(12) United States Patent
`US 6,411,649 B1
`(10) Patent N0.:
`Arslan et al.
`
`(45) Date of Patent: Jun. 25, 2002
`
`USOO6411649B1
`
`(54) ADAPTIVE CHANNEL TRACKING USING
`PILOT SEQUENCES
`
`(75)
`
`Inventors: Hiiseyin Arslan, Raleigh; Rajaram
`Ramésh, Cary, both of NC (US)
`
`(73) Assignee: Ericsson Inc., Research Triangle Park,
`NC (US)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/175,699
`
`(22)
`
`Filed:
`
`Oct. 20, 1998
`
`(51)
`
`Int. Cl.7 .................................................. H03H 7/30
`
`....................... 375/232; 375/342; 375/350;
`(52) US. Cl.
`375/364; 375/368; 370/512; 370/514; 370/520
`
`(58) Field of Search ................................. 375/231, 232,
`375/233; 328; 340; 342; 346; 350; 363;
`364; 368; 370/506; 509; 510; 511; 512;
`514; 503; 520; 528; 708/305; 322
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`.......... 375/267
`5/1995 Marchetto et a1.
`5,414,734 A
`........ 375/231
`5,479,444 A * 12/1995 Malkamaki et a1.
`5,517,524 A *
`5/1996 Sato ........................... 375/230
`5,692,015 A * 11/1997 Higashi et a1.
`375/340
`
`......
`5,850,393 A * 12/1998 Adachi
`370/335
`5/1999 Hassan ......
`5,901,185 A *
`375/346
`
`.......
`6,028,852 A *
`2/2000 Miya et a1.
`370/335
`8/2000 Okawa et a1.
`.............. 370/335
`6,097,711 A *
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`EP
`EP
`WO
`WO
`
`0520969 A2
`0529585 A2
`0715440 A1
`WO95/35615
`WO97/39557
`
`12/1992
`3/1993
`6/1996
`12/1995
`10/1997
`
`........... H04L/25/30
`..... H04L/25/30
`
`........... H04L/27/22
`
`........... H04L/25/03
`
`OTHER PUBLICATIONS
`
`Chong et al., “An Analysis of Pilot Symbol Assisted 16
`QAM in the Rayleigh Fading Channel”, IEEE, vol. 41, No.
`4.
`
`Cavers, “Pilot Symbol Assisted Modulation and Differential
`Detection in Fading and Delay Spread”, IEEE, vol. 43, No.
`7.
`
`Cavers, Pilot Symbol Assisted Modulation in Fading and
`Delay Spread, IEEE, 1993, pp. 13—16.
`Cavers, “An Analysis of Pilot Symbol Assisted Modulation
`for Rayleigh Fading Channels”,
`IEEE Transactions on
`Vehicular Technology, vol. 40, No. 4, Nov. 1991, pp.
`686—693.
`
`Cavers et al., “Cochannel Interference and Pilot Symbol
`Assisted Modulation” IEEE, vol. 42, No. 4.
`Lodge, et al., “Time Diversity for Mobile Satellite Channels
`Using Trells Coded Modulations”, IEEE Global Telecom-
`munications Conference, 1987, pp. 303—307.
`Moher et al., “TCMP—A Modulation and Coding Strategy
`for Rician Fading Channels”, IEEE Journal on Selected
`Areas in Communications, vol. 7, No. 9, Dec. 1989, pp.
`1347—1355.
`
`(List continued on next page.)
`
`Primary Examiner—Stephen Chin
`Assistant Examiner—Dac V. Ha
`
`(74) Attorney, Agent, or Firm—Myers Bigel Sibley &
`Sajovec
`
`(57)
`
`ABSTRACT
`
`Methods and systems are provided which utilize pilots in an
`information sequence to periodically retrain a channel esti-
`mator. Thus, a channel tracker may be synchronized using a
`synchronization sequence and then periodically retrained
`using known pilot symbols. Furthermore, the utilization of
`pilots may allow for the detection of errors in previous
`channel estimates. When errors are detected, a new channel
`estimate may be used based on the retraining using the pilot
`symbols and, optionally, previous errors in symbol estima-
`tion may be corrected. Thus, by retraining based on pilot
`symbols, the propagation of errors may be reduced.
`
`28 Claims, 6 Drawing Sheets
`
`Channel Estimation
`500
`Estimals Channel from
`synchronization symbols
`502
`Next symboi
`
`~>
`
`
`
`
`
`505
`
`Pilul
`Revise channel estimate
`?
`symbol
`
`in decision dimmed mode
`
`
`506
`
`Estimate channel In
`training mode
`
`
`
`
`
`
`
`Next frame
`
`—-
`
`
`
`
`ERIC-1003
`
`Ericsson v IV
`
`Page 1 of 16
`
`ERIC-1003
`Ericsson v IV
`Page 1 of 16
`
`

