 0 days.
`
`(31)
`
`Appl. No.1
`
`091673.266
`
`(22
`
`PCT Filed:
`
`Apr. 14, 1998
`
`(86)
`
`PCT No;
`
`PCTIEP981'02167
`
`§ 371 (CXU.
`(2). (4) Date:
`
`Nov. 29. 2000
`
`(87)
`
`PCT Pub. No: W0991'53664
`
`5332.105 A "
`5.771.224 A
`5.7‘N].516 A "
`5.889.?5‘1 A ‘
`5.946.292 A ‘
`5.999.129 A "
`6,202.??? B] “
`6.3?1683 Bl ’
`6.466.958 Bl ‘
`(5:34.283 Bl ‘
`(1.600.745 B] '
`
`3751226
`311998 Andrea eta].
`370.5206
`631998 Seki at at.
`3Tfh’210
`8.11098 Gudmundson et al.
`STUIZUT
`3519‘}?! Mdiihne)’
`3105204
`3!“qu 'l'sujishita et al.
`342.394
`1211999 Rose
`
`................ T042311
`9.1‘2(IIII
`Inoue el al.
`412002 Dohson el al.
`BYE-"406.12
`1012002 Van chhel et al.
`7113,3422
`6.12003 Sakoda el al.
`3T5.-"262
`22'2“.”
`Ilorii cl nl.
`37533”)
`
`
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`J P
`
`“93934 A2 ‘
`8265 293
`
`41-2002
`1031995
`
`HML 2?.=’2fi
`
`(Continued)
`OTHER PUBLICATIONS
`
`Moose. “Difl'erenlially Coded Multi-Frequency Modulation
`for
`Digital
`Communications".
`Signet
`Processing
`V—Illtearies and Appt'icnn'mts. Proceedings of EUSIPt.‘O-
`90 Fifth Eumpean Signal Processing Conference. Barce-
`lona. Spain, vol. III, pp. 18(17-l810(Sep. 1990).
`
`(Continued)
`
`Primary Examinerfi‘hephen (.‘h in
`Assistant Examiner—Harry Vananian
`(74}Attormn'. Agent, or Ii'inn—Roylance, Abrams. Berdo &
`Goodman, LLP.
`
`PCT I’llh. Date: Oct. 21, 1999
`
`(57')
`
`ABSTRACT
`
`(51)
`
`(52)
`(58)
`
`(56)
`
`H031) 1104;110:113 1.1116;
`Int. CU
`HOBK 5101; HOEK 6,.-'n4;H0413 1110;H(14L 11m
`U.S.CI.
`3751346
`Field of Search
`375346; 370.13%:
`3791-9110
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4.963355 A “
`5.166.024 A '
`5202,1143 A ‘
`
`1111900 Yoshida Ct at.
`111’1092 Moose
`41’1993 Sarto
`
`3201304
`3711,1280
`32911309
`
`A method of performing an echo phase ofl'set correction in
`a mulli-carn'er demodulation system involves the step of
`difl‘erential phase decoding phase shifts based on a phase
`diti‘erence between simultaneous carriers having dill'erent
`frequencies. An echo phase oITsel
`is determined for each
`decoded phase shift by eliminating phase shift uncertainties
`related to the transmitted information from the decoded
`
`phase shift. The echo phase offsets are averaged in order to
`generate an averaged ofl'set. Finally. each decoded phase
`shift is corrected based on the averaged oils-ct.
`
`16 Claims, 7 Drawing Sheets
`
`
`
`signalfromMOMtransmitter
`
`142
`402
`140
`132
`122
`
`difiarentiai de-
`Echo Phase Ofise
`
`
`
`Correction (EPDC
`mapping along the
`
`
`
`algenthm
` lrequancy axis
`
`
`
`
`
`forthememecalculation
`slgnal
`
`400
`
`404
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 1
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 1
`
`

