Patent Publication Number: US-2006003792-A1

Title: Controlling multiple modems in a wireless terminal using energy-per-bit determinations

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §120  
      The present Application for Patent is a Continuation and claims priority to patent application Ser. No. 10/283,935 entitled “CONTROLLING MULTIPLE MODEMS IN A WIRELESS TERMINAL USING ENERGY-PER-BIT DETERMINATIONS”filed Oct. 29, 2002, now allowed, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
    
    
     REFERENCE TO CO-PENDING APPLICATIONS FOR PATENT  
      This application is related to commonly-owned applications, entitled “Wireless Terminal Operating Under An Aggregate Transmit Power Limit Using Multiple Modems Having Fixed Individual Transmit Power Limits” having U.S. patent application Ser. No. 10/283,676, filed on Oct. 29, 2002, and “Controlling Multiple Modems In A Wireless Terminal Using Dynamically Varying Modem Transmit Power Limits” having U.S. patent application Ser. No. 10/283,934, filed Oct. 29, 2002, which are incorporated herein by reference.  
     BACKGROUND  
      1. Field  
      The present invention relates generally to mobile wireless terminals, and particularly, to mobile wireless terminals having multiple modems which are constrained to operate under an aggregate transmit power limit for all of the modems.  
      2. Background  
      In a data call established between a mobile wireless terminal (MWT) and a remote station, the MWT can transmit data to the remote station over a “reverse” communication link. Also, the MWT can receive data from the remote station over a “forward” communication link. There is an ever pressing need to increase the transmit and receive bandwidth, that is, the data rates, available over both the forward and reverse links.  
      Typically, the MWT includes a transmit power amplifier to power-amplify a radio frequency (RF) input signal. The power amplifier produces an amplified, RF output signal having an output power responsive to the input power of the input signal. An inordinately high input power may over-drive the power amplifier, and thus cause the output power to exceed an acceptable operating transmit power limit of the power amplifier. In turn, this may cause undesired distortion of the RF output signal, including unacceptable out-of-band RF emissions.  
      Therefore, there is a need to carefully control the input and/or output power of the transmit power amplifier in an MWT so as to avoid over-driving the power amplifier. There is a related need to control the output power as just mentioned, while minimizing to the extent possible, any reduction of the forward and reverse link bandwidth (that is, data rates).  
     SUMMARY  
      A feature of the present invention is to provide an MWT that maximizes an overall communication bandwidth in both the reverse and forward link directions using a plurality of concurrently operating communication links, each associated with a respective one of a plurality of modulator-demodulators (modems) of the MWT.  
      Another feature of the present invention is to provide an MWT that combines multiple modulator-demodulator (modem) transmit signals into an aggregate transmit signal (that is, an aggregate reverse link signal) so that a single transmit power amplifier can be used. This advantageously reduces power consumption, cost, and space requirements compared to known systems using multiple power amplifiers.  
      Another feature of the present invention is to carefully control an aggregate input and/or output power of the transmit power amplifier, thereby avoiding signal distortion at the power amplifier output. A related feature is to control the aggregate input and/or output power in such a manner as to maximize bandwidth (that is, data through-put) in both the reverse and forward link directions.  
      These features are achieved in several ways. First, individual transmit power limits are established in each of the plurality of modems of the wireless terminal, to limit the respective, individual modem transmit powers. Each individual transmit power limit is derived, in part, from an aggregate transmit power limit for all of the modems. Together, the individual transmit power limits collectively limit the aggregate transmit power of all of the modems.  
      Second, the present invention controls the total number of modems permitted to transmit data at any given time, so as to maximize an aggregate transmit data rate of the wireless terminal while maintaining the aggregate transmit power of all of the modems below the aggregate transmit power limit. To do this, the present invention collects and/or determines modem transmit statistics corresponding to a previous transmit period or cycle of the wireless terminal. The modem transmit statistics can include individual modem transmit data rates, individual modem transmit powers, the aggregate transmit data rate of all of the modems, and an aggregate transmit power for all of the modems combined.  
      The statistics are used to determine an average energy-per-transmitted bit across all of the modems, or alternatively, individual energy-per-transmitted bits for each of the modems, corresponding to the previous transmit cycle of the wireless terminal. Then, either the average or individual energy-per-transmitted-bits are used to determine a maximum number of “active” modems that can be scheduled to transmit data concurrently, and preferably at their respective maximum data rates, without exceeding the aggregate transmit power limit of the wireless terminal. This maximum number of active modems are scheduled to transmit data during the next transmit cycle of the wireless terminal. The invention repeats the process periodically, to update the maximum number of active modems over time. In this manner, the present invention attempts, proactively, to avoid “over-limit” conditions in the modems of the wireless terminal. An over-limit modem has an actual transmit power, or alternatively, a required transmit power, that exceeds the individual transmit power limit established in the modem.  
      In the present invention, only active modems are scheduled to transmit data in the reverse link direction. “Inactive” modems are modems that are not scheduled to transmit data. However, in the present invention, inactive modems are able to receive data in the forward link direction, thereby maintaining a high forward link through-put in the wireless terminal, even when modems are inactive in the reverse link direction.  
      The present invention is directed to an wireless terminal including a plurality (N) of wireless modems. The N modems have their respective transmit outputs combined to produce an aggregate transmit output. The N modems can concurrently transmit data in the reverse link direction and receive data in the forward link direction. The wireless terminal is constrained to operate within an aggregate transmit power limit. One aspect of the present invention is a method, comprising: scheduling a plurality, M, of active ones (that is active individual members) of the N modems to transmit payload data, where M is less than or equal to N; monitoring status reports from at least the active modems; determining, based on the status reports, whether to adjust/modify the number of active modems in order to maximize an aggregate transmit data rate of the N modems while maintaining an aggregate transmit power of the N modems at or below the aggregate transmit power limit; and modifying the number of active modems when it is determined that the number of active modems should be modified to maintain the aggregate transmit power level of the N modems at or below the aggregate transmit power level. This and further aspects of the present invention are described below.  
      The step of determining can comprise determining a maximum number of active modems that can concurrently transmit data, each at a predetermined maximum data rate, while maintaining the aggregate transmit power of the N modems at or below the aggregate transmit power limit, and comparing the maximum number of active modems to the number M of active modems. The maximum number can be determined by determining an average energy-per-transmitted-bit across at least the M active modems and the aggregate transmit power limit. Here, the status reports being monitored indicate a respective transmit data rate for each of the N modems while determining the average energy-per-transmitted-bit can comprise determining an aggregate transmit data rate across the N modems based on their respective transmit data rates and determining the aggregate transmit power. The status reports monitored can indicate a respective transmit power for each of the N modems.  
      In further aspects of the method, next active modems can be selected as the maximum number of modems having the lowest individual energy-per-transmitted-bits among the N modems, and the scheduling process is repeated using these next active modems. The number of active modems can be increased to the maximum number when the maximum number is greater than M, and decreased to the maximum number when the maximum number is less than M.  
      The method can include activating a selected, previously inactive one of the N modems, thereby increasing the number of active modems, and increasing the respective transmit power limit in the selected one of the N modems. Alternatively, a selected, previously active one of the N modems, is deselected thereby decreasing the number of active modems; and the respective transmit power limit in the selected one of the N modems is decreased. Each of the N modems is adapted to transmit data at at least one of a maximum transmit data rate and a minimum transmit data rate; and the maximum number of active modems is based on the minimum and maximum transmit data rates as well as the average energy-per-transmitted-bit and the aggregate transmit power limit.  
