

Apparatus and methods for providing efficient spacetime structures for preambles, pilots and data for multiinput, multioutput communications systems 
RE44867 
Apparatus and methods for providing efficient spacetime structures for preambles, pilots and data for multiinput, multioutput communications systems


Patent Drawings:  

Inventor: 
Stuber, et al. 
Date Issued: 
April 29, 2014 
Application: 

Filed: 

Inventors: 

Assignee: 

Primary Examiner: 
Bocure; Tesfaldet 
Assistant Examiner: 

Attorney Or Agent: 

U.S. Class: 
375/260; 375/267; 375/299; 375/347; 455/101; 455/132 
Field Of Search: 
;375/267; ;375/295; ;375/299; ;455/91; ;455/101; ;370/203; ;370/208; ;370/209; ;370/210 
International Class: 
H04L 27/08; H04B 7/08; H04L 1/06; H04L 27/36; H04B 7/02; H04J 11/00; H04B 7/06 
U.S Patent Documents: 

Foreign Patent Documents: 

Other References: 
AN. Mody and G.L. Stuber, "Efficient Training and Synchronization Sequence Structures for MIMO ODFM," 6th International OFDMWorkshop,Hamburg, Germany, Sep. 2001, 6 pages. cited by applicant. A.N. Mody and G.L. Stuber, "Parameter Estimation for OFDM with TransmitReceive Diversity," Proceedings of VTC Rhodes, Greece, 2001, 5 pages. cited by applicant. A.N. Mody and G.L. Stuber, "Receiver Implementation for a MIMO OFDM System," Proceedings of GLOBECOM 2002, Taipei, Taiwan, Nov. 2002, 5 pages. cited by applicant. A.N. Mody and G.L. Stuber, "Synchronization of MIMO OFDM Systems," Proceedings of GLOBECOM 2001, San Antonio, 2001, vol. 1, pp. 509513. cited by applicant. Biedka, T.E., "A comparison of initialization schemes for blind adaptive beamforming"; Acoustics, Speech, and Signal Processing, 1998. ICASSP '98. Proceedings of the 1998 IEEE International Conference on; vol. 3, May 1215, 1998 pp. 16651668 vol.3. cited by applicant. Eric W. Weisstein. "GramSchmidt Orthonormalization." From MathWorldA Wolfram Web Resource. http://mathworld.wolfram.com/GramSchmidtOrthonormalization.html; 19992006, 4 pages. cited by applicant. Jafarkhani, H., "A quasiorthogonal spacetime block code"; Communications, IEEE Transactions on vol. 49, Issue 1, Jan. 2001 pp. 14. cited by applicant. JianKang Zhang et al., "TraceOrthonormal FullDiversity Cyclotomic SpaceTime Codes," IEEE transaction on signal processing, vol. 55 No. 2, pp. 618630, Feb. 2007. cited by applicant. Naguib, A.F. et al., "Spacetime coded modulation for high data rate wireless communications"; Global Telecommunications Conference, 1997. GLOBECOM '97., IEEE vol. 1, Nov. 38, 1997 pp. 102109. cited by applicant. NonFinal Office Action on U.S. Appl. No. 12/555,332, mailed Feb. 4, 2011. cited by applicant. NonFinal Office Action on U.S. Appl. No. 12/555,332, mailed Jul. 27, 2011. cited by applicant. Notice of Allowance on U.S. Appl. No. 12/555,332, mailed Dec. 5, 2011. cited by applicant. Siavash M. Alamouti, "A Simple Transmit Delivery Technique for Wireless Communications," IEEE Journal on Select Areas in Communications, vol. 16, No. 8, Oct. 1998, pp. 14511458. cited by applicant. Timothy M. Schmidl and Donald C. Cox, "Robust Frequency and Timing Sychronization for OFDM," IEEE Transactions on Communications, vol. 45, No. 12, Dec. 1997, pp. 16131621. cited by applicant. Vahid Tarokh, Hamid Jafarkhani, and A. Robert Calderbank, "SpaceTime Block Coding for Wireless Communications: Performance Results," IEEE Journal on Selected Areas in Communications, vol. 17, No. 3, Mar. 1999, pp. 451460. cited by applicant. Vahid Tarokh, Hamid Jafarkhani, and A.R. Calderbank, "SpaceTime Block Codes from Orthogonal Designs," IEEE Transactions on Information Theory, vol. 45, No. 5, Jul. 1999, pp. 14561467. cited by applicant. Ye (Geoffrey) Li, Nambirajan Seshadri, and Sirikiat Ariyavisitakul, "Channel Estimation for OFDM Systems with Transmitter Diversity in Mobile Wireless Channels," IEEE Journal on Selected Areas in Communications, vol. 17, No. 3, Mar. 1999, pp.461471. cited by applicant. Eric W. Weisstein. "GramSchmidt Orthonormalization." From MathWorldA Wolfram Web Resource. http://mathworld.wolfram.com/GramSchmidtOrthonormalization.html; .COPYRGT. 19992006. cited by examiner. A quasiorthogonal spacetime block code Jefarkhani, H.; Communications, IEEE Transactions on vol. 49, Issue 1, Jan. 2001 pp. 14. cited by examiner. A comparison of initialization schemes for blind adaptive beamforming Biedka, T.E.; Acoustics, Speech, and Signal Processing, 1998. ICASSP '98. Proceedings of the 1998 IEEE International Conference on; vol. 3, May 1215, 1998 pp. 16651668 vol. 3.cited by examiner. Spacetime coded modulation for high data rate wireless communications Naguib, A.F.; Tarokh, V.; Seshadri, N.; Calderbank, A.R.; Global Telecommunications Conference, 1997. GLOBECOM '97., IEEE vol. 1, Nov. 38, 1997 pp. 102109 vol. 1. cited byexaminer. Vahid Tarokh, Hamid Jafarkhani, and A. Robert Calderbank, "SpaceTime Block Coding for Wireless Communications: Performance Results," IEEE Journal of Selected Areas in Communications, vol. 17, No. 3, Mar. 1999, pp. 451460. cited byapplicant. 

Abstract: 
Apparatus and methods for providing efficient spacetime structures for preambles, pilots and data for multiinput, multioutput (MIMO) communications systems are provided. One such embodiment includes providing a computer program that includes logic configured to provide an initial structure. The computer program further includes logic configured to verify that the rows of the initial structure are linearly independent and logic configured to apply an orthonormalization procedure to the initial structure to obtain a spacetime structure. Methods are also provided for providing efficient spacetime structures for preambles, pilots and data for MIMO communications systems. 
Claim: 
We claim:
.[.1. A computer program embodied in a computer readable medium for providing efficient spacetime structures for preambles, pilots and data for multiinput, multioutputcommunications systems, the computer program comprising: logic configured to provide an initial structure; logic configured to verify that rows of said initial structure are linearly independent; logic configured to apply an orthonormalizationprocedure to said initial structure to obtain a spacetime structure for a preamble or pilot in a time or frequency domain; and logic configured to insert the spacetime structure as a preamble or pilot in the time or frequency domain with one or moredata symbols for transmission in the multiinput, multioutput communications system..].
.[.2. The computer program of claim 1, wherein said logic configured to provide an initial structure comprises: logic configured to choose a symbol alphabet to provide symbols for said initial structure; and logic configured to choose aninitial configuration of said initial structure..].
.[.3. The computer program of claim 1, further comprising: logic configured to confirm that symbols of said spacetime structure are within a predetermined distance of symbols of said initial structure; logic configured to construct aspacetime sequence structure from a plurality of said spacetime structures; and logic configured to verify that a peaktoaverage power ratio of said spacetime structure is less than a predetermined value..].
.[.4. The computer program of claim 3, wherein said logic configured to confirm chat the symbols of said spacetime structure are within a predetermined distance of the symbols of said initial structure comprises logic configured to apply aEuclidean distance metric to determine the distance between the symbols of said spacetime structure and the symbols of said initial structure..].
.[.5. The computer program of claim 1, wherein said logic configured to verify that the rows of said initial structure are linearly independent comprises logic configured to determine rank of said initial structure..].
