US007426175B2
`
`(12) United States Patent
`Zhuang et a].
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 7,426,175 B2
`Sep. 16, 2008
`
`(54) METHOD AND APPARATUS FOR PILOT
`SIGNAL TRANSMISSIoN
`
`(75) Inventors: Xiangyang Zhuang, Hoffman Estates,
`,
`.
`IL (Us); Kevm L‘ Baffmi R011“?
`Meadows,1L(U$);VlJayNaI{gla,
`Schaumburg, IL (Us); Frederlck W-
`V00k, Schaumhurg, IL (US)
`(73) Assigneez Motorola’ Inc” Schaumburg IL (Us)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`5,493,563 A *
`2/1996 Rozanski et a1. .......... .. 370/332
`6,430,166 B1* 8/2002 Bejjanietal. ......... .. 370/320
`6,704,552 B1* 3/2004 Matsumoto ............ .. 455/1641
`6,744,807 B1* 6/2004 Linde e161. ............... .. 375/140
`6,804,307 B1 * 10/2004 Popovic
`375/299
`2002/0009125 A1 *
`1/2002 S111 ......... ..
`375/139
`2002/0181509 A1* 12/2002 Mody et a1.
`370/480
`2003/0152136 A1* 55/2003 Roman
`375/140
`2007/0165588 A1 *
`7/2007 McCoy ..................... .. 370/344
`2007/0217530 A1* 9/2007 Hosseinian et a1. ....... .. 375/260
`
`(21) Appl. No.: 10/813,476
`
`(22) Filed:
`
`Mar. 30, 2004
`
`(65)
`
`PI‘iOI‘ PllblicatiOIl Data
`US 2005/0226140 A1
`Oct. 13, 2005
`
`(51) Int CL
`(2006.01)
`H04J 11/00
`(52) US. Cl. ..................... .. 370/203; 370/331; 370/342;
`375/134; 375/137; 375/140; 375/148; 455/436
`(58) Field of Classi?cation Search ............... .. 370/203
`370/252’ 310’ 331’ 332’ 334’ 335’ 342’ 350;
`3'75/134a 137’ 140’ 142’ 143’ 144’ 145’ 148’
`375/ 149, 299
`See application ?le for complete search history.
`
`* Cited by examiner
`
`Primary ExamineriChi H. Pham
`Assistant Examinerishick Hom
`
`(57)
`
`ABSTRACT
`
`Pilot Sequences are Constructed from distinct “Classes” Of
`chirp sequences that have an optimal cross correlation prop
`eny-UtilizationofchirpSequencesforpilotsequencesresuhs
`in pilot sequences that have optimal or nearly-optimal cross
`correlation and auto-correlation properties.
`
`28 Claims, 1 Drawing Sheet
`
`[30/
`
`DETERMINE NUMBER OF
`PILOT SEQUENCES NEEDED
`[303
`COMPUTE PILOT SEQUENCES
`r305
`ASSIGN PILOT SEQUENCES
`T0 BASE UNITS
`
`ERIC-1004
`Ericsson v IV
`Page 1 of 9
`
`

`
`US. Patent
`
`Sep. 16, 2008
`
`US 7,426,175 B2
`
`A Y ,103
`MOBILE UNIT
`TETE
`
`105
`
`02 FIG- 1
`
`BASE
`UNIT
`
`r201
`PILOT SEQUENCE
`
`[202
`REMAINING TRANSMISSION/DATA
`
`I
`
`FIG. 2
`
`DETERMINE NUMBER OF
`PILOT SEQUENCES NEEDED
`I
`fans
`COMPUTE PILOT SEQUENCESI
`1,
`flos
`ASSIGN PILOT SEQUENCES
`T0 BASE UNITS
`
`FIG- 3
`
`ERIC-1004 / Page 2 of 9
`
`

`
`US 7,426,175 B2
`
`1
`METHOD AND APPARATUS FOR PILOT
`SIGNAL TRANSMISSION
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to pilot signal trans
`mission, and in particular to a method and apparatus for pilot
`signal transmission in a communication system.
