US00724.8559B2
`
`(12) United States Patent
`Ma et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,248.559 B2
`Jul. 24, 2007
`
`(54) SCATTERED PILOT PATTERN AND
`CHANNEL ESTMLATION METHOD FOR
`MIMO-OFDM SYSTEMS
`
`(75) Inventors: Jianglei Ma, Kanata (CA); Ming Jia,
`Ottawa (CA); Peiying Zhu, Kanata
`(CA); Wen Tong, Ottawa (CA)
`
`(73) Assignee: Nortel Networks Limited, St. Laurent,
`Quebec (CA)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 898 days.
`
`(*) Notice:
`
`1-1.
`(22) Filed:
`
`Jan. 8, 2002
`
`6,473,393 B1 * 10/2002 Ariyavisitakul et al. .... 370/203
`6,654,429 B1 * 1 1/2003 Li .............................. 375.316
`2002/0003774 A1* 1/2002 Wang et al. ................ 370,208
`2002/0034213 A1* 3/2002 Wang et al. ......
`... 375,132
`2002/0080887 A1* 6/2002 Jeong et al. ................ 375,295
`2002/O122383 A1* 9, 2002 Wu et al. ................... 370/210
`2002/0144.294 A1* 10, 2002 Rabinowitz et al. ........ 725,139
`2002/0181390 A1* 12/2002 Mody et al. ................ 370,208
`2003/0016621 A1
`1/2003 Li .............................. 370,203
`
`OTHER PUBLICATIONS
`Fernández-Getino Garcia, M. Julia et al; Efficient Pilot Patterns for
`Channel Estimation in OFDM Systems Over HF Channels; pp.
`
`Jones, V.K.; Raleigh, Gregory G.; Channel Estimation for Wireless
`OFDM Systems; pp.980-985.
`
`* cited by examiner
`Primary Examiner Chi Pham
`Assistant Examiner Melanie Jagannathan
`
`(57)
`
`ABSTRACT
`
`(65)
`
`(56)
`
`Prior Publication Data
`US 2003 FOOT2254A1
`Apr. 17, 2003
`Related U.S. Application Dat
`elated U.S. Application Uata
`(60) Provisional application No. 60/329.509, filed on Oct.
`17, 2001.
`A method and app
`provided for reducing th
`(51) Int. Cl.
`method and apparatus are orOV1Cled Or reduc1ng the
`number of pilot symbols within a MIMO-OFDM commu
`(2006.01)
`HO4. IIM
`nication system, and for improving channel estimation
`(2006.01)
`H04O 700
`within Such a system. For each transmitting antenna in an
`(2006.01)
`H04B 7/26
`OFDM transmitter, pilot symbols are encoded so as to be
`(2006.01)
`H04B 7/02
`unique to the transmitting antenna. The encoded pilot sym
`(2006.01)
`H04L 27/00
`bols are then inserted into an OFDM frame to form a
`370/208: 370/328: 370/335:
`(52) U.S. Cl
`diamond lattice, the diamond lattices for the different trans
`AV
`370,343.370,441: 375,299. 375,316. 455/101
`mitting antennae using the same frequencies but being offset
`.
`.
`.
`.
`.
`.
`s
`s
`(58) Field of Classislist Sh 5 437.465 ... from each other by a single symbol in the time domain. At
`S
`lication file f s
`s let s
`h hist
`s
`the OFDM receiver, a channel response is estimated for a
`ee appl1cauon Ille Ior complete searcn n1story.
`symbol central to each diamond of the diamond lattice using
`References Cited
`a two-dimensional interpolation. The estimated channel
`responses are Smoothed in the frequency domain. The chan
`U.S. PATENT DOCUMENTS
`nel responses of remaining symbols are then estimated by
`interpolation in the frequency domain.