.[.6. The computer program of claim 1, wherein said logic configured to apply an orthonormalization procedure to said initial structure to obtain a spacetime structure comprises logic configured to apply a rowwise GramSchmidt procedure tosaid initial structure to obtain a spacetime structure..].
.Iadd.7. A nontransitory computer readable medium having instructions stored thereon, the instructions comprising: instructions to produce a plurality of pilot symbols derived from at least one sequence, wherein the at least one sequence is aFrankZadoff or Chu sequence; instructions to produce a plurality of data symbols; instructions to provide a space time structure for transmission, wherein the space time structure includes a time dimension including a plurality of symbol timeintervals and a space dimension including a plurality of antenna outputs; and instructions to insert the plurality of pilot symbols and the plurality of data symbols into the space time structure for transmission in a multiinput, multioutputcommunications system, wherein the plurality of pilot symbols are inserted in the time and frequency domain for each antenna of the plurality of antenna outputs..Iaddend.
.Iadd.8. The computer readable medium of claim 7, wherein the multiinput, multioutput communications system is an orthogonal frequency division multiple access or single carrier frequency domain equalization communication system..Iaddend.
.Iadd.9. A method comprising: producing a plurality of pilot symbols derived from at least one sequence, wherein the at least one sequence is a FrankZadoff or Chu sequence; producing a plurality of data symbols; providing a space timestructure for transmission, wherein the space time structure includes a time dimension including a plurality of symbol time intervals and a space dimension including a plurality of antenna outputs; and inserting the plurality of pilot symbols and theplurality of data symbols into the space time structure for transmission in a multiinput, multioutput communications system, wherein the plurality of pilot symbols are inserted in the time and frequency domain for each antenna of the plurality ofantenna outputs..Iaddend.
.Iadd.10. The method of claim 9, wherein the multiinput, multioutput communications system is an orthogonal frequency division multiple access or single carrier frequency domain equalization communication system..Iaddend.
.Iadd.11. A multiinput, multioutput communications system comprising: an encoder configured to: produce a plurality of pilot symbols derived from at least one sequence, wherein the at least one sequence is a FrankZadoff or Chu sequence; receive a plurality of data symbols; provide a space time structure for transmission, wherein the space time structure includes a time dimension including a plurality of symbol time intervals and a space dimension including a plurality of antennaoutputs; and insert the plurality of pilot symbols and the plurality of data symbols into the space time structure, wherein the plurality of pilot symbols are inserted in the time and frequency domain for each antenna of the plurality of antennas; anda plurality of modulators configured to receive an output of the encoder; and a plurality of antennas configured to receive outputs of the plurality of modulators..Iaddend.
.Iadd.12. The system of claim 11, wherein the system is an orthogonal frequency division multiple access or single carrier frequency domain equalization communication system..Iaddend.
.Iadd.13. A multiinput, multioutput communications system comprising: a plurality of antennas; a plurality of demodulators configured to receive outputs from the plurality of antennas; and a decoder configured to receive the output of theplurality of demodulators and further configured to: recover data from a received space time structure, wherein the space time structure includes a time dimension including a plurality of symbol time intervals and a space dimension associated with theplurality of antennas; wherein a plurality of pilot symbols and a plurality of data symbols are inserted into the space time structure; wherein the pilot symbols are inserted in the time and frequency domain for each antenna of the plurality ofantennas; wherein the pilot symbols are derived from at least one sequence; and wherein the at least one sequence is a FrankZadoff or Chu sequence..Iaddend.
.Iadd.14. The system of claim 13, wherein the system is an orthogonal frequency division multiple access or single carrier frequency domain equalization communication system..Iaddend. 
Description: 
TECHNICAL FIELD OF THE INVENTION
The present invention is generally related to communications systems and, more particularly, to MultiInput, MultiOutput (MIMO) communications systems.
BACKGROUND OF THE INVENTION
Significant developments in communications have been made by the introduction of technologies that increase system operating efficiency (i.e., system "throughput"). One example of these technologies is the use of two or more transmit antennasand two or more receive antennas (i.e., multiple antennas) in a wireless communications system that employs multiple frequencies (i.e., multiple carriers). Such systems are typically referred to as MultiInput, MultiOutput (MIMO) communicationssystems. In contrast, traditional wireless communications systems typically employ one transmit antenna and one receive antenna operating at a single signalcarrier frequency (SC), and such systems are referred to accordingly as SingleInput,SingleOutput (SISO) systems.
In the operation of MIMO communications systems, signals are typically transmitted over a common path (i.e., a channel) by multiple antennas. The signals are typically preprocessed to avoid interference from other signals in the commonchannel. There are several techniques that may be used to preprocess the signals in this regard, and some of these techniques may be combined to further improve system throughput. One such technique, known as SpaceTime Processing (STP), processes andcombines "preambles" and "data symbols" into "spacetime signal structures." Wireless communications systems typically transmit data or information (e.g., voice, video, audio, text, etc.) as formatted signals, known as data symbols (or informationsymbols), which are typically organized into groups, known as data frames (or information frames).
Training symbols (or preamble symbols) are another type of symbol, which are typically added as prefixes to data symbols (e.g., at the beginning of data frames), to enable training (i.e., synchronization) of the data symbols between thetransmitters and receivers of a MIMO communications system. These training symbol prefixes can be referred to as preambles or preamble structures. The combination of the preambles and data symbols can be referred to as spacetime signal structures. Spacetime structures may also be constructed using STP for preambles and data symbols individually. Furthermore, pilot structures (or pilots) are spacetime structures that are also constructed by STP and have the same structure as preambles, althoughthey are periodically arranged within groups of data symbols for different purposes. Certain properties incorporated into spacetime signal structures make it possible to recover the data symbols from them through postprocessing by a receiver, forexample. Moreover, the formation and processing of spacetime signal structures in a wireless communications system may provide increased strength (i.e., gain) in the recovered signal, which typically enhances the performance of the communicationssystem.
Another technique that may be used to preprocess signals in MIMO communications systems is called Frequency Division Multiplexing (FDM). FDM involves dividing the frequency spectrum of a wireless communications system into subchannels andtransmitting modulated data or information (i.e., formatted signals for voice, video, audio, text, etc.) over these subchannels at multiple signalcarrier frequencies ("subcarrier frequencies"). Orthogonal Frequency Division Multiplexing (OFDM) hasemerged as a popular form of FDM in which the subcarrier frequencies are spaced apart by precise frequency differences. The application of OFDM technologies in SISO communications systems (i.e., SISO OFDM systems) provides the capability, among others,to transmit and receive relatively large amounts of information. The application of OFDM in MIMO communications systems (i.e., MIMO OFDM systems) provides the added capability of increased capacity to transmit and receive information using, generally,the same amount of bandwidth (i.e., transmission line capacity) as used in SISO OFDM systems. MIMO OFDM communications systems also offer improved performance to overcome some of the difficulties experienced in other FDM communications systems, such asperformance degradation due to multiple versions of a transmitted signal being received over various transmission paths (i.e., multipath channel interference).
In wireless communications systems (e.g., SISO or MIMO), synchronization of data symbols is typically required in both time and frequency. Estimation of noise variance and channel parameters is also typically required. Thus, efficient preamblestructures and pilot structures for use in wireless communications systems should provide both synchronization and parameter estimation. Furthermore, efficient preamble structures and pilot structures should possess a low peaktoaverage power ratio(PAPR) (i.e., at or approaching unity) to facilitate efficient system operation. In their application to MIMO communications systems, however, existing preamble structures and pilot structures have shortcomings in their capability to provide theforegoing functions of time and frequency synchronization, estimation of noise variance and channel parameters, and low PAPR. For example, the IEEE Standard 802.11a preamble structure includes a short sequence, which provides time synchronization andcoarse frequency offset estimation, followed by a long sequence, which provides fine frequency and channel estimation. Although this preamble has direct application to SISO communications systems, it is not directly applicable to MIMO communicationssystems to provide the above mentioned functions, without the need for significant modifications.
