Patent Publication Number: US-8116253-B2

Title: Controlling forward and reverse link traffic channel power

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §120 
     The present Application for Patent is a Continuation in Part and claims priority to U.S. patent application Ser. No. 11/781,883 filed Jul. 23, 2007, entitled “Controlling Forward Link Traffic Channel Power”, which is a continuation of U.S. patent application Ser. No. 10/267,289, filed Oct. 8, 2002, entitled “Controlling Forward Link Traffic Channel Power”; now allowed, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The present aspects of the present invention generally relate to wireless communications networks, and more particularly to methods and apparatus for controlling transmission power of both the forward and reverse links. 
     2. Background 
     There are a variety of wireless communication systems having multiple beam or sector communication links. A satellite-based communication system is one such example. Another example is a cellular communication system. 
     A satellite-based communication system includes one or more satellites to relay communications signals between gateways and user terminals. Gateways provide communication links for connecting a user terminal to other user terminals or users of other communications systems, such as a public switched telephone network (PSTN). User terminals can be fixed or mobile, such as a mobile telephone, and positioned near a gateway or remotely located. 
     A satellite can receive signals from and transmit signals to a user terminal provided the user terminal is within the “footprint” of the satellite. The footprint of a satellite is the geographic region on the surface of the earth covered by the satellite communications system. In some satellite systems, a satellite&#39;s footprint is geographically divided into “beams,” through the use of beam forming antennas. Each beam covers a particular geographic region within a satellite&#39;s footprint. 
     Some satellite communications systems employ code division multiple access (CDMA) spread-spectrum signals, as disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled “Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,” and U.S. Pat. No. 5,691,174, which issued Nov. 25, 1997, entitled “Method and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy,” both of which are assigned to the assignee of the present invention, and are incorporated herein by reference. 
     The method for providing CDMA mobile communications was standardized in the United States by the Telecommunications Industry Association in TIA/EIA/IS-95-A entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” referred to herein as IS-95. Combined AMPS &amp; CDMA systems are described in TIA/EIA Standard IS-98. Other communications systems are described in the IMT-2000UM, or International Mobile Telecommunications System 2000/Universal Mobile Telecommunications System, standards covering what are referred to as wideband CDMA (WCDMA), cdma2000 (such as cdma2000 1x or 3x standards, for example) or TD-SCDMA. 
     Cellular communications may also employ CDMA techniques. However, instead of receiving signals from gateways that are relayed through one or more satellites, user terminals receive signals from a fixed position base station that supports multiple sectors, each corresponding to a particular geographic region, similar to having multiple beams. 
     Gateways and base stations transmit information in the form of wireless signals to user terminals across forward link communications channels. These wireless signals need to be transmitted at power levels sufficient to overcome noise and interference so that the transfer of information occurs within specified error rates. In addition, these wireless signals need to be transmitted at power levels that are not excessive so that they do not interfere with communications involving other user terminals. Faced with this challenge, gateways and base stations employ dynamic forward link power control techniques to establish appropriate forward link transmit power levels. 
     Conventional forward and reverse link power control techniques involve closed loop approaches where user terminals provide gateways and base stations with feedback that specifies particular transmit power adjustments. For example, one such approach involves a user terminal determining signal-to-noise ratios (SNRs) of received forward link traffic signals. Based on these determined SNRs, the user terminal transmits commands that direct the gateway or base station to either increase or decrease the transmit power of traffic signals sent to the user terminal. 
     These commands are referred to as up/down commands because they direct either a power increase or a power decrease. Up/down commands are transmitted to the gateway or base station across an up/down power control channel. This channel is typically implemented by “puncturing” the up/down commands into frames of user terminal data that are transmitted to the gateway or base station. This puncturing can limit the data rates at which user terminals transmit information to gateways and base stations. Additionally, punctured channels may not be as reliable because punctured commands may introduce a higher bit error rate for a given signal-to-noise ratio and punctured channels are sending uncoded bits reducing the reliability of the up/down commands. 
     In addition to transmitting up/down commands, user terminals typically transmit other types of information to gateways and base stations. For example, many user terminals periodically transmit various power measurements and noise measurements to support operations, such as “handoffs” between beams during an active call. To eliminate the less reliable transmission of data rate limiting power adjustment commands, it is desirable for gateways and base stations to utilize such transmitted measurements to control forward link transmit power levels. 
     In addition, it is desirable to conserve forward link transmission power to maximize capacity and minimize interference. It is desirable to conserve reverse link transmission power to minimize interference and conserve battery life. Since satellite and cellular communications systems employ multiple beams, transmissions received by user terminals in a particular beam are susceptible to interference from transmissions designated for neighboring beams. A user terminal&#39;s interference susceptibility is related to its proximity to adjacent beams. A user terminal&#39;s reverse link is also susceptible to other users transmitting in the same beam or sector (an orthogonal interferer). Namely, the closer a user or user terminal is to an adjacent beam (a non-orthogonal interferer), the more susceptible the user is to interference from neighboring beams. Additionally, narrow band or wide band jammers may exist in close proximity to the user, increasing their interference susceptibility. 
     In a satellite-based communications system where the satellites are not stationary, the geographic area covered by a given satellite is constantly changing. As a result, a user terminal positioned within a particular beam of a particular satellite at one point in time can later be positioned within a different beam of the same satellite and/or within a different beam of a different satellite. Furthermore, because satellite communication is wireless, a user terminal is free to move about. As a result, user terminals typically have varying positions within a beam while receiving transmissions across forward link channels. Accordingly, their susceptibility to interference may vary over time. 
     One technique for reducing interference received by user terminals is to boost the power of signals that are transmitted by satellites and/or cellular base stations to user terminals by a fixed margin. However, since user terminals can experience varying degrees of interference susceptibility, this approach has the drawback of wasting power on users that are not as susceptible to interference as others. In addition, this approach can cause additional interference with other user terminals. 
     Accordingly, as with the elimination of user terminals needing to transmit closed loop power adjustment commands, techniques for reducing interference while conserving transmit power are desirable, especially in systems having limited power budgets. 
     SUMMARY 
     The aspects of the present invention are directed to apparatus and methods for controlling forward or reverse link transmission power, P transmit , to or from a user terminal in a wireless communications system. The systems and methods determine a baseline power level, P baseline , from a received active pilot channel signal-to-noise ratio (SNR); determine a power margin, P margin , from an identified interference susceptibility; determine a power level correction, P correction , based on an identified Quality of Service Metric (QSM) such as packet error rate (PER), determine a fade attack power margin, P fade ; and set P transmit  based on P baseline , P margin , P correction , and P fade . For example, P transmit  may be set to a power level that is substantially equal to the sum of P baseline , P margin , P correction , and P fade . For that matter, P transmit  may be any function of P baseline , P margin , P correction , and P fade . The determination of each of these components may be performed using independently running control loops or processes. 
     Determining P baseline  may include calculating a power level offset, P o , and adding P o  to a pilot channel transmit power level. Identifying a user terminal interference susceptibility may include receiving from the user terminal a plurality of signal power measurements. 
     Determining a power level correction, P correction , may include identifying a packet error rate (PER) associated with a user terminal determining P correction  may include increasing P correction  when the identified PER is greater than a desired PER, and decreasing P correction  when the identified PER is less than the desired PER. 
     Each of these signal power measurements corresponds to one of a plurality of beams. For example, these measurements may be pilot signal power measurements conveyed in a pilot strength measurement message (PSMM). Alternatively, these measurements may be conveyed using other types of signals such as a paging message, or an interference susceptibility message. The differences between a first of the signal power measurements, (such as one corresponding to the active beam, or the strongest measurement) and each of the other signal power measurements are calculated. 
     P margin  is set to a first power level when the smallest of the calculated differences is greater than a predetermined threshold. Alternatively, P margin  is set to a second power level when the smallest of the calculated differences is less than or equal to the predetermined threshold. This first power level is less than the second power level. Additionally, there may be N levels or thresholds that differentiate the P margin  mapping. 
     P margin  may also be dependent on interference outside the current communication system, otherwise known as jammers. The identification of jammers may be based upon mobile assisted measurements of interferers and their associated bandwidths (narrow or wide band). 
     Determining P fade  may include calculating or detecting when the signal is attenuated in a fade. This may be accomplished by monitoring the signal to noise ratio (SNR) and comparing it to previous or filtered historical values, using commonly accepted filtering practices. When the presence of a signal fade is identified, increasing P fade  will help the signal overcome the current fade. 
     Identifying a user terminal interference susceptibility may alternatively include determining the location of the user terminal within one of the plurality of beams. In this case, P margin  is set to a first power level when the identified location is within a beam crossover region. Otherwise, P margin  is set to a second power level when the identified location is within a beam central region. Here, the first power level is greater than the second power level. 
     A system for controlling P transmit  includes a selector that determines P baseline , P margin , P correction  and P fade . A transceiver sets P transmit  based on P baseline , P margin , P correction , and P fade . For example, by setting P transmit  to a power level that is substantially equal to the sum or any other function of P baseline , P margin , P correction , and P fade . 
     An advantage of the present invention is that it eliminates the need for closed loop forward link power control techniques where user terminals transmit up/down commands that specify particular forward or reverse link transmit power adjustments. 
     Another advantage of the present invention is that it keeps interference levels within acceptable ranges, while conserving transmit power. 
     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary wireless communication system; 
         FIG. 2  illustrates an exemplary footprint having a plurality of beams; 
         FIG. 3  illustrates an operational scenario within a satellite footprint; 
         FIG. 4  is a graph showing signal power as seen by the user terminal; 
         FIGS. 5-7  are flowcharts illustrating operational sequences of an embodiment; 
         FIG. 8  is a flow chart showing the operation of setting P margin ; 
         FIG. 9  is a flow chart showing the operation of setting P fade ; 
         FIG. 10  is a block diagram of an exemplary gateway implementation; and 
         FIG. 11  is a block diagram of a forward link transceiver implementation. 
     
