(19) United States
`(12) Patent Application Publication (10) Pub. N0.: US 2002/0181509 A1
`(43) Pub. Date:
`Dec. 5, 2002
`Mody et al.
`
`US 20020181509A1
`
`(54) TIME AND FREQUENCY
`SYNCHRONIZATION IN MULTI-INPUT,
`MULTI-OUTPUT (MIMO) SYSTEMS
`
`(76) Inventors: Apurva N. Mody, Atlanta, GA (US);
`Gordon L. Stuber, Atlanta, GA (US)
`
`Correspondence Address:
`THOMAS, KAYDEN, HORSTEMEYER &
`RISLEY, LLP
`100 GALLERIA PARKWAY, NW
`STE 1750
`ATLANTA, GA 30339-5948 (US)
`
`(21) Appl. No.:
`
`10/128,821
`
`(22) Filed:
`
`Apr. 24, 2002
`
`Related US. Application Data
`
`(60) Provisional application No. 60/286,180, ?led on Apr.
`24, 2001. Provisional application No. 60/286,130,
`?led on Apr. 24, 2001.
`
`Publication Classi?cation
`
`(51) Int. Cl? ...................................................... .. H04J 3/06
`(52) vs. C]. .......................................... .. 370/480; 370/350
`
`(57)
`
`ABSTRACT
`
`In a communication system, and in particular a Wireless
`Orthogonal Frequency Division Multiplexing (OFDM)
`communication system, the present invention provides sys
`tems for synchronizing data transmitted across a channel.
`The present invention may be used in a Multi-Input, Multi
`Output (MIMO) system in Which the data is transmitted
`from any number of transmitting antennas and received by
`any number of receiving antennas. The number of transmit
`ting and receiving antennas does not necessarily have to be
`the same. Circuitry is provided for synchronizing the data in
`both the time domain and frequency domain. Time synchro
`nization involves coarse time synchronization and ?ne time
`synchronization. Frequency synchronization involves coarse
`frequency offset estimation, ?ne frequency offset estimation,
`and frequency offset correction.
`
`6
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`SOURCE
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`DEMODULATOR
`j
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`I
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`22
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`OFDM
`DEMODULATOR
`
`ERIC-1005
`Ericsson v IV
`Page 1 of 23
`
`

`
`Patent Application Publication
`
`Dec. 5, 2002 Sheet 1 0f 9
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`ERIC-1005 / Page 2 of 23
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`Patent Application Publication
`
`Dec. 5, 2002 Sheet 2 0f 9
`
`US 2002/0181509 A1
`
`26X
`
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`
`ERIC-1005 / Page 3 of 23
`
`

`
`Patent Application Publication
`
`Dec. 5, 2002 Sheet 3 0f 9
`
`US 2002/0181509 A1
`
`DATA PQ(G+N) K‘ 56
`
`FIG. 4
`
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`54 i
`
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`
`<——————— PREAMBLE
`
`ANTENNA 1
`
`ANTENNA 2
`
`ANTENNA Q
`
`ERIC-1005 / Page 4 of 23
`
`

`
`Patent Application Publication
`
`Dec. 5, 2002 Sheet 4 0f 9
`
`US 2002/0181509 A1
`
`HTS
`
`OFDM
`MODULATOR
`
`OFDM
`MODULATOR
`
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`MODULATOR
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`OFDM
`MODULATOR
`
`OFDM
`DEMOD.
`
`OFDM
`DEMOD.
`
`OFDM
`DEMOD.
`
`OFDM
`DEMOD.
