United States Patent
`Andren et al.
`
`p19
`
`US005732105
`
`(11) Patent Number:
`
`5,732,105
`
`[45] Date of Patent:
`
`Mar. 24, 1998
`
`METHODOF ESTIMATING SIGNAL
`QUALITY IN A DPSK DEMODULATOR
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`[54]
`
`[75]
`
`Inventors: Carl Frank Andren, Indialantic; Perry
`Wesley Frogge, Palm Bay; Leonard
`Victor Lucas, Palm Bay; Jim Snell,
`Palm Bay,all of Fla.
`
`[73]
`
`Assignee: Harris Corporation, Melbourne, Fla.
`
`[21]
`
`Appl. No.: 509,586
`
`[22]
`
`Filed:
`
`Jul. 31, 1995
`
`:
`
`[51]
`
`[52]
`
`[58]
`
`Wt CES asacsisssccscisccicacacccscas HO4B 3/46; HO4B 17/00
`TS. Ch.
`cecsesccssscteennneeesees 375/226; 375/327; 375/330;
`375/340; 375/355; 375/371; 329/304
`Field of Search.
`..............
`375/226, 224,
`375/227, 283, 284, 285, 324, 327, 330,
`340, 346, 355, 371, 373, 376; 329/304
`
`5,001,724
`5,490,177
`
`..scssessenee 375/226
`3/1991 Birgenheier et al.
`2/1996 La Rosa et al. scsssmsenneen 375/340
`
`Primary Examiner—Don N. Vo
`Attorney, Agent, or Firm—Rogers & Killeen
`[57]
`ABSTRACT
`
`Amethod ofestimating signal quality in a radio demodulator
`receiving an input stream of symbols includes the steps of
`sampling a phase-only portion of each of the symbols in the
`input stream, determining a phase error for each of the
`samples of the phase-only portions, and calculating a signal
`quality estimate from a plurality of the sample phase errors.
`Thesignal quality estimate may be an average magnitude of
`a predetermined number of sample phase errors. The input
`stream may be symbols in the preamble, or symbols in the
`data signal that follows.
`
`15 Claims, 1 Drawing Sheet
`
`ASSESSMENT
`
`
`CLEAR CHANNEL
`
`INPUT TO CHIP
`
` SIGNAL QUALITY 2
`
`SUBRATE
`SAMP
`
`
`
`Be
`
`
`Petitioner Sirius XM Radio Inc. - Ex. 1005, p. 1
`
`Petitioner Sirius XM Radio Inc. - Ex. 1005, p. 1
`
`

`

`U.S. Patent
`
`Mar. 24, 1998
`
`5,732,105
`
`
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`Petitioner Sirius XM Radio Inc. - Ex. 1005, p. 2
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`Petitioner Sirius XM Radio Inc. - Ex. 1005, p. 2
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`5,732,105
`
`1
`METHOD OF ESTIMATING SIGNAL
`QUALITY IN A DPSK DEMODULATOR
`BACKGROUND OF THE INVENTION
`This application is related to application Ser. No. 08/509,
`587 entitled Short Burst Acquisition Circuit for Direct
`Sequence Spread Spectrum Links; application Ser. No.
`08/509,588 entitled Fast Acquisition Bit Timing Loop
`Method and Apparatus; application Ser. No. 08/509,589
`entitled Short Burst Direct Acquisition Direct Sequence
`Spread Spectrum Receiver; application Ser. No. 08/509,590
`entitled A/D Reference Level Adjustment Circuit to Main-
`tain Optimum Dynamic Range at the A/D all of which are
`incorporated by reference, and are filed concurrently here-
`with and assigned to the assignee hereof.
`Thepresent inventionis related to demodulators for radio
`receivers, and more particularly to a DPSK demodulator for
`a direct sequence spread spectrum radio receiver in which
`signal quality estimates include phase-only processing.
`Design of demodulators for differential binary phase shift
`keyed (DBPSK) and differential quadrature phase shift
`keyed (DQPSK)data typically involves a trade-off between
`circuit complexity and performance. More specifically, sig-
`nal quality estimates may be used in the demodulator to set
`the performance level of the demodulator; the more accurate
`the signal quality estimates, the better the performance of the
`demodulator. Additionally, the signal quality may be used in
`the acquisition decision process. The more accurate the
`signal quality estimates, the closer the decision point can be
`set to optimize the ratio of the probability of (correct)
`acquisition to the probability of false alarm. However, the
`circuit in the demodulator for providing the signal quality
`estimates increases in complexity and size as the accuracy of
`the signal quality estimate improves. Thus, the size and
`complexity of the demodulator are undesirably increased as
`its performance is improved.
