United States Patent [19J
`Mujtaba
`
`I lllll llllllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US006091781A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,091,781
`Jul. 18, 2000
`
`[54] SINGLE SIDEBAND TRANSMISSION OF
`QPSK, QAM AND OTHER SIGNALS
`
`[75]
`
`Inventor: Syed Aon Mujtaba, Berkeley Heights,
`N.J.
`
`[73] Assignee: Lucent Technologies Inc., Murray Hill,
`N.J.
`
`[21] Appl. No.: 08/970,987
`
`[22] Filed:
`
`Nov. 14, 1997
`
`[51]
`Int. Cl.7 ..................................................... H04L 27/18
`[52] U.S. Cl. ........................... 375/279; 375/270; 375/321
`[58] Field of Search ..................................... 375/270, 279,
`375/321, 280, 308, 260, 329, 276, 340;
`332/103, 108
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,241,451
`4,358,853
`4,803,700
`5,699,404
`5,729,575
`5,892,774
`5,909,434
`5,943,361
`
`12/1980 Maixner et al. ........................ 455/202
`11/1982 Qureshi ................................... 375/296
`2/1989 Dewey et al.
`.......................... 375/321
`12/1997 Satyamurti .. ... .... ... ... ... ... ... ... 340/311.1
`3/1998 Leitch ..................................... 375/268
`4/1999 Zehavi et al. ........................... 370/527
`6/1999 Odenwalder et al.
`.................. 370/342
`8/1999 Gilhousen et al.
`..................... 375/200
`
`OTHER PUBLICATIONS
`
`T.S. Rappaport, "Wireless Communications: Principles and
`Practice," Prentice-Hall, NJ, pp. 243-247, 1996.
`AV. Oppenheim and R.W. Schafer, "Discrete-Time Signal
`Processing," Prentice-Hall, NJ, pp. 676-688, 1989.
`
`J.G. Proakis and M. Salehi, "Communication Systems Engi(cid:173)
`neering," Prentice-Hall, NJ, pp. 310-317, 1994.
`R.D. Gitlin et al., "Data Communications Principles," Ple(cid:173)
`num Press, NY, pp. 305-312 and pp. 322-325, 1992.
`
`Primary Examiner-Stephen Chin
`Assistant Examiner-Kevin M Burd
`Attorney, Agent, or Firm-Ryan & Mason, L.L.P.
`
`[57]
`
`ABSTRACT
`
`Methods, apparatus and system for transmitting signals in
`QPSK, QAM and other similar modulation formats as single
`sideband (SSE) signals. An exemplary SSB-QPSK trans(cid:173)
`mitter receives an in-phase data signal and a quadrature(cid:173)
`phase data signal. The in-phase data signal and a Hilbert
`transform of the quadrature-phase data signal are modulated
`onto a cosine carrier signal, the quadrature-phase data signal
`and a Hilbert transform of the in-phase data signal are
`modulated onto a sine carrier signal, and the modulated sine
`and cosine carrier signals are combined to provide a modu(cid:173)
`lated SSB-QPSK signal. The in-phase and quadrature-phase
`data signals are time-aligned signals which are interpolated
`prior to modulation so as to include zero values at alternating
`instants of time. Their corresponding Hilbert transforms
`therefore also exhibit alternating zero values. During
`modulation, the in-phase data signal can thus be interleaved
`with Hilbert transforms of the quadrature-phase data signal,
`and the quadrature-phase data signal can be interleaved with
`Hilbert transforms of the in-phase data signal, without any
`interference between the signals. Coherent quadrature detec(cid:173)
`tion may be used to recover both the in-phase and
`quadrature-phase data signals at a receiver.
`
`19 Claims, 8 Drawing Sheets
`
`50~
`
`70
`
`66
`
`DELAY
`
`78
`
`PULSE(cid:173)
`SHAPING
`FILTER
`
`INTERPOLATE
`WITH ZEROS
`
`62
`
`INTERPOLATE
`WITH ZEROS
`
`74
`
`86
`
`z (ti
`
`64
`
`DELAY
`
`PULSE(cid:173)
`SHAPING
`FILTER
`
`84
`
`76
`
`72
`
`82
`
`sinlwctl
`
`Ericsson v. IV II LLC
`Ex. 1023 / Page 1 of 14
`
`

