Patent Publication Number: US-2005141459-A1

Title: Apparatus and associated methods to reduce management overhead in a wireless communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This disclosure is related to the following pending U.S. patent applications Ser. No. TBD, entitled “An Efficient Channel Estimator for SDMA”, by Qinghua Li, Xintian E. Lin, filed on TBD; and Ser. No.: TBD (P17922), entitled “Communication Overhead Reduction Apparatus, Systems and Methods” filed on Dec. 15, 2003 by Qinghua Li, Xintian E. Lin, each of which is assigned to the assignee of the embodiments disclosed herein, Intel Corporation. 
    
    
     TECHNICAL FIELD  
      Various embodiments described herein relate to communications generally, including apparatus, systems, and methods to reduce management overhead in a wireless communication system and, in particular, to reduce calibration and training overhead associated with a wireless communication channel.  
     BACKGROUND INFORMATION  
      Spatial multiplexing communications system performance, including SDMA (space division, multiple access) and MIMO (multiple-input, multiple-output) systems, may be improved by the activities of training and calibration. Training may include transmitting known signals to a receiver to increase the reliability of estimating channel state information. While longer training sequences may provide increased reception accuracy, the use of such sequences may also reduce the advantage to be gained by using spatial multiplexing in the first place (i.e., high data rates). Similarly, while calibrating transmitter power and receiver gains can contribute to improved data transmission rates, the additional time required for periodic calibration may decrease the ultimate system capability to communicate large amounts of data in a short time span.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements and in which:  
       FIG. 1  is a block diagram of apparatus and a system operating according to various embodiments;  
       FIG. 2  is a block diagram of apparatus and a system operating according to various embodiments;  
       FIG. 3  is a block diagram of apparatus and a system operating according to various embodiments;  
       FIG. 4  is a block diagram of exemplary packet formats that can be utilized by the apparatus and system of  FIG. 3 ;  
       FIGS. 5A and 5B  are a block diagram of an apparatus operating according to various embodiments, as well as an exemplary packet format which may be implemented thereby, respectively;  
       FIG. 6  is a flow chart illustrating several training and calibration methods according to various embodiments;  
       FIG. 7  is a flow chart illustrating several alternative training and calibration methods according to various embodiments;  
       FIG. 8  is a block diagram of an article according to various embodiments;  
       FIG. 9  is a block diagram of an example apparatus and a system operating according to various embodiments;  
       FIG. 10  is a block diagram of an example apparatus and a system operating according to various embodiments; and  
       FIG. 11  is a block diagram of apparatus and a system operating according to various embodiments. 
    
    
     DETAILED DESCRIPTION  
      MIMO system techniques can multiply the effective data rate of a wireless local area network (WLAN) by nearly as many times as the number of antennas employed by an access point (AP) without the need for increased spectrum usage. MIMO systems exploiting channel state information (CSI) at the transmitter have the potential to reduce receiver complexity while achieving increased channel capacity. Common examples of such techniques include transmit beamforming (e.g., singular value decomposition or SVD), adaptive bit loading (ABL), and power allocation (e.g., tone puncturing). Sometimes relevant CSI cannot be obtained directly via training, because training symbol measurements are the aggregate response of several components, including the transmit chain response of the transmitting device, the wireless channel response, and the receive chain response of the receiving device. Therefore, accurate measurements of the wireless channel response may be assisted by calibration.  
      CSI at the transmitter may be obtained by having the transmitter send training symbols to a receiver, and then feeding back receiver measurements of the received channel response to the transmitter. Unfortunately, this time-consuming feedback process does not lend itself to situations where high throughput is desired, such as when various forms of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 protocols are employed, including those considered by the High Throughput (HT) Study Group. For example, the round-trip channel responses of 2-by-2 and 4-by-4 MIMO systems using such feedback typically require 62 μs and 247 μs, respectively, at a 54 Mbps channel data rate. For more information on the 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.  
      Thus, in many embodiments of the invention, mechanisms are disclosed that do not require CSI feedback from the receiver. In some embodiments, calibration schemes attempt to provide a ratio of transmit chain gain to corresponding receive chain gain that is substantially constant for each antenna, at both the transmitter and the receiver. In some embodiments, calibration on one side (i.e., a transmitter or receiver) and channel estimation on the other side (i.e., the corresponding receiver or transmitter) can be accomplished in a substantially simultaneous fashion using the same sets of symbols, or preambles. According to one example implementation, training symbols may well be embedded in, or concatenated to backward-compatiaable protocols using, e.g., existing RTS/CTS (request-to-send/clear-to-send) symbols or messages may be used (e.g., IEEE 802.11 and related amendments), although the invention is not limited in this respect. Thus, in many embodiments of the invention, calibration and training, including channel estimation, at both the transmitter and receiver may be accomplished during an exchange of content (e.g., symbols) generated for some purpose other than the exchange of training symbols (e.g., RTS/CTS symbols), eliminating the need for explicit CSI feedback. In other embodiments, the training symbol(s) may well be embedded within or concatenated to any packet(s), symbol(s) or message(s) that are generated for other purposes (i.e., non-training, or calibration purposes). As used herein, a “symbol” or “training symbol” may include any character, symbol, or message known to a receiver, including, for example, preambles, such as the long and short preambles defined with respect to an IEEE 802.11 a standard packet.  
      FIG  1  is a block diagram of apparatus  100  and a system  102  operating according to various embodiments. In the system  102 , a first device  104 , such as an access point (AP) or station (STA) may communicate with a second device  108 , such as a STA or AP. The first device  104  may have a plurality of antennas  112  (e.g., three antennas  112 ), with one or more transmit chains  114  and one or more receive chains  116  coupled to each antenna  112 . Each transmit chain  114 -receive chain  116  pair may be included in a communication chain  118 . The second device  108  may also have a plurality of antennas  120  (e.g., two antennas  120 ), where each antenna  120  also may be coupled to one or more transmit chains  124  and/or one or more receive chains  126 . Each transmit chain  124 -receive chain  126  pair may be included in a communication chain  128 . Each transmit chain  114 ,  124  in one device  104 ,  108 , respectively, may send training and calibration symbols to all receive chains  126 ,  116  included in another device  108 ,  104 , respectively. For the purposes of this disclosure, the term “transceiver” (e.g., a device including a transmitter and a receiver) may be used in place of either “transmitter” or “receiver” throughout this document, and a transceiver may be included in a transmit chain and/or a receive chain.  
      Some communication systems may employ CSI, which may be acquired by receiving symbols, including preambles. However, as noted previously, the measurements of received preambles may include more than just the response of the wireless channel. For example, such measurements may include the combined responses of the transmit chains sending the preambles, the wireless channel, and the receive chain receiving the preambles. Thus, in some MIMO downlinks, the beamforming matrix can be affected by the combined responses of the transmit chains of the AP, the wireless channel, and the receive chains of the STA. In some cases, the chain responses of the STA may not be available to the AP.  
      In some embodiments, based on the preambles sent by the station, the device  104  can estimate the aggregate channel matrix from the input of the device  108  transmit chains  124  to the output of the device  104  receive chains  116  for the n-th subcarrier as shown in Equation (1):  
               H   u     =       [           β   A1         0       0           0         β   A2         0           0       0         β   A3           ]     ⁢         [           h   11           h   12               h   21           h   22               h   31           h   32           ]       ︸   H       ⁡     [           a   S1         0           0         a   S2           ]                 (   1   )             
 
