Patent Publication Number: US-8543152-B2

Title: Power control for a wireless communication system utilizing orthogonal multiplexing

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §120 
     The present application for patent is a divisional of patent application Ser. No. 10/897,463 entitled “Power Control for a Wireless Communication System Utilizing Orthogonal Multiplexing” filed Jul. 22, 2004, pending, which claims priority to Provisional Application No. 60/580,819 entitled “Reverse-Link Power Control Algorithm” filed Jun. 18, 2004, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
     REFERENCE TO CO-PENDING APPLICATIONS FOR PATENT 
     The present application for patent is related to the following co-pending U.S. patent applications: 
     “Robust Erasure Detection and Erasure-Rate-Based Closed Loop Power Control” by Arak Sutivong et al, filed Jul. 13, 2004, assigned to the assignee hereof, and expressly incorporated by reference herein. 
    
    
     BACKGROUND 
     I. Field 
     The present invention relates generally to communication, and more specifically to power control for a wireless communication system. 
     II. Background 
     A wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. 
     Multiple terminals may simultaneously transmit on the reverse link by multiplexing their transmissions to be orthogonal to one another. The multiplexing attempts to achieve orthogonality among the multiple reverse link transmissions in time, frequency, and/or code domain. Complete orthogonality, if achieved, results in the transmission from each terminal not interfering with the transmissions from other terminals at a receiving base station. However, complete orthogonality among the transmissions from different terminals is often not realized due to channel conditions, receiver imperfections, and so on. The loss in orthogonality results in each terminal causing some amounts of interference to other terminals communicating with the same base station. Furthermore, the transmissions from terminals communicating with different base stations are typically not orthogonal to one another. Thus, each terminal may also cause interference to terminals communicating with nearby base stations. The performance of each terminal is then degraded by the interference from all other terminals in the system. 
     There is therefore a need in the art for techniques to mitigate the effects of interference so that improved performance may be achieved. 
     SUMMARY 
     Techniques for controlling transmit power for a data transmission from a wireless terminal in a manner to mitigate both “intra-sector” interference and “inter-sector” interference are described herein. The transmit power is adjusted such that the amount of intra-sector interference the terminal may cause to a “serving” base station and the amount of inter-sector interference the terminal may cause to “neighbor” base stations are both maintained within acceptable levels. (The terms in quote are described below.) The amount of inter-sector interference the terminal may cause may be roughly estimated based on (1) the total interference observed by each neighbor base station, (2) channel gains for the serving and neighbor base stations, (3) the current transmit power level used by the terminal, and (4) possibly other parameters. Each base station may broadcast a report (e.g., a single bit) indicative of the total interference observed by that base station. The channel gain for each base station may be estimated based on a pilot received from the base station. The transmit power may be adjusted in a probabilistic manner, a deterministic manner, or some other manner based on these various parameters. 
     In general, the transmit power may be decreased if high interference is observed by neighbor base stations and increased if low interference is observed. The transmit power may also be adjusted by a larger amount and/or more frequently if (1) the terminal is located closer to a neighbor base station observing high interference and/or (2) the current transmit power level is higher. The transmit power may be adjusted by a smaller amount and/or less frequently if (1) the terminal is located closer to the serving base station and/or (2) the current transmit power level is lower. The intra-sector interference caused by the terminal is maintained within an acceptable level by limiting the received signal quality (SNR) for the data transmission to be within a range of allowable SNRs. 
     Various aspects and embodiments of the invention are described in further detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and nature of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
         FIG. 1  shows a wireless multiple-access communication system; 
         FIG. 2  illustrates frequency hopping on a time-frequency plane; 
         FIG. 3  shows a process for adjusting transmit power in a probabilistic manner; 
         FIG. 4  shows a process for adjusting transmit power in a deterministic manner; 
         FIG. 5  shows a power control mechanism for a data channel; 
         FIG. 6  shows a power control mechanism for a control channel; and 
         FIG. 7  shows a terminal, a serving base station, and a neighbor base station. 
     
    
    
     DETAILED DESCRIPTION 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs. 
       FIG. 1  shows a wireless multiple-access communication system  100 . System  100  includes a number of base stations  110  that support communication for a number of wireless terminals  120 . Terminals  120  are typically dispersed throughout the system, and each terminal may be fixed or mobile. A terminal may also be referred to as a mobile station, a user equipment (UE), a wireless communication device, or some other terminology. A base station is a fixed station used for communicating with the terminals and may also be referred to as an access point, a Node B, or some other terminology. A system controller  130  couples to base stations  110 , provides coordination and control for these base stations, and further controls the routing of data for the terminals served by these base stations. 
     Each base station  110  provides communication coverage for a respective geographic area  102 . A base station and/or its coverage area may be referred to as a “cell”, depending on the context in which the term is used. To increase capacity, the coverage area of each base station may be partitioned into multiple (e.g., three) sectors  104 . Each sector is served by a base transceiver subsystem (BTS). The term “sector” can refer to a BTS and/or its coverage area, depending on the context in which the term is used. For a sectorized cell, the base station for that cell typically includes the BTSs for all sectors of that cell. For simplicity, in the following description, the term “base station” is used generically for both a fixed station that serves a cell and a fixed station that serves a sector. A “serving” base station or “serving” sector is one with which a terminal communicates. A “neighbor” base station or “neighbor” sector is one with which the terminal is not in communication. For simplicity, the following description assumes that each terminal communicates with one serving base station, although this is not a required limitation for the techniques described herein. 
     The power control techniques described herein may be used for various wireless communication systems. For example, these techniques may be used for a Time Division Multiple Access (TDMA) system, a Frequency Division Multiple Access (FDMA) system, an orthogonal frequency division multiple access (OFDMA) system, and so on. A TDMA system uses time division multiplexing (TDM), and transmissions for different terminals are orthogonalized by transmitting in different time intervals. An FDMA system uses frequency division multiplexing (FDM), and transmissions for different terminals are orthogonalized by transmitting in different frequency subbands. TDMA and FDMA systems may also use code division multiplexing (CDM). In this case, transmissions for multiple terminals may be orthogonalized using different orthogonal (e.g., Walsh) codes even though they are sent in the same time interval or frequency subband. An OFDMA system utilizes orthogonal frequency division multiplexing (OFDM), which effectively partitions the overall system bandwidth into a number of (N) orthogonal frequency subbands. These subbands are also referred to as tones, sub-carriers, bins, frequency channels, and so on. Each subband is associated with a respective sub-carrier that may be modulated with data. An OFDMA system may use any combination of time, frequency, and/or code division multiplexing. For clarity, the power control techniques are described below for an OFDMA system. 
     For an OFDMA system, multiple “traffic” channels may be defined whereby (1) each subband is used for only one traffic channel in any given time interval and (2) each traffic channel may be assigned zero, one, or multiple subbands in each time interval. The traffic channels may include “data” channels used to send traffic/packet data and “control” channels used to send overhead/control data. The traffic channels may also be referred to as physical channels, transport channels, or some other terminology. 
     The traffic channels for each sector may be defined to be orthogonal to one another in time and frequency so that no two traffic channels use the same subband in any given time interval. This orthogonality avoids intra-sector interference among multiple transmissions sent simultaneously on multiple traffic channels in the same sector. Some loss of orthogonality may result from various effects such as, for example, inter-carrier interference (ICI) and inter-symbol interference (ISI). This loss of orthogonality results in intra-sector interference. The traffic channels for each sector may also be defined to be pseudo-random with respect to the traffic channels for nearby sectors. This randomizes the inter-sector or “other-sector” interference caused by traffic channels in one sector to traffic channels in nearby sectors. Randomized intra-sector interference and inter-sector interference may be achieved in various manners. For example, frequency hopping can provide randomized intra-sector and inter-sector interference as well as frequency diversity against deleterious path effects. 
       FIG. 2  illustrates frequency hopping (FH) on a time-frequency plane  200  for an OFDMA system. With frequency hopping, each traffic channel is associated with a specific FH sequence that indicates the particular subband(s) to use for that traffic channel in each time interval. The FH sequences for different traffic channels in each sector are orthogonal to one another so that no two traffic channels use the same subband in any time interval. The FH sequences for each sector are also pseudo-random with respect to the FH sequences for nearby sectors. Interference between two traffic channels in two sectors occurs whenever these two traffic channels use the same subband in the same time interval. However, the inter-sector interference is randomized due to the pseudo-random nature of the FH sequences used for different sectors. 
     Data channels may be assigned to active terminals such that each data channel is used by only one terminal at any given time. To conserve system resources, control channels may be shared among multiple terminals using, e.g., code division multiplexing. If the data channels are orthogonally multiplexed only in frequency and time (and not code), then they are less susceptible to loss in orthogonality due to channel conditions and receiver imperfections than the control channels. 
     The data channels thus have several key characteristics that are pertinent for power control. First, intra-cell interference on the data channels is minimal because of the orthogonal multiplexing in frequency and time. Second, inter-cell interference is randomized because nearby sectors use different FH sequences. The amount of inter-cell interference caused by a given terminal is determined by (1) the transmit power level used by that terminal and (2) the location of the terminal relative to the neighbor base stations. 
     For the data channels, power control may be performed such that each terminal is allowed to transmit at a power level that is as high as possible while keeping intra-cell and inter-cell interference to within acceptable levels. A terminal located closer to its serving base station may be allowed to transmit at a higher power level since this terminal will likely cause less interference to neighbor base stations. Conversely, a terminal located farther away from its serving base station and toward a sector edge may be allowed to transmit at a lower power level since this terminal may cause more interference to neighbor base stations. Controlling transmit power in this manner can potentially reduce the total interference observed by each base station while allowing “qualified” terminals to achieve higher SNRs and thus higher data rates. 
     Power control for the data channels may be performed in various manners to attain the goals noted above. For clarity, a specific embodiment of power control is described below. For this embodiment, the transmit power for a data channel for a given terminal may be expressed as:
 
