Abstract:
A radio base station (RBS) reduces the signaling overhead associated with maintaining rapidly updated estimates of transmit power headroom for a plurality of mobile stations. According to an exemplary embodiment, the RBS receives relatively infrequently periodic full reports from each mobile station that include an indication of the current transmit power headroom of the mobile station. The RBS then tracks the changes in the mobile station&#39;s headroom over the intervals between full reports based on reverse link power control information associated with the mobile station. The RBS adjusts the stored headroom value based on the reverse link power control commands sent by it, or based on power control decision feedback from the mobile station. In either case, use of the power control information keeps the stored headroom value accurate even over relatively long full reporting intervals.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority under 35 U.S.C. § 119(e) from the following U.S. provisional application: Application Serial No. 60/433,937 filed on Dec. 17, 2002. That application is incorporated in its entirety by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention generally relates to wireless communication networks, and particularly relates to tracking mobile station power headroom.  
           [0003]    In many types of wireless communication networks, and particularly in Code Division Multiple Access (CDMA) networks, the reverse radio link from mobile stations to the network, e.g., to a given radio base station represents a “managed” resource. For example, because the number of mobile stations simultaneously transmitting on the reverse link affects total interference at the base station, use of the reverse link may be “scheduled.” 
           [0004]    Several scheduling approaches are used in existing networks, or are planned for various next-generation wireless networks. In general, however, reverse link scheduling involves designating which one or ones in a set of mobile stations can use the reverse link at what times and at what data rates. For example, with Dedicated Rate Control (DRC), the base station grants specific mobile stations permission to transmit at particular rates at particular times. By changing such permissions over time, the base station can schedule users to achieve a desired reverse link “fairness” objective, to achieve a “maximum throughput” objective, or to achieve some other service objective.  
           [0005]    In another approach to rate control, the base station broadcasts Common Rate Control (CRC) commands that indicate whether the mobile stations should increase, decrease, or hold their current reverse link transmission rates. Thus, if reverse link loading, i.e., noise plus interference, was relatively high at the base station, it might transmit one or a series of down commands. Conversely, it might transmit one or a series of up commands if the reverse link load was relatively light.  
           [0006]    Generally, knowledge of certain mobile station conditions improves the base station&#39;s ability to carry on user scheduling in an efficient manner. For example, knowing the power headroom of each mobile station subject to scheduling provides a basis for determining whether a particular mobile station does or does not have the ability to operate at a contemplated higher rate. For example, the base station&#39;s reverse link scheduler might, for each scheduling interval, select a subset of mobile stations to operate at higher reverse link data rates and, in that context, it would not select any mobile station that lacked sufficient power headroom to operate at a higher rate.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention comprises a method and apparatus for tracking mobile station transmit power headroom at a wireless communication network base station based on periodically receiving full power headroom reports from the mobile stations and, in the intervals between the full reports, using reverse link transmit power control information to track power headroom changes. Use of the reverse link power control information allows the base station to keep its estimated headroom values relatively accurate over several or many transmit frame times, and thus the frequency of full power headroom reporting can be reduced, thereby reducing the signaling overhead attendant with transmitting power headroom information from the mobile stations to the network.  
           [0008]    Thus, in an exemplary embodiment of the present invention, an exemplary method of tracking mobile station power headroom at a wireless communication network base station comprises receiving a power headroom report from a mobile station, storing a headroom value for the mobile station based on the power headroom report received from the mobile station, and updating the headroom value to track changes in a transmit power of the mobile station based on reverse link power control information associated with the mobile station. Such processing at the base station may comprise periodically receiving a full report from a mobile station that indicates a transmit power headroom of the mobile station, updating a headroom value maintained at the base station for the mobile station responsive to receiving each full report, and tracking changes in transmit power headroom between each full report using reverse link power control information associated with the mobile station.  
           [0009]    In one embodiment, the reverse link power control information comprises the reverse link power control commands being transmitted from the base station to the mobile station as part of ongoing operations. Thus, the headroom value tracks changing power conditions at the mobile station by decrementing it each time the base station transmits an up command to the mobile station, and by incrementing it each time the base station transmits a down command to the mobile station. In an alternative embodiment, the base station maintains the headroom value during the intervals between full reports based on receiving one or more differential reports from the mobile station. These differential reports indicate the power control adjustments being made at the mobile station as part of its ongoing reverse link power control.  
