Abstract:
In a communication device compatible with either a SISO or MIMO access point, a set of antennas is configured as either sector-directional or omni-directional, depending upon whether the communication channel is characterized by the communication device as either strongly line of sight, or strongly scattering, respectively. In some embodiments, for the case in which a MIMO access point is utilized, the singular values of an estimated channel matrix are generated, and based upon the singular values, the characteristics of the communication channel is estimated. Other embodiments are described and claimed.

Description:
FIELD 
   Embodiments relate to digital communication systems, and more particularly, to multiple-antenna communication systems. 
   BACKGROUND 
   Multi-path wireless channels are capable of large channel capacities, and may be properly exploited through the use of an MIMO (Multiple-Input-Multiple-Output) communication system. An MIMO system employs multiple transmit antennas and multiple receive antennas. Standard IEEE 802.16e, sometimes referred to as Mobile Worldwide Interoperability for Microwave Access (Mobile WiMAX), supports MIMO antenna technology. Future wireless networks will also support MIMO antenna technology. There have been efforts to introduce a unified proposal for the next generation of high performance wireless networks, developed under the guidelines of the IEEE Standards Association and submitted to the IEEE 802.11n Task Group N (TGn). One of the goals is to enable MIMO Spatial Division Multiplexing to provide reliable transmission speeds of up to 315 megabits per second (Mbps) with two antennas, and up to 630 Mbps with larger systems employing more than two antennas. 
   Access point density in an enterprise WLAN (Wireless Local Area Network) is expected to increase to where the typical distance between an access point and a client may at times be 10 feet or less. As a result, the effect of line-of-sight (LOS) communication is expected to become important. A strong LOS communication link increases spatial signal correlation, so as to diminish the benefit of an MIMO system. 
   Accordingly, it is useful to provide communication technology in a multiple antenna client that may be configured to perform well when a LOS communication path is dominant. Furthermore, because upgrading an access point to a MIMO system is more expensive than for a client, it is expected that there will be a period of time for which access points have not yet been upgraded from a SISO (Single-Input-Single-Output) system to a MIMO system. Consequently, it is also useful to provide communication technology in which a multiple antenna client is easily configured whether the access point is a SISO system or an MIMO system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a prior art communication system. 
       FIG. 2  illustrates a prior art communication channel. 
       FIG. 3  illustrates a computer system according to an embodiment of the present invention. 
       FIG. 4  is a flow diagram illustrating the processing of an antenna controller according to an embodiment of the present invention. 
   

   DESCRIPTION OF EMBODIMENTS 
   In the descriptions that follow, the scope of the term “some embodiments” is not to be so limited as to mean more than one embodiment, but rather, the scope may include one embodiment, more than one embodiment, or perhaps all embodiments. 
   Embodiments configure multiple antennas as either omni-directional or sector-directional, depending upon whether a communication channel is characterized as having a strong LOS component or a strong scattering component. When characterized as a LOS communication channel, the multiple antennas are configured as sector-directional. In some embodiments, when configured as sector-directional, each antenna effectively has an antenna pattern beamwidth covering 2π/m radians in azimuth, where m is the number of antennas. 
   Embodiments may find utility in a client (e.g., notebook computer) or in an access point, where the channel matrix may be an uplink channel matrix (from the client to the access point), or a down link channel matrix (from the access point to the client). A communication channel may be defined in various ways. Before describing an embodiment, it is useful to first consider one possible definition of a communication channel for an MIMO system, and its corresponding channel matrix. 
     FIG. 1  is a high-level system diagram of a portion of a MIMO system utilizing n transmit antennas  102  and m receive antennas  104 . The symbols d i , i=1, . . . , k represent k complex-valued data quantities to be transmitted. These data quantities may arise by de-multiplexing one or more data stream into k data streams, in which coding may have been applied. 
   Vector Encoding functional unit  106  encodes d i ,i=1, . . . , k into the n complex-valued quantities x i , i=1, . . . , n. Defining  d   D  as a k-dimensional column vector having components [  d   D ] i =d i ,i=1, . . . , k, and defining  x   D  as an n-dimensional column vector having components [  x   D ] i =x i ,i=1, . . . , n, one may write the vector encoding as
 
  x   D =T D   d   D ,
 
where T D  denotes a complex-valued n by k matrix. (The significance of the subscript D will be discussed later.)
 