`
`5,867.478 A * 2/1999 Baum et al. ................ 370,203
`6,298,035 B1 * 10/2001 Heiskala ..................... 370,206
`6,359,938 B1* 3/2002 Keevill et al. .............. 375.316
`
`44 Claims, 7 Drawing Sheets
`
`OFDM TRANSMITTERTTTTTTTTTTTTTTTTTTTT
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`1O
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`- - - - - - - - - - - - - - - -
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`CODENG
`MODULATION
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`PLOT
`INSERTER
`24
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`PILOT STBC
`23
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`P1, P2
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`PILOT
`NSEER
`40
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`GUARD
`INSERTER
`30
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`GUARD
`INSERTER
`30
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`VWGoA EX1008
`U.S. Patent No. 10,965,512
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`

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`U.S. Patent
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`Jul. 24, 2007
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`Sheet 1 of 7
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`US 7,248,559 B2
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`PILOT EXTRACTION IN FREOUENCY
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`DeAN
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`INTERPOLATION IN FREOUENCY
`1B-2
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`FFT
`1B-3
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`SMOOTHING/DE-NOISE
`1B-4
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`FFT
`1B-5
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`FIG. 1B
`(PRIOR ART)
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`PILOT EXTRACTION IN FREOUENCY
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`DyAN
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`INTERPOLATION IN TIME
`1A-2
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`INTERPOLAT y FREOUENCY
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`FIG. 1A
`(PRIOR ART)
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`PILOT EXTRACTION IN FREOUENCY
`DeAN
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`INTERPOLATION INTIME
`1 C-2
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`INTERPOLATO y FREOUENCY
`
`FIG. 1C
`(PRIOR ART)
`
`

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`U.S. Patent
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`Jul. 24, 2007
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`Sheet 2 of 7
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`U.S. Patent
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`Jul. 24,
`2007
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`Sheet 3 of 7
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`248,559 B2 US 7,
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`H300|03|0
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`G/
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`U.S. Patent
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`Jul. 24, 2007
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`Sheet 4 of 7
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`US 7,248,559 B2
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`RECEIVE DATA SYMBOLS
`1OO
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`GENERATE PILOT SYMBOLS
`102
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`APPLY STBCENCODING TO PILOT SYMBOLS
`104
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`INSERT ENCODED PILOT SYMBOLS
`106
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`FIG. 4
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`FREOUENCY
`SOOOOOSOOO
`1SQQQQQQ
`SN
`KX-XCXCXCXCXNXCXCXCX-C Car(XCX-C)
`124-1922-22-22
`---------------- S&S
`s
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`120
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`S-S-S- N
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`SS
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`U.S. Patent
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`Jul. 24, 2007
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`Sheet S of 7
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`US 7,248,559 B2
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`140 s
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`150
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`Tx2
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`H
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`144
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`RX2
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`FIG. 6
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`

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`U.S. Patent
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`Jul. 24, 2007
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`Sheet 6 of 7
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`US 7.248,559 B2
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`O2D INTERPOLATION
`QPILOT CHANNELESTIMATE
`
`FIG. 7
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`FREOUENCY
`
`O2D INTERPOLATION
`QPILOTCHANNELESTIMATE
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`O2D INTERPOLATION
`QPILOT CHANNELESTIMATE
`O 1D INTERPOLATION
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`FREOUENCY
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`FIG. 8
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`FIG. 9
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`

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`U.S. Patent
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`Jul. 24, 2007
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`Sheet 7 of 7
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`US 7,248,559 B2
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`PILOT EXTRACTION AT FREOUENCY DOMAINFOR
`EACH RECEIVE ANTENNA
`500
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`CHANNEL RESPONSE MATRIX COMPUTING
`
`FIG. 10
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`1.OOE+00
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`
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`1.OOE-02
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`1.OOE-01 S N
`1.OOE-03 | | | SSS Na
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`SNCN
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`NSN
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`1.OOE-05
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`SNR (dB)
`O DPPOLER-100Hz PROPOSED METHOD
`O DOPPLER=3OOHz PROPOSEDMETHOD
`{) DPPOLER-100Hz IDEAL CHANNEL
`{XDOPPLER-3OOHz IDEAL CHANNEL
`FIG. 11
`
`

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`US 7,248,559 B2
`
`1.