`

`US 6,411,649 B1
`Page 2
`
`OTHER PUBLICATIONS
`
`Sampei, et al., “Rayleigh Fading Compensation Method for
`16QAM in Digital Land Mobile Radio Channels”, Proceed-
`ings of the IEEE Vehicular Technology Conference, 1989,
`pp. 640—646.
`Lindbom, Lars, A Wiener Filtering Approach to the Design
`of Tracking Algorithms With Applications in Mobile Radio
`Communications, pp. 1—255 (Uppsala University 1995).
`Lindbom, Lars, Simplified Kalman Estimation of Fading
`Mobile Radio Channels: High Performance at LMS Com-
`putational Load, Int. Conf. on Acoustics, Speech and Signal
`Processing, 4 pages (Apr. 1993).
`
`Lin, Jingdong, et al., Optimal Tracking of Time—Varying
`Channels: A Frequency Domain Approach for Known and
`New Algorithms, IEEE Transactions on Selected Areas in
`Communications, vol. 13, No. 1, pp. 141—154 (Jan. 1995).
`Gooch, Richard P., et al., Demodulation of Cochannel QAM
`Signals, IEEE, pp. 1392—1393, 1395 (1989).
`D’Andrea, Aldo N., et al., Symbol—Aided Channel Estima-
`tion With Nonselective Rayleigh Fading Channels, IEEE, 9
`pages (1995).
`International Search Report, International Application No.
`PCT/US99/21609.
`
`* cited by examiner
`
`ERIC-1003 / Page 2 of 16
`
`ERIC-1003 / Page 2 of 16
`
`

`

`US. Patent
`
`Jun.25,2002
`
`Sheet170f6
`
`US 6,411,649 B1
`
`ccmammmm
`
`.ommmooi
`
`ll'l'llllllllll'll'll
`
`
`
` EEEGN>nnxa;
`coszEEEcoszBE
`
`mmoo
`
`mEEmfi
`
`m_an>m
`
`v.OE
`
`ERIC-1003 / Page 3 of 16
`
`ERIC-1003 / Page 3 of 16
`
`
`
`
`

`

`US. Patent
`
`Jun. 25, 2002
`
`Sheet 2 0f 6
`
`US 6,411,649 B1
`
`FIG. 2
`
`interface
`
`Antenna
`
`Feed
`
`Structure
`
`User
`
`.
`Transmitter
`
`Receiver
`
`Correcting
`Channel
`
`Estimator
`
`ERIC-1003 / Page 4 of 16
`
`ERIC-1003 / Page 4 of 16
`
`

`

`US. Patent
`
`Jun. 25, 2002
`
`Sheet 3 0f 6
`
`US 6,411,649 B1
`
`
` Symbol or
`
`Sequence
`Estimator
`
`Received
`
`Signal
`
` Detected
`
`Symbols
`
` Adaptive Channel
`31 Estimator
`
`
`
`
`Channel Tracker
`
`
`
`
`Mode
`
`Selector
`
`Known
`
`Symbols
`
`ERIC-1003 / Page 5 of 16
`
`ERIC-1003 / Page 5 of 16
`
`

