Abstract:
A method and apparatus for managing beam selection in a mobile station. The mobile station determines that, using a currently selected beam, the mobile station would have to transmit at too high of a power level in order to maintain a balance between a forward link path and a reverse link path. The mobile station computes a transmit power on the currently selected beam and compares the computed transmit power to a predefined threshold value. If the computed transmit power exceeds the predefined threshold value, the mobile station may switch to another beam. Otherwise, the mobile station will stay with the current beam, regardless of whether the current beam has the highest receive power.

Description:
FIELD OF THE INVENTION 
   The present invention relates to wireless communications and, more particularly, to management of air interface communications between a mobile station and a base station. 
   DESCRIPTION OF RELATED ART 
   Cellular wireless is an increasingly popular means of personal communication in the modern world. People are using cellular wireless networks for the exchange of voice and data over cellular telephones, personal digital assistants (“PDAs”), cellular telephone modems, and other devices. In principle, a user can seek information over the Internet or call anyone over the public switched telephone network (“PSTN”) from any place inside the coverage area of the cellular wireless network. 
   In a typical cellular wireless system, an area is divided geographically into a number of wireless coverage areas (such as cells and cell sectors), each defined by a radio frequency (“RF”) radiation pattern from a respective base transceiver station (“BTS”) antenna. The base station antennae in the wireless coverage areas are in turn coupled to a base station controller (“BSC”), which is then coupled to a telecommunications switch or gateway that provides connectivity with a transport network. For instance, the BSC may be coupled with a mobile switching center (“MSC”), which provides connectivity with the PSTN, and/or the BSC may be coupled with a packet data serving node (PDSN) that provides connectivity with the Internet. 
   When a mobile station (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) (“MS”) is positioned in wireless coverage area, the MS and BTS can communicate with each other in various channels over the RF air interface. Communications from the BTS to an MS are considered to be in a “forward” direction, so the air interface channels used to carry such communications are referred to as the “forward link” channels. Conversely, communications from an MS to the BTS are considered to be in a “reverse” direction, so the air interface channels used to carry such communications are referred to as “reverse link” channels. 
   Traditional mobile stations include a fixed position antenna that radiates to provide both forward link and reverse link coverage. Typically, such an antenna would provide largely omni-directional or 360 degree coverage, to enable the mobile station to communicate with a base station regardless of the mobile station&#39;s physical orientation. Unfortunately, however, such a fixed beam antenna configuration tends to sacrifice signal strength for breadth of coverage. 
   To provide improved coverage, newer mobile stations may include an adaptive antenna system, such as a switched beam antenna. A switched beam antenna comprises a number of antenna elements that can be applied in various combinations and weights in order to produce different beams pointing in different directions. 
   A typical switched beam antenna, for instance, may include an array of antenna elements, and RF control logic in the mobile station may add together signals from some or all of the elements in phase or out of phase to produce a single beam direction. By applying complex weights and changing the combinations of antenna elements and phases, the mobile station can then change the beam direction. 
   Typically, a switched beam antenna system will define a discrete number of beams, each with a predefined combination or use of antenna elements. In operation, a mobile station equipped with such a system will periodically select a desired beam for use on both the forward and reverse link, by testing a received signal on all beams and selecting the beam that provides the highest received signal strength. 
   SUMMARY 
   The present invention stems from a realization that having a mobile station switch from one beam to another can, at least in theory, disrupt communications and be otherwise undesirable. The invention helps to solve that problem by conditioning a beam switch upon a determination that, using the currently selected beam, the mobile station would have to transmit at too high of a power level in order to maintain a balance between the forward and reverse links. 
   More particularly, when the mobile station is operating with a given one of its beams (e.g., one selected because it has the highest receive power), the mobile station will receive from the base station a set of control information including an indication of the base station&#39;s receive power and the base station&#39;s transmit power. Given knowledge of the base station&#39;s transmit power, the mobile station&#39;s receive power, and the base station&#39;s receive power, the mobile station can compute what mobile station transmit power would be necessary in order to balance the forward link path loss with the reverse link path loss—assuming (whether right or wrong) that forward and reverse transmission characteristics are substantially equivalent. 
   If the computed mobile station transmit power on the current beam would exceed a predefined threshold value (such as a maximum transmission power of the beam, or a designated percentage of that maximum transmission power), then the mobile station may switch to another beam, such as the beam that has the highest receive power. Otherwise, the mobile station will stay with the current beam, regardless of whether the current beam has the highest receive power. 
   This as well as other aspects, advantages, and alternatives will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. Further, it should be understood that the foregoing summary and the description provided below are set forth for purposes of example only and that many variations are possible, within the scope of the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the present invention is described herein with reference to the drawings, in which: 
       FIG. 1  illustrates a block diagram of a cellular wireless network arranged to carry out the exemplary embodiment; 
       FIG. 2  illustrates multiple beams enabling communication between a mobile station and a base station in accordance with the exemplary embodiment; 
       FIG. 3  illustrates operational parameters related to signaling on the communication path in accordance with the exemplary embodiment, and; 
       FIG. 4  depicts a flow chart of functions in accordance with the exemplary embodiment. 
   