    
    
     DETAILED DESCRIPTION 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. 
     I. EXEMPLARY OPERATIONAL ENVIRONMENT 
     Before describing aspects of the invention in detail, it is helpful to describe an example environment in which the claimed invention may be implemented. The presently claimed invention is particularly useful in mobile communications environments.  FIG. 1  illustrates such an environment. 
       FIG. 1  is a block diagram of an exemplary wireless communication system (WCS)  100  that includes a base station  112 , two satellites  116   a  and  116   b , and two associated gateways (also referred to herein as hubs)  120   a  and  120   b . These elements engage in wireless communications with user terminals  124   a ,  124   b , and  124   c . Typically, base stations and satellites/gateways are components of distinct terrestrial and satellite based communication systems. However, these distinct systems may interoperate as an overall communications infrastructure. 
     Although  FIG. 1  illustrates a single base station  112 , two satellites  116 , and two gateways  120 , any number of these elements may be employed to achieve a desired communications capacity and geographic scope. For example, an exemplary implementation of WCS  100  includes 48 or more satellites, traveling in eight different orbital planes in Low Earth Orbit (LEO) to service a large number of user terminals  124 . 
     The terms base station and gateway are also sometimes used interchangeably, each being a fixed central communication station, with gateways, such as gateways  120 , being perceived in the art as highly specialized base stations that direct communications through satellite repeaters while base stations (also sometimes referred to as cell-sites), such as base station  112 , use terrestrial antennas to direct communications within surrounding geographical regions. However, the claimed invention is not limited to multiple access communication systems, and may be employed in other types of systems that employ other access techniques. 
     In this example, user terminals  124  each have or include apparatus or a wireless communication device such as, but not limited to, a cellular telephone, wireless handset, a data transceiver, or a paging or position determination receiver. Furthermore each of user terminals  124  can be hand-held, portable as in vehicle mounted (including cars, trucks, boats, trains, and planes) or fixed, as desired. For example,  FIG. 1  illustrates user terminal  124   a  as a fixed telephone, user terminal  124   b  as a hand-held device, and user terminal  124   c  as a vehicle-mounted device. Wireless communication devices are also sometimes referred to as user terminals, mobile stations, mobile units, subscriber units, mobile radios or radiotelephones, wireless units, terminals, or simply as ‘users’, subscribers, and ‘mobiles’ in some communication systems, depending on preference. 
     User terminals  124  engage in wireless communications with other elements in WCS  100  using code division multiple access (CDMA) techniques. However, the presently claimed invention may be employed in systems that employ other communications techniques, such as time division multiple access (TDMA), and frequency division multiple access (FDMA), or other waveforms or techniques listed above (WCDMA, CDMA2000 . . . ). 
     Generally, beams from a beam source, such as base station  112  or satellites  116 , cover different geographical areas in predefined patterns. Beams at different frequencies, also referred to as CDMA channels, frequency division multiplexed (FDM) signals or channels, or ‘sub-beams’ can be directed to overlap the same region. It is also readily understood by those skilled in the art that beam coverage or service areas for multiple satellites, or antenna patterns for multiple base stations, might be designed to overlap completely or partially in a given region depending on the communication system design and the type of service being offered, and whether space diversity is being achieved. 
       FIG. 1  illustrates several exemplary signal paths. For example, signal paths  130   a - c  provide for the exchange of signals between base station  112  and user terminals  124 . Similarly, signal paths  138   a - d  provide for the exchange of signals between satellites  116  and user terminals  124 . Communications between satellites  116  and gateways  120  are facilitated by signal paths  146   a - d.    
     User terminals  124  are capable of engaging in bi-directional communications with base station  112  and/or satellites  116  across various channels. These channels can be traffic or data channels. These communications are carried across one or more forward link channels and one or more reverse link channels. These channels convey radio frequency (RF) signals across signal paths  130 ,  138 , and  146 . 
     Forward link channels transfer information to user terminals  124 . For example, forward link traffic channels convey signals carrying information, such as digitally encoded voice and data. To receive and process this information, a user terminal  124  needs to acquire the forward link traffic channel timing. This timing acquisition is performed through the reception of a corresponding forward link pilot channel that conveys a pilot signal. 
       FIG. 1  illustrates several exemplary forward and reverse link channels. A forward link traffic channel conveys information signals from base station  112  to user terminal  124   a . User terminal  124   a  acquires the timing of forward link traffic channel through the reception of pilot signals by base station  112  on a forward link pilot channel. Both traffic channel and pilot channel signals are transferred over signal path  130   a  (not shown). Similarly, a reverse link traffic channel conveys information signals from user terminal  124   a  to base station  112  over signal path  130   a  (not shown). 
     Within the context of satellite-based communications involving user terminal  124   c , satellite  116   a , and gateway  120   a , a forward link traffic channel, a forward link pilot channel, and a reverse link traffic channel transfer signals over signal paths  146   a  and  138   c  (not shown). Thus, terrestrial-based links typically involve a single wireless signal path between the user terminal and base station, while satellite-based links typically involve two, or more, wireless signal paths between the user terminal and a gateway through at least one satellite (ignoring multipath). 
     As described above, WCS  100  performs wireless communications according to CDMA techniques. Thus, signals transmitted across the forward and reverse links of signal paths  130 ,  138 , and  146  convey signals that are encoded, spread, and channelized according to CDMA transmission standards. In addition, block interleaving may be employed across these forward and reverse links. These blocks are transmitted in frames (also referred to herein as packets) having a predetermined duration, such as 20 milliseconds. 
     Base station  112 , satellites  116 , and gateways  120  may adjust the power of the signals that they transmit across the forward link traffic channels of WCS  100 . This power (referred to herein as forward traffic channel transmit power) may be varied according to commands, requests, or feedback from user terminal  124 , or according to time. This time varying feature may be employed on a periodic basis. For example, this feature may be employed on a frame-by-frame basis. Alternatively, this feature may be employed on other time boundaries that are either larger or smaller than a frame. Such power adjustments are performed to maintain forward link bit error rates (BER) and/or packet error rates (PER) within specific requirements, reduce interference, and conserve transmission power. 
     For example, gateway  120   a , through satellite  116   a , may transmit forward link traffic channel signals to user terminal  124   b  at a different transmit power than it does for user terminal  124   c . Additionally, gateway  120   a  may vary the forward traffic channel transmit power of each of the forward links to user terminals  124   b  and  124   c  for each successive frame. 