Existing techniques for spacetime processing of preamble symbols, pilot symbols, and data symbols into spacetime structures also have shortcomings in their applications to MIMO communications systems. For example, existing spacetimestructures (i.e., preamble, pilot, or data) are typically limited to applications in MIMO communications systems that employ two, four, or eight transmit antennas. However, MIMO communications systems may be required that employ other numbers oftransmit antennas to satisfy various applications. As another example, existing spacetime structures do not support the "full diversity" performance of MIMO communications systems. That is, existing spacetime structures do not support the optimalsignal transmission performance that MIMO communications systems can provide. For example, a MIMO communications system that employs four transmit antennas can provide a full diversity signal transmission performance of four spacetime structures overfour time periods. However, typical existing spacetime structures are limited to support a signal transmission performance of no better than three spacetime structures over four time periods in a fourantenna MIMO system.
Therefore, there is a need for apparatus and methods for providing efficient preamble structures and pilot structures that provide time and frequency synchronization, estimation of noise variance and channel parameters, and low PAPR in theirapplication to MIMO communications systems. Moreover, there is a need for an apparatus and methods for providing spacetime structures (i.e., preamble, pilot, or data) that can be applied to MIMO communications systems with any number of transmit andreceive antennas and that facilitate full diversity performance of MIMO communications systems.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and methods for providing efficient spacetime structures for preambles, pilots and data for multiinput, multioutput (MIMO) communications systems.
Briefly described, one embodiment of the present invention, among others, includes providing a computer program that includes logic configured to provide an initial structure. The computer program further includes logic configured to verifythat the rows of the initial structure are linearly independent and logic configured to apply an orthonormalization procedure to the initial structure to obtain a spacetime structure.
The present invention can also be viewed as providing methods for providing efficient spacetime structures for preambles, pilots and data for MIMO communications systems. In this regard, one embodiment of such a method, among others, can bebroadly summarized by the following: providing an initial structure, verifying that the rows of the initial structure are linearly independent, and applying an orthonormalization procedure to the initial structure to obtain a spacetime structure.
Another embodiment of a method of the present invention can be broadly described by the following: selecting a data structure, verifying that the data structure is a unitary transmission matrix, and applying the data structure as a spacetimepreamble structure.
Other apparatus, methods, features and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additionalapparatus, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Many aspects of the invention can be better understood with reference to the following drawings. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
FIG. 1 is a block diagram of an exemplary MultiInput, MultiOutput (MIMO) communications system.
FIG. 2 is a block diagram of an exemplary encoder with respect to the communications system depicted in FIG. 1.
FIG. 3 is a diagram illustrating exemplary signal transmissions and associated signal sample matrices with respect to the communications system depicted in FIG. 1.
FIG. 4 is a graphical illustration of a version of the receive sample matrix shown in FIG. 3 that is applicable to the MIMO communications system of FIG. 1 when employing Orthogonal Frequency Division Multiplexing (OFDM).
FIG. 5 illustrates an exemplary frame that may be implemented in the MIMO communications system depicted in FIG. 1.
FIG. 6 is a flow chart illustrating a method for providing efficient spacetime structures for preambles, pilots and data that may be implemented in the MIMO communications system depicted in FIG. 1.
FIG. 7 is a flow chart illustrating an exemplary method to determine an initial structure for use in the method described with respect to FIG. 6.
FIG. 8 is a flow chart illustrating an alternative method for providing efficient spacetime structures for preambles, pilots and data that may be implemented in a MIMO communications system, such as the system depicted in FIG. 1.
DETAILED DESCRIPTION
The invention now will be described more fully with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather,these embodiments are intended to convey the scope of the invention to those skilled in the art. Furthermore, all "examples" given herein are intended to be nonlimiting.
FIG. 1 shows a block diagram of an exemplary MultiInput, MultiOutput (MIMO) communications system 6. The exemplary MIMO communications system 6 and its subcomponents will be described hereinafter to facilitate the description of the presentinvention. In that regard, the exemplary MIMO communications system 6 may be implemented as a wireless system for the transmission and reception of data across a wireless channel 19, as depicted in FIG. 1. For example, the MIMO communications system 6may be implemented as part of a wireless Local Area Network (LAN) or Metropolitan Area Network (MAN) system, a cellular telephone system, or another type of radio or microwave frequency system incorporating oneway or twoway communications over a rangeof distances.
The MIMO communications system 6 may transmit and receive signals at various frequencies. For example, the MIMO communications system 6 may transmit and receive signals in a frequency range from 2 to 11 GHz, such as in the unlicensed 5.8 GHzband, using a bandwidth of about 3 to 6 MHz. Further, the MIMO communications system 6 may employ various signal modulation and demodulation techniques, such as SingleCarrier Frequency Domain Equalization (SCFDE) or Orthogonal Frequency DivisionMultiplexing (OFDM), for example. However, throughout this description, references may be made with respect to a MIMO OFDM communications systems merely to facilitate the description of the invention.
The MIMO communications system 6 may also be implemented as part of a communications system (not shown) that includes an array of subchannel communications links, which convey one or more signals transmitted by one or more transmitting elementsto one or more receiving elements. The subchannel communications links may include wires (e.g., in a wiring harness) or other forms of tangible transmission medium that span between a data source and a receiver within the communications system.
The MIMO communications system 6 includes a transmitter 8 and a receiver 10. The transmitter 8 typically transmits signals across a channel 19 to the receiver 10. As depicted in FIG. 1, the transmitter 8 typically includes several components. In this regard, the transmitter 8 includes an encoder 14. The encoder 14 typically encodes data and/or other types of signals received, for example, from a data source 12. Such signals may alternatively be referred to collectively hereinafter as"data," "signals," or "data signals." The data source 12 may be a device, system, etc. that outputs such signals. The encoder 14 may also perform functions such as employing a channel code on data for transmission and forming sequence structures byspacetime processing (STP) techniques. Further, the encoder 14 may separate the received signals onto one or more signal paths 15, included in the transmitter 8, which will be referred to hereinafter as transmit diversity branches (TDBs) 15. Each TDB15 may correlate to a different subchannel within the MIMO communications system 6. The encoder 14 typically facilitates the transmission of signals across the channel 19 by bundling the signals into groups, which are typically referred to as a"frame." Details of a frame, with respect to the present invention, will be discussed further below.
The transmitter 8 also includes one or more modulators 16 that are configured to modulate signals for transmission over the channel 19. In this regard, the modulators 16 may employ various modulation techniques, such as SCFDE or OFDM. Themodulators 16 are typically connected to the encoder 14 by the TDBs 15. The transmitter also includes one or more transmit antennas 18 connected respectively to the one or more modulators 16. Thus, each TDB 15 directs signals from the encoder 14 to oneor more modulators 16, and the modulators 16 modulate the signals for transmission by a respective transmit antenna 18.
As discussed above, the exemplary MIMO communications system 6, shown in FIG. 1, also includes a receiver 10. The receiver 10 also typically includes several components. The receiver includes one or more receive antennas 20 that are connectedto one or more demodulators 22, respectively. The receive antennas 20 typically receive modulated signals that are transmitted across the channel 19 from the transmit antennas 18. The received signals are typically directed to the demodulators 22 fromthe respective receive antennas 20. The demodulators 22 demodulate signals that are received by the respective receive antennas 20.
The receiver 10 also includes a decoder 24, which is connected to the demodulators 22. The decoder 24 typically combines and decodes demodulated signals from the demodulators 22. In this regard, the decoder 24 typically recovers the originalsignals that were input to the transmitter 8 from the data source 12 and transmitted across the channel 19. As depicted in FIG. 1, the original signals recovered by the decoder 24 may be transmitted to a connected data sink 25, which may include one ormore devices configured to utilize or process the original signals.
As discussed above, the transmitter 8 of the MIMO communications system 6 includes one or more modulators 16 that are connected to one or more transmit antennas 18, respectively. Further, the receiver 10 of the communications system 6 includesone or more demodulators 22 that are connected to one or more receive antennas 20, respectively. In this regard, the number of modulators 16 and respective transmit antennas 18 that are implemented in the transmitter 8 may be represented by a firstvariable, "Q." Similarly, the number of demodulators 22 and respective receive antennas 20 that are implemented in the receiver 10 may be represented by a second variable, "L." In the exemplary MIMO communications system 6, the number (Q) of modulators16 and respective transmit antennas 18 may be equivalent or nonequivalent to the number (L) of demodulators 22 and respective receive antennas 20. In this regard, the communications system 6 may be said to have "Q.times.L" transmitreceive diversity.
