UNOUETA
`
`US005345440A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,345,440
`Sep. 6, 1994
`
`OTHER PUBLICATIONS
`
`Hirosaki et.al., “Advanced Groupband Data Modem
`Using Orthogonally Multiplexed QAM technique,”
`Jun. 1986, pp. 587-592, IEEE Transactions on Commu-
`nications, vol. 34, No. 6.
`Le Flochetal., “Digital Sound Broadcasting to Mobile
`Receivers,” Aug., 1989, pp. 493-503, IEEE Transac-
`tions on Consumer Electronics, vol. 35, No.3.
`:
`
`Primary Examiner—Douglas W. Olms
`Assistant Examiner—Melvin Marcelo
`Attorney, Agent, or Firm—Watson Cole Grindle &
`Watson
`
`ABSTRACT
`[57]
`In an OFDM< signal
`individual OFDM carriers are
`modulated by samples of signals which can only take a
`limited range of allowed values and a block of samples
`modulates the group ofcarriers during a time period T.
`In a receiver,
`the signal modulating an individual
`OFDM carrier may be demodulated by estimating the
`position of reference axes serving as the frameofrefer-
`ence against which the allowed modulating values are
`defined, and multiplying the value of the demodulated
`samples by the complex conjugate of a point on one of
`the estimated reference axes. The spread of the groups
`of samples demodulated from each OFDMcarrier gives
`an indication of whether the receiver is synchronised to
`the block boundaries. Preferably spread of the complex
`samples is evaluated in the radial direction only and
`used to steer the sample clock frequency. Phase drift of
`the demodulated samples from one block to the next
`indicates the degree of local oscillator frequencyerror.
`Preferably phase drift is assessed by multiplying com-
`plex values by the complex conjugate of an earlier sam-
`ple demodulated from the same OFDM carrier and
`using the resulting measure to steer the local oscillator
`frequency via a frequency locked loop.
`
`17 Claims, 11 Drawing Sheets
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 1
`
`United States Patent 9
`Gledhill et al.
`
`[54] RECEPTION OF ORTHOGONAL
`FREQUENCY DIVISION MULTIPLEXED
`SIGNALS
`
`[75]
`
`Inventors:
`
`Jeffrey J. Gledhill, Chandlers Ford;
`Santosh V. Anikhindi, Southampton;
`Peter A. Avon, Peterfield, all of
`England
`
`(73] Assignee:
`
`National Transcommunications
`Limited, Winchester, England
`
`[21] Appl. No.:
`
`934,653
`
`[22] PCT Filed:
`
`Sep. 13, 1991
`
`[86] PCT No::
`
`PCT/GB91/01571
`
`§ 371 Date:
`
`Jan. 7, 1993
`
`§ 102(e) Date:
`
`Jan. 7, 1993
`
`[87] PCT Pub. No.: W0O92/05646
`
`PCT Pub. Date: Apr. 2, 1992
`
`Foreign Application Priority Data
`[30]
`Sep. 14, 1990 [GB] United Kingdom ..-scssessesss... 9020170
`
`Umt. CLS vaecccsssssssssseeeeesese HOST 1/00; HO4J 11/00
`[51]
`SZ} WER MS scessccessicesencasvsionissensusss 370/19; 370/23;
`370/69.1; 375/120
`. 370/19, 20, 21, 23,
`[58] Field of Search .
`370/69.1, 121,122;375/119, 120, 83; 364/725,
`726
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,884,139 11/1989 Pommier ....
`5,166,924 11/1992 Moose .....
`5,197,061
`3/1993 Halbert-Lassalle e'
`
`we 358/142
`+ 370/32.1
`« 370/19 X
`
`FOREIGN PATENT DOCUMENTS
`
`6/1989 European Pat. Off.
`0321021
`61-129936 10/1986 Japan .
`
`.
`
`|||||!|II
`
`BASEBAND
`
`SIGNAL
`
`
`
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 1
`
`

