Patent Publication Number: US-7725085-B2

Title: Space-time communications determination

Description:
This application claims the benefit of priority to Indian Patent Application No. 1187/DEL/2005, filed on May 10, 2005, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     Various embodiments described herein relate to communications generally, including apparatus, systems, and methods used to transmit and receive information, such as space-time communications systems. 
     BACKGROUND INFORMATION 
     Space-time communication techniques, including the use of multiple-input, multiple-output (MIMO) systems, can make it possible to multiply the data rate of a wireless local area network (WLAN) by nearly as many times as the number of antennas employed, without the need for increased spectrum usage. However, evaluating the suitability of space-time communication techniques for use in a particular environment may involve significant signaling overhead. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of apparatus and systems according to various embodiments of the invention. 
         FIG. 2  is a flow diagram illustrating several methods according to various embodiments of the invention. 
         FIG. 3  is a block diagram of an article according to various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments disclosed herein may operate to determine whether space-time communications should be employed (or continue to be employed) by processing a short time sequence (STS), perhaps embedded in one or more packets transmitted from a plurality of antennas by a client station (STA) to an access point (AP). The packets may be formatted according to an IEEE (Institute of Electrical and Electronic Engineers) 802.11 standard, such as an IEEE 802.11b, 802.11h, or 802.11n standard. For more information with respect to IEEE 802.11 standards, please refer to “IEEE Standards for Information Technology—Telecommunications and Information Exchange between Systems—Local and Metropolitan Area Network—Specific Requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY), ISO/IEC 8802-11; 1999” and related amendments/versions. 
       FIG. 1  is a block diagram of apparatus  100 ,  104  and systems  110 ,  114  according to various embodiments of the invention, each of which may operate as previously described. Thus, the apparatus  100  may include an indication module  120  to provide an indication  124  (e.g., GO MIMO, NO-GO MIMO, etc.) of whether to communicate (or continue communicating) using a space-time communications technique in response to processing an STS  128 . 
     For example, a method of processing the STS can be implemented by assuming that l=1, . . . , 12 represents twelve non-zero subcarriers in an STS, and that k snapshots (making up a group of samples) can be obtained for each subcarrier frequency f l . Each snapshot may include four samples corresponding to four antennas, perhaps forming a portion of a multiple-input, multiple-output (MIMO) system. This, a matrix X l  of size 4×k may then be constructed with respect to the four antennas. 
     The correlation matrix for subcarrier l may be designated as: C l =E[X l X H     l   ], with a QR decomposition computed as C l =Q l R l , where Q l  is designated as an orthogonal matrix for the l th  subcarrier, and R l  is designated as an upper triangular matrix for the l th  subcarrier. Here E may comprise the averaging operator, and X H     l    may comprise the conjugate transpose of X l . Since X l  may comprise a column vector and X H     l    may comprise a row vector, multiplication of the two vectors X l X H     l    can provide a matrix realization for one snapshot. Averaging over several snapshots may then provide the correlation matrix C l . 
     At this point, the diagonal elements of R l  may be extracted as r l =diag(R l )=[r l   11  r l   22  . . . r l   44 ]. Averaging r l  over all 12 subcarriers may be accomplished according to the formula: 
     
       
         
           
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     The determination of whether to communicate using space-time communications may now be made. For example, if r 11 &lt;&lt;r nn , for n=2, 3, 4 (e.g., r 11  is more than about two times larger than r nn ) then the channel should not be used to support space-time communications (e.g., MIMO communications) since it is likely that only a single dominant loss of signal path exists for both the STA and AP. The indication, which may form the state of a reserved bit in a packet, may thus comprise “NO-GO MIMO” in this case. Of course, other boundaries may be established with respect to the amount r 11  is greater than r 44 . For example, r n  may be more than about five times, or even ten times greater than r nn  before the decision is made that space-time communications should not be supported by the channel. 