`

`US. Patent
`
`Jun. 25, 2002
`
`Sheet 4 0f 6
`
`US 6,411,649 B1
`
`Channel Estimation
`
`
`
`
`
`500
`
`Estimate channel from
`
`synchronization symbols
`
`FIG. 5
`
`
`
`
`
`
` s
`
`508
`
`Revise channel estimate
`
`in decision directed mode
`
`Next symbol
`
`Estimate channel in
`
`training mode
`
`512
`
`
`
`ERIC-1003 / Page 6 of 16
`
`ERIC-1003 / Page 6 of 16
`
`

`

`US. Patent
`
`Jun. 25, 2002
`
`Sheet 5 0f 6
`
`US 6,411,649 B1
`
`Channel Estimation
`
`500
`
`Estimate channel from
`
`FIG. 6
`
`synchronization symbols
`
`502
`
`508
`
`
`
`Next symbol
`
`Revise channel estimate
`in decision directed mode
`
`
`
`
`
`
`
`
`Pilot
`previous
`
`symbol
`?
`
`
`Estimate channel in
`training mode
`
`Backtrack through pilots
`to determine channel
`
`
`
`estimate of first pilot
`
`
`
`
`Determine difference
`
`between channel estimate
`
`for first pilot and channel
`estimate for preceding Symbol
`
`
`
`Phase
`
`
`slip or
`Yes
`512
`. deviation
`
`
`Next frame
`
`?
`
`
`
`
`Yes
`
`608
`
`Use channel estimate from
`
`pilot for subsequent symbols
`
`ERIC-1003 / Page 7 of 16
`
`ERIC-1003 / Page 7 of 16
`
`

`

`US. Patent
`
`Jun. 25, 2002
`
`Sheet 6 0f 6
`
`US 6,411,649 B1
`
`Channel Estimation
`
`FIG 7
`
`508
`
`500
`
`Revise channel estimate
`
`
`
`Estimate channel from
`
`synchronization symbols
`
`in decision directed mode
`
`502
`
`600
`
`Pilot
`
`
`
`
`Next symbol
`
`
`previous
`symbol
`
`?
`
`
`Backtrack through pilots
`to determine channel
`
`estimate of first pilot
`
`
`
`
`
`Determine difference
`
`
`between channel estimate
`
`
`
`
`
`
`Yes
`
`506
`
`Estimate channel in
`
`training mode
`
`for first pilot and channel
`
`estimate for preceding symbol
`
`Yes
`
`512
`
`700
`
` slip or
`
`deviation
`
`?
`
`es
`
`N0
`
`
`Channel
`estimate within
`
`threshold
`
`
`Backtrack from the first pilot
`
`correcting channel and symbol
`
`608
`
`Yes
`
`Use channel estimate from
`pilot for subsequent symbols
`
`ERIC-1003 / Page 8 of 16
`
`ERIC-1003 / Page 8 of 16
`
`