`
`BACKGROUND OF THE INVENTION
`
`A pilot signal (or preamble) is commonly used for com
`munication systems to enable the receiver to perform a num
`ber of critical functions, including but not limited to, the
`acquisition and tracking of timing and frequency synchroni
`zation, the estimation and tracking of desired channels for
`subsequent demodulation and decoding of the information
`data, the estimation and monitoring of the characteristics of
`other channels for handoff, interference suppression, etc.
`Several pilot schemes can be utilized by communication sys
`tems, and typically comprise the transmission of a knoWn
`sequence at knoWn time intervals. A receiver, knowing the
`sequence and time interval in advance, utilizes this informa
`tion to perform the above-mentioned functions.
`Several criteria are important When determining pilot
`sequences for communication systems. Among these criteria
`is the ability to have good auto -correlation for each of the pilot
`sequences utilized, and at the same time the ability to have
`good cross-correlation betWeen any tWo different pilot
`sequences. Auto- and cross-correlation are sequences them
`selves corresponding to different shifts. Auto-correlation at
`shift-d is de?ned as the result of summing over all entries after
`an element-Wise multiplication betWeen the sequence and its
`conjugated replica after shifting it by d entries (d can be
`positive or negative for right or left shift). Cross-correlation at
`shift-d is de?ned as the result of summing over all entries after
`an element-Wise multiplication betWeen a sequence and
`another sequence that is conjugated and shifted by d entries
`With respect to the ?rst sequence. “Good” auto-correlation
`results in each pilot sequence having a minimal auto-correla
`tion value at all shifts of interest (i.e., a range of d, except for
`dIO). “Good” cross-correlation results in the pilot sequence
`having a minimal cross-correlation value at all shifts of inter
`est. When the auto-correlation is zero at all d, except for dIO,
`it is referred to as “ideal” auto-correlation. Since the cross
`correlation of tWo sequences that have ideal auto-correlation
`cannot be zero at all d, the minimum of the maximum cross
`correlation values at all shifts can be reached only When the
`cross-correlation at all d is equal in amplitude, Which is
`referred to as having “optimal” cross-correlation.
`Since the received signal after propagation consists of rep
`licas of the delayed pilot sequence after some scaling factors,
`the ideal auto-correlation property of the pilot makes the
`estimation of the channel scaling factors at different delays
`possible. The optimal cross-correlation property betWeen any
`tWo pilot sequences Will minimize the interference effect seen
`at the receiver that is caused by any pilot sequences other than
`the desired one (i.e., one that the receiver is tuned to). Good
`cross-correlation makes the detection of the desired pilot
`signal and the estimation of the desired channel characteris
`tics more reliable, Which enables the receiver to perform
`synchronization and channel estimation more reliably.
`Various techniques have been used in the past to design
`systems With e?icient pilot sequences. For example, in the
`current CDMA-based cellular system, the pilot sequence in a
`cell is a Walsh code that is scrambled by a cell-speci?c scram
`bling code (long code). This effectively randomizes the pilot
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`sequence for each cell. Channel estimation of the neighboring
`base stations, When required during a soft handoff, is simply
`performed by correlating the received signal With the neigh
`boring base station’s long code scrambled pilot sequences.
`But the cross-correlation property of tWo random pilot
`sequences is not optimal, and thus a larger channel estimation
`error can be expected. Therefore, a need exists for a method
`and apparatus for pilot signal or preamble transmission that
`optimizes both the cross correlation betWeen pilot signals, as
`Well as optimizing each pilot signal’s auto correlation.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of a communication system.
`FIG. 2 illustrates pilot signal transmission for the commu
`nication system of FIG. 1.
`FIG. 3 is a How chart shoWing pilot sequence assignment
`for the communication system of FIG. 1.