     If r 11 ≈r 22  (e.g., r 11  is approximately equal to r 22 ) and R 11 &lt;&lt;r 33  and r 11 &lt;&lt;r 44  then the channel may be used to support space-time communications (e.g., 2×2 MIMO), since it is likely that two independent dominant paths (e.g., multipaths) are present. If r 11 ≈r 22 ≈r 33  (e.g., r 11  is approximately equal to r 22  and r 33 ) and r 11 &lt;&lt;r 44  then the channel may be used to support space-time communications (e.g., 3×3 MIMO), since it is likely that three dominant independent paths (e.g., multipaths) are present. If r 11 ≈r 22 ≈r 33 ≈r 44  (e.g., r 11  is approximately equal to r 22 , r 33 , and r 44 ), then the channel may be used to support space-time communications (e.g., 4×4 MIMO), since it is likely four or more dominant independent paths (e.g., multipaths) are present. In these cases, the indication (e.g., the state of a reserved bit in a packet) may comprise “GO MIMO”. In some embodiments, a determination of whether to implement space-time communications can be made through a QR decomposition of STS samples obtained from the antennas involved. Fast-QR decomposition, and Gram-Schmidt orthogonalization, as well as other techniques known to those of skill in the art may be used for finding [r 11  r 22  . . . r 44 ]. 
     Assuming space-time communications are to be supported (e.g., the indication comprises GO MIMO), the AP may respond with a packet, including a clear to send (CTS) packet indicating the GO MIMO status, along with the diversity order, if desired. The STA may then respond, in turn, with a MIMO transmission according to the diversity order indicated in the packet transmitted by the AP. However, if space-time communications are not to be supported, the AP may respond with a packet, including a clear to send (CTS) packet, indication the NO-GO MIMO status. 
     In GO MIMO situation, perhaps indicated using a CTS packet, the STA may respond with a MIMO transmission (e.g., 4×4 MIMO) where the data packet format, including the STS and long term sequences (LTSs) are compatible with legacy IEEE 802.11a and 802.11g formats. The training pattern of the packet transmission may occur using any number of suitable methods, such as frequency orthogonality, time orthogonality, repetition, or a cyclic delay diversity method. 
     Many configurations of the apparatus  100  are possible. For example, in some embodiments, the apparatus  100  may include a plurality of reception chains  132  to couple to the indication module  120  and to receive the STS  128 . The reception chains  132  may include a number of component elements, such as a bandpass filter BPF, and RF down-converter DCV, an analog-to-digital conversion device ADC, and a demodulator DEM (e.g., an orthogonal frequency-division multiplexing (OFDM) demodulator) to provide the STS  128  to the indication module  120 . The apparatus  100  may also include a media access control (MAC) module  136  to receive the indication  124 . 
     In some embodiments, the apparatus  100  may include a transmitter  140  to transmit information  144  in response to the indication  124 , which may be included in a packet  148 , such as a clear to send (CTS) packet. The transmitter  140  may comprise a number of components, such as a modulator, digital to analog converter, up-converter, and bandpass filter. The transmitter may have a number of baseband-RF paths connected to antennas  164 , similar to the four baseband-RF paths (reception chains  132 ) shown in  FIG. 1 . The packet  148  may be formatted according to an IEEE 802.11 standard. The information  144  may include any number of components. For example, the information  144  may comprise an indication selected from GO MIMO and NO-GO MIMO, corresponding to an indication that a channel with support space-time communications, or an indication that a channel should not be used to support space-time communication, respectively. 
     Many embodiments may be realized. For example, consider a WLAN  146  including an AP  154  capable of communicating according to IEEE 801.11 standard, such as an IEEE 802.11n standard. Assuming the presence of mixed client stations, including STA  150  that can communicate according to a variety of IEEE 802.11 standards (e.g., IEEE 802.11b, 802.11g, and 802.11n standard), various embodiments may include formatting a packet  152 , such as an RTS (request to send) packet, to be transmitted by the STA  150  so as to be understood by the AP  154  and legacy STAs (not shown). Some embodiments include processing and STS  128  included in the packet  152  by the AP  154  to estimate the channel suitability for engaging in (e.g., beginning, continuing, and/or resuming) space-time communications, such as MIMO communications. 