 where H is the wireless channel matrix for the uplink; β A1 , β A2  and β A3  are the responses of the device  104  receive chains  116 ; and α S1  and α S2  are the responses of the transmit chains for the device  108 . The subcarrier index, n, has been omitted for simplicity. It should be noted that H may not be observed by the device  104 , although it may be contained within H u , where H u  is the measurement of the received training symbols (e.g., preambles). However, even when H is not available directly, in some embodiments, the matrix H u  may be used without further processing. 
 
      For example, consider the prior art, where transmit beamforming (including techniques such as SVD and SDMA) may utilize explicit feedback from the receiver. For medium size packets, including those having about 500 bytes, feedback overhead can reduce physical layer efficiency by more than 40%. Thus, in various embodiments, reducing or removing feedback can significantly improve physical layer efficiency. To effect such a mechanism, several backward compatible protocols will be described, employing the exchange of existing RTS/CTS symbols, as well as various calibration techniques, some of which operate to adjust transmit/receive chain power and gain levels so that the ratio of a transmit gain to the corresponding receive gain comprises two constants (one for each device  104  antenna  112 , and the other for each device  108  antenna  120 ).  
      Given the parameters established in Equation (1), the signals received at the device  108  from the device  104  in the downlink of  FIG. 1  may be illustrated by Equation (2) below:  
               [           y   S1               y   S2           ]     =             [           β   S1         0           0         β   S2           ]     ⁡     [           h   11           h   21           h   31               h   12           h   22           h   32           ]       ⁡     [           α   A1         0       0           0         α   A2         0           0       0         α   A3           ]         ︸     H   d         ⁡     [           x     A1   5                 x   A2               x   A3           ]               (   2   )             
 
 where y s1  and y s2  signify the received signal at the output of the device  108  receive chains  126 ; x A1 , x A2 , and x A3  are the symbols sent to the device  108 ; α A1 , α A2  and α A3  are the device  104  transmit chain  114  gains; and, β S1  and β S2  are the device  108  receive chain  126  gains. As a matter of contrast, the signals received at the device  104  from the device  108  in the uplink may be illustrated by Equation (3) below:  
               [           y   A1               y   A2               y   A3           ]     =             [           β   A1         0       0           0         β   A2         0           0       0         β   A3           ]     ⁡     [           h   11           h   12               h   21           h   22               h   31           h   32           ]       ⁡     [           α   S1         0           0         α   S2           ]         ︸     H   u         ⁡     [           x   S1               x   S2           ]               (   3   )             
 
 where x S1  and x S2  are the symbols sent to the device  104 ; y A1 , y A2 , and y A3  are the signals received at the output of the device  104  receive chains  116 ; α S1  and α S2  are the device  108  transmit chain  124  gains; and β A1 , β A2  and β A3  are the device  104  receive chain  116  gains. 
 
      Two aggregate channels, H d  and H u , may be defined as shown in Equations (2) and (3). If the aggregate channels H d  and H u  maintain reciprocity, (i.e., H d =H u   T ), the estimated aggregate channel may be employed without decomposition to perform transmit beamforming.  
      A sufficient condition for reciprocity may be shown in Equations (4) and (5) as follows:  
                 α   A1       β   A1       =         α   A2       β   A2       =         α   A3       β   A3       =     c   n                 (   4   )                   α   S1       β   S1       =         α   S2       β   S2       =     b   n               (   5   )             
 
 where c n  and  n  are two constants for the n-th subcarrier. To satisfy the condition of reciprocity exactly, c n  may be set equal to b n . However, in many embodiments, it may be sufficient that H d =k n H u   T , where H d  and H u   T  differ by the product of a scalar k n . To satisfy the conditions set by Equations (4) and (5) then, calibration and compensation may be effected at device  104  and device  108 . Two exemplary schemes that may be used to achieve these conditions are described next. 
 