`SCATTERED PLOT PATTERN AND
`CHANNEL ESTMLATION METHOD FOR
`MIMO-OFDM SYSTEMS
`
`RELATED APPLICATION
`
`This application claims the benefit of U.S. provisional
`application No. 60/329,509 filed Oct. 17, 2001.
`
`FIELD OF THE INVENTION
`
`This invention relates to OFDM communication systems,
`and more particularly to a more efficient use of pilot symbols
`within Such systems.
`
`BACKGROUND OF THE INVENTION
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`The variations in phase and amplitude resulting from
`propagation along the channel are referred to as the channel
`response. The channel response is usually frequency and
`time dependent. If the receiver can determine the channel
`response, the received signal can be corrected to compensate
`for the channel degradation. The determination of the chan
`nel response is called channel estimation. The inclusion of
`pilot symbols in each OFDM symbol allows the receiver to
`carry out channel estimation. The pilot symbols are trans
`mitted with a value known to the receiver. When the receiver
`receives the OFDM symbol, the receiver compares the
`received value of the pilot symbols with the known trans
`mitted value of the pilot symbols to estimate the channel
`response.
`The pilot symbols are overhead, and should be as few in
`number as possible in order to maximize the transmission
`rate of data symbols. Since the channel response can vary
`with time and with frequency, the pilot symbols are scattered
`amongst the data symbols to provide as complete a range as
`possible of channel response over time and frequency. The
`set of frequencies and times at which pilot symbols are
`inserted is referred to as a pilot pattern. The optimal tem
`poral spacing between the pilot symbols is usually dictated
`by the maximum anticipated Doppler frequency, and the
`optimal frequency spacing between the pilot symbols is
`usually dictated by the anticipated delay spread of multi
`path fading.
`The existing pilot-assisted OFDM channel estimation
`approaches are designed for conventional one transmitter
`system. With a scattered pilot arrangement, there are three
`classes of algorithms:
`1-D frequency interpolation or time interpolation
`Transformed frequency 1-D interpolation
`Independent time and frequency 1-D interpolation
`The first class of algorithms is based on the pilot OFDM
`symbol (all the sub-carriers are used as the pilots) or
`comb-type of pilots. This approach shown in the flow chart
`of FIG. 1A is simple but only suitable for channels with high
`frequency selectivity or channels with high time fading. The
`method involves pilot extraction in the frequency domain
`(step 1A-1) followed by interpolation in time (step 1A-2), or
`interpolation in frequency (step 1A-3).
`The second method shown in the flow chart of FIG. 1B is
`aimed for channels with slow Doppler fading and fast
`frequency fading. It improves the first method by using FFT
`to reconstruct the channel response back to time domain for
`noise reduction processing at the expense of FFT/IFFT
`computing for the channel estimation separately. The
`method begins with pilot extraction in the frequency domain
`(step 1B-1), which may be followed by interpolation in
`frequency (step 1B-2). Then an inverse fast Fourier trans
`form (step 1B-3), Smoothing/de-noise processing (step
`1B-4), and finally a fast Fourier transform (1B-5) steps are
`executed.
`The third method shown in the flow chart of FIG. 1C can
`be used to estimate channel for mobile applications, where
`both fast time fading and frequency fading exist. However
`it needs a relatively high density of pilots and a completed
`interpolator. This method involves pilot extraction in the
`frequency domain (step 1C-1) this is followed by interpo
`lation in time (step 1C-2) and interpolation in frequency
`(step 1C-3).
`In the propagation environment with both high frequency
`dispersion and temporal fading, the channel estimation
`performance can be improved by the increase of pilot
`symbol density at the price of the reduction of the spectral
`efficiency of the data transmission. To interpolate and recon
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`Multiple Input Multiple Output-Orthogonal Frequency
`Division Multiplexing (MIMO-OFDM) is a novel highly
`spectral efficient technology used to transmit high-speed
`data through radio channels with fast fading both in fre
`quency and in time.