`
`DETAILED DESCRIPTION OF THE DRAWINGS
`
`To address the above-mentioned need, a method and appa
`ratus for pilot signal transmission is disclosed herein. In par
`ticular, pilot sequences are constructed from distinct
`“classes” of chirp sequences that have an optimal cyclic cross
`correlation property While satisfying the ideal cyclic auto
`correlation requirement. Utilization of chirp sequences for
`pilot sequences results in pilot channels that have good cross
`correlation as Well as having good auto-correlation.
`The present invention encompasses a method for assigning
`a pilot sequence to communication units Within a communi
`cation system. The method comprises the steps of assigning a
`?rst communication unit a ?rst pilot sequence, Wherein the
`?rst pilot sequence is selected from a group of pilot sequences
`constructed from a set of Generalized Chirp-Like (GCL)
`sequences, and then assigning a second communication unit a
`secondpilot sequence taken from the group of pilot sequences
`constructed from the set of GCL sequences.
`The present invention additionally encompasses a method
`comprising the steps of receiving a pilot sequence as part of
`an over-the air transmission, Wherein the pilot sequence is
`constructed from a set of Generalized Chirp-Like (GCL)
`sequences, and utilizing the pilot sequence for at least acqui
`sition and tracking of timing and frequency synchronization,
`estimation and tracking of desired channels for subsequent
`demodulation and decoding, estimation and monitoring of
`characteristics of other channels for handoff purposes, and
`interference suppression.
`Finally, the present invention encompasses a communica
`tion unit comprising pilot channel circuitry for transmitting or
`receiving a pilot channel sequence, Wherein the pilot channel
`sequence comprises a sequence unique to the communication
`unit and is constructed from a GCL sequence.
`Turning noW to the draWings, Where like numerals desig
`nate like components, FIG. 1 is a block diagram of commu
`nication system 100 that utilizes pilot transmissions. Com
`munication system utilizes an Orthogonal Frequency
`Division Multiplexing (OFDM) protocol; hoWever in alter
`nate embodiments communication system 100 may utilize
`other digital cellular communication system protocols such
`as a Code Division Multiple Access (CDMA) system proto
`col, a Frequency Division Multiple Access (FDMA) system
`protocol, a Spatial Division Multiple Access (SDMA) system
`protocol or a Time Division Multiple Access (TDMA) system
`protocol, or various combinations thereof.
`As shoWn, communication system 100 includes base unit
`101 and 102, and remote unit 103. A base unit comprises a
`
`ERIC-1004 / Page 3 of 9
`
`

`
`US 7,426,175 B2
`
`3
`transmit and receive unit that serves a number of remote units
`Within a sector. As known in the art, the entire physical area
`served by the communication netWork may be divided into
`cells, and each cell may comprise one or more sectors. When
`multiple antennas are used to serve each sector to provide
`various advanced communication modes (e.g., adaptive
`beamforming, transmit diversity, transmit SDMA, and mul
`tiple stream transmission, etc.), multiple base units can be
`deployed. These base units Within a sector may be highly
`integrated and may share various hardWare and softWare
`components. For example, all base units co-located together
`to serve a cell can constitute What is traditionally knoWn as a
`base station. Base units 101 and 102 transmit doWnlink com
`munication signals 104 and 105 to serving remote units on at
`least a portion of the same resources (time, frequency, or
`both). Remote unit 103 communicates With one or more base
`units 101 and 102 via uplink communication signal 106.
`It should be noted that While only tWo base units and a
`single remote unit are illustrated in FIG. 1, one of ordinary
`skill in the art Will recogniZe that typical communication
`systems comprise many base units in simultaneous commu
`nication With many remote units. It should also be noted that
`While the present invention is described primarily for the case
`of doWnlink transmission from multiple base units to multiple
`remote units for simplicity, the invention is also applicable to
`uplink transmissions from multiple remote units to multiple
`base units. A base unit or a remote unit may be referred to
`more generally as a communication unit.
