United States Patent [19]
`Marchetto et al.
`
`Illlll||||l|||Illllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`
`USOO5414734A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,414,734
`May 9, 1995
`
`[75]
`
`[73]
`
`Assignee:
`
`[54] COMPENSATION FOR MULTI-PATH
`INTERFERENCE USING PILOT SYMBOLS
`Inventors: Robert F. Marchetto, Burnaby; Todd
`A. Stewart, Vancouver; Paul K.-M.
`Ho, Surrey, all of Canada
`Glenayre Electronics, Inc., Charlotte,
`NC
`App]. No.: 1,061
`Filed:
`Jan. 6, 1993
`
`[21]
`[22]
`[5 1]
`
`[52]
`
`[58]
`
`[56]
`
`Int. Cl.6 ..................... .. H04L 1/02; H04L 25/08;
`H04B l/10; H04B 15/00
`US. Cl. .................................. .. 375/267; 375/285;
`375/346; 375/347; 375/350
`Field of Search ................. .. 375/99, 100, 102, 40,
`375/58, 94, 103, l2, 14, 11; 455/303, 304, 306,
`65, 521; 364/7241, 577; 370/1101, 110.2,
`110.3, 110.4
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`2,858,529 10/ 1958 Black et al. .
`
`................... .. 340/248
`
`
`
`
`
`3,717,814 2/1973 Gans ...... .. 4,146,838 3/ 1979 Takada 4,675,880 6/1987 Davarian ...... ..
`
`5,018,166 5/1991 Tiahjadi et al.
`5,091,918 2/1992 Wales ............... ..
`5,109,390 3/ 1992 Gilhousen et al. .
`5,127,051 6/1992 Cham et a1. ...... ..
`5,140,615 8/1992 Tasker et al. .... ..
`5,170,413 12/1992 Hess et al. ........ ..
`5,191,598 3/1993 B'zickstriim et a1. ............... .. 375/100
`
`OTHER PUBLICATIONS
`“Adaptive Equalization and Diversity Combining for a
`Mobile Radio Channel”, 1990 IEEE, Lo et al. pp.
`507A.2.1-2.5.
`Cavers, James K., “An Analysis of Pilot Symbol As
`sisted Modulation for Rayleigh Fading Channels,”
`IEEE Transactions on Vehicular Technology, vol. 40,
`No. 4, Nov. 1991, ©1991 IEEE, pp. 686-693.
`Lo, Norm W. K. Falconer, David D., and Sheikh,
`Asrar U. H., “Adaptive Equalization and Diversity
`
`Combining for a Mobile Radio Channel,” @1990,
`IEEE, pp. 923-927.
`Moher, Michael L. and Lodge, John H., “TCMP -A
`Modulation and Coding Strategy for Rician Fading
`Channels,” Reprinted from IEEE Journal on Selected
`Areas in Communications, vol. 7, No. 9, Dec. 1989,
`@1989, pp. 1347-1355 plus cover page.
`Primary Examiner-Stephen Chin
`Assistant Examiner-Tesfaldet Bocure
`Attorney, Agent, or Firm-Christensen, O’Connor,
`Johnson & Kindness
`
`ABSTRACT
`[57]
`A method and apparatus for compensating fading and
`interference in a radio signal. A plurality of pilot sym
`bols are appended to a plurality of data symbols to form
`successive frames that are modulated at a transmitter.
`The transmitted modulated signal is subject to loss of
`data due to simple fading and multi-path and simulcast
`interference. The received signals are demodulated by a
`receiver and processed to provide a data signal compris
`ing the data symbols and a pilot signal comprising the
`pilot symbols. The data signal is delayed for suf?cient
`time to enable channel impulse response estimates to be
`made of successive blocks of pilot symbols, preferably
`using pilot symbol blocks that both precede and follow
`the data symbols in the frame being processed. The
`channel impulse response estimates for blocks of pilot
`symbols are buffered and used by an interpolator to
`determine an interpolated channel impulse response
`estimate for each data symbol as a function of both the
`pilot symbols and of prede?ned channel characteristics.