`In wireless communication systems that employ OFDM,
`a transmitter transmits data to a receiver using many Sub
`carriers in parallel. The frequencies of the sub-carriers are
`orthogonal. Transmitting the data in parallel allows the
`symbols containing the data to be of longer duration, which
`reduces the effects of multi-path fading. The orthogonality
`of the frequencies allows the sub-carriers to be tightly
`spaced, while minimizing inter-carrier interference. At the
`transmitter, the data is encoded, interleaved, and modulated
`to form data symbols. Overhead information is added,
`including pilot symbols, and the symbols (data plus over
`head) are organized into OFDM symbols. Each OFDM
`35
`symbol typically uses 2" frequencies. Each symbol is allo
`cated to represent a component of a different orthogonal
`frequency. An inverse Fast Fourier Transform (IFFT) is
`applied to the OFDM symbol (hence the preference of 2"
`frequencies) to generate time samples of a signal. Cyclic
`extensions are added to the signal, and the signal is passed
`through a digital-to-analog converter. Finally, the transmitter
`transmits the signal to the receiver along a channel.
`When the receiver receives the signal, the inverse opera
`tions are performed. The received signal is passed through
`an analog-to-digital converter, and timing information is
`then determined. The cyclic extensions are removed from
`the signal. The receiver performs an FFT on the received
`signal to recover the frequency components of the signal,
`that is, the data symbols. Error correction may be applied to
`the data symbols to compensate for variations in phase and
`amplitude caused during propagation of the signal along the
`channel. The data symbols are then demodulated, de-inter
`leaved, and decoded, to yield the transmitted data.
`In systems employing differential detection, the receiver
`compares the phase and/or amplitude of each received
`symbol with an adjacent symbol. The adjacent symbol may
`be adjacent in the time direction or in the frequency direc
`tion. The receiver recovers the transmitted data by measur
`ing the change in phase and/or amplitude between a symbol
`and the adjacent symbol. If differential detection is used,
`channel compensation need not be applied to compensate for
`variations in phase and amplitude caused during propagation
`of the signal. However, in Systems employing coherent
`detection the receiver must estimate the actual d phase and
`amplitude of the channel response, and channel compensa
`tion must be applied.
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`US 7,248,559 B2
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`3
`struct the channel response function from the limited pilots
`to achieve reliable channel estimation with the minimum
`overhead is a challenging task.
`There are a variety of existing standard pilot patterns. In
`environments in which the channel varies only slowly with
`time and frequency, the pilot symbols may be inserted
`cyclically, being inserted at an adjacent frequency after each
`time interval. In environments in which the channel is highly
`frequency dependent, the pilot symbols may be inserted
`periodically at all frequencies simultaneously. However,
`Such a pilot pattern is only Suitable for channels that vary
`very slowly with time. In environments in which the channel
`is highly time dependent, the pilot symbols may be inserted
`continuously at only specific frequencies in a combarrange
`ment to provide a constant measurement of the channel
`response. However, such a pilot pattern is only Suitable for
`channels that vary slowly with frequency. In environments
`in which the channel is both highly frequency and highly
`time dependent (for example, mobile systems with much
`multi-path fading), the pilot symbols may be inserted peri
`odically in time and in frequency so that the pilot symbols
`form a rectangular lattice when the symbols are depicted in
`a time-frequency diagram.
`In OFDM communication systems employing coherent
`modulation and demodulation, the receiver must estimate
`the channel response at the frequencies of all Sub-carriers
`and at all times. Although this requires more processing than
`in systems that employs differential modulation and
`demodulation, a significant gain in signal-to-noise ratio can
`be achieved using coherent modulation and demodulation.
`The receiver determines the channel response at the times
`and frequencies at which pilot symbols are inserted into the
`OFDM symbol, and performs interpolations to estimate the
`channel response at the times and frequencies at which the
`data symbols are located within the OFDM symbol. Placing
`pilot symbols more closely together (in frequency if a comb
`pattern is used, in time if a periodic pattern is used, or in both
`frequency and in time if a rectangular lattice pattern is used)
`within a pilot pattern results in a more accurate interpolation.