FIG. 2 is a block diagram of an exemplary encoder 14 with respect to the communications system 6 depicted in FIG. 1. The elements of the encoder 14 shown in FIG. 2 will be described below with respect to several elements that were describedabove for FIG. 1. The exemplary encoder 14 includes a channel encoder 26. The channel encoder 26 typically converts data and/or other types of signals to channel encoded versions of the signals, which may also be referred to collectively as "channelencoded data" or "channel encoded signals." These signals may be received by the channel encoder 26 from a data source 12, for example. The channel encoder 26 is typically configured to encode signals using an encoding scheme that can be recognized bythe decoder 24 of the receiver 10 that is intended to receive the channel encoded signals. In the process of encoding signals, the channel encoder 26 also typically adds parity to the signals so that the receiving decoder 24 can detect errors in thereceived channel encoded signals, which may occur, for example, due to environmental conditions of the channel 19 or inadvertent noise injection by the transmitter 8 or receiver 10, for example.
The exemplary encoder 14 depicted in FIG. 2 also includes a symbol mapper 28, which receives channel encoded data from the channel encoder 26. The symbol mapper 28 is typically configured to map channel encoded data into data symbols. Thesymbol mapper 28 typically maps channel encoded data into data symbols by grouping a predetermined number of bits of the data so that each group of bits constitutes a specific symbol that is selected from a predetermined symbol alphabet. In thisregard, a symbol alphabet typically includes a finite set of values. For example, a symbol alphabet of a binary phase shift keying (BPSK) system typically consists of the values +1 and 1, and a symbol alphabet for a quadrature phase shift keying (QPSK)system typically consists of the values 1+j, 1+j, 1j, and 1j. The symbol mapper 28 is also typically configured to structure a stream of data symbols into a data section, which will be discussed further below.
The exemplary encoder 14 also includes a spacetime processor 30. The spacetime processor 30 is typically configured to encode data symbol streams (i.e., data sections), received from the symbol mapper 28, through spacetime processing to formspacetime structures with properties that enhance the performance of the communication systems 6. The encoded data sections are output from the spacetime processor 30 over Q lines 13, where Q represents the number of modulators 16 and respectivetransmit antennas 18 of the transmitter 8, as discussed above.
As illustrated in FIG. 2, the Q output lines from the spacetime processor 30 input respectively to Q adders 34. The encoder 14 also includes a pilot/training symbol inserter 32, which also has Q output lines 17 that input respectively to the Qadders 34. As depicted in FIG. 2, the Q adders 34 output to Q transmit diversity branches (TDBs) 15, which input respectively to Q modulators 16. The pilot/training symbol inserter 32 typically provides pilot symbols and training symbols that areinserted into (or combined with) data sections by the adders 34, which then output the modified data sections as spacetime structures over the TDBs 15.
The term pilot symbols, as used in this description, refers to symbols provided by the pilot/training symbol inserter 32, which are inserted periodically into data sections. Typically, pilot symbols may be inserted at any point in a datasection. The term training symbols, as also used in this description, refers to one or more continuous sections of training symbols provided by the pilot/training symbol inserter 32, which are inserted into data sections. Training symbols arepreferably inserted into data sections at the beginning of the section and transmitted once per frame. However, training symbols may also be inserted at other parts of data sections, such as the middle or end of the sections. Preambles (or preamblestructures) are symbol structures formed of training symbols. Pilot structures (or pilots) are symbol structures formed of pilot symbols.
Pilot symbols are typically transmitted with data sections to perform minor adjustments to the calibration (i.e., synchronization and channel parameter estimation) of the receiver 10 to the transmitter 8 to accommodate, for example, the timevarying nature of the channel 19. Training symbols, however, are typically used for periodic calibration of the receiver 10 to the transmitter 8. The training symbols that are transmitted for each subchannel may be unique. Moreover, different sets oftraining symbols and/or pilot symbols may be provided by the pilot/training symbol inserter 32, depending on the operating criteria of the communications system 6, which may be determined, for example, by the user. However, although pilot symbols andtraining symbols have different purposes, the structure of preambles and pilot structures are the same. Therefore, all descriptions made hereinafter, in accordance with the present invention, with respect to preambles or preambles structures also applyto pilots or pilot structures unless otherwise specified).
FIG. 3 is a diagram that illustrates exemplary signal transmissions and associated signal sample matrices with respect to the modulator/demodulator configuration of the communications system 6 depicted in FIG. 1. As shown in FIG. 3, theconfiguration includes one or more modulators 16 and one or more demodulators 22. As discussed above with respect to FIG. 1, the modulators 16 and the demodulators 22 may be configured to modulate and demodulate signals, respectively, by varioustechniques, such as SCFDE or OFDM.
Each modulator 16 is connected to one or more respective transmit antennas 18, and each demodulator 22 is connected to one or more respective receive antennas 20. As discussed above with respect to FIG. 1, the transmit antennas 18 are typicallyconfigured to transmit modulated signals across a channel 19, and the receive antennas 20 are typically configured to receive modulated signals via the channel 19. In this regard, exemplary signal transmissions are depicted in FIG. 3, which will bediscussed further below.
Similar to the above discussion with respect to the MIMO communications system 6 of FIG. 1, in the modulator/demodulator configuration of FIG. 3, the number of modulators 16 and respective transmit antennas 18 that are implemented may berepresented by the variable, "Q." Accordingly, the number of demodulators 22 and respective receive antennas 20 in the arrangement of FIG. 3 may be represented by the variable, "L." Thus the modulator/demodulator arrangement depicted in FIG. 3 may alsobe described as having "Q.times.L" transmitreceive diversity. Moreover, the variables, Q and L, may be equivalent or nonequivalent in various configurations of the modulators 16 and demodulators 22.
Exemplary signal transmissions from the Q transmit antennas 18 to the L receive antennas 20, across the channel 19, are also depicted in FIG. 3. For example, a first of the L receive antennas 20 may receive each of the Q transmitted signalsfrom the Q transmit antennas 18. These Q transmitted signals are typically transmitted over channel impulse responses h.sub.11, h.sub.21, h.sub.31, . . . , h.sub.Q1, that are transmitted from the 1.sup.st to the Q.sup.th transmit antennas 18,respectively, as depicted in FIG. 3. In this regard, the term h.sub.ij (where i=1, 2, 3, . . . , Q and j=1, 2, 3, . . . , L) is used to refer to the channel impulse response, in the time domain, that is transmitted from the i.sup.th transmit antenna18 to the j.sup.th receive antenna 20. Thus, as a further example, the L.sup.th receive antenna 20 may receive each of the Q transmitted signals, over the channel impulse responses h.sub.1L, h.sub.2L, h.sub.3L, . . . , h.sub.QL, from the 1.sup.st tothe Q.sup.th transmit antennas 18, respectively, as depicted in FIG. 3. Although, for simplicity, exemplary signal transmissions are depicted in FIG. 3 from the Q transmit antennas 18 to only the 1.sup.st and the L.sup.th receive antennas 20, in atypical MIMO communications system, signals transmissions may occur from any of the Q transmit antennas 18 to any of the L receive antennas 20.
A transmit sample matrix S is illustrated in FIG. 3. The matrix S is associated with the signals that are modulated by the Q modulators 16 and transmitted over the channel 19 from the Q transmit antennas 18. In this regard, the sample matrix Smay be associated with signals that are transmitted by a MIMO communications system. Thus, the elements of the transmit sample matrix S may represent Q spacetime symbols (i.e., preamble or data), which are simultaneously transmitted from the Q transmitantennas 18 during Q or more symbol periods ("T.sub.S"). For example, the elements of the first row of the transmit sample matrix S may represent the symbols S.sub.1, S.sub.2, . . . , S.sub.Q, which are transmitted from the 1.sup.st through theQ.sup.th transmit antennas 18, respectively, at a first time ("t"). Similarly, the elements of the second row of the transmit sample matrix S may represent the symbols S.sub.Q+1, S.sub.Q+2, . . . , S.sub.2Q, which are transmitted from the 1.sup.stthrough the Q.sup.th transmit antennas 18, respectively, at a second time ("t+T.sub.S"). The elements of the last row of the transmit sample matrix S may represent the final set of symbols, S.sub.(Q1)Q+1, S.sub.(Q1)Q+2, . . , S.sub.QQ, which aretransmitted from the 1.sup.st through the Q.sup.th transmit antennas 18, respectively, at a final time ("t+(Q1)T.sub.S").