`
`FIG. 5
`
`ERIC-1005 / Page 5 of 23
`
`

`
`Patent Application Publication
`
`Dec. 5, 2002 Sheet 5 0f 9
`
`US 2002/0181509 A1
`
`95%
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`
`ERIC-1005 / Page 6 of 23
`
`

`
`Patent Application Publication
`
`Dec. 5, 2002 Sheet 6 0f 9
`
`US 2002/0181509 A1
`
`57
`
`58
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`20 TU»
`
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`CIRCUIT
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`
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`CIRCUIT
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`CORRECTION CIRCUIT
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`ESTIMATION
`CIRCUIT
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`DFT 64
`
`TO THE
`‘I PRE-AMPLIFIER 57
`
`To THE
`LOCAL OSCILLATOR 59
`
`FIG. 8
`
`ERIC-1005 / Page 7 of 23
`
`

`
`Patent Application Publication
`
`Dec. 5, 2002 Sheet 7 0f 9
`
`US 2002/0181509 A1
`
`rn
`
`7
`
`magnitude
`
`SUMMING
`CIRCUIT
`Gil
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`TIME
`' AVERAGING ———>
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`frequency offset (7)
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`FIG. 10
`
`ERIC-1005 / Page 8 of 23
`
`

`
`Patent Application Publication
`
`Dec. 5, 2002 Sheet 8 0f 9
`
`US 2002/0181509 A1
`
`SUMMING
`CIRCUIT
`
`11:0
`
`—‘———-—>
`
`BUFFER
`
`FIG. 11
`
`100
`
`BUFFER
`
`104
`
`SUMMING
`CIRCUIT
`
`#0
`
`108
`
`FIG. 12
`
`ERIC-1005 / Page 9 of 23
`
`

`
`Patent Application Publication
`
`Dec. 5, 2002 Sheet 9 0f 9
`
`US 2002/0181509 A1
`
`- L
`
`114
`
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`:
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`;
`PROCESSOR
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`ESTIMATOR
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`FIG. 13
`
`ERIC-1005 / Page 10 of 23
`
`

`
`US 2002/0181509 A1
`
`Dec. 5, 2002
`
`TIME AND FREQUENCY SYNCHRONIZATION IN
`MULTI-INPUT, MULTI-OUTPUT (MIMO)
`SYSTEMS
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`[0001] This application claims priority to copending US.
`provisional application entitled, “Synchronization for
`MIMO OFDM Systems,” having serial No. 60/286,180,
`?led Apr. 24, 2001, Which is entirely incorporated herein by
`reference.
`
`[0002] This application is related to copending US. pro
`visional application entitled “Parameter Estimation for
`MIMO OFDM Systems,” having serial No. 60/286,130,
`?led on Apr. 24, 2001, Which is entirely incorporated herein
`by reference.
`
`TECHNICAL FIELD OF THE INVENTION
`
`[0003] The present invention is generally related to Wire
`less communication systems that employ Orthogonal Fre
`quency Division Multiplexing (OFDM) and, more particu
`larly, to an apparatus and method for providing time and
`frequency synchroniZation in a Multi-Input, Multi-Output
`(MIMO) OFDM system.
`
`BACKGROUND OF THE INVENTION
`
`[0004] In Wireless communication systems, recent devel
`opments have been made using technologies Wherein mul
`tiple signals are simultaneously transmitted over a single
`transmission path. In Frequency Division Multiplexing
`(FDM), the frequency spectrum is divided into sub-chan
`nels. Information (e.g. voice, video, audio, text, etc.) is
`modulated and transmitted over these sub-channels at dif
`ferent sub-carrier frequencies.
`[0005] In Orthogonal Frequency Division Multiplexing
`(OFDM) schemes, the sub-carrier frequencies are spaced
`apart by precise frequency differences. Because of the ability
`of OFDM systems to overcome the multiple path effects of
`the channel, and to transmit and receive large amounts of
`information, much research has been performed to advance
`this technology. By using multiple transmitting antennas and
`multiple receiving antennas in OFDM systems, it is possible
`to increase the capacity of transmitted and received data
`While generally using the same amount of bandWidth as in
`a system With one transmit and one receive antenna.
