USRE43294E1  Apparatus and methods for providing efficient spacetime structures for preambles, pilots and data for multiinput, multioutput communications systems  Google Patents
Apparatus and methods for providing efficient spacetime structures for preambles, pilots and data for multiinput, multioutput communications systems Download PDFInfo
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 USRE43294E1 USRE43294E1 US12555332 US55533209A USRE43294E US RE43294 E1 USRE43294 E1 US RE43294E1 US 12555332 US12555332 US 12555332 US 55533209 A US55533209 A US 55533209A US RE43294 E USRE43294 E US RE43294E
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 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L25/00—Baseband systems
 H04L25/02—Details ; Arrangements for supplying electrical power along data transmission lines
 H04L25/0202—Channel estimation
 H04L25/0224—Channel estimation using sounding signals
 H04L25/0226—Channel estimation using sounding signals sounding signals per se

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04B—TRANSMISSION
 H04B7/00—Radio transmission systems, i.e. using radiation field
 H04B7/02—Diversity systems; Multiantenna systems, i.e. transmission or reception using multiple antennas
 H04B7/04—Diversity systems; Multiantenna systems, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
 H04B7/06—Diversity systems; Multiantenna systems, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
 H04B7/0613—Diversity systems; Multiantenna systems, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
 H04B7/0667—Diversity systems; Multiantenna systems, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of delayed versions of same signal
 H04B7/0669—Diversity systems; Multiantenna systems, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of delayed versions of same signal using different channel coding between antennas

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L1/00—Arrangements for detecting or preventing errors in the information received
 H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
 H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/26—Systems using multifrequency codes
 H04L27/2601—Multicarrier modulation systems
 H04L27/2602—Signal structure
 H04L27/261—Details of reference signals
 H04L27/2613—Structure of the reference signals per se

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/0014—Carrier regulation
 H04L2027/0083—Signalling arrangements
 H04L2027/0089—Inband signals
 H04L2027/0093—Intermittant signals
 H04L2027/0095—Intermittant signals in a preamble or similar structure
Abstract
Description
This application claims priority to copending U.S. provisional application entitled, “Efficient Training and Synchronization Sequence Structures for MIMO OFDM,” having serial No. 60/322,786, filed Sep. 17, 2001, which is entirely incorporated herein by reference.
This is a broadening Reissue application of U.S. patent application Ser. No. 10/245,090, filed Sep. 17, 2002 (now U.S. Pat. No. 7,269,224, granted Sep. 11, 2007). U.S. patent application Ser. No. 10/245,090 claims priority to U.S. Provisional Patent Application No. 60/322,786, filed on Sep. 17, 2001, and entitled “Efficient Training and Synchronization Sequence Structures for MIMO OFDM,” which is entirely incorporated herein by reference.
This application is related to copending U.S. provisional application entitled “Preamble Structures for SISO and MIMO OFDM Systems,” having Ser. No. 60/327,145, filed on Oct. 4, 2001, which is entirely incorporated herein by reference.
The present invention is generally related to communications systems and, more particularly, to MultiInput, MultiOutput (MIMO) communications systems.
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 antennas and 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) communications systems. 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 common channel. 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 and combines “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 information symbols), 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 the transmitters 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, although they 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, for example. 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 communications system.
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 and transmitting 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) has emerged 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 as performance 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 preamble structures 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 the foregoing 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 and coarse 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 communications systems 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 spacetime structures (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 of transmit 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 optimal signal 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 over four 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 their application 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 and receive antennas and that facilitate full diversity performance of MIMO communications systems.
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 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.
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 be broadly 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 spacetime preamble 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 additional apparatus, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.
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.
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.
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 GHz band, 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 Division Multiplexing (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 elements to 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
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. The modulators 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 one or 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
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 original signals that were input to the transmitter 8 from the data source 12 and transmitted across the channel 19. As depicted in
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 includes one 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 first variable, “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 modulators 16 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×L” transmitreceive diversity.