      The N modems can be sorted according to their respective individual energy-per-transmitted-bits and scheduling includes using the maximum number of active modems having the lowest individual energy-per-transmitted-bits among the N modems.  
      The invention also includes a method of dynamically calibrating a data terminal including N wireless modems having their respective transmit outputs combined to produce an aggregate transmit output, the method comprising scheduling each of the N modems to concurrently transmit respective data; receiving respective reported transmit powers P Rep (i) from the N modems corresponding to when the N modems concurrently transmit, where i designates a respective modem from 1 to N; measuring an aggregate transmit power P Agg  corresponding to when the N modems concurrently transmit; generating an equation representing the aggregate transmit power as a cumulative function of each reported transmit power P Rep (i) and a corresponding, undetermined, modem dependent gain factor g(i); repeating these steps N times to generate N simultaneous equations; and determining all of the modem dependent gain factors from the N simultaneous equations. Furthermore these steps can be periodically repeated so that the modem dependent gain factors are updated periodically.  
      In further aspects of the invention, a wireless terminal is provided which is constrained to operate under an aggregate transmit power limit, having N wireless modems with their respective transmit outputs combined together to produce an aggregate transmit output. The terminal comprises means for scheduling a plurality, M, of active ones of the N modems to transmit payload data, where M is less than or equal to N; means for monitoring status reports from at least the active modems; means for determining, based on the status reports, whether to modify the number of active modems in order to maximize an aggregate transmit data rate of the N modems while maintaining an aggregate transmit power of the N modems at or below the aggregate transmit power limit; and means for modifying the number of active modems when it is determined the number should be modified to maintain the aggregate transmit power level at or below the aggregate transmit power level.  
      The determining means in the wireless terminal may comprise means for determining a maximum number of active modems that can concurrently transmit data, each at a predetermined maximum data rate, while maintaining the aggregate transmit power of the N modems at or below the aggregate transmit power limit, and means for comparing the maximum number of active modems to the number M of active modems.  
      In further embodiments, the means for determining the maximum number comprises means for determining an average energy-per-transmitted-bit across at least the M active modems or an individual energy-per-transmitted-bit for each of the N modems, and means for determining the maximum number of active modems based on the average or individual energy-per-transmitted-bits, respectively, and the aggregate transmit power limit. The monitored status reports indicate a respective transmit data rate or transmit power for each of the N modems. The means for determining the average energy-per-transmitted-bit comprises means for determining an aggregate transmit data rate across the N modems based on their respective transmit data rates, means for determining the aggregate transmit power, and means for determining the average energy-per-transmitted-bit based on the aggregate transmit data rate and the aggregate transmit power.  
      The wireless terminal may include means for selecting as next active modems the maximum number of modems having the lowest individual energy-per-transmitted-bits among the N modems. The modifying means can comprise means for increasing the number of active modems to the maximum number when the maximum number is greater than M, or means for decreasing the number of active modems to the maximum number when the maximum number is less than M. The modifying means can include means for activating a selected, previously inactive one of the N modems, thereby increasing the number of active modems, and means for increasing the respective transmit power limit in the selected one of the N modems. The modifying means can comprise means for deactivating a selected, previously active one of the N modems, thereby decreasing the number of active modems; and decreasing the respective transmit power limit in the selected one of the N modems.  
      In further aspects, each of the N modems is adapted to transmit data at at least one of a maximum transmit data rate and a minimum transmit data rate, and the means for determining the maximum number comprises determining the maximum number based on the minimum and maximum transmit data rates as well as the average energy-per-transmitted-bit and the aggregate transmit power limit.  
      A wireless terminal constrained to operate within an aggregate transmit power limit, having N wireless modems with their respective transmit outputs combined to produce an aggregate transmit output, comprising means for determining an individual energy-per-transmitted-bit for each of the N modems that was previously transmitting, means for determining, based on individual energy-per-transmitted-bits and the aggregate transmit power limit, a maximum number of active modems that can concurrently transmit data at a maximum data rate without exceeding the aggregate transmit power limit, and means for scheduling the maximum number of active modems to transmit data.  
      In further aspects the wireless terminal further comprises means for sorting the N modems according to their respective individual energy-per-transmitted-bits, while the means for scheduling comprises means for scheduling the maximum number of active modems having the lowest individual energy-per-transmitted-bits among the N modems. The wireless terminal further comprises means for monitoring status reports from at least the active modems, which collectively include a transmit power estimate of each active modem, wherein the means for determining the individual energy-per-transmitted-bits comprises means for determining, from each transmit power estimate, the corresponding individual energy-per-transmitted-bit.  
      Apparatus for dynamically calibrating a wireless terminal including N wireless modems having their respective transmit outputs combined to produce an aggregate transmit output. The apparatus comprises means for scheduling each of the N modems to concurrently transmit respective data, means for receiving respective reported transmit powers P Rep (i) from the N modems, a power meter, coupled to the aggregate transmit output, for measuring an aggregate transmit power P Agg  of the N modems, means for generating a representation of the aggregate transmit power as a cumulative function of each reported transmit power P Rep (i) and a corresponding, undetermined, modem dependent gain factor g(i), wherein the scheduling means, the receiving means, the power meter, and the generating means repeat their respective functions N times to generate N simultaneous equations, and means for determining all of the modem dependent gain factors from the N simultaneous equations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The features, objects, and advantages 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 the same or similar elements throughout and wherein:  
       FIG. 1  is an illustration of an example wireless communication system.  
       FIG. 2  is a block diagram of an example mobile wireless terminal.  
       FIG. 3  is a block diagram of an example modem representative of individual modems of the mobile wireless terminal of  FIG. 2 .  
       FIG. 4  is an illustration of an example data frame that may be transmitted or received by any one of the modems of  FIGS. 2 and 3 .  
       FIG. 5  is an illustration of an example status report from the modems of  FIGS. 2 and 3 .  
       FIG. 6  is a flowchart of an example method performed by each of the modems of  FIGS. 2 and 3 .  
       FIG. 7  is a flowchart of an example method performed by the mobile wireless terminal.  
       FIG. 8  is a flowchart expanding on the method of  FIG. 7 .  
       FIG. 9  is a flowchart expanding on the method of  FIG. 7 .  
       FIG. 10  is a flowchart of another example method performed by the mobile wireless terminal.  
       FIG. 11  is an example plot of Power versus Modem index(i) identifying respective ones of the modems of  FIG. 2 , wherein uniform modem transmit power limits are depicted.  FIG. 11  also represents an example transmit scenario of the mobile wireless terminal of  FIG. 2 .  
       FIG. 12  is another example transmit scenario similar to  FIG. 11 .  
       FIG. 13  is an illustration of an alternative, tapered arrangement for the modem transmit power limits.  
       FIG. 14  is a flowchart of an example method of calibrating modems in the mobile wireless terminal of  FIG. 2 .  
       FIG. 15  is a flowchart of an example method of operating the mobile wireless terminal, using dynamically updated individual modem transmit power limits.  
       FIG. 16  is a flowchart of an example method expanding on the method of  FIG. 15 .  
       FIG. 17  is a flowchart of an example method of determining a maximum number of active modems using an average energy-per-transmitted-bit of the modems.  
       FIG. 18  is a flowchart of an example method of determining a maximum number of active modems, using an individual energy-per-transmitted-bit for each of the modems.  
       FIG. 19  is a graphical representation of different modem transmit limit arrangements.  
       FIG. 20  is a functional block diagram of an example controller of the mobile wireless terminal of  FIG. 2 , for performing the methods of the present invention. 
    