`
`[0006] OFDM technologies are typically divided into tWo
`categories. The ?rst category is the Single-Input, Single
`Output (SISO) scheme, Which utiliZes a single transmitting
`antenna to transmit radio frequency (RF) signals and a single
`receiving antenna to receive the RF signals. The second
`category is the Multi-Input, Multi-Output (MIMO) scheme,
`Which uses multiple transmitting antennas and multiple
`receiving antennas.
`[0007] In typical communication systems, training sym
`bols, or preamble, at the beginning of data frames, are
`usually added as a pre?x to the data symbols. The data
`symbols, of course, include the useful data or information
`(e.g., voice, data, video, etc.), Which is meant to be trans
`mitted to a remote location. The training symbols in SISO
`systems are used to provide synchroniZation of the received
`
`signals With respect to the transmitted signals, as Well as to
`provide channel parameter estimation.
`[0008] Although training symbols used for SISO systems
`can be used to provide synchroniZation in a MIMO system,
`the training symbols cannot provide for channel parameter
`estimation in the MIMO system. In fact, no method or
`apparatus exists for MIMO systems that are capable of
`providing time and frequency synchroniZation as Well as
`channel parameter estimation. Thus, a need exists for a
`method and apparatus that is capable of providing time and
`frequency synchroniZation in MIMO systems and can fur
`ther perform channel estimation.
`
`SUMMARY OF THE INVENTION
`
`[0009] The present invention provides systems and meth
`ods that overcome the de?ciencies of the prior art as
`mentioned above. The present invention utiliZes a sequence
`of training symbols or preambles that may be used in both
`Single-Input, Single-Output (SISO) and Multi-Input, Multi
`Output (MIMO) systems, using any number of transmitting
`and receiving antennas. Also, the present invention can be
`used to synchroniZe a received data frame With a transmitted
`data frame in a MIMO system in both the time domain and
`frequency domains. In order to make MIMO systems opera
`tional, synchroniZation is essential. HoWever, no scheme has
`been developed Which is capable of time and frequency
`synchroniZation in MIMO systems. The present invention
`achieves synchroniZation in the time domain and frequency
`domain and, therefore, enables MIMO systems to operate
`acceptably.
`[0010] One MIMO Orthogonal Frequency Division Mul
`tiplexing (OFDM) system of the present invention includes
`a number of OFDM modulators, Which provide data frames
`to be transmitted across a channel. The data frames of the
`present invention comprise one or more training symbols, a
`plurality of data symbols, and cyclic pre?xes inserted
`betWeen the data symbols. A number of transmitting anten
`nas corresponding to the number of modulators is used to
`transmit the modulated signals over the channel. A number
`of receiving antennas is used to receive the transmitted
`signals. The received signals are demodulated by a number
`of OFDM demodulators corresponding to the number of
`receiving antennas and decoded by an OFDM decoder,
`Which processes the data frames. By utiliZing the structure
`embedded in the training symbols, the MIMO system of the
`present invention is capable of providing time and frequency
`synchroniZation as Well as perform channel estimation.
`
`[0011] A method of the present invention is also provided,
`Wherein synchroniZation is carried out in the time and
`frequency domains in a MIMO system. The method includes
`producing data frames comprising at least one training
`symbol, multiple data symbols and cyclic pre?xes. The data
`frames are transmitted over the channel, received, and
`demodulated and processed. By processing the training
`symbol of the data frame, the data frame can be synchro
`niZed in both the time and frequency domains.
`[0012] Other systems, methods, features, and advantages
`of the present invention Will become apparent to a person
`having skill in the art upon examination of the folloWing
`draWings and detailed description. All such additional sys
`tems, methods, features, and advantages are Within the scope
`of the present invention.
`
`ERIC-1005 / Page 11 of 23
`
`

`
`US 2002/0181509 A1
`
`Dec. 5, 2002
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0013] Many aspects of the invention can be better under
`stood with reference to the following drawings. Moreover, in
`the drawings, like reference numerals designate correspond
`ing parts throughout the several views.