`

`US 6,931,084 Bl
`Page 2
`
`JP
`JP
`W0
`W0
`
`FOREIGN PM'EN'I' DOCUMENTS
`9149092
`(HEW?
`1041991
`2;:998
`9205646
`4ft992
`”WW
`“£993
`OTHER PUBUCKI'IONS
`
`Moose. “A Technique for thogonal Frequency Division
`Multiplexing Frequency Oflsct Correction”, IEEE Transac-
`tions on Coimiitmications, vol. 42, N0. 10, pp. 2908-2914
`(Oct. 1994).
`
`“Orthogonal Frequency Division
`l-Ianm;
`and
`Keller
`Multiplex Synchronisalion 'I'cchniqucs for Wireless Local
`Area Networks“.
`IEEE International Symposiwu
`on
`Pars-watt Indoor and Mobile Radio Communications. pp.
`963-967 (Oct. 1996).
`_
`Zuu and Wu, “COI-‘DM: An Overview". ”SEE Tram'actions
`{m Broadcasting. vol. 41‘ N0. 1. PP- 1-8 (Mar. 1995)
`
`* cited by examiner
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 2
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 2
`
`

`

`US. Patent
`
`Aug. 16, 2005
`
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`US 6,931,084 B1
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`

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`US. Patent
`
`Aug. 16, 2005
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`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 4
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`US. Patent
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`Aug. 16, 2005
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`US 6,931,084 B1
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`

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`US. Patent
`
`Aug. 16, 2005
`
`Sheet 4 of 7
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`US 6,931,084 B1
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`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 6
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 6
`
`

`

`US. Patent
`
`Aug. 16, 2005
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`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 7
`
`

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`US. Patent
`
`Aug. 16, 2005
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`US 6,931,084 B1
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`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 8
`
`

`

`US. Patent
`
`Aug. 16, 2005
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`Sheet 7 0f 7
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`US 6,931,084 B1
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`