   The complex-valued quantities x i ,i=1, . . . , n represent the in-phase and quadrature components of a baseband signal, such as a voltage signal, to be transmitted over a channel. Functional units  108  (e.g., transmitters) indicate modulators for up-converting a baseband signal to an RF (radio-frequency) signal before transmission by antennas  102 , although the scope of the embodiment is not limited in this regard. 
   Receivers  110  down-convert the received signals provided by antennas  104  into the m complex-valued baseband signals y i ,i=1, . . . , m. Vector Decoding functional unit  112  indicates that the m complex-valued baseband signals y i , i=1, . . . , m are decoded into the k complex-valued baseband signals {circumflex over (d)} i ,i=1, . . . , k. Defining 
               d   ^     _     D         
as a k-dimensional column vector having components
 
                 [         d   ^     _     D     ]     i     =       d   ^     i       ,     i   =   1     ,   …   ⁢           ,   k   ,         
and defining  y   D  as an m-dimensional column vector having components [  y   D ] i =y i ,i=1, . . . , m, one may write the vector decoding as
 
                   d   ^     _     D     =       R   D     ⁢       y   _     D         ,         
where R D  denotes a complex-valued k by m matrix. It is desirable that the quantities {circumflex over (d)} i ,i=1, . . . , k are in some sense a “good” estimate of d i ,i=1, . . . , k.
 
   There are various ways to define a communication channel. In  FIG. 1 , a communication channel may be defined to include the components within dashed box  114 . For this model of a communication channel, the channel inputs are x i ,i=1, . . . , n and the channel outputs are y i ,i=1, . . . , m. If Vector Encoding  106  and transmitters  108  are associated with an access point, and Vector Decoding  112  and receivers  110  are associated with a client, then the channel defined by box  114  may be referred to as the downlink channel. If on the other hand, Vector Encoding  106  and transmitters  108  are associated with a client, and Vector Decoding  112  and receivers  110  are associated with an access point, then the channel defined by box  114  may be referred to as the uplink channel. Although  FIG. 1  shows only a one-way channel, in practice there is a downlink channel in addition to an uplink channel. For convenience, the channel in  FIG. 1  will be referred to as a downlink channel. This is the reason for using the subscript D in the above discussion. 
   Although the above example is described with respect to a client and an access point, the methods and apparatuses described herein may be readily applicable to other communication devices, such as subscriber stations and base stations, for example. 
   The downlink channel defined by box  114  may be abstracted as shown in  FIG. 2A . For simplicity, the downlink channel depicted in  FIG. 2A  is a stationary, noiseless channel. However, in practice, there will be noise sources, and the channel transfer function may be fading. 
   In  FIG. 2A , h ij ,i=1, . . . , m; j=1, . . . , n are complex-valued scalars representing the channel gains due to the gains of the transmit antennas, receive antennas, transmitters, and receivers. It is convenient to define h ij  as
 
 h   ij =( Tx   j )( TG   ji ) w   ji ( RG   ji )( Ry   1 ),
 
where Tx j  is the gain of the transmitter for symbol x j ; TG ji  is the antenna gain for the transmit antenna associated with x j  in the direction toward the receive antenna associated with symbol y i ; RG ji  is the antenna gain for the receive antenna associated with symbol y i  for a signal received from the direction of the antenna associated with symbol x j ; Ry i  is the gain for the receiver for the symbol y j ; and w ji  is the response of the physical transmission medium between transmit antenna j and receive antenna i.
 
   Defining the m by n downlink channel matrix H to have components [H] ij =h ij ,i=1, . . . , m; j=1, . . . , n, the input-output relationship defined by the downlink channel matrix H is
 
  y   D =H  x   D .
 
   In general, the uplink channel matrix, where y i ,i=1, . . . , m are now being transmitted and x i ,i=1, . . . , n are received, is different from H. This is so because for the uplink channel, receivers are used to generate the x i ,i=1, . . . , n instead of transmitters  108 , and transmitters are used to generate the y i ,i=1, . . . , m instead of receivers  110 , and as a result, the overall channel gains may be different. However, if one assumes that the channel is calibrated to take into account the differences in transmitter and receiver gains for uplink and downlink communication, then the same channel gains h ij ,i=1, . . . , m; j=1, . . . ,n as indicated in  FIG. 2A  also hold for the uplink channel of  FIG. 2B . With this assumption, the two-way channel is said to be reciprocal, and the input-output relationship for the uplink channel is given by
 
  x   U =H T   y   U ,
 
where  T  denotes transposition, and the subscript U denotes uplink communication.
 