   DETAILED DESCRIPTION 
   1. Exemplary Architecture 
     FIG. 1  illustrates a simplified block diagram of a cellular wireless network in which an exemplary embodiment of the present invention may be employed. As shown in  FIG. 1 , the network may include a mobile station (MS)  102 , a base station  112 , a mobile switching center (MSC)  118 , a packet data serving node (PDSN)  120 , a public switched telephone network (PSTN)  122 , and an internet protocol (IP) network  124 . The base station  112  may be segmented into base transceiver station (BTS)  114 , and a base station controller (BSC)  116 . Further, the base station  112 , MSC  118 , and PDSN  120  may be collectively referred to as a radio access network (RAN). The arrangement and functionality of these components are well known in the art and therefore will not be described here in detail. 
   Preferably, the mobile station  102  includes a switched beam antenna  104 , a wireless communication interface  106 , a processing unit  108 , and a data storage  110 . The switched beam antenna  104  (also referred to as a switched beam antenna system  104 ) includes a number of antenna elements that can be applied in various combinations and weights in order to produce different beams pointing in different directions, allowing communication with base station  112 . 
   The processing unit  108  may include one or more processors, such as a general purpose processor, special purpose processor, and/or a digital signal processor. Other types of processors may also be used. 
   The wireless communication interface  106  may be a wireless chipset, enabling air interface communication with a base station  112  according to a CDMA air interface protocol as set forth in TIA/EIA-95 or TIA/EIA/IS-2000. Other protocols may also be used. 
   The data storage  110  may be any medium or media, such as any volatile or non-volatile mass storage system, such as disc, tape storage drive, memory or other storage means readable by the wireless communication interface  106 . Further, the data storage  22  may be an add-on module that is temporarily situated within or otherwise connected with the mobile station  102 . The data storage  22  may be used to store data and/or machine-readable instructions. 
   The processing unit  108  may execute RF control logic (e.g., software routine and/or machine code) stored in data storage  110  to add together signals from some or all of the elements in phase or out of phase to produce a single beam direction on the switched beam antenna  104 . 
   The mobile station  102  and the BTS  114  may use another technology, such as AMPS, TDMA, DECT, GSM, PCS, or PWT; the cellular technology used is not necessarily critical to the functioning of the present invention. 
   The BTS  114  preferably includes a transceiver and an antenna for communicating with mobile station  102 . Further, the BTS  114  antenna may also be coupled to BSC  116 . It is not necessary that BTS  114  and BSC  116  be separate entities, since the functionality of both a BTS  114  and BSC  116  may be integrated into one unit. 
   The MSC  118  may serve as an interface between base station  112  and PSTN  122 . Similarly, PDSN  120  may serve as an interface between base station  112  and an IP network  124 , such as an Intranet or the Internet. It is not necessary that BSC  116  and MSC  118  be separate entities, since the functionality of both a BSC  116  and an MSC  118  could be integrated into one unit. 
   For clarity only, multiple network entities, such as BTSs and BSCs, have been omitted from the drawings, although normally a network in which the invention may be implemented would include, for example, more than one BTS  114 , MSC  118 , mobile station  102 , etc. 
   Referring next to  FIG. 2 , an illustration  200  of multi-beam communication between the mobile station  102  and the base station  112  is provided. Beams A, B, C, D, E, F, G, and H are transmitted to enable communication between mobile station  102  and base station  112 . The mobile station  102  is preferably equipped with a switched beam antenna  104 . The base station  112  antenna may transmit beams A, B, C, and D to the mobile station  102  antenna in a forward direction, such that the air interface channels used to carry such communications may be referred to as forward link path channels. On the other hand, the mobile station  102  antenna may transmit beams E, F, G, and H to the base station  112  antenna in the reverse direction, such that the air interface channels used to carry such communications may be referred to as reverse link path channels. As an example, the mobile station  102  and the base station  112  may initially communicate with a predetermined beam A on the forward link path, and corresponding predetermined beam E on the reverse link path. If the mobile station  102  switches to another beam, the mobile station  102  may switch to (i) beam B on the forward link path, and corresponding beam F on the reverse link path, (ii) beam C on the forward link path, and corresponding beam G on the reverse link path, or (iii) beam D on the forward link path, and corresponding beam H on the reverse link path. 