`US 6,931,084 Bl
`
`1
`DIFFERENTIAL CODING AND CARRIER
`RECOVERY FOR MULTICARRIER
`SYSTEMS
`
`FIELD OF THE INVENTION
`
`The present invention relates to methods and apparatus
`for performing modulation and de-modulation in multi-
`carrier modulation systems (MCM systems) and. in particu-
`lar, to methods and apparatus for differential mapping and
`etc-mapping of information onto carriers of multi-carrier
`modulation symbols in such systems. Furthermore.
`the
`present
`invention relates to methods and apparatus for
`performing an echo phase offset correction when decoding
`information encoded onto carriers of multi-carrier modula-
`
`tion symbols in multi-carrier modulation systems.
`
`BACKGROUND OF THE INVENTION
`
`The present invention generally relates to broadcasting of
`digital data to mobile receivers over time-variant multipath
`channels. More specifically. the present invention is particu-
`larly useful
`in multipath environments with low channel
`coherence time, i.e. rapidly changing channels. In preferred
`embodiments. the present invention can be applied to sys-
`tems
`implementing a multicarrier modulation scheme.
`Multi-carrier modulation (MCM) is also known as orthogo-
`nal frequency division multiplexing (OFDM ).
`in a MCM transmission system binary information is
`represented in the form ofa complex spectrum. Le. a distinct
`number of complex subcarricr 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.
`FIG. 7 shows a MCM system overview. At 100 a MCM
`transmitter is shown. A description of such a MCM trans-
`mitter 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
`applied to a bit~carrier mapper 106 which produces a
`sequence of spectra 108 from the incoming serial bitstream
`104. An inverse fast Fourier transform (HT) 110 is per-
`formed on the sequence of spectra 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 (15]) caused by
`multipath distortion. in unit 11-1 is provided for inserting a
`guard interval of fixed 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 guard interval by placing same in front
`of the useful symbol. The resulting MCM symbol is shown
`at 115 in FIG. 7.
`
`for each
`A unit 116 for adding a reference symbol
`predetermined number of MCM symbols is provided in
`order to produce a MCM signal having a frame structure.
`Using this frame structure comprising useful symbols. guard
`intervals and reference symbols it is possible to recover the
`useful information from the MCM signal at the receiver side.
`The resulting MCM signal having the structure shown at
`118 in FIG. 7 is applied to the transmitter front end 120.
`Roughly speaking, at the transmitter front end 120, a digital;r
`
`2
`
`analog conversion and an up-converting ofthe MCM signal
`is performed. Thereafter,
`the MGM signal
`is transmitted
`through a channel 122.
`Following, the mode of operation of a MCM receiver 130
`is shortly described referring to FIG. 7. The MCM signal is
`received at the receiver front end 132. In the receiver front
`end 132. the MCM signal is clown-converted and. further-
`more. a digitallanalog conversion of thc down-convened
`signal is performed. The down-converted MCM signal is
`provided to a frame synchronization unit 134. The frame
`synchronization unit 134 determines the location of the
`reference symbol in the MGM symbol. Based on the deter-
`mination of the frame synchronization unit [.34. a reference
`symbol extracting unit 136 extracts the framing information.
`i.c. 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 so far 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 of spectra from the useful symbols.
`Thereafter, the sequence of spectra is provided to a carrier-
`bit mapper 142 in which the serial hitstream is recovered.
`This serial bitstream is provided to a data sink 144.
`As it is clear from FIG. '7, every MCM transmitter 1w
`must contain a device which performs mapping of the
`transmitted bitstreams onto the amplitudes andlor phases of
`the sub-carriers. In addition. at
`the MGM receiver 130. a
`device is needed for the inverse operation, Le. retrieval of
`the transmitted bitstream from the amplitudes andlor phases
`of the sub-carriers.
`
`For a better understanding of MCM mapping schemes. it
`is preferable to think of the mapping as being the assignment
`of one ore more hits to one or more sub-carrier symbols in
`the time-frequency plane. In the following. the term symbol
`or signal point
`is used for the complex number which
`represean the amplitude andjor phase modulation of a
`subearrier in the equivalent haschand. Whenever all com-
`plex numbers representing all subcarricr symbols are des-
`ignated, the term MCM symbol is used.
`
`DESCRIPTION OF PRIOR ART
`
`In principle. two methods for mapping the hitstrcam into
`the time-frequency plane are used in the prior art:
`A first method is a differential mapping along the time
`axis. When using difl’erentiai mapping along the time axis
`one or more hits are encoded into phase andror amplitude
`shifts. between two subcarriers of the same center frequency
`in adjacent MCM symbols. Such an encoding scheme is
`shown in FIG. 8. The arrows depicted between the sub-
`carricr symbols conespond to information encoded in ampli-
`tude andlor phase shifts between two subcarrier symbols.
`A system applying such a mapping scheme is defined in
`the European 'l‘elecommunication standard ETS 300 40}
`(EUl47-DAB). A system compliant to this standard uses
`Differential Quadrature Phase Shift Keying (DQPSK) to
`encode every two bits into a 0. 90. 180 or 270 degrees phase
`difference between two subcarriers of the same center fre-
`
`quency which are located in MCM symbols adjacent in time.
`A second method for mapping the bitstream into the
`time-frequency plane is a non-differential mapping. When
`using non-dilIerential mapping the information carried on a
`sub-carrier is independent of information transmitted on any
`other subcarricr. and the other subcarricr may differ either in
`
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`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 10
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 10
`
`