Also illustrated in FIG. 3 is a receive sample matrix R, which is associated with the signals that are received over the channel 19 by the L receive antennas 20 and demodulated by the L demodulators 22. Similar to the elements of the transmitsample matrix S, described above, the elements of the receive sample matrix R may represent L spacetime symbols, which are simultaneously received by the L receive antennas 20 during Q or more symbol periods ("T.sub.S"). For example, the elements ofthe first row of the receive sample matrix R may represent the symbols R.sub.1, R.sub.Q+1, . . . , R.sub.(L1)Q+1, which are demodulated by the 1.sup.st through the L.sup.th demodulators 22, respectively, at a first time ("t"). Similarly, the elementsof the second row of the receive sample matrix R may represent the symbols R.sub.2, R.sub.Q+2 . . . , R.sub.(L1)Q+2, which are demodulated by the 1.sup.st through the L.sup.th demodulators 22, respectively, at a second time ("t+T.sub.S"). The elementsof the last row of the receive sample matrix R may represent the final set of symbols, R.sub.Q, R.sub.2Q, . . . , R.sub.QL, which are demodulated by the 1.sup.st through the L.sup.th demodulators 22, respectively, at a final time ("t+(Q1)T.sub.S"). Itis noted that although references are made to the same time instances (e.g., t, t+T.sub.S, etc.) in the foregoing descriptions, as well as in FIG. 3, with respect to the transmit sample matrix S and the receive sample matrix R, there is typically a timedelay between the transmission and reception of the signals represented by these matrices.
In addition to the transmit sample matrix S and the receive sample matrix R, there are at least two other matrices that are relevant to represent the transmission and reception of signals in a MIMO communications system, such as the systemdepicted in FIGS. 1 and 3. The channel matrix .eta. typically includes elements that represent channel coefficients, which are determined based on characteristics of the channel 19. The channel matrix .eta. typically has a dimension of Q.times.L. The noise matrix W typically includes elements that represent additive white Gaussian noise, which typically causes distortion and corruption of received signals that are represented, for example, by the receive sample matrix R. The noise matrix Wtypically has a dimension of Q.times.L.
The relationship between the receive sample matrix R, the transmit sample matrix S, the channel matrix .eta., and the noise matrix W can be expressed by the following equation:R.sub.k,T.times.L=S.sub.k,T.times.Q.eta..sub.k,Q.times.L+W.sub.k,T.times. L EQ. 1 With respect to EQ. 1, k represents the subcarrier or subchannel of received demodulated signals and T represents a dimension variable that is typically equivalent toQ, although it may have other values. As discussed above, Q and L represent, respectively, the number of modulators 16 and respective transmit antennas 18 and the number of demodulators 22 and respective receive antennas 20 with respect to a typicalMIMO communications system 6.
FIG. 4 is a graphic illustration of a version of the receive sample matrix R' shown in FIG. 3 that is applicable to the MIMO communications system of FIG. 1, when employing Orthogonal Frequency Division Multiplexing (OFDM). As shown, the x axisrepresents space, the y axis represents time, and the z axis represents frequency. Each receive sample matrix R.sub.k that is depicted in the spacetime dimensions is similar to the receive sample matrix R discussed above with respect to FIG. 3. However, each element of the receive sample matrix R' illustrated in FIG. 4 also has N frequency components that are each represented by an index, "k". As k varies from 0 to N1 for the elements of each receive sample matrix R.sub.k in FIG. 4, thefrequency component of the received symbol varies accordingly. Thus, the threedimensional receive sample matrix R' can be viewed as including N receive sample matrices R.sub.k of dimensions Q.times.L or Q*L vectors R.sub.1,j of length N. For example,with respect to the symbol received by the 1.sup.st antenna and demodulated by the 1.sup.st demodulator, there is a vector of elements R.sub.1,0, R.sub.1,1, . . . , R.sub.1,N1, as depicted in FIG. 4.
FIG. 5 illustrates an exemplary frame 50 that may be implemented in a MIMO communications system that has Q transmit antennas, such as the communications system depicted in FIGS. 1 and 3. As depicted in FIG. 5, the frame 50 typically includes Qsignal structures 52, which correspond respectively to the Q antennas. Each signal structure 52 typically includes a preamble 54 and a data section 56. As discussed above for FIG. 2, the preamble 54 is typically inserted into the data section 56 by thepilot/training symbol inserter 32. The preamble 54 typically includes one or more training blocks 58 of length N.sub.I and cyclic prefixes 57 of length G, as depicted in FIG. 5. The combination of a cyclic prefix 57 and a training block 58 forms atraining symbol 53 that has a length of G+N.sub.I samples in the time domain. Thus, as depicted, the preamble 54 typically includes Q training symbols 53 that have an overall length of Q*(G+N.sub.I) samples in the time domain. A cyclic prefix 57 mayalso be referred to as a guard interval, since the cyclic prefix 57 typically functions to guard the signal structures 52 from intersymbol interference (ISI) during transmission as a frame 50 across the channel 19. The time length of the cyclic prefix57 is typically greater than the maximum length of the channel impulse response h.sub.ij, which was discussed above for FIG. 3.
As also depicted in FIG. 5, the data section 56 typically includes one or more data blocks 59 of length N and cyclic prefixes 57 of length G. The combination of a cyclic prefix 57 and a data block 59 forms a data symbol 55 that has a length ofG+N samples in the time domain. Therefore, the data section 56 of the signal structure 52 typically includes Q or more data symbols 55 that have an overall length of P*Q*(G+N) samples in the time domain, as depicted in FIG. 5, where P is some positiveinteger. Although not depicted in FIG. 5, for simplicity, pilot symbols may also be intermittently inserted into the data symbols 55 by the pilot/training symbol inserter 32, as discussed above.
The length N.sub.I of a training block 58 may be shorter than the length N of a data block 59 in a signal structure 52. Typically, the length N.sub.I of a training block 58 in the preamble 54 is established as a fraction of the length N of adata block 59 in the data section 56 to provide the relationship of N.sub.I being equivalent to N/I, where I is some positive integer. For example, N.sub.I may be equivalent to N/4 (i.e., I=4). If the length N.sub.I of a training block 58 is notestablished, the length N.sub.I may be assumed to be equivalent to N (i.e., I=1). Typically, the length of a training symbol 53 (i.e., G+N.sub.I) is equivalent to the length of a data symbol 55 (i.e., G+N). However, it is feasible for the trainingsymbol 53 to be shorter than the data symbol 55 in the context of the signal structure 52.
A primary purpose of the preamble 54 is to enable the receiver 10 (FIG. 1) to identify the arrival of the signal structure 52. Thus, the preamble 54 may facilitate time synchronization, frequency synchronization, channel parameter estimation,and noise variance estimation. Efficient spacetime structures for the preamble 54 ("spacetime preamble structures"), in accordance with the present invention, provide time synchronization, frequency synchronization, channel parameter estimation, andnoise variance estimation through synchronization signals that have low peaktoaverage power ratios (PAPR) (e.g., at or approaching unity).
A spacetime preamble structure, which may also be referred to as a spacetime training structure, may be represented by a signal transmission matrix S.sub.k. In accordance with an embodiment of the present invention, the signal transmissionmatrix S.sub.k of an efficient spacetime preamble structure should be a unitary transmission matrix in the frequency domain and have a low PAPR in the time domain. In this regard, efficient spacetime preamble structures provide enhanced performance inMIMO communications systems.