     In another example, a STA  150  may operate to seize a WLAN  146  channel by sending a packet  152 , such as an RTS packet, to an AP  154 . The packet  152  format may conform to a variety of legacy modem formats, such as an IEEE 802.11 standard format. An STS  128  within the packet  152  may be transmitted by a plurality of antennas  156  (e.g., the four antennas  156  shown in  FIG. 1 ). An LTS and control date  160  may be transmitted by a single antenna  158  with the same format as a legacy STA format (e.g., an IEEE 802.11 format). Other STAs (not shown) listening to the STA  150  may implement a backoff function to facilitate the exchange of packets between the AP  154  and the STA  150  after receiving the packet  148 , such as an RTS packet, and the NAV (Network Allocation Vector) may by set accordingly. 
     The AP  154  may receive the packet  152 , including the STS  128  using a plurality of antennas  164  (e.g., for antennas shown in  FIG. 1 ) as well as receiving the LTS and control data using a single antenna  168 . The AP  154  may operate to process the packet  152 , including the STS  128 , to estimate the feasibility of channel support for continued space-time communications, perhaps providing an indication (e.g., GO MIMO or NO-GO MIMO)  124  as a result. The AP  154  may then respond by transmitting a packet  148 , such as a CTS packet, including the information  144 , such as an indication of GO MIMO or NO-GO MIMO. 
     Other embodiments may be realized. For example, the apparatus  104  may include a transmission module  172  to transmit an STS  128  and the remainder of an associated packet  152  using a single antenna  158  selected from a plurality of antennas  156 . The STS  128  may also be substantially simultaneously transmitted without the remainder of the associated packet  152  using the remainder of the plurality of antennas  156  (e.g., some of the antennas  156 , not including antenna  158 ). The associated packet  152  may comprise an RTS packet formatted according to an IEEE 802.11 standard. The transmission module  172  may be included in an access point, a hand-held computer, a laptop computer, a personal digital assistant, a cellular telephone, a STA, and an AP, among others. 
     Other embodiments may be realized. For example, a system  110  may include one or more apparatus  100 , as described above, as well as a plurality of antennas  164  and a display  176  to display information derived from packets  152  received by a plurality of reception chains  132  (coupled to the plurality of antennas  164 ). As noted previously, the some of the reception chains  132  may include one or more demodulators DEM to provide an STS  128  (included in the packet  142 ) to the indication module  120  for processing. The display  176  may comprise a cathode ray tube display, as well as a solid-state display (e.g., liquid crystal), and may be included in a hand-held computer, a laptop computer, a person digital assistant, a cellular telephone, a STA, and an AP, among others. 
     Other embodiments may be realized. For example, a system  114  may include one or more apparatus  104 , as described above, as well as a plurality of antennas  156  and a display  180  to display information derived from packets  148  received by the plurality of antennas  156 . The display  180 , as well as the transmission module  712 , may be included in a hand-held computer, a laptop computer, a personal digital assistant, a cellular telephone, a STA, and an AP, among others. The display  180  may comprise a cathode ray tube display, as well as a solid-state display (e.g., liquid crystal). 
     The apparatus  100 ,  104 ; systems  110 ,  114 ; indication module  120 ; indication  124 ; STS  128 ; reception chains  132 ; bandpass filter BPF; RF down-converter DVC; analog-to-digital conversion device ADC; demodulator DEM; MAC module  136 ; transmitter  140 ; information  144 ; packets  148 ,  152 ; STA  150 ; AP  154 ; WLAN  146 ; antennas  156 ,  158 ,  164 ,  168 ; transmission module  172 ; and displays  176 ,  180  may all be characterized as “modules” herein. Such modules may include hardware circuitry, single and/or multi-processor circuits, memory circuits, software program modules and objects, and/or firmware, and combinations thereof, as desired by the architect of the apparatus  100 ,  104  and systems  110 ,  114 , and as appropriate for particular implementations of various embodiments. For example, such modules may be included in a system operation simulation package, such as a software electrical signal simulation package, a power usage and distribution simulation package, a capacitance-inductance simulation package, a power/heat dissipation simulation package, a signal transmission-reception simulation package, and/or a combination of software and hardware used to simulate the operation of various potential embodiments. 