`

`US 6,931,084 Bl
`
`3
`i.e.
`the same MCM symbol, or in time,
`i.e.
`frequency,
`adjacent MCM symbols. Asystem applying such a mapping
`scheme is defined in the European Telecommunication stan-
`dard ETS 300 744 (DVD-"D. A system compliant to this
`standard uses 4. 16 or 64 Quadrature Amplitude Modulation
`(0AM) to assign bits to the amplitude and phase of a
`subc'arricr.
`The quality with which transmitted multi~carrier modu-
`lated signals. can be recovered at the receiver depends on the
`properties of the channel. The most
`interesting property
`when transmitting MCM signals is the time interval at which
`a mobile channel changes its characteristic. considerably.
`The channel coherence time 'I‘r is normally used to deter-
`mine the time interv'al at which a mobile channel changes its
`characteristics considerably. 'I'C depends on the maximum
`Doppler shift as follows:
`Limp-‘1’”... :l’fcanlfl‘lr
`with
`v: speed of the mobile receiver in [mr’s]
`fcarrier: carrier frequency of the RF signal [Hz]
`c: speed of light (3-10“ ms)
`The channel coherence time Tc is often defined to be
`
`(Fit I}
`
`[Br]. 3]
`
`it becomes clear from the existence of more than one
`definition. that the channel coherence time 'I'r is merely a
`rule-of—thumb value for the stationarity of the channel. As
`explained above. the prior art time-axis differential mapping
`requires that the mobile channel be quasi stationary during
`several MGM symbols periods, i.e. required channel coher-
`ence time 'l'c>>M(‘.M symbol period. The prior art non-
`differential MCM mapping only requires that
`the mobile
`channel be quasi stationary during one symbol interval, i.e.
`required channel coherence time>MCM symbol period.
`Thus. both prior art mapping schemes have specific
`disadvantages. For differential mapping into time axis direc-
`tion the channel must be quasi stationary. i.c. the channel
`must not change during the transmission of two MCM
`symbols adjacent in time. lfthis requirement is not met. the
`channel
`induced phase and amplitude changes between
`MCM symbols will yield an increase in bit error rate.
`With non-differential mapping exact knowledge of the
`phase of each subcarrier is needed (Le. coherent reception).
`For multipath channels, coherent reception can only be
`obtained if the channel impulse response is known. There-
`fore. a channel estimation has to be part of the receiver
`algorithm. The channel estimation usually needs additional
`sequences in the transmitted waveform which do not carry
`information. in case of rapidly changing channels, which
`necessitate update of the channel estimation at short inter-
`vals,
`the additional overhead can quickly lead to instilli-
`ciency of non-diflerential mapping.
`P.
`ll. Moose: “Differentially Coded Multi-l-‘requency
`Modulation for Digital Communications", SIGNAL PRO-
`CESSING THEORIES AND APPLICATIONS. 18.—21.
`September 1990, pages
`1807—1810, Amsterdam, NL,
`teaches a differentially coded multi-frequency modulation
`for digital communications. A meld-frequency differential
`modulation is described in which symbols are differentially
`encoded within each baud between adjacent tones. At
`the
`receiver. following a digital Fourier transform (DPT). the
`complex product between the DFT coeflicient of digital