`

`_U.S. Patent
`
`Sep. 6, 1994
`
`Sheet 1 of 11
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`5,345,440
`
`AoNannaesCLYVsoiVl9)/J
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 2
`
`Gl914d33S
`
`WNYLIFdSWOI0
`
`t:Glold
`
`(LYVYOIdd)
`
`JONLINIVW
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 2
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`

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`USS. Patent
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`Sep. 6, 1994
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`Sheet 2 of 11
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`5,345,440
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`E.G. INITIAL STATE=+1+/
`
`TRANSITION
`
`FINAL STATE
`
`Fig.ZA
`
`(PRIOR ART)
`
`IMAGINARY
`
`REAL
`
`Fig. 2C
`
`(PRIOR ART)
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 3
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 3
`
`

`

`U.S. Patent
`
`Sep. 6, 1994
`
`Sheet 3 of 11
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`5,345,440
`
`Ww
`
`WO)
`Gjerornror coer oer crore Rw
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`Qicoooooocoe er er eee e Sy275 tt
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`stoke
`
`CLOCK
`
`Fig.2B (PRIORART)
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 4
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 4
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`

`

`U.S. Patent
`
`Sep. 6, 1994
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`Sheet 4 of 11
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`5,345,440
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`Ly|AININOFIASjYIN
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`gebly
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`614
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`9¢bly
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`
`b-*AdOI{|
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 5
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 5
`
`

`

`U.S. Patent
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`Sep. 6, 1994
`
`Sheet 5 of 11
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`5
`
`345,440
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`9
`
`+ i ! | I 1 ! ! | ! \ { J
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`a ee oe oe ee ee eee
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`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 6
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 6
`
`
`
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`

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`U.S. Patent
`
`Sep. 6, 1994
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`Sheet 6 of 11
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`5,345,440
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`IMAGINARY
`
`\
`
`7
`
`REFERENCE AXES
`
`REAL
`
`:
`
`‘Fig. 5(a)
`
`\
`
`SS" "|
`
`REFERENCE AXES
`
`—=
`
`REAL
`
`8:
`
`‘Fig.5(b)
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 7
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 7
`
`

`

`U.S. Patent
`
`Sep. 6, 1994
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`Sheet 7 of 11
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`5,345,440
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`0
`
`CRUDE ESTIMATE OFREF AXIS
`
` /
`
`Fig. 5C
`
`7
`
`P(@)
`
`Fig.7
`
`REFERENCE AXIS
`
`A.
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 8
`
`/
`
`
`NEW REF AXIS
`rl
`DEMODULATED
`DATA= 10
`LY o -~op REF AXIS
`“a “a+jb
`
`-
`
`INPUT SAMPLE. ~~
`x+y gt
`”
`o~
`
`STEP 1
`
`7
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 8
`
`

`

`U.S. Patent
`
`Sep. 6, 1994
`
`Sheet 8 of 11
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`5,345,440
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`RATE OF CARRIER ROTATION
`
`CLOCK FREQUENCY ERROR
`IS PROPORTIONAL TO
`
`CARRIER FREQUENCY
`
`Fig.6(a)
`
`THE SLOPE OF THIS LINE
`CARRIER FREQUENCY
`
`RATE OF CARRIER ROTATION
`
`INTERCEPT
`PROPORTIONAL
`TO ERROR IN
`UHF CARRIER
`
`Fig. 6(b)
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 9
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 9
`
`

`

`U.S. Patent
`
`Sep. 6, 1994
`
`Sheet 9 of 11
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`5,345,440
`
`wW.
`
`
`
`QNIGOIS0YOSINIWAFIAWVS
`
`OL
`
`BOFJEVMLIOSTEE
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`OVIddS
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`
`NOILVONTWAI
`
`ONVEsISVE
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`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 10
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 10
`
`
`
`
`

`

`U.S. Patent
`
`Sep. 6, 1994
`
`Sheet 10 of 11
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`5,345,440
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`Fig.9
`
`D200
`
`s
`
`
`
`BLOCKOFFSET
`
`(SAMPLES)
`
`Ree
`
`NAN
`=2eS88
`RRR
`OGM st
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 11
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 11
`
`