`

`US 6,411,649 B1
`
`1
`ADAPTIVE CHANNEL TRACKING USING
`PILOT SEQUENCES
`
`FIELD OF THE INVENTION
`
`The present invention relates to digital communications
`and, in particular,
`to the tracking of channel response in
`digital wireless mobile radio systems.
`BACKGROUND OF THE INVENTION
`
`The radio channel in mobile wireless communications
`
`may be one of the most harsh mediums to operate. The
`transmitted signals are often reflected, scattered, diffracted,
`delayed and attenuated by the surrounding environment.
`Moreover, the environment through which the signal passes
`from the transmitter to the receiver is not stationary due to
`the mobility of the user and surrounding objects. Charac-
`teristics of the channel environment also differ from one area
`
`to another. Radio propagation in such environments is
`characterized by multi-path fading, shadowing, and path
`loss. Among those, multi-path fading may be the most
`important. Multi-path fading may be characterized by enve-
`lope fading, Doppler spread and time-delay spread.
`Multi-path waves are combined at the receiver antenna to
`give a resultant signal which can vary widely in amplitude
`and phase. Therefore, signal strength may fluctuate rapidly
`over a small distance traveled or time interval, causing
`envelope fading. Rayleigh distribution is commonly used to
`describe the statistical time varying nature of the received
`envelope of a flat fading signal, or the envelope of an
`individual multi-path component. In satellite mobile radio
`and in micro-cellular radio, in addition to the many multi-
`path waves, a dominant signal, which may be a line-of-sight
`(LOS) signal, arrives at the receiver and gives rise to a
`Ricean distributed signal envelope. This dominant path
`significantly decreases the depth of fading depending on the
`Ricean parameter, K, which is defined as the ratio of the
`power in the dominant path to the power in the scattered
`paths.
`Doppler shift is the frequency shift experienced by the
`radio signal when a wireless receiver, such as a wireless
`mobile terminal, is in motion. Doppler spread is a measure
`of the spectral broadening caused by the time rate of change
`of the mobile radio channel. Doppler spread may lead to
`frequency dispersion. The Doppler spread in the frequency
`domain is closely related to the rate of change in the
`observed signal. Hence, the adaptation time of the processes
`which are used in the receivers to track the channel varia-
`
`tions should be faster than the rate of change of the channel
`to be able to accurately track the fluctuations in the received
`signal.
`Each of these characteristics of the radio channel present
`difficulties in tracking the channel to allow for decoding of
`information contained in the received signal. Often,
`in
`wireless mobile radio systems, known data sequences are
`inserted periodically into the transmitted information
`sequences. Such data sequences are commonly called syn-
`chronizing sequences or training sequences and are typically
`provided at
`the beginning of a frame of data. Channel
`estimation may be carried out using the synchronizing
`sequences and other known parameters to estimate the
`impact
`the channel has on the transmitted signal. After
`determining the channel response,
`the channel estimator
`enters a “decision directed” mode where the symbol esti-
`mates are used to estimate the channel.
`
`For systems where fading changes very slowly, generally,
`least square estimation may be an efficient way of estimating
`
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`the channel impulse response in the presence of additive
`white Gaussian noise. Because the fading rate is slow