`The interpolated channel impulse response estimates are
`applied to successive data symbols in the delayed data
`signal, enabling the data to be decoded, compensating
`for fading and interference. Interpolation using prede
`fined channel characteristics based on worst case condi
`tions substantially improves the bit error rate (BER) for
`the data recovered, compared to the prior art.
`
`22 Claims, 8 Drawing Sheets
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`US. Patent
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`May 9, 1995
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`US. Patent
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`May 9, 1995
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`US. Patent
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`May 9, 1995
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`May 9, 1995
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`US. Patent
`
`May 9, 1995
`
`Sheet 6 of 8
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`5,414,734
`
`GET DATA BLOCK
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`BLOCK TO FORM A FRAME
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`May 9, 1995
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`Sheet 7 of 8
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`5,414,734
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`SIGNAL FROM DATA SIGNAL
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`US. Patent
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`May 9, 1995
`
`Sheet 8 of 8
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`5,414,734
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`1
`
`5,414,734
`
`COMPENSATION FOR MULTI-PATH
`INTERFERENCE USING PILOT SYMBOLS
`
`FIELD OF THE INVENTION
`The present invention generally relates to apparatus
`and a method for minimizing the effects of multi-path
`and simulcast interference on a radio signal, and more
`speci?cally, to apparatus and a method for using pilot
`symbols to determine the effects of propagation on a
`received radio signal.
`
`BACKGROUND OF THE INVENTION
`Radio signals are subject to several propagation phe
`.nomena that greatly impact on the strength of the signal
`at a receiver. If the signal re?ects from buildings or
`other man-made or natural surfaces, the re?ected signal,
`which follows a different, longer path than a direct
`signal, can arrive at the receiver out of phase with the
`direct signal; the resulting interference between the two
`or more signals reduces the signal strength at the re
`ceiver. This phenomenon is multi-path interference.
`A related type of interference arises in simulcast pag
`ing systems wherein the same signal is broadcast from a
`plurality of transmitters. A receiver disposed in a zone
`where identical transmitted signals from two or more
`transmitting stations overlap can experience a reduction
`in the strength of the overall received signal, due to a
`destructive interference that occurs when the radio
`signals received from the different transmitters are
`summed together, since the propagation distance, and
`therefore, the phase of the signals can be quite different.
`Simulcast interference is thus similar in its effect, but
`differs from multi-path interference in that the former
`involves interference between signals from different
`transmitters, while the latter involves interference be
`tween signals from the same transmitter that travel
`along different propagation paths. In addition, since it is
`virtually impossible to precisely set the transmit center
`RF carder frequencies of the overlapping transmissions,
`the receiver in a simulcast paging system experiences an
`additional distortion problem that is not evident in mul
`ti-path interference of signals transmitted from a single
`transmitter. In ?at fading, the frequency response of the
`received signal is ?at and only the gain and phase ?uc
`tuate.
`Flat fading represents yet another phenomenon by
`which received signal strength can be diminished. How
`ever, when tlat fading occurs, only a signal from one
`transmitter is involved, and this signal is propagated
`without re?ection, directly to the receiver.
`To mitigate the effects of multi-path interference at
`the receiver, multi-level low baud rate transmissions are
`sometimes employed. This technique tries to minimize
`the effect of multi-path distortion by making the relative
`time/phase difference in the received multi~path signals
`relatively small compared to the data baud rate. The
`most signi?cant drawback of this approach is its limita
`tion on data rate, which causes poor system perfor
`mance. Further, the bit error rate (BER) of the received
`signal suffers, even when a relatively strong received
`signal is available, and the modulation of a signal at
`multiple levels is complicated by the need to split the
`signal into different sub-bands, which increases proces
`sor loading. Use of multi-level, low baud rate modula
`tion techniques is discussed in “MTEL Petition for Rule
`Making to Allocate Frequencies for New Nationwide
`
`2
`Wireless Network Services,” a petition submitted to the
`FCC on Nov. 12, 1991.