`However, because pilot symbols are overhead, a tighter pilot
`pattern is at the expense of the transmitted data rate.
`Existing pilot patterns and interpolation techniques are
`usually sufficient if the channel varies slowly with time (for
`example for nomadic applications). However, if the channel
`varies quickly with time (for example, for mobile applica
`tions), the time interval between pilot symbols must be
`reduced in order to allow an accurate estimation of the
`channel response through interpolation. This increases the
`overhead in the signal.
`The problem of minimizing the number of pilot symbols
`while maximizing the accuracy of the interpolation is also
`particularly cumbersome in Multiple-Input Multiple-Output
`(MIMO) OFDM systems. In MIMO OFDM systems, the
`transmitter transmits data through more than one transmit
`ting antenna and the receiver receives data through more
`than one receiving antenna. The binary data is usually
`divided between the transmitting antennae, although the
`same data may be transmitted through each transmitting
`antenna if spatial diversity is desired. Each receiving
`antenna receives data from all the transmitting antennae, so
`if there are M transmitting antennae and N receiving anten
`nae, then the signal will propagate over MxN channels, each
`of which has its own channel response. Each transmitting
`antenna inserts pilot symbols into the same Sub-carrier
`location of the OFDM symbol which it is transmitting. In
`order to minimize interference at the receiver between the
`pilot symbols of each transmitting antenna, each transmit
`
`4
`ting antenna typically blinks its pilot pattern on and off. This
`increases the temporal separation of the pilot symbols for
`each transmitter, reducing the accuracy of the interpolation
`used to estimate the channel response. In MIMO-OFDM
`systems a simple and fast channel estimation method is
`particularly crucial because of the limitation of the compu
`tational power for estimating MXN channels, while in SISO
`OFDM system only one channel needs to be estimated.
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`SUMMARY OF THE INVENTION
`
`Channel estimation methods are provided which are based
`on the partial interpolation of a scattered pilot by using true
`2-D interpolation; and additionally, simple 1-D interpolation
`is used reconstruct the entire channels. This method has a
`reduced scattered pilot overhead, and is at least an order of
`magnitude less computationally complex than some existing
`methods. In general, the proposed method of channel esti
`mation is more robust in channels with high Doppler spread,
`and provides better performance than Some existing methods
`and requires the less buffering of the OFDM symbols for the
`coherent detection at the receiver than in Some methods.
`The methods allow fewer pilot symbols to be placed
`within each OFDM symbol, while still allowing accurate
`interpolation of the channel response. The data rate of an
`MIMO-OFDM system is thereby improved.
`A first broad aspect of the invention provides a method of
`inserting pilot symbols into Orthogonal Frequency Division
`Multiplexing (OFDM) frames at an OFDM transmitter hav
`ing at least one transmitting antenna, the OFDM frames
`having a time domain and a frequency domain, each OFDM
`frame comprising a plurality of OFDM symbols. The
`method involves, for each antenna, inserting scattered pilot
`symbols in an identical scattered pattern in time-frequency.
`In some embodiments, the identical scattered pattern is a
`regular diagonal-shaped lattice.
`In some embodiments, inserting pilot symbols in an
`identical diagonal-shaped lattice involves for each point in
`the identical diagonal shaped lattice inserting a number of
`pilot symbols on a single Sub-carrier for N consecutive
`OFDM symbols, where N is the number of transmitting
`antennae.
`In some embodiments, diagonal shaped lattice is a dia
`mond shaped lattice.
`In some embodiments for each point in the diagonal
`shaped lattice, Luncoded pilot symbols are generated. Space
`time block coding (STBC) is performed on the group of L
`uncoded pilot symbols to produce an NxN STBC block, L
`and N determining an STBC code rate. Then, one row or
`column of the STBC block is transmitted on each antenna on
`a specific Sub-carrier.
`In some embodiments, transmitting the pilot symbols is
`done with a power level greater than a power level of data
`symbols, depending upon a value reflective of channel
`conditions.