`

`U.S. Patent
`
`Jul. 18, 2000
`
`Sheet 1 of 8
`
`6,091,781
`
`FIG.
`.1A
`!PRIOR ARTI
`
`10,
`
`12
`
`cos!wctl
`
`IN-PHASE x[ n]
`
`PULSE SHAPING
`FILTER g!tl
`
`QUADRATURE-
`PHASE
`y[ n]
`
`PULSE SHAPING
`FILTER g!tl
`
`20
`
`z (ti
`
`14
`
`18
`
`16
`
`sin(wctl
`
`FIG. 18
`(PRIOR ARTI
`
`34
`
`PULSE SHAPING
`FILTER g!tl
`
`30,
`
`32
`
`DELAY
`
`cos (we ti
`
`36
`
`42
`
`44
`
`w!tl
`
`x[ n]
`
`HILBERT
`FILTER
`
`38
`
`PULSE SHAPING
`FILTER g I ti
`
`40
`
`sin(wctl
`
`Ex. 1023 / Page 2 of 14
`
`

`

`U.S. Patent
`
`Jul. 18, 2000
`
`Sheet 2 of 8
`
`6,091,781
`
`FIG. 2A
`
`x[ n]
`
`1. 0
`
`0.8
`
`0.6
`
`0.4
`
`0.2
`
`0
`0
`
`1
`
`0.5
`
`H{ x[ n]l 0
`
`-0.5
`
`-1
`
`0
`
`2
`
`4
`
`6
`
`8
`
`10
`
`12
`
`14
`
`16
`
`18
`
`n
`
`FIG. 28
`
`2
`
`4
`
`6
`
`8
`
`10
`
`12
`
`14
`
`16
`
`18
`
`n
`
`Ex. 1023 / Page 3 of 14
`
`

`

`U.S. Patent
`
`Jul. 18, 2000
`
`Sheet 3 of 8
`
`6,091,781
`
`FIG. 3A
`
`1
`
`0.5
`
`xi n] 0
`
`-0.5
`
`-1
`
`0
`
`2
`
`4
`
`6
`
`8
`
`10
`
`n
`
`12
`
`14
`
`16
`
`18
`
`20
`
`FIG. 38
`
`3
`
`2
`
`1
`
`0
`
`H{ x[ n]}
`
`-1
`
`-2
`
`0
`
`2
`
`4
`
`6
`
`8
`
`10
`
`n
`
`12
`
`14
`
`16
`
`18
`
`20
`
`Ex. 1023 / Page 4 of 14
`
`

`

`U.S. Patent
`
`Jul. 18, 2000
`
`Sheet 4 of 8
`
`6,091,781
`
`FIG. 4A
`
`1
`
`0.5
`
`xi n] 0
`
`-0.5
`
`-1
`
`0
`
`5
`
`10
`
`15
`
`25
`
`30
`
`35
`
`40
`
`20
`n
`
`FIG. 48
`
`H{x[ n]}
`3
`
`2
`
`-1
`
`-2+-~---.-~~--.-~~,--~~~~-,-~--r~~~~~
`40
`15
`35
`25
`10
`20
`30
`0
`5
`n
`
`Ex. 1023 / Page 5 of 14
`
`

`

`U.S. Patent
`
`Jul. 18, 2000
`
`Sheet 5 of 8
`
`6,091,781
`
`FIG. SA
`
`FIG. SB
`
`DATA AT MODULATOR INPUT
`
`DATA AT MODULATOR OUTPUT
`
`I
`
`'
`
`I-CHANNEL
`
`x[n]
`
`x[n+1]
`
`x[n+2]
`
`x[n+3]
`
`x[ nl I x[ n+1]
`
`x[ n+2]
`
`x[ n+3]
`
`0-CHANNEL :
`
`I
`
`FIG. SC
`
`DATA AT MODULATOR INPUT
`
`DATA AT MODULATOR OUTPUT
`
`I-CHANtEL I x[ n]
`
`I
`
`'
`
`0-CHANNEL I
`
`y[ n]
`
`I
`I
`I
`
`x[ n]
`
`Hy[ nl
`
`x[ n+ 1] Hy[ n+ 1]
`
`y[ n]
`
`Hx[ nl
`
`y[ n+ 1] Hx[ n+ 1]
`
`I xi"' II I
`I y[ n· 1] I
`
`I
`I
`
`I
`I
`I
`
`Ex. 1023 / Page 6 of 14
`
`

`

`U.S. Patent
`
`Jul. 18, 2000
`
`Sheet 6 of 8
`
`6,091,781
`
`FIG. 6
`
`60~
`
`xi n]
`
`INTERPOLATE
`WITH ZEROS
`
`62
`
`y[ n]
`
`INTERPOLATE
`WITH ZEROS
`
`64
`
`66
`
`DELAY
`
`HILBERT
`FILTER
`
`74
`
`68
`
`HILBERT
`FILTER
`
`DELAY
`
`76
`
`78
`
`cos(wctl
`
`70
`
`80
`
`PULSE-
`SHAPING
`FILTER
`
`86
`
`z ( tl
`
`PULSE-
`SHAPING
`FILTER
`
`84
`
`72
`
`82
`
`sin( we tl
`
`Ex. 1023 / Page 7 of 14
`
`