     As described above, pilot signals provide timing and phase references for corresponding traffic signals. These timing references include a phase reference of codes that enables user terminals  124  to become synchronized with the spreading and channelizing functions performed by gateways  124  and base station  112 . In addition, this phase reference allows user terminals  124  to coherently demodulate received traffic signals. 
     WCS  100  may feature different communications offerings across these forward links, such as low data rate (LDR) and high data rate (HDR) services. An exemplary LDR service provides forward links having data rates from 3 kilobits per second (kbps) to 9.6 kbps, while an exemplary HDR service supports typical data rates as high as 604 kbps or more. 
     HDR service may be bursty in nature. That is, traffic transferred across HDR links may suddenly begin and end in an unpredictable fashion. Thus, in one instant, an HDR link may be operating at zero kbps, and in the next moment operating at a very high data rate, such as 604 kbps. 
       FIG. 2  illustrates an exemplary satellite beam pattern  202 , also known as a footprint. As shown in  FIG. 2 , the exemplary satellite footprint  202  includes sixteen beams  204   1 - 204   16 . Each beam covers a specific geographical area, although there usually is some beam overlap. The satellite footprint shown in  FIG. 2  includes an inner beam (beam  204   1 ), middle beams (beams  204   2 - 204   7 ), and outer beams (beams  204   8 - 204   16 ). Beam pattern  202  is a configuration of particular predefined gain patterns that are each associated with a particular beam  204 . 
     Beams  204  are illustrated as having non-overlapping geometric shapes for purposes of illustration only. In fact, beams  204  each have gain pattern contours that extend well beyond the idealized boundaries shown in  FIG. 2 . However, these gain patterns are attenuated beyond these illustrated boundaries such that they do not typically provide significant gain to support communications with user terminals  124  outside of a given “boundary”. 
     Beams  204  may each be considered to have different regions based on their proximity to one or more other beams and/or position within other beam gain patterns. For example,  FIG. 2  illustrates beam  204   2  having a central region  206  and a crossover region  208 . Crossover region  208  includes portions of beam  204   2  that are in close proximity to beams  204   1 ,  204   3 ,  204   7 ,  204   8 ,  204   9 , and  204   10 . Because of this proximity, user terminals  124  within crossover region  208  (as well as similar regions in other beams) are more likely to handoff to an adjacent beam, than are user terminals  124  in central region  206 . However, user terminals  124  within handoff probable regions, such as crossover region  208 , are also more likely to receive interference from communications links in adjacent beams  204 . 
     To illustrate this principle,  FIG. 3  shows an exemplary operational scenario within footprint  202 . This operational scenario involves user terminals  124   d - f  communicating through different beams of a satellite  116 . In particular, user terminals  124   d  and  124   e  are communicating with satellite  116  through beam  204   2 , while user terminal  124   f  is communicating with satellite  116  through beam  204   7 . As shown in  FIG. 3 , user terminal  124   d  is within central region  206  of beam  204   2  and user terminal  124   e  is within crossover region  208  of beam  204   2 . 
     As described above, crossover region  208  is closer to beam  204   7  than is central region  206 . Because of this proximity, user terminal  124   e  within crossover region  208  can be within a higher gain portion of the beam  204   7  gain pattern than can user terminal  124   d  within central region  206 . For instance, in the operational scenario of  FIG. 3 , user terminal  124   f  receives a forward link transmission  302  from satellite  116 . In addition, user terminals  124   d  and  124   e  receive this transmission as attenuated transmissions  302 ′ and  302 ″. Although both are weaker than transmission  302 , transmission  302 ″ is stronger than transmission  302 ′. 
     In addition to receiving these attenuated transmissions, user terminals  124   d  and  124   e  also receive forward link transmissions from satellite  116  that are intended for their reception. In particular, user terminal  124   d  receives a forward link transmission  304  from satellite  116  and user terminal  124   e  receives a forward link transmission  306  from satellite  116 . 
     In the context of exemplary WCS  100 , downlink CDMA transmissions within a particular beam  204  are orthogonally encoded. That is, they are not generally interfering with each other. However, downlink CDMA transmissions from different beams are not necessarily orthogonal, and may interfere with each other. Thus, in the operational scenario of  FIG. 3 , the reception of transmission  304  is susceptible to interference from transmission  302 ′. Similarly, the reception of transmission  306  is susceptible to interference from transmission  302 ″. Furthermore, the reception of  304  is susceptible to interference from jammer  305 . 
       FIG. 4  graphically shows the signal power as seen by user terminal  124 D. Power is in the first axis and frequency is shown in the other. User terminal  124 D will have information regarding the forward link interference, measurements of orthogonal signals within the beam/sector  308 , measurements of non-orthogonal system interference  310  from users outside the current sector/beam, and it is possible for the user terminal to measure jammers within RF band of interest  312 . For example, beam  204   7  can be an in-band source of interference and will be measurable by terminal  124 D. Jammer  305  will also be a source of interference and terminal  124 D may be able to measure the impact of this jammer  305  as well. An adjacent communication system, may also be a source of interference  306  as shown in  FIG. 4 . User terminal  124 D may or may not be able to characterize this source of interference  306 . All this information, collected by terminal  124 D, may be accumulated into a forward link interference message that can be exchanged with the base station for purposes of interference mitigation techniques or transmit power control. As an example, the base station can reduce ‘other sector’ traffic from sector  204   7 , to reduce interference, based on the aggregate of measurements from user terminals. Additional knowledge of interference power from adjacent channel  306  and jammer  305  allows the base station to take necessary mitigation techniques, which may involve, but are not limited to, increasing the power control set points, determining P margin  for user terminals impacted, or limiting the maximum expected throughput for users and only using more reliable lower data rate services in the presence of these interferers, thus preventing a poor quality of service that would be present with higher data rates. 
     Jammer  305  may potentially be located within the signal band of interests as indicated by the jammer labeled  305 ′, making its detection more difficult and its impact greater. The knowledge of whether interference is narrow band, jammer  305 , or wide band adjacent channel  306 , will allow the base station to further optimize the required P margin  necessary for terminal  124 D to have proper quality of service. 
     II. POWER CONTROL ARCHITECTURE 
     Communications systems, such as WCS  100 , specify certain maximum BERs and/or PERs for signals transmitted across their wireless communications channels as being useful for desired link quality of service (QoS). The metrics used to measure QoS are often known as Quality of Service Metrics (QSM). For a channel to perform as intended, these error rates must not be exceeded, at least not for an appreciable amount of time. A channel&#39;s error rates depend on a ratio of power levels that is referred to herein as a signal-to-noise ratio (SNR). This ratio is expressed below in Equation (1). 
     