       FIG. 2  is a block diagram of apparatus  200  and a system  202  operating according to various embodiments. Each device  204 ,  208  (which may be similar to or identical to devices  104 ,  108 , respectively, as shown in  FIG. 1 , and may include an AP and/or a STA) may have multiple transmit power control (TPC) levels and multiple receive gain control levels, including automatic gain control (AGC) levels, for each of the included communication chains. Further, transmit and receive responses, α and β, may vary with selected TPC and AGC settings. Thus, implementing a series of training exchanges for each possible combination of TPC and AGC (e.g., when there is no prior information about the desired setting) may be time-consuming if there are a large number of combinations. However, as explained hereinbelow, in various embodiments, desired combinations of TPC and AGC settings may be established relatively quickly with respect to the devices  204 ,  208 , such that calibration can occur rapidly.  
      In a first scheme, one device  204  may send one or more symbols  230 , such as a request to transmit (e.g., a legacy RTS symbol or message) to the device  208  using a default TPC. Then, after the device  208  receives the transmitted symbol(s) (e.g., the RTS)  230 , the device  208  may determine a set of desired AGC and TPC settings for the link to the device  204 .  
      At this point, the device  208  may send a symbol  234  in response, such as a clear to transmit response (e.g., a legacy CTS symbol or message) and N r  training symbols  238 , where N r  is the number of receive antennas (or RF chains) employed by the device  208 , which may use the same N r  antennas to receive one or more MIMO modulated data packets. The N r  symbols  238 , which may be used for training, can be sent in turn by each one of the N r  antennas, perhaps using one symbol per antenna.  
      After the device  204  receives the response  234  (e.g., the CTS symbol), the device  204  may determine a set of desired AGC and TPC settings for the link to the device  208 . Reception of the N r  training symbols  238  may be used by the device  204  to estimate the N t ×N r  channel, which may be a MIMO channel, where N t  is the number of transmit antennas (or RF chains) included in the device  204 . The device  204  may use the same N t  antennas for channel estimation and data transmission, including MIMO data transmission. The N r  training symbols  238  received by the device  208  may also be used to calibrate the communication chains (e.g., chains  128  shown in  FIG. 1 ) included in the device  208  for the newly determined set of TPC and AGC settings.  
      The device  204  may subsequently transmit N t  training symbols  240  and data  244 , including MIMO modulated data, to the device  208 . The N t  training symbols  240  may be sent by N t  antennas (or RF chains), perhaps using one symbol per antenna at a desired TPC setting. The device  204  may receive the N t  training symbols  240  at a set of desired AGC settings and calibrate the communication chains included in the device  204  (e.g., chains  118  in  FIG. 1 ). The communication chains included in the device  208  may likewise be calibrated after transmission of the N r  training symbols  238 . Beamforming, perhaps as a form of MIMO or SDMA system modulation, may be performed by the device  204  with respect to data sent by the device  204  to the device  208  using the channel information obtained as a result of receiving the response  234  from the device  208 .  
      During reception of the N t  training symbols  240 , the device  208  may set a desired AGC level and perform channel estimation. The resulting channel estimates may permit the device  208  to demodulate beamformed data provided by the device  204 . After all data  244  has been received from the device  204 , an acknowledgment  248  (e.g., a legacy ACK response) may be sent from the device  208  to the device  204  at a desired TPC setting.  
       FIG. 3  is a block diagram of apparatus  300  and a system  302  operating according to various embodiments. Each device  304 ,  308  may be similar to or identical to devices  104 ,  108 , respectively, shown in  FIG. 1 , and may include an AP and/or a STA.  FIG. 4  is a block diagram of exemplary packet formats that can be utilized by the apparatus and system of  FIG. 3 .  
      In a second scheme, advantage is taken of the fact that, according to some implementations of the IEEE 802.11 standards, RTS and CTS symbols can be transmitted in such a way as to protect long data packets from collision. Thus, the N t  and N r  training symbols may be attached directly to the request to transmit (e.g., legacy RTS) symbol and the clear to transmit response (e.g., legacy CTS) symbol, respectively, where N t  and N r  are the number of antennas at the devices (or the number of communication chains), as described previously. In each case, the training symbols may be used to both calibrate the transmitter and enable the channel estimation of the receiver in one or more of the communication chains included in the apparatus  300 .  
      Referring now to  FIGS. 3 and 4 , it can be seen that a device prepared to send data, for example, device  304 , may transmit a symbol  330 ,  430  or packet, such as a legacy RTS packet, to another device, such as device  308 . N t  training symbols  340  may be attached to the end of the packet  330 , where N t  can be the number of transmit chains included in the device  304 . The length field  448  in the packet  330 ,  430  may be set to protect up to the end of the pad bits  450 , as specified in the IEEE 802.11 standard for legacy RTS packets. Thus, a legacy device may receive the RTS packet  330 ,  430  correctly and perform collision avoidance operations as needed.  
      The N t  symbols  340  may be sent in turn by the N t  communication chains included in the device  304 . A calibration algorithm may be performed as the N t  symbols  340  are sent to calibrate both the transmit and the receive chains of the device  304 . The device  308  receiving the N t  symbols  340 ,  440  and the symbol  330 ,  430  may estimate the associated channels and compute demultiplexing matrices to enhance data reception, as is known to those of ordinary skill in the art.  
      In some embodiments, calibration of M transmit/receive or communication chains at either of the devices  304 ,  308  may occur in such a way as to satisfy the criterion set by Equation (4). First, a training symbol x 0  for the n-th sub-carrier may be sent using a first transmit chain (e.g., transmit chain # 1 ), and the output of a second receive chain (e.g., receive chain # 2 ) may be measured. The measured output may be characterized by t 12 =α A1 C 12 β A2 x 0 , where C 12  is the response from the input of a first antenna (e.g., antenna # 1  coupled to transmit chain # 1 ) to the output of a second antenna (e.g., antenna # 2  coupled to receive chain # 2 ).  
      Second, a training symbol x 0  for the n-th sub-carrier may be sent using a second transmit chain (e.g., transmit chain # 2 ), and the output of a first receive chain (e.g., receive chain # 1 ) may be measured. The measured output may be characterized by t 21 =α A2 C 21 β A1 x 0 , where C 21  is the response from the input of the second antenna to the output of the first antenna.  
      Third, the variables α A1 , α A2 , β A1  and β A2  may be adjusted so as to render t 12 =t 21 . In some cases, this may be accomplished by changing only the variable β A2 . The adjustments of the chain gains can be implemented in the digital domain, if desired. After compensation is effected in this manner, the result should be: 
 