`

`U.S. Patent
`
`Jul. 18, 2000
`
`Sheet 7 of 8
`
`6,091,781
`
`c::
`)::(
`
`C>
`C>
`........
`
`C'U
`........
`........
`
`........
`'
`
`LL :c
`
`C>
`........
`........
`
`CD
`en
`
`LL
`:::c
`
`CD
`C>
`........
`
`LL
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`
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`tL
`
`(.!)
`
`Cl
`
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`
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`
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`en
`
`LL
`C1...
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`
`u
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`en
`a
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`
`( .......
`
`c::>
`en
`
`Cl
`
`-
`
`<C
`
`LL
`C1...
`__J
`
`--
`
`c::>
`........
`
`C'U
`C>
`........
`
`.......
`u
`J
`c::
`en
`
`C'U
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`
`Ex. 1023 / Page 8 of 14
`
`

`

`Jul. 18, 2000
`
`wn
`
`—~
`
`—
`o
`
`3c—
`
`_
`
`U.S. Patent
`
`Sheet 8 of 8
`
`6,091,781
`
`Ex. 1023 / Page 9 of 14
`
`

`

`1
`SINGLE SIDEBAND TRANSMISSION OF
`QPSK, QAM AND OTHER SIGNALS
`
`6,091,781
`
`2
`
`(2)
`
`The complex baseband-equivalent representation of the
`transmitted QPSK signal z(t) may be defined as:
`
`(3)
`
`Similarly, the complex baseband-equivalent representation
`of the SSE signal w(t) can be written as:
`
`(4)
`
`where x(t)=H { x(t)} and His the Hilbert transform operator.
`If the conventional QPSK signal as defined in (3) is trans(cid:173)
`formed into an SSE signal, the resulting signal is given by:
`
`(x-y)+j(x+y).
`
`(5)
`
`It can be seen from (5) that a conventional SSE transfor(cid:173)
`mation of a QPSK signal results in a catastrophic interfer(cid:173)
`ence between the I and Q components that cannot be
`removed at the receiver. As a result, SSE transmission is
`generally not utilized in QPSK communication systems.
`Similar problems have prevented the use of SSE transmis(cid:173)
`sion with other types of similar modulation techniques,
`including quadrature-amplitude modulation (QAM).
`
`SUMMARY OF THE INVENTION
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to communication
`systems and more particularly to communication systems
`which utilize quaternary-phase-shift-keying (QPSK),
`quaternary-amplitude modulation (QAM) or other similar
`signal transmission techniques.
`
`5
`
`10
`
`BACKGROUND OF THE INVENTION
`
`Modulation techniques based on QPSK are commonly
`used in cellular, personal communication service (PCS) and
`other types of wireless communication systems. For 15
`example, QPSK and offset QPSK (OQPSK) are used in
`digital wireless systems configured in accordance with the
`IS-95 standard as described in TIAJEINIS-95, "Mobile
`Station - Base Station Compatibility Standard for Dual(cid:173)
`Mode Wideband Spread Spectrum Cellular System," June 20
`1996. Other wireless system standards, including IS-54,
`IS-136 and GSM, also make use of QPSK or a variant
`thereof. FIG. lAshows a conventional QPSK modulator 10.
`An in-phase (I) signal x[ n] is passed through a pulse-shaping
`filter 12, and the output of filter 12 is modulated onto a 25
`cosine carrier signal cos(wct) in mixer 14. A quadrature(cid:173)
`phase ( Q) signal y[ n] is passed through a pulse-shaping filter
`16, and the output of filter 16 is modulated onto a sine carrier
`signal sin(wct) in mixer 18. The I and Q radio frequency
`(RF) signals from mixers 14 and 18 are supplied as inputs to 30
`a signal combiner 20. The signal combiner 20 combines the
`I and Q RF signals to form an output QPSK signal z(t) which
`may be transmitted over a communication channel to a
`receiver. QPSK modulation thus involves transmitting inde(cid:173)
`pendent signals on the I and Q components of the signal z(t). 35