           [0010]    Knowledge of each mobile station&#39;s current transmit power headroom may be used in a number of ways by the base station and by the network at large, and the present invention is not limited to a specific use of such information. However, in an exemplary embodiment of the present invention, the base station uses its knowledge of mobile station transmit power headroom to avoid attempts to increase the reverse link rate of mobile stations that lack sufficient power headroom to operate at higher rates. For example, in reverse link scheduling or Dedicated Rate Control (DRC) procedures, the base station would avoid picking mobile stations for rate increases if they lacked sufficient power headroom.  
           [0011]    According to an exemplary embodiment of the present invention, a base station for use in a wireless communication network comprises transceiver circuits to communicate with a plurality of mobile stations via wireless signaling, and one or more processing circuits to control communications with the plurality of mobile stations. The one or more processing circuits include a headroom tracking circuit configured to track transmit power headroom for each mobile station by periodically receiving a full report from the mobile station that indicates a transmit power headroom of the mobile station, updating a headroom value maintained at the base station for the mobile station responsive to receiving each full report, and tracking changes in transmit power headroom between each full report using reverse link power control information associated with the mobile station.  
           [0012]    The present invention is not limited by these exemplary embodiments. Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a diagram of a wireless communication network according to one or more exemplary embodiments of the present invention.  
         [0014]    [0014]FIG. 2 is a diagram of typical variations in a mobile station&#39;s transmit power and headroom over several reverse link transmit frames.  
         [0015]    [0015]FIG. 3 is a diagram of exemplary transmit power headroom tracking according to one embodiment of the present invention.  
         [0016]    [0016]FIGS. 4A and 4B are exemplary diagrams for power headroom tracking based on transmitted power control commands.  
         [0017]    [0017]FIGS. 5A and 5B are exemplary diagrams for power headroom tracking based on power control decision feedback.  
         [0018]    [0018]FIG. 6 is a diagram of an exemplary mobile station and radio base station configured to support the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    [0019]FIG. 1 is a diagram of an exemplary wireless communication network  10  according to one or more embodiments of the present invention. Network  10  may be a cdma2000 network configured according to IS-2000 standards but the present invention is applicable to networks configured according to other standards, such as Wideband CDMA (WCDMA). Regardless, as illustrated, network  10  communicatively couples a plurality of mobile stations  12  to one or more Public Data Networks (PDNs)  14 , such as the Internet.  
         [0020]    Network  10  comprises a Radio Access Network (RAN)  16  that is coupled to the PDN(s)  14  through a Packet Switched Core Network (PSCN)  18 . RAN  16  comprises at least one Base Station (BS) that includes a Base Station Controller  20  and one or more associated Radio Base Stations (RBSs)  22 . BSC  20  may include packet control interface circuits to communicate with PSCN  18 , or may couple to PSCN  18  through a Packet Control Function  24 . While only one BSC  20  and RBS  22  are illustrated for clarity, it should be understood that RAN  16  may include a plurality of BSCs  20 , each controlling one or more RBSs  22 . Further, network  10  may include additional entities not illustrated, such as a Circuit Switched Core Network (CSCN) to communicatively couple RAN  16  to the Public Switched Telephone Network (PSTN).  
         [0021]    RBS  22  transmits signals to mobile stations  12  on one or more forward link channels, and receives signals from them on one or more reverse link channels. In an exemplary embodiment, RBS  22  provides closed-loop reverse link power control wherein it controls the transmit power of each mobile station  12  to receive the mobile station&#39;s transmissions at a targeted received signal quality. For example, according to IS-2000 standards, RBS  22  transmits power control commands to each mobile station  12  at a defined power control rate, e.g., 400 Hz, 800 Hz, etc. Each power control command transmitted to a given mobile station  12  tells the mobile station  12  to increment (up command) or decrement (down command) its reverse link transmit power. Thus, by streaming the appropriate mix of up and down commands to each mobile station  12  at the defined power control rate, the RBS  22  maintains each mobile stations&#39; reverse link power at the appropriate level.  
         [0022]    [0022]FIG. 2 illustrates the reverse link power control process for a given mobile station  12  over several reverse link transmit frames, shown as N, N+1, and so on. The mobile station&#39;s transmit power varies up and down over time responsive to incoming reverse link power commands received from RBS  22  and, possibly, from other RBSs  22 , such as when mobile station  12  is in soft handoff. In an exemplary embodiment, the reverse link transmit frames are 10 ms, and the mobile station  12  receives eight power control commands per frame, i.e., it receives a PCB every 1.25 ms.  