       
         
           
             
               
                 
                   
                     E 
                     b 
                   
                   
                     N 
                     t 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In Equation (1), E b  represents the energy per transmitted bit and N t  represents a noise energy. N t  includes two components: N 0  and I t . N o  represents thermal noise and I t  represents interference power. 
     N o  is relatively constant in wireless communications environments, such as the environment of WCS  100 . However, I t  can vary greatly. Since I t  can vary greatly, the ratio of Equation (1), as well as the associated link error rates, can fluctuate across a large range of values. 
     Error rates, such as BER and PER, are functions of SNR. Namely, as SNR increases, these error rates decrease. Therefore, increasing E b  by boosting the power of signals transmitted across a forward link channel is one way to keep error rates beneath specified maximum levels. Unfortunately, wireless communications systems, such as WCS  100 , include components, such as satellites  116 , which have limited available transmit power. The present aspect efficiently allocates this available power to multiple traffic channels. 
     This provides a power control architecture that efficiently allocates transmit power to communications channels, such as forward link traffic channels.  FIG. 5  is a flowchart illustrating an operation according to this architecture. This operation is described in the context of forward link traffic channel communications from gateway  120   a  to user terminal  124   a . However, this operation may be applied to communications between a variety of user terminals  124  and gateways  120  or base stations  112 . 
     As described above, conventional techniques for forward link power control involve closed loop approaches where user terminals provide gateways or base stations with commands, such as up/down commands, that specify particular forward link traffic channel power adjustments. Such commands are typically transmitted across a reverse link up/down command channel. The power control architecture of  FIG. 5  advantageously eliminates the need for such channels. 
     In a step  402 , gateway  120   a  performs a noise based power control. As shown in  FIG. 5 , step  402  includes steps  408  and  410 . In step  408 , gateway  120   a  receives an active pilot channel SNR measurement from a user terminal  124   a . Gateway  120   a  transmits pilot channel signals at constant power. Therefore, this received SNR estimate provides a frame of reference for determining transmit power levels for forward link traffic channels. Accordingly, from this received SNR, gateway  120   a  determines a baseline power level, P baseline , in step  410 . This determination is described in greater detail below with reference to  FIG. 5 . 
     In a step  404 , gateway  120   a  performs an interference based power control. Step  404  includes steps  412  and  414 . In step  412 , gateway  120   a  identifies a susceptibility of user terminal  124   a  to interfering transmissions that involve other user terminals  124 . Although such interfering transmissions are difficult to predict and can have fluctuating levels, the operational environment of user terminal  124   a  determines the interference susceptibility of user terminal  124   a . Due to the nature of these signals, commonly accepted filtering practices may be necessary to properly assess the average interference levels for their given purpose. The filtering operation, will help minimize the changes to a time period that reduces overhead messaging and prevents changes that are based on a noisy measurement versus a real signal trend. This determination is described in greater detail below with reference to  FIG. 6 . 
     The interference susceptibility of user terminal  124   a  corresponds to a range of possible interference power levels. From this determined interference susceptibility, gateway  120   a  determines a corresponding power margin, P margin , in step  414 . 
     In a step  406 , gateway  120   a  performs an error rate based power control. As shown in  FIG. 5 , step  406  includes steps  416  and  418 . In step  416 , gateway  120   a  identifies a forward link Quality of Service Metric (QSM), such as a packet error rate (PER) to effectively measure the Quality of Service (QoS) of the user terminal. In step  418 , gateway  120   a  determines a power level correction, P correction , from the identified error rate. 
     Step  422  is signal fade adjustment. In this step, gateway  120   a  identifies the fade  424  and calculates the fade correction  426 . 
     In a step  420 , gateway  120   a  sends forward link traffic channel transmissions to user  124   a  having a transmit power, P transmit , that is based on P baseline , P margin , and P correction  according to a relationship, such as the one expressed below in Equation (2), but it could easily be any function or operation of these elements.
 