 P   dch ( n )= P   ref ( n )+Δ P ( n ),  Eq (1)
 
     where P dch (n) is the transmit power for the data channel for update interval n; 
     P ref (n) is a reference power level for update interval n; and 
     ΔP(n) is a transmit power delta for update interval n. 
     The power levels P dch (n) and P ref (n) and the transmit power delta ΔP(n) are given in units of decibels (dB). 
     The reference power level is the amount of transmit power needed to achieve a target signal quality for a designated transmission (e.g., on a control channel). Signal quality (denoted as SNR) may be quantified by a signal-to-noise ratio, a signal-to-noise-and-interference ratio, and so on. The reference power level and the target SNR may be adjusted by a power control mechanism to achieve a desired level of performance for the designated transmission, as described below. If the reference power level can achieve the target SNR, then the received SNR for the data channel may be estimated as:
 
SNR dch ( n )=SNR target   +ΔP ( n ).  Eq (2)
 
     Equation (2) assumes that the data channel and the control channel have similar interference statistics. This is the case, for example, if the control and data channels from different sectors may interfere with one another. The reference power level may be determined as described below. 
     The transmit power for the data channel may be set based on various factors such as (1) the amount of inter-sector interference the terminal may be causing to other terminals in neighbor sectors, (2) the amount of intra-sector interference the terminal may be causing to other terminals in the same sector, (3) the maximum power level allowed for the terminal, and (4) possibly other factors. Each of these factors is described below. 
     The amount of inter-sector interference each terminal may cause may be determined in various manners. For example, the amount of inter-sector interference caused by each terminal may be directly estimated by each neighbor base station and sent to the terminal, which may then adjust its transmit power accordingly. This individualized interference reporting may require extensive overhead signaling. For simplicity, the amount of inter-sector interference each terminal may cause may be roughly estimated based on (1) the total interference observed by each neighbor base station, (2) the channel gains for the serving and neighbor base stations, and (3) the transmit power level used by the terminal. Quantities (1) and (2) are described below. 
     Each base station can estimate the total or average amount of interference observed by that base station. This may be achieved by estimating the interference power on each subband and computing an average interference power based on the interference power estimates for the individual subbands. The average interference power may be obtained using various averaging techniques such as, for example, arithmetic averaging, geometric averaging, SNR-based averaging, and so on. 
     For arithmetic averaging, the average interference power may be expressed as: 
                       I     meas   ,   m       ⁡     (   n   )       =       1   N     ⁢       ∑     k   =   1     N     ⁢       I   m     ⁡     (     k   ,   n     )                   Eq   ⁢           ⁢     (   3   )                 
where I m (k,n) is the interference power estimate for sector m on subband k in time interval n; and
 
     I meas,m (n) is the average interference power for sector m in time interval n. The quantities I m (k,n) and I meas,m (n) are in linear units in equation (3) but may also be given in decibel (dB). With arithmetic averaging, a few large interference power estimates can skew the average interference power. 
     For geometric averaging, the average interference power may be expressed as: 
     
       
         
           
             
               
                 
                   
                     
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     Geometric averaging can suppress large interference power estimates for a few subbands, so that the average interference power is lower than with arithmetic averaging. 
     For SNR-based averaging, the average interference power may be expressed as: 
                       log   ⁡     (     1   +       P   nom         I     meas   ,   m       ⁡     (   n   )           )       =       1   N     ·       ∑     k   =   1     N     ⁢     log   ⁡     (     1   +       P   nom         I   m     ⁡     (     k   ,   n     )           )             ,           Eq   ⁢           ⁢     (   5   )                 
where P nom  denotes a nominal received power assumed for each subband. Equation (5) determines the theoretical capacity of each subband based on the nominal received power, computes the average capacity for all N subbands, and determines an average interference power that gives the average capacity. SNR-based averaging (which may also be called capacity-based averaging) also suppresses large interference power estimates for a few subbands.
 
     Regardless of which averaging technique is used, each base station may filter the interference power estimates and/or the average interference power over multiple time intervals to improve the quality of the interference measurement. The filtering may be achieved with a finite impulse response (FIR) filter, an infinite impulses response (IIR) filter, or some other types of filter known in the art. The term “interference” may thus refer to filtered or unfiltered interference in the description herein. 
     Each base station may broadcast its interference measurements for use by terminals in other sectors. The interference measurements may be broadcast in various manners. In one embodiment, the average interference power (or the “measured” interference) is quantized to a predetermined number of bits, which are then sent via a broadcast channel. In another embodiment, the measured interference is broadcast using a single bit that indicates whether the measured interference is greater than or below a nominal interference threshold. In yet another embodiment, the measured interference is broadcast using two bits. One bit indicates the measured interference relative to the nominal interference threshold. The other bit may be used as a distress/panic bit that indicates whether the measured interference exceeds a high interference threshold. The interference measurements may also be sent in other manners. 
     For simplicity, the following description assumes the use of a single other-sector interference (OSI) bit to provide interference information. Each base station may set its OSI bit (OSIB) as follows: 
                     OSIB   ⁡     (   n   )       =     {             ‘   1   ’     ,               if   ⁢           ⁢       I     meas   ,   m       ⁡     (   n   )         ≥     I   target       ,   and                 ‘   0   ’     ,               if   ⁢           ⁢       I     meas   ,   m       ⁡     (   n   )         &lt;     I   target       ,                     Eq   ⁢           ⁢     (   6   )                 
where I target  is the nominal interference threshold.
 