     It should also be understood that the apparatus and systems of various embodiments can be used in application other than MIMO communication systems, and thus, various embodiments are not to be so limited. The illustrations of apparatus  100 ,  104  and systems  110 ,  114  are intended to provide a general understanding of the structure of various embodiments, and they are not intended to serve as a complete description of all the elements and features of apparatus and systems that might make use of the structures described herein. 
     Applications that may include the novel apparatus and systems of various embodiment include electronic circuitry used in high-speed computers, communication and signal processing circuitry, modems, single and/or multi-processor modules, single and/or multiple embedded processors, data switches, and application-specific modules, including multilayer, multi-chip modules. Such apparatus and systems may further be included as sub-components within a variety of electronic systems, such as televisions, cellular telephones, personal computers, workstations, radios, video players, vehicles, and others. 
     Some embodiments may include a number of methods. For example,  FIG. 2  is a flow diagram illustrating several methods  211  according to various embodiments of the invention. A method  211  may begin at block  221  with transmitting a packet, such as an RTS packet, from a STA, for example. A first antenna included in a plurality of antennas may be used to transmit the packet, including an STS and the remainder of the packet. Other antennas in the plurality of antennas (e.g., not including the first antenna) may be used to transmit the STS, without the remainder of the packet, at substantially the same time. Thus, the method  211  may include transmitting an RTS packet including an STS, as well as transmitting just the STS at substantially the same time, at block  221 . 
     The method  211  may include receiving the packet at block  225 , including the STS, and then determining whether to communicate using a space-time communications technique at block  231 , perhaps in response to an indication derived from processing the STS. Thus, the method  211  may include receiving the STS as a portion of an RTS packet at block  225 , as well as receiving the STS without the remainder of the packet at substantially the same time. 
     The method  211  may include processing the STS to derive the indication at block  235 . Processing the STS may include determining the indication using a decomposition of a correlation matrix, as described previously, wherein the correlation matrix (e.g., C l =E[X l X H     l   ]) includes samples of l non-zero subcarriers included in the STS. Thus, the method  211  may include comparing a first averaged value of a first element (e.g., r l   11  selected from a triangular matrix R l ) to a second averaged value of a second element (e.g., r l   22  selected from the triangular matrix R l ), wherein the triangular matrix forms a portion of the decomposition. 
     The method  211  may include transmitting a packet, such as a CTS packet, including information responsive to the indication so derived, at block  239 . The packet may be transmitted from an AP, for example. The information may be selected from one of GO MIMO and NO-GO MIMO, as described above. 
     Other embodiments may be realized. For example, a method  241  may include transmitting an STS without the remainder of an associated packet using some of a plurality of antennas at block  255 . The method  241  may include substantially simultaneously transmitting the STS and the remainder of the associated packet using a single antenna selected from the plurality of antennas at block  259  (e.g., the single antenna may be a different antenna from those used to send the STS at block  255 ). The associated packet may be formatted according to an IEEE 802.11 standard, and may be transmitted as an RTS packet. 
     In some embodiment, the method  241  may include receiving the STS as a portion of an RTS packet transmitted along with a remainder of the RTS packet using a single antenna selected from a plurality of antennas at block  261 . 
     The method  241  may also include deriving an indication as to whether space-time communications should be continued by processing the STS included in the received packet, such as an RTS packet, at block  265 . Processing the STS may include any of the activities described above with respect to block  235 . Thus, the method  241  may include determining whether to communicate using a space-time communications technique in response to the indication derived from processing the STS at block  269 . 
     The method  241  may also include transmitting a packet including information indicating whether to use one of a non-MIMO transmission and a MIMO transmission in response to the indication derived from processing the STS at block  271 . The packet may be transmitted using a non-MIMO transmission, a MIMO transmission, or both (responsive to the indication derived from processing the STS). Transmitting the packet at block  271  may include transmitting a CTS packet including information responsive to the indication, wherein the information is selected from one of GO MIMO and NO-GO MIMO. 