`Single sideband (SSE) modulation is a modulation tech(cid:173)
`nique that has historically received considerably more atten(cid:173)
`tion for analog rather than digital transmission applications,
`and is described in greater detail in, for example, W. E. Sabin
`and E. 0. Schoenike (Eds.) "Single Sideband Systems &
`Circuits," 2nd Edition, McGraw-Hill, New York, 1995. FIG.
`lB shows a conventional discrete-time SSE modulator 30.
`An in-phase signal x[ n] is passed through a delay element 32
`and a pulse-shaping filter 34, and the output of filter 34 is
`modulated onto cos(wct) in mixer 36. Unlike QPSK
`modulation, which as described above transmits indepen(cid:173)
`dent signals x[ n] and y[ n] in its respective I and Q
`components, SSE modulation transmits x[n] in the I com(cid:173)
`ponent and the Hilbert transform of x[ n] in the Q compo(cid:173)
`nent. The Q component in SSE modulator 30 is therefore
`generated by passing x[ n] through a Hilbert filter 38 and a
`pulse-shaping filter 40, and modulating the output of filter 40
`onto sin( wet) in mixer 42. A signal combiner 44 combines
`the I and Q RF signals from mixers 36 and 42 to generate an 55
`SSE signal w(t) for transmission. While SSE modulation
`transmits half the number of bits as QPSK modulation, it
`also utilizes half the bandwidth, such that SSE and QPSK
`modulation have the same spectral efficiency.
`A conventional QPSK signal generally cannot be trans(cid:173)
`mitted as an SSE signal. For example, the QPSK signal z(t)
`generated by QPSK modulator 10 may be expressed as:
`
`The present invention provides techniques which allow
`signals modulated using QPSK, QAM or other similar
`modulation formats to be transmitted as SSE signals. As a
`result, the invention provides the benefits of SSE transmis(cid:173)
`sion in communication systems utilizing QPSK, QAM and
`other modulation formats. In an illustrative embodiment of
`the invention, an in-phase data signal x[ n] and a Hilbert
`transform HY of a quadrature-phase data signal y[ n] are
`modulated onto a cosine carrier signal, and the quadrature-
`40 phase data signal y[ n] and a Hilbert transform Hx of the
`in-phase data signal x[ n] are modulated onto a sine carrier
`signal. The x[ n] and y[ n] signals are time-aligned signals
`which are interpolated prior to modulation so as to include
`zero values at alternating instants of time. Their correspond-
`45 ing Hilbert transforms Hx and HY therefore also exhibit
`alternating zero values. This arrangement of alternating
`zeros allows x[ n] to be interleaved with H and y[ n] to be
`interleaved with Hx, without creating ;ny interference
`between x[ n] and y[ n] in the modulation process. The
`modulated cosine and sine carrier signals are then combined
`50 to generate a modulated SSB-QPSK signal for transmission.
`The SSB-QPSK signal can be demodulated in a receiver
`which uses coherent quadrature detection to recover both the
`x[ n] and y[ n] data signals.
`The modulation techniques of the invention provide sub-
`stantially the same spectral efficiency as conventional SSE
`and QPSK modulation, but can provide advantages over
`both SSE and QPSK in particular applications. For example,
`in the presence of equalization imperfections on Rayleigh-
`60 faded mobile radio channels, the SSB-QPSK modulation of
`the invention can provide better bit error rate (BER) per(cid:173)
`formance than conventional SSE or QPSK modulation.
`
`z(t)~{ x[ n ]*g(t) }cos(w ct)+{y[ n ]*g(t) }sin( w ct).
`
`(1)
`
`65
`
`Given that x[ n ], y[ n ]E { ±1} for QPSK signaling, the trans(cid:173)
`mitted signal z(t) can be written as:
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. lA shows a conventional QPSK modulator.
`FIG. lB shows a conventional discrete-time SSE modu(cid:173)
`lator.
`
`Ex. 1023 / Page 10 of 14
`
`

