United States Patent
`
`1191
`
`Gledhill et a].
`
`U5005345440A
`
`[11] Patent Number:
`
`5,345,440
`
`[45} Date of Patent:
`
`Sep. 6, 1994
`
`[54] RECEPTION 0F ORTHDGONAL
`FREQUENCY DIVISION MULTIPLEXED
`SIGNALS
`
`[75]
`
`Inventors:
`
`Jeffrey J. Gledhill, Chandlers Ford;
`Santos]: V. Anilthindi, Southampton;
`Peter A. Avon1 Peterfield, all of
`England
`
`[73] Assignee: National Transcommnnicatlons
`Limited, Winchester, England
`
`[21] App]. No.:
`
`934,653
`
`[22] PCT Filed:
`
`Sep. 13, 1991
`
`OTHER PUBLICATIONS
`
`Hirosalo’ et 111., “Advanced Groupband Data Modem
`Using Orthogonally Multiplexed QAM technique,"
`Jun. 1986, pp. 587—592, IEEE Transactions on Commu-
`nications, vol- 34, No. 6.
`Le FIoch et 211., “Digital Sound Broadcasting to Mobile
`Receivers,” Aug., 1989, pp. 493—503, IEEE Transac-
`tions on Consumer Electronics, vol. 35, No. 3.
`—
`
`Pdmary Examiner—Douglas W. Olms
`Assistant Examiner—Melvin Marcelo
`Attorney, Agent, or Firm—Watson Cole Griddle d‘r.
`WatSOn
`
`[86] PCT No.:
`
`PCT/GBQl/OISTI
`
`15?]
`
`ABSTRACT
`
`§ 371 Date:
`
`Jan. 7, 1993
`
`§ 102(e) Date:
`
`Jan. 7, 1993
`
`[87] PCT Pub. No.: WO92/05646
`
`PCT Pub. Date: Apr. 2, 1992
`
`[30]
`
`Foreign Application Priority Data
`
`Sep. 14. 1990 [GB] United Kingdom
`
`9020170
`
`1104.1 1/oo;11041 11/00
`111:. C1.5
`[51]
`[52] us. (:1. ........................................ arm/19; 310/23;
`BTU/69.1; 375/120
`370/19, 20, 21, 23,
`[53] Field of Search
`370/69.1, 121, 122; 315/119. 120'. 33; 364/725,
`126
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,884,139 11/1939 Pommier
`5,166,924 11/1992 Moose
`5,197,061 3/1993 Halbert-Lassalle e
`
`358/142
`370/311
`370/ 19 X
`
`FOREIGN PATENT DOCUMENTS
`
`6/1989 European Pat. oft.
`0321021
`61.129936 10/1936 Japan.
`
`individual OFDM carriers are
`In an OFDM signal
`modulated by samples of signals which can only take a
`limited range of allowed values and a block of samples
`modulates the group of carriers 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 frame of refer-
`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 OFDM carrier 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 frequency error.
`Preferably phase drift is assessed by multiplying con»
`plea 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
`
`
`
`
`
`SAMPLE (LOCK
`WENEY
`GENERATOR
`
`
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 1
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 1
`
`

`

`. US. Patent
`
`Sep. 6, 1994
`
`Sheet 1 of 11
`
`5,345,440
`
`Luzmbcmkk
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`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 2
`
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 2
`
`
`

`

`U.S. Pafent
`
`Sep. 6, 1994
`
`Sheet 2 of 11
`
`5,345,440
`
`5.0. INITIAL STATE=+1+j
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`
`(PRIOR ART)
`
`IMAGINARY
`
`
`
`Fig. 20
`
`(PRIOR ART)
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 3
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 3
`
`

`

`US. Patent
`
`Sep. 6, 1994
`
`Sheet 3 of 11
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`5,345,440
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 4
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 4
`
`

`

`US. Patent
`
`Sep. 6, 1994
`
`Sheet 4 of 11
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`5,345,440
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 5
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 5
`
`

`

`US. Patent
`
`Sep. 6, 1994
`
`Sheet 5 of 11
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 6
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 6
`
`
`
`

`

`US. Patent '
`
`Sep. 6, 1994
`
`Sheet 6 of 11
`
`5,345,440
`
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 7
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 7
`
`