`Another technique applied to reduce the effect of
`multi-path interference is the use of equalizers, includ
`ing fast Kalman equalizers that attempt to rapidly track
`and converge on dynamically changing conditions.
`However, this technique has previously not been pro
`posed for use in a simulcast system. Conventional equal
`izers are relatively slow in adapting to distortion, caus
`ing a relatively high BER until the equalizer properly
`converges. Although faster Kalman equalizers can re
`duce this problem, the algorithm employed in the de
`vices is so computationally complicated that it requires
`significant processing overhead. Also, the algorithm
`employed in Kalman equalizers is somewhat unstable.
`In fact, when rapid fading occurs due to multi-path
`distortion, equalizers tend to become unstable and are
`more likely to fail. A textbook entitled Digital Commu
`nications, 2nd edition, by J. Proakis, published by
`McGraw Hill in 1989, describes the use of equalizers for
`this purpose in Chapter 6, pages 519 through 693.
`Use of pilot symbol assisted modulation (PSAM) is a
`technique well known in the prior art for minimizing
`the effect of ?at fading, and is particularly effective for
`use with mobile receivers. Several references describe
`how BER caused by ?at fading can be substantially
`reduced using pilot symbols, including, for example,
`“An Analysis of Pilot Symbol Assisted Modulation for
`Rayleigh Fading Channels,” by I. K. Cavers, in IEEE
`Transactions on Vehicular Technology, Vol. 40, No. 4,
`November 1991, and “TCMP-A Modulation and Cod
`ing Strategy for Rician. Fading Channels,” IEEE Jour
`nal on Selected Areas of Communications, L. Moher
`and J. H. Lodge, Vol. 7, pp. 1347-1355, December
`1989. This technique uses a single pilot symbol in each
`frame transmitted. These references only discuss the use
`of pilot symbols in connection with reducing the fading
`of a single ray following a direct path from a single
`transmitter site and do not teach or suggest how pilot
`. symbols might be used to deal with multi-path or simul
`cast interference problems.
`A paper entitled “Adaptive Equalization and Diver
`sity Combining for a Mobile Radio Channel,” by N. Lo,
`D. Falconer, and A. Sheikh, Proc. IEEE Globecom ’90,
`December 1990, discloses a digital cellular radio sys
`tem, which employs a jointly adaptive decision-feed
`back equalizer and diversity combiner to mitigate Dop
`pler fading rates up to 100 Hz. As disclosed in this refer
`ence, current estimates of channel impulse response are
`interpolated and the interpolated values applied to sue
`cessive data symbols in blocks of data, to compensate
`for changes in the channel impulse response with time.
`The technique provides for transmitting a plurality of
`prede?ned pilot symbols before each block of data and
`interpolating the estimated channel impulse response
`for the symbols received before and after each block of
`data. The interpolation attempts to minimize the effect
`of relatively fast Doppler fading on the data. However,
`this approach achieves only a modest performance gain,
`because it fails to consider channel conditions, such as
`relative signal strength of the received signals, the Dop
`pler fading frequency, propagation delay differences
`between the interfering signals, frequency offsets be
`tween the interfering signals, and the signal-to-noise
`ratio of the received signals, when carrying out the
`interpolation process. Consequently, the method dis
`closed in this paper can not provide an optimal compen
`
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`5,414,734
`3
`sation under extreme signal fading conditions for the
`channel.
`Accordingly, it should be evident that compensation
`for multi-path and simulcast interference is required
`that enables high-speed data transfer. The prior art
`systems are either limited to low data rates, have too
`high a BER, require too much processing overhead, or
`fail to provide optimal mitigation of the fading problem.
`
`5
`
`4
`and pilot symbols in each frame, and L is a duration for
`the channel impulse response of the received signals.
`Note that throughout this speci?cation and in the
`claims, the notation “[X]” is used to represent X as a
`vector or a matrix.
`In the case where fading is due to an interference
`between at least two signals traveling over different
`propagation paths to the receiver, the interpolation
`?lter means comprise matrix operator means for deter
`mining the interpolated channel impulse response as a
`function of a vector product of a vector representing
`the prede?ned channel characteristics and a vector of
`channel estimates derived from the received pilot sym
`bols. The decoder means preferably comprise a Viterbi
`decoder and preferably implement a reduced complex
`ity decoder algorithm.