`As discussed above, pilot assisted modulation is com
`monly used to aid in many functions such as channel estima
`tion for subsequent demodulation of transmitted signals. With
`this in mind, base units 101 and 102 transmit known
`sequences at knoWn time intervals as part of their doWnlink
`transmissions. Remote unit 103, knoWing the sequence and
`time interval, utiliZes this information in demodulating/de
`coding the transmissions. Such a pilot transmission scheme is
`illustrated in FIG. 2. As shoWn, doWnlink transmissions 200
`from base units 101 and 102 typically comprise pilot
`sequence 201 folloWed by remaining transmission 202. The
`same or a different sequence can shoW up one or multiple
`times during the remaining transmission 202. Thus, each base
`unit Within communication system 100 comprises pilot chan
`nel circuitry 107 that transmits one or more pilot sequences
`along With data channel circuitry 108 transmitting data.
`It should be noted that although FIG. 2 shoWs pilot
`sequence 201 existing at the beginning of a transmission, in
`various embodiments of the present invention, the pilot chan
`nel circuitry may include pilot sequence 201 anyWhere Within
`doWnlink transmission 200, and additionally may be trans
`mitted on a separate channel. Remaining transmission 202
`typically comprises transmissions such as, but not limited to,
`sending information that the receiver needs to knoW before
`performing demodulation/decoding (so called control infor
`mation) and actual information targeted to the user (user
`data).
`As discussed above, it is important for any pilot sequence
`to have optimal cross-correlation and ideal auto-correlation.
`With this in mind, communication system 100 utiliZes pilot
`sequences constructed from distinct “classes” of chirp
`sequences With ideal cyclic auto-correlation and optimal
`cyclic cross-correlation. The construction of such pilot
`sequences is described beloW.
`
`50
`
`55
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`Construction of a Set of Pilot Sequences to Use Within a
`Communication System
`The construction of the pilot sequences depends on at least
`tWo factors, namely, a desired number of pilot sequences
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`needed in a netWork (K) and a desired pilot length (N P) Where
`K cannot exceed NP. In fact, the number of pilot sequences
`available that has the ideal cyclic auto-correlation and opti
`mal cyclic cross-correlation is P-l Where P is the smallest
`prime factor of NP other than “1” after factoring NP into the
`product of tWo or more prime numbers including “1”. For
`example, the maximum value that P can be is Np—l When NP
`is a prime number. But When NP is not a prime number, the
`number of pilot sequences often Will be smaller than the
`desired number K. In order to obtain a maximum number of
`sequences, the pilot sequence Will be constructed by starting
`With a sequence Whose length NG is a prime number and then
`performing modi?cations. In the preferred embodiment, one
`of the folloWing tWo modi?cations is used:
`1. Choose NG to be the smallest prime number that is
`greater than NP and generate the sequence set. Truncate
`the sequences in the set to NP; or
`2. Choose NG to be the largest prime number that is smaller
`than NP and generate the sequence set. Repeat the begin
`ning elements of each sequence in the set to append at
`the end to reach the desired length NP.
`The above design of requiring NG to be a prime number
`Will give a set of NG—l sequences that has ideal auto corre
`lation and optimal cross correlation. HoWever, if only a
`smaller number of sequences is needed, NG does not need to
`be a prime number as long as the smallest prime factor of NG
`excluding “l” is larger than K.
`When a modi?cation such as truncating or inserting is
`used, the auto-correlation Will not be precisely ideal and the
`cross-correlation Will not be precisely optimal anymore.
`HoWever, the auto- and cross-correlation properties are still
`acceptable. The modi?ed pilot sequence can be referred to as
`nearly-optimal pilot sequences that are constructed from
`GCL sequences With optimal auto- and cross-correlation.
`Further modi?cations to the truncated/extended sequences
`may also be applied, such as applying a unitary transform to
`them.