   Referring next to  FIG. 3 , an illustration  300  of operational parameters related to signaling on the forward and the reverse link paths is provided. In  FIG. 3 , the base station  112  antenna may transmit a beam on the forward link path to the mobile station  102  antenna with signaling indicating operational parameters such as (i) a transmit power of the base station  112  antenna (B T ), (ii) a receive power of the base station  112  antenna (B R ), (iii) a gain due to amplification by the base station  112  antenna (B G ), (iv) a loss due to attenuation on the line between the base station  112  antenna and a base station  112  receiver (B L ). On the other hand, the mobile station  102  antenna may transmit a beam on the reverse link path to the base station  112  with signaling indicating operational parameters such as (i) a transmit power of the mobile station  102  antenna (M T ), (ii) a receive power of the mobile station  102  antenna (M R ), (iii) a gain due to amplification by the mobile station  102  antenna (M G ), and (iv) a loss due to attenuation on the link between the mobile station  102  antenna and a mobile station  102  receiver (M L ). The base station  112  may, for instance, transmit values for the operational parameters to the mobile station  102  in overhead control signaling, such as in a page message, an access probe acknowledgement message, or a traffic channel control message. 
   2. Overview of the Exemplary Operation 
   In accordance with the exemplary embodiment, the mobile station  102  learns the base station  112  values of B T , B R , B G  and B L . The base station  112  preferably transmits the value of B T  to the mobile station  102  on the forward link path. For a given channel, the base station  112  will transmit to the mobile station  102  a power signal for the value of B T , which is typically defined in part by a Digital Gain Unit (“DGU”) parameter stored by the BSC  116 . Preferably, the power signal transmitted to the base station  112  is the value B R . Further, the values of B G  and B L  are characteristics of the base station  112  and are therefore specific to the base station  112  that transmits these values. 
   Referring next to  FIG. 4 , a generalized flowchart  400  of an exemplary embodiment is provided. At block  402 , the process starts with mobile station  102  operating on a currently selected beam of a switched beam antenna  104  on the forward link path. The mobile station  102  may select the current beam because it has the highest receive power. 
   At block  404 , the base station  112  transmits a signal to the mobile station  102 . The signal indicates operational parameters such as (i) the transmit power of the base station  112  (B T ), and (ii) the receive power of the base station  112  (B R ). The operational parameters may also include (i) a gain of an antenna of the base station  112  (B G ), and/or (ii) a loss due to attenuation on the line between the base station  112  antenna and the base station  112  receiver (B L ). 
   At block  406 , the mobile station  102  computes a transmit power for the currently selected beam of the switched beam antenna  104  by balancing the forward link path loss (FPL) with the reverse link path loss (RPL). By equating FPL with RPL, the mobile station  102  may determine what transmit power (M T ) is necessary to provide a balance between the FPL and RPL. 
   When FPL equals RPL, the mobile station  102  may compute the transmit power given that the mobile station  102  knows the values for B T , B R , and M R . Alternatively, when FPL equals RPL, the mobile station  102  may compute the proposed transmit power (M T ) given that the mobile station  102  knows the values for B T , B R , M R , B G , B L , M G , and M L . 
   At block  408 , the mobile station  102  compares the computed mobile station  102  transmit power on the currently selected beam to a predefined threshold value. The predefined threshold may be a maximum transmission power on the beam or a designated percentage of that maximum transmission power. The maximum transmission power is the total power transmitted by the mobile station on the beam. The predefined threshold may also extend to lower thresholds. 
   In turn, at block  410 , the mobile station  102  determines if the computed mobile station transmit power exceeds the predefined threshold for the currently selected beam. The computed transmit power may exceed the threshold by being (i) greater than or equal to the threshold, or (ii) greater than the threshold. 
   At block  412 , the mobile station  102  may switch to another beam if mobile station  102  determines that the computed transmit power for the currently selected beam exceeds the predefined threshold. Otherwise, at block  414 , the mobile station  102  will stay on the current beam, regardless of whether the current beam has the highest receive power. 
   3. Mathematical Description of the Exemplary Operation 
   The exemplary embodiment of the present invention, as illustrated in  FIG. 4 , will be described in mathematical detail below. 
   Discounting other factors and assuming for simplicity that the base station&#39;s transmit power is B T , the mobile station&#39;s receive power is M R , the mobile station&#39;s transmit power is M T , and the base station&#39;s receive power is B R , it follows that the forward link path loss, FPL, will be:
 
