Abstract:
A method of controlling reverse link feedback in a mobile communication system comprises detecting reverse link load for at least one sector in the network; and dynamically updating a control setting for a reverse link control channel responsive to the detected load.

Description:
BACKGROUND  
       [0001]     In high data rate CDMA systems, such as 1×EV-DV, 1×EV-DO and WCDMA systems, the forward traffic channel is time-multiplexed and transmitted at the full power available to the sector, but with data rates and slot times that vary depending on downlink channel conditions. The data rate that can be supported by the downlink is proportional to the SNR, which changes continuously. The mobile terminals measure the instantaneous signal to noise ratio (SNR) of the pilot signal received from each sector in its active set and requests service from the access network providing the strongest signal. The mobile terminal  100  transmits the SNR value, or equivalently the supportable data rate, for the sector providing the strongest signal on a reverse control channel referred to generically as the rate control channel. In 1×EV-DO systems, the mobile terminal measures the SNR and transmits data rate requests to the serving sector on the Data Rate Channel (DRC). The mobile terminal applies a Walsh cover to the DRC to indicate its selection of a serving sector for forward link communications.  
         [0002]     The access network designates a DRC information length denoted by the system variable DRC Length indicating a number of slots over which the DRC information is repeated and transmits the designated DRC information length to the access terminal. The mobile terminal transmits updated DRC information to the access network in every DRCLength slots. In general, fast channel feedback (low DRCLength) is beneficial for forward link operations. However, faster feedback implies higher overhead, which can negatively impact reverse link transmissions.  
       SUMMARY  
       [0003]     The present invention relates to a method implemented by an access network (also known as a base station) for controlling reverse link feedback in a mobile communication system. The access network determines reverse link load for at least one sector in the network, and dynamically adjusts a control setting for a reverse link control channel responsive to the detected load. In one exemplary embodiment, the access network monitors the reverse link load and adjusts the repetition frequency of a rate control channel. In 1×EV-DO systems according to the Telecommunications Industry Association (TIA) standard TIA-856A, for example, the repetition frequency of a Data Rate Channel (DRC) is dynamically adjusted based on reverse link load by adjusting the system variable DRCLength. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is a block diagram of an exemplary mobile communication network.  
         [0005]      FIG. 2  is a block diagram of an exemplary access network.  
         [0006]      FIG. 3  is an exemplary procedure for adjusting a control setting for a reverse link control channel. 
     
    
     DETAILED DESCRIPTION  
       [0007]     Referring now to the drawings, the present invention will be discussed in the context of an exemplary wireless communications network  10 .  FIG. 1  illustrates a CDMA network  10  according the TIA-856A standard, which is commonly known as 1×EV-DO. The network  10  comprises a core network (CN)  20  and a radio access network (RAN) including a plurality of access networks  32  providing services to one or more mobile terminals  100 . The core network  20  includes a packet data serving node  22  that connects the access networks  32  to external Packet Data Networks (PDN)  12 , such as the Internet. Each access network  32  is located in and provides wireless communication services to a geographic region referred to as a cell, which may comprise one or more sectors. In general, there is one access network  32  for each cell or sector. A single access network  32  may serve multiple sectors.  
         [0008]      FIG. 2  illustrates an exemplary access network  32 . The access network  32  comprises a radio base station (RBS)  34 , an access network controller (ANC)  36 , and a packet control function (PCF)  38 . The RBS  34  comprises the radio equipment for communicating over the air interface with the mobile terminals  32 . The ANC  36  controls operation of the access network  32  and the use of communication resources. The PCF  38  provides connection to the PDSN  22  in the core network  20 .  
         [0009]     In 1×EV-DO systems, packet data is transmitted on the forward link over a shared packet data channel called the Forward Traffic Channel (FTC). Packet data transmissions to different users are time multiplexed and transmitted at full power. Only one user in a sector receives transmissions from the access network  32  at a time. Due to the complexity of coordinating packet data transmissions between sectors, soft handoff is not used on the FTC channel. Instead, a process known as sector selection or sector switching is used. The mobile terminal  100  monitors the signal power from all sectors in its active set and selects the sector that provides the strongest signal as the serving sector. As the mobile terminal  100  moves away from the serving sector toward a non-serving sector, the signal strength from the serving sector will diminish while the signal strength from the non-serving sector will increase. When the signal strength from a candidate sector in the mobile terminal&#39;s active set exceeds the signal strength from the serving sector by a predetermined amount, the mobile terminal  100  sends a signal to the network  10  to switch sectors.  
         [0010]     A virtual handoff or cell-switching occurs when the mobile terminal  100  switches from a serving sector belonging to a first access network  32  to a new serving sector belonging to a different access network  32 . In this case, there may be a small delay in the delivery of packets to the mobile terminal  100  while the target sector prepares for communications with the mobile terminal  100 . Many packet data applications are delay tolerant and the small delays due to cell switching may be acceptable for these applications. However, some packet data applications, such as voice-over IP, are delay intolerant and even small delays will negatively impact the perceived quality of the connection. Therefore, it is desirable to minimize delays in delivering packet data for these delay-sensitive applications when switching from a sector belonging to one access network  32  to a sector belonging to a different access network  32 . To reduce such delays, the mobile terminal  100  may give an early indication of its intention to change cells by sending a signal to the access network.  