     Alternatively, each base station may obtain a measured interference-over-thermal (IOT), which is a ratio of the total interference power observed by the base station to the thermal noise power. The total interference power may be computed as described above. The thermal noise power may be estimated by turning off the transmitter and measuring the noise at the receiver. A specific operating point may be selected for the system and denoted as IOT target . A higher operating point allows the terminals to use higher transmit powers (on average) for the data channels. However, a very high operating point may not be desirable since the system can become interference limited, which is a situation whereby an increase in transmit power does not translate to an increase in received SNR. Furthermore, a very high operating point increases the likelihood of system instability. In any case, each base station may set its OSI bit as follows: 
                     OSIB   ⁡     (   n   )       =     {             ‘   1   ’     ,               if   ⁢           ⁢       IOT     meas   ,   m       ⁡     (   n   )         ≥     IOT   target       ,   and                 ‘   0   ’     ,               if   ⁢           ⁢       IOT     meas   ,   m       ⁡     (   n   )         &lt;     IOT   target       ,                     Eq   ⁢           ⁢     (   7   )                 
where IOT meas,m (n) is the measured IOT for sector m in time interval n; and
 
     IOT target  is the desired operating point for the sector. 
     For both cases, the OSI bit may be used for power control as described below. 
     Each terminal can estimate the channel gain (or propagation path gain) for each base station that may receive a reverse link transmission from the terminal. The channel gain for each base station may be estimated by processing a pilot received from the base station via the forward link, estimating the received pilot strength/power, and filtering pilot strength estimates over time (e.g., with a filter having a time constant of several hundred milli-seconds) to remove the effects of fast fading and so on. If all base stations transmit their pilots at the same power level, then the received pilot strength for each base station is indicative of the channel gain between that base station and the terminal. The terminal may form a channel gain ratio vector, G, as follows:
 
 G=[r   1 ( n ) r   2 ( n ) . . .  r   M ( n )],  Eq (8)
 
                         r   i     ⁡     (   n   )       =         g   s     ⁡     (   n   )           g   ni     ⁡     (   n   )           ,           Eq   ⁢           ⁢     (   9   )                 
where
 
     g s (n) is the channel gain between the terminal and the serving base station; 
     g ni (n) is the channel gain between the terminal and neighbor base station i; and 
     r i (n) is the channel gain ratio for neighbor base station i. 
     Since distance is inversely related to channel gain, the channel gain ratio g s (n)/g ni (n) may be viewed as a “relative distance” that is indicative of the distance to a neighbor base station i relative to the distance to the serving base station. In general, the channel gain ratio for a neighbor base station, r i (n), decreases as the terminal moves toward the sector edge and increases as the terminal moves closer to the serving base station. The channel gain ratio vector, G, may be used for power control as described below. 
     Although the data channels for each sector are multiplexed such that they are orthogonal to one another, some loss in orthogonality may result from inter-carrier interference (ICI), intersymbol interference (ISI), and so on. This loss of orthogonality causes intra-sector interference. To mitigate intra-sector interference, the transmit power of each terminal may be controlled such that the amount of intra-sector interference that this terminal may cause to other terminals in the same sector is maintained within an acceptable level. This may be achieved, for example, by requiring the received SNR for the data channel for each terminal to be within a predetermined SNR range, as follows:
 
SNR dch (n)ε[SNR min ,SNR max ],  Eq (10)
 
     where SNR min  is the minimum received SNR allowable for a data channel; and 
     SNR max  is the maximum received SNR allowable for a data channel. 
     The minimum received SNR ensures that all terminals, especially those located near the sector edge, can achieve a minimum level of performance. Without such a constraint, terminals located near the sector edge may be forced to transmit at an extremely low power level, since they often contribute a significant amount of inter-sector interference. 
     If the received SNRs for the data channels for all terminals are constrained to be within the range [SNR min , SNR max ], then the amount of intra-sector interference caused by each terminal due to a loss of orthogonality may be assumed to be within the acceptable level. By limiting the received SNRs to be within this SNR range, there can still be as much as (SNR max −SNR min ) dB difference in received power spectral density between adjacent subbands (assuming that similar amounts of inter-sector interference are observed on the subbands, which is true, e.g., if the control and data channels hop randomly so that the control and data channels from different sectors may collide with one another). A small SNR range improves the robustness of the system in the presence of ICI and ISI. An SNR range of 10 dB has been found to provide good performance in most operating scenarios. Other SNR ranges may also be used. 
     If the transmit power for the data channel is determined as shown in equation (1), then the received SNR for the data channel may be maintained within the range of [SNR min , SNR max ] by constraining the transmit power delta, ΔP(n) to be within a corresponding range, as follows:
 
ΔP(n)ε[ΔP min ,ΔP max ],  Eq (11)
 
     where ΔP min  is the minimum transmit power delta allowable for a data channel, and 
     ΔP max  is the maximum transmit power delta allowable for a data channel. 
     In particular, ΔP min =SNR min −SNR target  and ΔP max =SNR max −SNR target . In another embodiment, the transmit power P dch (n) may be constrained to be within a range that is determined, for example, based on the received signal power for the data channel. This embodiment may be used, for example, if interference power is statistically different among the subbands. 
     The transmit power for the data channel for each terminal may then be adjusted based on the following parameters: 
     The OSI bit broadcast by each base station; 
     The channel gain ratio vector, G, computed by the terminal; 
     The range of received SNRs allowable for the data channels, [SNR min , SNR max ] or equivalently the range of allowable transmit power deltas, [ΔP min , ΔP max ]; and 
     The maximum power level, P max , allowed for the terminal, which may set by the system or the power amplifier within the terminal. 
     Parameters 1) and 2) relate to the inter-sector interference caused by the terminal. Parameter 3) relates to the intra-sector interference caused by the terminal. In general, a terminal located close to a neighbor sector that reports high interference may transmit with a lower transmit power delta so that its received SNR is closer to SNR min . Conversely, a terminal located close to its serving base station may transmit with a higher transmit power delta so that its received SNR is closer to SNR max . A gradation of received SNRs may be observed for the terminals in the system based on their proximity to the serving base stations. A scheduler at each base station can take advantage of the distribution of received SNRs to achieve high throughput while ensuring fairness for the terminals. 
     The transmit power for the data channel may be adjusted in various manners based on the four parameters noted above. The power control mechanism does not need to maintain equal SNR for all terminals, especially in an orthogonal system like an OFDMA system, where terminals closer to a base station may transmit at higher power levels without causing much problem to other terminals. For clarity, a specific embodiment for adjusting transmit power is described below. For this embodiment, each terminal monitors the OSI bits broadcast by neighbor base stations and only responds to the OSI bit of the strongest neighbor base station, which has the smallest channel gain ratio in the vector G. If the OSI bit of a given base station is set to ‘1’ (due to the base station observing higher than nominal inter-sector interference), then the transmit powers of terminals having this base station as their strongest neighbor base station may be adjusted downward. Conversely, if the OSI bit is set to ‘0’, then the transmit powers of terminals having this base station as their strongest neighbor base station may be adjusted upward. For other embodiments, each terminal may adjust its transmit power based on one or multiple OSI bits obtained for one or multiple base stations (e.g., serving and/or neighbor base stations). 
     The OSI bit thus determines the direction in which to adjust the transmit power. The amount of transmit power adjustment for each terminal may be dependent on (1) the current transmit power level (or the current transmit power delta) of the terminal and (2) the channel gain ratio for the strongest neighbor base station. Table 1 lists some general rules for adjusting the transmit power based on the transmit power delta and the channel gain ratio for the strongest base station. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 OSI Bit 
                 Transmit Power Adjustment 
               