   Embodiments may find application in an access point as well as in a client. Without loss of generality, subscripts distinguishing a downlink quantity from an uplink quantity may be dropped. Accordingly, in describing the embodiments, for convenience the downlink communication channel matrix is considered, so that the input-output channel matrix transformation is written simply as  y =H  x , and adaptive antenna configuration is performed on the client side. It should be appreciated that the embodiments are not necessarily limited to a client, but may find utility on both sides of a communication channel. 
   An application of an embodiment to a computer system is illustrated in  FIG. 3 .  FIG. 3 . illustrates, in simplified form, a portion of a computer system, comprising microprocessor  302 , chipset  304 , system memory  306 , RF (Radio Frequency) module  308 , and adaptive antennas  310 . In the particular embodiment of  FIG. 3 , chipset  304  includes MCH (Memory Controller Hub)  304 A and ICH (Input/Output Controller Hub)  304 B. Microprocessor  302  communicates with chipset  304  via front-side bus  312 . MCH  304 A serves as a controller for system memory  306 . Integrated in ICH  304 B is MAC (Media Access Control) layer  314 , baseband module  316 , and antenna controller  318 . The protocol layers higher than MAC layer  314  are not explicitly shown. 
   In the particular embodiment of  FIG. 3 , RF module  308  is part of the PHY (Physical) layer in the communication protocol stack. RF module  308  down-converts the RF signals received by adaptive antennas  310  to baseband signals, which are provided to baseband module  316 . Antenna controller  318  processes the baseband signals, and configures adaptive antennas  310  as either omni-directional or sector-directional, depending upon how the communication channel is characterized. 
   For some computer systems, MAC layer  314  and baseband module  316  may reside off chipset  304 . Chipset  304  may itself comprise one or more discrete integrated chips, or some or all of its functional units may be integrated on microprocessor  302 . System memory  306  may comprise a hierarchical structured memory, where some portions may be integrated on microprocessor  302 . 
   For some embodiments, antenna controller  318 , or portions thereof, may be implemented as an ASIC (Application Specific Integrated Circuit) integrated within chipset  304 , on a discrete chip off chipset  304 , or as a software module running on microprocessor  302 . The processing performed by antenna controller  318  is described in reference to  FIG. 4 . 
     FIG. 4  is a flow diagram illustrating the processing performed by antenna controller  318  according to an embodiment. A timer is used, so that each time the timer expires, control is brought to block  402  in which each antenna is switched to its omni-directional mode. Block  404  directs which branch in the flow diagram is taken, depending upon whether the access point supports MIMO. This information is transmitted by the access point, so that it is available to the client. If MIMO is supported, then control is brought to block  406  in which the channel is characterized as either a strongly scattering channel or strongly LOS channel. In some embodiments, this characterization of the channel is determined by analyzing the channel matrix H. 
   The channel matrix H may be analyzed in several ways. For some embodiments, the rank of H is determined. A low rank channel matrix indicates a communication channel with weak scattering. Accordingly, for some embodiments, the communication channel is characterized as having a strong scattering component if
 
rank{H}&gt;N,
 
where N is a positive integer threshold.
 
   It should be noted that in practice, the channel matrix H is not known a priori, but is estimated. Accordingly, the symbol H is to be understood as representing either the actual channel matrix, or an estimate of the channel matrix, and it should be clear from context which representation applies. Various methods may be used to estimate the channel matrix. For example, the columns of the channel matrix H may easily be observed by transmitting a  x  such that only one component of  x  is non-zero. For example, if the first component of  x  is 1 and all the other components are zero, then (in the noiseless case)  y  is the first column of H. To estimate H in the presence of noise, more than one measurement may be taken, followed by a sample-data average of the measurements. 
   It also should be noted that if H where known a priori, in practice it will generally be full rank even if there is no scattering. Accordingly, it is to be understood that the expression rank{H} may be interpreted to represent the number of singular values of H that are greater than some threshold ε. That is, the m by n matrix H may be decomposed as 
             H   =       ∑     i   =   1       min   ⁢     {     m   ,   n     }         ⁢       σ   i     ⁢     u   i     ⁢     v   i   H           ,         
where σ 1 ≧σ 2 ≧ . . . σ min{m,n} ≧0 are the singular values of H, u i  is the i th  column of an m by m unitary matrix U, and v i  is the i th  column of an n by n unitary matrix V. In the expression for the singular value decomposition, the superscript H denotes complex conjugate transposition, a mnemonic for Hermitian, and should not be confused with the channel matrix. The rank of H may be defined as the maximum index j for which σ j &gt;ε, where ε is some suitably chosen small number, which may be found by simulation or experiment to provide acceptable performance.
 