    
     DETAILED DESCRIPTION  
      A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users. However, spread spectrum modulation techniques, such as those used in code division multiple access (CDMA) communication systems provide significant advantages over other modulation schemes, especially when providing service for a large number of communication system users. Such techniques are disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title “Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters” to Gilhousen et al., and U.S. Pat. No. 5,691,974, 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” to Carter et al., both of which are assigned to the assignee of the present invention, and are incorporated herein by reference in their entirety.  
      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. Other communications systems are described in other standards such as the IMT-2000/UM, or International Mobile Telecommunications System 2000/Universal Mobile Telecommunications System, standards covering what are referred to as wideband CDMA (WCDMA), cdma2000 (such as cdma2000 1× or 3× standards, for example) or TD-SCDMA.  
      I. Example Communication Environment  
       FIG. 1  is an illustration 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 inter-operate 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.  
      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 for example cars, trucks, boats, trains, and planes), or fixed, as desired. For example,  FIG. 1  illustrates user terminal  124   a  as a fixed telephone or data transceiver, user terminal  124   b  as a hand-held device, and user terminal  124   c  as a portable vehicle-mounted device. Wireless communication devices are also sometimes referred to as mobile wireless terminals, user terminals, mobile wireless communication devices, subscriber units, mobile units, mobile stations, mobile radios, or simply “users,” “mobiles,” “terminals,” or “subscribers” in some communication systems, depending on preference.  
      User terminals  124  engage in wireless communications with other elements in WCS  100  through CDMA communications systems. However, the present 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) 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, communication links  130   a - c  provide for the exchange of signals between base station  112  and user terminals  124 . Similarly, communications links  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 communications links  146   a - d.    
      User terminals  124  are capable of engaging in bi-directional communications with base station  112  and/or satellites  116 . As such, communications links  130  and  138  each include a forward link and a reverse link. A forward link conveys information signals to user terminals  124 . For terrestrial-based communications in WCS  100 , a forward link conveys information signals from base station  112  to a user terminal  124  across a link  130 . A satellite-based forward link in the context of WCS  100  conveys information from a gateway  120  to a satellite  116  over a link  146  and from the satellite  116  to a user terminal  124  over a link  138 . Thus, terrestrial-based forward 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).  
      In the context of WCS  100 , a reverse link conveys information signals from a user terminal  124  to either a base station  112  or a gateway  120 . Similar to forward links in WCS  100 , reverse links typically require a single wireless signal path for terrestrial-based communications and two wireless signal paths for satellite-based communications. 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 and higher.  
      As described above, WCS  100  performs wireless communications according to CDMA techniques. Thus, signals transmitted across the forward and reverse links of links  130 ,  138 , and  146  convey signals that are encoded, spread, and channelized according to CDMA transmission standards. In addition, block interleaving can be employed for these forward and reverse links. These blocks are transmitted in frames 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 over the forward links of WCS  100 . This power (referred to herein as forward link transmit power) may be varied according to user terminal  124  and according to time. This time varying feature may be employed on a frame-by-frame basis. Such power adjustments are performed to maintain forward link bit error rates (BER) within specific requirements, reduce interference, and conserve transmission power.  
      User terminals  124  may adjust the power of the signals that they transmit over the reverse links of WCS  100 , under the control of gateways  120  or base stations  112 . This power (referred to herein as reverse link transmit power) may be varied according to user terminal  124  and according to time. This time varying feature may be employed on a frame-by-frame basis. Such power adjustments are performed to maintain reverse link bit error rates (BER) within specific requirements, reduce interference, and conserve transmission power.  
      Examples of techniques for exercising power control in CDMA communication systems are found in U.S. Pat. No. 5,383,219 issued Jan. 17, 1995, entitled “Fast Forward Link Power Control In A Code Division Multiple Access System” to Padovani et al., U.S. Pat. No. 5,396,516 issued Mar. 7, 1995, entitled “Method And System For The Dynamic Modification Of Control Parameters In A Transmitter Power Control System” to Padovani et al., and U.S. Pat. No. 5,056,109 issued Oct. 8, 1991, entitled “Method and Apparatus For Controlling Transmission Power In A CDMA Cellular Mobile Telephone System” to Gilhousen et al., which are incorporated herein by reference.  
      II. Mobile Wireless Terminal  
       FIG. 2  is a block diagram of an example MWT  206  constructed and operated in accordance with the principles of the present invention. MWT  206  communicates wirelessly with a base station or gateway (referred to as a remote station), not shown in  FIG. 2 . Also, MWT  206  may communicate with a user terminal. MWT  206  receives data from external data sources/sinks, such as a data network, data terminals, and the like, over a communication link  210 , such as an Ethernet link, for example. Also, MWT  206  sends data to the external data sources/sinks over communication link  210 .  
      MWT  206  includes an antenna  208  for transmitting signals to and receiving signals from the remote station. MWT  206  includes a controller (that is, one or more controllers)  214  coupled to communication link  210 . Controller  214  exchanges data with a memory/storage unit  215 , and interfaces with a timer  217 . Controller  214  provides data-to-be-transmitted to, and receives data from, a plurality of wireless modems  216   a - 216   n  over a plurality of corresponding bi-directional data links  218   a - 218   n  between controller  214  and modems  216 . Data links  218  may be serial data connections. The number N of modems that may be used can be one of several values, as desired, depending on known design issues such as complexity, cost, and so forth. In an example implementation, N=16.  
      Wireless modems  216   a - 216   n  provide RF signals  222   a   T - 222   n   T  to and receive RF signals  222   a   R - 222   n   R  from a power combiner/splitter assembly  220 , over a plurality of bi-directional RF connections/cables between the modems and the power combiner/splitter assembly  220  (hereinafter “assembly  220 ”). In a transmit (that is, reverse link) direction, a power combiner included in assembly  220  combines together the RF signals received from all of modems  216 , and provides a combined (that is, aggregate) RF transmit signal  226  to a transmit power amplifier  228 . Transmit power amplifier  228  provides an amplified, aggregate RF transmit signal  230  to a duplexer  232 .  
      Duplexer  232  provides the amplified, aggregate RF transmit signal to antenna  208 . In MWT  206 , duplexing may be achieved by means other than duplexer  232 , such as using separate transmit and receive antennas. Also, a power monitor  234 , coupled to an output of power amplifier  228 , monitors a power level of amplified, aggregate transmit signal  230 . Power monitor  234  provides a signal  236  indicating the power level of amplified, aggregate transmit signal  230  to controller  214 . In an alternative arrangement of MWT  206 , power monitor  234  measures the power level of aggregate signal  226  at the input to transmit amplifier  228 . In this alternative arrangement, the aggregate transmit power limit of MWT  206  is specified at the input to transmit amplifier  228  instead of at its output, and the methods of the present invention, described below, take this into account.  
      In a receive (that is, forward link) direction, antenna  208  provides a received signal to duplexer  232 . Duplexer  232  routes the received signal to a receive amplifier  240 . Receive amplifier  240  provides an amplified received signal to assembly  220 . A power splitter included in assembly  220  divides the amplified received signal into a plurality of separate received signals and provides each separate signal to a respective one of the modems  216 .  
      MWT  206  communicates with the remote station over a plurality of wireless CDMA communication links  250   a - 250   n  established between MWT  206  and the remote station. Each of the communication links  250  is associated with a respective one of modems  216 . Wireless communication links  250   a - 250   n  can operate concurrently with one another. Each of wireless communication links  250  supports wireless traffic channels for carrying data between MWT  206  and the remote station in both forward and reverse link directions. The plurality of wireless communication channels  250  form part of an air interface  252  between MWT  206  and the remote station.  
      In the present embodiment, MWT  206  is constrained to operate under an aggregate transmit power limit (APL) at the output of transmit amplifier  228 . In other words, MWT  206  is required to limit the transmit power of signal  230  to a level that is preferably below the aggregate transmit power limit. All of modems  216 , when transmitting, contribute to the aggregate transmit power of signal  230 . Accordingly, the present invention includes techniques to control the transmit powers of modems  216 , and thereby cause the aggregate transmit power of modems  216 , as manifested in transmit signal  230 , to be under the aggregate transmit power limit.  
      Over-driving transmit amplifier  228  causes the power level of signal  230  to exceed the aggregate transmit power limit. Therefore, the present invention establishes individual transmit power limits (also referred to as transmit limits) for each of modems  216 . The individual transmit power limits are related to the aggregate transmit power limit in such a way as to prevent modems  216  from collectively over-driving transmit amplifier  228 . During operation of MWT  206 , the present invention controls a maximum number of active modems that can concurrently transmit data at any given time so as to maximize the aggregate transmit data rate of the MWT, while maintaining the aggregate transmit power of all of modems  216  at or below the aggregate transmit power limit. The present invention uses proactive techniques to avoid over-limit conditions in modems  216 . Further aspects of the present invention are described below.  
      Although MWT  206  is referred to as being mobile, it is to be understood that the MWT is not limited to a mobile platform, or portable platforms. For example, MWT  206  may reside in a fixed base station or gateway. MWT  206  may also reside in a fixed user terminal  124   a.    
      III. Modem  
       FIG. 3  is a block diagram of an example modem  300  representative of each of modems  216 . Modem  300  operates in accordance with CDMA principles. Modem  300  includes a data interface  302 , a controller  304 , a memory  306 , a modem signal processor or module  308 , such as one or more digital signal processors (DSP) or ASICs, an intermediate frequency IF/RF subsystem  310 , and an optional power monitor  312 , all coupled to one another over a data bus  314 . In some systems, the modems do not comprise transmit and receive processors coupled in pairs as in a more traditional modem structure, but may use an array of transmitters and receivers or modulators and demodulators which are interconnected, as desired, to handle user communications, and one or more signals, or otherwise time shared among users.  