`

`U.S. Patent
`
`Sep. 6, 1994
`
`Sheet 7 of 11
`
`5,345,440
`
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`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 8
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 8
`
`

`

`US. Patent
`
`Sep. 6, 1994
`
`Sheets of 11
`
`5,345,440
`
`RATE OF CARRIER ROTATION
`
`CLOCK FREQUENCY ERROR
`IS PROPORTIONAL TO
`THE SLOPE OF THIS LINE
`
`
`
`CARRIER FREQUENCY
`
`Fig.6IaI
`
`RATE OF CARRIER ROTATION
`
`CARRIER FREOUENC Y
`
`INTERCEPT
`PROPORTIONAI.
`TO ERROR IN
`UHF CARRIER
`
`Fig. 6IbI
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 9
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 9
`
`

`

`US. Patent
`
`Sep. 6, 1994
`
`Sheet 9 of 11
`
`. 5,345,440
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 10
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 10
`
`
`
`

`

`US. Patent
`
`Sep. 6, 1994
`
`Sheet 10 of 11
`
`5,345,440
`
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`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 11
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 11
`
`

`

`US. Patent
`
`Sep.6, 1994
`
`Sheet 11 of 11
`
`5,345,440
`
`f'_ ___________________ '2"_-_'1
`
`SOFTWARE
`
`
`
`
`
`
`
`
`FREQUENCY
`SENSITI VE
`DETECTOR
`
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`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 12
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 12
`
`

`

`I
`
`5,345,440
`
`RECEPTION OF ORTHOGONAL FREQUENCY
`DIVISION MULTIPLEXED SIGNALS
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates to the field of reception
`of orthogonal frequency division multiplexed (OFDM)
`signals. More particularly, the invention concerns de-
`modulation of received OFDM signals and synchroni-
`sation at an OFDM receiver.
`
`2. Description of The Prior Art
`Orthogonal
`frequency division multiplexing is a
`method of transmitting data which is being investigated,
`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 OFDM signal onto a carrier so as to reach an
`appropriate frequency for transmission. By adapting the
`processing which is performed in the frequency domain
`it becomes possible to simplify the modulation of the
`OFDM signal 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 which it 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 couventional methods of data transmis—
`sion, with OFDM signals it 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 promoted, in the article “Digital Im-
`plementation of High Speed HF Modems" by D.
`Harmer and 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 of cy-
`cles occurring during one black, a non-zero value for
`the integral will indicate block misalignment.
`Embodiments of the present invention may deal with
`any or all of black synchronisation, sample clock syn-
`chronisation and, where relevant, local oscillator syn-
`chr0nisation, 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 xiii)2 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
`
`5
`
`10
`
`15
`
`20
`
`.
`
`25
`
`35
`
`45
`
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`
`65
`
`2
`symbol rate of each modulating signal (assuming that all
`of the modulating signals have the same symbol rate).
`The overall spectrum of the OFDM signal is very
`close to rectangular when a large number of carriers are
`contained in the OFDM signal.
`During a given time period, T, the OFDM signal may
`be represented by a block of N samples. The value of
`the kth sample is, as follows:
`
`Jock) = ”firm-WW"
`5:0
`
`The N values X(n) represent the respective values,
`during period T, of the discretely-varying signals modu-
`lating the OFDM carriers ezf'WN.
`It may be seen from the above equation that the
`OFDM signal 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 OFDM signal
`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, f...
`SUMMARY OF THE INVENTION
`
`According to the present invention it 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,
`fa. In particularly preferred embodiments of the inven-
`tion the set of values to which the samples X(n) are
`restricted comprises values +1+j, +l—j, —l+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
`OFDM signal produced in this way will also tolerate
`non-phase-equalised channels much better than would
`conventional signals.
`Since the data samples cantained 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 l:j. One way of doing this is
`to first convert the input data into a binary bitstream
`and then to code each 2-bit portion of the bitstream into
`one of the four allowed values. Thus when converting a
`digital datastream into an OFDM signal, in which each
`OFDM carrier is QPSK modulated as desoribed 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 ilij. The resulting array of
`complex values for one block is then subjected to an
`inverse Discrete Fourier Transform so as to produce
`the OFDM signal.
`A consequence of using QPSK modulation of the
`carriers in the OFDM signal is that only two bits of data
`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
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 13
`
`

