Abstract:
Systems and methods of controlling transmission of communications from a base station to a wireless communication station are provided. When the signal quality of communications between the base station and the wireless communication station are above a predetermined signal quality level, multiple antennas are employed for transmitting such communications. When the signal quality of transmissions by the base station to the wireless communication station are below a predetermined signal quality level, a single antenna is employed for transmitting such communications.

Description:
BACKGROUND OF THE INVENTION 
     In wireless communication systems base stations and wireless communication stations communicate with each other using radio frequency signals that are transmitted and received using antennas. Conventionally, base stations and wireless communication stations used a single antenna for transmitting and receiving signals. Recently, multiple input-multiple output (MIMO) techniques have been employed in which multiple antennas are used for transmitting and receiving signals. Processing circuitry coupled to the multiple antennas combine the signals received over the antennas and/or provide copies of the signals to the multiple antennas for transmission. 
     SUMMARY OF THE INVENTION 
     Typically, wireless communication systems that employ multiple antennas use multiple antennas for all communications. For example, in WiMAX when a wireless communication station is experiencing good radio frequency conditions, e.g., a high signal-to-noise ratio (SNR), then the base station transmits communications to the wireless communication station over two antennas using spatial multiplexing (SM), and when the wireless communication station is experiencing poor radio frequency conditions, then the base station transmits communications to the wireless communication station over two antennas using space-time coding (STC). In either situation, the base station transmits communications over both antennas at the same power level. 
     It has been recognized, however, that when a wireless communication station is located at the edge of the base station&#39;s coverage area and when the signal quality is low, it is preferable to use a single antenna. For example, when STC is used for a wireless communication station with poor signal quality, the effective power can be 10 Watts, even though each of the two antennas are fed with 10 Watts of power, whereas when the wireless communication station has good signal quality, the effective power can be 20 Watts, i.e., the sum of the 10 Watts of power used for each of the two antennas. Accordingly, in accordance with exemplary embodiments of the present invention, when the signal quality is low, then a single antenna is employed for transmissions to the wireless communication station. 
     Accordingly, exemplary embodiments of the present invention provide systems and methods for selecting antennas for communications between a base station and a wireless communication station. Specifically, the base station can transmit signals to the wireless communication station using a first communication mode in which multiple antennas are employed. When the signal quality is equal to, or less than, a signal quality threshold, the base station can then transmit signals to the wireless communication station using a second communication mode in which a single antenna is employed. In the second communication mode the power level used for transmissions over the single antenna can be a sum of the power levels used for transmissions over the multiple antennas. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  is a block diagram of an exemplary base station in accordance with the present invention; 
         FIGS. 2A and 2B  are flow diagrams of exemplary methods in accordance with the present invention; and 
         FIGS. 3A and 3B  are block diagrams of exemplary systems in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a block diagram of an exemplary base station in accordance with the present invention. Base station  100  includes processor  120 , which is coupled to memory  105  and transceiver  110 . Transceiver  110  is coupled to at least two antennas  115   a  and  115   b . Processor  120  includes logic  122 - 126 , which will be described in more detail below in connection with  FIGS. 2A and 2B . Processor  120  can be any type of processor, such as a microprocessor, field programmable gate array (FPGA) and/or an application specific integrated circuit (ASIC). When processor  120  is a microprocessor then logic  122 - 126  can be processor-executable code loaded from memory  105 . Base station  100  can communicate with mobile stations in accordance with, for example, WiMAX protocols. 
     Although.  FIG. 1  illustrates only two antennas  115   a  and  115   b  being coupled to transceiver  110 , more than two antennas can be employed. 
       FIGS. 2A and 2B  are flow diagrams of exemplary methods in accordance with the present invention. Referring now to  FIG. 2A , base station  100  communicates with a wireless communication station in one of a first or second communication mode (step  205 ). The first communication mode can employ more than one antenna for transmitting communications to the wireless communication station, and the second communication mode can employ a single antenna for transmitting communications to the wireless communication station.  FIG. 3A  is a block diagram of base station  100  transmitting communications to wireless communication station  205  in the first communication mode and  FIG. 3B  is a block diagram of base station  100  transmitting communications to wireless communication station  305  in the second communication mode. As illustrated in  FIG. 3A , in the first communication mode base station  100  transmits communications to wireless communication station  305  at a power level of X Watts over each of the antennas  115   a  and  115   b . As illustrated in  FIG. 3B , in the second communication mode base station  100  transmits communications to wireless communication station  305  over antenna  115   a  at a power level of X+X Watts, i.e., a sum of the power levels used for transmitting over the two antennas. As also illustrated, antenna  115   a  is used for both transmitting to, and receiving from, wireless communication station  305 , while antenna  115   b  is only used for receiving from wireless communication station  305 . 