         [0023]    The mobile station&#39;s total reverse link transmit power includes the power allocated to its pilot signal transmissions, control channel signals, etc., which collectively is referred to as “overhead” power. An additional amount of the mobile station&#39;s transmit power is allocated to transmitting data as needed or desired on, for example, a reverse link packet data channel. The remaining unused transmit power may be considered “power headroom,” as it represents the amount by which the mobile station  12  could increase its transmit power if commanded by RBS  22 . Note that FIG. 2 illustrates a constant data power proportionality over time for simplicity, but that proportionality might vary, such as where varying data rates are used.  
         [0024]    As will be explained later herein, knowledge of each mobile station&#39;s transmit power headroom provides the RBS  22  with the ability to make reverse link rate control decisions, make scheduling decisions, etc. The present invention provides a method and apparatus whereby the RBS  22  tracks the power headroom at each mobile station  12 , while simultaneously reducing the signaling overhead that would arise if the mobile stations  12  simply transmitted power headroom reports to the RBS  22  at a high rate. FIG. 3 illustrates exemplary power headroom tracking at the RBS  22  with respect to a particular mobile station  12 . It should be understood that RBS  22  is configured to perform like power headroom tracking for a plurality of mobile stations  12 .  
         [0025]    Processing begins with receipt of a “full” power headroom report from a mobile station  12 . The full report may comprise multiple bits in a Protocol Data Unit (PDU) header that indicates the mobile station&#39;s power headroom (Step  100 ). For example, a five-bit indicator provides thirty-two (2 n ) measurement levels that may be used to indicate transmit power headroom. RBS  22  maintains a stored headroom value for the mobile station that is updated based on the power headroom indicated in the full power headroom report (Step  102 ). Essentially, this step resets the stored headroom value to the value indicated by the full report. Thus, the stored headroom value is reset responsive to receiving each full report.  
         [0026]    To track changes in the mobile station&#39;s power headroom over the intervals between full reports, RBS  22  uses reverse link power control information associated with the mobile station  12  (Step  104 ). That is, it uses information associated with ongoing reverse link power control of the mobile station&#39;s reverse link transmit power to keep the stored headroom value current over the intervals between full reports and then resets the stored value upon receiving the next full report (Step  106 ).  
         [0027]    [0027]FIGS. 4A and 4B illustrate a first exemplary embodiment, wherein RBS  22  receives periodic full reports from the mobile station  12 , e.g., every 20 ms. In between full reports, the RBS  22  updates the headroom value stored for the mobile station  12  such that it tracks changes in the mobile station&#39;s transmit power based on the power control commands sent to the mobile station  12  during the intervals between full reports. In an exemplary embodiment, RBS  22  is configured to incrementally adjust the stored headroom value up or down for each power control command transmitted to the mobile station  12 . For example, if RBS  22  transmits an up command to the mobile station  12 , it decrements the headroom value by the amount by which the mobile station  12  is assumed to have increased its transmit power responsive to receiving the up command. Conversely, if a down command is transmitted to the mobile station  12 , RBS  22  increments the headroom value by the amount by which the mobile station  12  is assumed to have decreased its transmit power responsive to receiving the down command.  
         [0028]    Thus, the illustrated processing begins with receipt of a full report at RBS  22  (Step  110 ), which is used to set the stored headroom value (Step  112 ). Then, in response to each PCB transmitted to the mobile station (Step  114 ), RBS  22  increments or decrements the headroom value accordingly (Step  116 ). Note that such decrementing and incrementing of the headroom value generally should be based on the known step size changes that mobile station  12  makes in its transmit power responsive to the RBS&#39;s power control commands. These incremental adjustments continue until receipt of the next full report (Step  118 ), which then resets the headroom value based on the value received in the full report.  
         [0029]    [0029]FIGS. 5A and 5B illustrate another exemplary embodiment that may be used in conjunction with that described immediately above, such as during soft handoff conditions, or may be used as an alternative regardless of whether the mobile station  12  is in soft handoff. According to the illustrated logic, the RBS  22  tracks power headroom at mobile station  12  during the intervals between full reports based on feedback from the mobile station  12 . That is, rather than assuming that its transmitted power control commands accurately represent actual transmit power adjustments at mobile station  12 , RBS  22  monitors power control decisions as feedback from the mobile station.  