`
`4
`
`frequency k and the complex conjugate of the DFT coeffi-
`cient of digital frequency k—l
`is formed. Thereafter, the
`result
`is multiplied by appropriate terms such that
`the
`differentially encoded phase bits are realigned to the original
`constellations. Thus. the constellation following the dilfer-
`ential decoding must correspond to the original constella-
`tron.
`
`SUMMARY OF THE INVENTION
`
`It is an object of the present invention to provide methods
`and devices for performing an echo phase offset correction
`in a multi-carrier demodulation system.
`In accordance with a first aspect, the present invention
`provides a method of performing an echo phase offset
`correction in a multi-carrier demodulation system. compris-
`ing the steps of:
`differential phase decoding phase shifts based on a phase
`diflcmncc between simultaneous carriers having diaer-
`ent frequencies;
`determining an echo phase olfset for each decoded phase
`shift by eliminating phase shift uncertainties related to
`the transmitted information from the decoded phase
`shift;
`averaging the echo phase offsets in order to generate an
`averaged offset; and
`correcting each decoded phase shift based on the averaged
`ofllset.
`
`In accordance with a second aspect. the present invention
`provides a method of performing an echo phase olfsut
`correction in a multi-carrier demodulation system. compris»
`ing the steps of:
`dilferential phase decoding phase shifts based on a phase
`dilferenee between simultaneous carriers having differ»
`crtl frequencies. the phase shifts defining signal points
`in a complex plane;
`pro-rotating the signal points into the sector of the com-
`plex plane between —45° and +45“;
`determining parameters of a straight line approximating
`the location of the pre-rotated signal points in the
`complex plane;
`determining a phase offset based on the parameters: and
`correcting each decoded phase shift based on the phase
`oflset.
`
`In accordance with a third aspect. the present invention
`provides an echo phase otl‘sct correction device for a multi-
`carrier demodulation system. comprising:
`a differential phase decoder for decoding phase shifts
`based on a phase difference between simultaneous
`carriers having dilferent frequencies;
`means for determining an echo phase offset for each
`decoded phase shift by eliminating phase shift uncer-
`tainties related to the transmitted information from the
`decoded phase shift;
`means for averaging the echo phase offsets in order to
`generate an averaged offset; and
`means for correcting each decoded phase shift based on
`the averaged ofl'set.
`In accordance with a fourth aspect, the prese nt invention
`provides an echo phase offset correction device for a multi-
`carrier demodulation system, comprising:
`a differential phase decoder for decoding phase shifts
`based on a phase difference between simultaneous
`carriers having tlilfercnt frequencies, the phase shifts
`defining signal points in a complex plane;
`means for pre-rotating the signal points into the sector of
`the complex plane between -45° and +45°;
`
`t;
`
`10
`
`30
`
`30
`
`35
`
`4|)
`
`45
`
`5|]
`
`'Jt‘J'I
`
`an
`
`65
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 11
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 11
`
`