`Also in the preferred embodiment, the interpolation
`?lter means use prede?ned channel characteristics, in
`cluding: a Doppler fading frequency, relative signal
`strengths of interfering signals at the receiver, propaga
`tion delay differences between the interfering signals,
`frequency offsets between the interfering signals, and a
`signal-to-noise ratio of the received signals. These pre—
`de?ned channel characteristics are selected based on a
`worst case scenario for fading and interference between
`the received signals.
`Another aspect of the present invention is a method
`for compensating a receiver for fading of signals propa
`gating from at least one transmitter to the receiver as
`frames of data. This method comprises steps that are
`generally consistent with the functions of the elements
`comprising the circuit described above.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The foregoing aspects and many of the attendant
`advantages of this invention will become more readily
`appreciated as the same becomes better understood by
`reference to the following detailed description, when
`taken in conjunction with the accompanying drawings,
`wherein:
`FIG. 1 is a schematic diagram of a radio system in
`which a receiver receives a direct and a reflected signal,
`causing multi-path interference and fading;
`FIG. 2 is a schematic diagram of a simpli?ed simul
`cast radio system‘ in which a receiver receives simulcast
`transmissions from different transmitters, and experi
`ences simulcast interference and fading caused by sum
`ming signals that have propagated over different length
`paths and which may have slightly different frequen
`cies;
`FIG. 3 is a schematic block diagram of a'radio trans
`mitter and receiver in a radio system that incorporates
`the present invention to compensate interference and
`fading between radio signals received at a receiver;
`FIG. 4 is a functional diagram illustrating interfer
`ence and fading in transmitted radio signals;
`FIG. 5 is a discrete time model diagram of a modula
`tion system, showing a received signal (data vector
`[S(k)]), sampled at intervals of time T, and a channel
`state vector [H(k)]);
`FIG. 6 is a diagram showing the relationship between
`the pilot symbols and data symbols in each frame of a
`received signal;
`FIG. 7 is a flow chart showing the steps undertaken
`at the transmitter to include a plurality of pilot symbols
`with data symbols in each frame that is modulated and
`transmitted;
`
`SUMMARY OF THE INVENTION
`In accordance with the present invention, a radio
`system is de?ned for compensating a receiver for fading
`of signal propagating from at least one transmitter to the
`receiver as frames of data. The radio system comprises
`pilot symbol generation means, coupled to transmitters
`in the radio system, for providing a prede?ned plurality
`of pilot symbols in each frame of data transmitted to the
`receiver. Separating means, coupled to the receiver for
`input of a received signal, separate the plurality of pilot
`symbols from a plurality of data symbols in each frame
`of data, producing a pilot symbol signal comprising the
`plurality of pilot symbols and a data signal comprising
`the plurality of data symbols. Delay means, coupled to
`receive the data signal, delay the data signal from a
`current frame until after the pilot symbol signal from at
`least one subsequent frame is received, producing a
`delayed data signal. Pilot signal processing means,
`which are coupled to receive the pilot symbol signal,
`are provided for determining an estimated channel im
`pulse response for the plurality of pilot symbols in each
`frame of the received signal. Interpolation ?lter means,
`coupled to receive the estimated channel impulse re
`sponse for each frame of data include storage means for
`storing an estimated channel impulse response from at
`least one prior frame. The interpolation ?lter means
`interpolate values of the estimated channel impulse
`response between the current and prior frames to deter
`mine, as a function of prede?ned channel characteris
`tics, an interpolated channel impulse response for each
`data symbol in a frame. Decoder means, coupled to
`receive the interpolated channel impulse response and
`the delayed data signal, recover the data transmitted to
`the receiver as a function of the interpolated channel
`impulse response and the delayed data signal, thereby
`substantially compensating for any fading and interfer‘
`ence in the received signal.