 P   transmit   =P   baseline   +P   margin   +P   correction   +P   fade   (2)
 
     As described above, a forward link traffic channel&#39;s error rates depend on its SNR. P baseline , P margin , P correction  and P fade  are each determined in steps  402 ,  404 ,  406 , and  422  to maintain forward link traffic channel quality of service estimates (QSM), such as bit error rate (BER) and packet error rate (PER), within specific requirements. The requirements may be selected as desired, and alternatively may be dynamically adjusted over time. 
     III. NOISE BASED POWER CONTROL 
     As described above with reference to  FIG. 5 , P baseline  is determined by gateway  120   a  in step  410 . Gateway  120   a  adjusts P baseline  so that, in the absence of interference from other RF energy sources, forward link information error rates are maintained within specified requirements. P baseline  is determined from SNR measurements taken by user terminal  124   a  that characterize the reception quality of active beam pilot channel signals. 
     As shown in  FIG. 1 , gateway  120   a  communicates with user terminal  124   a  through satellite  116   a . Satellite  116   a  supports communications across a footprint that includes a plurality of beams, such as beams  204 . Gateway  120   a  transmits a plurality of forward link pilot channel signals. Each of these pilot channel signals is relayed by satellite  116   a  in a respective one of the plurality of beams. 
     These pilot channel signals employ time-based offsets of a given PN code sequence. Furthermore, gateway  120   a  transmits these pilot signals at a substantially constant power. 
     User terminal  124   a  is serviced by one of the plurality of beams of satellite  116   a . This beam is referred to herein as the active beam of user terminal  124   a . User terminal  124   a  measures an active beam pilot signal SNR and transmits the results of this measurement to gateway  120   a . This transmitted measurement may be in the form of a message that user terminal  124   a  periodically sends to gateway  120   a.    
     Since forward link pilot channel signals are transmitted at a constant power, these SNR measurements transmitted by user terminal  124   a  provide gateway  120   a  with a frame of reference for determining adequate forward link traffic channel transmit power levels in the absence of interference. 
     The active pilot channel SNR measurements received from user terminal  124   a  are each expressed herein as E cp /N t , where E cp  represents the energy per pilot signal chip. As described above, gateway  120   a  receives E cp /N t  in step  408 . From E cp /N t , gateway  120   a  determines a power level for P baseline . In the absence of interference, forward link traffic channel signals transmitted at P baseline  will be within specified error rate limits when received by user terminal  124   a.    
       FIG. 6  is a flowchart illustrating a performance of step  410  in greater detail. This performance begins with a step  502 , where gateway  120   a  calculates a power level offset, P o , according to the relationship (3) below.
 
 P   o   =E   bt   /N   t +10 log( R/W )− E   cp   /N   t   (3)
 
     In Equation (3), E bt /N t  is a desired forward link traffic channel SNR in decibels (dB), R is the forward link traffic channel data rate, W is the forward link traffic channel spreading bandwidth, E cp /N t  is the received active pilot channel SNR measurement in dB and R/W is the processing gain. E bt /N t  is selected to achieve a desired BER for forward link traffic channel transmissions to user terminal  124   a.    
     A step  504  follows step  502 . In step  504 , gateway  124   a  adds PO to the power level used to transmit pilot channel signals to user terminal  124   a . Next, in a step  506 , gateway  120   a  sets P baseline  to the result of the addition performed in step  504 . 
     Two examples of these steps are now described in the context of Equation (3). For both of these examples, the desired forward link traffic channel SNR (E b t/N t ) is 1 dB. In the first example, R=6.048 kbps and W=1.2288 MHz. If gateway  120   a  receives from user terminal  124   a  an E cp /N t  value of −21 dB, then P o  is approximately −1 dB. Thus, in this example, gateway  120   a  sets P baseline  at 1 dB less than the corresponding pilot channel transmit power. 
     In the second example, R=9.6 kbps and W=1.2288 MHz. If gateway  120   a  receives from user terminal  124   a  an E cp /N t  value of −21 dB, then P o  is approximately 1 dB. Thus, in this example, gateway  120   a  sets P baseline  at 1 dB greater than the corresponding pilot channel transmit power. These two examples illustrate that, as data rates increase, so does the differential between pilot transmit power and traffic transmit power. 
     IV. INTERFERENCE BASED POWER CONTROL 
     As described with reference to the operational scenario of  FIG. 3 , the signal of interest,  206 , is impacted through interference susceptibility of signals,  302 ′,  302 ″, and  305 . Accordingly, within the scenario of  FIG. 3 , if the signal strength of  302 ″ is stronger than  302 ′, the reception of transmission  306  by user terminal  124   e  is susceptible to a greater amount of interference than is the reception of transmission  304  by user terminal  124   d . Gateway  120   a  applies this principle in step  404  to reduce such interference while conserving transmit power. 
     Forward link traffic channel signals that are directed to other user terminals  124  in different beams may interfere with traffic channel signals directed to user terminal  124   a . As described above with reference to Equation (1), interference power levels (expressed as I t ) may vary greatly. Such variations cause the forward link traffic channel SNR, as well as the associated error rates, to fluctuate over a large range of values. 
     The reason for such fluctuations is described with reference to Equation (4), below. Equation (4) expresses the interference noise component, I t,i , that a user, i, receives from the forward link traffic channel transmissions of a set of interfering users (indexed by the variable j). 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       t 
                       , 
                       i 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         j 
                         ≠ 
                         t 
                       
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         
                           P 
                           j 
                         