               
                   
                   
               
             
            
               
                   
                 ‘1’ 
                 A terminal with a smaller channel gain 
               
               
                   
                 (high inter- 
                 ratio for (and is thus closer to) the 
               
               
                   
                 ference level) 
                 base station transmitting the OSI bit, 
               
               
                   
                   
                 in general, decreases its transmit power 
               
               
                   
                   
                 delta by a larger amount in comparison to 
               
               
                   
                   
                 a terminal with a larger channel gain ratio 
               
               
                   
                   
                 for (and is thus farther away from) this 
               
               
                   
                   
                 base station. 
               
               
                   
                   
                 A terminal with a larger transmit power 
               
               
                   
                   
                 delta, in general, decreases its transmit 
               
               
                   
                   
                 power delta by a larger amount in comparison 
               
               
                   
                   
                 to a terminal with a similar channel gain 
               
               
                   
                   
                 ratio for this base station but a smaller 
               
               
                   
                   
                 transmit power delta. 
               
               
                   
                 ‘0’ 
                 A terminal with a larger channel gain ratio 
               
               
                   
                 (low inter- 
                 for (and is thus farther away from) the base 
               
               
                   
                 ference level) 
                 station transmitting the OSI bit, in general, 
               
               
                   
                   
                 increases its transmit power delta by a larger 
               
               
                   
                   
                 amount in comparison to a terminal with a 
               
               
                   
                   
                 smaller channel gain ratio for (and is thus 
               
               
                   
                   
                 closer to) this base station. 
               
               
                   
                   
                 A terminal with a smaller transmit power delta, 
               
               
                   
                   
                 in general, increases its transmit power delta 
               
               
                   
                   
                 by a larger amount in comparison to a terminal 
               
               
                   
                   
                 with a similar channel gain ratio for this base 
               
               
                   
                   
                 station but a larger transmit power delta. 
               
               
                   
                   
               
            
           
         
       
     
     The transmit power may be adjusted in a deterministic manner, a probabilistic manner, or some other manner. For deterministic adjustment, the transmit power is adjusted in a pre-defined manner based on the pertinent parameters. For probabilistic adjustment, the transmit power has a certain probability of being adjusted, with the probability being determined by the pertinent parameters. Exemplary deterministic and probabilistic adjustment schemes are described below. 
       FIG. 3  shows a flow diagram of a process  300  for adjusting transmit power in a probabilistic manner. Process  300  may be performed by each terminal and for each time interval in which an OSI bit is transmitted. Initially, the terminal processes the OSI bit of the strongest neighbor base station (block  312 ). The terminal then determines whether the OSI bit is ‘1’ or ‘0’ (block  314 ). If the OSI bit is ‘1’, indicating a higher than nominal interference level, then the terminal determines a probability for decreasing the transmit power, Pr dn (n) (block  322 ). Pr dn (n) may be computed based on the current transmit power delta, ΔP(n), and the channel gain ratio for the strongest neighbor base station, r osib (n), as described below. The terminal then randomly selects a value x between 0.0 and 1.0 (block  324 ). In particular, x is a random variable uniformly distributed between 0.0 and 1.0. If the randomly selected value x is less than or equal to the probability Pr dn (n), as determined in block  326 , then the terminal decreases its transmit power delta by a ΔP dn  down step (block  328 ), as follows:
 
Δ P ( n+ 1)=Δ P ( n )×Δ P   dn .  Eq (12)
 
     Otherwise, if x is greater than Pr dn (n), then the terminal maintains the transmit power delta at the current level (block  330 ). From blocks  328  and  330 , the process proceeds to block  342 . 
     If the OSI bit is ‘0’ in block  314 , indicating a lower than nominal interference level, then the terminal determines a probability for increasing the transmit power, Pr up (n), e.g., based on ΔP(n) and r osib (n) as also described below (block  332 ). The terminal then randomly selects a value x between 0.0 and 1.0 (block  334 ). If the randomly selected value x is less than or equal to the probability Pr up (n), as determined in block  336 , then the terminal increases its transmit power delta by an ΔP up  up step (block  338 ), as follows:
 
Δ P ( n+ 1)=Δ P ( n )+Δ P   up .  Eq (13)
 
     The step sizes for ΔP up  and ΔP dn  may both be set to the same suitable value (e.g., 0.25 dB, 0.5 dB, 1.0 dB, and so on). If x is greater than Pr up (n) in block  336 , then the terminal maintains the transmit power delta at the same level (block  330 ). From blocks  330  and  338 , the process proceeds to block  342 . 
     In block  342 , the terminal limits the transmit power delta, ΔP(n+1), to be within the allowable range [ΔP min , ΔP max ]. The terminal then computes the transmit power for the next time interval, P dch (n+1), based on the transmit power delta, ΔP(n+1), and the reference power level, P ref (n+1), for the next time interval, as shown in equation (1) (block  344 ). The terminal then limits the transmit power P dch (n+1) to be within the maximum power level (block  346 ), as follows: 
     
       
         
           
             
               
                 
                   
                     
                       P 
                       dch 
                     
                     ⁡ 
                     
                       ( 
                       
                         n 
                         + 
                         1 
                       
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               
                                 P 
                                 dch 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   n 
                                   + 
                                   1 
                                 
                                 ) 
                               
                             
                             , 
                           
                         
                         
                           
                             
                               
                                 if 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
                                     P 
                                     dch 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       n 
                                       + 
                                       1 
                                     
                                     ) 
                                   
                                 
                               
                               ≤ 
                               
                                 P 
                                 max 
                               
                             
                             , 
                           
                         
                       
                       
                         
                           
                             
                               P 
                               max 
                             
                             , 
                           
                         
                         
                           
                             otherwise 
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Eq 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     14 
                     ) 
                   
                 
               
             
           
         
       
     
     The terminal uses the transmit power P dch (n+1) for the next time interval. 
     The probabilities Pr dn (n) and Pr up (n) may be a function of the transmit power delta, ΔP(n), and the channel gain ratio for the strongest neighbor base station, r osib (n). Various functions may be used for Pr dn (n) and Pr up (n). Each function may have a different impact on various power control characteristics such as (1) the convergence rate of the transmit power adjustment and (2) the distribution of transmit power deltas for the terminals in the system. 
     In an embodiment, the probabilities Pr dn (n) and Pr up (n) are defined as follows:
 
 Pr   up ( n )=max( Pr   up,min ,[1− Pr   Δp ( n )]·[1 −Pr   gain ( n )]), and  Eq (15a)
 
 Pr   dn ( n )=max( Pr   dn,min   ,Pr   Δp ( n )· Pr   gain ( n )),  Eq (15b)
 