      In the transmit direction, controller  304  receives data-to-be-transmitted from controller  214  over data connection  218   i  (where “i” indicates any one of the modems  216   a - 216   n ), and through interface  302 . Controller  304  provides the data-to-be-transmitted to modem processor  308 . A transmit (Tx) processor  312  of modem processor  308  encodes and modulates the data-to-be-transmitted, and packages the data into data frames that are to be transmitted. Transmit processor  312  provides a signal  314  including the data frames to IF/RF subsystem  310 . Subsystem  310  frequency up-converts and amplifies signal  314 , and provides a resulting frequency up-converted, amplified signal  222   i   T  to power combiner/splitter assembly  220 . Optional power meter  320  monitors a power level of signal  222   i   T  (that is, the actual transmit power at which modem  300  transmits the above-mentioned data frames). Alternatively, modem  300  can determine the modem transmit power based on gain/attenuator settings of IF/RF subsystem  310  and the data rate at which modem  300  transmits the data frames.  
      In the receive direction, IF/RF subsystem  310  receives a received signal  222   i   R  from power combiner/splitter assembly  220 , frequency down-converts signal  222   i   R  and provides the resulting frequency down-converted signal  316 , including received data frames, to a receive (Rx) processor  318  of modem processor  308 . Receive processor  318  extracts data from the data frames, and then controller  304  provides the extracted data to controller  214 , using interface  302  and data connection  218   i.    
      Modems  216  each transmit and receive data frames in the manner described above and further below.  FIG. 4  is an illustration of an example data frame  400  that may be transmitted or received by any one of modems  216 . Data frame  400  includes a control or overhead field  402  and a payload field  404 . Fields  402  and  404  include data bits used to transfer either control information ( 402 ) or payload data ( 404 ). Control field  402  includes control and header information used in managing a communication link established between a respective one of modems  216  and the remote station. Payload field  404  includes payload data (bits  406 ), for example, data-to-be-transmitted between controller  214  and the remote station during a data call (that is, over the communication link established between the modem and the remote station). For example, data received from controller  214 , over data link  218   i,  is packaged into payload field  404 .  
      Data frame  400  has a duration T, such as 20 milliseconds, for example. The payload data in payload field  404  is conveyed at one of a plurality of data rates, including a maximum or full-rate (for example, 9600 bits-per-second (bps)), a half-rate (for example, 4800 bps), a quarter-rate (for example, 2400 bps), or an eighth-rate (for example, 1200 bps). Each of the modems  216  attempts to transmit data at the full-rate (that is, at a maximum data rate). However, an over-limit modem rate-limits, whereby the modem reduces its transmit data rate from the maximum rate to a lower rate, as will be discussed below. Also, each of the modems  216  may transmit a data frame (for example, data frame  400 ) without payload data. This is referred to as a zero-rate data frame.  
      In one modem arrangement, each of the data bits  406  within a frame carries a constant amount of energy, regardless of the transmit data rate. That is, within a frame, the energy-per-bit, E b , is constant for all of the different data rates. In this modem arrangement, each data frame corresponds to an instantaneous modem transmit power that is proportional to the data rate at which the data frame is transmitted. Therefore, the lower the data rate, the lower the modem transmit power.  
      Each of the modems  216  provides status reports to controller  214  over respective data connections  218 .  FIG. 5  is an illustration of an example status report  500 . Status report  500  includes a modem data rate field  502 , a modem transmit power field  504 , and an optional over-limit (also referred to as a rate-limiting) indicator field  506 . Each modem reports the data rate of the last transmitted data frame in field  502 , and the transmit power of the last transmitted data frame in field  504 . In addition, each modem can optionally report whether it is in a rate-limiting condition in field  506 .  
      In another alternative modem arrangement, the modem can provide status signals indicating the over-limit/rate-limiting condition, the transmit power, and transmit data rate of the modem.  
      IV. Example Method  
       FIG. 6  is a flowchart of an example method or process  600  representative of an operation of modem  300 , and thus, of each of modems  216 . Method  600  assumes a data call has been established between a modem (for example, modem  216   a ) and the remote station. That is, a communication link including a forward link and a reverse link has been established between the modem and the remote station.  
      At a first step  602 , a transmit power limit P L  is established in the modem (for example, in modem  216   a ).  
      At a next step  604 , the modem receives a power control command from the remote station over the forward link indicating a requested transmit power P R  at which the modem is to transmit data frames in the reverse link direction. This command may be in the form of an incremental power increase or decrease command.  
      At a decision step  606 , the modem determines whether any payload data has been received from controller  214 , that is, whether or not there is any payload data to transmit to the remote station. If not, processing of the method proceeds to a next step  608 . At step  608 , the modem transmits a data frame at the zero-rate, that is, without payload data. The zero-rate data frame may include control/overhead information used to maintain the communication link/data call, for example. The zero-rate data frame corresponds to a minimum transmit power of the modem.  
      On the other hand, if there is payload data to transmit, then processing (control) proceeds from step  606  to a next step  610 . At step  610 , the modem determines whether or not it is not over-limit, that is, whether the modem is under-limit. In one arrangement, determining whether the modem is under-limit includes determining whether the requested transmit power P R  is less than the transmit power limit P L . In this arrangement, the modem is considered over-limit when the requested transmit power P R  is greater than or equal to P L . In an alternative arrangement, determining whether or not the modem is under-limit includes determining whether an actual transmit power P T  of the modem is less than the transmit power limit P L . In this arrangement, the modem is considered over-limit when P T  is greater than or equal P L . The modem may use power meter  320  in determining whether its transmit power P T , for example, the transmit power of signal  222   i   T , is less than the transmit power limit P L .  
      While the modem is not-over limit, the modem transmits a data frame, including payload data and control information, at a maximum data rate (for example, the full-rate) and at a transmit power level P T  that is in accordance with the requested transmit power P R . In other words, the modem transmit power P T  tracks the requested transmit power P R .  
      When P T  or P R  is equal to or greater than P L , the modem is over-limit, and thus rate-limits from a current rate (for example, the full-rate) to a lower transmit data rate (for example, to the half-rate, quarter-rate, eighth-rate or even the zero-rate), thereby reducing the transmit power P T  of the modem relative to when the modem was transmitting at the full-rate. Therefore, rate-limiting in response to either of the over-limit conditions described above is a form of modem self power-limiting, whereby the modem maintains its transmit power P T  below the transmit power limit P L . Also, the over-limit/rate-limiting condition, as reported in status report  500 , indicates to controller  214  that the requested power P R , or the actual transmit power P T  in the alternative arrangement, is greater than or equal to the transmit power limit P L . It should be appreciated that while the modem may be operating at the zero-rate in the transmit (that is, reverse link) direction, because it either is rate-limiting (for example, in step  610 ) or has no payload data to transmit (step  608 ), it may still receive full-rate data frames in the receive (that is, forward link) direction.  
      Although it can be advantageous for the modem to self rate-limit in response to the over-limit condition, an alternative arrangement of the modem does not rate-limit in this manner. Instead, the modem reports the over-limit condition to controller  214 , and then waits for the controller to impose rate-limiting adjustments. A preferred arrangement uses both approaches. That is, the modem self rate-limits in response to the over-limit condition, and the modem reports the over-limit condition to controller  214 , and in response, the controller imposes rate-limiting adjustments on the modem.  
      After both step  608  and step  610 , the modem generates a status report (for example, status report  500 ) at a step  612 , and provides the report to controller  214  over a respective one of data links  218 .  
      V. Fixed Transmit Power Limit Embodiments  
       FIG. 7  is a flowchart of an example method performed by MWT  206 , in accordance with the present embodiments. Method  700  includes an initializing step  702 . Step  702  includes further steps  704 ,  706 , and  708 . At step  704 , controller  214  establishes an individual transmit power limit P L  in each of modems  216 . The transmit power limits are fixed over time in method  700 .  
      At step  706 , controller  214  establishes a data call over each of modems  216 . In other words, a communication link, including both forward and reverse links, is established between each of the modems  216  and the remote station. The communication links operate concurrently with one another. In an exemplary arrangement of the present invention, the communication links are CDMA based communication links.  
      In the embodiments, a modem may be designated as an active modem or as an inactive modem. Controller  214  can schedule active modems, but not inactive modems, to transmit payload data. Controller  214  maintains a list identifying currently active modems. At a step  708 , controller  214  initially designates all of the modems as being active, by adding each of the modems to the active list, for example.  
      At a next step  710 , assuming controller  214  has received data that needs to be transmitted to the remote station, controller  214  schedules each of the active modems to transmit payload data. In a first past through step  710 , all of modems  216  are active (from step  708 ). However, in subsequent passes through step  710 , some of modems  216  may be inactive, as will be described below.  
      Controller  214  maintains a queue of data-to-be-transmitted for each of the active modems, and supplies each data queue with data received from the external data sources over link  210 . Controller  214  provides data from each data queue to the respective active modem. Controller  214  executes data-loading algorithms to ensure the respective data queues are generally, relatively evenly loaded, so that each active modem is concurrently provided with data-to-be-transmitted. After controller  214  provides data to each modem, each modem in turn attempts to transmit the data in data frames at the full-rate and in accordance with the respective requested transmit power P R , as described above in connection with  FIG. 6 .  
      At step  710 , controller  214  also de-schedules inactive modems by diverting data-to-be-transmitted away from such inactive modems and toward the active modems. However, there are no inactive modems in the first pass through step  710 , since all of the modems are initially active after step  708 , as mentioned above.  
      At a next step  712 , controller  214  monitors the modem status reports from all of the inactive and active modems.  