The exemplary encoder 14 depicted in
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 form spacetime 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 respective transmit antennas 18 of the transmitter 8, as discussed above.
As illustrated in
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 data section. 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 are preferably 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 preamble structures) 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 time varying 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 of training 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 and training 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 apply to pilots or pilot structures unless otherwise specified).
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
Similar to the above discussion with respect to the MIMO communications system 6 of
Exemplary signal transmissions from the Q transmit antennas 18 to the L receive antennas 20, across the channel 19, are also depicted in
A transmit sample matrix S is illustrated in
Also illustrated in
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 system depicted in
The relationship between the receive sample matrix R, the transmit sample matrix S, the channel matrix η, and the noise matrix W can be expressed by the following equation:
R_{k,T×L}=S_{k,T×Q}·η_{k,Q×L}+W_{k,T×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 to Q, 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 typical MIMO communications system 6.
As also depicted in
The length N_{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_{I }of a training block 58 in the preamble 54 is established as a fraction of the length N of a data block 59 in the data section 56 to provide the relationship of N_{I }being equivalent to N/I, where I is some positive integer. For example, N_{I }may be equivalent to N/4 (i.e., I=4). If the length N_{I }of a training block 58 is not established, the length N_{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_{I}) is equivalent to the length of a data symbol 55 (i.e., G+N). However, it is feasible for the training symbol 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 (
A spacetime preamble structure, which may also be referred to as a spacetime training structure, may be represented by a signal transmission matrix S_{k}. In accordance with an embodiment of the present invention, the signal transmission matrix S_{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 in MIMO 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 properties represented by the following equations:
where S_{i,j }represents the constituent symbols of the unitary transmission matrix.
Providing a spacetime preamble structure that is a unitary signal transmission matrix S_{k }reduces or eliminates noise enhancement during channel estimation of the received signals. Moreover, providing a spacetime preamble structure that possesses 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 transmission matrix 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 first example, 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_{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:
The foregoing diagonal spacetime preamble structure S_{D1 }can be simplified so that the same training symbol (e.g., S_{1}) can be transmitted from each antenna, instead of Q different training symbols (i.e., S_{1}, S_{2}, etc.), as shown by the following simplified diagonal structure S_{DS }that can be applied, in accordance with the present invention, as a spacetime preamble structure in a MIMO communications system with Q antennas:
When the foregoing diagonal structures S_{D}, S_{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 the parameters of the received symbols are estimated by the receivers connected to each receive antenna. Due to their unitary characteristic, the diagonal structures S_{D}, S_{DS}, provide simplified signal acquisition (i.e., synchronization) and parameter estimation when applied as a spacetime preamble structure in a MIMO communications system. These diagonal structures S_{D}, S_{DS}, are preferably applied as spacetime preamble structures, in MIMO communications systems that use two transmit 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 the diagonal spacetime preamble structures S_{D}, S_{DS }may decrease in MIMO systems with more than two transmit antennas, since the diagonal structures S_{D}, S_{DS }only include symbols on the main diagonal (i.e., spanning from the topleft to the 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_{A}. This data structure is a unitary transmission matrix, and it can be applied as a spacetime preamble structure S_{A}, in MIMO communications systems that employ two transmit antennas. The spacetime preamble structure S_{A }has the following form:
In the above spacetime preamble structure S_{A}, the “*” symbol indicates a complex conjugate operation. The foregoing spacetime preamble structure S_{A }can also be simplified, in accordance with the present invention, so that the same training symbol is transmitted from each of the two antennas of the MIMO system, as shown by the following simplified spacetime preamble structure S_{AS}:
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 unitary transmission 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, the following spacetime preamble structure S_{T4 }can be applied, in accordance with present invention, when the constituent symbols have real number values:
The foregoing spacetime preamble structure S_{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 MIMO system, as shown by the following simplified spacetime preamble structure S_{T4S}. The symbols of this structure S_{T4S }may have complex values (e.g., W+jX):
The foregoing simplified structures S_{AS}, S_{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 simplified spacetime preamble structures S_{AS}, S_{T4S }are typically unity when the symbols consist of chirptype sequences, such as:
Therefore, these simplified structures S_{AS}, S_{T4S }are typically efficient (i.e., they provide time and frequency synchronization, estimation of noise variance and channel parameters, and low PAPR) when applied, in accordance with the present invention, as spacetime preamble structures.