`

`U.S. Patent
`
`Sep. 6, 1994
`
`Sheet 11 of 11
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`5,345,440
`
`SOFT WARE
`
`
`
`FREQUENCY
`SENSITIVE
`DETECTOR
`
`
`
`
`
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 12
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 12
`
`

`

`1
`
`5,345,440
`
`RECEPTION OF ORTHOGONAL FREQUENCY
`DIVISION MULTIPLEXED SIGNALS
`
`BACKGROUND OF THE INVENTION
`
`20
`
`.
`
`35
`
`1. Field of the Invention
`The present inventionrelates to the field of reception
`of orthogonal frequency division multiplexed (OFDM)
`signals. More particularly, the invention concerns de- j¢
`modulation of received OFDMsignals and synchroni-
`sation at an OFDM receiver.
`2. Description of The Prior Art
`Orthogonal
`frequency division multiplexing is a
`method oftransmitting data which is being investigated, 15
`because of its good interference properties, for use in
`the UHF band.
`In the discussion that follows it is assumed that an
`inverse Discrete Fourier Transformation is used to gen-
`erate the OFDM signal.
`In general it will be necessary to modulate a gener-
`ated OFDMsignal onto a carrier so as to reach an
`appropriate frequency for transmission. By adapting the
`processing which is performedin the frequency domain
`it becomes possible to simplify the modulation of the
`OFDMsignal onto a carrier. This is described in our
`co-pending
`International
`patent
`application No.
`PCT/GB91/00513.
`In order to recover data from a received OFDM
`signal which has been modulated onto a carrier it is
`necessary to demodulate the OFDM signal from the
`carrier onto whichit is modulated before demodulating
`data from the individual OFDM carriers. Embodiments
`of the present invention may deal with one or both of
`these types of demodulation.
`As with more conventional methodsofdata transmis-
`sion, with OFDMsignalsit is necessary to synchronise
`the receiver to the transmission before useful data can
`be recovered. The normal way of achieving this is by
`using special framing signals, however this represents
`an overhead on the available bit rate and may compro-
`mise the interference properties of the signal.
`It has also been proposed, in the article “Digital Im-
`plementation of High Speed HF Modems” by D.
`Harmerand B. Hillam, to synchronise a receiver to the
`block start positions in a received OFDM signal by
`integrating each received modulated OFDM carrier
`and, because there should be an integer number ofcy-
`cles occurring during one block, a non-zero value for
`the integral will indicate block misalignment.
`Embodiments of the present invention may deal with
`any orall of block synchronisation, sample clock syn-
`chronisation and, where relevant, local oscillator syn-
`chronisation, without the need to use special framing
`signals, by looking at the distribution of the demodu-
`lated sample values.
`As shown in FIG.1, an orthogonal frequency divi-
`sion multiplexed (OFDM)signal consists of a large
`number of carriers each of which is modulated by a
`signal whose level varies discretely rather than continu-
`ously and thus the resulting power spectrum of each
`carrier follows a (Sin x/x)* distribution. The symbol
`rate of the modulating signals, and the carrier frequen-
`cies, are such that the peak of power of each modulated
`carrier occurs at frequencies corresponding to nulls in
`the power spectrum of the other modulated carriers.