                         · 
                         
                           R 
                           j 
                         
                       
                       W 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In Equation (4), P j  is the forward link transmit power directed to a user, j, R j  is the data rate of the forward link power to user j, and W is the CDMA spreading bandwidth. 
     As expressed in Equation (4), the contribution of an interfering user terminal  124  to the forward link interference noise component of user terminal  124   a  is directly proportional to the interfering user terminal&#39;s forward link data rate, R j . In accordance with the relationship of Equation (1), as forward link data rates increase, the interference noise component, I t , of N t  becomes progressively dominant over the corresponding thermal noise component, N 0 . 
     As described herein with reference to  FIG. 1 , WCS  100  may offer both LDR and HDR services. Because of its substantially lower data rate, interference noise variations from LDR links are relatively small when compared to interference noise variations from HDR links that transfer bursty traffic at higher data rates. The base station or gateway will have information regarding the user&#39;s data rates and sectors affected in the forward link, this data may be used to evaluate the interference of any given user in the system. Additionally, the user terminal will have measurements of forward link active sectors, adjacent sectors and jammers, enabling the gateway to properly compute P margin  for the forward link. In the reverse link, the gateway or base station will be able to determine the amount of traffic and the measured noise floor at the receiver, thereby determining the interference levels that may then be fed back to the user terminal so it may adjust its P margin  for the reverse link transmit power. 
     To make sure that such interference variations do not compromise communications across or over wireless links, gateway  120   a  includes P margin  as a forward link traffic channel transmit power component. P margin  mitigates interference from adjacent beam forward link traffic channels. 
     As described above with reference to the operational scenario of  FIG. 3 , the location of a user terminal  124  within a beam affects its susceptibility to interference. More specifically, a user terminal  124  near the interface between two beams, such as a user terminal  124  in crossover region  208 , is likely to receive more interference than a user terminal  124  further away from beam interfaces, such as a user terminal  124  in central region  206 . Therefore, to mitigate interference, gateway  120   a  may employ a smaller P margin  when user terminal  124   a  is in a central region than when user terminal  124   a  is in a crossover region. Additionally, with today&#39;s location technology, the base station or gateway will have knowledge of the location within the sector or beam of the greatest sources of interference (other users). This knowledge may aid in the determination of the required P margin . Location will provide information regarding how disadvantaged the interferer&#39;s channel may be and thus indicate the potential transmission power of that interfering signal. A signal on the edge of the beam or sector will potentially interfere with more than one user terminal in the highest geometry location, the center of the sector. 
     Accordingly, gateway  120   a  determines P margin  based on the location of user terminal  124   a  within its active beam  204 . As described above with reference to  FIG. 5 , P margin  is determined by gateway  120  in step  414 . Accordingly, step  414  may comprise setting P margin  to a first power level when the identified location is within a beam crossover region, and setting P margin  to a second power level when the identified location is within a beam central region. Since user terminals  124  within beam crossover regions are more susceptible to interference, the first power level in this example is greater than the second power level.  FIG. 7  is a flowchart that illustrates a performance of step  412  that implements this position-based feature. This performance begins with a step  602 , where gateway  120   a  receives a plurality of signal power measurements from user terminal  124   a . Each of these power measurements corresponds to one of a plurality of beams. For example, each of these measurements may be pilot signal power measurements. These measurements may be in the form of a formatted message, such as a pilot strength measurement message (PSMM). 
     Next, in a step  604 , gateway  120   a  calculates the differences between a first of the signal power measurements and each of the other signal power measurements. This first power measurement may be of the active beam pilot signal power or the largest power measurement. In this case, the smallest of these differences indicates the ability of user terminal  124   a  to receive forward link transmissions, such as interfering forward link traffic channel transmissions, from another beam. Accordingly, the smallest of these differences indicates the interference susceptibility of user terminal  124   a.    
     In a step  606 , gateway  120   a  determines whether the smallest of the differences calculated in step  604  is greater than a predetermined threshold. If so, then a step  608  is performed, where gateway  120   a  concludes that user terminal  124   a  has a first interference susceptibility. Otherwise, a step  610  is performed, where gateway  120   a  concludes that user terminal  124   a  has second interference susceptibility, which is greater than the first interference susceptibility. 
     The forward link interference susceptibility may be computed by determining the smallest difference as mentioned above, or it may be any function of any or all the differences of interfering signal strengths with the signal of interest. From this determination, values of P margin  may be determined based on a single threshold mapping to two values of P margin , or N thresholds mapping to N+1 values for P margin , or P margin  may determined directly from a formula of the susceptibilities. The calculation of P margin  will be greatly facilitated through the use of a message exchange with the user terminal, whereby the terminal communicates the levels of susceptibility of orthogonal, non-orthogonal and jammer based interference. This adds the additional benefit that the base station or gateway may then decide to reduce the interference levels by feeding back information to the offending user terminals, which are interfering with the signal of interest, and throttling back the data traffic and hence the interference for the user terminal of interest. 
     From this identified interference susceptibility, gateway  120   a  determines a corresponding P margin  value, as described above with reference to step  414  of  FIG. 5 . In particular, gateway  120   a  determines a P margin  according to a relationship where P margin  increases as the interference susceptibility identified in step  412  increases. 
     For instance, as described above with reference to  FIG. 7 , gateway  120   a  determines the interference susceptibility of user terminal  124   a . Namely, gateway  120   a  identifies a higher interference susceptibility in step  608  than in step  610 . Thus, gateway  120   a  sets P margin  to a greater value when step  414  follows step  608  than when step  414  follows step  610 . 
     Note that interference determination and calculation of P margin  may be done by a gateway only mechanism, whereby the gateway notes the type and power levels of data connections in adjacent beams/sectors, or it can be a mobile assisted mechanism, whereby the user terminal sends a message indicating the power levels of interfering signals which may include orthogonal, non-orthogonal and jammer classifications of the interference. The mobile assisted techniques will provide a more robust performance than the gateway only approach. It is also possible for a power control mechanism to use a combination of both base station only and mobile assisted interference detection/mitigation. 
     In addition to directly controlling the P margin  of a given user, the base station may use the interference estimates to institute an interference mitigation policy by lowering data rates, ending connections and lower power transmission levels to users that are causing interference to the given user.  FIG. 8  is a flow chart showing the operation of setting P margin . From  FIG. 8 , the base station receives a message which includes multiple signal power and interference measurements  1100 , enabling a segregation of interference into orthogonal noise, non-orthogonal noise or jammer noise  1110 . The base station or gateway may then choose which techniques to use  1120  which may be one or both techniques. The first technique, implementing interference mitigation by lowering orthogonal or non-orthogonal sources  1130  enables better user performance by lowering the interference in the other users  1140 , such as lowering transmissions from  204   7  and  204   2  users in  FIGS. 4 and 3 . Additionally, the gateway may decide to increase the value of P margin  to boost the signal power by calculating a difference between the signal of interest and the interference signals  1150 , comparing the difference with N thresholds  1160  and mapping the appropriate threshold to the required P margin  value  1170 . These steps will be necessary in the presence of an uncontrollable noise source such as  305  and  306  as seen in  FIGS. 3 and 4 . 
     V. ERROR RATE BASED POWER CONTROL 
     As described above with reference to  FIGS. 6 and 7 , P baseline  and P margin  are determined in response to SNR and power measurements. For instance, gateway  120   a  determines P baseline  in step  410  in response to active pilot channel SNR measurements so that a desired forward link traffic channel SNR (expressed in Equation (3) as Ebt/Nt) is achieved. This desired SNR corresponds to target error rate(s) based on a relationship that is determined by the modulation scheme and error correction coding techniques employed by gateway  120   a  in forward link traffic channel transmissions. 
     Similarly, in step  414  gateway  120   a  determines P margin  according to a comparison of pilot signal power measurements received from user terminal  124   a  that identifies interference susceptibility. However, this identified interference susceptibility does not indicate actual interference received by user terminal  124   a.    
     In contrast to P baseline  and P margin , P correction  is determined by gateway  120   a  in step  418  from actual link Quality of Service Metric that user terminal  124   a  encounters. As described above with reference to  FIG. 5 , gateway  120   a  identifies a forward link error rate, such as PER, in step  416 . 
     Gateway  120   a  sends information across the forward link traffic channel to user terminal  124   a  in the form of packets. Each of these packets is marked with a sequence identification number (sequence ID) that is assigned in a predetermined manner. User terminal  124   a  monitors the sequence IDs of received packets and sends a message to gateway  120   a  when packets are received out of sequence. 
     This message, referred to herein as a negative acknowledgement (NAK) message, indicates a sequence ID that was missing in a series of packets that user terminal  124   a  received from gateway  120   a . A missing sequence ID indicates a packet error. Gateway  120   a  collects statistics on the number of NAK messages received from user terminal  124   a  to compute the forward link traffic channel PER in step  416 . 
     Accordingly, step  416  includes gateway  120   a  counting the number of negative acknowledgement (NAK) messages received over a data collection interval. In addition, step  416  includes gateway  120   a  calculating a PER according to a relationship, such as the one expressed below in Equation (5). 
     