         [0011]     The Data Rate Control (DRC) channel, Acknowledgement (ACK), and Data Source Control (DSC) channels on the reverse link support the forward traffic channel operation. The mobile terminal  100  informs the access network  32  of the supportable data rate on the FTC and the best serving sector for the mobile terminal  100  on the DRC channel. In 1×EV-DO systems, the mobile terminal  100  indicates the best serving sector by the Walsh cover applied to the DRC. The mobile terminal  100  informs the access network  12  whether transmitted packets have been correctly received on the ACK channel. The DSC channel is a new channel introduced to reduce delays in delivering packets during a virtual handoff. The mobile terminal  100  uses the DSC channel to indicate the data source, e.g. access network  32 , responsible for delivering packets on the forward link. More particularly, the mobile terminal  100  gives an early indication of its intention to switch between sectors in different cells by the Walsh cover applied to the DSC channel. The DRC and DSC channels are repeated over a predetermined number of slots as indicated by the system variables DRCLength and DSCLength respectively.  
         [0012]     In general, fast channel feedback is beneficial for forward link operations, particularly when channel conditions are changing rapidly. However, when the reverse link is heavily loaded, some mobile terminals  100  may be power limited and thus unable to close the reverse link. Consequently, some of the mobile terminals  100  may not be able to reliably transmit information to the access network  32  over the reverse link control channels. If the ACK and/or DRC channels are not reliably received, the efficiency of forward link transmissions over the FTC will be negatively impacted. Therefore, it is desirable to have a relatively slow channel feedback during periods when the sector is heavily loaded. When the reverse link is lightly loaded, however, fast feedback is possible without substantially affecting reverse link operation. A faster feedback implies a smaller DRCLength and DSCLength, while a slower feedback implies a larger DRCLength/DSCLength.  
         [0013]     According to the present invention, the access network  32  monitors the reverse link load and adjusts DRCLength and DSCLength accordingly. Equivalently, the access network  32  could adjust the gain of the DRC and DSC channels relative to the pilot channel. The control variables DRCLength and DSCLength control the number of slots over which a DRC message or DSC message is repeated. A low DRCLength/DSCLength value corresponds to a fast channel feedback, while a large DRCLength/DSCLength corresponds to a slow channel feedback. In operation, the access network  32  gradually reduces DRCLength and/or DSCLength as system load decreases, and gradually increases DRCLength and/or DSCLength as system load increases.  
         [0014]      FIG. 3  is a flow diagram illustrating a procedure executed by the access network controller  36  to update DRCLength and/or DSCLength. The access network controller  36  determines the reverse link load (block  50 ). Measurement of the reverse link load can be done, for example, by measuring the rise over thermal (RoT). The access network controller  36  may use indirect measures of the load. For example, rate control commands generated by the access network controller  36  and/or the number of reverse link channels allocated can serve as an indirect measure of the load. In 1×EV-DO systems, for example, rate control commands or reverse activity bits (RABs) generated by rate control algorithms to control the data transmission rate on reverse link channels can be monitored. The RABs can be processed/filtered to generate a load indication. Performance parameters, such as sector/user throughput, delay, FER outage, ROT outage, etc., can be used as an indirect measure of the reverse link load.  
         [0015]     Once the reverse link load is determined, the access network controller  36  uses the load information to update DRCLength and/or DSCLength (block  52 ). DRCLength and/or DSCLength can be updated on a per sector basis using load information for each sector. Alternatively, the access network controller  36  may aggregate load information for a plurality of sectors and use the aggregate load information to update DRCLength and/or DSCLength for all sectors.  
         [0016]     In some embodiments, the same DRCLength/DSCLength may be used for all mobile terminals  100  within a sector. In this case, DRCLength and/or DSCLength may be transmitted over a broadcast channel to the mobile terminals  100  within the sector. In other embodiments, DRCLength and/or DSCLength may be adjusted for individual mobile terminals  100  or groups of mobile terminals  100 . Adjusting DRCLength and/or DSCLength for individual mobile terminals  100  and/or groups of mobile terminals  100  allows the access network controller  36  to take into account other criteria such as the relative priority assigned to individual mobile terminals  100  or groups of mobile terminals  100 , QoS requirements, and/or channel conditions. For example, different classes of users or different types of applications may be assigned different priority levels that affect the DRC/DSCLength. For delay tolerant applications, a large DRCLength and/or DSCLength may be acceptable. Delay-sensitive applications may require a shorter DRC/DSCLength. As another example, channel conditions may be used in addition to the load information to adjust DRCLength and/or DSCLength. Slow channel feedback may suffice for a stationary mobile terminal  100  with relatively stable channel conditions. On the other hand, when the mobile terminal  100  is moving, the channel may be changing rapidly. In this situation, fast channel feedback may be more desirable.  
         [0017]     After updating DRCLength/DSCLength, the access network controller  36  controls the radio base station to send the DRCLength and/or DSCLength to one or more mobile terminals  100  (block  54 ). In embodiments where the same DRCLength/DSCLength is used for all mobile terminals  100 , the access network  32  can send the updated DRCLength/DSCLength over a broadcast channel. In embodiments where the DRCLength and/or DSCLength is separately controlled for individual mobile terminals  100  or groups of mobile terminals  100 , the updated DRCLength/DSCLength can be transmitted over a dedicated control channel. One simple and straightforward implementation is to send the new DRCLength/DSCLength to a mobile terminal  100  within a traffic channel assignment (TCA) message when the mobile terminal  100  performs a soft handoff in the reverse link.  
         [0018]     The basic inventive concept of using load information to control channel feedback over a reverse link control channel can be easily extended to other standards, such as 1×EV-DV and WCDMA.  
         [0019]     The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects 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.