`compared to the frame rate, the channel estimates can be
`updated frame by frame. However, for many wireless mobile
`radio systems, the channel impulse response changes very
`rapidly over a small travel distance or time interval. With the
`trend in wireless communications being to move to higher
`frequency bands, such as in the Personal Communication
`Systems (PCS),
`the Doppler spread, hence,
`the rate of
`change in the observed signal may be further increased.
`Even during the reception of the synchronizing sequences,
`the mobile radio channel response may not be constant.
`Therefore, the ability to track the channel parameters for fast
`time-varying systems provides more robust receiver struc-
`tures and enhances the receiver performance.
`The most commonly used channel tracking methods are
`the Least Mean Square (LMS) and Recursive Least Square
`(RLS) based algorithms. See for example, “Optimal Track-
`ing of Time-varying Channels: A Frequency Domain
`Approach for known and new algorithms,” IEEE transac-
`tions on selected areas in communications, Vol. 13, NO. 1,
`January 1995, Jingdong Lin, John G. Proakis, Fuyun Ling.
`By incorporating prior knowledge about the channel coef-
`ficient
`in the estimation, stochastic based methods have
`recently been introduced. In contrast to the LMS and RLS,
`these methods provide for the extrapolation of the channel
`coefficients in time. More details on these approaches can be
`obtained in, “A wiener filtering approach to the design of
`tracking algorithms”, Uppsala University Department of
`technology and signal processing group, Lars Lindbom,
`1995.
`
`One difficulty with the adaptive channel tracker methods
`is that during the decision directed mode the estimated
`symbols are used for
`the channel response adaptation.
`Therefore, the effect of using potentially incorrect decisions
`needs to be considered for parameter selection. Tuning of
`design parameters may result in a trade-off between tracking
`capability and sensitivity to noise. For example,
`if the
`adaptation gain of the channel tracker is very large, then, the
`channel tracker may become very sensitive to noise and to
`incorrect symbol decisions. On the other hand, if the adap-
`tation gain is chosen to have a small magnitude, the ability
`to track the variation of the channel parameters may be lost.
`Specifically, in those systems where coherent modulation
`and coherent demodulation schemes are used, these issues
`become more serious compared to systems where differen-
`tial modulations are implemented.
`In coherent modulation schemes (like coherent Quadra-
`ture Phase Shift Keying (QPSK)), even if the channel tracker
`tracks the magnitude of the channel response well,
`the
`channel phase may frequently slip (i.e., the tracker can lock
`on a wrong phase offset) during a deep fade of the in-phase
`and/or quadrature phase component of the channel, and the
`phase would be off by kZJ'c/m. In other words, the tracker
`actually tracks well but with an offset, which consequently
`causes symbol rotation and error propagation. Because the
`channel phase rotation and symbol rotation are in the
`opposite direction, a conventional
`tracker is not able to
`correct the problem. Thus, all the remaining information
`symbols may be lost because of this phase rotation until a
`new frame and synchronization sequence is received.
`SUMMARY OF THE INVENTION
`
`it is an object of the
`In light of the above discussion,
`present invention to provide channel tracking which com-
`pensates for variations in the channel
`including channel
`fade.
`
`ERIC-1003 / Page 9 of 16
`
`ERIC-1003 / Page 9 of 16
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`