A unitary transmission matrix contains rows and columns that are orthogonal to each other, and the energy of the signals represented by each row or column is unity. In mathematical terms, a unitary transmission matrix has the propertiesrepresented by the following equations:
.times..times.''.noteq.'.times..times..times..times.''.noteq.'.times..tim es. ##EQU00001## where S.sub.i,j represents the constituent symbols of the unitary transmission matrix.
Providing a spacetime preamble structure that is a unitary signal transmission matrix S.sub.k reduces or eliminates noise enhancement during channel estimation of the received signals. Moreover, providing a spacetime preamble structure thatpossesses a low PAPR reduces or eliminates signal nonlinearities and spurious, outofband signal transmissions. As will be discussed below, data structures formed by spacetime processing (i.e., spacetime data structures) to be a unitary transmissionmatrix also provide enhanced performance in MIMO communications systems.
The following descriptions present several examples of data structures that, in accordance with the present invention, can be applied and/or modified to provide spacetime preamble structures that are unitary transmission matrices. As a firstexample, a diagonal data structure can be applied and/or modified to provide a spacetime preamble structure in accordance with the present invention. In this regard, the resulting diagonal spacetime preamble structure is a unitary transmission matrix. The following diagonal structure S.sub.D1 is an example of this unitary transmission matrix that can be applied as a spacetime preamble in a MIMO communications system with Q antennas:
.times. ##EQU00002##
The foregoing diagonal spacetime preamble structure S.sub.D1 can be simplified so that the same training symbol (e.g., S.sub.1) can be transmitted from each antenna, instead of Q different training symbols (i.e., S.sub.1, S.sub.2, etc.), asshown by the following simplified diagonal structure S.sub.DS that can be applied, in accordance with the present invention, as a spacetime preamble structure in a MIMO communications system with Q antennas:
.times. ##EQU00003##
When the foregoing diagonal structures S.sub.D, S.sub.DS are applied as spacetime preamble structures in a MIMO communications system, the training symbols are transmitted sequentially in time from each corresponding transmit antenna, and theparameters of the received symbols are estimated by the receivers connected to each receive antenna. Due to their unitary characteristic, the diagonal structures S.sub.D, S.sub.DS, provide simplified signal acquisition (i.e., synchronization) andparameter estimation when applied as a spacetime preamble structure in a MIMO communications system. These diagonal structures S.sub.D, S.sub.DS, are preferably applied as spacetime preamble structures, in MIMO communications systems that use twotransmit antennas. As the number (Q) of transmit antennas in the MIMO system is increased, the power output from each transmit antenna typically has to be reduced by a factor of Q due to the nature of MIMO systems. As a result, the efficiency of thediagonal spacetime preamble structures S.sub.D, S.sub.DS may decrease in MIMO systems with more than two transmit antennas, since the diagonal structures S.sub.D, S.sub.DS only include symbols on the main diagonal (i.e., spanning from the topleft tothe bottom).
A data structure that was introduced by S. Alamouti is another example of a data structure that can be applied and/or modified, in accordance with the present invention, to provide a spacetime preamble structure S.sub.A. This data structure isa unitary transmission matrix, and it can be applied as a spacetime preamble structure S.sub.A, in MIMO communications systems that employ two transmit antennas. The spacetime preamble structure S.sub.A has the following form:
.times. ##EQU00004##
In the above spacetime preamble structure S.sub.A, the "*" symbol indicates a complex conjugate operation. The foregoing spacetime preamble structure S.sub.A can also be simplified, in accordance with the present invention, so that the sametraining symbol is transmitted from each of the two antennas of the MIMO system, as shown by the following simplified spacetime preamble structure S.sub.AS:
.times. ##EQU00005##
Several orthogonal structures that were introduced by V. Tarokh, et al. are examples of data structures that can be applied and/or modified, in accordance with the present invention, to provide spacetime preamble structures that are unitarytransmission matrices. These data structures can be applied as spacetime preamble structures, in accordance with the present invention, in MIMO communications systems that employ four or eight transmit antennas. For a fourantenna MIMO system, thefollowing spacetime preamble structure S.sub.T4 can be applied, in accordance with present invention, when the constituent symbols have real number values:
.times..times..times. ##EQU00006## The foregoing spacetime preamble structure S.sub.T4 can be simplified, in accordance with the present invention, so that the same training symbol is transmitted from each of the four antennas of the MIMOsystem, as shown by the following simplified spacetime preamble structure S.sub.T4S. The symbols of this structure S.sub.T4S may have complex values (e.g., W+jX):
.times..times..times..times. ##EQU00007##
The foregoing simplified structures S.sub.AS, S.sub.T4S (i.e., EQ. 6 and EQ. 8) typically form unitary transmission matrices when applied as spacetime preamble structures, without further modification. Furthermore, the PAPR of the simplifiedspacetime preamble structures S.sub.AS, S.sub.T4S are typically unity when the symbols consist of chirptype sequences, such as:
.function..times..times..pi..times..times..times..times. .times. ##EQU00008## Therefore, these simplified structures S.sub.AS, S.sub.T4S are typically efficient (i.e., they provide time and frequency synchronization, estimation of noisevariance and channel parameters, and low PAPR) when applied, in accordance with the present invention, as spacetime preamble structures.
The foregoing structures S.sub.A, S.sub.T4 (i.e., EQ. 5 and EQ. 7) are also typically efficient when applied as spacetime preamble structures, in accordance with the present invention. The structure S.sub.T4 is typically not efficient whenapplied as spacetime data structures in a MIMO communications system. However, both structures S.sub.A, S.sub.T4 can be modified and then applied as efficient spacetime data structures, in accordance with the present invention. Since the structuresS.sub.A, S.sub.T4 will include symbols with complex values when they are applied as spacetime data structures, the resultant data structures will typically not be unitary transmission matrices. Therefore, the structures S.sub.A, S.sub.T4 can bemodified, in accordance with the present invention, to form unitary transmission matrices and, thus, provide efficient spacetime data structures. Methods, in accordance with the present invention, to transform these structures S.sub.A, S.sub.T4 andother structures into efficient spacetime data structures will be described below.
The following spacetime preamble structure S.sub.T8 is based on another data structure by Tarokh, et al., and the structure S.sub.T8 can be applied in eightantenna MIMO communications systems, in accordance with present invention, when theconstituent symbols have real number values:
.times..times..times. ##EQU00009##
The foregoing spacetime preamble structure S.sub.T8 can be simplified, in accordance with the present invention, so that the same training symbol is transmitted from each of the eight antennas of the MIMO system, as shown by the followingsimplified spacetime preamble structure S.sub.T8S
.times..times..times..times. ##EQU00010##
The foregoing structures S.sub.T8, S.sub.T8S (i.e., EQ. 10 and EQ. 11) are typically efficient when applied as spacetime preamble structures, in accordance with the present invention. However, these structures S.sub.T8, S.sub.T8S aretypically not efficient when applied as spacetime data structures in a MIMO communications system. The structure S.sub.T8 preferably can be modified and then applied as efficient spacetime data structures, in accordance with he present invention. Since the structure S.sub.T8 will include symbols with complex values when it is applied as a spacetime data structure, the resultant data structure will typically not be a unitary transmission matrix. Therefore, the structure S.sub.T8 can be modified,in accordance with the present invention, to form a unitary transmission matrix and, thus, provide an efficient spacetime data structure. Methods, in accordance with the present invention, to transform this structure S.sub.T8 and other structures intoefficient spacetime data structures will be described below.
Orthogonal structures, such as those introduced by Tarokh, et al., typically only have applications to MIMO communications systems that employ two, four, or eight transmit antennas. As described above, some of the orthogonal structures can beapplied, in accordance with the present invention, in twoantenna MIMO systems as spacetime data structures, with complex symbols, that are unitary transmission matrices. However, the application of existing orthogonal structures using complex symbols(e.g., for spacetime data structures) in MIMO systems having more than two transmit antennas typically results in a loss of the system diversity gain and/or system bandwidth. For example, the following orthogonal structure S.sub.T3 was introduced byTarokh, et al. for use as a data structure with complex symbols in a threeantenna MIMO system:
.times..times..times. ##EQU00011##
When the foregoing structure S.sub.T3 is applied in a threeantenna MIMO system, it does not provide the full diversity performance of the system, which is the capability to transmit three symbols over three symbol periods. Instead, thestructure S.sub.T3 only provides for the transmission of three symbols over four symbol periods, which is apparent since the structure has a four rows instead of three. This lack of full diversity may result in a loss of as much as 25% of systemthroughput. However, methods, in accordance with the present invention, will be discussed below to transform such inefficient structures into efficient spacetime structures (for preambles or data) that provide full diversity performance in MIMOcommunications systems.