`The carrier spacing is equal to the reciprocal of the
`
`60
`
`6:A
`
`2
`symbolrate of each modulatingsignal (assumingthatall
`of the modulating signals have the same symbolrate).
`The overall spectrum of the OFDMsignal is very
`close to rectangular when a large number ofcarriers are
`contained in the OFDMsignal.
`During a given time period, T, the OFDMsignal may
`be represented by a block of N samples. The value of
`the kth sample is, as follows:
`
`ath) = NE!x(nye2mkin
`n=o
`
`The N values X(n) represent the respective values,
`during period T,ofthe discretely-varying signals modu-
`lating the OFDMcarriers e2/7k/N,
`It may be seen from the above equation that the
`OFDMsignal corresponds to the inverse Discrete Fou-
`rier Transform of a set of data samples, X(n). Thus, a
`stream of data may be converted into an OFDMsignal
`by splitting the data stream up into blocks of N samples
`X(n) and subjecting each block of data samples to an
`inverse Discrete Fourier Transform.
`The succession of data samples, X(n), which appear
`at a particular sample position over time constitute a
`discretely-varying signal which modulates a carrier at a
`frequency,fr.
`SUMMARY OF THE INVENTION
`
`According to the present inventionit is preferred to
`have only a restricted set of values which the samples
`X(n) may take, the set of values representing a set of
`phase states and amplitudes to be imparted to carriers,
`fn. In particularly preferred embodiments of the inven-
`tion the set of values to which the samples X(n) are
`restricted comprises values +-1+j, +1—j, —1+j, and
`—1—j. This set of values corresponds to four allowable
`equally spaced phase states for the modulated carriers
`f,, with the same amplitude. Thus, the modulation of
`each carrier, f,, in these embodiments amounts to quad-
`rature phase shift keying (QPSK). QPSK has the advan-
`tage of simplicity and good performance. Further ad-
`vantages may be gained by differentially coding the
`data (this avoids the need for carrier references). An
`OFDMsignal produced in this way will also tolerate
`non-phase-equalised channels much better than would
`conventional signals.
`Since the data samples contained in a stream of data
`to be transmitted will not necessarily be restricted to
`taking one of four possible values it is necessary to use
`an indirect process to code the input data into the four
`allowed sample values +1+j. One wayofdoingthis is
`to first convert the input data into a binary bitstream
`and then to code each 2-bit portion ofthe bitstream into
`oneof the four allowed values. Thus when converting a
`digital datastream into an OFDMsignal, in which each
`OFDMcarrier is QPSK modulated as described above,
`the datastream may be broken up into blocks 2N bits
`long and then each group of 2 bits may be coded into
`one of the four values +1-+j. The resulting array of
`complex values for one block is then subjected to an
`inverse Discrete Fourier Transform so as to produce
`the OFDMsignal.
`A consequence of using QPSK modulation of the
`carriers in the OFDMsignal is that only twobits ofdata
`are “modulated onto”each carrier per inverse Discrete
`Fourier Transform. Without increasing the number of
`Fourier transformations this bit rate can be increased by
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 13
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 13
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`