The foregoing structures S_{A}, S_{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_{T4 }is typically not efficient when applied as spacetime data structures in a MIMO communications system. However, both structures S_{A}, S_{T4 }can be modified and then applied as efficient spacetime data structures, in accordance with the present invention. Since the structures S_{A}, S_{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_{A}, S_{T4 }can be modified, 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_{A}, S_{T4 }and other structures into efficient spacetime data structures will be described below.
The following spacetime preamble structure S_{T8 }is based on another data structure by Tarokh, et al., and the structure S_{T8 }can be applied in eightantenna MIMO communications systems, in accordance with present invention, when the constituent symbols have real number values:
The foregoing spacetime preamble structure S_{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 following simplified spacetime preamble structure S_{T8S }
The foregoing structures S_{T8}, S_{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_{T8}, S_{T8S }are typically not efficient when applied as spacetime data structures in a MIMO communications system. The structure S_{T8 }preferably can be modified and then applied as efficient spacetime data structures, in accordance with he present invention. Since the structure S_{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_{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_{T8 }and other structures into efficient 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 be applied, 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_{T3 }was introduced by Tarokh, et al. for use as a data structure with complex symbols in a threeantenna MIMO system:
When the foregoing structure S_{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, the structure S_{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 system throughput. 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 MIMO communications 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
In general the transmission matrix for Q transmit antennas over Q symbol intervals can be represented by the following matrix S_{Q} ^{2}:
This general transmission matrix S_{Q} ^{2 }can be composed using Q^{2 }different symbols (or sequences in the case of OFDM modulation). However, in general, only Q sequences are used to form a structure. 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 performance indicates the optimal use of the system resources.
In order to utilize the structure of the foregoing general transmission matrix S_{Q} ^{2 }to construct efficient spacetime sequence structures for preambles, pilots and data to be applied in MIMO communications systems, the matrix S_{Q} ^{2 }is preprocessed and/or preconditioned in accordance with the present invention.
Following step 122, the method 120 proceeds to step 124 in which the rows of the initial structure S_{in }are verified to be linearly independent. The check for linear independence of the rows of the initial structure S_{in }may be performed by various methods and techniques, which may be known in the art. For example, the rows of the initial structure S_{in }can be tested for linear independence by determining the rank of the initial structure S_{in}. If the rank of the initial structure S_{in }is determined to be Q, the rows of the initial structure S_{in }are linearly independent. If the rows of the initial structure S_{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_{in }are determined not to be linearly independent, the method 120 returns to step 122, in which one or more different initial structures S_{in }are provided and the method 120 proceeds again to step 124.
In the step 126, an orthonormalization (i.e., orthogonalization and normalization) procedure is applied to the initial structure S_{in}. The orthonormalization procedure may be any procedure that transforms the initial structure S_{in }to a spacetime structure S_{out }that has the properties of a unitary signal transmission matrix. As discussed above, a unitary transmission matrix has the following mathematical properties:
where S_{i,j }represents the constituent symbols of the unitary transmission matrix. One example of an orthonormalization procedure that may be applied to the initial structure S_{in }to obtain a spacetime structure S_{out }that is a unitary signal transmission matrix is known as a rowwise GramSchmidt procedure. An example application of a rowwise GramSchmidt procedure will be presented below.