`[0014] FIG. 1 is a block diagram illustrating an example
`embodiment of a Multi-Input, Multi-Output (MIMO)
`Orthogonal Frequency Division Multiplexing (OFDM) sys
`tem.
`
`[0015] FIG. 2 is a block diagram illustrating an example
`embodiment of the MIMO encoder shown in FIG. 1.
`
`[0016] FIG. 3 is a block diagram illustrating an example
`embodiment of one of the OFDM modulators shown in FIG.
`1.
`
`[0017] FIG. 4 illustrates an example frame structure for a
`MIMO OFDM system.
`
`[0018] FIG. 5 is a block diagram illustrating an example
`matrix of a transmitted sequence structure and an example
`matrix of a received sequence structure using the modulator/
`demodulator arrangement shown in FIG. 1.
`
`[0019] FIG. 6 illustrates a three-dimensional representa
`tion of the received sequence structure in detail.
`
`[0020] FIG. 7 is a block diagram illustrating an example
`embodiment of one of the OFDM demodulators shown in
`FIG. 1.
`
`[0021] FIG. 8 is a block diagram illustrating an example
`embodiment of the synchroniZation circuit shown in FIG. 7.
`
`[0022] FIGS. 9A and 9B are block diagrams illustrating
`example embodiments of the coarse time synchroniZation
`circuit shown in FIG. 8.
`
`[0023] FIG. 10 is a block diagram illustrating an example
`embodiment of the ?rst frequency offset estimation circuit
`shown in FIG. 8.
`
`[0024] FIG. 11 is a block diagram illustrating an example
`embodiment of the ?ne time synchroniZation circuit shown
`in FIG. 8.
`
`[0025] FIG. 12 is a block diagram illustrating an example
`embodiment of the second frequency offset estimation cir
`cuit shown in FIG. 8.
`
`[0026] FIG. 13 is a block diagram illustrating an example
`embodiment of the decoder shown in FIG. 1.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`[0027] In FIG. 1, an example embodiment of a Multi
`Input, Multi-Output (MIMO) Orthogonal Frequency Divi
`sion Multiplexing (OFDM) communication system 6 of the
`present invention is shown. The communication system 6 in
`this example embodiment may be implemented as a wireless
`system for the transmission and reception of data across a
`wireless channel 19. The communication system 6, for
`example, may be part of a wireless Local Area Network
`(LAN) system or wireless Metropolitan Area Network
`(MAN) system, cellular telephone system, or other type of
`radio or microwave frequency system incorporating either
`one-way or two-way communication over a range of dis
`tances. The communication system 6 may transmit in a
`
`range from 2 to 11 GHZ, for example, such as in the
`unlicensed 5.8 GHZ band using a bandwidth of about 3-6
`MHZ.
`
`[0028] It is also possible for the present invention to be
`used in a system that comprises an array of sub-channel
`communication links that carry a number of signals trans
`mitted by a number of transmitting elements to each of a
`number of receiving elements. In this latter case, commu
`nication links, such as wires in a wiring harness or some
`alternative wired transmission system, for example, could be
`used over the distance between a data source and a receiver.
`
`[0029] In the example embodiment of FIG. 1, a transmit
`ter 8 transmits signals across the wireless channel 19 and a
`receiver 10 receives the transmitted signals. The transmitter
`8 comprises a data source 12, which provides the original
`binary data to be transmitted from the transmitter 8. The data
`source 12 may provide any type of data, such as, for
`example, voice, video, audio, text, etc. The data source 12
`applies the data to an encoder 14, which encodes the data to
`allow for error correction. The encoder 14 further processes
`the data so that certain criterion for space-time processing
`and OFDM are satis?ed. The encoder 14 separates the data
`onto multiple paths in the transmitter 8, each of which will
`hereinafter be referred to as a transmit diversity branch
`(TDB). The separate TDBs are input into OFDM modulators
`16, each of which modulates the signal on the respective
`TDB for transmission by the transmitting antennas 18. The
`present invention may be used in a Single-Input, Single
`Output (SISO) system, which may be considered as a special
`case of MIMO wherein the number of transmitting and
`receiving antennas is one. In the SISO system example,
`separation of the data by the encoder 14 is not necessary
`since only one OFDM modulator 16 and antenna 18 is used.