      At a next step  714 , controller  214  determines whether any of the modems  216  are over-limit, and thus rate-limiting, based on the modem status reports. If controller  214  determines that one or more (that is, at least one) of the modems are over-limit, then controller  214  deactivates only these over-limit modems, at a step  716 . For example, controller  214  can deactivate an over-limit modem by removing it from the active list.  
      If none of the modems are determined to be over-limit at step  714 , the method or processing proceeds to a step  718 . Processing also proceeds to step  718  after any over-limit modems are deactivated in step  716 . At step  718 , controller  214  determines whether or not any of the modems previously deactivated at step  716  need to be activated (that is, reactivated). Several techniques for determining whether modems should be activated are discussed below. If the answer at step  718  is yes (modems need to be reactivated), then processing proceeds to a step  720 , and controller  214  activates the previously deactivated modems that need to be activated, for example, by reinstating the modems on the active list.  
      If none of the previously deactivated modems need to be activated, then processing proceeds from step  718  back to step  710 . Also, processing proceeds from step  720  to step  710 . Steps  710  through  720  are repeated over time, whereby over-limit ones of modems  216  are deactivated at step  716  and then reactivated at step  718  as appropriate, and correspondingly de-scheduled and re-scheduled at step  710 .  
      When an over-limit modem is deactivated at step  716  (that is, becomes inactive), and remains deactivated through step  718 , the modem will be de-scheduled in the next pass through step  710 . In other words, controller  214  will no longer provide data to the deactivated modem. Instead, controller  214  will divert data to active modems. If it is assumed that the data call associated with the deactivated modem has not been torn-down (that is, terminated), then de-scheduling the modem at step  710  will cause the deactivated modem to have no payload data to transmit, and will thus cause the modem to operate at the zero-rate and at a corresponding minimum transmit power level on the reverse link (see steps  606  and  608 , described above in connection with  FIG. 6 ). This keeps the data call alive or active on the deactivated/descheduled modem, so the modem can still receive full-rate data frames on the forward link. When a data call associated with a modem is torn-down, that is, terminated or ended, the modem stops transmitting and receiving data altogether.  
      Deactivating the over-limit modem at step  716  ultimately causes the modem to reduce its transmit data rate and corresponding transmit power in the reverse link direction. In this manner, controller  214  individually controls the modem transmit power limits (and thus modem transmit powers), and as a result, can maintain the aggregate transmit power of signal  230  at a level below the aggregate transmit power limit of MWT  206 .  
      Alternative arrangements of method  700  are possible. As described above, deactivating step  716  includes deactivating an over-limit modem by designating the modem as inactive, for example, by removing the modem from the active list. Conversely, activating step  720  includes reinstating the deactivated modem to the active list. In an alternative arrangement of method  700 , deactivating step  716  further includes tearing-down (that is, terminating) the data call (that is, the communication link) associated with the over-limit modem. Also, in this alternative arrangement, activating step  720  further includes establishing another data call over the previously deactivated modem, so that the modem can begin to transmit data to and receive data from the remote station.  
      In another alternative arrangement of method  700 , deactivating step  716  further includes deactivating all of the modems, whether over-limit or not over-limit, when any one of the over-limit modems is detected at step  714 . In this arrangement, deactivating the modems may include designating all of the modems as inactive, and may further include tearing-down all of the data calls associated with the modems.  
       FIG. 8  is a flowchart expanding on transmit limit establishing step  704  of method  700 . At a first step  802 , controller  214  derives the transmit power limit for each of modems  216 . For example, controller  214  may calculate the transmit power limits, or simply access predetermined limits stored in a memory look-up table. At a next step  804 , controller  214  provides each of the modems  216  with a respective one of the transmit power limits, and in response, the modems store their respective transmit power limits in their respective memories.  
       FIG. 9  is a flowchart expanding on determining step  718  of method  700 . Controller  214  monitors (at step  712 , for example) the respective reported transmit powers of the deactivated/inactive modems that are transmitting at the zero-rate. At a step  902 , controller  214  derives, from the reported modem transmit powers, respective extrapolated modem transmit powers representative of when the modems transmit at the maximum transmit data rate.  
      At a next step  904 , controller  214  determines whether each extrapolated transmit power is less than the respective modem transmit power limit P L . If yes, then processing proceeds to step  720  where the respective modem is activated, because it is likely the modem will not exceed the power limit. If not, the modem remains deactivated, and the method proceeds back to step  710 .  
       FIG. 10  is a flowchart of another example method  1000  performed by MWT  206 . Method  1000  includes many of the method steps described previously in connection with  FIG. 7 , and such method steps will not be described again. However, method  1000  includes a new step  1004  following step  716 , and a corresponding determining step  1006 . At step  1004 , controller  214  initiates an activation timeout period (for example, using timer  217 ) corresponding to each modem deactivated at step  716 . Alternatively, controller  214  can schedule a future activation time/event corresponding to each modem deactivated in step  716 .  
      At determining step  1006 , controller  214  determines whether it is time to activate any of the previously deactivated modems. For example, controller  214  determines whether any of the activation timeout periods have expired, thereby indicating it is time to activate the corresponding deactivated modem. Alternatively, controller  214  determines whether the activation time/event scheduled at step  1004  has arrived.  
      Alternative arrangements of method  1000 , similar to the alternative arrangements discussed above in connection with method  700 , are also envisioned.  
      VI. Fixed Transmit Power Limit Arrangements  
      1. Uniform Limits  
      In one fixed limit arrangement, a uniform set of fixed transmit power limits is established across all of modems  216 . That is, each modem has the same transmit power limit as each of the other modems.  FIG. 11  is an example plot of Power versus Modem index(i) identifying respective ones of the modems  216 , wherein uniform, modem transmit power limits P Li  are depicted. As depicted in  FIG. 11 , modem( 1 ) corresponds to power limit P L1 , modem( 2 ) corresponds to power limit P L2 , and so on.  
      In one arrangement of uniform limits, each transmit power limit P L  is equal to the aggregate transmit power limit APL divided by the total number N of modems  216 . Under this arrangement of uniform limits, when all of the modems have respective transmit powers equal to their respective transmit power limits, the aggregate transmit power for all of the modems will just meet, and not exceed, the APL. An example APL in the present invention is approximately 10 or 11 decibel-Watts (dBW).  
       FIG. 11  also represents an example transmit scenario for MWT  206 . Depicted in  FIG. 11  are representative, requested modem transmit powers P R1  and P R2  corresponding to modem( 1 ) and modem( 2 ). The example transmit scenario depicted in  FIG. 11  corresponds to the scenario in which all of the requested modem transmit powers are below the respective, uniform transmit power limits. In this situation, none of the modems are over-limit, and thus rate-limiting.  
       FIG. 12  is another example transmit scenario similar to  FIG. 11 , except that modem( 2 ) has a requested power P R2  exceeding respective transmit power limit P L2 . Therefore, modem( 2 ) is over-limit, and thus rate-limiting. Since modem( 2 ) is over-limit, controller  214  deactivates modem( 2 ) in accordance with method  700  or method  1000 , thereby causing modem( 2 ) to transmit at a zero-data rate, and at a correspondingly reduced transmit power level  1202 .  
      2. Tapered Limits  
       FIG. 13  is an illustration of an alternative, tapered arrangement for the fixed modem transmit power limits. As depicted, the tapered arrangement includes progressively decreasing transmit power limits P Li  in respective successive ones of the N modems, where i=1 . . . N. For example, transmit power limit P L1  for modem(l) is less than transmit power limit P L2  for modem( 2 ), which is less than transmit power limit P L3 , and so on down the line.  
      In one tapered arrangement, each of the transmit power limits P Li  is equal to the APL divided by the total number of modems having transmit power limits greater than or equal to P Li . For example, transmit power limit P L5  is equal to the APL divided by five (5), which is the number of modems having transmit power limits greater than or equal to P L5 . In another tapered arrangement, each transmit power limit P Li  is equal to the transmit power limit mentioned above (that is, the APL divided by the total number of modems having transmit power limits greater than or equal to P Li ) less a predetermined amount, such as one, two or even three decibels (dB). This permits a safety margin in the event that the modems tend to transmit at an actual transmit power level that is slightly higher than the respective transmit power limits, before they are deactivated.  
      Assume a transmit scenario where all of the modems transmit at approximately the same power, and all of the transmit powers are increasing over time. Under the tapered arrangement, modem(N) rate-limits first, modem(N- 1 ) rate limits next, modem(N- 2 ) rate-limits third, and so on. In response, controller  214  deactivates/deschedules modem(N) first, modem(N- 1 ) second, modem(N- 3 ) third, and so on.  
      VII. Modem Calibration—Determining Gain Factors g(i)  
      As described above in connection with  FIG. 2 , each modem  216   i  generates a transmit signal  222   i   T  having a respective transmit power level. Also, each modem  216   i  generates a status report including a modem transmit power estimate P Rep (i) of the respective transmit power level. Each modem transmit signal  222   i   T  traverses a respective transmit path from modem  222   i  to the output of transmit amplifier  228 . The respective transmit path includes RF connections, such as cables and connectors, power combiner/splitter assembly  220 , and transmit amplifier  228 . Therefore, transmit signal  222   i   T  experiences a respective net power gain or loss g(i) along the respective transmit path. An example gain for the above-mentioned transmit path is approximately 29 dB.  
      Accordingly, the gain or loss g(i) of the respective transmit path may cause the power level of respective transmit signal  222   i   T  at the output of modem  222   i  to be different from the transmit power level at the output of transmit amplifier  228 . Therefore, the respective modem transmit power estimate P Rep (i) may not accurately represent the respective transmit power at the output of transmit amplifier  228 . A more accurate estimate P O (i) of the transmit power at the output of transmit amplifier  228  (due to modem  222   i ), is the reported power P Rep (i) adjusted by the corresponding gain/loss amount g(i). Therefore, g(i) is referred to as a modem dependent gain correction factor g(i), or the modem gain factor g(i) for modem  222   i.    
      When reported modem transmit power estimate P Rep (i) and modem gain correction factor g(i) both represent power terms (as expressed in decibels or Watts, for example), the corrected transmit power estimate P O (i) is given by: 
 