   In some other embodiments, the communication channel may be characterized by observing the distribution of singular values. For example, the communication channel may be characterized as strongly LOS if 
                 σ   1         ∑     j   ≠   1               ⁢     σ   j         &gt;   λ     ,         
where λ is some threshold. (We have assumed that the singular values are ordered so that σ 1  is the maximum of the singular values.)
 
   Other methods for observing the distribution of singular values may be implemented. Generally stated, a communication channel may be characterized as strongly LOS if most of the energy of H is concentrated among a relatively small number of singular values. That is, if 
             ∑     i   =   1     N     ⁢       σ   i     ⁢     u   i     ⁢     v   i   H             
is close to H, where N&lt;&lt;min{m, n}, then the channel may be characterized as strongly LOS.
 
   If in block  406  it is determined that the channel is strongly LOS, then control is brought to block  408  in which the antennas are configured to be sector-directional. In block  410 , one of the sector-directional antennas is chosen to be used for communication to the access point, and in block  412  the timer is re-set and activated, whereby control is brought to block  414 . 
   If, however, block  406  determines that the communication channel is not strongly LOS, then control is brought to block  415 , whereby antennas  310  are configured to be omni-directional. In this way, the advantages of the MIMO system are exploited. Control is then brought to block  412 . 
   If block  404  determines that the access point does not support MIMO, e.g., a SISO transmitter, then control is brought from block  404  to block  416 . Block  416  characterizes the channel as either strongly LOS or strongly scattering. However, in general block  416  will employ a different algorithm than block  406 . This is so because H in this case is an m by 1 vector, which is rank  1 . 
   For some embodiments, the channel is characterized by observing the cross-correlation of channel gains from one antenna to another. For example, the sample data averages of the products h i h j  for i≠j, denoted as E{h i h j }, are formed, where because H in this case is an m by 1 vector, h i  is the i th  component of H. E{ } denotes an ensemble average, but in practice, the ensemble average of a random variable is estimated by forming the average of a large number of samples of the random variable, i.e., a sample average. Accordingly, E{ } will also be used to denote a sample average. For some embodiments, the communication channel is characterized as strongly LOS if 
                 ∑     l   ≠   j               ⁢          E   ⁢     {       h   i     ⁢     h   j       }              &gt;   Λ     ,         
where Λ is some threshold.
 
   If the communication channel is characterized as strongly LOS, then control is brought to block  408 . If, however, block  416  determines that the communication channel is scattering, then control is brought to block  415 . 
   Various methods may be employed by block  410  to choose one of the multiple antennas for communication with the access point. As one simple example, a beacon may be received from the access point, and the signal strength is sampled at each antenna. The antenna providing the largest signal power is chosen. 
   By choosing a single sector-directional antenna when the benefits of MIMO are diminished due to a strongly LOS channel, it is expected that interference to or from adjacent clients will be reduced, thus increasing spatial re-use, or channel re-usability in the same contention space, and thereby contributing to a higher channel capacity. Whereas for those cases in which MIMO provides a benefit, such as a communication channel characterized as strongly scattering, the antennas are configured as omni-directional so as to take advantage of the MIMO system. 
   Adaptive antennas  310  may be designed by standard antenna design principles. Each adaptive antenna  310  may comprise an array of simple antenna elements, such as dipoles. An array may be a phased-array, so that it may be configured to be omni-directional or sector-directional, depending upon the phase shifts applied to the individual array elements and the way in which their outputs are summed. The phase shifting and summing may be performed in the RF domain, or baseband domain, or in the digital domain after the baseband signals are quantized and represented as digital numbers. 
   Various modifications may be made to the disclosed embodiments without departing from the scope of the invention as claimed below. 
   Various mathematical relationships may be used to describe relationships among one or more quantities. For example, a mathematical relationship or mathematical transformation may express a relationship by which a quantity is derived from one or more other quantities by way of various mathematical operations, such as addition, subtraction, multiplication, division, etc. Or, a mathematical relationship may indicate that a quantity is larger, smaller, or equal to another quantity. These relationships and transformations are in practice not satisfied exactly, and should therefore be interpreted as “designed for” relationships and transformations. One of ordinary skill in the art may design various working embodiments to satisfy various mathematical relationships or transformations, but these relationships or transformations can only be met within the tolerances of the technology available to the practitioner. 
   Accordingly, in the following claims, it is to be understood that claimed mathematical relationships or transformations can in practice only be met within the tolerances or precision of the technology available to the practitioner, and that the scope of the claimed subject matter includes those embodiments that substantially satisfy the mathematical relationships or transformations so claimed.