`[0030] During the encoding by the encoder 14 and modu
`lating by the OFDM modulators 16, data is normally
`bundled into groups such that the collection of each group of
`data is referred to as a “frame.” Details of the frame as used
`in the present invention will be described in more detail
`below with reference to FIG. 4. Each frame along each TDB
`is output from a respective OFDM modulator 16. As illus
`trated in FIG. 1, any number of OFDM modulators 16 may
`be used. The number of OFDM modulators 16 and respec
`tive transmitting antennas 18 may be represented by a
`variable “Q.” The OFDM modulators 16 modulate the
`respective frames at speci?c sub-carrier frequencies and
`respective transmitting antennas 18 transmit the modulated
`frames over the channel 19.
`
`[0031] On the side of the receiver 10, a number “L” of
`receiving antennas 20 receives the transmitted signals,
`which are demodulated by a number L of respective OFDM
`demodulators 22. The number L may represent any number
`and is not necessarily the same as the number Q. In other
`words, the number Q of transmitting antennas 18 may be
`different from the number L of receiving antennas 20, or
`they may alternatively be the same. The outputs of the
`demodulators 22 are input into a decoder 24, which com
`bines and decodes the demodulated signals. The decoder 24
`outputs the original data, which may be received by a device
`(not shown) that uses the data.
`
`[0032] The communication system 6 may comprise one or
`more processors, con?gured as hardware devices for execut
`ing software, particularly software stored in computer-read
`
`ERIC-1005 / Page 12 of 23
`
`

`
`US 2002/0181509 A1
`
`Dec. 5, 2002
`
`able memory. The processor can be any custom made or
`commercially available processor, a central processing unit
`(CPU), an auxiliary processor among several processors
`associated With a computer, a semiconductor based micro
`processor (in the form of a microchip or chip set), a
`macroprocessor, or generally any device for executing soft
`Ware instructions. Examples of suitable commercially avail
`able microprocessors are as folloWs: a PA-RISC series
`microprocessor from Hewlett-Packard Company, an 80x86
`or Pentium series microprocessor from Intel Corporation, a
`PoWerPC microprocessor from IBM, a Sparc microproces
`sor from Sun Microsystems, Inc, a 68xxx series micropro
`cessor from Motorola Corporation, or a 67xxx series Digital
`Signal Processor from the Texas Instruments Corporation.
`[0033] When the communication system 6 is implemented
`in softWare, it should be noted that the communication
`system 6 can be stored on any computer-readable medium
`for use by or in connection With any computer-related
`system or method. In the context of this document, a
`computer-readable medium is an electronic, magnetic, opti
`cal, or other physical device or means that can contain or
`store a computer program for use by or in connection With
`a computer related system or method. The communication
`system 6 can be embodied in any computer-readable
`medium for use by or in connection With an instruction
`execution system, apparatus, or device, such as a computer
`based system, processor-containing system, or other system
`that can fetch the instructions from the instruction execution
`system, apparatus, or device and execute the instructions. In
`the context of this document, a “computer-readable
`medium” can be any means that can store, communicate,
`propagate, or transport the program for use by or in con
`nection With the instruction execution system, apparatus, or
`device. The computer-readable medium can be, for example
`but not limited to, an electronic, magnetic, optical, electro
`magnetic, infrared, or semiconductor system, apparatus,
`device, or propagation medium. Examples of the computer
`readable medium include the folloWing: an electrical con
`nection having one or more Wires, a portable computer
`diskette, a random access memory (RAM), a read-only
`memory (ROM), an erasable programmable read-only
`memory (EPROM, EEPROM, or Flash memory), an optical
`?ber, and a portable compact disc read-only memory
`(CDROM). Note that the computer-readable medium could
`even be paper or another suitable medium upon Which the
`program is printed, as the program can be electronically
`captured, via for instance optical scanning of the paper or
`other medium, then compiled, interpreted or otherWise pro
`cessed in a suitable manner if necessary, and then stored in
`a computer memory.