`A typical data transmission received at a DPSKreceiver
`includes a data signal preceded by a preamble. The receiver
`is expected to acquire the preamble before attempting to
`demodulate the data signal that follows. The signal quality
`estimate is used to set various demodulator features, and is
`determined first during the preamble and updated during the
`data signal. In the prior art, signal quality estimates typically
`are bit sync amplitudes derived from correlators that receive
`Land Q components of the input stream of symbols (the term
`symbol is used herein to refer to the units used in the
`preamble format, e.g., spread spectrum BPSK, or in the data
`signal format,e.g., DBPSK or DQPSK). However,bit sync
`amplitudes provide an incomplete picture of signal quality.
`For example, they do not directly indicate any phase rela-
`tionships.
`Accordingly, it is an object of the present invention to
`provide a novel method of estimating signal quality in a
`radio receiver that
`takes advantage of phase-only
`processing, thereby obviating the problems oftheprior art.
`It is another object of the present invention to provide a
`novel method of estimating signal quality in a DPSK
`demodulator in which the phase-only portions of input
`symbols are sampled so that a phase error may be deter-
`mined for each of the symbols.
`It is yet another object of the present invention to provide
`a novel method of estimating signal quality in a DPSK
`demodulator in which phase errors derived from a plurality
`of samples of phase-only portionsof input symbols are used
`as signal quality estimates.
`It is still another object of the present invention to provide
`a novel method of estimating signal quality in a DPSK
`
`2
`demodulator in which each symbol in a stream of symbols
`is sampled once and converted to polar coordinate form, in
`which a phase error for each of the samples is determined
`using a phase-only portion ofthe polar coordinate form, and
`in which a signal quality estimate is an average magnitude
`of a predetermined number of the sample phase errors.
`It is a further object of the present invention to provide a
`novel method of estimating signal quality in a radio
`demodulator in which phase errors derived from a plurality
`of samples of phase-only portions of input symbols are
`provided from a phase locked loop.
`These and many other objects and advantages of the
`present invention will be readily apparent to one skilled in
`the art to which the invention pertains from a perusal of the
`claims, the appended drawings, and the following detailed
`description of the preferred embodiments.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a block diagram of an embodiment of the
`demodulator of the present invention.
`DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`An embodiment of the present invention for estimating
`signal quality in a radio demodulator receiving an input
`stream of symbols may include the steps of sampling a
`phase-only portion of each of the symbols in the input
`stream, determining a phase error for each of the samples of
`the phase-only portions, and calculating a signal quality
`estimate from a plurality of the sample phase errors. The
`input stream may be symbols in the preamble, or symbols in
`the data signal that follows.
`In a preferred embodiment, the input stream of symbolsis
`provided in I and Q components, and the I and Q compo-
`nents are combined for each of the symbols. The combined
`Tand Q componentsfor each of the symbols may be sampled
`at the rate of one sample per symbol, and the samples
`converted to a form that separates the magnitude and the
`phase, such as polar coordinate form. The phase-only por-
`tion of the polar coordinate form may be provided to a phase
`locked loop for determination of a phase error for each of the
`samples. Thereafter, an estimate of the signal quality may be
`provided by calculating an average magnitude of a plurality
`of the sample phase errors.
`During the period when the signal is still being acquired
`(the preambleis still being received), the average magnitude
`may include the sample phase errors in a block of the
`symbols from the preamble received from an antenna. When
`more than one antenna is present, signal quality estimates
`may be provided for each antenna by using subsequent
`blocks of symbols from the preamble. Once the signal has
`been acquired and data are being demodulated, the signal
`quality estimate may be the average magnitude of a prede-
`termined numberof the sample phase errors, for example the
`average magnitude of phaseerrors from samples of 128 data
`symbols.
`The use of the phase errors provides a robust signal
`quality estimate, even when the input signal is weak because
`the phase errors do not rely on signal amplitude. Further, the
`conversion to phase-only provides advantages to the rest of
`the demodulation functions. For example, the samples are
`real numbers, rather than complex numbers, so that phase
`rotation of the carrier can be compensated by a simple
`addition rather than a complex multiply. The decision struc-
`ture for removing the DBPSK/DQPSK modulation involves
`additions and does not require comparators or complicated
`logic.
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`15
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`Petitioner Sirius XM Radio Inc. - Ex. 1005, p. 3
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`Petitioner Sirius XM Radio Inc. - Ex. 1005, p. 3
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`

`

`5,732,105
`
`15
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`55
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`4
`3
`3. The method of claim 1 wherein the step of determining
`In a further embodiment, an average phase ofa predeter-
`a phase error comprises thestep of providing the phase-only
`mined number of the symbols may be determined from the
`portion to a phase locked loop for determination of the phase
`phase-only portions of the symbols. The average phase may
`be provided to the phase locked loop to remove frequency
`error for each of the samples.
`offsets.
`4. The method of claim 3 further comprising the step of
`determining an average phase of a predetermined number of
`In yet a further embodiment. an average of the magnitude-
`the symbols from the phase-only portions of the symbols.
`only portions of the polar coordinate form of the symbols
`(also denoted the bit sync amplitude) may be provided as a ©
`5. The method of claim 4 further comprising the step of
`providing the average phase to the phase locked loop.
`further signal quality estimate.