     
       
         
           
             
               
                 
                   
                     
                       
                         Pr 
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           P 
                         
                       
                       ⁡ 
                       
                         ( 
                         n 
                         ) 
                       
                     
                     = 
                     
                       
                         
                           min 
                           ⁡ 
                           
                             ( 
                             
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   P 
                                   ⁡ 
                                   
                                     ( 
                                     n 
                                     ) 
                                   
                                 
                               
                               , 
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
                                     P 
                                     ~ 
                                   
                                   max 
                                 
                               
                             
                             ) 
                           
                         
                         - 
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             
                               P 
                               ~ 
                             
                             min 
                           
                         
                       
                       
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             
                               P 
                               ~ 
                             
                             max 
                           
                         
                         - 
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             
                               P 
                               ~ 
                             
                             min 
                           
                         
                       
                     
                   
                   , 
                   
                     
 
                   
                   ⁢ 
                   where 
                 
               
               
                 
                   Eq 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     
                       15 
                       ⁢ 
                       c 
                     
                     ) 
                   
                 
               
             
             
               
                 
                   
                     
                       
                         Pr 
                         gain 
                       
                       ⁡ 
                       
                         ( 
                         n 
                         ) 
                       
                     
                     = 
                     
                       
                         
                           min 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   r 
                                   osib 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   n 
                                   ) 
                                 
                               
                               , 
                               
                                 r 
                                 max 
                               
                             
                             ) 
                           
                         
                         - 
                         
                           r 
                           min 
                         
                       
                       
                         
                           r 
                           max 
                         
                         - 
                         
                           r 
                           min 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   Eq 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     
                       15 
                       ⁢ 
                       d 
                     
                     ) 
                   
                 
               
             
           
         
       