`It should also be noted that While only sequence truncation
`and cyclic extension Were described above, in alternate
`embodiments of the present invention there exist other Ways
`to modify the GCL sequences to obtain the ?nal sequences of
`the desired length. Such modi?cations include, but are not
`limited to extending With arbitrary symbols, shortening by
`puncturing, etc. Again, further modi?cations to the extended/
`punctured sequences may also be applied, such as applying a
`unitary transform to them.
`The length-NP sequences are assigned to base units in com
`munication system 100 as the time-domain pilot sequence, or
`as the frequency-domain pilot sequence (i.e., the entries of the
`sequence or its discrete IDFT Will be assigned onto a set of
`subcarriers in the frequency domain). If the sequences
`obtained are used as the time-domain pilot, option 2 Will be
`preferred because the autocorrelation over a siZe-NG WindoW
`is still ideal. If the sequences obtained are used as the fre
`quency-domain pilot and the channel estimation is performed
`in the frequency domain, the autocorrelation is irrelevant (but
`the cross-correlation properties of the sequences can still be
`important in many situations). In this case, either modi?ca
`tion 1 or 2 is acceptable With a preference to choosing NG as
`the closest to NP.
`The ?nal pilot sequences transmitted in time domain can be
`cyclically extended Where the cyclic extension is typically
`longer than the expected maximum delay spread of the chan
`nel (LD). In this case, the ?nal sequence sent has a length
`equal to the sum of NP and the cyclic extension length. The
`cyclic extension can comprise a pre?x, post?x, or a combi
`nation of a pre?x and a post?x. The cyclic extension may also
`
`ERIC-1004 / Page 4 of 9
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`

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`US 7,426,175 B2
`
`5
`be an inherent part of the communication system used such as
`an Orthogonal Frequency Division Multiplexing (OFDM)
`protocol. The inserted cyclic pre?x makes the ordinary auto
`or cross-correlation appear as a cyclic correlation at any shift
`that ranges from 0 to the cyclic pre?x length. If no cyclic
`pre?x is inserted, the ordinary correlation is approximately
`equal to the cyclic correlation if the shift is much smaller than
`the pilot sequence length.
`As discussed above, in the preferred embodiment of the
`present invention Generalized Chirp-Like (GCL) sequences
`are utiliZed for constructing pilot sequences. There exists a
`number of “classes” of GCL sequences and if the classes are
`chosen carefully (see GCL property 3 beloW), sequences With
`those chosen classes Will have optimal cross-correlation and
`ideal autocorrelation. Class-u GCL sequence (S) of length NG
`
`Where b can be any complex scalar of unit amplitude and
`
`am : exp(_?,mw}
`No
`
`(2)
`
`where,
`u:l, .
`.
`. NG—l is knoWn as the “class” of the GCL sequence,
`kIO, l, .
`.
`. NG—l are the indices of the entries in a sequence,
`q:any integer.
`
`Each class of GCL sequence can have in?nite number of
`sequences depending on the particular choice of q and b, but
`only one sequence out of each class is used to construct one
`pilot sequence.
`It should also be noted that if an NG-point DFT (Discrete
`Fourier Transform) or IDFT (inverse DFT) is taken on each
`GCL sequence, the member sequences of the neW set also
`have optimal cyclic cross-correlation and ideal autocorrela
`tion, regardless of Whether or not the neW set can be repre
`sented in the form of (l) and (2). In fact, sequences formed by
`applying a matrix transformation on the GCL sequences also
`have optimal cyclic cross-correlation and ideal autocorrela
`tion as long as the matrix transformation is unitary. For
`example, the NG-point DFT/IDFT operation is equivalent to a
`siZe-NG matrix transformation Where the matrix is an NG by
`NG unitary matrix. As a result, sequences formed based on
`unitary transformations performed on the GCL sequences
`still fall Within the scope of the invention, because the ?nal
`sequences are still constructed from GCL sequences. That is,
`the ?nal sequences are substantially based on (but are not
`necessarily equal to) the GCL sequences.