The resultant spacetime structure S_{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 of the 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 spacetime data 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_{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

 Binary Phase Shift Keying (BPSK) alphabet: +1, −1
 Quadrature Phase Shift Keying (QPSK) alphabet: 1+j, −1+j, −1−j, 1−j
 8Phase Shift Keying (8PSK) alphabet: exp(j*2πi/8), i=0, 1, 2, . . . , 7
 16Phase Shift Keying (16PSK) alphabet: exp(j*2πi/16), i=0, 1, 2, . . . , 15
 32Phase Shift Keying (32PSK) alphabet: exp(j*2πi/32), i=0, 1, 2, . . . , 31
 In general, MPhase Shift Keying (MPSK) alphabet: exp(j*2πi/M), i=0, 1, 2, . . . , M−1; 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 the art.
Following the step 142, the method 140 concludes with a step 144 in which the initial configuration of the initial structure S_{in }is chosen. The determination of the initial configuration may add certain specific characteristics to the structure. For example, the initial configuration typically reduces the number of possible symbol combinations from Q^{2 }to Q. The initial configuration may be chosen from any structure configuration. The following are several examples of a possible initial configuration of the initial structure S_{in}:

 Circular configuration, S_{C}:

 Symmetric configuration, S_{S}:
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_{in }can be determined. This initial structure S_{in }can be used in the method 120, depicted in
If the rows of the initial structure S_{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_{in }to transform the initial structure S_{in }to a spacetime structure S_{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
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_{out }are checked to be within a tolerable distance of the alphabet points of the constituent symbols of the initial structure S_{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 components of the MIMO communications system 6, such as digitaltoanalog (D/A) converters. The constituent symbols of the spacetime structure S_{out }may be checked to be within a tolerable distance of the original alphabet points by various methods and techniques, which are known in the art. For example, the constituent symbols of the spacetime structure S_{out }may be checked to be within a tolerable distance by application of a Euclidean distance metric represented, for example, by the following equation:
d_{t,l}=∥S_{t}−S_{l}∥^{2} EQ. 16
If the constituent symbols of the spacetime structure S_{out }are found to be within a tolerable distance from the original alphabet points of the initial structure S_{in}, the spacetime structure S_{out }is stored in a memory or other device for application in a MIMO communications system. However, if the constituent symbols of the spacetime structure S_{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_{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_{out }that are unitary signal transmission matrices are obtained and stored, as discussed above.
If the symbols are within a tolerable distance, in step 170, the stored spacetime structure S_{out }used to construct spacetime sequence structures S_{out,k}, where k represents a subcarrier or subchannel index of the OFDM setup. The spacetime sequence structures S_{out,k }may be constructed by an encoder, as described above with respect to
In the final step 172 of the method 160, the peaktoaverage power ratio (PAPR) of the spacetime sequence structures S_{out,k }are tested to determine if the PAPR of the structures is low enough to provide efficient signal transmission and reception 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_{out,k }may be tested by converting 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_{out,k }is found to be acceptable (e.g., at or approaching 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 spacetime sequence structures S_{out,k }are found to be unacceptably high, the method 160 returns to step 162, in which one or more different initial structures S_{in }are provided and the method 160 proceeds again as described above.
In the case of some orthogonal polyphase sequences, complex coefficients b_{i }that are used to modulate the sequences may be useful to form efficient spacetime sequence structures S_{out,k}. In this regard, modulation of the orthogonal polyphase sequences by the complex coefficients b_{i }may make the rows of the corresponding spacetime structures S_{out }linearly independent. Furthermore, the modulation by the complex coefficients b_{i }may also reduce the PAPR of the resulting spacetime sequence structures S_{out,k }that are formed from the spacetime structures S_{out}.
In the step 126 of the method 120 and the step 166 of the method 160, described above with respect to
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 a computer 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 of the 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 combinational logic 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 for implementing 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 the block 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 in connection 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 execute the 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, or device. 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 nonexhaustive list) 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), an erasable 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 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 processed in a suitable manner if necessary, and then stored in 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 and modifications 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 of this disclosure and the invention, and protected by the following claims.
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