     
     Pr Δp (n) is a probability related to the transmit power level; 
     Pr gain (n) is a probability related to the channel gain ratio for the strongest neighbor base station; 
     Δ{tilde over (P)} max , Δ{tilde over (P)} min , r max , and r min  are normalizing constants selected to achieve the desired power control characteristics; 
     Pr up,min  is a minimum probability for upward adjustment of transmit power; and 
     Pr dn,min  is a minimum probability for downward adjustment of transmit power. 
     For the embodiment shown by equation set (15), Pr dn (n) and Pr up (n) are joint probabilities determined by the transmit power level and the channel gain ratio for the strongest neighbor base station. The minimum probabilities Pr up,min  and Pr dn,min  improve steady-state characteristics and promote some movement for points in the extremes (e.g., very high or very low channel gain values). The probabilities Pr dn (n) and Pr up (n) derived as shown in equation set (15) conform to the general transmit power adjustment rules given in Table 1. The probabilities Pr dn (n) and Pr up (n) may also be derived with some other functions, and this is within the scope of the invention. 
       FIG. 4  shows a flow diagram of a process  400  for adjusting transmit power in a deterministic manner. Process  400  may also be performed by each terminal and for each time interval in which an OSI bit is transmitted. The terminal processes the OSI bit of the strongest neighbor base station (block  412 ) and determines whether the OSI bit is ‘1’ or ‘0’ (block  414 ). If the OS bit is ‘1’, then the terminal determines the amount of decrease in transmit power, ΔP dn (n+1), for the next time interval (block  422 ). The variable down step size may be determined based on the current transmit power delta, ΔP(n), and the channel gain ratio for the strongest base station, r osib (n). The terminal then decreases the transmit power delta by ΔP dn (n+1) (block  424 ). Otherwise, if the OSI bit is ‘0’, then the terminal determines the amount of increase in transmit power, ΔP up (n+1), for the next time interval, e.g., based on ΔP(n) and r osib (n) (block  432 ). The terminal then increases the transmit power delta by ΔP up (n+1) (block  434 ). After blocks  424  and  434 , the terminal limits the transmit power delta for the next time interval, ΔP(n+1), to be within the allowable range of [ΔP min , ΔP max ] (block  442 ) and further computes and limits the transmit power for the next time interval to be within the maximum power level (blocks  444  and  446 ). 
     The variable step sizes ΔP dn (n+1) and ΔP up (n+1) may be determined based on a predetermined function of ΔP(n) and r osib (n) e.g., similar to the function expressed by equation set (15). The variable step sizes may be defined to be proportional to ΔP(n) and inversely proportional to r osib (n). The adjustment probabilities and variable step sizes may also be determined based on a look-up table of different probabilities and step size values for different ΔP(n) and r osib (n) values, or by some other means. 
       FIGS. 3 and 4  show exemplary embodiments for adjusting transmit power in a probabilistic and a deterministic manner, respectively. For the probabilistic embodiment shown in  FIG. 3 , the adjustment probability is determined based on the parameters ΔP(n) and r osib (n), and fixed-size up and down steps are used for transmit power adjustment. For the deterministic embodiment shown in  FIG. 4 , the adjustment probability is fixed at 1.0, and the up and down step sizes are determined based on the parameters ΔP(n) and r osib (n). Various modifications may also be made to these embodiments. For example, variable up and down step sizes may also be used for the probabilistic embodiment. As another example, fixed-size up and down steps may be used for the deterministic embodiment. 
     The power delta ΔP(n) for the data channel may be adjusted based on the OSI bit, the channel gain, the prior power delta ΔP(n×1), the range of allowable power deltas, and the maximum power level for the terminal, as described above. In general, the power delta ΔP(n) may be adjusted based on any one or any combination of parameters. Other parameters that may be used to adjust ΔP(n) include the current transmit power P dch (n), a peak-to-average backoff factor ΔP bo , a “designated” set of base stations that may potentially observe high interference from the terminal, and so on. The peak-to-average backoff factor may be determined by the number of subbands used by the terminal for transmission, and a higher value may be used for ΔP bo  if more subbands are used for transmission. The transmit power for the data channel may be constrained to be less than Pmax minus this backoff factor, or P dch (n)≦(P max −ΔP bo ) 
     The transmit power for the terminal may also be adjusted based on OSI bits sent by multiple base stations (e.g., the serving and/or neighbor base stations). The transmit power may be adjusted in the same manner or different manners for the serving base station and a neighbor base station. The terminal may be orthogonal to other terminals communicating with the serving base station, but may nevertheless cause some interference to these other terminals if complete orthogonality is not achieved. The transmit power for the terminal may be adjusted lower if the OSI bit for the serving base station is set to 1′. The amount of transmit power adjustment due to the OSI bit from the serving base station may be determined based on computer simulation, empirical measurement, and so on, to achieve good performance. 
     The transmit power for the terminal may also be adjusted based on other parameters, criteria, and information. For example, the terminal may consider only OSI bits from the base stations in the designated set. The terminal may also consider or not consider a given base station for transmit power adjustment based on the channel gain and/or other parameters for that base station. The terminal may also adjust the transmit power by different amounts and/or in different manners based on all of the information available for the base station(s) to be considered for transmit power adjustment. 
       FIG. 5  shows a power control mechanism  500  that may be used to adjust the transmit power for a terminal  120   x  in system  100 . Terminal  120   x  communicates with a serving base station  110   x  and may cause interference to neighbor base stations  110   a  through  110   m  (albeit by different amounts). Power control mechanism  500  includes a reference loop  510  and a second loop  520 . Reference loop  510  operates between terminal  120   x  and serving base station  110   x . Second loop  520  operates between terminal  120   x  and neighbor base stations  110   a  through  110   m  and possibly serving base station  110   x . For simplicity,  FIG. 5  shows only the portion of loops  510  and  520  residing at terminal  120   x.    
     Reference loop  510  adjusts the transmit power for a control channel (or some other traffic channel) and attempts to maintain the received SNR for this control channel, as measured at serving base station  110   x , as close as possible to a target SNR. For reference loop  510 , serving base station  110   x  estimates the received SNR for the control channel, compares the received SNR against the target SNR, and generates transmit power control (TPC) commands based on the comparison results, as described below. Each TPC command may be either (1) an UP command to direct an increase in transmit power for the control channel or (2) a DOWN command to direct a decrease in transmit power. Serving base station  110   x  transmits the TPC commands on the forward link (cloud  570 ) to terminal  120   x.    
     Terminal  120   x  receives and processes the forward link transmission from serving base station  110   x  and provides “received” TPC commands to a TPC command processor  542 . Each received TPC command is a noisy version of a TPC command transmitted by serving base station  110   x . Processor  542  detects each received TPC command and obtains a “TPC decision”, which may be (1) an UP decision if the received TPC command is deemed to be an UP command or (2) a DOWN decision if the received TPC command is deemed to be an DOWN command. A control channel transmit (TX) power adjustment unit  544  adjusts the transmit power for the control channel, P cch (n), based on the TPC decisions from TPC command processor  542 . For example, unit  544  may increase P cch (n) by a ΔP cch,up  up step for each UP decision and decrease P cch (n) by a ΔP cch,dn  down step for each DOWN decision. A TX data processor/modulator  560  sets the transmit power for the control channel to the P cch (n) level indicated by unit  544 . The transmission on the control channel is sent to serving base station  110   x.    
     Due to path loss, fading, and multipath effects on the reverse link (cloud  540 ), which typically vary over time and especially for a mobile terminal, the received SNR for the control channel continually fluctuates. Reference loop  510  attempts to maintain the received SNR at or near the target SNR in the presence of changes in the reverse link channel condition. 
     Second loop  520  adjusts the transmit power for a data channel (or some other traffic channel) such that a power level that is as high as possible is used for the data channel while keeping inter-sector and intra-sector interference to within acceptable levels. For second loop  520 , an OSI bit processor  552  receives and processes the OSI bits broadcast by neighbor base stations  110   a  through  110   m  and possibly serving base station  110   x . OSI bit processor  552  provides detected OSI bits from the base stations to a transmit power delta adjustment unit  556 . A channel estimator  554  receives pilots from the serving and neighbor base stations, estimates the channel gain for each base station, and provide the estimated channel gains for all base stations to unit  556 . Unit  556  determines the channel gain ratios for the neighbor base stations and identifies the strongest neighbor base station. Unit  556  further adjusts the transmit power delta ΔP(n) for the data channel based on the detected OSI bit and the channel gain ratio for the strongest neighbor, as described above. Unit  556  may implement process  300  or  400  and may adjust ΔP(n) in a probabilistic or deterministic manner. In general, unit  556  may adjust the transmit power delta ΔP(n) based on detected OSI bits and/or other pertinent information for any number of base stations, which may include the serving and/or neighbor base stations. 
     A data channel transmit power computation unit  558  receives the control channel transmit power, P cch (n), which is used as the reference power level, P ref (n), and the transmit power delta, ΔP(n). Unit  558  computes the transmit power P dch (n) for the data channel based on P cch (n) and ΔP(n). Unit  560  sets the transmit power for the data channel to the P dch (n) level indicated by unit  558 . The transmission on the data channel is sent to serving base station  110   x . The transmissions on the data and control channels may cause interference to neighbor base stations  110   a  through  110   m.    
     Each base station  110  receives transmissions from terminals on the reverse link, estimates the interference observed by that base station, compares the measured interference against the nominal interference threshold, sets the OSI bit accordingly based on the comparison result, and broadcasts the OSI bit on the forward link. 
     Reference loop  510  and second loop  520  may operate concurrently but may be updated at different rates, with loop  510  being a faster loop than loop  520 . The update rates for the two loops may be selected to achieve the desired power control performance. As an example, reference loop  510  may be updated at a rate of, e.g., 150 times per second, and second loop may be updated at a rate of, e.g., 10 to 20 times per second. Reference loop  510  and second loop  520  may operate on transmissions sent on the control channel and the data channel, respectively. The control and data channels may be assigned different subbands in each hop period, as shown in  FIG. 2 . In this case, reference loop  510  and second loop  520  may operate simultaneously on transmissions sent on different subbands. The control channel may also be multiplexed with the data channel (e.g., using TDM and/or CDM) and sent on the same subbands. 
       FIG. 6  shows a power control mechanism  600  that may be used for the control channel. Power control mechanism  600  (which may be used for reference loop  510  in  FIG. 5 ) includes an inner loop  610 , an outer loop  620 , and a third loop  630 . Inner loop  610  attempts to maintain the received SNR for the control channel as close as possible to the target SNR. For inner loop  610 , an SNR estimator  642  at serving base station  110   x  estimates the received SNR for the control channel and provides the received SNR to a TPC command generator  644 . Generator  644  compares the received SNR against the target SNR and generates TPC commands based on the comparison results. Serving base station  110   x  transmits the TPC commands on the forward link (cloud  570 ) to terminal  120   x . Terminal  120   x  receives and processes the TPC commands from serving base station  110   x  and adjusts the transmit power for the control channel, as described above for  FIG. 5 . 
     Data may be sent in blocks on the control channel, and each data block may be coded with a block code to obtain a corresponding codeword (or coded data block). An error detection code may not be used for the control channel. In this case, the serving base station may perform erasure detection for each received codeword to determine whether the codeword is erased or non-erased. An erased codeword may be deemed to be unreliable and processed accordingly (e.g., discarded). The erasure detection may be performed by computing a metric for each received codeword, comparing the computed metric against an erasure threshold, and declaring the received codeword to be erased or non-erased based on the comparison result. 
     Outer loop  620  adjusts the target SNR such that a target erasure rate, Pr erasure , is achieved for the control channel. The target erasure rate indicates a desired probability (e.g., 10%) of declaring a received codeword as erased. A metric computation unit  652  computes the metric for each received codeword. An erasure detector  654  performs erasure detection for each received codeword based on its computed metric and the erasure threshold and provides the status of the received codeword (erased or non-erased) to a target SNR adjustment unit  656 . Unit  656  then adjusts the target SNR for the control channel as follows: 
     
       
         
           
             
               
                 
                   
                     
                       SNR 
                       target 
                     
                     ⁡ 
                     
                       ( 
                       
                         k 
                         + 
                         1 
                       
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               
                                 
                                   SNR 
                                   target 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   k 
                                   ) 
                                 
                               
                               + 
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   SNR 
                                   up 
                                 
                               
                             
                             , 
                           
                         
                         
                           
                             
                               for 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               an 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               erasedcodeword 
                             
                             , 
                           
                         
                       
                       
                         
                           
                             
                               
                                 
                                   SNR 
                                   target 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   k 
                                   ) 
                                 
                               
                               - 
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   SNR 
                                   dn 
                                 
                               
                             
                             , 
                           
                         
                         
                           
                             
                               for 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               a 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               non 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               erasedcodeword 
                             
                             , 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Eq 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     16 
                     ) 
                   
                 
               
             
           