The foregoing spacetime preamble structures, in accordance with the present invention, can be applied in a Qantenna MIMO communications system, such as the system 6 depicted in FIG. 1, using any applicable technique. For example, thespacetime preamble structure S.sub.T4 may be stored in a pilot/training symbol inserter 32 of the transmitter 8 of a fourantenna MIMO communications system 6 and combined with one or more data symbols for transmission over a channel 19, as discussedabove.
In general the transmission matrix for Q transmit antennas over Q symbol intervals can be represented by the following matrix S.sub.Q.sup.2:
.times..function..times. ##EQU00012## This general transmission matrix S.sub.Q.sup.2 can be composed using Q.sup.2 different symbols (or sequences in the case of OFDM modulation). However, in general, only Q sequences are used to form astructure. As discussed above, the transmission performance of Q symbols over Q symbol periods indicates full diversity performance of the MIMO system and also indicates the utilization of the full bandwidth of the system. Thus, such performanceindicates the optimal use of the system resources.
In order to utilize the structure of the foregoing general transmission matrix S.sub.Q.sup.2 to construct efficient spacetime sequence structures for preambles, pilots and data to be applied in MIMO communications systems, the matrixS.sub.Q.sup.2 is preprocessed and/or preconditioned in accordance with the present invention.
FIG. 6 is a flow chart illustrating a method 120 for providing efficient spacetime structures for preambles, pilots and data that may be implemented in a MIMO communications system, such as the system 6 depicted in FIG. 1. The method 120begins with a step 122 in which one or more initial structures S.sub.in are provided for conversion into efficient spacetime structures for preambles or data. The structure S.sub.in will typically have a form that is applicable to a Q.times.L MIMOcommunications system, where Q represents the number of transmit antennas and L represents the number of receive antennas, as discussed above. Thus, if the initial structure S.sub.in is to be applied to a MIMO system that has 4 transmit antennas (i.e.,Q=4), the structure will typically have 4 columns and 4 rows, similar to the general transmission matrix S.sub.Q.sup.2 described above. Typically, the initial structure S.sub.in is formed of symbols from a known symbol alphabet. As discussed above, asymbol alphabet typically includes a finite set of values. In general, the initial structure S.sub.in may be any structure that has a possible application to a MIMO communications system with Q transmit antennas. One method, among others, to determinean initial structure S.sub.in will be discussed below with respect to FIG. 7.
Following step 122, the method 120 proceeds to step 124 in which the rows of the initial structure S.sub.in are verified to be linearly independent. The check for linear independence of the rows of the initial structure S.sub.in may beperformed by various methods and techniques, which may be known in the art. For example, the rows of the initial structure S.sub.in can be tested for linear independence by determining the rank of the initial structure S.sub.in. If the rank of theinitial structure S.sub.in is determined to be Q, the rows of the initial structure S.sub.in are linearly independent. If the rows of the initial structure S.sub.in are determined to be linearly independent, the method 120 proceeds to the next step 126. However, if the rows of the initial structure S.sub.in are determined not to be linearly independent, the method 120 returns to step 122, in which one or more different initial structures S.sub.in are provided and the method 120 proceeds again to step124.
In the step 126, an orthonormalization (i.e., orthogonalization and normalization) procedure is applied to the initial structure S.sub.in. The orthonormalization procedure may be any procedure that transforms the initial structure S.sub.in to aspacetime structure S.sub.out that has the properties of a unitary signal transmission matrix. As discussed above, a unitary transmission matrix has the following mathematical properties:
.times..times..times.''.noteq.'.times..times..times..times..times.''.note q.'.times..times. ##EQU00013## where S.sub.i,j represents the constituent symbols of the unitary transmission matrix. One example of an orthonormalization procedurethat may be applied to the initial structure S.sub.in to obtain a spacetime structure S.sub.out that is a unitary signal transmission matrix is known as a rowwise GramSchmidt procedure. An example application of a rowwise GramSchmidt procedure willbe presented below.
The resultant spacetime structure S.sub.out that is obtained by the step 126 may be applied as an efficient spacetime preamble structure or an efficient spacetime data structure, depending on the characteristics of the constituent symbols ofthe structure. For example, as discussed above, an efficient spacetime preamble structure includes symbols that provide time and frequency synchronization and estimation of noise variance and channel parameters. In contrast, an efficient spacetimedata structure typically includes symbols that have complex values, as also discussed above. Further, if OFDM modulation is employed in the communications system, the constituent symbols will be symbol sequences, as also discussed above.
The resultant spacetime structure S.sub.out may be applied accordingly as a spacetime preamble or data structure in a Qantenna MIMO communications system, such as the system 6 depicted in FIG. 1, using any applicable technique, which may beknown in the art. For example, a resultant spacetime preamble structure S.sub.out may be stored in a pilot/training symbol inserter 32 of MIMO communications system transmitter 8 and combined with one or more data symbols for transmission over achannel 19, as discussed above.
FIG. 7 is a flow chart illustrating an exemplary method 140, among others, to determine an initial structure S.sub.in for use in the step 122 described above for FIG. 6. The exemplary method 140 begins with a step 142 in which a symbol alphabetis chosen to provide the symbols for the initial structure S.sub.in. Preferably, the symbols (or symbol sequences in the case of OFDM modulation) are derived from a complex alphabet on the unit circle, that is, all of the alphabet points have the sameenergy. The following are exemplary alphabets in this regard: Binary Phase Shift Keying (BPSK) alphabet: +1, 1 Quadrature Phase Shift Keying (QPSK) alphabet: 1+j, 1+j, 1j, 1j 8Phase Shift Keying (8PSK) alphabet: exp(j*2.pi.i/8), i=0, 1, 2, . . ., 7 16Phase Shift Keying (16PSK) alphabet: exp(j*2.pi.i/16), i=0, 1, 2, . . . , 15 32Phase Shift Keying (32PSK) alphabet: exp(j*2.pi.i/32), i=0, 1, 2, . . . , 31 In general, MPhase Shift Keying (MPSK) alphabet: exp(j*2.pi.i/M), i=0, 1, 2, . . ., M1; M=8, 16, 32, . . .
The symbols or symbols sequences may also be derived from polyphase sequences, such as Chirp sequences; Milewski sequences; FrankZadoff sequences; Chu sequences; Suehiro polyphase sequences; and Ng et al. sequences, among others known in theart.
Following the step 142, the method 140 concludes with a step 144 in which the initial configuration of the initial structure S.sub.in is chosen. The determination of the initial configuration may add certain specific characteristics to thestructure. For example, the initial configuration typically reduces the number of possible symbol combinations from Q.sup.2 to Q. The initial configuration may be chosen from any structure configuration. The following are several examples of a possibleinitial configuration of the initial structure S.sub.in: Circular configuration, S.sub.C:
.times. ##EQU00014## Symmetric configuration, S.sub.S:
.times. ##EQU00015##
Based on the determination of the symbol alphabet and the initial structure configuration in the step 142 and the step 144, respectively, an initial structure S.sub.in can be determined. This initial structure S.sub.in can be used in the method120, depicted in FIG. 6, to obtain an efficient spacetime structure, as discussed above.
FIG. 8 is a flow chart illustrating an alternative method 160 for providing efficient spacetime structures for preambles, pilots and data that may be implemented in a MIMO communications system, such as the system 6 depicted in FIG. 1. Themethod 160 begins with a step 162 in which one or more initial structures S.sub.in are provided for conversion into efficient spacetime structures for preambles or data. The step 162 is at least substantially similar to the step 122 discussed abovewith respect to FIG. 6. Following the step 162, the method 160 proceeds to a step 164 in which the rows of the initial structure S.sub.in are verified to be linearly independent. This step 164 is at least substantially similar to the step 124 discussedabove with respect to FIG. 6.