α A1 C 12 β A2 x 0 =α A2 C 21 β A1 x 0 tm ( 6) 
 
 Equation (6) may be simplified as follows, since C 12=C   21  due to reciprocity:  
                 α   A1       β   A1       =       α   A2       β   A2               (   7   )             
 
      At this point, a loop may be executed with respect to the remaining communication chains, that is, for i=3, . . ., M. Each execution of the loop may involve sending a training symbol x 0  for the n-th sub-carrier using the first transmit chain and measuring the output of receive chain i. The measured output, characterized by t li =α A1 C li β Ai x 0 , where C li  may be seen as the response from the input of the first antenna to the output of antenna i. Then loop execution may involve sending a training symbol x 0  for the n-th sub-carrier using transmit chain i, and measuring the output of the first receive chain. The measured output may be characterized as t il =α Ai C il β Al x 0 , where C il  can be seen as the response from the input of antenna i to the output of the first antenna.  
      Finally, the variables α Ai  and β Ai  may be adjusted so as to render t li =t il . Again, in some cases, this may be accomplished by changing only the variable β Ai . The adjustments of the communication chain gains may be implemented in the digital domain, if desired. After compensation is effected in this manner, the result may be: 
 
α A1 C li β Ai x 0 =α Ai C il β Al x 0    (8) 
 
 Since C li =C il  due to reciprocity, Equation (8) may be simplified as follows:  
                 α   A1       β   A1       =       α   Ai       β   Ai               (   9   )             
 
 The loop may be repeated for each value of i in this manner until all of the chains M have been calibrated. 
 