`

`US 6,411,649 B1
`
`3
`invention are
`These and other objects of the present
`provided by utilizing pilots in an information sequence to
`periodically retrain a channel estimator. Thus, a channel
`tracker may be synchronized using a synchronization
`sequence and then periodically retrained using known pilot
`symbols. Furthermore, the utilization of pilots may allow for
`the detection of errors in previous channel estimates. When
`errors are detected, a new channel estimate may be used
`based on the retraining using the pilot symbols and,
`optionally, previous errors in symbol estimation may be
`corrected. Thus, by retraining based on pilot symbols, the
`propagation of errors may be reduced.
`In a particular embodiment, methods and systems are
`provided which track the channel impulse response of a
`signal received by a wireless device by estimating the
`channel impulse response of the received signal during a
`synchronizing period of a received frame and retraining the
`channel impulse response during a pilot period within the
`received frame. Furthermore, the channel impulse response
`may also be estimated based on estimated symbol values
`during information periods of the received frame.
`In a particular embodiment, the estimate of the channel
`impulse response is determined by a channel tracker. In such
`an embodiment,
`the channel tracker may be placed in a
`training mode during at least one pilot period of the received
`frame. Furthermore, the channel tracker may be placed in
`decision directed mode during at
`least one information
`period the received frame. Such a two mode operation may
`also allow for increasing the gain of the channel tracker
`when the channel tracker is placed in training mode and
`decreasing the gain of the channel tracker when the channel
`tracker is placed in decision directed mode.
`In another embodiment of the present invention the chan-
`nel impulse response of the received signal is estimated
`during the synchronizing period of the received frame by
`first determining an average channel impulse response esti-
`mate based on a plurality of symbols in during the synchro-
`nizing period. The estimated channel impulse responses for
`symbols in the plurality of symbols, in symbol order, and
`wherein the initial symbol estimate for a symbol in the
`plurality of symbols is based on the determined average
`channel impulse response.
`In yet another embodiment of the present invention, phase
`slip occurrences or channel estimate deviations from the real
`channel are detected in the signal received by the wireless
`device. Such a determination may be made by comparing a
`determined channel impulse response determined during an
`information period of the received frame to a determined
`channel impulse response determined during a pilot period
`of a frame to determine the difference in phase between the
`channel
`impulse responses. A phase slip occurrence or
`channel estimate deviation may then be detected based on
`the determined difference.
`
`Furthermore, the comparison may be made by comparing
`a determined channel impulse response determined during
`an information period of the received frame corresponding
`to a first
`information symbol
`to a determined channel
`impulse response determined during a pilot period of a
`frame corresponding to a pilot symbol immediately subse-
`quent
`to the first
`information symbol
`to determine the
`difference in phase between the channel impulse responses.
`In such a case, the determined channel impulse response
`determined during a pilot period of a frame corresponding to
`a pilot symbol immediately subsequent to the first informa-
`tion symbol may be determined by re-determining channel
`impulse responses in a reverse direction from determined
`
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`
`4
`channel impulse responses corresponding to pilot symbols
`subsequent to the pilot symbol immediately subsequent to
`the first information symbol.
`In yet another embodiment of the present invention, the
`determined channel impulse response corresponding to the
`first information symbol may be discarded and subsequent
`tracking performed utilizing the determined channel esti-
`mate corresponding to a pilot symbol subsequent to the first
`information symbol.
`In a symbol correcting embodiment of the present
`invention,
`the determined channel impulse responses for
`symbols prior to the pilot symbol subsequent to the first
`information symbol are compared. Backwards re-tracking to
`prior symbols from the pilot symbol immediately subse-
`quent to the first information symbol is carried out until the
`difference between the determined channel
`impulse
`response corresponding to the pilot symbol subsequent to
`the first
`information symbol and a determined channel
`impulse response corresponding to a symbol prior to the first
`information symbol is less than a predefined threshold so as
`to determine an initial symbol. Symbol estimates may be
`revised utilizing the determined channel impulse response
`corresponding to the pilot symbol subsequent to the initial
`symbol and until and including the first information symbol.
`As will be appreciated by those of skill in the art, the
`present invention may also be embodied in a radiotelephone.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 illustrates a radio communication system in which
`the teaching of the present invention can be utilized;
`FIG. 2 is a block diagram of a radiotelephone according
`to the present invention;
`FIG. 3 is a block diagram of a baseband processor
`according to the present invention;
`FIG. 4 illustrates a specific frame structure according to
`one embodiment of the present invention;
`FIG. 5 is a flow chart illustrating operations according to
`one embodiment of the present invention;
`FIG. 6 is a flow chart illustrating operations according to
`a first alternative embodiment of the present invention; and
`FIG. 7 is a flow chart illustrating operations according to
`a second alternative embodiment of the present invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`The present invention now will be described more fully
`hereinafter with reference to the accompanying drawings, in
`which preferred embodiments of the invention are shown.
`This invention may, however, be embodied in many different
`forms and should not be construed as limited to the embodi-
`
`ments set forth herein; rather, these embodiments are pro-
`vided so that this disclosure will be thorough and complete,
`and will fully convey the scope of the invention to those
`skilled in the art. Like numbers refer to like elements
`
`throughout. As will be appreciated by one of skill in the art,
`the present
`invention may be embodied as methods or
`devices. Accordingly, the present invention may take the
`form of an entirely hardware embodiment, an entirely soft-
`ware embodiment or an embodiment combining software
`and hardware aspects. Like numerals refer to the same items
`throughout the present disclosure.
`FIG. 1 depicts a radio communications system 100 such
`as a cellular or satellite telephone system,
`in which the
`teachings of the present invention can be utilized. As shown
`
`ERIC-1003 / Page 10 of 16
`
`ERIC-1003 / Page 10 of 16
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`