`

`6,091,781
`
`3
`FIGS. 2A and 2B show an impulse function and its Hilbert
`transform, respectively.
`FIGS. 3A and 3B show an impulse train and its Hilbert
`transform, respectively.
`FIGS. 4A and 4B show a zero-interpolated impulse train
`and its Hilbert transform, respectively.
`FIGS. SA, SB and SC compare modulation formats for
`conventional QPSK, conventional SSE, and SSB-QPSK in
`accordance with an illustrative embodiment of the invention.
`FIG. 6 shows an exemplary SSB-QPSK transmitter in
`accordance with the invention.
`FIG. 7 shows a dual-branch SSE receiver in accordance
`with the invention.
`FIG. 8 shows an SSB-QPSK receiver in accordance with 15
`the invention.
`
`4
`given application. Certain windowing techniques can also be
`used to further reduce the number of taps. These and other
`details regarding Hilbert transformers are described in, for
`example, A. V. Oppenheim and R. W. Schafer, "Discrete-
`5 Time Signal Processing," Prentice-Hall, N.J., 1989, which is
`incorporated by reference herein.
`The ideal Hilbert transformer characterized above may be
`made causal by introducing a delay ofna=(N-1)/2, assuming
`that N is odd. The resulting impulse response is given by:
`
`10
`
`h[n] =
`
`{
`
`2sin 2 [n(n-nd)/2]
`n(n - nd)
`
`0
`
`(8)
`
`Due to the rt/2 term appearing in the argument of the sin
`function in (8), the impulse response h[ n] goes to zero every
`other time instant, i.e., h[ n] is zero for n=na, n=na±2, n=na±4
`and so on. By way of example, FIG. 2A shows an impulse
`function x[n] and FIG. 2B shows the corresponding impulse
`20 response h[ n ]=H { x[ n ]} of the Hilbert transformer for na=9.
`It can be seen from FIG. 2B that the impulse response h[n]
`goes to zero for values of n=l, 3, 5, 7, 9, 11 and so on.
`If the input to a causal Hilbert transformer is a train of
`impulse functions, such as that shown in FIG. 3A, then the
`25 alternating zeros do not appear in the corresponding Hilbert
`transform H{x[n]}, as shown in FIG. 3B. The train of
`impulse functions in FIG. 3Amay be expressed as ~ko(n-k).
`The corresponding Hilbert transform in FIG. 3B is given by:
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`The invention will be illustrated below in conjunction
`with an exemplary communication system utilizing single
`sideband (SSE) quaternary-phase-shift-keying (QPSK)
`modulation. It should be understood, however, that the
`invention is not limited to use with any particular type of
`communication system or modulation format, but is instead
`more generally applicable to any system in which it is
`desirable to transmit signals modulated using QPSK,
`quaternary-amplitude modulation (QAM) or other similar
`modulation techniques in an SSE format. For example, the
`invention may be used in a variety of wireless communica- 30
`tion systems, including systems configured in accordance
`with the IS-54, IS-95, IS-136 and GSM standards. Addi(cid:173)
`tional details regarding these and other communication
`systems in which the invention may be utilized can be found
`in, for example, T. S. Rappaport, "Wireless Communica- 35
`tions: Principles and Practice," Prentice-Hall, N.J., 1996,
`which is incorporated by reference herein.
`The invention provides techniques which enable signals
`modulated in QPSK, QAM and other similar modulation
`formats to be transmitted as SSE signals. As a result, the 40
`invention provides the benefits of SSE transmission in
`communication systems utilizing QPSK, QAM and other
`similar modulation formats. In order to illustrate the opera(cid:173)
`tion of the invention, the discrete Hilbert transform will first
`be described in greater detail. An ideal Hilbert transformer 45
`may be considered an all-pass filter that rotates an input
`signal by 90 degrees. The frequency response of the ideal
`Hilbert transformer is therefore given by:
`
`-j O<w:;;n
`
`H(w) =
`
`{
`
`0 w = 0
`j -Jr< w < 0
`
`(6) 50
`
`The resulting impulse response of the ideal Hilbert trans- 55
`former is then given by:
`
`h[n] =
`
`{
`
`2sin 2 (nn/2)
`nn
`
`0
`
`n * O
`
`n=O
`
`(7)
`
`60
`
`' \ ' 2sin 2 [n(n -nd -k)/2]
`U
`n(n-nd -k)
`
`k
`
`(9)
`
`It can be seen from (9) that, since the index k increments by
`1 during the summation process, only half of the terms
`contributing to the final sum will be zero. Therefore, for any
`value of n, the sum would not be zero, and the alternating
`zeros illustrated in conjunction with FIG. 2B would there(cid:173)
`fore not appear in the Hilbert transform. This provides an
`illustration of why a QPSK signal cannot be transformed
`into an SSE signal using conventional techniques. More
`particularly, if the signal x[ n] modulates the cosine carrier
`and its Hilbert transform modulates the sine carrier, as in the
`conventional SSE modulator 30 of FIG. lB, modulating the
`QPSK quadrature-phase signal y[n] on the sine carrier
`would lead to catastrophic interference with the Hilbert
`transform, as was previously described.
`The invention allows a QPSK signal to be transmitted as
`an SSE signal by recovering the alternating zeros in the
`Hilbert transform of in-phase signal x[ n ], and interleaving
`the quadrature-phase signal y[ n] into Hilbert transform at the
`locations of the alternating zeros. In other words, in accor(cid:173)
`dance with the invention, the quadrature-phase signal y[n]
`can be inserted in locations where the Hilbert transform of
`x[n] is zero. For example, if x[n] is an impulse train
`containing alternating zeros as shown in FIG. 4A, x[n] can
`be expressed as ~ko(n-2k), and the corresponding Hilbert
`transform illustrated in FIG. 4B is then given by:
`
`' \ ' 2sin 2 [n(n -nd - 2k)/2]
`U
`n(n- nd - 2k)
`
`k
`
`(10)
`
`As can be seen from (7), the impulse response of the Hilbert
`transformer is non-causal and infinite in duration. In prac(cid:173)
`tical applications, the Hilbert transformer includes a finite
`number N of filter taps, where N is selected based on the
`degree of SSE suppression that needs to be achieved in a
`
`65 In the summation (10), if n-na is even, then the argument of
`the sin function will remain even over the entire summation
`process and a zero sum would be obtained, as can be seen
`
`Ex. 1023 / Page 11 of 14
`
`