`
`[0034] In an alternative embodiment, Where the commu
`nication system 6 is implemented in hardWare, the commu
`nication system can be implemented With any or a combi
`nation of the folloWing technologies, Which are each Well
`knoWn in the art: one or more discrete logic circuits having
`logic gates for implementing logic functions upon data
`signals, an application speci?c integrated circuit (ASIC)
`having an appropriate combination of logic gates, a pro
`grammable gate array (PGA), a ?eld programmable gate
`array (FPGA), etc.
`[0035] The encoder 14 and OFDM modulators 16 of the
`transmitter 8 Will noW be described With respect to FIGS. 2
`and 3. FIG. 2 shoWs details of an example embodiment of
`
`the encoder 14 shoWn in FIG. 1. The encoder 14 may be
`con?gured such that data from the data source 12 is encoded
`by a channel encoder 26, Which adds parity to the original
`data to produce channel encoded data. The channel encoder
`26 encodes the data using a scheme that is recogniZed by the
`decoder 24 of the receiver 10 and enables the decoder 24 to
`detect errors in the received data. Errors may arise as a result
`of environmental conditions of the channel 19 or noise
`inadvertently added by the transmitter 8 or receiver 10.
`
`[0036] The encoder 14 further includes a symbol mapper
`28, Which maps the channel-encoded data into data symbols.
`The symbol mapper 28 groups a predetermined number of
`bits such that each group of bits constitutes a speci?c symbol
`chosen from a pre-determined alphabet. The symbol mapper
`28 further lays out a stream of data symbols Within the
`structure of a frame.
`
`[0037] The encoder 14 further includes a space-time pro
`cessor 30 that processes the data symbol stream received
`from the symbol mapper 28 and outputs the processed data
`symbols via the respective TDBs. The space-time processor
`30 encodes the data symbol stream in a manner such that the
`receiver 10 is capable of decoding the signals. The data
`symbols in the TDBs are distributed over Q lines that Will
`eventually be transmitted at precise frequencies spaced apart
`from each other by a predetermined difference in frequency.
`By providing a speci?c frequency difference betWeen the
`multiple sub-channels, orthogonality can be maintained,
`thereby preventing the OFDM demodulators 22 from pick
`ing up frequencies other than their oWn designated fre
`quency.
`
`[0038] Each TDB provides an input to a respective adder
`34. The other input into each of the adders 34 is connected
`to the output of a pilot/training symbol inserter 32, Which
`provides pilot symbols and training symbols to be inserted
`into the frames on the TDBs. Symbols inserted periodically
`Within the data symbols Will be referred to herein as “pilot
`symbols.” These periodic pilot symbols may be inserted
`anyWhere in the stream of the data symbols. If a continuous
`burst of symbols is inserted by the pilot/training symbol
`inserter 32, this type of symbol Will be referred to herein as
`“training symbols” Which constitute the preamble. The train
`ing symbols preferably are inserted at the beginning of the
`frame. HoWever, the training symbols may be inserted onto
`the frame in a location other than at the beginning of the
`frame, such as at the end or in the middle of the frame.
`
`[0039] The pilot/training symbol inserter 32 may be con
`?gured so that it is capable of storing multiple sets of
`training symbols or pilot symbols. In this case, a particular
`set may be selected, for example, based on desirable com
`munication criteria established by a user. The training sym
`bols for each respective sub-channel may preferably be
`unique to the particular sub-channel. In order to accommo
`date amplitude differences betWeen the sub-channels, the
`training symbols may be designed and adjusted to maintain
`a constant amplitude at the output of each sub-channel.