`

`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
`OFDM carriers, such as 8-PSK or 16 QAM could be
`used. However, the greater the number of allowed
`phase states the more sensitive the OFDM signal is to
`noise and to channel distortions. Also, if when produc—
`ing an OFDM signal 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
`OFDM spectrum can very nearly fill a channel. This
`allows it to achieve close to the theoretical maximum
`data rate, eg. 2 bits/second/Hz for QPSK. Conven-
`tional digital modulation schemes require wider band-
`widths because of practical difficulties in implementing
`suitable filters
`
`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/61391100513.
`in the case of
`PAL television signals most of the power is concen-
`trated around the vision and sound carriers. When using
`OFDM signals for TV near existing channels, setting
`the OFDM carriers to zero near these vision and sound
`
`carriers frequencies results in a spectrum with rectangu-
`lar holes and a dramatic improvement in bit error rate
`caused by PAL co—channel interference. It is also possi-
`ble to reduce adjacent channel interference by not using
`oaniers 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 duodulation using a Dis-
`crete Fourier Transformation or an inverse Discrete
`Fourier Transformation to recover sample values, a
`complex multiplication and a decoding of the sign of the
`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 or not they are also restricted in
`amplitude) the sample values output from the DFT, in a
`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 of the 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 tangent of the phase angle, this would
`mean that the sample demodulation process would in-
`volve the performance of a division followed by refer-
`ence to a look-up table. Division is difficult to imple-
`ment in digital signal processing apparatus and would
`require a large amount of processing power.
`
`4
`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 OFDM signal.
`Where successive portions of the OFDM signal rep-
`resent respective sucwssive blocks of data modulating
`the OFDM carriers, an advantageous method for 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 of each OFDM carrier and altering 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 of a “block start” signal or, preferably, comprises
`an alteration in the frequency of a clock rumu'ng at
`sample rate.
`In preferred embodiments, where block synchronisa-
`tion is achieved by steering the sample clock frequency,
`it is not necessary to have a separate sample clock syn-
`chronisation sup.
`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 OFDM carrier frequency to an-
`other (eliminating from the consideration these phase
`changes that are attributable to the value of the 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 ermr
`that there may be arising 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 “spr
`" of samples should give an indicatiou of the
`direction in which the receiver parameters should be
`changed in order to improve the block synchronisation
`An advantageous method of achieving this is to assess
`the “spread" of samples when the block start position is
`early compared with the receiver setting, to assess the
`“spread” of samples when the block start 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 away the receiver setting is from
`the true block start position and indicates the direction
`in which the receiver setting departs from the b10ck
`start position.
`A preferred technique for achieving local oscillator
`synchronisation at an OFDM receiver is to evaluate the
`rate of change of phase of the sample values with time
`for each OFDM carrier and to alter the receiver param-
`eters to reduce or eliminate this phase change. If the
`
`10
`
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`
`2|]
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 14
`
`Petitioner Sirius XM Radio Inc. - Ex. 1004, p. 14
`
`