`

`5,345,440
`
`3
`increasing the number of allowed phase states and/or
`allowed amplitudes of the modulated carrier. For exam-
`ple, alternative methods of modulating the individual
`OFDMcarriers, such as 8-PSK or 16 QAM could be
`used. However, the greater the number of allowed
`phase states the more sensitive the OFDMsignal is to
`noise and to channeldistortions. Also, if when produc-
`ing an OFDMsignal portion representing a single block
`of data different carriers can have different sizes of
`amplitude, then the overall spectrum of that OFDM
`signal portion may not be rectangular.
`OFDM has several important advantages over more
`conventional modulation techniques. Among these are:
`1) Comprehensive immunity from multipath inter-
`ference—this arises because the symbol time is long
`compared to the time between typical echoes.
`2) Efficient
`spectrum usage—The
`rectangular
`OFDMspectrum can very nearly fill a channel. This
`allows it to achieve close to the theoretical maximum
`data rate, e.g. 2 bits/second/Hz for QPSK. Conven-
`tional digital modulation schemes require wider band-
`widths because of practical difficulties in implementing
`suitablefilters.
`3) Good interference properties—It is possible to
`modify the OFDM spectrum to take account of the
`distribution of power in the spectrum of an interfering
`signal. As described in corresponding International
`patent application PCT/GB91/00513,
`in the case of
`PAL television signals most of the power is concen-
`trated around the vision and soundcarriers. When using
`OFDMsignals for TV near existing channels, setting
`the OFDMcarriers to zero near these vision and sound
`carriers frequencies results in a spectrum with rectangu-
`lar holes and a dramatic improvementin bit error rate
`caused by PAL co-channel interference.It is also possi-
`ble to reduce adjacent channel interference by not using
`carriers near to the channel edges.
`In one aspect the present invention provides methods
`and apparatus for demodulating data modulated onto
`OFDM carriers via sample values taking one of four
`possible phase values, the demodulation using a Dis-
`crete Fourier Transformation or an inverse Discrete
`Fourier Transformation to recover sample values, a
`complex multiplication and a decoding ofthe sign ofthe
`real and imaginary parts of the resultant signal into first
`and second data bits respectively.
`When demodulating data from an OFDM signal in
`which the OFDM carriers are modulated by sample
`values which are restricted to taking one of four phases
`(regardless of whether ornot they are also restricted in
`amplitude) the sample values output from the DFT,ina
`complex vector representation, appear as four group of
`points spaced roughly 90° from one another and 45°
`from a set of reference axes derived from the received
`signal. To demodulate an individual sample value it is
`necessary to determine in which of four quadrants
`formed by the reference axes the sample lies—this re-
`quires comparing the phase angles of the sample with
`those ofthe reference axes. Although in theory it would
`be possible to recover the phase angle information by
`making use of the fact that the ratio of the imaginary
`component to the real component of the sample value
`gives the inverse tangentof the phase angle, this would
`mean that the sample demodulation process would in-
`volve the performanceofa division followed by refer-
`ence to a look-up table. Division is difficult to imple-
`mentin digital signal processing apparatus and would
`require a large amountof processing power.
`
`10
`
`30
`
`40
`
`60
`
`It is possible to reduce the amount of processing