`

`US 6,931,084 Bl
`
`5
`line
`means for determining parameters of a straight
`approximating the location of the pre-rotated signal
`points in the complex plane;
`means for determining a phase offset based on the param-
`eters; and
`means for correcting each decoded phase shift based on
`the phase offset.
`The present invention provides methods and devices for
`performing an echo phase offset correction. suitable for
`multi-carrier (OFDM) digital broadcasting over rapidly
`changing multipath channels. comprising differential encod-
`ing ofthe data along the frequency axis such that there is no
`need for channel stationarity exceeding one multicarrier
`symbol.
`When using the mapping process along the frequency axis
`it is preferred to make use of a receiver algorithm that will
`correct symbol phase offsets that can be caused by channel
`echmas.
`
`The mapping scheme along the frequency axis for multi-
`carrier modulation renders the transmission to a certain
`extent independent of rapid changes in the multipath channel
`without introducing a large overhead to support channel
`estimation. Especially systems with high carrier frequencies
`andfor high speeds of the mobile carrying the receiving unit
`can benefit from such a mapping scheme.
`Thus,
`the mapping scheme of a differential encoding
`along the frequency axis does not exhibit the two problems
`of the prior art systems described above. The mapping
`scheme is robust with regard to rapidly changing multipath
`channels which may occur at high frequencies andior high
`speeds of mobile receivers.
`The controlled respective parameters of the subcarriers
`are the phases thereof, such that the information is differ-
`entially phase encoded.
`In accordance with the mapping described above, map-
`ping is also differential, however, not into time axis direction
`but into frequency axis direction. Thus, the information is
`not contained in the phase shift between subcarriers adjacent
`in time but in the phase shift between subcarriers adjacent in
`frequency. Differential mapping along the frequency axis
`has two advantages when compared to prior art mapping
`schemes.
`
`Because of differential mapping, no estimation of the
`absolute phase of the subcarriers is required. Therefore,
`channel estimation and the related overhead are not neces-
`sary. By choosing the frequency axis as direction for differ-
`entially encoding the information hitstream, the requirement
`that the channel must be stationary during several MCM
`symbols can be dropped. The channel only has to remain
`unchanged during the current MCM symbol period. There-
`fore, like for non-differential mapping it holds that
`
`required channel coherence timeéMCM symbol period.
`The present invention provides methods and apparatus for
`correction ofphase distortions that can be caused by channel
`echoes. As described above, differential mapping into fre-
`quency axis direction solves problems related to the station-
`arity of the channel. However, differential mapping into
`frequency axis direction may create a new problem. In
`multipath environments, path echoes succeeding or preced-
`ing the main path can lead to systematic phase offsets
`between sub-carriers in the same MCM symbol.
`ln this
`context, the main path is thought ol'heing the path echo with
`the highest energy content. The main path echo will deter-
`mine the position of the FFI‘ window in the receiver of an
`MCM system.
`
`6
`According to the premnt invention, the information will
`be contained in a phase shift between adjacent subca rriers of
`the same MCM symbol. If not corrected for. the path echo
`induced phase offset between two subcarriers can lead to an
`increase in bit error rate. Therefore. application of the MCM
`mapping scheme presented in this invention will preferably
`be used in combination with a correction of the systematic
`subcarrier phase offsets in case of a multipath channel.
`The introduced phase offset can be explained from the
`shifting property of the Discrete Fourier Transform (DPT):
`
`t."
`
`if)
`
`3:
`air
`xlttu —mti,| H xii-p- M h“
`with
`
`IE]. 3}
`
`x|n|zsantpled time donnin signal [U 5:: s N— II
`
`Xlkl: DI'T transformed frequency domain signal
`
`in s k s N — 11
`
`N: length of BF?
`
`t...t~:eyelic shift of the OFT window in the time
`
`In: length of Uni—Shift. in the time domain
`
`30
`
`35
`
`4|)
`
`45
`
`5|]
`
`'JI J:
`‘
`
`an
`
`65
`
`in a multipath channel, echoes
`Equation 3 shows. that
`following the main path will yield a subcarrier dependent
`phase oll'set. After differential demapping in the frequency
`axis direction at the receiver, a phase offset between two
`neighboring symbols remains. Because the channel induced
`phase offsets between differentially demodulated symbols
`are systematic errors, they can be corrected by an algorithm.
`In the context of the following specification, algorithms
`which help correcting the phase shift are called Echo Phase
`offset correction (EPOC) algorithms. Two such algorithms
`are described as preferred embodiments for the correction of
`phase distortions that can be caused by channel echoes.
`These algorithms yield a sufficient detection security for
`MCM frequency axis mapping even in channels with echoes
`close to the limits of the guard interval.
`In principle, an EPOC‘ algorithm must calculate the echo
`induced phase offset from the signal space constellation
`following the differential demodulation and subsequently
`correct this phase ofi‘set.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In the following, preferred embodiments of the present
`invention will be explained in detail on the basis of the
`drawings enclosed. in which:
`FIG. 1 shows a Schematic view representing a mapping
`scheme used according to the invention;
`FIG. 2 shows a functional block diagram of an embodi-
`ment of a mapping device;
`FIGS. 3A and 3B show scatter diagrams of the output of
`an differential de-mapper of a MCM receiver for illustrating
`the effect of an echo phase offset correction;
`FIG. 4 shows a schematic block diagram for illustrating
`the position and the functionality of an echo phase offset
`correction unit;
`
`FIG, 5 shows a schematic block diagram of an embodi-
`ment of an echo phase offset correction device according to
`the present invention;
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 12
`
`Petitioner Sirius XM Radio Inc. - Ex. 1001, p. 12
`
`

