Abstract:
Mobile terminals in a high data rate CDMA system may be placed in a control hold mode wherein reverse link control channels are gated, or transmitted at a reduced duty cycle. Mode decisions are based on the activity of the mobile terminal in both the forward and reverse links. A forward link inactivity timer is maintained at the base station that schedules forward link communications to a mobile terminal. The reverse link inactivity timer may be maintained at the same base station, at a different base station in the mobile terminal&#39;s active set, or at the base station controller. The forward link inactivity timer may follow the mobile terminal&#39;s selection of best forward link serving base station in handoff. When both the forward and reverse link inactivity timers have expired, and no forward link data is pending, the mobile terminal may be commanded to a control hold mode.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to the field of wireless communications and specifically to methods for controlling the operational mode of mobile terminals in a wireless communication system.  
           [0002]    Numerous access schemes exist to allow multiple users to share a communication medium. One such access scheme is known as Code Division Multiple Access (CDMA). In CDMA systems, multiple users share the same carrier frequency and may transmit simultaneously. Each user has its own pseudo-noise (PN) sequence, which is approximately orthogonal to the PN sequences of other users. Transmissions to or from individual users are imprinted with that user&#39;s PN sequence. The receiver selects the desired signal, which combines in the communication with unwanted signals, by correlating the received signal with the PN sequence of the desired signal. All other signals are spread by the PN sequence and appear as noise to the receiver.  
           [0003]    CDMA systems are interference-limited systems. Since all mobile terminals operate at the same frequency, internal interference generated within the system plays a critical role in determining system capacity and voice quality. The transmit power from each mobile terminal must be controlled to limit interference while maintaining desired performance objectives, e.g., bit error rate (BER), frame error rate (FER), capacity, dropped-call rate, coverage, etc. Normally a mobile terminal transmits a pilot signal continuously on a reverse pilot channel (R-PICH) to enable closed loop power control by the network.  
           [0004]    In some implementation of cdma2000, such as 1xEV-DV and 1xEV-DO systems, the forward link is time-multiplexed and transmitted at the full power available to the base station, but with data rates and slot times that vary depending on forward link channel conditions. The data rate that can be supported by the forward link is proportional to the Signal to Noise Ratio (SNR), which changes continuously. The mobile terminal measures the instantaneous SNR of the pilot signal received from each base station in its active set and requests service from the base station providing the strongest signal. The mobile terminal transmits the SNR value, or equivalently the supportable data rate, for the base station providing the strongest signal on a reverse control channel referred to generically herein as the rate control channel.  
           [0005]    It has been proposed to reduce interference and hence increase system capacity by introducing a control hold state for mobile terminals with low transmit activity factors. In the control hold state, the mobile terminal suspends or reduces transmissions on many of the reverse link channels, such as a reverse rate control channel or reverse pilot channel. Gating or suspending transmission on the reverse link channels reduces interference, thus increasing the reverse link throughput and capacity. It also results in lower power consumption at the mobile terminal and thus increased battery life.  
           [0006]    One method for identifying which mobile terminals should be placed in a control hold state exploits the phenomenon known as temporal locality. Communications to and from a particular user tend to be grouped in time. A mobile terminal that has very recently sent or received a transmission is more likely to receive or send another transmission in the near term, than is a mobile terminal that has not sent or received a transmission for some time. Following this principle, mobile terminals that have recently successfully received or sent transmissions are maintained in an active mode, and those to and from whom no transmissions have been directed for a predetermined time are commanded to enter a control hold state. A straightforward manner of implementing this approach is to maintain countdown timers for each transmission direction, which are loaded with a predetermined value and started upon each successful transmission. Such timers are referred to herein as the forward link inactivity timer and the reverse link inactivity timer. If an inactivity timer counts down to zero before another transmission in its respective direction, it is said to have expired, or timed-out. If both the forward link and the reverse link inactivity timers for a particular mobile terminal have expired, and if there is no data queued awaiting transmission to that mobile terminal, the network may command the mobile terminal to transition to the control hold state. Traditionally, a base station controller (BSC) in the network maintains and manages both inactivity timers, and controls the operational modes of the mobile terminals.  