`In some embodiments, transmitting the pilot symbols is
`done with a power level which is dynamically adjusted to
`ensure Sufficiently accurate reception as a function of a
`modulation type applied to the Sub-carriers carrying data.
`In some embodiments, the diagonal shaped lattice pattern
`has a first plurality of equally spaced Sub-carrier positions,
`and a second plurality of equally spaced Sub-carrier posi
`tions offset from said first plurality. The pilot symbols are
`inserted alternately in time using the first plurality of equally
`spaced sub-carrier positions and the second plurality of
`equally spaced Sub-carrier positions.
`
`

`

`5
`In some embodiments, the second plurality of Sub-carriers
`is offset from the first plurality of equally spaced-sub-carrier
`positions by half the spacing between adjacent Sub-carriers
`of the first plurality of sub-carrier positions thereby forming
`a diamond shaped lattice pattern.
`In some embodiments, the pilot pattern is cyclically
`offset, both in a time direction and in a frequency direction,
`for at least one adjacent base station to form re-use patterns.
`Another broad aspect of the invention provides an OFDM
`transmitter. The OFDM transmitter has a plurality of trans
`10
`mit antennas, and is adapted to insert pilot symbols into
`Orthogonal Frequency Division Multiplexing (OFDM)
`frames having a time domain and a frequency domain, each
`OFDM frame comprising a plurality of OFDM symbols by,
`for each antenna, inserting pilot symbols in an identical
`scattered pattern in time-frequency.
`In some embodiments, the identical scattered pattern is a
`diagonal-shaped lattice.
`In some embodiments, inserting pilot symbols in an
`identical scattered pattern involves for each point in the
`identical scattered pattern inserting a number of pilot sym
`bols on a single sub-carrier for N consecutive OFDM
`symbols, where N is the number of transmitting antennae,
`where Na=1.
`In some embodiments, the scattered pattern is a diamond
`shaped lattice.
`In some embodiments, for each point in the scattered
`pattern, the OFDM transmitter is adapted to generate L
`uncoded pilot symbols, perform space time block coding
`30
`(STBC) on the group of L. pilot symbols to produce an NXN
`STBC block, and transmit one row or column of the STBC
`block on each antenna.
`In some embodiments, the OFDM transmitter is further
`adapted to transmit the pilot symbols with a power level
`greater than a power level of data symbols depending on a
`value reflective of channel conditions.
`In Some embodiments in which the diamond shaped
`lattice pattern is employed, the diamond shaped lattice
`pattern has a first plurality of equally spaced sub-carrier
`positions and a second plurality of equally spaced Sub
`carrier positions offset from said first plurality. The pilot
`symbols are inserted alternately in time using the first
`plurality of equally spaced sub-carrier positions and the
`second plurality of equally spaced sub-carrier positions.
`Another broad aspect of the invention provides a method
`of estimating a plurality of channel responses at an Orthogo
`nal Frequency Division Multiplexing (OFDM) receiver hav
`ing at least one receive antenna. The method involves at each
`receive antenna receiving OFDM frames transmitted by at
`least one transmitting antenna, the OFDM frames having a
`time domain and a frequency domain, the OFDM frames
`transmitted by each antenna having pilot symbols inserted in
`an identical scattered pattern in time-frequency, each OFDM
`frame comprising a plurality of OFDM symbols. For each
`55
`transmit antenna, receive antenna combination: a) the pilot
`symbols of the received OFDM frames are used to estimate
`a channel response for each point in the scattered pattern; b)
`the channel response is estimated for of a plurality of points
`not on the scattered pattern by performing a two-dimen
`sional (time direction, frequency direction) interpolation of
`channel responses determined for points in the scattered
`pattern; c) an interpolation is performed in the frequency
`direction to estimate the channel responses corresponding to
`remaining OFDM sub-carriers within each OFDM symbol.
`In some embodiments, a filtering function is performed on
`the channel responses prior to performing the interpolation
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`in the frequency direction to estimate the channel responses
`corresponding to remaining OFDM sub-carriers within each
`OFDM symbol.