`required during demodulation of data relating to one
`OFDM carrier by multiplying the recovered sample
`values for that carrier by the complex conjugate of a
`point appearing on the reference axes; the quadrant into
`which the resultant signal falls determines two bits of
`decoded data (whether or not further bits of data are
`obtainable from a consideration of the amplitude of the
`sample value).
`invention provides
`the present
`In another aspect
`methods and apparatus for synchronising a receiver to a
`received OFDMsignal.
`Where successive portions of the OFDM signal rep-
`resent respective successive blocks of data modulating
`the OFDMcarriers, an advantageous methodfor syn-
`chronising the receiver to the block boundaries in the
`signal involves evaluating the spread in distribution of
`sample values which are recovered at the receiver in
`respect ofeach OFDMcarrier andaltering the receiver
`parameters so as to cause the subsequent recovered
`sample values to fall into more precisely defined groups.
`This alteration may take the form of a change in the
`timing ofa “blockstart” signal or, preferably, comprises
`an alteration in the frequency of a clock running at
`sample rate.
`In preferred embodiments, where block synchronisa-
`tion is achieved bysteering the sample clock frequency,
`it is not necessary to have a separate sample clock syn-
`chronisation step.
`Where a separate sample clock synchronisation is
`required or desired this may be obtained by evaluating
`how the rate of change of phase of the samples with
`time varies from one OFDMcarrier frequency to an-
`other (eliminating from the consideration those phase
`changes that are attributableto the value ofthe data bits
`modulating the respective OFDM carriers).
`It is preferable that the method chosen for evaluating
`the “spread”of received sample values for each OFDM
`carrier should be independent of any frequency error
`that there may bearising from differences between the
`frequencies of the various local oscillators used in the
`receiver compared with the corresponding frequencies
`used at the transmitter. In preferred embodiments of the
`invention this is achieved by evaluating the sample
`spread looking at the variation in sample position in the
`radial direction only. Additionally, reductions in pro-
`cessing may be achieved by assessing sample spread in
`respect of only a subset of the OFDM carriers, rather
`than for all of the carriers of the received signal.
`It is further preferred that the technique for evaluat-
`ing “spread” of samples should give an indication of the
`direction in which the receiver parameters should be
`changed in order to improvethe block synchronisation.
`An advantageous method of achieving this is to assess
`the “spread” of samples whenthe block start position is
`early compared with the receiver setting, to assess the
`“spread” of samples whentheblockstart position is late
`compared with the receiver setting and to subtract the
`“late” value from the “early” value. The resultant mea-
`sure indicates how far awaythe receiver setting is from
`the true block start position and indicates the direction
`in which the receiver setting departs from the block
`start position.
`A preferred technique for achieving local oscillator
`synchronisation at an OFDMreceiveris to evaluate the
`rate of change of phase of the sample values with time
`for each OFDMcarrier andtoalter the receiver param-
`eters to reduce or eliminate this phase change. If the
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 14
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 14
`
`