           [0007]    In some cdma2000 networks, such as 1xEV-DV and 1xEV-DO systems, the forward link is a time-shared channel and the base station transmits to only one mobile terminal at any given time. In systems of this type, a scheduler is usually maintained at each base station to schedule forward link transmissions for all mobile terminals served by that base station. When the serving base station schedules data packets for transmission to a mobile terminal, however, the forward link inactivity timer maintained at the BSC may not be started or reset at the proper time due to scheduling delays at the serving base station, or signaling delays between the base station and the BSC, and thus may not accurately reflect the timing of the last forward link transmission to the mobile terminal. This problem may be further complicated by the fact that, in soft handoff, the mobile terminal may select a different base station in its active set as its forward link serving base station, based on received signal quality. This further complicates the synchronization of message scheduling to the mobile terminal at the base station and the state of the forward link inactivity timer at the base station controller.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention relates to a method of controlling the operational modes of mobile terminals in a wireless communication system, based on their activity in both the forward and reverse links. The method comprises maintaining a forward link inactivity timer and a reverse link inactivity timer. The forward link inactivity timer is located at a serving base station that schedules forward link communications to a mobile terminal. The reverse link inactivity timer is preferably located at a base station controller that controls the operating mode of the mobile terminal, or at a serving base station. Less preferably, the reverse link inactivity timer could be places at a non-serving base station in the active set of the mobile terminal, however, this solution would require signaling between the base stations. The status of the timers, including the value or whether they have expired, are communicated to the base station controller. When both timers have expired and no forward link data is queued, the mobile terminal is commanded to transition to a control hold mode where reverse link control channels are gated, reducing interference and increasing system capacity on the reverse link.  
           [0009]    In soft handoff, when the mobile terminal selects a new forward link serving base station, the former forward link serving base station may communicate the state and the value of the forward link inactivity timer to the new forward link serving base station, which manages the timer. The reverse link inactivity timer may be transferred as well, or may be maintained by any base station in the active set of the mobile terminal, or at the base station controller. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a functional block diagram of a mobile communication network.  
         [0011]    [0011]FIG. 2 is a functional block diagram of a base station.  
         [0012]    [0012]FIG. 3 is a state diagram for managing the operating mode of mobile terminal.  
         [0013]    [0013]FIG. 4 is a state diagram for managing a forward link inactivity timer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Referring now to the drawings, the mode control method of the present invention is shown in the context of an IS-2000 mobile communication network, which is indicated generally by the numeral  10 . IS-2000 is a communications protocol for code division multiple access (CDMA) radio networks published by the Telecommunications Industry Association (TIA) and the Electronics Industry Association (EIA). FIG. 1 illustrates the logical architecture of a mobile communication network  10  and is not intended to imply a particular physical implementation. The following description is intended to describe how to implement the present invention in an exemplary mobile communication system. Those skilled in the art will recognize that the present invention can be adapted for use in mobile communication systems employing different standards, such as IS-95 in Wideband CDMA (W-cdma) systems.  
         [0015]    The mobile communication network  10  includes a plurality of base transceiver stations  20  (referred to herein simply as base stations), one or more base station controllers (BSCs)  30 , and a core network  14 . Each base station  20  communicates with mobile terminals  100  within its coverage area over an air interface. Each base station  20  is connected to a BSC  30  that supervises the operation of multiple base stations  20 . The BSC  30  connects to the core network  14 , which provides a gateway to the public switch telephone network (PSTN)  16  and packet data networks  18 , such as the Internet. The BSC  30 , in the exemplary embodiment of the invention, includes mode control logic  32  to control the operational mode of the mobile terminals  100  in its service area. The BSC  30  may also include reverse link inactivity timers  34 , the purpose of which is discussed below.  
         [0016]    In current implementations of cdma2000 systems, the reverse link control channels, such as the reverse pilot channel and the reverse rate control channel are transmitted continuously. It has been proposed to reduce interference and hence increase system capacity by introducing a control hold mode for mobile terminals  100  with low transmit activity factors. In the control hold mode, the mobile terminal  100  suspends or reduces transmissions on one or more of the reverse link control channels. The control hold mode is described in the standard Upper Layer (Layer 3) Signaling For cdma2000 Spread Spectrum Systems, TIA/EIA 2000.5—C issued in May 2002.  