`In some embodiments, estimating the channel response of
`a plurality of points not on the scattered pattern by perform
`ing a two-dimensional (time direction, frequency direction)
`interpolation of channel responses determined for points in
`the scattered pattern lattice involves for each sub-carrier to
`be estimated averaging channel responses of the given
`channel estimation period of a sub-carrier before the sub
`carrier to be estimated in frequency (when present) and a
`sub-carrier after the sub-carrier to be estimated in frequency
`(when present) and the channel response for a previous
`estimation period (when present) and a following estimation
`period (when present).
`In some embodiments, the method is applied to a single
`transmitter, single receiver system.
`In other embodiments the method is applied to a single
`transmitter system wherein each point in the scattered pat
`tern contains a single pilot symbol.
`In some embodiments, the method is applied to a system
`in which there are N>=2 antennas, and each point in the
`scattered pattern contains a number N of consecutive
`encoded pilot symbols transmitted on a Sub-carrier, a single
`channel estimate being determined for each N encoded pilot
`symbols.
`In some embodiments, the N encoded pilot symbols
`contain L. pilot symbols which were STBC block coded,
`where N and L together determine a STBC code rate.
`Other aspects and features of the present invention will
`become apparent to those ordinarily skilled in the art upon
`review of the following description of specific embodiments
`of the invention in conjunction with the accompanying
`Figures.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention will now be described in greater detail with
`reference to the accompanying Figures, in which:
`FIG. 1 illustrates flow-charts for three examples of con
`ventional OFDM Channel Estimation;
`FIG. 2 is a block diagram of a Multiple-Input Multiple
`Output Orthogonal Frequency Division Multiplexing
`(OFDM) transmitter provided by an embodiment of the
`invention;
`FIG. 3 is a block diagram of an OFDM receiver;
`FIG. 4 is a flowchart of a method by which an OFDM
`transmitter inserts pilot symbols into an OFDM frame
`according to one embodiment of the invention;
`FIG. 5 is a diagram of a pilot pattern generated using the
`method of FIG. 4;
`FIG. 6 is a block diagram of a MIMO system showing the
`channel transfer functions between two transmit antennas
`and two receive antennas;
`FIG. 7 is a time frequency diagram showing channel
`estimate positions for pilot channel estimation;
`FIG. 8 schematically illustrates a step of filtering esti
`mated and interpolated pilot channel estimates;
`FIG. 9 shows schematically the step of interpolating
`between the channel estimates previously determined to
`provide channel estimates for all Sub-carriers and all times;
`FIG. 10 is a flow chart summarizing the overall channel
`estimation method provided by an embodiment of the inven
`tion; and
`FIG. 11 is an example of a set of performance results
`obtained using the method of FIG. 10.
`
`

`

`US 7,248,559 B2
`
`7
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`8
`The data symbols sent along the second processing path
`18 are sent to a second OFDM component 38 which includes
`processors similar to those included in the first OFDM
`component 20. However, the pilot inserter 40 inserts
`encoded pilot symbols from the second row of the STBC
`block produced by the pilot STBC function 23. The symbols
`sent along the second processing path 18 are ultimately
`transmitted as a signal through a second transmitting
`antenna 42.
`Referring now to FIG. 3, a block diagram of an MIMO
`OFDM receiver is shown. An OFDM receiver 50 includes a
`first receiving antenna 52 and a second receiving antenna 54
`(although more generally there will be one or more receiving
`antennae). The first receiving antenna 52 receives a first
`received signal. The first received signal is a combination of
`the two signals transmitted by the two transmitting antennae
`37 and 42 of FIG. 2, although each of the two signals will
`have been altered by a respective channel between the
`respective transmitting antenna and the first receiving
`antenna 52. The second receiving antenna 54 receives a
`second received signal. The second received signal is a
`combination of the two signals transmitted by the two
`transmitting antennae 37 and 42 of FIG. 2, although each of
`the two signals will have been altered by a respective
`channel between the respective transmitting antenna and the
`second receiving antenna 54. The four channels (between
`each of the two transmitting antennae and each of the two
`receiving antennae) may vary with time and with frequency,
`and will usually be different from each other.