`The separating means preferably comprise timing
`means that extract a portion of each frame for the pilot
`symbol signal, as a function of a prede?ned time interval
`during which the plurality of pilot symbols in each
`frame are temporally present in the received signal, and
`then extract a temporal remainder of each frame of the
`received signal for the data signal. Further, the separat
`ing means, the delay means, the pilot signal processing
`means, the interpolation ?lter means, and the decoder
`means preferably comprise a digital signal processor.
`The decoder means select a data vector for each data
`symbol from a set of possible values of data vectors, [S],
`by determining a minimum value for an expression,
`D([S]), after evaluating the expression for each data
`vector in the set. The expression is de?ned as:
`
`s W 2
`— ME“ _1
`D([SD — i=L+1 1'0) — [ (OH (1)]!
`
`where r(i) is the i,;, received data symbol, [W(i)] is the
`interpolated channel impulse response for the i,;, re
`ceived data symbol, M is a total number of data symbols
`
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`FIG. 8 is flow chart illustrating the logical steps im
`plemented at the receiver to recover data from received
`signals that are subject to fading and interference;
`FIG. 9 is a graph of the BER versus signal to noise
`ratio (SNR) at different root mean squared delays, in a
`prior art system that also compensates for fading and
`interference using pilot symbols; and
`FIG. 10 is a graph of the BER versus SNR, at differ
`ent root mean squared delays, for the present invention.
`
`6
`
`where f(t), g(t), and w(t) are all independent complex
`Gaussian processes and d is the relative delay between
`two propagation paths. Specifically, f(t) and g(t) repre
`sent fading and w(t) represents a channel additive white
`Gaussian noise (AWGN), as shown in FIG. 4. In this
`FIGURE, the complex envelope, s(t) represented by
`line 120 is transmitted along separate paths 122 and 124,
`each of which are subject to fading processes f(t) and
`g(t) at multiplier blocks 126 and 128, respectively. The
`signal subject to fading, which is transmitted along a
`path 130, is added to other signals at the receiver an
`tenna (as represented by the summation of signals in an
`adder 140), including the signal traversing a path 132
`that is subject to delay d, as represented in a block 134.
`The resulting delayed signal traverses a path 136 and is
`summed in adder 140 with both the signal traversing
`path 130 and the AWGN, represented by w(t), which is
`represented as an input signal conveyed on a line 138.
`Adder 140 combines theses signals, providing a re
`ceived complex envelope r(t) to receiver 30 for demod
`ulation, as indicated by a line 142. The present invention
`provides a circuit and a method for processing the re
`ceived signal r(t) to recover the data originally transmit
`ted, which would otherwise be subject to a substantial
`BER and loss of transmitted information, due to the
`fading and/or interference between the received sig
`nals.
`Turning now to FIG. 3, a radio system that is com
`pensated for fading and interference in accordance with
`the present invention is generally illustrated at reference
`numeral 38. Radio system 38 includes a receiver 40 and
`one or more transmitters 42 (only one transmitter 42
`being shown in FIG. 3). Transmitter 42 comprises a
`data source 44, which generates a plurality of data sym
`bols that are to be transmitted to receiver 40. In addi
`tion, a pilot symbol generator 46 produces a plurality of
`pilot symbols that are grouped together in a block that
`is transmitted preceding the plurality of data symbols
`generated by data source 44. The block of pilot symbols
`followed by the plurality of data symbols comprise a
`?ame of the transmitted signal.
`FIG. 6 illustrates an exemplary frame 170 of M (total)
`symbols, including (2L+ 1) pilot symbols 174, ranging
`from P(—L) through P(L), and (M-—(2L— 1)) data
`symbols 172. Each successive frame 178 (only a portion
`of the next successive frame is shown in FIG. 6) simi
`larly includes a block of (2L+ 1) pilot symbols 174 and
`a plurality of (M—2L— 1) data symbols 172.