         [0030]    Thus, the mobile station  12  is configured to transmit a decision indicator at each power control decision point, i.e., that indicates whether it incremented or decremented its transmit power for that control interval. With this approach, tracking accuracy may be improved because adjustments to the headroom value stored for the mobile station  12  at RBS  22  are made based on the actual power control adjustment made by the mobile station  12  rather than on the power control commands being transmitted by RBS  22 , which the mobile station  12  may or may not follow. Mobile station  12  may not follow the RBS&#39;s power control commands due to reception errors at the mobile station  12 , or due to soft handoff conditions, wherein the mobile station  12  receives power control commands from one or more additional RBSs  22 .  
         [0031]    Exemplary processing according to this embodiment thus comprises receiving periodic full reports from mobile station  12  as before (Step  120 ), and setting the power headroom value stored for the mobile station  12  based on the headroom indicated by the full report (Step  122 ). Then, in the intervals between full reports, RBS  22  increments or decrements the stored headroom value responsive to each power control decision feedback value received from the mobile station  12  (Steps  124  and  126 ). As before, the stored headroom value is reset responsive to receiving the next full report (Step  128 ).  
         [0032]    Mobile station  12  may be configured to stream power control decision feedback to the RBS  22  at the same rate it receives power control commands. Thus, for example, the mobile station  12  may transmit a decision indicator for each power control interval, e.g., a single bit value, that indicates whether the mobile station  12  incremented or decremented its power for that interval. Nominally, then, the mobile station  12  transmits indicators at the same rate that it receives power control commands, e.g., 400 Hz, 800 Hz, etc. However, those skilled in the art should appreciate that it could send decision feedback at a half-rate, or other rate as needed or desired.  
         [0033]    [0033]FIG. 6 illustrates exemplary mobile station and radio base station configurations that may be used to carry out the present invention in any of its exemplary embodiments. The exemplary mobile station  12  comprises an antenna assembly  30 , a receiver circuit  32 , a transmitter circuit  34 , baseband processor circuits  36 , including a transmit power controller  38  and transmit power headroom estimator  40 , a system controller  42  and its associated user interface  44 , i.e., display, keypad, etc. RBS  22  comprises a receive/transmit antenna assembly  50 , transceiver circuits  52 , including receiver circuits  54  and transmitter circuits  56 , forward/reverse link signal processing circuits  58 , and interface/control circuits  60 , including headroom tracking circuit  62 , memory  64 , and scheduler/rate controller  66 . Those skilled in the art should recognize that other functional arrangements for either or both the mobile station  12  and RBS  22  could be used to support the present invention.  
         [0034]    Further, those skilled in the art should appreciate that the present invention can be embodied in hardware, software, or some combination thereof. For example, headroom tracking circuit  62  and scheduler/rate controller circuit  66  may be implemented as stored program instructions for execution on a microprocessor or other logical processing circuit included in RBS  22 .  
         [0035]    In any case, headroom tracking circuit  62  maintains a headroom value in memory  64  for each mobile station  12  being tracked. During ongoing radio service, RBS  22  receives periodic full reports from a plurality of mobile stations  12 , and signal processing circuits  58  extract the reported headroom values from that received information and provide them to headroom tracking circuit  62 , which uses the received information to refresh the corresponding stored headroom values. As explained above, headroom tracking circuit  62  compensates the headroom values between full reports based either on the power control commands being transmitted from the RBS  22  to the mobile stations  12 , or based on power control decision feedback received from the mobile stations  12 .  
         [0036]    In either case, scheduler/rate controller circuit  66  may use the stored headroom information to improve its ongoing rate control operations. For example, scheduler/rate controller  66  may be configured to grant higher reverse link data rates to a subset of the mobile stations  12  at any given time. For example, scheduler/rate controller  66  would avoid selecting a mobile station  12  for a contemplated reverse link rate increase if it had insufficient power headroom to support the rate increase as indicated by the headroom value stored for it in memory  64 .  
         [0037]    Of course, the stored power headroom values may be used to additional advantage in RBS operations and the present invention is not limited to the exemplary usage in ongoing reverse link rate control as described above. Indeed, the present invention is not limited by the foregoing discussion but rather is limited by the following claims and their reasonable equivalents.