`The OFDM receiver 50 includes a first OFDM component
`56 and a second OFDM component 58 (although in general
`there will be N OFDM components, one for each receiving
`antenna). The first OFDM component 56 includes a RF
`receiver 59, and an analog-to-digital converter 60, which
`converts the first received signal into digital signal samples.
`The signal samples are passed to a frequency synchronizer
`62 and a frequency offset corrector 64. The signal samples
`are also fed to a frame/time synchronizer 66. Collectively,
`these three components produce synchronized signal
`samples.
`The synchronized signal samples represent a time
`sequence of data. The synchronized signal samples are
`passed to a demultiplexer 68, then passed in parallel to a Fast
`Fourier Transform (FFT) processor 70. The FFT processor
`70 performs an FFT on the signal samples to generate
`estimated received symbols which are multiplexed in MUX
`76 and sent as received symbols to decoder 78. Ideally, the
`received symbols would be the same as the symbols fed into
`the IFFT processor 26 at the OFDM transmitter 10. How
`ever, as the received signals will have likely been altered by
`the various propagation channels, the first OFDM compo
`nent 56 must correct the received symbols by taking into
`account the channels. The received symbols are passed to a
`channel estimator 72, which analyses received pilot symbols
`located at known times and frequencies within the OFDM
`frame. The channel estimator 72 compares the received pilot
`symbols with what the channel estimator 72 knows to be the
`values of the pilot symbols as transmitted by the OFDM
`transmitter 10, and generates an estimated channel response
`for each frequency and time within the OFDM symbol. The
`estimated channel responses are passed to decoder 78. The
`channel estimator 72 is described in detail below.
`The second OFDM component 58 includes similar com
`ponents as are included in the first OFDM component 56,
`and processes the second received signal in the same manner
`
`10
`
`15
`
`25
`
`30
`
`35
`
`The following sections describe a MIMO-OFDM trans
`mitter/receiver and scattered pilot insertion. By way of 5
`introduction, a OFDM frame consists of the preamble
`OFDM symbols and regular OFDM symbols. Each OFDM
`symbol uses a set of orthogonal sub-carriers. When there are
`two transmit antennas, two OFDM symbols form a STTD
`block. For regular OFDM symbols, some sub-carriers are
`used as pilot sub-carriers to carry pilot symbols while the
`others are used as data Sub-carriers to carry data symbols.
`The pilot sub-carriers are modulated by pilot symbols gen
`erated by QPSK. The data sub-carriers are modulated by
`complex data symbols generated by QAM mapping. STTD
`coding is applied to the pilot Sub-carrier pairs located at the
`same frequency within one STTD block.
`Referring to FIG. 2, a block diagram of a Multiple-Input
`Multiple-Output (MIMO) Orthogonal Frequency Division
`Multiplexing (OFDM) transmitter provided by an embodi
`ment of the invention is shown. The OFDM transmitter
`shown in FIG. 2 is a two-output OFDM transmitter, though
`more generally there may be a plurality of M transmitting
`antennae. An OFDM transmitter 10 takes binary data as
`input but data in other forms may be accommodated. The
`binary data is passed to a coding/modulation primitive 12
`responsible for encoding, interleaving, and modulating the
`binary data to generate data symbols, as is well known to
`those skilled in the art. The coding/modulation primitive 12
`may include a number of processing blocks, not shown in
`FIG. 2. An encoder 14 applies Space-Time Block Coding
`(SBTC) to the data symbols. The encoder 14 also separates
`the data symbols into a first processing path 16 and a second
`processing path 18, by sending alternate data symbols along
`each of the two processing paths. In the more general case
`in which the OFDM transmitter 10 includes M transmitting
`antennae, the encoder 14 separates the data symbols into M
`processing paths.
`The data symbols sent along the first processing path 16
`are sent to a first OFDM component 20. The da