`The baud rate of data source 44 applies equally to
`data source 44 and pilot symbol generator 46, so that the
`overall baud rate at which successive frames of M sym
`bols are transmitted is substantially constant. Data
`source 44 is coupled through lines 48 to a logic switch
`54 that is controlled by a timer (or counter) 52. As a
`function of the total number of symbols M in each
`frame, and the relative number of pilot symbols 174 and
`data symbols 172 in each flame, timer 52 changes the
`condition of logic switch 54, selecting between data
`source 44 and pilot symbol generator 46 to determine
`the type of symbol that is transmitted during portions of
`successive frames. The selected symbols are conveyed
`from logic switch 54 to a framing block 56 that compiles
`the frame of M symbols to be transmitted. Each succes
`sive ?ame 178 is conveyed sequentially on a line 58 to a
`modulator 60 in transmitter 42. Modulator 60 modulates
`a carrier signal with the data and pilot symbols, and the
`resulting modulated signal is conveyed by a line 62 to a
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`As described in the Background of the Invention
`above, a radio receiver is subject to several types of
`fading and interference. Flat fading occurs when a di
`rect signal fades in and out at the receiver. Two other
`types of fading result from interference of a plurality of
`signals, including multi-path and simulcast interference,
`as shown by block diagrams 20 and 32 in FIGS. 1 and 2,
`respectively. In schematic block diagram 20, a transmit
`ter 22 transmits an RF signal from an antenna 24, and
`the signal propagates along a path td directly to an an
`tenna 28, which is coupled to a receiver 30. In addition,
`the signal from transmitter 22 also propagates along a
`path t, and is reflected from a man-made object such as
`a building, or a natural object such as a mountain 26.
`The reflected signal then travels along a path t’rtoward
`antenna 28 at receiver 30. The signal travelling along
`path t’rand the direct signal travelling along path td can
`interfere, depending upon the phase relationship be
`tween the signals at antenna 28. Due to the different
`path lengths followed by the re?ected and direct radio
`signals, a phase shift and gain change can occur be
`tween the two signals. If these reflected and direct sig
`nals are 180° out of phase, maximum fading occurs in
`the signal demodulated by receiver 30.
`Similarly, as shown in schematic block diagram 32 in
`FIG. 2, a ?rst simulcast transmitter 22a and a second
`simulcast trans'initter 22b, which are provided with the
`same input signal, transmit corresponding linearly mod
`ulated RF signals along paths tda and tab, respectively,
`to antenna 28, at receiver 30. The distance traveled by
`each of these nominally identical RF signals and any
`slight differences in frequencies of the two signals can
`cause phase differences at antenna 28 that contribute to
`fading of the signal demodulated by receiver 30, much
`like the interference and fading of signals in the multi
`path example shown in FIG. 1. Furthermore, flat fading
`and multi-path interference may combine with simul
`cast interference, although not speci?cally shown, to
`further exacerbate the fading problem at receiver 30.
`The preferred embodiment of the present invention is
`disclosed in an application wherein it is used to recover
`simulcast paging data at a receiver subject to the three
`types of fading discussed above. The method is particu
`larly applicable to linear modulation, such as in a l6-ary
`quadrature amplitude modulated (16QAM) system. In
`describing the apparatus and the method implemented
`in the present invention, a two-ray model with indepen
`dent Rayleigh fading in each ray is adopted, since this
`channel model is speci?ed by the Telecommunications
`Industry Association (TIA) for evaluating the perfor
`mance of the North American Digital Cellular System.
`Based upon this channel model, if s(t) is a transmitted
`complex envelope, a corresponding received base band
`signal r(t) is de?ned by the following expression:
`
`65
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`Juniper Ex 1008-p. 12
`Juniper v MTel891
`
`

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`5
`
`20
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`25
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`7
`transmitter power ampli?er 64, which is coupled to a
`transmit antenna 66.