`

`6,091,781
`
`5
`from FIG. 4B. If the signal x[n] appears as a delta function
`at nd, i.e., x[ n ]=(n-na), then the value of the Hilbert trans(cid:173)
`form at n=na would be zero. Similarly, when x[n]=o(n-na-
`2), the value of the Hilbert transform at n=na+2 would be
`zero. Thus, wherever x[ n] is non-zero, its Hilbert transform
`at that corresponding instant is zero and vice-versa. If y[ n]
`can be generated such that it is zero at alternating instants in
`time, then its Hilbert transform would also exhibit alternat(cid:173)
`ing zeros.
`An illustrative embodiment of the invention thus gener(cid:173)
`ates two signals x[ n] and y[ n] which have a value of zero at
`alternating instants of time, such that their Hilbert trans(cid:173)
`forms also exhibit alternating zeros. If the non-zero values
`of x[n] and y[n] are time aligned, then the non-zero values
`of their respective Hilbert transforms are also time aligned.
`As described in conjunction with FIG. lA, conventional
`QPSK modulation generally transmits x[ n] on the cosine
`carrier and y[ n] on the sine carrier. A QPSK signal can be
`transmitted as an SSE signal in accordance with the illus(cid:173)
`trative embodiment of the invention by transmitting the
`Hilbert transform ofx[ n ], designated Hx, on the sine carrier,
`and the Hilbert transform ofy[ n ], designated HY, on the
`cosine carrier. Thus, if x[ n] and y[ n] are time aligned, HY will
`interleave with x[ n] without any interference and similarly
`Hx will interleave with y[ n] without any interference. These
`and other techniques of transmitting a QPSK signal in an
`SSE format in accordance with the invention will be gen(cid:173)
`erally referred to herein as SSB-QPSK modulation.
`FIGS. SA, SB and SC compare transmission formats for
`conventional QPSK modulation (FIG. SA) and conventional
`SSE modulation (FIG. SB) with SSB-QPSK modulation in
`accordance with the invention (FIG. SC). It is assumed that
`the transmission bandwidth is the same for each of the three
`transmission formats. In the case of conventional QPSK
`transmission, signal data introduced in the I-channel (i.e.,
`x[ n ], x[ n+ 1 ], . . . ) and signal data introduced in the
`Q-channel (i.e., y[ n ], y[ n+ 1 ], ... ) are modulated by a QPSK
`modulator onto respective cosine and sine carriers after
`pulse shaping, as illustrated in FIG. SA In the case of
`conventional SSE transmission, signal data is introduced 40
`only in the I-channel (i.e., x[n], x[n+l], ... ), and the SSE
`modulator extracts the signal data for the Q-channel by
`generating Hilbert transforms (i.e., Hx[n], HJn+l], ... ) of
`the I-channel data, as illustrated in FIG. SB.
`In the case of SSB-QPSK modulation, signal data is 45
`introduced in both the I-channel and the Q-channel as shown
`in FIG. SC. An SSB-QPSK modulator, to be described in
`greater detail below, interpolates between the introduced
`data with zeros, and then extracts the Hilbert transforms,
`such that the introduced data and the corresponding Hilbert 50
`transforms are arranged as shown for transmission. The
`I-channel in the SSB-QPSK transmission format includes
`the data introduced in the I-channel (i.e., x[n], x[n+l], ... )
`interleaved with the Hilbert transforms (i.e., Hy[n],
`Hy[ n+ 1 ], ... ) of the data introduced in the Q-channel (i.e., 55
`y[n], y[n+l], ... ). Similarly, the Q-channel in the SSB(cid:173)
`QPSK transmission format includes the data introduced in
`the Q-channel interleaved with the Hilbert transforms (i.e.,
`Hx[n], Hx[n+l], ... ) of the I-channel data.
`FIG. 6 shows an exemplary SSB-QPSK transmitter 60 60
`which implements the above-described illustrative embodi(cid:173)
`ment of the invention. The transmitter 60 includes interpo(cid:173)
`lation devices 62 and 64 for interpolating with zeros
`between the introduced data of the input signals x[ n] and
`y[ n ], respectively. The interpolated signal x[ n] is separated 65
`into two parts. One part is delayed in a delay element 66, and
`the other part is Hilbert transformed in a Hilbert filter 68.