      According to one embodiment, variations of the process in block  28  and  29  are anticipated. For example, after a first calibration between chain  1  and  2  (e.g., Eq. (7)), chain  2  may be used to perform the calibration with chain  3  (i.e., not chain  1 ). In other words, the subscript  1 , may be replaced with any “i” such that chain i has been calibrated. When one chain is sending a calibration symbol, the remaining chains within the same device can receive it and perform calibrations, substantially simultaneously. In this regard, the calibration “loop” of blocks  28  and  29  may be shortened.  
      The device  308  receiving the symbol  330  may respond by sending another symbol (or symbols, and/or packets, such as a legacy CTS symbol). This transmission may occur if the status of a network allocation vector (NAV) indicates the channel is idle. N r  training symbols  338 ,  438  may be attached to the end of the symbol or packet  334 ,  434 , where N r  is the number of the receive chains included in the device  308 . The N r  symbols  338 ,  438  may be sent in turn by N r  antennas coupled to the receive chains included in the device  308  to receive data packets  344 ,  444 . As noted above, the length field  454  in the packet  334 ,  434  may be set to protect up to the end of the pad bits  458 , as specified in the IEEE 802.11 standard for legacy CTS packets. Thus, a legacy device may receive the CTS packet  334 ,  434  correctly and perform collision avoidance operations as needed.  
      As described above, a calibration algorithm may be performed as the N r  symbols  338 ,  438  are sent, in order to calibrate the transmit and the receive chains included in the device  308 . In turn, the device  304  receiving the N r  symbols  338 ,  438  and the response symbol  334 ,  434  (e.g., a legacy CTS packet) may estimate the associated channels and determine beamforming matrices for transmission of the data  344 ,  444 .  
      The device  304  may then send the data  344 ,  444  using transmit beamforming, adaptive bit loading, and/or power allocation techniques, as is known to those of skill in the art. A symbol of acknowledgment (e.g., a legacy ACK symbol or packet)  348  may be received by the device  304  after the data  344 ,  444  is sent.  
      Upon reading this disclosure, those of skill in the art will realize that the device  308  receiving the request to send  330 ,  430  symbol or packet may estimate the channel matrix (e.g., for each orthogonal frequency division multiplexing (OFDM) tone) and form a corresponding demultiplexing matrix (e.g., the “U” matrix in SVD techniques) by exploiting the attached training symbols  340 ,  440 . Since channel estimation and matrix computation are completed beforehand, the preambles at the beginning of the data packet  344 ,  444  may be used only for synchronization, and may not be needed for channel estimation. Thus, since the preambles of the data  344 ,  444  are used only for synchronization, they may be shortened. Similarly, upon reading this disclosure, those of skill in the art will realize that the device  304  receiving the clear to send response  334 ,  434  symbol or packet may also estimate the associated channel and compute a beamforming matrix (e.g., the “V” matrix in SVD techniques) by exploiting the attached training symbols  338 ,  438 .  
      Thus, referring now to  FIGS. 1, 2 , and  3 , it can be seen that an apparatus  100 ,  200 ,  300  may be similar to or identical to the devices  104 ,  108 ,  204 ,  208 , and  304 ,  308 , including devices such as an AP and/or STA. Such apparatus  100 ,  200 ,  300  may therefore include a device  104 ,  204 ,  304  having a first number of communication chains  118  to transmit to a second apparatus  100 ,  200 ,  300  or device  108 ,  208 ,  308  a first number of training symbols corresponding to the first number of communication chains  118  and to solicit a response from the second apparatus  100 ,  200 ,  300  or device  108 ,  208 ,  308  including a second number of training symbols corresponding to a number of communication chains  128  included in the second device  108 ,  208 ,  308 .  
      The first number of communication chains  118  may correspond to a number of transmit chains  114 , and the second number of communication chains  128  may correspond to a number of receive chains  126 . Similarly, the first number of communication chains  118  may correspond to a number of receive chains  116 , and the second number of communication chains  128  may correspond to a number of transmit chains  124 . The apparatus  100 ,  200 ,  300  may include a calibration module  160  to calibrate the transmit chains  114 ,  124  and/or the receive chains  116 ,  126 . The apparatus  100 ,  200 ,  300  may also include an estimation module  162  to estimate one or more channels associated with the number of receive chains  116 ,  126 .  
      A system  102 ,  202 ,  302  may include a first apparatus  100 ,  200 ,  300  or device  104 ,  204 ,  304 , similar to or identical to those described previously. The system  102 ,  202 ,  302  may also include a second apparatus  100 ,  200 ,  300  or device  108 ,  208 ,  308 , similar to or identical to those described previously. The first apparatus  100 ,  200 ,  300  or device  104 ,  204 ,  304  may include a number of communication chains  118  to transmit a number of training symbols corresponding to the number of communication chains  118  to the second device  108 ,  208 ,  308 . In turn, the second apparatus  100 ,  200 ,  300  or device  108 ,  208 ,  308  may include a number of communication chains  128  to receive the training symbols from the first device  104 ,  204 ,  304 , and may respond by transmitting to the first device  104 ,  204 ,  304  a number of training symbols corresponding to the number of communication chains  128 .  
      The system  102 ,  202 ,  302  may include a first number of antennas  112  corresponding to a first number of communication chains  118 , and a second number of antennas  120  corresponding to a second number of communication chains  128 . The system  102 ,  202 ,  302  may also include one or more calibration modules  160  to calibrate the communication chains  118 ,  128 , as well as one or more estimation modules to estimate one or more channels associated with the communication chains  118 ,  128 . In some embodiments, the communication chains  118 ,  128  may be capable of being coupled to a number of antennas  112 ,  120  to form a portion of a multiple-input, multiple-output (IMO), or SDMA system.  
       FIGS. 5A and 5B  are a block diagram of an apparatus  500  operating according to various embodiments, as well as an exemplary packet format which may be implemented thereby, respectively. Calibration of the apparatus  100 ,  200 ,  300  and devices  104 ,  108 ,  204 ,  208 ,  304 ,  308  may be accomplished in many ways other than those described with respect to the first and second schemes explicitly described herein. For example, with respect to the second scheme outlined above, since some apparatus  500  (which may be similar to or identical to apparatus  100 ,  200 ,  300  and/or devices  104 ,  204 ,  304  and devices  108 ,  208 ,  308 ) periodically operate in a sleep mode, calibration may sometime be accomplished during this mode, such as after the apparatus  500  announces an upcoming sleep period. The apparatus  500  may include a communication chain  518 .  
      In such circumstances, calibration may begin with sending a symbol or packet  530  from the apparatus  500  to the apparatus  500  itself (i.e., self-calibration). Then calibration and/or training symbols  540  can also be sent from and to itself. This type of calibration can be accomplished using antennas  512  and on-air signals  566 , or via an internal switching network  570 . On-air calibration may provide increased accuracy, but it may also generate interference. Use of the switching network  570  may reduce accuracy due to mismatch among switches.  
      Transmit gains (β Ai  and β Si ) may vary with the TPC setting  572 . Similarly, receive gains (α Ai  and α Si ) may vary with the gain control setting  574 , such as the AGC setting. Therefore, calibration may be used to find a set of values for one chain (typically a number of receive gain settings) for each pair of TPC and AGC settings on other chains. Assuming there are N T  and N R  levels for TPC and AGC respectively, then a compensation and calibration algorithm may step through all N T ×N R  settings. Gains may be selected independently of actual transmit and receive signal magnitudes.  