 P   O ( i )= g ( i )+ P   Rep ( i ). 
 
      Alternatively, when reported transmit power estimate P Rep (i) and modem gain correction factor g(i), in Watts, for example, the transmit power P O (i) is given by: 
 
 P   O ( i )= g ( i ) P   Rep ( i ). 
 
      It is useful to be able to calibrate MWT  206  dynamically, to determine the gain correction factors g(i) corresponding to all of the N modems. Once the factors g(i) are determined, they can be used to calculate more accurate individual and aggregate modem transmit power estimates from the modem transmit power reports.  
       FIG. 14  is a flowchart of an example method of calibrating modems  216  in MWT  206 . At a first step  1405 , controller  214  schedules all N modems  216  to transmit data, so as to cause all of the modems to transmit data, concurrently.  
      At a next step  1410 , controller  214  collects status reports  500 , including respective reported transmit powers P Rep (i), where i represents modem i, and i=1 . . . N.  
      At a next step  1420 , controller  214  receives an aggregate transmit power measurement P Agg  for all of the N modems, for example, as determined by transmit power monitor  234 .  
      At a next step  1425 , controller  214  generates an equation representing the aggregate transmit power as a cumulative function of reported transmit powers P Rep (i) and corresponding unknown, modem dependent gain correction factors g(i). For example, aggregate transmit power P Agg  is represented as:  
         P   Agg     =       ∑     i   =   1       N   N       ⁢       g   ⁡     (   i   )       ⁢         P   Rep     ⁡     (   i   )       .             
 
      At a next step  1430 , previous steps  1405 ,  1410 ,  1420  and  1425  are repeated N times to generate N simultaneous equations in P Rep (i) and unknown gain correction factors g(i).  
      At a next step  1435 , controller  214  determines the N gain correction factors g(i) by solving the N equations generated in step  1430 . Determined gain correction factors g(i) are stored in memory  215  of MWT  206 , and used as needed to adjust/correct modem transmit power estimates P Rep (i) in the methods of the invention, described below. Method  1400  may be scheduled to repeat periodically to update factors g(i) over time.  
      VIII. Methods Using Dynamically Updated Transmit Limits  
       FIG. 15  is a flowchart of an example method  1500  of operating MWT  206 , using dynamically updated individual modem transmit power limits. In method  1500 , controller  214  initializes (step  702 ), schedules and deschedules active and inactive ones of modems  216  (step  710 ), and monitors status reports from the modems (step  712 ), as described above. At a next step  1502 , controller  214  determines whether to modify (for example, increase or decrease) or maintain the number of active modems of MWT  206 , in order to maximize an aggregate reverse link data rate (that is, the aggregate transmit data rate) without exceeding the aggregate transmit power limit of the MWT.  
      At a next step  1504 , controller  214  increases, decreases, or maintains the number of active modems, as necessary, in accordance with step  1502 . To increase the number of active modems, controller  214  adds one or more previously inactive modems to the active list. Conversely, to decrease the number of active modems, controller  214  deletes one or more previously active modems from the active list.  
      At a next step  1506 , controller  214  updates/adjusts individual transmit power limits in at least some of modems  216 , as necessary. Techniques for adjusting individual transmit power limits will be described further below. In step  1506 , the individual transmit power limits are adjusted across modems  216  such that when all of the individual transmit limits are combined together into a combined transmit power limit, the combined transmit power limit does not exceed the aggregate transmit power limit of MWT  206 . Exemplary transmit power limit arrangements that may be used with method  1500  are described later in connection with Table 1 and  FIG. 19 . A reason for varying modem transmit power limits in method  1500  is to avoid rate-limiting conditions in the modems. Also, a reason for deactivating modems (that is, decreasing the number of active modems) includes avoiding rate-limiting conditions so as to increase the overall transmit data rate on the reverse-link while operating under the aggregate transmit power limit.  
      At first blush, it might appear that deactivating modems would decrease, not increase, the transmit data rate. However, operating a number of modems, for example, 16 modems, at their rate-limited data rates (for example, at 4800 bps) achieves a lower effective data rate than operating a lesser number modems, for example 8 modems, at their full rates (for example, 9600 bps), even though each case may have the same aggregate transmit power. This is because the ratio of overhead information (used to manage the data calls, for example) to actual/useful data (used by end users, for example) is disadvantageously greater for rate limiting modems compared to non-rate limiting modems.  
       FIG. 16  is a flowchart of an example method  1600  expanding on method  1500 . Method  1600  includes a step  1602  expanding on step  1502  of method  1500 . Step  1602  includes further steps  1604  and  1606 . At step  1604 , controller  214  determines a maximum number N Max  of active modems that can concurrently transmit at their respective maximum data rates (for example, at 9600 bps), without exceeding the aggregate transmit power limit of MWT  206 . It is assumed that N Max  is less than or equal to a total number N of modems  216 .  
      At next step  1606 , controller  214  compares the maximum number N Max  to a number M of previously active modems (that is, the number of active modems used in a previous pass through step  710 , described above).  
      A next step  1610 , corresponding to step  1504  of method  1500 , includes further steps  1612 ,  1614  and  1616 . If the maximum number N Max  of active modems from step  1604  is greater than the number M of previously active modems, then the method proceeds from step  1606  to next step  1612 . At step  1612 , controller  214  increases the number M of active modems to the maximum number N Max  of active modems. To do this, controller  214  selects an inactive modem to activate from among the N modems.  
      Alternatively, if the maximum number N Max  of modems is less than M, then processing proceeds from step  1606  to step  1614 . At step  1614 , controller  214  decreases the number of active modems. To do this, controller  214  selects an active modem to deactivate. Steps  1612  and  1614  together represent an adjusting step (also referred to as a modifying step) where the number M of previously active modems is modified in preparation for a next pass through steps  710 ,  712 , and so on.  
      Alternatively, if the maximum number N Max  is equal to M, then processing proceeds from step  1606  to step  1616 . In step  1616 , controller  214  simply maintains the number of active modems at M, for the next pass through steps  710 ,  712 , and so on.  
      The method proceeds from both modifying steps  1612  and  1614  to a next, limit adjusting step  1620 . At step  1620 , controller  214  increases the individual transmit power limits in the one or more modems that were activated at step  1612 . Conversely, controller  214  decreases the individual power limits in the one or more modems that were deactivated in step  1614 .  
      The method proceeds from steps  1610  and  1620  back to scheduling/descheduling step  710 , and the process described above repeats.  
       FIG. 17  is a flowchart of an example method  1700  of determining the maximum number N Max  of active modems using an average energy-per-transmitted-bit of the N modems. Method  1700  expands on step  1604  of method  1600 . At a first step  1702 , controller  214  determines an aggregate transmit data rate based on the respective transmit data rates reported by the N modems. For example, controller  214  adds together all of the transmit data rates reported by the N modems in respective status reports  500 .  
      At a next step  1704 , controller  214  determines an aggregate power level of transmit signal  230 , at the output of transmit amplifier  228 . For example, controller  214  may receive transmit power measurements (signal  236 ) from transmit power monitor  234 . Alternatively, controller  214  may aggregate individual modem transmit power estimates P Rep (i) (as corrected using factors g(i)) received from the individual modems in respective status reports  500 .  
      At a next step  1706 , controller  214  determines the average energy-per-transmitted-bit across the N modems  216  based on the aggregate data rate and the aggregate transmit power. In one arrangement of the embodiments, controller  214  determines the average energy-per-transmitted-bit in accordance the following relationships: 
 