`If NG is a prime number, the cross-correlation betWeen any
`tWo sequences of distinct “class” is optimal and there Will
`NG—l sequences (“classes”) in the set (see properties beloW).
`The original GCL sequences have the folloWing properties:
`
`Property 1: The GCL sequence has constant amplitude, and
`its NG-point DFT has also constant amplitude.
`Note that constant amplitude in both the time and fre
`quency domain is desired for a pilot signal. Constant ampli
`tude of the temporal Waveform is ideal for a poWer ampli?er
`to operate at higher output poWer Without causing clipping.
`Constant amplitude in the frequency domain means that the
`subcarriers are equally excited and hence the channel esti
`mates Will not be biased. HoWever, for multi-carrier systems
`such as OFDM, some of the subcarriers (typically those at the
`edges of the band) are unoccupied to form the guard band.
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`The corresponding time-domain pilot Waveform is not of
`constant modulus anymore, but is essentially the result of
`interpolating the time-domain, i.e., over sampling the
`sequence to obtain a longer sequence after running it through
`a “sinc” ?lter. The resulting Waveform still enjoys loW peak
`to-average ratio (PAPR is typically <3 dB).
`
`Property 2: The GCL sequences of any length have an “ideal”
`cyclic autocorrelation (i.e., the correlation With the circularly
`shifted version of itself is a delta function)
`Property 3: The absolute value of the cyclic cross-correlation
`function betWeen any tWo GCL sequences is constant and
`equal to 1/, C16, When |u 1—u2|, u 1, and u2 are relatively prime
`to NG.
`
`Assignment of Pilot Sequences Within a Communication Sys
`tem
`Each communication unit may use one or multiple pilot
`sequences any number of times in any transmission interval
`or a communication unit may use different sequences at dif
`ferent times in a transmission frame. Additionally, each com
`munication unit can be assigned a different pilot sequence
`from the set of K pilot sequences that Were designed to have
`nearly-optimal auto correlation and cross correlation proper
`ties. One or more communication units may also use one pilot
`sequence at the same time. For example Where multiple com
`munication units are used for multiple antennas, the same
`sequence can be used for each signal transmitted form each
`antenna. HoWever, the actual signals may be the results of
`different functions of the same assigned sequence. Examples
`of the functions applied are circular shifting of the sequence,
`rotating the phase of the sequence elements, etc.
`
`Receiver Functions that May Bene?t from the Pilot Design:
`A number of critical receiver functions are described that
`can bene?t from the above-described pilot design. The
`examples given here are not exhaustive, and it Will be under
`stood by those skilled in the art that various changes in form
`and details may be made therein Without departing from the
`spirit of utiliZing the good auto- and/or cross-correlation of
`the designed sequence.
`1. Single Channel Estimation:
`This section shoWs hoW the channel estimation can bene?t
`from the above pilot design strategy. In essence, channel
`estimation can be performed easily by correlating the
`received data With the pilot sequence. Thanks to the ideal
`auto-correlation of GCL sequences, the output of the corre
`lation provides the channel estimate. The channel estimate
`can then be re?ned, if desired, using a “tap selection” process.
`An example tap selection process is provided beloW. Also,
`time synchronization With the desired base station (BS) can
`be achieved straightforwardly because the arrival path can be
`detected easily. If channel information to an interference BS
`is also needed, it can be obtained from the correlation of the
`received data With the pilot sequence of that BS. The cross
`correlation property increases the accuracy and detection reli
`ability of the signi?cant channel taps and reduces the false
`detections, as Will be explained here.