If the rows of the initial structure S.sub.in are determined to be linearly independent, the method 160 proceeds from the step 164 to a step 166 in which an orthonormalization procedure is applied to the initial structure S.sub.in to transformthe initial structure S.sub.in to a spacetime structure S.sub.out that has the properties of a unitary signal transmission matrix. This step 166 is at least substantially similar to the step 126 discussed above with respect to FIG. 6. However, if therows of the initial structure S.sub.in are determined not to be linearly independent, the method 160 returns to step 162.
Following the step 166, the method 160 proceeds to a step 168 in which the alphabet points of the constituent symbols of the resultant spacetime structure S.sub.out are checked to be within a tolerable distance of the alphabet points of theconstituent symbols of the initial structure S.sub.in. The amplitude of the alphabet points may be modified during the orthonormalization procedure in the step 166. The tolerable distance is typically dependent on the operating capability of componentsof the MIMO communications system 6, such as digitaltoanalog (D/A) converters. The constituent symbols of the spacetime structure S.sub.out may be checked to be within a tolerable distance of the original alphabet points by various methods andtechniques, which are known in the art. For example, the constituent symbols of the spacetime structure S.sub.out may be checked to be within a tolerable distance by application of a Euclidean distance metric represented, for example, by the followingequation: d.sub.t,l=.parallel.S.sub.tS.sub.l.parallel..sup.2 EQ. 16
If the constituent symbols of the spacetime structure S.sub.out are found to be within a tolerable distance from the original alphabet points of the initial structure S.sub.in, the spacetime structure S.sub.out is stored in a memory or otherdevice for application in a MIMO communications system. However, if the constituent symbols of the spacetime structure S.sub.out are not determined to be within a tolerable distance from the original alphabet points, the method 160 returns to step 162,in which one or more different initial structures S.sub.in are provided and the method 160 proceeds again as described above.
In the case of a MIMO communications system that employs OFDM modulation, the steps 162 through 168 may be repeated until a sufficient number of spacetime structures S.sub.out that are unitary signal transmission matrices are obtained andstored, as discussed above.
If the symbols are within a tolerable distance, in step 170, the stored spacetime structure S.sub.out used to construct spacetime sequence structures S.sub.out,k, where k represents a subcarrier or subchannel index of the OFDM setup. Thespacetime sequence structures S.sub.out,k may be constructed by an encoder, as described above with respect to FIGS. 2 and 5, or other methods, which may be known in the art, may be utilized to construct the spacetime sequence structures S.sub.out,k.
In the final step 172 of the method 160, the peaktoaverage power ratio (PAPR) of the spacetime sequence structures S.sub.out,k are tested to determine if the PAPR of the structures is low enough to provide efficient signal transmission andreception in a MIMO OFDM communications system. The PAPR of the training sequences may be tested by various methods and techniques, which may be known in the art. For example, the PAPR of the spacetime sequence structures S.sub.out,k may be tested byconverting the structures to the time domain (e.g., by inverse Fourier transform or "IFT") and calculating the PAPR of the resultant signal samples. If the PAPR of the spacetime sequence structures S.sub.out,k is found to be acceptable (e.g., at orapproaching unity), the structures have been determined to be efficient, in accordance with the present invention, and may be used for preambles or data in a MIMO communications system 6 employing OFDM modulation. However, if the PAPR of the spacetimesequence structures S.sub.out,k are found to be unacceptably high, the method 160 returns to step 162, in which one or more different initial structures S.sub.in are provided and the method 160 proceeds again as described above.
In the case of some orthogonal polyphase sequences, complex coefficients b.sub.i that are used to modulate the sequences may be useful to form efficient spacetime sequence structures S.sub.out,k. In this regard, modulation of the orthogonalpolyphase sequences by the complex coefficients b.sub.i may make the rows of the corresponding spacetime structures S.sub.out linearly independent. Furthermore, the modulation by the complex coefficients b.sub.i may also reduce the PAPR of theresulting spacetime sequence structures S.sub.out,k that are formed from the spacetime structures S.sub.out.
In the step 126 of the method 120 and the step 166 of the method 160, described above with respect to FIGS. 6 and 8, respectively, an orthonormalization procedure is applied to the initial structure S.sub.in to transform the initial structureS.sub.in to a spacetime structure S.sub.out that has the properties of a unitary signal transmission matrix. As discussed above, one example of such an orthonormalization procedure is a rowwise GramSchmidt procedure. In general, when a matrixS.sub.k is subjected to the GramSchmidt procedure, the resulting matrix S'.sub.k will be unitary, so long as the rank of S.sub.k is Q or the rows of S.sub.k are linearly independent. In a rowwise application of the GramSchmidt procedure to a matrixS.sub.k, the first row of the matrix S.sub.k is unchanged and used as a reference to make the remaining rows orthonormal (i.e., orthogonal and normal). The following matrices illustrate the application of a rowwise GramSchmidt procedure to a 4.times.4matrix S.sub.k to obtain the orthogonalized unitary matrix S'.sub.k:
.times.e.times..times.e.times..times.e.times..times.e.times..times.e.time s..times.e.times..times.e.times..times.e.times..times.e.times..times.e.tim es..times.e.times..times.e.times..times.e.times..times.e.times..times.e.times..times.e.times..times.'.times.e.times..times.e.times..times.e.times..t imes.e.times..times.e.times..times.e.times..times.e.times..times.e.times.. times.e.times..times.e.times..times.e.times..times.e.times..times.e.times..times.e.times..times.e.times..times.e.times..times. ##EQU00016##
It is noted that embodiments of the present invention, such as those described above, may be implemented in hardware, software, firmware, or a combination thereof. For example, in some embodiments, the present invention may be implemented as acomputer program or application in software or firmware that is stored in a memory and that is executed by a suitable instruction execution system. In other embodiments the present invention may be implemented, for example, with one or a combination ofthe following technologies, which may be known in the art: one or more discrete logic circuit(s) having logic gates for implementing logic functions upon data signals, an application specific integrated circuit (ASIC) having appropriate combinationallogic gates, a programmable gate array(s) (PGA), a field programmable gate array (FPGA), etc.
It is further noted that any process descriptions or blocks in flow charts described above may represent modules, segments, and/or portions of a computer program or application code that includes one or more executable instructions forimplementing specific logical functions or steps in the process. Alternate implementations are included within the scope of the present invention in which functions may be executed out of order from that shown in the figures and/or discussed above,including substantially concurrently or in reverse order, depending at least in part on the functionality involved, as will be understood by those skilled in the art.
With regard to any block diagrams described above, although the flow of data or other elements may be depicted as unidirectional or bidirectional, such depictions are merely exemplary and not limiting. Variations of the flows depicted in theblock diagrams are included within the scope of the present invention. Furthermore, the functionality of some of the blocks may be implemented by a single combined block within the scope of the present invention.
Moreover, embodiments of the present invention, such as those described above, may comprise an ordered listing of executable instructions for implementing logical functions which can be embodied in any computerreadable medium for use by or inconnection with an instruction execution system, apparatus, or device, such as a computerbased system, processorcontaining system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and executethe instructions. In the context of this disclosure, a "computerreadable medium" may be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, ordevice. The computer readable medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (i.e., a nonexhaustivelist) of the computerreadable medium include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (electronic), a readonly memory (ROM) (electronic), anerasable programmable readonly memory (EPROM or Flash memory) (electronic), an optical fiber (optical), and a portable compact disc readonly memory (CDROM) (optical). It is noted that the computerreadable medium may even be paper or another suitablemedium 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 processed in a suitable manner if necessary, and then storedin a computer memory.
Finally, it should be emphasized that the abovedescribed embodiments of the present invention are merely possible examples of implementations set forth for a clear understanding of the principles of the invention. Many variations andmodifications may be made to the abovedescribed embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope ofthis disclosure and the invention, and protected by the following claims.
* * * * * 