`10
`6. The method of claim 1 wherein the signal quality
`By way of further explanation, and with reference to FIG.
`1, symbols in a received signal may be provided to a
`estimate is an average magnitude of a predetermined number
`demodulator 10 for processing. I and Q components may be
`of the sample phase errors.
`provided through analog to digital converters 12 and 14 to
`7. A method of estimating signal quality in a DPSK
`correlators 16 and 18 to remove the symbol spreading
`demodulator in which the estimate of signal quality is used
`sequence. Outputs from correlators 16 and 18 may be
`to establish a level of demodulation acquisition
`combined and converted from cartesian to polar coordinate
`performance, the method comprising the steps of:
`form in converter 20. The stream from converter 20 may be
`(a) sampling each symbolin a stream of symbolsinputto
`decimated to the symbolrate and the phase maybe corrected
`a DPSK demodulator;
`for frequency offset before PSK demodulation in PSK
`demodulator 22. Phase errors from PSK demodulator 22 are
`(b) converting the samples to polar coordinate form;
`fed to a numerically controlled oscillator 24 through a
`(c) determining a phase error for each of the samples
`lead/lag filter 26 to achieve phase lock in phase locked loop
`using a phase-only portion of the polar coordinate
`27.
`form; and
`Frequency offset is measured during signal acquisition
`(d) calculating a signal quality estimate fromaplurality of
`and stored in processor 28 for use in demodulation. The
`the sample phase errors.
`frequency offset is estimated by averaging the phaserotation
`8. The method of claim 7 wherein the stream of symbols
`from symbol to symbol and then provided to phase locked
`is a stream of data to be demodulated.
`loop 27 as an initial condition to accelerate phase lock,
`9. The method of claim 8 wherein the signal quality
`thereby saving time over the classical phase locked loops
`estimate is an average magnitude of a predetermined number
`used in the prior art. The average magnitude of the phase
`of the sample phase errors.
`errors for the symbols may be determined in calculating
`10. The method of claim 7 wherein the stream of symbols
`block 30 and provided as a signal quality estimate.
`is a preambleto a data signal.
`The magnitude of the output from converter 20 also may
`11. The method of claim 10 wherein the signal quality
`be provided to integrator 32 for integration. As will be
`estimate is an average magnitude of the sample phase errors
`appreciated, this eliminates the need (and the hardware) in
`in a block of the symbols received fromafirst antenna.
`the prior art to Doppler shift or to correct for oscillator offset
`prior to the correlator. The integrated output is provided to
`12. The method of claim 7 wherein each symbol
`is
`timer 34 for determination of symbol timing and bit sync
`sampled once.
`amplitude. The bit sync amplitude is an average magnitude
`13. A method ofestimating signal quality in a data radio
`of the symbols in polar coordinates and is a measure of
`demodulator comprising the steps of:
`signal amplitude that also may be provided as a signal
`(a) providing I and Q components of a stream of data
`quality estimate.
`symbols to a data radio demodulator,
`During data demodulation the phase locked loop tracks
`(b) combining the I and Q components for each of the
`and removes frequency offsets by subtracting the average
`symbols;
`phase. The demodulator switches from differential demodu-
`(c) sampling the combined I and Q components for each
`lation during acquisition to coherent demodulation followed
`of the symbols at the rate of one sample per symbol
`by differential detection during data demodulation. This
`switch improves bit error rate performance.
`(d) converting the samples to polar coordinate form;
`While preferred embodiments of the present invention
`(e) providing a phase-only portion of the polar coordinate
`have been described,it is to be understood that the embodi-
`form to a phase locked loop for determination of a
`ments described are illustrative only and the scope of the
`phase error for each of the samples;
`invention is to be defined solely by the appended claims
`(f) calculating an average magnitude of a plurality of the
`when accorded a full range of equivalence, many variations
`sample phase errors; and
`and modifications naturally occurring to those of skill in the
`(g) providing the average magnitude to an input to the
`art from a perusal hereof.
`Whatis claimed is:
`demodulator as a signal quality estimate.
`14, The method of claim 13 further comprising the steps
`1. A method of estimating signal quality in a DPSK
`of determining an average phase of a predetermined number
`demodulator receiving an input stream of symbols,
`the
`of the symbols from the phase-only portions of the symbols,
`method comprising the steps of sampling a phase-only
`and providing the average phase to the phase locked loop.
`portion of each of the symbols in the input stream, deter-
`15. The method of claim 14 further comprising the step of
`mining a phase error for each of the samples, and calculating
`providing an average of magnitude-only portions of the
`a signal quality estimate from a plurality ofthe sample phase
`errors.
`symbols to the input to the demodulator as a further signal
`2. The method of claim 1 further comprising the step of
`quality estimate.
`converting the symbols to a magnitude and phase form
`before sampling.
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`Petitioner Sirius XM Radio Inc. - Ex. 1005, p. 4
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`Petitioner Sirius XM Radio Inc. - Ex. 1005, p. 4
`
`