     It should be noted that the methods described herein do not have to be executed in the order described, or in any particular order. Moreover, various activities described with respect to the methods identified herein can be executed in repetitive, simultaneous, serial, or parallel fashion. Information, including parameters, commands, operands, and other data, can be sent and received in the form of one or more carrier waves. 
     Upon reading and comprehending the content of this disclosure, one of ordinary skill in the art will understand the manner in which a software program can be launched from a computer-readable medium in a computer-based system to execute the functions defined in the software program. One of ordinary skill in the art will further understand the various programming languages that may be employed to create one or more software programs designed to implement and perform the methods disclosed herein. The programs may be structured in an object-orientated format using an object-oriented language such as Java or C++. Alternatively, the programs can be structured in a procedure-orientated format using a procedural language, such as assembly or C. The software components may communicate using a number of mechanisms well known to those skilled in the art, such as application program interfaces or interprocess communication techniques, including remote procedure calls. The teachings of various embodiments are not limited to any particular programming language or environment. 
     Thus, other embodiments may be realized. For example,  FIG. 3  is a block diagram of an article  385  according to various embodiments of the invention. Examples of such embodiments may comprise a computer, a memory system, a magnetic or optical disk, some other storage device, and/or any type of electronic device or system. The article  385  may include one or more processors  387  coupled to a machine-accessible medium such as a memory  389  (e.g., a memory including an electrical, optical, or electromagnetic conductor) having associated information  391  (e.g., computer program instructions and/or data), which, when accessed, results in a machine (e.g., the processor(s)  387 ) performing such actions as determining whether to communicate using a space-time communications technique responsive to an indication derived from processing an STS. Other activities may include receiving the STS as a portion of an RTS packet transmitted along with a remainder of the RTS packet using a single antenna selected from a plurality of antennas, as well as transmitting a CTS packet including information in response to the indication, wherein the information is selected from one of GO MIMO and NO-GO MIMO, for example. 
     In some embodiments, the article  385  may include one or more processors  387  coupled to a machine-accessible medium  389  having associated information  391 , which, when accessed, results in a machine performing such actions as transmitting an STS and a remainder of an associated packet using a single antenna selected from a plurality of antennas, and substantially simultaneously transmitting the STS without the remainder of the associated packet using a remainder of the plurality of antennas. Other activities may include transmitting the associated packet as a packet formatted according to an IEEE 802.11 standard and/or as an RTS packet. 
     Implementing the apparatus, systems, and methods disclosed herein may aid in determining whether space-time communication techniques should continue to be utilized, perhaps without incurring significant signaling overhead. Increased system response time and reduced power consumption may result, since both the AP and STA may acquire knowledge as to whether such communications should be used (including the diversity order) by virtue of a legacy-compatible RTS/CTS packet exchange. 
     Although the inventive concept may be discussed in the exemplary context of an 802.xx implementation (e.g., 802.11a, 802.11g, 802.11HT, 802.16, etc.), the claims are not so limited. Indeed, embodiments of the present invention may well be implemented as part of any wired and/or wireless system. Examples may also include embodiments comprising multi-carrier wireless communication channel (e.g., orthogonal frequency-division multiplexing (OFDM), discrete multi-tone modulation (DMT), etc.), such as may be used within, without limitation, a wireless personal area network (WPAN), a wireless local area network (WLAN), a wireless metropolitan are network (WMAN), a wireless wide area network (WWAN), a cellular network, a third generation (3G) network, a fourth generation (4G) network, a universal mobile telephone system (UMTS), and the like communication systems. 
     The accompanying drawings that form a part hereof show by way of illustration, and not of limitation, specific embodiments in which the subject matter may be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments id defined only by the appended claims, along with the full range of equivalents to which such claims are entitled. 
     Such embodiments of the inventive subject matter may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept it more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description. 
     The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b), requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.