 BE   b     —     avg   =P ( t )Δ t=E   T , 
 
and, therefore, 
 
 E   b     —     avg =( P ( t )Δ t )/ B=E   T   /B,  
 
 where: 
      Δt is a predetermined measurement time interval (for example, the duration of a transmitted frame, such as 20 ms),     B is the aggregate data rate during time interval Δt,     E b     —     avg  is the average energy-per-transmitted-bit during time interval Δt,     P(t) is the aggregate transmit power during time interval Δt, and     E T  is the total energy of all the bits transmitted during time interval Δt.    

      At a next step  1708 , controller  214  determines the maximum number N Max  based on the average energy-per-transmitted-bit and the aggregate transmit power limit. In one arrangement, controller  214  determines the maximum number N Max  in accordance with the following equations: 
 
(( R   max   N   Max   +R   min ( N−N   Max )) E   b     —     avg   =APL,  
 
and, therefore, 
 
 N   Max =(( APL/E   b     —     avg )− P   min   N)/ ( R   max   −R   min ), 
 
 where: 
      APL is the aggregate transmit power limit of MWT  206  (for example, 10 or 11 decibel-Watts (dBW)),     R max  is a maximum data rate of the N modems (for example, 9600 bps),     R min  is a minimum data rate of the N modems (for example, 2400 bps),     E b     —     avg  is the average energy-per-transmitted-bit during time interval Δt,     N is the total number of modems  216 , and     N Max  is the maximum number of active modems to be determined.    

       FIG. 18  is a flowchart of an example method  1800  of determining the maximum number N Max  of active modems, using an individual energy-per-transmitted-bit for each of modems  216 . Method  1800  expands on step  1604  of method  1600 . At a first step  1802 , controller  214  determines an individual energy-per-transmitted-bit E b (i) for each modem using modem reports  500 . In one arrangement of the embodiment, controller  214  determines each energy-per-transmitted-bit E b (i) in accordance the following relationship: 
   E   b ( i )= g ( i ) P   Rep ( i )Δ t/Bi,    
 where: 
      Δt is a predetermined measurement time interval,     E b (i) is the individual energy-per-transmitted-bit for modem i, where i=1 . . . N, over time interval Δt,     P Rep (i) is a reported modem transmit power (that is, a transmit power estimate for modem i), and     g(i) is a modem dependent gain correction factor, also referred to as a gain calibration factor (described above in connection with  FIG. 14 ), and     Bi is the transmit data rate of modem i.    
      At a step  1804 , controller  214  sorts the modems according to their respective energy-per-transmitted-bits E b (i).  
      At a next step  1805 , controller  214  determines the maximum number N Max  of active modems based on the individual modem energy-per-transmitted-bits, using an iterative process. In one arrangement, the iterative process of step  1805  determines the maximum number N Max  of active modems that can be supported, using the following equation:  
         APL   =         ∑     i   =   1       N   Max       ⁢       P   max     ⁢       E   b     ⁡     (   i   )           +       ∑     i   =     N   Max       N     ⁢       P   min     ⁢       E   b     ⁡     (   i   )               ,       
 
 where: 
      APL is the aggregate transmit power limit,     P max  is the maximum data rate for each modem,     P min  is the minimum data rate for each modem, and     E b (i) is the individual energy-per-transmitted-bit for modem i.    

      Step  1805  is now described in further detail. A step  1806  within step  1805  is an initializing step in the iterative process, wherein modem  214  sets a test number N Act  of active modems equal to one (1). Test number N Act  represents a test, maximum number of active modems. At a next step  1808 , modem  214  determines an expected transmit power P Exp  using the test number N Act  of modems. In step  1808 , it is assumed that the test number N Act  of modems having the-lowest individual energy-per-transmitted-bits among the N modems each transmit at a maximum data rate (for example, 9600 bps). In the arrangement mentioned above, step  1808  determines the expected transmit power in accordance with the following relationship:  
           P   Exp     =         ∑     i   =   1       N   act       ⁢       P   max     ⁢       E   b     ⁡     (   i   )           +       ∑     i   =     N   act       N     ⁢       P   min     ⁢       E   b     ⁡     (   i   )               ,       
 
      At a next step  1809 , controller  214  compares the expected transmit power P Exp  to the APL. If P Exp &lt;APL, then more active modems can be supported. Thus, the test number N Act  of active modems is incremented (step  1810 ), and the method proceeds back to step  1808 .  
      Alternatively, if P Exp =APL, then the maximum number N Max  of active modems is set equal to the present test number N Act  (step  1812 ).  
      Alternatively, if P Exp &gt;APL, then the maximum number N Max  is set equal to the previous test number of active modems, that is, N Act −1 (step  1814 ).  
      If P Exp  is neither equal to nor greater than APL then the process returns to step  1810  and step  1809 . At some point a maximum number of modems may be reached or exceeded and either step  1812  or  1814 , respectively, are reached. The process for recalculating APL checking the current N (number of access terminals in use), or checking P Exp  relative to APL, may be repeated every so often or on a periodic basis as part of an iterative procedure to prevent overdriving the power amplifier.  
      IX. Example Transmit Power Limits  
      Table 1, below, includes exemplary modem transmit power limits that may be used in the present invention.  
                           TABLE 1                       A                   No. active   B   C   D       modems   Active Modem   Active Modem   Active Modem       (Total   Limits (dBm)   Limits (dBm)   Limits (dBm)       N = 16)   APL = 10 dBW   APL = 11 dBW   APL = 10 dBW                                                1.0   5.0   5.2   4.2       2.0   5.0   4.6   3.6       3.0   5.0   4.0   3.0       4.0   5.0   3.5   2.5       5.0   4.0   3.1   2.1       6.0   3.2   2.7   1.7       7.0   2.5   2.3   1.3       8.0   2.0   2.0   1.0       9.0   1.5   1.7   0.7       10.0   1.0   1.4   0.4       11.0   0.6   1.1   0.1       12.0   0.2   0.9   −0.1       13.0   −0.1   0.6   −0.4       14.0   −0.5   0.4   −0.6       15.0   −0.8   0.2   −0.8       16.0   −1.0   0.0   −1.0                  
 