`

`US 6,411,649 B1
`
`5
`in FIG. 1, the radio communications system 100 includes a
`radio transmitter 102 having a transmit antenna 104, and a
`radio receiver 114. The radio receiver 114 includes a receive
`
`antenna 106, a radio frequency processor 108, and a base-
`band processor 110. An output of the radio transmitter 102
`is coupled to the transmit antenna 104. The receive antenna
`106 is coupled to a radio frequency processor 108. The
`output of the radio processor is provided to the inputs of the
`baseband processor 110.
`In operation, the transmitter 102 transmits an information
`signal (modulated at a carrier frequency specified by the
`system and regulatory agencies and suitable for
`radio
`communication). The transmitted signal reaches the radio
`receiver 114 after passing through a propagation medium.
`The transmitted signal plus any noise are received at the
`receiver antenna 106. The received signal is processed by
`the radio frequency processor 108 to produce a baseband
`signal corresponding to the desired carrier frequency.
`Specifically, the radio processor 108 amplifies, mixes,
`filter, samples, and quantizes the signal to extract the base-
`band signal corresponding to the desired carrier frequency.
`The resulting baseband signal is provided to the baseband
`processor 110 for demodulation of the transmitted informa-
`tion signal.
`The present invention utilizes pilot symbols inserted in a
`frame of data in a received signal to periodically retrain a
`channel tracker during communication. Thus, the channel
`tracker may more accurately track changes in the channel
`impulse response of the channel corresponding to the
`received signal. Thus, the present invention may be incor-
`porated in the baseband processor 110 and be utilized in
`demodulating the received signal to extract the transmitted
`information.
`
`An embodiment of a radiotelephone 10 which includes a
`correcting channel estimator 15 according to the present
`invention is depicted in the block diagram of FIG. 2. As
`shown in FIG. 2, radiotelephone 10 typically includes a
`transmitter 12, a receiver 14, a user interface 16 and an
`antenna system 18. The antenna system 18 may include an
`antenna feed structure 22 and an antenna 20. As is well
`known to those of skill in the art, transmitter 12 converts the
`information which is to be transmitted by radiotelephone 10
`into an electromagnetic signal suitable for radio communi-
`cations. Receiver 14 demodulates electromagnetic signals
`which are received by radiotelephone 10 so as to provide the
`information contained in the signals to user interface 16 in
`a format which is understandable to the user. Awide variety
`of transmitters 12, receivers 14, user interfaces 16 (e.g.,
`microphones, keypads, displays) which are suitable for use
`with handheld radiotelephones are known to those of skill in
`the art, and such devices may be implemented in radiotele-
`phone 10. Other than the correcting channel estimator 15
`according to the present invention, the design of radiotele-
`phone 10 is well known to those of skill in the art and will
`not be further described herein.
`
`FIG. 3 depicts a baseband processor 110 according to the
`present invention. As shown, the received baseband signal is
`provided to both the inputs of adaptive channel estimator 30
`(which includes a channel
`tracker 31) and symbol or
`sequence estimator 32. The output of the adaptive channel
`estimator 30 is also provided to the symbol or sequence
`estimator 32. The output of symbol or sequence estimator 32
`is provided to a mode selector 34. The output of the mode
`selector 34 is provided as an input to the adaptive channel
`estimator 30. As is further seen in FIG. 3, the mode selector
`34 is also provided known symbols. These known symbol
`
`6
`may correspond to the known symbols of a synchronizing
`period or of a pilot period in a received frame of information.
`The mode selector 34 selects between a training mode where
`known symbols are utilized to train the channel tracker 31
`and a decision directed mode where the channel tracker 31
`tracks the channel based on previous channel estimates.
`FIG. 4 illustrates a frame structure which may be utilized
`with the present invention. As seen in FIG. 4, the frame
`includes a synchronizing portion 0 to A using training
`symbols, information portions B—C, F—G and J—K and pilot
`portions D—E, H—I and Y—Z. According to the present
`invention, the pilot portions are interspersed between infor-
`mation portions so as to allow retraining of the adaptive
`channel estimator 30 during the frame. The synchronizing
`portion of the frame is a series of predefined symbols, from
`0 to Ain FIG. 4, which are the same for each received frame.
`The information portion of the frame, from B to C, F to G
`and J to K, contains symbols which may vary from frame to
`frame and contain the information to be transmitted in the
`frame. The pilot portions of the frame, from D to E, H to I
`and Y to Z, contain predefined symbols which may be used
`to retrain the adaptive channel estimator 30. As will be
`appreciated by those of skill in the art in light of the present
`disclosure, the frame structure of FIG. 4 is merely illustra-
`tive and the present invention should not be construed as
`limited to any particular frame structure but may be used
`with any frame structure which includes pilots.
`Preferably, the pilot sequences inserted into a go frame are
`of sufficient length so as to allow for accurate retraining of
`the channel estimate. However, in a fixed frame length, the
`length of the pilot sequences represents a trade off between
`channel estimate accuracy and the amount of information
`symbols in a frame. Accordingly, while the pilot sequence