     Returning now to  FIG. 2A , while communicating in one of the two communication modes, logic  124  monitors the signal quality (step  210 ) and determines whether the signal quality is equal to, or less than, a predetermined signal quality (step  215 ). The predetermined signal quality can be, for example, 12-15 dB. However, the actual predetermined signal quality can be selected depending upon implementation. The signal quality can be measured by, and/or calculated by, the base station and/or the wireless communication station. The present invention can use any type of signal quality measure, including, but not limited to, signal-to-noise ratio (SNR), signal to-interference-plus noise ratio (SINR), carrier to interference-plus-noise ratio (CINR), and/or the like. When the signal quality is greater than the predetermined signal quality (“No” path out of decision step  215 ), the base station continues to transmit communications to the wireless communication station in the currently selected communication mode, as indicated by logic  122  (step  205 ). 
     If, however, the signal quality is equal to, or less than, the predetermined signal quality (“Yes” path out of decision step  215 ), then logic  122  determines whether the base station has previously switched modes of communication when transmitting to the mobile station (step  220 ). When the base station has not previously switched modes (“No” path out of decision step  220 ), then logic  122  causes base station  100  to switch communication modes (step  225 ), and base station  100  transmits communications to the wireless communication station in the selected communication mode (step  205 ). 
     When the base station has previously switched communication modes when transmitting to the wireless communication station (“Yes” path out of decision step  220 ), then logic  124  determines whether the monitored signal quality is less than the predetermined signal quality by less than a predetermined amount (step  230 ). The predetermined amount is a hysteresis value that is used to prevent the switching of between communication modes when there has been only a small change in the signal quality. However, this step can be omitted, if desired. As an alternative to, or in addition to, using a predetermined amount of signal quality as a hysteresis value, a predetermined amount of time in which the signal quality is equal to, or less than, the threshold can be employed. 
     When the monitored signal quality is less than the predetermined signal quality by less than the predetermined amount (“No” path out of decision step  230 ), then base station  100  continues to transmit communications to the wireless communication station using the currently selected communication mode as indicated by logic  122  (step  205 ). When the monitored signal quality is less than the predetermined signal quality by more than the predetermined amount (“Yes” path out of decision step  230 ), then logic  122  causes base station  100  to switch communication modes (step  225 ), and the base station transmits communications to the wireless communication station using the newly selected communication mode (step  205 ). 
     The method of  FIG. 2B  is similar to that of  FIG. 2A , with the addition of a distance determination in step  235 . Specifically, when the signal quality is less than the predetermined signal quality by less than a predetermined amount (“Yes” path out of decision step  230 ), then logic  126  determines a distance between the wireless communication station and base station  100 , and whether the distance is greater than or equal to a predetermined distance (step  235 ). When the determined distance is less than the predetermined distance (“No” path out of decision step  235 ), then base station  100  continues to transmit communications to the wireless communication station using the currently selected communication mode (step  205 ). If, however, the distance is equal to or greater than the predetermined distance (“Yes” path out of decision step  235 ), then logic  122  switches communication modes (step  225 ) and base station  100  transmits communications to the wireless communication station using the currently selected communication mode (step  205 ). 
     Step  235  can be added to the method of  FIG. 2A  to avoid switching communication modes due to changes in signal quality that are not related to distance from the base station. Specifically, signal quality may deteriorate due to fading, interference and/or obstructions, and it may be desired to switch from multiple antennas to a single antenna only when the signal quality changes are due to distance. 
     It should be recognized that if the method of  FIG. 2A  is employed, base station  100  would not need to include logic  126 , but could, if desired. Moreover, it should be recognized that the steps of  FIGS. 2A and 2B  need not necessarily be performed in the particular order as illustrated. For example, step  220  can be performed before step  215 . Similarly, step  235  can be performed before step  215 . Furthermore, step  235  can replace step  215 . In this case, step  230  can be omitted. 
     Although  FIGS. 2A and 2B  have been described as using a determination of whether the signal quality is equal to, or less than, a predetermined signal quality, the present invention can instead employ a determination of whether the signal quality is merely less than a predetermined signal quality. Similarly, the distance determination can employ a criteria of whether the wireless communication station is at a distance greater than a predetermined distance, instead of greater than, or equal to, the predetermined distance. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.