      The transmit power limits of Table 1 may be stored in memory  215  of MWT  206 . Table 1 assumes MWT  206  includes a total of N=16 modems. Each row of table 1 represents a corresponding number (such as 1, 2, 3, and so on, down the rows) of active ones of the N modems, at any given time. Each row of Column A identifies a given number of active modems. The number of inactive modems corresponding to any given row of Table 1 is the difference between the total number of modems (16) and the number of active modems specified in the given row.  
      Columns B, C and D collectively represent three different individual transmit power limit arrangements of the present invention. The transmit limit arrangement of column B assumes an APL of 10 dBW in MWT  206 . Also, the arrangement of column B assumes that, in any given row, all of the active modems receive a common maximum transmit limit, while all of the inactive modems receive a common minimum transmit limit equal to zero. For example in column B, when the number of active modems is six (6), a common maximum transmit limit of 3.2 decibel-milliwatt (dBm) is established in each of the active modems, and a common minimum transmit limit of zero is established in each of the ten (10) inactive modems. The sum of the maximum transmit power limits in all of the active modems corresponding to any given row is equal to the APL.  
      The transmit limit arrangement of column C assumes an APL of 11 dBW in MWT  206 . Also, the arrangement of column C assumes that, for any given number of active modems (that is, for each row in Table 1), all of the active modems receive a common maximum transmit limit, while all of the inactive modems receive a common minimum transmit limit equal to the maximum transmit limit less six (6) dB. For example in column C, when the number of active modems is six (6), a maximum transmit limit of 2.7 dBm is established in each of the six (6) active modems, and a minimum transmit limit of (2.7-6) dBm is established in each of the ten (10) inactive modems. The sum of the maximum transmit power limits in all of the active modems, together with the sum of the minimum transmit power limits in all of the inactive modems, corresponding to any given row is equal to the APL. Since the transmit power limit in each of the inactive modems is greater than zero, the inactive modems may be able to transmit at respective minimum data rates, or at least at the zero-data rate, in order to maintain their respective data links active.  
      The transmit limit arrangement of column D is similar to that of column C, except a lower APL of 10 dBW is assumed in the arrangement of column D. The arrangement of column D assumes that, for any given number of active modems (that is, for each row in Table 1), all of the active modems receive a common maximum transmit limit, while all of the inactive modems receive a common minimal transmit limit equal to the maximum transmit limit less six (6) dB. For example, from column D, when the number of active modems is six (6), a maximum transmit limit of 1.7 dBm is established in each of the active modems, and a transmit limit of (1.7-6) dBm is established in each of the ten (10) inactive modems.  
      Controller  214  can use the limits specified in Table 1 to establish and adjust individual transmit limits in modems  216  in methods  1500  and  1600 , described above in connection with  FIGS. 15 and 16 . For example, assume the transmit limit arrangement of Table 1, column D, is being used with method  1600 . Assume the number of active modems in a previous pass through step  710  is seven. During the previous pass, a transmit limit of 1.3 dBm is established in each of the seven active modems, and a transmit limit of (1.3-6) dBm is established in the other nine, inactive modems (see the entry in column D corresponding to seven active modems). Also assume that in the next pass through steps  1602  and  1614 , the number of active modems is decreased from seven down to six. Then, at limit adjusting step  1620 , a new transmit limit of 1.7 dB is established in each of the six active modems, and a transmit limit of (1.7-6) dB is established in each of the ten remaining inactive modems.  
       FIG. 19  is a graphical representation of the information presented in Table 1.  FIG. 19  is a plot of transmit limit power (in dBm) versus the number of active modems (labeled as N) for each of the transmit limit arrangements listed in columns B, C and D of Table 1. In  FIG. 19 , the transmit limit arrangement of column B is represented by a curve COL B, the limit arrangement of column C is represented by a curve COL C, and the limit arrangement of column D is represents by a curve COL D.  
      X. MWT Computer Controller  
       FIG. 20  is a functional block diagram of an example controller (which can also be a plurality of controllers)  2000  representing controller  214 . Controller  2000  includes a series of controller modules for performing the various method steps of the embodiments discussed above.  
      A scheduler/descheduler  2002  schedules active modems to transmit payload data, and de-schedules inactive modems; a call manager  2004  establishes data calls and tears-down data calls over the plurality of modems  216 ; and a status monitor  2006  monitors status reports from modems  216 , for example, to determine when various ones of the modems are over-limit, and to collect modem transmit data rates and transmit powers. Status monitor  2006  may also determine an aggregate data rate and an aggregate transmit power based on the modem reports.  
      A deactivator/activator module  2008  acts to deactivate over-limit ones (in the fixed limit arrangement of the present invention) of the modems (for example by removing the modems from the active list) and to activate deactivated ones of the modems by reinstating the modems on the active list. Module  2008  also activates/deactivates selected ones of the modems in accordance with steps  1504 ,  1612 , and  1614  of methods  1500  and  1600 .  
      A limit calculator  2010  operates to calculate/derive transmit power limits for each of the modems  216 . Limit calculator also accesses predetermined transmit power limits stored in memory  215 , for example.  
      An initializer  2012  supervises/manages initialization of the system, such as establishing initial transmit power limits in each modem, setting up calls over each modem, initializing various lists and queues in MWT  206 , and so on.  
      A modem interface  2014  receives data from and transmits data to modems  216 , and a network interface  2016  receives and transmits data over interface  210 .  
      A module  2020  determines whether to modify the number of active modems in accordance with steps  1502  and  1602  of methods  1500  and  1600 . Module  2020  includes a sub-module  2022  for determining a maximum number of active modems that can be supported based on either an average-energy-per-transmitted-bit or individual modem energy-per-transmitted-bits. Sub-module  2022  includes comparison or comparing logic (such as a comparator) configured to operate in accordance with comparing step  1606  of method  1600 . Module  2020  also includes sub-modules  2024  and  2026  for determining the average-energy-per-transmitted-bit and the individual modem energy-per-transmitted-bits, respectively. Sub-modules  2024  and  2026 , or alternatively, status monitor  2006 , also determine an aggregate data rate and an aggregate transmit power based on modem reports.  
      A calibration module  2040  controls calibration in MWT  206  in accordance with method  1400 , for example. The calibration module includes an equation generator to generate simultaneous equations and an equation solver to solve the equations to determine modem correction factors g(i). The calibration module can also call/incorporate other modules, as necessary, to perform calibration of MWT  206 .  
      A software interface  2050  is used for interconnecting all of the above mentioned modules to one another.  
      Features of the present invention can be performed and/or controlled by processor/controller  214 , which in effect comprises a programmable or software-controllable element, device, or computer system. Such a computer system includes, for example, one or more processors that are connected to a communication bus. Although telecommunication-specific hardware can be used to implement the present invention, the following description of a general purpose type computer system is provided for completeness.  
      The computer system can also include a main memory, preferably a random access memory (RAM), and can also include a secondary memory and/or other memory. The secondary memory can include, for example, a hard disk drive and/or a removable storage drive. The removable storage drive reads from and/or writes to a removable storage unit in a well known manner. The removable storage unit, represents a floppy disk, magnetic tape, optical disk, and the like, which is read by and written to by the removable storage drive. The removable storage unit includes a computer usable storage medium having stored therein computer software and/or data.  
      The secondary memory can include other similar means for allowing computer programs or other instructions to be loaded into the computer system. Such means can include, for example, a removable storage unit and an interface. Examples of such can include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM, or PROM) and associated socket, and other removable storage units and interfaces which allow software and data to be transferred from the removable storage unit to the computer system.  
      The computer system can also include a communications interface. The communications interface allows software and data to be transferred between the computer system and external devices. Software and data transferred via the communications interface are in the form of signals that can be electronic, electromagnetic, optical or other signals capable of being received by the communications interface. As depicted in  FIG. 2 , processor  214  is in communications with memory  215  for storing information. Processor  214 , together with the other components of MWT  206  discussed in connection with  FIG. 2 , performs the methods of the present invention.  
      In this document, the terms “computer program medium” and “computer usable medium” are used to generally refer to media such as a removable storage device, a removable memory chip (such as an EPROM, or PROM) within MWT  206 , and signals. Computer program products are means for providing software to the computer system.  
      Computer programs (also called computer control logic) are stored in the main memory and/or secondary memory. Computer programs can also be received via the communications interface. Such computer programs, when executed, enable the computer system to perform certain features of the present invention as discussed herein. For example, features of the flow charts depicted in  FIGS. 7, 8 ,  9  and  10 , can be implemented in such computer programs. In particular, the computer programs, when executed, enable processor  214  to perform and/or cause the performance of features of the present invention. Accordingly, such computer programs represent controllers of the computer system of MWT  206 , and thus, controllers of the MWT.  
      Where the embodiments are implemented using software, the software can be stored in a computer program product and loaded into the computer system using the removable storage drive, the memory chips or the communications interface. The control logic (software), when executed by processor  214 , causes processor  214  to perform certain functions of the invention as described herein.  
      Features of the invention may also or alternatively be implemented primarily in hardware using, for example, a software-controlled processor or controller programmed to perform the functions described herein, a variety of programmable electronic devices, or computers, a microprocessor, one or more digital signal processors (DSP), dedicated function circuit modules, and hardware components such as application specific integrated circuits (ASICs) or programmable gate arrays (PGAs). Implementation of the hardware state machine so as to perform the functions described herein will be apparent to persons skilled in the relevant art(s).  
      The previous description of the preferred embodiments is provided to enable a person skilled in the art to make or use the present invention. While the invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.  
     XI. CONCLUSION  
      The present invention has been described above with the aid of functional building blocks illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Any such alternate boundaries are thus within the scope and spirit of the claimed invention. One skilled in the art will recognize that these functional building blocks can be implemented by discrete components, application specific integrated circuits, processors executing appropriate software and the like or many combinations thereof. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.