`
`[0040] Training symbols are preferably transmitted once
`for every frame. Training symbols are used for periodic
`calibration (synchroniZation and channel parameter estima
`tion) Whereas pilot symbols are used for minor adjustments
`to deal With the time-varying nature of the channel. The
`training symbols may be indicative of calibration values or
`knoWn data values. These calibration values or knoWn
`
`ERIC-1005 / Page 13 of 23
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`US 2002/0181509 A1
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`Dec. 5, 2002
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`values may be transmitted across the channel, and used to
`calibrate the communication system 6. Any necessary
`re?nements may be made to the communication system 6 if
`the received calibration values do not meet desirable speci
`?cations.
`
`[0041] Furthermore, the training symbols may be used as
`speci?c types of calibration values for calibrating particular
`channel parameters. By initially estimating these channel
`parameters, offsets in the time domain and frequency
`domain may be accounted for so as to calibrate the com
`munication system 6. The training sequence may or may not
`bypass an Inverse Discrete Fourier Transform (IDFT) stage
`38, Which is a part of the embodiment of the OFDM
`modulator 16 of FIG. 3. A training sequence that bypasses
`the IDFT stage 38 and is directly input into a digital to
`analog converter (DAC) 44 is referred to herein as a directly
`modulatable training sequence. Examples of such training
`sequences may be “chirp-like” sequences. These sequences
`cover each portion of the bandWidth used by the commu
`nication system 6. Hence, channel response can be easily
`determined. In general, a chirp sequence in the time domain
`is given by the equation:
`
`[0042] Where j is given by \/——1 and is used to denote the
`quadrature component of the signal. It should be noted that
`the term sn refers to a time domain signal on the side of the
`transmitter 8. Frequency domain signals on the transmitter
`side Will hereinafter be referenced by capital letters Sk. Time
`and frequency domain signals on the receiver side Will
`hereinafter be Written as rn and Rk, respectively. Other
`modi?cations of the chirp-like sequence may be Frank
`Zadoff sequences, Chu sequences, MileWski sequences,
`Suehiro polyphase sequences, and sequences given by Ng et
`al. By observing the response of the receiver 10 to the chirp
`signals, the channel parameters maybe estimated.
`
`[0043] In the case When the IDFT stage 38 is not bypassed,
`a training sequence may be generated by modulating each of
`the symbols on the TDBs With a knoWn sequence of symbols
`in the frequency domain and passing the symbols through
`the IDFT stage 38. Generally, such a knoWn sequence of
`symbols is obtained from an alphabet Which has its con
`stituents on the unit circle in the complex domain and such
`that the resultant sequence in the time domain has a suitable
`Peak to Average PoWer Ratio (PAPR). An alphabet in
`communication systems is de?ned as a ?nite set of complex
`values that each of the symbols can assume. For example, an
`alphabet of a binary phase shift keying (BPSK) system
`consists of values +1 and —1 only. An alphabet for a
`quaternary phase shift keying (QPSK) system consists of the
`values 1+j, —1+j, 1-j, and —1—j. For example, the training
`sequence may be generated by modulating each of the tones
`of the OFDM block using a BPSK alphabet, Which consists
`of symbols +1 and —1. The synchroniZation scheme may be
`very general such that any knoWn sequence having suitable
`properties, such as loW PAPR, may be used to form the
`training sequence.
`
`[0044] With reference again to FIG. 2, the adders 34 add
`the training symbols and pilot symbols to the frame. Other
`embodiments may be used in place of the adders 34 for
`combining the training symbols and pilot symbols With the
`data symbols in the frame. Furthermore, the adders 34 may
`include additional inputs to alloW for ?exibility When adding
`
`the pilot/training symbols or in the combining of multiple
`training symbols or even selectable training symbols. After
`the training symbols are inserted into frames on the respec
`tive TDBs, the frames are output from the encoder 14 and
`input in respective OFDM modulators 16.