`The modulated frames comprising pilot symbols and
`data symbols are radiated from transmit antenna 66. A
`Rayleigh fading channel 1, identi?ed graphically by a
`“lightning bolt 68” in FIG. 3, represents the effects of
`fading experienced by the signal ?nally received at a
`receive antenna 72. Similarly, a Simulcast Rayleigh
`fading channel 2 (identi?ed by “lightning bolt” 70)
`represents the fading experienced by a signal transmit
`ted from another transmitter and/or the signal from
`transmit antenna 66 after it is reflected from a man-made
`or natural object. Interference between these two Ray
`leigh fading channels can cause substantial fading, mak
`ing dif?cult the recovery of the data symbols transmit
`ted in a conventional receiver. However, receiver 40
`includes circuitry that makes use of the pilot symbols
`produced by pilot symbol generator 46 and transmitter
`42, to recover the data symbols affected by fading and
`interference, thereby substantially compensating for
`such undesired effects.
`Receive antenna 72 is coupled to a receiver demodu-.
`lator 74, which demodulates the signal r(t), producing a
`demodulated signal rK. The demodulated signal rK is
`input to a circuit 36 over a line 76. In the preferred
`embodiment, circuit 36 comprises a digital signal pro
`cessor. The demodulated signal on line 76 is conveyed
`to a logic switch 80, which is controlled by a timer (or
`counter) 78 that is synchronized with timer 52 in each
`transmitter 42 (at least in terms of determining the dura
`tion of each frame and the portion of each frame that
`respectively comprises pilot symbols and data symbols).
`Timer 78 causes logic switch 80 to divert the pilot sym
`bol portion of the received demodulated signal to a line
`86 that is coupled to a channel estimator 96, and to
`divert the data symbol portion of each frame to a line
`82, which is coupled to a delay block 84. Thus, logic
`switch 80 separates the pilot symbols from the data
`symbols in each frame received.
`In the preferred embodiment, delay block 84 delays
`40
`successive frames of data symbols by K/2 frames of
`data. This delay enables interpolation of an estimated
`channel impulse response that is applied to successive
`data symbols in each frame to compensate for fast fad
`ing (fast fading being de?ned as fading that occurs at a
`rate in excess of 100 Hz) and to provide more than 80 us
`of equalization for simulcast signals, as will be apparent
`from the following discussion.
`While the data symbols in successive frames are being
`delayed by delay block 84, channel estimator block 96
`processes the current pilot symbols to derive a channel
`impulse response estimate for the current 2L+1 pilot
`symbols that will be used with the channel impulse
`response estimate for corresponding 2L+l pilot sym
`bols in successive and previous frames. The channel
`impulse response estimates for the current frame are
`conveyed via a line 98 for temporary storage in a buffer
`100. Buffer 100 stores K channel impulse response esti
`mates that are input over a line 102 to an interpolator 92.
`Interpolated channel impulse response estimates are
`determined using the K channel impulse response esti
`mates, including the K/2 channel impulse response
`estimates from the previous frames, and the K/2 Chan
`nel impulse response estimates from the current and
`successive frames. The delayed data symbols from
`delay block 84 are then processed with the interpolated
`channel impulse response estimates to recover the data
`symbols subject to fading.
`
`5,414,734
`8
`Interpolator 92 carries out a relatively straightfor
`ward interpolation operation to achieve greater accu
`racy in recovering the data symbols. Under optimum
`conditions, a received signal might be subject to rela
`tively slow fading. Slow fading conditions mean that a
`channel impulse response estimate applied to each of
`the data symbols in a frame would be substantially con
`stant over the duration of the frame. However, fading
`rates up to and exceeding 100 Hz are quite common,
`causing a substantially different channel impulse re
`sponse estimate to apply to the data symbols early in a
`frame, as compared to that which should be applied to
`the data symbols later in the frame. To accommodate
`the rapidly changing channel estimate and minimize the
`BER of the data recovered from the received signal
`during fast fading, it is important that an interpolation of
`the channel impulse response estimate be applied to the
`data symbols over the duration of each frame. In the
`simplest case, a channel impulse response estimate for
`the pilot symbols in the frames immediately before and
`after the data symbols being processed could be applied
`to interpolate a channel impulse response estimate for
`each of the data symbols in the frame. However, a sub
`stantially lower BER can be obtained by using the chan
`nel impulse response estimates from two or three frames
`before and alter the frame of data symbols being pro
`cessed.
`Interpolator 92 in the present invention, unlike the
`prior art system discussed