       
         
           
             
               
                 
                   PER 
                   = 
                   
                     NumberofreceivedNAKmessages 
                     Numberoftransmittedpackets 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In Equation (5), gateway  120   a  divides the number of NAK messages received during the data collection interval by the number of packets that gateway  120   a  transmitted during the data collection interval. 
     An alternative way to calculate a PER involves user terminal  124   a  receiving packets containing cyclical redundancy check (CRC) bits. For each packet, user terminal  124   a  uses these CRC bits to determine whether the packet contains bit errors. If so, then user terminal  124   a  increments a packet error counter. User terminal  124   a  may determine a PER by calculating the ratio of counted packet errors to received errors. This PER may periodically transmit such calculated PERs to gateway  120   a . In addition, other known methods of calculating a PER, or other quality of service metrics, may be used within the embodiments without departing from the scope of the claimed invention. 
     As described above with reference to  FIG. 5 , gateway  120   a  determines in step  418  a power level correction component, P correction , from the identified error rate. Step  418  comprises comparing the PER identified in step  416  with a desired PER, and adjusting P correction  accordingly. In particular, this adjustment comprises gateway  120   a  increasing P correction  when the identified PER is greater than the desired PER, and gateway  120   a  decreasing P correction  when the identified PER is less than the desired PER. 
     Additionally, the base station or gateway may associate a timer with a quality of service metric (PER for example) threshold, such that if service moves below a certain marginal level of performance for a given amount of time, P correction  may be increased to ensure an acceptable level of performance. Furthermore, N levels of thresholds may be defined to map to N different correction factors, P correction . 
     VI. FADE CORRECTION 
     For implementation on the reverse or forward link in a terrestrial communication system, the need exists to compensate for signal fades that happen relatively fast in the channel of interest. The term P fade  is computed to compensate for these fast fades on the forward or reverse link. P fade  may be computed based on information fed back from the user terminal such as Eb/Nt measurements or any other SNR metric, including filtered responses. This SNR is then used to compute the required P fade  to overcome the signal deficiency that may exist. 
     In the reverse link, the base station will use information from its receiver to gauge the fading that may exist and will then send a message to the user terminal indicating what level of correction, P fade , is needed to compensate for the fading environment or fade that is present at the moment.  FIG. 9  is a flow chart showing the steps to set P fade . Referring to  FIG. 9 , the gateway receives a periodic update of the SNR metric  1000 . The gateway then process a history of the SNR metrics to determine if a fade is present  1010  and route the determination  1020 . If no fade is detected  1030 , the P fade  aspect may be ignored and the algorithm may proceed  1040 . If a fade is detected  1050 , the gateway must determine the magnitude of correction, P fade , necessary  1060  and adjust this value to set P fade  appropriately  1070 . 
     VII. TIMING 
     As shown in  FIG. 5 , steps  402 ,  404 , and  406  may be performed sequentially. However, these steps may also be performed independently of each other. As described above, steps  402 ,  404 , and  406  each involve receiving information from user terminal  124   a . In response to this information, these steps each set a corresponding transmit power component. 
     As described above, noise based power control is performed in step  402 . This power control involves gateway  120   a  receiving SNR measurements, such as E cp /N t , from user terminal  124   a , and in response setting P baseline . User terminal  124   a  may periodically transmit these SNR measurements, such as once every second. Therefore, gateway  120   a  may periodically set P baseline . 
     Interference based power control is performed in step  404 . Changes in interference susceptibility often change more slowly than changes in a user terminal&#39;s noise environment because interference based changes are due to slower geometry changes that are caused by satellite motion and/or user terminal motion. Therefore, step  404  may involve gateway  120   a  receiving a set of pilot signal power measurements. These measurements may be in the form of a PSMM, which is also transmitted periodically, such as once every 10 seconds. Accordingly, gateway  120   a  may periodically adjust P margin . 
     Gateway  120   a  performs error rate based power control in step  406 . As described above, this power control involves the receipt of NAK messages over a data collection interval. This data collection interval may have various durations, as desired, as would be known. More reliable PER statistics are gathered when longer data collection intervals are employed. Therefore, gateway  120   a  may periodically adjust P correction  once every data collection interval. An exemplary data collection interval is 60 seconds. 
     VIII. EXEMPLARY GATEWAY IMPLEMENTATION 
       FIG. 10  is a block diagram of an exemplary gateway  120  implementation that performs the techniques described herein. Although described in the context of satellite communications, this exemplary implementation may also be employed in cellular base stations, such as base station  112  of  FIG. 1 . As shown in  FIG. 10 , this implementation includes an antenna segment  702  that is coupled to a radio frequency (RF) subsystem  704 , and a CDMA subsystem  706  that is coupled to RF subsystem  704 . In addition, gateway  120  further includes a switch  708  that is coupled to CDMA subsystem  706 . 
     Antenna segment  702  includes one or more antennas that exchange RF signals with one or more user terminals  124  through satellite(s)  116 . In particular, antenna segment  702  receives reverse link RF signals and transmits forward link RF signals. To enable the transmission and reception of RF signals through a single antenna, antenna segment  702  may also include a diplexer (not shown). 
     RF subsystem  704  receives electrical signals from antenna segment  702  within an RF frequency band. Upon reception, RF subsystem  704  down converts these electrical signals from the RF frequency band to an intermediate frequency (IF). In addition, RF subsystem  704  may filter the electrical signals received from antenna segment  702  in accordance with a predetermined bandwidth. 
     To increase the power of the RF signals received from antenna segment  702 , RF subsystem  704  also includes amplification components (not shown). Exemplary amplification components include a low noise amplifier (LNA) that initially amplifies signals received from antenna segment  702 , and a variable gain amplifier (VGA) that further amplifies these signals after they are mixed down to IF during the aforementioned down conversion process. 
     As a result of these filtering, down conversion, and amplification operations, RF subsystem  204  produces an IF signal  720  that is sent to a reverse link transceiver  712  within CDMA subsystem  706 . 
     In addition to receiving reverse link RF signals from antenna segment  702 , RF subsystem  704  receives a forward link IF signal  722  from a forward link transceiver  710  within CDMA subsystem  706 . RF subsystem  704  amplifies and up converts this signal into a corresponding RF signal for transmission by antenna segment  702 . 
     As shown in  FIG. 10 , CDMA subsystem  706  includes a forward link transceiver  710 , a reverse link transceiver  712 , a router  714 , and a selector bank subsystem (SBS)  716 . As described above, transceivers  710  and  712  exchange IF signals  720  and  722  with RF subsystem  704 . In addition, transceivers  710  and  712  perform CDMA operations. 
     In particular, forward link transceiver  710  receives one or more forward link information sequences  724  from router  714 . Upon reception, forward link transceiver  710  converts these sequences into IF signal  722 , which is in a CDMA transmission format. This conversion is described in greater detail below with reference to  FIG. 11 . 