      To accomplish compensation and calibration in the sleep mode, then, an apparatus  500  may begin by announcing a coming sleep period. This announcement may be asserted by setting a value in an associated power management field of a frame. Then, for i=1, . . . , N T  a loop involving the following activities may be entered: set the TPC to level i for all transmit chains, then loop j times for j=1, . . . , N R , setting the AGC to level j for all receive chains except chain i, sending training symbols (e.g., OFDM training symbols) having a magnitude to optimize the received signal-to-noise ratio (SNR) without saturation in the receive chains while minimizing interference with other devices. These activities may be followed with calibrating as described for the second scheme above.  
      As shown in  FIG. 5B , the training symbols  540  may be sent in a packet format to prevent nearby devices (e.g., other AP or STA devices) from interfering with calibration for the apparatus  500 . For example, the packet length field in the physical layer convergence protocol (PLCP) header  578  may be used to indicate to nearby devices that calibration is in effect, and to prevent them from transmitting during that time. Training symbols  540  may be included in the data portion of the packet  530 , where S ij  is the training symbol for TPC setting i and AGC setting j. The packet  530  may be addressed to the device  500  itself.  
      Path loss between two calibrating antennas  512  coupled to the same apparatus  500  may be about 30-40 dB, and the path loss between two apparatus  500  or devices may be about 60-90 dB. Therefore, devices not in calibration mode should be able to operate while other devices are engaged in self-calibration. However, in some cases non-calibrating devices may interfere with self-calibrating devices, because calibration and training AGC levels may be set to normal operating levels, so that interfering signals have about the same level as training signals. Such difficulties may be resolved by sending additional calibration packets during the sleep mode, since the time spent in sleep mode by some apparatus  500  may be much longer than the time spent in active mode.  
      The apparatus  100 ,  200 ,  300 ,  500 , systems  102 ,  202 ,  302 , devices  104 ,  108 ,  204 ,  208 ,  304 ,  308 , antennas  112 ,  120 ,  512 , transmit chains  114 ,  124 , receive chains  116 ,  126 , communication chains  118 ,  128 ,  518 , symbols  230 ,  234 ,  238 ,  240 ,  430 ,  434 ,  438 ,  440 ,  530 ,  540 , data  244 ,  444 , fields  448 ,  454 , bits  450 ,  458 , calibration module  160 , estimation module  162 , on-air signals  566 , switching network  570 , TPC setting  572 , gain control setting  574 , and PLCP header  578  may all be characterized as “modules” herein. Such modules may include hardware circuitry, and/or one or more processors and/or memory circuits, software program modules, including objects and collections of objects, and/or firmware, and combinations thereof, as desired by the architect of the apparatus  100 ,  200 ,  300 ,  500  and the systems  102 ,  202 ,  302 , and as appropriate for particular implementations of various embodiments.  
      It should also be understood that the apparatus and systems of various embodiments can be used in applications other than transmitters and receivers, and other than for wireless systems, and thus, various embodiments are not to be so limited. The illustrations of apparatus  100 ,  200 ,  300 ,  500  and systems  102 ,  202 ,  302  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 embodiments include electronic circuitry used in high-speed computers, communication and signal processing circuitry, modems, processor modules, 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, personal digital assistants (PDAs), workstations, radios, video players, vehicles, and others.  
       FIG. 6  is a flow chart illustrating several training and calibration methods according to various embodiments. With respect to this figure, it should be noted that any of the numbers of communication chains discussed may correspond to a number of receive chains, and/or to a number of transmit chains, as desired for particular implementations of the method  611 . Therefore, in light of the previous discussion with respect to the first scheme, it can be seen that a method  611  directed to the operation of various embodiments embodiments of the invention disclosed may (optionally) begin with receiving a request to transmit at a first number of communication chains at block  621  and determining one or more transmit power levels and/or receive gain levels associated with the first number of communication chains at block  625 . The method  611  may include transmitting a clear to transmit response and a first number of training symbols from the first number of communication chains at block  631  and calibrating some number of transmit and receive chains included in the first number of communication chains at block  635 . Thus, the method  611  may include transmitting a first number of training symbols corresponding to a first number of communication chains to solicit a response including a second number of training symbols corresponding to a second number of communication chains.  
      The method  611  may continue with receiving a clear to transmit response and the first number of training symbols at a second number of communication chains at block  641  and estimating one or more communications channels associated with the second number of communication chains based on the first number of training symbols at block  645 . The method  611  may also include transmitting the second number of training symbols and data at block  651 . Thus, the method  611  may include transmitting a second number of training symbols corresponding to a second number of communication chains in response to receiving a first number of training symbols corresponding to a first number of communication chains.  
      The method  611  may include calibrating some number of transmit and receive chains included in the second number of communication chains based on the second number of training symbols at block  655 . The method  611  may continue with receiving the second number of training symbols and data at block  661  and estimating one or more communications channels associated with the first number of communication chains based on the second number of training symbols at block  665 .  
       FIG. 7  is a flow chart illustrating several alternative training and calibration methods according to various embodiments. With respect to this figure, it should be noted that any of the numbers of communication chains discussed. may correspond to a number of receive chains, and/or to a number of transmit chains, as desired for particular implementations of the method  711 . Therefore, in light of the previous discussion with respect to the second scheme, it can be seen that a method  711  directed to the operation of various embodiments of the invention disclosed may (optionally) begin with transmitting a request to transmit and the first number of training symbols at block  721  and calibrating one or more of the first number of communication chains at block  725 . Calibrating the first number of communication chains may occur during a sleep mode. The method  711  may also include transmitting a header including a length specification corresponding to the first number of training symbols. Thus, the method  711  may include transmitting a first number of training symbols corresponding to a first number of communication chains to solicit a response including a second number of training symbols corresponding to a second number of communication chains.  
      The method  711  may continue with receiving a request to transmit and the first number of training symbols at block  731  and estimating one or more channels associated with the second number of communication chains at block  735 . The method  711  may include transmitting a clear to transmit response and the second number of training symbols at block  741  and calibrating one or more of the second number of communication chains at block  745 . Calibrating the second number of communication chains may occur during a sleep mode. Thus, the method  711  may include transmitting a second number of training symbols corresponding to a second number of communication chains in response to receiving a first number of training symbols corresponding to a first number of communication chains.  
      The method  711  may continue with receiving a clear to transmit response and the second number of training symbols at block  751  and estimating one or more channels associated with the first number of communication chains at block  755 . The method  711  may also include transmitting a header including a length specification corresponding to the second number of training symbols.  