 FPL=B   T   −M   R  
 
and the reverse link path loss, RPL, will be:
 
 RPL=M   T   −B   R .
 
Assuming, as desired, that the forward link path loss equals the reverse link path loss, it then follows that the mobile station  102  transmit power necessary to provide a base station  112  receive power of B R  will be the sum of the forward link power and the base station  112  receive power, or:
 
 M   T   =B   T   −M   R   +B   R .
 
   In accordance with the exemplary embodiment, the base station  112  will report its transmit power B T  and receive power B R  to the mobile station  102 . The mobile station  102  will then use those values together with its receive power M R  on its current beam to compute a necessary transmit power M T  on that beam. 
   The mobile station  102  will then determine whether that value of M T  exceeds a designated upper threshold. If the computed M T  does not exceed the threshold, then the mobile station  102  will continue to use the current beam, regardless of whether the current beam has the highest receive power of the mobile station&#39;s beams. (In fact, in that scenario, the mobile station  102  may not even determine receive levels of its other beams.) On the other hand, if the computed M T  would exceed the threshold, then the mobile station  102  may switch to another beam in any manner, such as by conventionally switching to the beam that has the highest receive power. 
   In a preferred embodiment, the mobile station  102  will take into account more than just transmit and receive power levels. In particular, the mobile station  102  will preferably consider the forward link path to extend from the base station  112  antenna to the mobile station  102  receiver, so that the forward link path loss will also include a gain M G  and a loss M L . Similarly, the mobile station  102  will preferably consider the reverse link path to extend from the mobile station  102  antenna to the base station  112  receiver, so that the reverse link path loss will also include a gain B G  and a loss B L . 
   Consequently, in the preferred embodiment, the equations for FPL and RPL will be:
 
 FPL=B   T   +M   G   −M   L   −M   R  
 
and
 
 RPL=M   T   +B   G   −B   L   −B   R .
 
In turn, it then follows that the mobile station  102  transmit power M T  needed to have the base station  112  receive at receive-power B R , with all other values being constant, would be:
 
 M   T   =B   T   +M   G   −M   L   −M   R   −B   G   +B   L   +B   R  
 
   In this preferred embodiment, as with the more generalized embodiment above, the mobile station  102  will compute its transmit power for its current beam and will determine if the computed transmit power exceeds a designated threshold. If the computed mobile station  102  transmit power does not exceed the exceed the threshold, then the mobile station  102  will continue to use the current beam, regardless of whether the current beam has the highest receive power of the mobile station&#39;s beams. On the other hand, if the computed M T  would exceed the threshold, then the mobile station  102  may switch to another beam in any manner, such as by conventionally switching to the beam that has the highest receive power. 
   4. Conclusion 
   An exemplary embodiment of the present invention has been described above. Those skilled in the art will understand, however, that changes and modifications may be made to this embodiment without departing from the true scope and spirit of the present invention, which is defined by the claims. For example, although the invention has been described in the context of a switched beam system in a mobile station  102 , the invention could equally be applied to other switched beam systems in other places, such as in a base station  112  or elsewhere. Other examples are possible as well.