     Reverse link transceiver  712  converts IF signal  720 , which is in a CDMA transmission format, into information sequences  726   a - 726   n . For example, forward link transceiver  710  despreads and decovers IF signal  720  with one or more PN sequences and channelizing codes. In addition, forward link transceiver  710  may perform decoding and de-interleaving operations to produce information sequences  726 , which are sent to router  714 . 
     Router  714  handles the transfer of information sequences  724  and  726 , which may be in the form of packets, between SBS  716  and transceivers  710  and  712 . This transfer is performed across interface  728 , which may be a data network, such as a local area network (LAN), or any other well known mechanism for transferring information. 
     SBS  716  processes the forward link and reverse link traffic handled by gateway  120 . This traffic includes both payload traffic and signaling traffic. For example, SBS  716  exchanges signaling traffic in the performance of call processing operations, such as call setup, call teardown, and beam hand-offs. SBS  716  also forwards traffic to switch  708 , which provides an interface to a public switched telephone network (PSTN). 
     SBS  716  includes a plurality of selectors  718   a - n  for processing forward and reverse link traffic. Each selector  718  handles active communications for a corresponding user terminal  124 . However, selectors  718  may be reassigned to other user terminals  124  upon the termination of such active communications. For example, selectors  718  evaluate PSMMs, pilot signal SNR measurements, and NAK messages sent from user terminals  124  to perform appropriate forward link traffic channel transmit power adjustments. 
     Each selector  718  may be implemented in a software-controlled processor programmed to perform the functions described herein. Such implementations may include well known standard elements or generalized function or general purpose hardware including a variety of digital signal processors (DSPs), programmable electronic devices, or computers that operate under the control of software instructions perform the desired functions. 
     Each selector  718  controls forward link power control operations. To adjust the power of forward link transmissions, selectors  718  each send a power control command  730  to forward link transceiver  710 . Power control commands  730  each designate a forward link transmit power. In response to these commands, forward link transceiver  710  sets the transmit power for the forward links controlled by the selectors  718  originating these commands. 
     For example, selector  718   a  generates a power control command  730   a  that is sent to transceiver  710  through interface  728  and router  714 . Upon receipt of power control command  730   a , forward link transceiver  710  sets the power of the forward link controlled by selector  718   a . Details regarding this feature are described below with reference to  FIG. 11 . 
     Accordingly, each selector  718  operates with forward link transceiver  710  to perform the steps described above with reference to  FIGS. 5-7 . For example, as described above with reference to steps  402 ,  404 , and  406 , each selector  718  determines P baseline , P margin , and P correction . 
     Additionally, each selector  718  operates with forward link transceiver  710  to set the corresponding P transmit  based on P baseline , P margin , and P correction . Thus, these components perform step  420 . 
       FIG. 11  is a block diagram of a forward link transceiver  710  implementation. As shown in  FIG. 11 , transceiver  710  includes a plurality of transceiver paths  802   a - 802   n , a summer  804 , and an output interface  805 . Each transceiver path  802  receives a forward link information sequence  724  and a power control command  730  from a corresponding selector  718 . Although  FIG. 11  only shows implementation details for transceiver path  802   a , transceiver paths  802   b - 802   n  may include similar or identical features. 
     As shown in  FIG. 11 , transceiver path  802   a  includes an interleaver  806 , an encoder  808 , and a gain module  810 . Interleaver  806  receives an information sequence  724  and block interleaves this sequence to produce an interleaved sequence  820 . 
     Interleaved sequence  820  is sent to encoder  808 , which performs error correction encoding, such as turbo block encoding, to produce an encoded information sequence  822 . 
     Gain module  810  receives encoded sequence  822 , which is a forward link information sequence. Additionally, gain module  810  receives power control command  730   a  from selector  718   a . Gain module  810  scales encoded sequence  822  based on the transmit power level designated by power control command  730   a . Thus, gain module  810  may increase or decrease the power of encoded sequence  822 . This scaling produces a scaled sequence  824 . 
     Encoded sequence  822  is a sequence of digital symbols. This sequence may be scaled by multiplying each of the symbols with a gain factor determined by power control command  730 . Such scaling operations may be implemented digitally through hardware techniques, and/or software instructions operating on well known elements or generalized function or general purpose hardware including a variety of programmable electronic devices, or computers that operate under the control of commands, firmware, or software instructions to perform the desired functions. Examples include a software-controlled processor, controller or device, a microprocessor, one or more digital signal processors (DSP), dedicated function circuit modules, application specific integrated circuits (ASIC), and field programmable gate arrays (FPGA). Accordingly, power control command  730   a  may include one or more software instructions transferred between selector  718   a  and gain module  810 . 
     As shown in  FIG. 11 , transceiver path  802  further includes spreading combiners  812   a - 812   b , channelizing combiners  814   a - 814   b , and a quadrature phase shift keying (QPSK) modulator  816 . Spreading combiners  812   a - 812   b  each receive scaled sequence  824  and combine (for example, multiply) this sequence with a respective PN sequence  834  to produce spread sequences  828   a  and  828   b.    
     Spread sequences  828   a  and  828   b  are each transferred to a respective channelizing combiner  814 . Each channelizing combiner  814  combines (for example, multiplies) the corresponding spread sequence  828  with a channelizing code, such as a Walsh code. As a result, combiners  814  each produce a channelized sequence  830 . In particular, combiner  814   a  produces an in-phase (I) channelized sequence  830   a  and combiner  814   b  produces a quadrature (Q) channelized sequence  830   b.    
     Channelized sequences  830   a  and  830   b  are sent to QPSK modulator  816 . QPSK modulator  816  modulates these sequences to generate a modulated waveform  832   a . Modulated waveform  832   a  is sent to summer  804 . Summer  804  adds modulated waveform  832   a  and waveforms  832   b - 832   n  produced by transceiver paths  802   b - 802   n . This operation results in a combined signal  834 , which is sent to output interface  805 . 
     Output interface  805  up converts combined signal  834  from baseband to an IF, thereby generating forward link IF signal  722 . Output interface  805  may additionally perform filtering and amplification operations in the generation of IF signal  722 . 
     IX. CONCLUSION 
     While various aspects have been described above, it should be understood that they have been presented by way of example only, and not limitation. For example, the presently claimed invention is not limited to satellite-based communications systems, but also may be applied to terrestrial-based systems, such as where there are multiple sectors (beams) and cross-over regions between such sectors. Furthermore, the presently claimed invention is not limited to CDMA systems, but may be extended to other types of communications systems and air interfaces, such TDMA, FDMA, CDMA2000, WCDMA, and OFDMA systems. Moreover, while the aspects describe wireless CDMA transmission in the context of QPSK modulation, other modulation techniques may employed. 
     Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the aspects disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the presently claimed invention. 
     The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The steps of a method or algorithm described in connection with the aspects disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
     The previous description of the disclosed aspects is provided to enable any person skilled in the art to make or use the presently claimed invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the spirit or scope of the claimed invention. Thus, the presently claimed invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.