`
`6
`The delayed version of x[ n] from delay element 66 is applied
`to a signal combiner 70, and the Hilbert transform of x[ n] is
`applied to another signal combiner 72. The delay of delay
`element 66 is selected to match the delay introduced by the
`5 Hilbert filter 68.
`The interpolated signal y[ n] is similarly processed using
`a Hilbert filter 74 and a delay element 76, with the Hilbert
`transform of y[ n] applied to signal combiner 70, and the
`delayed version ofy[n] applied to a signal combiner 70. The
`10 delay in delay element 76 is selected to match the delay
`introduced by the Hilbert filter 74. The signal combiner 70
`thus sums the Hilbert transform of y[ n] with the delayed
`version of x[ n ], and the signal combiner 72 sums the Hilbert
`transform of x[ n] with the delayed version ofy[ n ], to produce
`15 I-channel and Q-channel data signals similar to those shown
`in FIG. SC. The summation operations performed by signal
`combiners 70 and 72 in FIG. 6 may thus be viewed as
`time-interleaving operations. The I-channel data signal is
`then pulse shaped in a filter 78 and the pulse-shaped signal
`20 is modulated on a cosine carrier signal cos( wet) in a mixer
`80. Similarly, the Q-channel data signal is pulse shaped in a
`filter 82 and modulated on a sine carrier signal sin( wet) in a
`mixer 84. The I-channel and Q-channel RF signals from
`mixers 80 and 84 are combined in a signal combiner 86 to
`25 generate an SSB-QPSK signal z(t) in accordance with the
`invention.
`The operation of an SSB-QPSK receiver in accordance
`with the invention will be described below in conjunction
`with FIGS. 7 and 8. A conventional single-branch SSE
`30 receiver implementing a coherent analog demodulation pro(cid:173)
`cess mixes a received SSE signal with a locally-generated
`cosine carrier, and then low pass filters the result to recover
`x(t). Information arriving on the sine term of the SSE signal
`is usually ignored. FIG. 7 shows a dual-branch SSE receiver
`35 90 in which a received SSE signal w(t) is quadrature
`demodulated in accordance with the invention to recover
`information from both the cosine and sine terms of the SSE
`signal. The I-channel information arriving on the cosine
`term of the SSE signal w(t) is coherently demodulated by
`mixing w(t) with cos( wet) in mixer 92, and low pass filtering
`the result in low pass filter (LPF) 94. The output of the LPF
`94 is converted to a digital signal in analog-to-digital (ND)
`converter 96, and the digital signal is passed through a
`matched filter (MF) 98 and then applied to an input of a
`signal combiner 100. The Q-channel information arriving on
`the sine term of the SSE signal w(t) is coherently demodu-
`lated by mixing w(t) with sin(wet) in mixer 102, and low
`pass filtering the result in LPF 104. The output of the LPF
`104 is converted to a digital signal in AID converter 106, and
`the digital signal is passed through an MF 108. The output
`of the MF 108 is then Hilbert transformed in a Hilbert filter
`(HF) 110.
`From equation ( 4) above it can be seen that the Q-channel
`information on the sine term of the SSE signal corresponds
`generally to x=H {x}. In order to obtain x from H {x}, the
`receiver 90 makes use of the property of the Hilbert trans-
`form that H{H {x}}=-x. Therefore, the output of HF 110,
`which corresponds to H{H{x}} or -x, is inverted by mul(cid:173)
`tiplying it with -1 in multiplier 112, so as to obtain x. The
`output of multiplier 112 is summed with the output of MF 98
`in signal combiner 100, and the result is thresholded in a
`threshold device 114 to recover x[ n]. Since the signals
`applied to signal combiner 100 add coherently while the
`noise adds incoherently, the signal-to-noise ratio is effec(cid:173)
`tively doubled after the summation in signal combiner 100.
`The receiver 90 thus delivers substantially the same bit error
`rate (BER) performance as a conventional QPSK receiver. In
`
`Ex. 1023 / Page 12 of 14
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`