`The GCL sequence effectively spreads the poWer of each
`tap of the interference channel evenly across NG taps thanks
`to the cross-correlation properties of GCL sequences. There
`fore, after correlating With the desired sequence, the interfer
`ence Will be more evenly distributed in the time domain. The
`signi?cant tap of the desired channel Will be preserved better
`than the smaller taps. In comparison, if non-GCL sequences
`are used, the poWer of each tap of the interference Will not be
`evenly distributed across NG taps. The distortion effect on the
`
`ERIC-1004 / Page 5 of 9
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`US 7,426,175 B2
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`7
`desired channel varies from tap to tap With an unpredictable
`behavior. Hence, With non-GCL sequences, the detected sig
`ni?cant taps are more likely to be false due to the interference,
`or the true signi?cant taps can be distorted so much that they
`become undetectable. The interference power on each desired
`tap is PI/NG With P, being the interference poWer; in other
`Words, the spreading factor for each interference channel tap
`is NG.
`The correlation is typically performed in the time domain.
`But correlation can also be performed in the frequency
`domain as Will be described beloW. Frequency domain esti
`mation may be more computationally e?icient because of the
`FFT operation and is preferred for multicarrier systems such
`as OFDM systems. The example beloW is for an OFDM
`system.
`First, assume the frequency domain received data is Y(m)
`Where m is a pilot subcarrier. Assume SG(m) is the pilot When
`m is a pilot subcarrier and zero otherWise, then a “noisy”
`channel estimate at the pilot subcarriers can be obtained as:
`
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`
`11,,(m) = { Saw)
`
`if Sc(m) # 0
`
`0
`
`if saw) = 0
`
`(3)
`
`25
`
`The noisy estimates Will be transformed to the time domain
`through an IDFT as
`
`EWIIDFT ({Hr.(m)w(m)}),
`
`30
`
`(4)
`
`Where W is a Weighting WindoW applied onto the noisy fre
`quency response. The WindoW is to reduce the poWer leakage
`problem caused by the discontinuity from the edge to null
`subcarriers (since zeros are inserted in place of the null sub
`carriers before the IDFT). A “Hanning” WindoW can be used,
`1.e.,
`
`35
`
`27rm
`w(m) : [0.5 + O.5cosT],
`
`(5)
`
`40
`
`Where the parameter F controls the shape of the WindoW (an
`in?nite F means a ?at WindoW).
`The resulting hW Will then be truncated to length-LD to
`obtain hmL. Furthermore, only the “signi?cant” channel taps
`in hW>L should be included before being DFT’d back to get the
`frequency domain response, i.e.,
`
`(6)
`HM IIDFTNWWL).
`The tap selection procedure is important, as described ear
`lier, for exploiting the cross-correlation property of the pilot
`sequences. Tap selection also tries to enforce the frequency
`correlation according to the instantaneous channel delay pro
`?le, Which can improve the channel estimation especially in
`the case of a sparse channel.
`A threshold (denoted as 11) used in tap selection should be
`determined according the noise-plus-interference poWer esti
`mated previously, or the total noise-plus-interference poWer
`over the used bandWidth can be estimated from the samples in
`hW that Will be discarded (after L D+l). Note that compensa
`tion for the WindoWing effect is recommended during noise
`poWer estimation. Based on the above noise-plus-interfer
`ence poWer over the occupied bandWidth, the corresponding
`time-domain reference noise-plus interference poWer at each
`
`45
`
`50
`
`55
`
`60
`
`65
`
`(7)
`
`8
`tap (denoted as 02) can be easily derived after accounting for
`the zeros at those null-subcarrier positions.
`Finally hW>L Will be transformed back to the frequency
`domain With a DFT to obtain the frequency channel response,
`and the WindoWing effect of (5) is preferably “de-empha
`sized”, i.e.,
`HESAMFHMLWVWW)
`2. Multiple Channel Estimation:
`When multiple channels corresponding to different pilot
`sequences are needed, the above single channel estimation
`process is conducted for the different pilot sequences one at a
`time or simultaneously. The characteristics learned about
`other channels can be useful to improve the speed and per
`formance of handoff, to perform interference suppression at
`the receiver for better demodulation and decoding, to enable
`the base unit to intelligently schedule transmission to avoid
`interference, etc.