`

`5
`receiver local oscillator is misaligned with the corre-
`sponding transmitter local oscillator then forall samples
`there will be a phase drift in sample location from one
`block to the next. There may also be a phase change
`from one blockto the next if the sample signal modulat-
`ing the relevant OFDM carrier changes from one block
`to the next, however the receiver can detect this gross
`variation and deduct the relevant multiple of 90° from
`the sample phase.
`Again it is preferable that the evaluation of the rate of
`change of phase should be performed keeping process-
`ing simple. In particular, it is advantageous to evaluate
`the change ofphase between successive blocks avoiding
`functions such as inverse tangents and minimising the
`numberofdivisions.
`Thus according to certain preferred embodiments of
`the invention the change of phase from oneblock to the
`next in the demodulated sample values modulating one
`OFDM carrieris evaluated by multiplying one sample
`value by the complex conjugate of the previous sample;
`the imaginary part of the result corresponds to the
`phase difference between the two sample values. This
`measure is dependent on the signal level but, if greater
`accuracyis desired, the calculated valueof phase differ-
`ence may be divided by the signal level.
`This method for assessing phase difference between
`blocks for one OFDMcarrier mayalso be used,if sepa-
`rate sample clock synchronisation is being performed,
`during the sample clock synchronisation.
`Whenassessing phase difference between blocks for
`the purpose oflocal oscillator synchronisation ideally
`the phase difference commontoall of the OFDM carri-
`ers would be evaluated. However,in practice it may be
`sufficient to work with only a subset of the OFDM
`carriers. Similarly, in order to reduce the amount of
`processing required to maintain synchronisation (once
`lock has been achieved) the phase difference between
`two blocks may be found by comparing the phases of
`samples several blocks apart and dividing the phase
`difference by the number of blocks separating the sam-
`ples.
`It may be seen that the synchronisation techniques
`offered by aspects of the present invention involve fur-
`ther processing of sample values which have already
`been extracted from a received OFDMsignal by a Dis-
`crete Fourier Transform or an Inverse Discrete Fourier
`Transform,this further processing preferably involving
`a complex multiplication step.
`
`Brief Description of the Drawings
`Further features and advantages provided by aspects
`of the present invention will become clear from the
`following description of embodiments thereof, given by
`way of example, and illustrated by the accompanying
`drawings, in which:
`FIGS. 1A and 1Billustrates the power spectrum of
`an orthogonal frequency division multiplexed (OFDM)
`signal;
`FIG. 2A illustrates a data coding used to produce
`differential quadrature phase shift keying (DQPSK)
`modulation of OFDM carriers;
`FIG. 2C uses a complex vector representation to
`illustrate differential coding; and
`FIG. 2B shows an example of a differential coding
`device for producing signals for DOQPSK modulation of
`OFDMcarriers;
`FIG. 2D shows the coding associated with the cod-
`ing device of FIG. 2B;
`
`5
`
`10
`
`25
`
`40
`
`50
`
`55
`
`60
`
`65
`
`5,345,440
`
`6
`FIGS.3A to 3C illustrates two methods of generating
`an OFDMsignal modulated onto a carrier;
`FIG. 4 showsin block diagrammatic form elements
`of a receiver according to a first embodiment of the
`invention, for receiving a transmitted OFDM signal
`modulating a carrier;
`FIGS.5(a) 5(5) and 5(c) show graphs whichillustrate
`how the successive values found at the receiver for the
`signal QPSK. modulating a single OFDMcarrier differ
`from the “allowed” values used at the transmitter and
`indicate how the received data can be demodulated and
`in particular, FIG. 5(a) indicates where received sam-
`ples would lie in an “ideal” transmission/reception sys-
`tem; FIG. 5(b) shows where received samples lie in
`practice; and FIG. 5(c) indicates how received sample
`values are demodulated;
`FIGS. 6(@) and 6(5) show graphs which illustrate
`how lack of proper synchronisationaffects the received
`sample values; and wherein FIG. 6{a) indicates how
`error in synchronisation to clock frequency affects re-
`ceived samples, and FIG. 6(6) indicates how error in
`synchronisation to a UHF carrier signal frequency com-
`bined with error in synchronisation to the clock fre-
`quencyaffects received samples;
`FIG.7 illustrates a typical distribution of the phase of
`received sample values with respect to reference axes,
`for a single OFDM carrier frequency;
`FIG.8 shows in block diagrammatic form a preferred
`embodiment of block synchronisation elements for an
`OFDMreceiver;
`FIG. 9 shows graphs of the characteristic of the
`phase sensitive detector of FIG. 8 for different phase
`locked loop settings; and
`FIG. 10 shows in block diagrammatic form a pre-
`ferred embodimentoflocal oscillator synchronisation
`elements for an OFDM receiver.
`Before discussing detailed embodiments of the inven-
`tion it is useful to consider how an OFDMsignal may
`be generated.
`
`Modulation
`
`As mentioned above, the OFDMsignal can be gener-
`ated using an inverse Discrete (preferably Fast) Fourier
`Transform, which operates on an array of complex
`samples in the frequency domain to produce an array of
`complex samples in the time domain. Preferred embodi-
`ments of the invention operate on OFDM signals in
`which the real parts of the frequency domain samples
`are constrained to take the values +1 or —1, and the
`imaginary parts +j or —j. The four possible combina-
`tions of real and imaginary parts correspondto the four
`QPSKphase states.
`If the data is differentially coded (as is preferred) then
`it is the transitions between successive phase states
`which define the two data bits being coded. As shown
`in FIGS.2A and 2Cthe four possible transitions may be
`considered in a vector representation to be rotations of
`+0°, +90°, + 180° and +270° respectively, and each of
`these transitions represents a pair of binary digits (for
`example, as shown in the table under FIG. 2D).
`An example of circuitry which could be used to gen-
`erate the successive values of the complex number used
`to modulate one particular OFDM carrier is given in
`FIG. 2B.
`After transformation into the time domain the signal
`has to be shifted up in frequency for transmission. Two
`possible approaches to this are illustrated in FIG.3.
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 15
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 15
`
`