`

`5,345,440
`
`5
`receiver local oscillator is misaligned with the corre-
`sponding transmitter local oscillator then for all 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 block to 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 of phase between successive blocks avoiding
`functions such as inverse tangents and minimising the
`number of divisions.
`Thus according to certain preferred embodiments of
`the invention the change of phase from one block to the
`next in the demodulated sample values modulating one
`OFDM carrier is 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
`accuracy is desired. the calculated value of phase differ-
`ence may be divided by the signal level.
`This method for assessing phase difference between
`blocks for one OFDM carrier may also be used, if sepa-
`rate sample clock synchronisation is being performed,
`during the sample clock synchronisation.
`When assessing phase difference between blocks for
`the purpOSe of local oscillator synchronisation ideally
`the phase difference common to all 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 OFDM signal 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 13 illustrates 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. 21’: uses a complex vector representation to
`illustrate differential coding; and
`FIG. 28 shows an example of a differential coding
`device for producing signals for DQPSK modulation of
`OFDM carriers;
`FIG. 2D shows the coding associated with the cod-
`ing device of FIG. 2B;
`
`6
`FIGS. 3A to 3C illustrates two methods of generating
`an OFDM signal modulated onto a carrier;
`FIG. 4 shows in 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(b) and 5(6) show graphs which illustrate
`how the successive values found at the receiver for the
`signal QPSK modulating a single OFDM carrier 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(a) indicates how received sample
`values are demodulated;
`FIGS. 6(a) and 6(6) show graphs which illustrate
`how lack of proper synchronisation affects the received
`sample values; and wherein FIG. 6(a) indicates how
`error in synchronisation to clock frequency affects re—
`ceived samples, and FIG. 6(b) indicates how error in
`synchronisation to a UHF carrier signal frequency com-
`bined with error in synchronisation to the clock fre-
`quency affects 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
`OFDM receiver;
`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 embodiment of local oscillator synchronisation
`elements for an OFDM receiVer.
`
`IO
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`Before disoussing detailed embodiments of the inven-
`tion it is useful to consider how an OFDM signal may
`be generated.
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`Modulation
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`As mentioned above, the OFDM signal can be gener-
`ated using an inverse Discrete (preferany 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 talte the values +1 or — l, and the
`imaginary parts +j or --j. The four possible combina-
`tions of real and imaginary parts correspond to the four
`QPSK phase states.
`If the data is differentially coded (as is preferred) then
`is the transitions between successive phase states
`it
`which define the two data bits being coded. As shown
`in FIGS. 2A and 2C the 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. 213.
`
`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
`The first 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 summed and after suitable filtering 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 approach relies on producing a time do-
`main signal with no imaginary part. To achieve this the
`real part of the frequency domain signal must be even
`symmetrical about its 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. At first sight it may seem that each FFT pro-
`cesses halfas many data hits as in the first approach. In
`practice, however, it is possible to use each FFT to
`transform two sets of data at once with only a small loss
`inefficiency.Thisreelsignalcanthenbefedtoasingle
`modulator which produces a signal with two sets of
`sidebauds symmetrical about the carrier. Since they
`contain no additional information, one set of sidebands
`can be filtered off, leaving a signal which occupies the
`same bandwidth per bit as the first approach.
`This second approach, although slightly 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 OFDM signal,
`rather than the real and imaginary signals of the first
`approach.
`
`Demodulation
`
`FIG. 4 shows a simplified block diagram of an
`OFDM receiver according to a first embodiment of the
`invention. In the discussion that follows it will be as-
`sumed that the incoming signal is a QPSK modulated
`OFDM signal, 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.
`The received signal is isolated by a UHF tuner which
`outputs the received OFDM signal at an intermediate
`frequency, IF, which in this detailed example is given as
`39 MHz. The intermediate frequency signal is mixed
`down to baseband in a mixer I, 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 synchronisation is
`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
`poaition of the start samples in each data block, then the
`first part of the digital processing is that the Discrete
`Fourier Transform (DFI') 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 OFDM carrier (which, where QPSK modula-
`tion is used, code two data bits).
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`8
`If the transmitter circuitry and transmission channel
`were noise-less and introduced no distortion of phase or
`amplitude then the set of complex numbers output from
`the DFT in the receiver would correspond to the values
`X(n) used at the transmitter. However, in practice both
`the amplitude and the phase of each received OFDM
`carrier is 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.
`The efi'ects of noise and distortion may be illustrated
`by plotting the successive received sample values for a
`single OFDM carrier frequency against a pair of axes
`(Real, Imaginary). FIG. 5b shows such a plotting for an
`OFDM signal 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 another and 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
`OFDM carrier can be properly demodulated.
`In order to fmd 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-
`quency to 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 of lines
`through the origin which bisect the angles between the
`groups.
`Knowing the position of the reference axes, received
`samples can be decoded into pairs of bits by determining
`which quadrant of 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 most of the calculations involved
`in demodulation are concerned with angles. At first
`sight these calculations appear to require use of division
`and inverse tangents which would entail expenditure of
`a large amount of processing time. However, in OFDM
`systems where the signals modulating the OFDM cani-
`ers are only showed to take four different phase states
`for the carrier all of the calc