`3. Synchronization Acquisition and Tracking:
`One Way to achieve good initial acquisition of synchroni
`zation to the desired base unit is to ?rst correlate the received
`signal With the pilot sequence candidates. The results Will be
`assessed to ?nd the desired base unit (e.g., the strongest one).
`The characteristics learned from the correlation With the
`desired pilot sequence may be used to adjust the timing and
`frequency of the receiver to achieve synchronization. For
`example, the channel knoWledge Will give a good indication
`of the arrival time of the propagation paths and their strength,
`so the sample timing can be adjusted accordingly. The corre
`lation results may also be used to adjust the frequency offset
`of the receiver. For example, correlation results from pilot
`sequences received at nearby but different times can be com
`pared to identify the frequency offset. In another example,
`When the pilot sequence is mapped onto a set of OFDM
`subcarriers, a frequency domain correlation can identify fre
`quency offsets to the nearest integer number of subcarriers.
`The tracking of the synchronization to the desired signal
`can also be accomplished through the correlation results
`Where only correlation With the desired pilot is required. The
`?ne tuning of the timing and frequency offset can be achieved
`as in the initial acquisition step.
`Another type of synchronization required is frame syn
`chronization. Since a frame consists of many symbols, the
`information content at different locations Within a frame may
`be different. The ability to detect the frame boundary is a
`prerequisite for decoding the information. The pilot
`sequences can be used to support this function as Well. For
`example, if multiple pilot sequences are assigned in a frame
`and the location of each sequence relative to the frame bound
`ary is designed to be ?xed, When a certain pilot sequence is
`detected, the frame boundary can then be determined.
`FIG. 3 is a How chart shoWing the assignment of pilot codes
`to various base units Within communication system 100. The
`logic ?oW begins at step 301 Where a number of needed pilots
`(K), desired pilot length (N P) and a candidate length (NG) of
`each pilot sequence are determined. Based on NP and NG, the
`pilot sequences are computed (step 303). As discussed above,
`the pilot sequences are constructed from the Generalized
`Chirp-Like (GCL) sequences of length NP, With each GCL
`sequence being de?ned as shoWn in equation (1). Finally, at
`step 305, the pilot sequences are assigned to base units Within
`communication system 100. It should be noted that each base
`unit may receive more than one pilot sequence from the K
`available pilot sequences. HoWever, at a minimum a ?rst base
`unit is assigned a ?rst pilot sequence taken from a group of
`GCL sequences While a second base unit is assigned a differ
`ing pilot sequence from the group of GCL sequences. During
`
`ERIC-1004 / Page 6 of 9
`
`

`
`US 7,426,175 B2
`
`operation, pilot channel circuitry Within each base unit Will
`transmit the pilot sequence as part of an overall strategy for
`coherent demodulation. Particularly, each remote unit Within
`communication With the base units Will receive the pilot
`sequence and utiliZe the pilot sequence for many functions,
`such as channel estimation as part of a strategy for coherent
`demodulation of the received signal.
`As described above, the pilot sequences of the present
`invention have a loW peak-to-average ratio (PAPR). As a
`result, the PAPR of a pilot signal/sequence of the present
`invention is loWer than the PAPR of data signals that are also
`transmitted by a communication unit. The loW PAPR property
`of the pilot signal enables pilot channel circuitry 107 to trans
`mit the pilot signal With a higher poWer than the data in order
`to provide improved signal-to-noise/interference ratio on the
`pilot signal received by another communication unit, thereby
`providing improved channel estimation, synchronization,
`etc.
`While the invention has been particularly shoWn and
`described With reference to a particular embodiment, it Will
`be understood by those skilled in the art that various changes
`in form and details may be made therein Without departing
`from the spirit and scope of the invention. For example,
`although the above discussion Was relate