         [0017]    One method used to determine when mobile terminals  100  should placed in a control hold mode exploits the property of temporal locality. Communications to and from a particular user tend to be grouped in time. A mobile terminal  100  that has very recently sent or received a transmission is more likely to receive or send another transmission in the near term, than is a mobile terminal  100  that has not sent or received a transmission for some time. Following this principle, mobile terminals  100  that have recently received or sent transmissions are maintained in an active mode, and those to and from whom no transmissions have been directed for a predetermined time are commanded to enter a control hold mode. A straightforward manner of implementing this approach is to maintain inactivity timers for each transmission direction, which are loaded with a predetermined value and started upon each successful transmission. Such timers are referred to herein as the forward link inactivity timer and the reverse link inactivity timer.  
         [0018]    If an inactivity timer counts down to zero before another transmission in its respective direction, it is said to have expired, or timed-out. If both the forward link and the reverse link inactivity timers for a particular mobile terminal have expired and if there is no data queued awaiting transmission to that mobile terminal  100 , the may safely signal the mobile terminal  100  to enter a control hold mode. Traditionally, the BSC  30  maintains and manages both inactivity timers, and controls the operational modes of the mobile terminals  100  within its service area, however, this function could also be performed by other network entities as well.  
         [0019]    In some cdma2000 networks, such as 1xEV-DV and 1xEV-DO systems, the forward link is a time-shared channel and the base station transmits to only one mobile terminal at any given time. In systems of this type, a scheduler is usually maintained at each base station  20  to schedule forward link transmissions for all mobile terminals served by that base station  20 . When the base station  20  schedules data packets for transmission to a mobile terminal  100 , however, the forward link inactivity timer maintained at the BSC  30  may not be started or reset at the proper time, and thus may not accurately reflect the timing of the last forward link transmission to the mobile terminal  100 . Maintaining a forward link inactivity timer at the BSC  30  is problematic because the BSC  30  does not know exactly when packet data for a particular mobile terminal  100  are transmitted over the forward link. The data packets may be queued for some time at the serving base station  20 . Additionally, the radio link protocol (RLP) uses negative acknowledgement (NACK) signaling between a base station  20  and the BSC  30 . Thus, when the BSC  30  forwards packet data to a serving base station  20  for a particular mobile terminal  100 , the BSC  30  cannot be assured of a successful transmission until the end of the NACK window. Thus, the BSC  30  maintaining a forward link inactivity timer must delay starting the forward link timer for a defined hold-off period to account for these scheduling and signaling delays. As a consequence, the decision to command a mobile terminal  100  to transition to a control hold mode may be significantly delayed and the mobile terminal  100  will unnecessarily remain in an active mode even though it may be currently inactive.  
         [0020]    According to the present invention, the forward link inactivity timer is maintained at the serving forward link base station  20  rather than the BSC  30 . The reverse link inactivity timer need not be maintained at the same network entity as the forward link inactivity timer. In an exemplary embodiment, the BSC  30  tracks and controls, through a plurality of base stations  20 , communications to and from mobile terminals  100 . The BSC  30  additionally decides upon and communicates to the mobile terminals  100  their preferred operational mode, as decided by recent communication activity. In this exemplary embodiment, forward link scheduling to a particular mobile terminal  100  is handled by the serving base station  20 , which also maintains the forward link inactivity timer associated with the mobile terminal  100 . The reverse link inactivity timer may be maintained by the serving base station  20 , by another base station  20  in the active set of the mobile terminal  100 , or by the BSC  30 . The base station(s)  20  managing the forward and reverse link inactivity timers communicate the value and/or status of the timers to the BSC  30  as required. In particular, the base station(s)  20  signal the BSC  30  when one of the inactivity timers maintained by the base station  20  expires.  