`[0045] FIG. 3 shoWs an example embodiment of an
`OFDM modulator 16, Which receives signals along one of
`the TDBs. The number of OFDM modulators 16 is prefer
`ably equal to the number of transmitting antennas 18. In
`SISO systems, there is only one OFDM modulator 16 and
`one transmitting antenna 18. In MIMO systems, there may
`be any number of OFDM modulators 16 and transmitting
`antennas 18.
`[0046] The respective signal from the encoder 14 is input
`into a serial-to-parallel converter 36 of the OFDM modula
`tor 16. The serial-to-parallel converter 36 takes N symbols
`received in a serial format and converts them into a parallel
`format. The variable N Will be referred to herein as the
`blocksiZe of the OFDM symbol. The N parallel symbols are
`processed by an Inverse Discrete Fourier Transform (IDFT)
`stage 38, Which transforms the frequency signals to the time
`domain. The N number of transformed symbols in the time
`domain Will be referred to herein as samples.
`[0047] A method is proposed herein to design the training
`symbols such that the transforms of all the sequences from
`the IDFT stage 38 Will have a constant magnitude. By
`maintaining a constant magnitude at the output of each of the
`IDFT stages 38 Within their respective modulators, one of
`the main problems of OFDM, i.e., peak to average poWer
`ratio (PAPR), is solved. The receiver 10 can thus more
`accurately estimate the channel parameters, Which are used
`by the receiver 10 to synchroniZe the received signals in the
`time and frequency domains, as Will be described beloW in
`more detail.
`[0048] The output from the IDFT stage 38 is input into a
`cyclic pre?x inserter 40, Which inserts an additional number
`of samples for every N samples. The number of samples
`inserted by the cyclic pre?x inserter 40 Will be referred to
`herein by the variable “G.” The G samples are intended to
`be inserted as guard intervals to separate the N adjacent data
`symbols from each other in time by a separation adequate to
`substantially eliminate Inter Symbol Interference (ISI). The
`cyclic pre?x inserter 40 repeats G samples from a latter
`portion of the N samples output from the IDFT stage 38 and
`inserts the G samples as a pre?x to each of the data samples.
`Preferably, the time length of the cyclic pre?x is greater than
`the maximum time delay of a transmitted signal across the
`channel 19. Since the nature of the channel 19 may be
`susceptible to a variation in the delay time from the trans
`mitted antennas 18 to the receiving antennas 20, it may be
`desirable to increase, or even double, the length of cyclic
`pre?xes of the preamble to ensure that the time delay of the
`channel does not exceed the time of the cyclic pre?x,
`thereby eliminating ISI.
`[0049] The G+N samples, herein referred to as an OFDM
`symbol, are then converted from a parallel format to a serial
`format using parallel-to-serial converter 42, and then input
`ted to a digital-to-analog converter (DAC) 44 for conversion
`into analog signals. The output from the DAC 44 is input
`into a mixer 48. A local oscillator 46 provides a signal
`having the carrier frequency to the other input of the mixer
`48 to up-convert the respective OFDM symbol from base
`band to RF.
`
`ERIC-1005 / Page 14 of 23
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`US 2002/0181509 A1
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`Dec. 5, 2002
`
`[0050] After the respective frame has been mixed With a
`carrier frequency that is set by the respective local oscillator
`46, the frame is ampli?ed by an ampli?er 50. As indicated
`above, one of the draWbacks to any OFDM signal is that it
`generally has a high PAPR. To accommodate this drawback,
`the ampli?er 50 may be backed off to prevent it from going
`into its non-linear region. HoWever, the present invention
`may provide certain speci?c sequences that can be used in
`order to make the PAPR minimal or unity.
`
`[0051] Each OFDM modulator 16 preferably comprises
`the same components as the OFDM modulator 16 shoWn in
`FIG. 3. Other techniques for designing the OFDM modu