`

`5,345,440
`
`7
`Thefirst approach uses two modulators fed with
`in-phase and quadrature local oscillator signals. One
`modulator handles the real part of the time domain
`signal, the other the imaginary part. The modulator
`outputs are summedand after suitablefiltering the sig-
`nal can be transmitted.
`Although this first approach is workable we have
`developed a second approach because it provides cer-
`tain advantages.
`The second approachrelies on producing a time do-
`main signal with no imaginary part. To achievethis the
`real part of the frequency domain signal must be even
`symmetrical aboutits centre, while the imaginary part
`must be odd symmetrical. This may be achieved by
`writing data into only half of the available space in the
`real and imaginary arrays and copying the data, with
`appropriate sign changes, into the other half of each
`array. Atfirst sight it may seem that each FFT pro-
`cesses half as many data bits as in the first approach. In
`practice, however, it is possible to use each FFT to
`transform twosets of data at once with only a small loss
`in efficiency. This real signal can then be fed to a single
`modulator which produces a signal with two sets of
`sidebands symmetrical about the carrier. Since they
`contain no additional information, one set of sidebands
`can befiltered off, leaving a signal which occupies the
`same bandwidth perbit as the first approach.
`This second approach,althoughslightly more com-
`plicated, has two advantages. Firstly, it simplifies the
`demodulator in the receiver since there is no need to
`accurately demodulate signals in quadrature. Secondly,
`there is a simple baseband version of the OFDMsignal,
`rather than the real and imaginary signals of the first
`approach.
`
`Demodulation
`
`FIG. 4 shows a simplified block diagram of an
`OFDMreceiver accordingto a first embodimentof the
`invention. In the discussion that follows it will be as-
`sumed that the incoming signal is a QPSK modulated
`OFDMsignal, modulated up to the UHF band, coding
`a television signal. Also,
`in order to be concise, the
`described embodiment includes demodulation process-
`ing, block synchronisation and intermediate frequency
`synchronisation all according to aspects of the present
`invention, whereas in practice although all three pro-
`cesses may be used only one, or some subcombination,
`of these three processes could alternatively be used in a
`receiver, perhaps also with a separate clock synchroni-
`sation step.
`Thereceived signal is isolated by a UHF tuner which
`outputs the received OFDMsignal at an intermediate
`frequency, IF, whichin this detailed exampleis given as
`39 MHz. The intermediate frequency signal is mixed
`down to basebandin a mixer 1,filtered in a filter 2 and
`converted to digital words by an analog-to-digital con-
`vertor 3 and a digital processing device 4.
`Assuming, for the moment, that no synchronisationis
`required,i.e. that the receiver local oscillator, 6, and the
`digital sample clock, 7, are correctly locked to the
`transmitter at all times, and that the receiver knows the
`position of the start samples in each data block, then the
`first part of the digital processing is that the Discrete
`Fourier Transform (DFT) of each block is taken. The
`output from the DFT will be a set of complex numbers
`each of which represents the amplitude and phase of a
`received OFDMcarrier (which, where QPSK modula-
`tion is used, code two data bits).
`
`8
`If the transmitter circuitry and transmission channel
`were noise-less and introduced nodistortion of phase or
`amplitude then the set of complex numbers output from
`the DFT in the receiver would correspondto the values
`X(n) used at the transmitter. However,in practice both
`the amplitude and the phase of each received OFDM
`carrieris likely to have been altered and so the complex
`numbers output from the DFT in the receiver will not
`correspond to the values X(n) used at the transmitter.
`Theeffects of noise and distortion may beillustrated
`by plotting the successive received sample values for a
`single OFDM carrier frequency against a pair of axes
`(Real, Imaginary). FIG. 55 shows such a plotting for an
`OFDMsignal where the carriers are QPSK modulated.
`In an “ideal” transmission/reception system each of
`the received samples would lie in one of four positions
`spaced 90° apart from one anotherand at 45° to the axes
`(as shown in FIG. 5a). In practice, however, the re-
`ceived samples fall into four groups of points (because
`of noise and channel distortion) and these groups will
`generally not lie at 45° to the axes but will be offset by
`some angle (because of phase changes in the channel),
`see FIG. 5b. It may be considered that the four groups
`of received samples are at 45° to a set of notional refer-
`ence axes whose position must be found before the
`OFDMcarrier can be properly demodulated.
`In order to find the orientation of the notional refer-
`ence axes the digital processor 4 at the receiver may
`assign the samples received for a given carrier fre-
`quencyto one of four groups and then find the centres
`of the groups by averaging over many received sam-
`ples. The reference axes are formed by a pair oflines
`through the origin which bisect the angles between the
`groups.
`Knowing the position of the reference axes, received
`samples can be decodedinto pairs of bits by determining
`which quadrantof the reference axes they lie in. There
`is in fact an ambiguity since there are four possible
`orientations of the reference axes, and which one is
`correct is not known. In practice, however, this does
`not matter if the data is differentially coded, then it is
`only the direction of transitions between quadrants that
`count. Alternatively, in embodiments where differential
`coding is not used and an overhead in the transmitted
`signal is allowed, a reference signal of known phase may
`be transmitted at a predetermined position or timing in
`the signal.
`It may be seen that mostof the calculations involved
`in demodulation are concerned with angles. At first
`sight these calculations appear to require use ofdivision
`and inverse tangents which would entail expenditure of
`a large amountofprocessing time. However, in OFDM
`systems where the signals modulating the OFDM carri-
`ers are only allowed to take four different phase states
`for the carrier all of the calculations can be performed
`using comparatively simple operations (ie complex mul-
`tiplication, addition and conjugation).
`The relevant techniques for simplifying the demodu-
`lation calculations occu