      Turning now to  FIGS. 9-12 , additional embodiments of the inventive aspects of the invention are introduced. Recall from  FIGS. 2 and 3 , that training symbols were selectively embedded within, or characterized by, communication symbols conventionally used for other purposes (e.g., handshaking, acknowledgment, link negotiation, etc.). That is, rather than generating and issuing dedicated training symbols to effect training and calibration, we propose leveraging the transmission of “other” symbols, traditionally used for purposes other than training, in which to include training symbol(s), or as training symbols themselves. As described above, legacy handshaking packets (e.g., RTS/CTS) were but one example embodiment, wherein training symbols associated with each transmit antenna(e) were issued from both devices  204 ,  208 . In  FIGS. 9-11 , this inventive concept is extended and modified to provide further reduction in communication overhead.  
       FIG. 9  is a block diagram of an example apparatus and a system operating according to various embodiments. As introduced above, an inventive aspect of the invention is that it leverages “known packets” such as, e.g., acknowledgment packets, clear to send (CTS) packets, and the like) as training symbols for training and/or calibration. The content of the known packet is known to the recipient to a high extent. For example, in a legacy system, the content of a CTS is known to an expected recipient, i.e. the sender of the RTS, except the only uncertainty is the code rate and modulation type used in the CTS packet. According to one example embodiment, the most accurate calibration and training results are achieved when performed on an antenna by antenna basis, i.e., when a symbol is sent from a single antenna at a time. In this regard, transmission from multiple antenna(e) is introduced wherein symbol transmission is sequentially stepped through at least a subset of the antenna(e), although the invention is not limited in this regard.  
      As introduced above, each device  902 ,  904  (which may be similar to or identical to devices  104 ,  108 , respectively, as shown in  FIG. 1 , and may include an AP and/or a STA) may have multiple transmit power control (TPC) levels and multiple receive gain control levels, including automatic gain control (AGC) levels, for at least a subset of the included communication chains. Further, transmit and receive responses, α and β, may vary with selected TPC and AGC settings.  
      As shown, device  902  may send one or more symbols  908  such as a request to transmit (e.g., a legacy RTS symbol or message) to the device  904 , e.g., using a default or previously determined TPC, although the invention is not limited in this regard. According to one aspect of the invention, the transmission  908  is sent via one or more antenna(e) predicted to provide the best (as compared to the other antenna options) signal characteristic (e.g., signal to noise ratio (SNR) at the receiving device ( 904 ). The determination of which antenna(e) to send symbol(s)  908  through may be made based on prior training, or predicted without training/calibration based on an estimate of channel conditions, although the invention is not limited in this regard.  
      In response to the received symbol (e.g., the RTS), the receiving device  904  may generate a response  910 , e.g., a clear to send (CTS) symbol if/when appropriate, for transmission to device  902 . According to one aspect of the invention, device  904  introduces a training symbol to the response  910 . According to one aspect of the invention, the training symbol(s) may well be integrated within, or appended to the response  910 . Unlike the system of  FIG. 2  that utilized at least one training symbol for each of the transmit antenna(e), device  904  of  FIG. 9  may select a mere subset of the available transmit antennae through which to transmit the response  910  and associated training symbol  912 . As introduced above, the training symbol(s)  912  may well be integrated within, or appended to response  910 . Utilizing the CTS  910  and training symbol  912 , device  902  may perform channel estimations, while device  904  may perform calibration. According to one aspect of the present invention, the response  910  is sent from the antenna(e) which is perceived, or estimated, to provide the best signal characteristics at the receiving device ( 902 ), although the invention is not limited in this regard.  
      Upon receipt of the response from device  904 , e.g., the CTS symbol, device  902  processes content (e.g., data)  916  for transmission to device  904 . According to one embodiment, device  902  includes one or more training symbol(s)  914 . In accordance with the illustrated example embodiment, device  902  includes at least one training symbol for each of the antenna(e) of device  902 . According to one embodiment, the first training symbol (TI) of training symbols  914  is sent via the antenna identified as providing the best performance at the receiving device  904 , although the invention is not limited in this regard.  
      According to one embodiment, upon receipt of data  916 , device  904  issues an acknowledgement, e.g., an ACK symbol  918 . Thus, embodiments of the invention limit the training/calibration overhead associated with managing a communication channel by reducing the number of training symbols utilized by the devices, and transmitting the training symbol from only a subset of the antenna(e) of the device identified to provide the best signal characteristics at the receiver, and that such training symbols may be embedded within, or appended to, any type of conventional transmission (e.g., a CTS symbol, a data symbol, etc.).  
      Turning to  FIG. 10 , a block diagram of an example apparatus and system according to embodiments of the invention is depicted. More particularly, an apparatus and system which combines the select transmission of training symbol(s) through a select subset of transmit antenna(e) using conventional data packets (e.g., RTS/CTS) is depicted. In this regard, the apparatus and system depicted in  FIG. 10  may, in some embodiments, represent a combination of at least a subset of the inventive elements of  FIGS. 3 and 9 .  
      In  FIG. 10 , device  1002  generates a message  1010  for transmission to a remote device  1004 . According to one embodiment, the message  1010  is a request to transmit (RTS) packet. According to one aspect of the invention, the message  1010  will be sent via the antenna perceived, or estimated, to provide the best performance at the receiving device  1004 . According to one aspect of the invention, the number of training symbols  1012  and the antenna(e) from which they are sent are similarly selected from the remaining options by device  1002  to provide the best performance at the receiving device  1004 . That is, since message  1010  will be sent from the antenna deemed to provide the best performance at receiving device  1004 , the training symbols will be sent from the next best two antenna options, although the invention is not limited in this regard.  
      In accordance with conventional operation, the device  1004  receiving the RTS message will generate, a clear to send (CTS) response  1014  when it is, in fact, clear for device  1002  to continue with the transmission of data. According to one aspect of the invention, device  1004  takes the opportunity of issuing the CTS message  1014  to issue its own training symbol(s)  1016 . Utilizing the CTS  1014  and training symbol  1016 , device  1002  may perform channel estimations, and device  1004  may perform calibration. According to one aspect of the invention, the CTS  1014  is transmit from the antenna perceived, or estimated, by device  1004  to provide the best receive performance at device  1002 . According to one aspect of the invention, the number of training symbols  1016  and the antenna(e) from which they are transmit are selected by device  1004  from the remaining options to provide the best receive performance at device  1002 .  
      Upon receiving the CTS message, device  1002  proceeds with the transmission of data  1018 . According to one embodiment, device  1002  selects the antenna(e) through which the data is transmit based, at least in part, on the channel information received/perceived as a result of receiving the training symbols  1016  from device  1004 . In response to the receipt of data  1018 , device  1004  issues an acknowledgement  1020 .  