         
       
     
     where SNR target (k) is the target SNR for outer loop update interval k; 
     ΔSNR up  is an up step size for the target SNR; and 
     ΔSNR dn  is a down step size for the target SNR. 
     The ΔSNR up  and ΔSNR dn  step sizes may be set based on the following: 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       SNR 
                       up 
                     
                   
                   = 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         SNR 
                         dn 
                       
                       · 
                       
                         
                           ( 
                           
                             
                               1 
                               - 
                               
                                 Pr 
                                 erasure 
                               
                             
                             
                               Pr 
                               erasure 
                             
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     17 
                     ) 
                   
                 
               
             
           
         
       
     
     Third loop  630  adjusts the erasure threshold such that a target conditional error rate, Pr error , is achieved for the control channel. The target conditional error rate indicates a desired probability of a received codeword being decoded in error when deemed to be non-erased. A small Pr error  (e.g., 1%) corresponds to high confidence in the decoding results for non-erased codewords. Terminal  110   x  and/or other terminals in communication with serving base station  110   x  may transmit known codewords on the control channel periodically or when triggered. Units  652  and  654  perform erasure detection for each received known codeword in the same manner as for a received codeword. For each received known codeword deemed to be non-erased, a decoder  662  decodes the received known codeword and determines whether the decoded data block is correct or in error. Decoder  662  provides the status of each received known codeword, which may be erased, “good”, or “bad”. A good codeword is a received known codeword deemed to be non-erased and decoded correctly. A bad codeword is a received known codeword deemed to be non-erased but decoded in error. An erasure threshold adjustment unit  664  adjusts the erasure threshold based on the status of each received known codeword, as follows: 
     
       
         
           
             
               
                 
                   
                     
                       TH 
                       erasure 
                     
                     ⁡ 
                     
                       ( 
                       
                         l 
                         + 
                         1 
                       
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               
                                 
                                   TH 
                                   erasure 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   l 
                                   ) 
                                 
                               
                               + 
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   TH 
                                   up 
                                 
                               
                             
                             , 
                           
                         
                         
                           
                             
                               fora 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               goodcodeword 
                             
                             , 
                           
                         
                       
                       
                         
                           
                             
                               
                                 
                                   TH 
                                   erasure 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   l 
                                   ) 
                                 
                               
                               - 
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   TH 
                                   dn 
                                 
                               
                             
                             , 
                           
                         
                         
                           
                             fora 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             badcodewordand 
                           
                         
                       
                       
                         
                           
                             
                               
                                 TH 
                                 erasure 
                               
                               ⁡ 
                               
                                 ( 
                                 l 
                                 ) 
                               
                             
                             , 
                           
                         
                         
                           
                             foran 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             erasedcodeword 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Eq 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     18 
                     ) 
                   
                 
               
             
           
         
       
     
     where TH erasure  ( ) is the erasure threshold for third loop update interval l; 
     ΔTH up  is an up step size for the erasure threshold; and 
     ΔTH dn  is a down step size for the erasure threshold. 
     Equation (18) assumes that a lower erasure threshold increases the likelihood of a received codeword being declared erased. 
     The ΔTH up  and ΔTH dn  step sizes may be set based on the following: 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       TH 
                       dn 
                     
                   
                   = 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         TH 
                         up 
                       
                       · 
                       
                         
                           ( 
                           
                             
                               1 
                               - 
                               
                                 Pr 
                                 error 
                               
                             
                             
                               Pr 
                               error 
                             
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     19 
                     ) 
                   
                 
               
             
           
         
       