`length may be any length which includes at
`least one
`symbol, pilot sequence length of from about 2 to about 4
`symbols may be suitable.
`Similarly, the frequency of occurrence of pilot sequences
`in a frame may also depend on the specific application, as the
`pilot frequency should be sufficient to allow accurate symbol
`estimation over the duration of a frame but not so often as
`
`to unacceptably degrade performance in the reduction of
`information in a frame. For example, more pilots may be
`beneficial in a phase shift encoded protocol which detects 4
`bits per symbol than one which detects 2 bits per symbol as
`a result of the higher accuracy needed to detect 24 levels of
`phase shift as opposed to 4. Thus, while the present inven-
`tion should not be construed as limited to any particular
`number of pilot sequences per frame, from about 2 to about
`6 pilot sequences per frame may be beneficial. As will be
`appreciated by those of skill in the art, depending on the rate
`of change of the channel (Doppler spread), the number of
`pilot sequences may change. For low Doppler spread, less
`sequences may be needed and for higher Doppler spread,
`more sequences are preferred.
`In operation, during the synchronizing period of each
`frame, the mode selector 34 provides known symbols to the
`adaptive channel estimator 30. These symbols may, for
`example, be stored in memory and accessed during the
`synchronization portion of a received frame. During the
`synchronization portion of the received frame, the channel
`estimator 30 estimates the channel impulse response using
`any number of well known methods of channel estimation.
`For example, a least square (LS) estimator may be utilized
`which minimizes the squared difference between the actual
`received signal and the reconstructed signal based on the
`known symbol of the synchronization portion of the frame.
`These initial estimates which are obtained by the LS
`estimator are used as initial estimates for the channel tracker
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
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`45
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`50
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`55
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`60
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`65
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`US 6,411,649 B1
`
`7
`31. Because the channel may change very rapidly, and an LS
`estimator provides an average channel parameter estimation
`during the training period, it is not desirable to use these
`initial estimates (starting from the first information symbol
`after the synchronizing period (represented as B in FIG. 4)
`of the information field just after the synchronization
`sequence)
`for
`tracking the channel
`impulse response.
`Instead, a channel tracker 31 portion of the adaptive channel
`estimator 30 begins tracking the channel from the first
`symbol (represented as 0 in FIG. 4) of the synchronization
`sequence using the initial estimates provided by least square
`determination and tracks the channel until the last symbol
`(A) of the synchronization sequence in the training mode.
`The channel estimate at this point (A) is more reliable than
`the average channel estimate obtained by the least square
`determination. Thus, during the synchronization sequence,
`the channel tracker 31 will typically converge to a reason-
`able value and the estimates at the end of the training period
`(A) will follow the changes in the channel during the
`training mode.
`After the training sequence, the mode selector 34 changes
`to decision directed mode and provides the output of the
`symbol or sequence estimator 32 to the adaptive channel
`estimator 30. Thus, starting from the first symbol (B) of the
`information sequence just after the training sequence, the
`estimated symbols at the output of the symbol or sequence
`estimator are used for the estimation of channel impulse
`response. The channel estimation in decision directed mode
`continues until the last symbol (C) of the first information
`sequence.
`
`While the channel tracker 31 is operating in decision
`directed mode, the channel tracker 31 may lose the ability to
`track the real radio channel depending on the parameter
`selection. Furthermore, the channel tracker 31 may be able
`to closely follow the amplitude of the channel but the phase
`of the estimated parameters might slip with respect to the
`actual channel. These errors may propagate as incorrect
`channel estimates which may result
`in incorrect symbol
`estimates which may further result
`in incorrect channel
`estimates. By inserting pilots in the sequence of the frame,
`an opportunity is provided to re-train the channel response.
`FIG. 5 further illustrates the operations of a first embodi-
`ment of the present
`invention utilizing the pilots in a
`received frame. This embodiment of the present invention
`utilizes the pilot symbols to re-train the channel impulse
`response with known symbols.
`As seen in FIG. 5, the process begins as described above
`by the estimation of the channel impulse response during the
`synchronizing sequence (0 to A) by the adaptive channel
`estimator (block 500). The process then continues a symbol
`at a time by obtaining the next symbol (block 502) and
`determining if the symbol is a pilot symbol (block 504). If
`the symbol is not a pilot symbol, then the symbol is an
`information symbol and the adaptive channel estimator 30 is
`placed in decision directed mode and the estimated symbols
`are fed back to the adaptive channel estimator 30 by the
`mode selector 34 (block 508). This operation continues until
`a pilot symbol is reached. At that point, the mode selector 34
`returns the