      Turning to  FIG. 11 , a block diagram of an example apparatus and system according to embodiments of the invention is presented. More particularly,  FIG. 11  illustrates a training scheme that utilizes the transmission of data packets, and subsequent acknowledgements to selectively effect training of the devices  1104 ,  1108 . According to one example embodiment,  FIG. 11  presupposes that there may be a sequence of DATA-ACK exchanges between the devices  1104 ,  1108  because, e.g., device  1104  may have a lot of data packets to download to device  1108 . Unlike the techniques introduced above that relied on conventional channel management packets (e.g., RTS/CTS) in which to transmit training symbols, the technique in  FIG. 11  does not require initiation through an RTS/CTS exchange. Rather, as shown, training symbol(s) are selectively embedded within, or appended to, an otherwise conventional DATA-ACK transmission exchange.  
      As shown, the technique begins with device  1104  generating a data packet  1130  for transmission to device  1108 . As shown, device  1104  will transmit the data packet  1130  to device  1108  with training symbols  1128  via each of the transmit antenna, although the invention is not limited in this regard.  
      In response to receipt of a data packet  1130 , device  1108  generates an acknowledgment packet (ACK)  1132  for transmission to device  1104 . According to one aspect of the invention, device  1108  generates one or more training symbol(s)  1134  to embed within, or append to, the ACK  1132 . According to one aspect of the invention, the ACK  1132  may include information regarding the antenna with the best reception quality at device  1108 , and one training symbol for each other antenna under two conditions: 1) device  1108  detects that device  1104  did not employ beamforming, or that any beamforming applied is not sufficiently accurate; and 2) device  1108  detects that more data is coming (from device  1104 ). According to one embodiment, the determination that additional data is coming may be identified from analysis of the received data packet  1130  (e.g., an indication embedded within a “more data” field of the received packet).  
      Using one antenna to send the ACK  1132  eliminates the need for one training symbol. Upon receiving the ACK  1132  and training symbol  1134 , device  1104  performs channel training and determines an appropriate transmit power control (TPC) and auto gain control (AGC) levels, e.g., in accordance with one or more techniques introduced above, although the invention is not limited in this regard.  
      After device  1104  performs initial channel training, it may issue another data packet  1138 . In accordance with the illustrated example embodiment, one or more training symbols  1136  may be embedded within, or appended to, data packet  1138 . As shown, the number of training symbols, their order, and the antenna from which each is sent may be selected by device  1104  to provide improved channel training for the receiving device  1108  based, at least in part, on the initial channel training previously performed. In this regard, the training symbols  1136  may be longer than those previously sent. Using at least these symbols  1136 , the device  1104  calibrates its transmit chains, while device  1108  may “perform channel estimations”. According to one aspect of the invention, insofar as device  1104  obtains both calibration and channel training, it may perform beamforming on the DATA  1138  portion of the second packet.  
      According to one aspect of the invention, device  1108  may well issue another training symbol along with the acknowledgment packet  1140 , the purpose of which to allow device  1104  to estimate the channel again and track variation in the channel. According to one embodiment, such additional training symbol(s) may be sent if 1) device  1108  detects that additional data may be sent from device  1104 , and/or 2) device  1108  detects a variation in the channel, although the invention is not so limited.  
      As shown, device  1104  may again issue training symbols  1142  along with a subsequent data packet  1144 , although the invention is not limited in this regard. According to one aspect of the invention, device  1104  may issue the subsequent training symbols if: 1) it detects variation in its chains, e.g., from an internal analysis of the reverse link, or if it receives an acknowledgement packet with training symbols from the remote device  1108 ; and 2) device  1108  has additional data to transmit to device  1108 , although the invention is not limited in this regard.  
      Turning now to  FIG. 12 , a block diagram of an example apparatus and system according to embodiments of the invention is depicted. More particularly, according to one example embodiment of the invention,  FIG. 12  illustrates an example implementation which is an extension to the embodiment of  FIG. 11  where, in responding to the receipt of data from a remote device ( 1204 ), a receiving device ( 1206 ) issues a data packet and an acknowledgment packet  1242 . In this regard, to send data from device  1206  using beamforming, the device may utilize channel training symbol(s) ( 1232 ,  1234 ) previously sent to device  1204 , e.g., in response to receipt of a first data packet ( 1230 ). According to one embodiment, the device may use a “piggy-back” mechanism to send the data  1242  as shown in  FIG. 12 , or it may use an ordinary data packet. According to one embodiment, the DATA+ACK packet  1242  may be similar to a CF−ACK+DATA packet used in the point coordination function (PCF) of an 802.11 media access controller (MAC), although the invention is not limited in this regard.  
      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 serial or parallel fashion. For the purposes of this document, the terms “information” and “data” may be used interchangeably. 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, Smalltalk, 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 any of a number of mechanisms well-known to those skilled in the art, such as application program interfaces or inter-process communication techniques, including remote procedure calls. The teachings of various embodiments are not limited to any particular programming language or environment, including Hypertext Markup Language (HTML) and Extensible Markup Language (XML). Thus, other embodiments may be realized.  
      For example,  FIG. 8  is a block diagram of an article  885  according to various embodiments, such as 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  885  may comprise a processor  887  coupled to a machine-accessible medium such as a memory  889  (e.g., a memory including an electrical, optical, or electromagnetic conductor) having associated information  891  (e.g., data or computer program instructions), which when accessed, results in a machine (e.g., the processor  887 ) performing such actions as transmitting a second number of training symbols corresponding to a second number of communication chains in response to receiving a first number of training symbols corresponding to a first number of communication chains. Other activities may include receiving a clear to transmit response and the first number of training symbols at the second number of communication chains, and estimating one or more communications channels associated with the second number of communication chains based on the first number of training symbols. Further activities may include transmitting the second number of training symbols and data, and calibrating some number of transmit and receive chains included in the second number of communication chains based on the second number of training symbols.  
      In some embodiments, an article including a machine-accessible medium having associated information, wherein the information, when accessed, results in a machine performing such activities as transmitting a first number of training symbols corresponding to a first number of communication chains to solicit a response including a second number of training symbols corresponding to a second number of communication chains. Additional activities may include transmitting a request to transmit and the first number of training symbols, and calibrating the first number of communication chains. Further activities may include receiving a clear to transmit response and the second number of training symbols, and estimating one or more channels associated with the first number of communication chains.  
      Implementing the apparatus, systems, and methods described herein may result in reducing the overhead used for calibration and training of various devices, including those forming a portion of a MIMO system. For packet sizes of approximately 500-1500 bytes, improvements in efficiency may be on the order of 30%-50%. Thus, this type of operation may in turn provide improved bandwidth utilization, and reduced communication costs.  
      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 is 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 if 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.