         [0021]    [0021]FIG. 2 is a functional block diagram of a base station  20  in accordance with the present invention. The base station  20  includes controller  202 , transceiver resources  204 , and antenna  206 . The transceiver resources  204  include a transceiver array  208  that is coupled to antenna  206  to communicate with mobile terminals  100  in its coverage area. The controller  202  sends and receives voice, data, and control signals to and from the network, and in particular to and from a BSC  30 . The controller  202  includes scheduler  212  and buffer memory  214 . The controller  202  may, for example, comprise an appropriately programmed processor, controller, or digital signal processor. Buffer memory may be any kind of computer memory, such as RAM. The scheduler  212  schedules transmissions to the mobile terminals  100  over the forward link. The buffer memory  214  stores data packets  216  in a queue prior to transmission to a mobile terminal  100 .  
         [0022]    According to the present invention, the controller  202  further includes one or more timers  218 , each corresponding to a mobile terminal  100  being served by the base station  20 . The timers  218  may comprise countdown timers that are independently reset, or loaded with a predetermined value, and thenceforth decrement to zero. A timer  218  may function as either a forward link or reverse link inactivity timer, or have dual timing modes so as to keep track of both forward link and reveries link transmissions. In the case of a forward link inactivity timer, the timer  218  is reset if a data packet  216  is transmitted to the associated mobile terminal  100 . The forward link inactivity timer  218  is started when the last acknowledge signal is received from the mobile terminal  100 , indicating successful reception of the last data packet  216 . That is, the forward link inactivity timer  218  is started upon acknowledgement of the last transmitted data packet. If and when an inactivity timer  218  expires, the controller  202  signals this fact to the BSC  30 . Additionally, the controller  202  may communicate the present value or other status of an inactivity timer  218 , as appropriate or required. The inactivity timers  218  may comprise dedicated hardware counters, or alternatively may comprise logical timers maintained by software, as will be readily apparent to one of skill in the art.  
         [0023]    The operation of the forward and reverse link inactivity timers, and the transition of mobile stations  100  between active and control hold operating modes, is explained with reference to the state diagrams of FIGS. 3 and 4. In an exemplary embodiment of the present invention, as described above, the BSC  30  or other network entity tracks and controls communications to and from mobile terminals  100 , and makes decisions as to the operating mode of each mobile terminals  100  based on activity levels. In this embodiment, forward link scheduling to a particular mobile terminal  100  is handled by a serving base station  20 , which also maintains the forward link inactivity timer associated with the mobile terminal  100 . The reverse link inactivity timer could be maintained at the BSC  30 . Alternatively, the reverse link inactivity timer for a given mobile terminal  100  may be maintained at a base station  20  in the active set of the mobile terminal  100 , which may or may not be the same base station  20  as the one serving the mobile terminal  100  on the forward link. While FIGS. 3 and 4 are explicated with reference to the configuration of this embodiment, one of skill in the art will readily recognize that the state diagrams are generally applicable to other network configurations.  
         [0024]    [0024]FIG. 3 depicts the state transitions for the BSC  30 , as related to a particular mobile terminal  100 . Initially, at  300 , the mobile terminal  100  is in an active mode, receiving and sending voice and/or data packets through the wireless communication system  10 . When the BSC  30  receives an indication that the forward link inactivity timer  218  has expired, it transitions along arc  302  to state  304 . If new forward link data arrives for the mobile terminal  100  through the core network  14 , the BSC  30  transitions along arc  306  back to state  300  (the forward link inactivity timer  218  will be reset if the selected forward link base station  20  sends a packet to the mobile terminal  100 ). If no new forward link data arrives, the BSC  30  remains in state  304  until it receives an indication that the reverse link inactivity timer  218  has expired. At that point, it transitions along arc  308  to state  310 . At state  310 , the mobile terminal  100  is commanded to enter a control hold mode, gating control signals (e.g. reducing or suspending transmissions) on one or more reverse link channels to minimize interference and increase system capacity. In particular, the mobile terminal  100  may transmit at a 50% duty cycle on the reverse pilot channel and/or reverse rate control channels in the control hold mode, or may suspend transmissions altogether on one or both of those channels. Gating may also be applied to other reverse link channels in control hold mode.  