     
     Inner loop  610 , outer loop  620 , and third loop  630  are typically updated at different rates. Inner loop  610  is the fastest loop of the three loops, and the transmit power for the control channel may be updated at a particular rate (e.g., 150 times per second). Outer loop  620  is the next fastest loop, and the target SNR may be updated whenever a codeword is received on the control channel. Third loop  630  is the slowest loop, and the erasure threshold may be updated whenever a known codeword is received on the control channel. The update rates for the three loops may be selected to achieve the desired performance for erasure detection and power control for the control channel. Power control mechanism  600  is further described in commonly assigned U.S. patent application Ser. No. , entitled “Robust Erasure Detection and Erasure-Rate-Based Closed Loop Power Control,” filed Jul. 13, 2004. 
     For clarity, specific embodiments have been described above for various aspects of power control. Numerous other embodiments may also be derived based on the description provided herein. Some examples are given below. 
     The same range of allowable transmit power deltas, [ΔP min , ΔP max ], may be used for all terminals in the system. Different ranges of [ΔP min , ΔP max ] may also be used for different terminals, e.g., depending on their locations. For example, terminals with smaller channel gain ratio for the strongest neighbor base stations may use a smaller range of transmit power deltas (e.g., the same ΔP min  but a smaller ΔP max ) than terminals located closer to the serving base stations. 
     The reference power level, P ref (n), used to derive the data channel transmit power, P dch (f), may be set to the transmit power for another power-controlled channel, as described above. The reference power level may also be obtained in other manners, e.g., estimated based on the channel gain for the serving base station. The data channel transmit power may also be adjusted directly, instead of via the transmit power delta. The serving base station may provide feedback to inform the terminal whether the data channel transmit power is within an allowable range. 
     A terminal may only respond to the OSI bit of the strongest neighbor base station, as described above. A terminal may also adjust its transmit power based on OSI bits of multiple neighbor base stations. For example, a terminal may perform process  300  or  400  for S strongest neighbor base stations, one base station at a time, where S&gt;1. The channel gain ratio for each neighbor base station may be taken into account in either the adjustment probabilities (for process  300 ) or the variable step sizes (for process  400 ). 
     A single OSI bit may be used to indicate the interference observed by each base station, as described above. Multiple bits may also be used to report interference. This may allow the terminals to more quickly and/or effectively adjust their transmit powers. This may, in turn, improve overall system stability and performance. For example, each base station may report information on “how far” the measured interference is from the nominal interference threshold. As another example, each base station may broadcast an additional bit (a distress/panic bit) that may be set to ‘1’ when the interference level exceeds a high interference threshold. This high threshold may be significantly higher (e.g., 2 to 3 standard deviations higher) than the nominal threshold. A rapidly rising or an unusually high interference level is often a sign of the system becoming unstable. Upon observing the panic bit set, each terminal may simply set its transmit power delta to the minimum value, ΔP min , and may remain at this transmit power level until the panic bit is reset to ‘0’. In conjunction with the power control for the control channel, this mechanism may be effective in ensuring system stability. 
     Each base station may broadcast its interference information to all terminals, if the interference observed by the base station is randomized, e.g., with frequency hopping. If the base stations have more specific interference information, then the transmit powers of the terminals may be adjusted in a manner to take advantage of this information. For example, each terminal may be assigned one or more specific subbands for data transmission (without frequency hopping). A base station may then observe different amounts of interference on different subbands. Terminals causing large amounts of interference may be specifically identified based on their assigned subbands, and the transmit powers of these terminals may be reduced accordingly. 
     The supported data rate for each terminal is determined by the received SNR for the data channel. This received SNR, for the embodiments described above, is dependent on (1) the target SNR associated with the reference power level and (2) the transmit power delta, ΔP(n), used by the terminal. The transmit power delta may be autonomously adjusted by the terminal without any input from the serving base station, as described above. The terminal may send the transmit power delta, the received SNR for the data channel, the supported data rate for the data channel, or equivalent information to the serving base station. The terminal may also send the maximum number of subbands, N sb,max (n), that the terminal can support at the current transmit power delta, the desired quality of service (QoS), the buffer size, and so on. To reduce the amount of signaling, the terminal may send ΔP(n) and N sb,max (n) every few update intervals, via in-band signaling on the data channel, and so on. 
     A scheduler at/for the serving base station may use all of the information reported by the terminal to allocate resources to the terminal and to schedule the terminal for data transmission on the reverse link. The scheduler may allocate N sb,max (n) subbands, less than N sb,max (n) subbands, or more than N sb,max (n) subbands to the terminal. If the scheduler allocates more than N sb,max (n) subbands, then the terminal can scale down the transmit power delta accordingly. For example, if 2N sb,max (n) subbands are allocated, then ΔP(n) may be scaled down by a factor of two. 
     The power control may be performed by each terminal based on various pieces of information the terminal obtains from its serving base station and neighbor base stations, as described above. The power control may also be performed by each base station for all terminals in communication with the base station. For example, each base station may obtain an interference report (e.g., the OSI bit) for each neighbor base station, e.g., via signaling between the base stations or transmissions from the terminals. Each base station may also obtain the channel gains determined by each terminal for the serving and neighbor base stations. Each base station may then compute the transmit power delta for each terminal based on the interference reports and the channel gains applicable for that terminal and may sent the transmit power delta to the terminal. Each terminal may then adjust its transmit power using the transmit power delta received from its serving base station. Alternatively, each base station may compute and send the transmit power for each terminal. The availability of the transmit power deltas for all terminals in communication with each base station can expedite the scheduling for the terminals. 
     The power control techniques described herein may be used for various types of wireless communication systems. These techniques are especially suited for systems with small intra-sector interference, e.g., OFDMA, TDMA and FDMA systems. 
     The techniques described herein may be used for power control of various types of traffic channels (e.g., data and control channels). These techniques are also well suited for a hybrid automatic retransmission (H-ARQ) scheme. With H-ARQ, each coded packet is partitioned into multiple (Nbl) subblocks, and one subblock is transmitted at a time for the coded packet. As each subblock for a given coded packet is received via the reverse link, the serving base station attempts to decode and recover the packet based on all subblocks received thus far for the packet. The serving base station is able to recover the packet based on a partial transmission because the subblocks contain redundant information that is useful for decoding when the received SNR is low but may not be needed when the received SNR is high. The serving base station transmits an acknowledgment (ACK) if the packet is decoded correctly, and the terminal may terminate the transmission of the packet early upon receiving the ACK. 
     With H-ARQ, each coded packet may be transmitted in a variable amount of time until decoded correctly. A conventional power control mechanism that adjusts the received SNR for the data channel based on packet error rate (PER) would reduce the transmit power for the data channel to a low level such that a target PER is achieved with all Nbl subblocks transmitted for each coded packet. This may severely reduce system throughput. The techniques described herein allow a high transmit power level to be used even with variable duration transmission supported by H-ARQ. 
       FIG. 7  shows a block diagram of an embodiment of terminal  120   x , serving base station  110   x , and neighbor base station  110   a . On the reverse link, at terminal  120   x , a TX data processor  710  processes (e.g., codes, interleaves, and modulates) reverse link (RL) traffic data and provides modulation symbols for the traffic data. TX data processor  710  also processes control data (e.g., a channel quality indicator) from a controller  720  and provides modulation symbols for the control data. A modulator (MOD)  712  processes the modulation symbols for the traffic and control data and pilot symbols and provides a sequence of complex-valued chips. The processing by TX data processor  710  and modulator  712  is dependent on the system. Modulator  712  performs OFDM modulation if the system utilizes OFDM. A transmitter unit (TMTR)  714  conditions (e.g., converts to analog, amplifies, filters, and frequency upconverts) the sequence of chips and generates a reverse link signal, which is routed through a duplexer (D)  716  and transmitted via an antenna  718 . 
     At serving base station  110   x , the reverse link signal from terminal  120   x  is received by an antenna  752   x , routed through a duplexer  754   x , and provided to a receiver unit (RCVR)  756   x . Receiver unit  756   x  conditions (e.g., filters, amplifies, and frequency downconverts) the received signal and further digitizes the conditioned signal to obtain a stream of data samples. A demodulator (DEMOD)  758   x  processes the data samples to obtain symbol estimates. A receive (RX) data processor  760   x  then processes (e.g., deinterleaves and decodes) the symbol estimates to obtain decoded data for terminal  120   x . RX data processor  760   x  also performs erasure detection and provides to a controller  770   x  the status of each received codeword used for power control. The processing by demodulator  758   x  and RX data processor  760   x  is complementary to the processing performed by modulator  712  and TX data processor  710 , respectively. 
     The processing for a forward link transmission may be performed similarly to that described above for the reverse link. The processing for the transmissions on the forward and reverse links is typically specified by the system. 
     For reverse link power control, at serving base station  110   x , an SNR estimator  774   x  estimates the received SNR for terminal  120   x  and provides the received SNR to a TPC command (cmd) generator  776   x . Generator  776   x  also receives the target SNR and generates TPC commands for terminal  120   x . The TPC commands are processed by a TX data processor  782   x  and a modulator  784   x , conditioned by a transmitter unit  786   x , routed through duplexer  754   x , and transmitted via antenna  752   x  to terminal  120   x . At neighbor base station  110   a , an interference estimator  774   a  estimates the interference observed by the base station and provides the measured interference to an OSI bit generator  776   a . Generator  776   a  also receives the nominal interference threshold and generates the OSI bit for base station  110   a . The OSI bit is processed and broadcast to terminals in the system. Generator  776   a  may also generate a panic bit or some other type of interference report. 
     At terminal  120   x , the forward link signals from the serving and neighbor base stations are received by antenna  718 . The received signal is routed through duplexer  716 , conditioned and digitized by a receiver unit  740 , and processed by a demodulator  742  and an RX data processor  744  to obtain received TPC commands and received OSI bits. A channel estimator within demodulator  742  estimates the channel gain for each base station. A TPC processor  724  detects the received TPC commands to obtain TPC decisions, which are used to update the transmit power for the control channel. TPC processor  724  also adjusts the transmit power for the data channel based on the received OSI bits for neighbor base stations, the channel gains for the serving and neighbor base stations, and the transmit powers for the data and control channels, as described above. TPC processor  724  (or controller  720 ) may implement process  300  in  FIG. 3  or process  400  in  FIG. 4 . TPC processor  724  provides transmit power adjustment controls for the control and data channels. Processor  710  and/or modulator  712  receive the controls from TPC processor  724  and adjust the transmit powers for control and data channels. 
     Controllers  720 ,  770   x , and  770   a  direct the operations of various processing units within terminal  120   x  and base station  110   x  and  110   a , respectively. These controllers may also perform various functions for power control for the reverse link. For example, controllers  720  and  770   x  may implement the processing units shown in  FIGS. 5 and 6  for terminal  120   x  and base station  110   x , respectively. Memory units  722 ,  772   x , and  772   a  store data and program codes for controllers  720 ,  770   x , and  770   a , respectively. A scheduler  780   x  schedules terminals for data transmission to/from serving base station  110   x.    
     The power control techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing units used to perform power control may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. 
     For a software implementation, the power control techniques may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit (e.g., memory unit  722  in  FIG. 7 ) and executed by a processor (e.g., controller  720 ). The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art. 
     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.