`6,091,781
`
`7
`contrast, the BER performance of the above-noted conven(cid:173)
`tional single-branch SSE receiver is approximately 3 dB
`worse than that of either the dual-branch SSE receiver 90 or
`the conventional QPSK receiver.
`FIG. 8 shows an SSB-QPSK receiver 120 in accordance 5
`with an illustrative embodiment of the invention. The SSB(cid:173)
`QPSK receiver 120 demodulates the above-described SSB(cid:173)
`QPSK signal using the dual-branch quadrature demodula(cid:173)
`tion techniques illustrated in FIG. 7. An incoming SSB(cid:173)
`QPSK signal is quadrature demodulated, with mixer 122, 10
`LPF 124, ND converter 126 and MF 128 used to recover the
`I-channel information on the cosine carrier, and mixer 132,
`LPF 134, ND converter 136 and MF 138 used to recover the
`Q-channel information on the sine carrier. The outputs of the
`MFs 128 and 138 are applied to an equalizer 130 which 15
`removes intersymbol interference (ISi) which may have
`been introduced in the transmission channel. The resulting
`output signals are converted from serial to parallel form in
`serial-to-parallel (SIP) converters 140 and 150. The cosine(cid:173)
`demodulated I-channel signal at the output of SIP converter 20
`140 corresponds to a real signal, while the sine-demodulated
`Q-channel signal at the output of SIP converter 150 corre(cid:173)
`sponds to an imaginary signal. The real signal from SIP
`converter 140 is split into an x[ n] data part and an HY Hilbert
`transform part. Similarly, the imaginary signal from SIP 25
`converter 150 is split into ay[ n] data part and an Hx Hilbert
`transform part. This composition of the I-channel and
`Q-channel signals was described above in conjunction with
`FIG. SC. The HY Hilbert transform part from SIP converter
`140 is processed through Hilbert filter 142 and multiplier 30
`144 in the manner described in conjunction with FIG. 7, and
`then combined in signal combiner 146 with the y[ n] data part
`from SIP converter 150. The resulting combined signal is
`thresholded in threshold device 148 to yield the output
`signal y[ n]. Similarly, the Hx Hilbert transform part from SIP 35
`converter 150 is processed through Hilbert filter 152 and
`multiplier 154, combined in signal combiner 156 with the
`x[n] data part from SIP converter 140, and the resulting
`combined signal is thresholded in threshold device 160 to
`yield the output signal x[ n].
`The SSB-QPSK modulation techniques of the invention
`provide substantially the same spectral efficiency as con(cid:173)
`ventional SSE and QPSK modulation, but can provide
`advantages over both SSE and QPSK in particular applica(cid:173)
`tions. For example, in the presence of equalization imper- 45
`fections on Rayleigh-faded mobile radio channels, the SSB(cid:173)
`QPSK modulation of the invention can provide better BER
`performance than conventional SSE or QPSK modulation.
`The above-described embodiments of the invention are
`intended to be illustrative only. Numerous alternative
`embodiments within the scope of the following claims will
`be apparent to those skilled in the art.
`What is claimed is:
`1. A method of generating a modulated single sideband
`signal for transmission in a communication system, the
`method comprising the steps of:
`modulating an in-phase data signal and a Hilbert trans(cid:173)
`form of a quadrature-phase data signal onto a first
`carrier signal; and
`modulating the quadrature-phase data signal and a Hilbert
`transform of the in-phase data signal onto a second
`carrier signal, such that the modulated first and second
`carrier signals when combined provide the modulated
`single sideband signal.
`2. The method of claim 1 wherein the modulated single
`sideband signal is a single sideband quaternary-phase-shift(cid:173)
`keying (QPSK) signal.
`
`8
`3. The method of claim 1 wherein the step of modulating
`an in-phase data signal and a transform of a quadrature(cid:173)
`phase data signal onto a first carrier signal further includes
`modulating an interpolated in-phase data signal and a Hil(cid:173)
`bert transform of an interpolated quadrature-phase data
`signal onto a cosine carrier signal.
`4. The method of claim 1 wherein the step of modulating
`the quadrature-phase data signal and a transform of the
`in-phase data signal onto a second carrier signal further
`includes modulating an interpolated quadrature-phase data
`signal and a Hilbert transform of an interpolated in-phase
`data signal onto a sine carrier signal.
`5. The method of claim 1 wherein the in-phase and the
`quadrature-phase data signals are time-aligned signals.
`6. The method of claim 1 wherein the step of modulating
`an in-phase data signal and a transform of a quadrature(cid:173)
`phase data signal onto a first carrier signal further includes
`time interleaving portions of the in-phase data signal with
`Hilbert transforms of portions of the quadrature-phase data
`signal.
`7. The method of claim 1 wherein the step of modulating
`the quadrature-phase data signal and a transform of the
`in-phase data signal onto a second carrier signal further
`includes time interleaving portions of the quadrature-phase
`data signal with Hilbert transforms of portions of the
`in-phase data signal.
`8. The method of claim 1 wherein the in-phase and the
`quadrature-phase signals are interpolated so as to include
`zero values at alternating instants of time, such that their
`corresponding Hilbert transforms also exhibit alternating
`zero values.
`9. An apparatus for of generating a modulated single
`sideband signal for transmission in a communication
`system, the apparatus comprising:
`an in-phase channel operative to modulate an in-phase
`data signal and a Hilbert transform of a quadrature(cid:173)
`phase data signal onto a first carrier signal; and
`a qu