         [0025]    Alternatively, the BSC  30  may receive an indication that the reverse link inactivity timer  218  has expired, in which case it transitions along arc  312  to state  314 . At state  314 , the BSC  30  may receive an indication that the mobile terminal  100  has transmitted new voice or data on the reverse link, in which case the reverse link inactivity timer  218  is reset, and the BSC  30  transitions along line  316  back to state  300 . Otherwise, the BSC  30  remains in state  314  until it receives an indication of the expiration of the forward link inactivity timer  218 , which causes a transition along arc  318  to state  310 . Upon entering state  310 , the BSC  30  commands the mobile terminal  100  to transition to the control hold mode. Upon either the arrival of new forward link data for the mobile terminal  100 , or the receipt of reverse link data from the mobile terminal  100 , the appropriate inactivity timer  218  is reset, the mobile terminal  100  is commanded to enter active mode, and the BSC  30  transitions along line  320  to state  300 , where it awaits the expiration of the active timer(s). Note that the mobile terminal  100  is in active mode in states  300 ,  304 , and  314 , and is only transitioned to control hold mode in state  310 .  
         [0026]    [0026]FIG. 4 depicts the state diagram for the selected forward link base station  20  (or other network entity) maintaining the forward link inactivity timer  218 . Upon transmitting one or more data packets to the mobile terminal  100 , the selected forward link base station  20  enters state  350 , where it waits for the last acknowledgement from the mobile terminal  100 , indicating receipt of the last packet. When the last acknowledgement is received, the selected forward link base station  20  transitions along arc  352  to state  354 , and starts the forward link inactivity timer  218 . If the selected forward link base station  20  receives new forward link data for the mobile terminal  100 , it will transition along arc  356  back to state  350 . Otherwise, upon the expiration of the forward link inactivity timer  218 , the selected forward link base station  20  will transition along arc  358  to state  360 , where it will notify the BSC  30  of the expiration of the forward link inactivity timer  218 . The selected forward link base station  20  will remain in state  360  (regardless of reverse link activity) until it receives new forward link data intended for mobile terminal  100 , at which point it will transition along arc  362  to state  350 . The network entity maintaining the reverse link inactivity timer  218  functions analogously.  
         [0027]    In the event that a mobile terminal  100  selects a new forward link serving base station  20  while it is in an active state, the former forward link serving base station  20  will communicate the value and status of the forward link inactivity timer  218  to the new forward link serving base station  20 . The new forward link serving base station  20  will set up a new forward link inactivity timer  218  and associate it with the mobile terminal  100  (such as in the controller  202 ). If the forward link inactivity timer  218  was in a reset state at the former forward link serving base station  20  (i.e., the last data packet not yet transmitted), the new forward link serving base station  20  will reset its forward link inactivity timer  218 . The new forward link serving base station  20  receives the queued data intended for mobile terminal  100 , enters state  350  (see FIG. 5), and begins transmitting data to the mobile terminal  100 . If the forward link inactivity timer  218  was running at the former forward link serving base station  20  (i.e., the last data transfer acknowledgement had been received from the mobile terminal  100 ), the new forward link serving base station  20  will load its forward link inactivity timer  218  with the value communicated from the former forward link serving base station  20 , and enter state  354 . In either case, the new forward link serving base station  20  will transition among the states depicted in FIG. 4, communicating with the BSC  30  as appropriate. In this manner, the forward link inactivity timer  218  is always maintained by the network entity performing forward link message scheduling, and thus maintains the most accurate inactivity timer status timing and communication.  
         [0028]    Since high data rate CDMA systems, such as 1xEV-DV and 1xEV-DO systems, use soft handoff on the reverse link, any base station  20  in the active set of a mobile terminal  100  may maintain the reverse link inactivity timer  218 , signaling its status to the BSC  30  as necessary. In particular, maintenance of the reverse link inactivity timer  218  may be transferred to the currently selected forward link serving base station  20  along with the forward link inactivity timer  218 , as discussed above. Alternatively, the reverse link inactivity timer  218  may simply be maintained by the same base station  20  for as long as the base station  20  remains in the active set of the mobile terminal  100 . As yet another alternative, the reverse link inactivity timer  218  may be maintained by all base stations  12  in the active set or by the BSC  30 .  
         [0029]    Although the present invention has been described herein with respect to particular features, aspects and embodiments thereof, it will be apparent that numerous variations, modifications, and other embodiments are possible within the broad scope of the present invention, and accordingly, all variations, modifications and embodiments are to be regarded as being within the scope of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.