`

`US 6,931,084 Bl
`
`8
`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 Hits
`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 used to prevent that differential detection
`phase errorssn'taller than 135 degreescaosc double bit errors
`at the receiver.
`
`Differential phase encoding of the K phases. is performed
`in a differential phase encoder 222. At this stage of process-
`ing.
`the K phases phi[k] generated by the OPSK Gray
`mapper are differentially encoded. ln principal. a feedback
`loop 224 calculates a cumulative sum over all K phases. As
`starting point for the first computation (k=0) the phase ofthe
`reference carrier 226 is used. A switch 228 is provided in
`order to provide either the absolute phase of the reference
`subcarricr 226 or the phase information encoded onto the
`preceding (i.e. 2'1, where 2" denotes the unit delay opera-
`tor) subcarrier to a summing point 230. At the output of the
`differential phase encoder 222, the phase information theta
`[k] with which the respective subcarriers are to be encoded
`is provided. In preferred embodiments of the present inven-
`tion. the subcarricrs of a MCM symbol are equally spaced in
`the frequency axis direction.
`The output of the differential phase encoder 222 is con-
`nected to a unit 232 for generating complex subcarrier
`symbols using the phase information theta[k]. To this end.
`the K differentially encoded phases are convened to com-
`plex symbols by multiplication with
`£82112)! 0 e1 ‘ I." {It ' [much] MPH-O]
`
`(En-4)
`
`t."
`
`10
`
`30
`
`7
`FIG. 6 shows schematic views for illustrating a projection
`performed by another embodiment of an echo phase ofl‘set
`correction device according to the present invention;
`FIG. 7 shows a schematic block diagram of a generic
`multi-carrier modulation system; and
`FIG. 8 shows a schematic view representing a prior art
`dificrcrttial mapping scheme.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`[n a preferred embodiment thereof, the present invention
`is applied to a MCM system as shown in FIG. 7. With
`respect to this MCM system, the present invention relates to
`the bit-carrier mapper 106 of the MCM transmitter 100 and
`the carrier-bit mapper 142 of the MCM receiver [30, which
`are depicted with a shaded background in FIG. '7.
`An preferred embodiment of an inventive mapping
`scheme used by the bit-carrier mapper 106 is depicted in
`FIG. 1. A number of MCM symbols 200 is shown in FIG. 1.
`Each MCM symbol 200 comprises a number of sub-carrier
`symbol5202. The arrow5204 in FIG. 1 illustrate information
`encoded between two sub-carrier symbols 202. As can be
`seen from the arrows 204, the bit-carrier mapper 106 uses a
`differential mapping within one MCM symbol along the
`frequency axis direction.
`In the embodiment shown in FIG. 1, the first sub-carrier
`(k=(l)
`in an MC‘M symbol 200 is used as a reference
`sub-carrier 206 (shaded) such that information is encoded
`beIWeen the reference sub-carrier and the first active carrier
`208. The other information of a MCM symbol 21])
`is
`encoded between active carriers, respectively.
`Thus, for every MCM symbol an absolute phase reference
`exists.
`In accordance with FIG.
`I,
`this absolute phase
`reference is supplied by a reference symbol inscned into
`every MCM symbol (k=0). The reference symbol can either
`have a constant phase for all MCM symbols or a phase that
`varies from MCM symbol to MCM symbol. Avarying phase
`can be obtained by replicating the phase from the last
`subcarrier of the MGM symbol preceding in time.
`In FIG. 2 a preferred embodiment of a device for pct“-
`forming a differential mapping along the frequency axis is
`shown. Referring to FIG. 2, assembly of MCM symbols in
`the frequency domain using differential mapping along the
`frequency axis according to the present
`invention is
`described.
`FIG. 2 shows the assembly of one MCM symbol with the
`following parameters:
`NFFT designates the number of complex coefficients of
`the discrete Fourier transform, number of subcarriers respec-
`tively.
`K designates the number of active carriers. The reference
`carrier is not included in the count for K.
`According to FIG. 2, a quadrature phase shift keying
`(QPSK) is used for mapping the bitstream onto the complex
`symbols. However, other M-ary mapping schemes (MI’SK)
`like 2-PSK, 8»PSK, lo-QAM, lb-APSK, 64-APSK etc. are
`possible.
`Furthermore, for ease of filtering and minimization of
`aliasing effects some subcarriers are not used for encoding
`information in the device shown in FIG. 2. 'lhese subcarri-
`
`ers, which are set to zero, constitute the so-called guard
`bands on the upper and lower edges of the MGM signal
`spectrum.
`At the input of the mapping device shown in FIG. 2,
`complex signal pairs b0[k], bllk] of an input bitstream are
`received. K complex signal pairs are assembled in order to
`
`35
`
`40
`
`wherein factor designates a scale factor and PHI designates
`an additional angle. The scale factor and the additional angle
`I’III are optional. By choosing PIII=45° a rotated DQI’SK
`signal constellation can be obtained.
`Finally, assembly of a MCM symbol is effected in an
`assembling unit 234. One MCM symbol comprising Np”
`subcat‘ricrs is assembled from NIL-NeK—l guard band sym-
`bols which are “rero”, one reference subcarricr symbol and
`K DQl’SK subcart'ior symbols. Thus. the assembled MCM
`symbol 20