Abstract:
A method for transmitting control information from multiple base stations in an active set to a mobile device in a wireless communication system reduces resource consumption by routing information originating from a plurality of active base stations through a single serving base station. The serving base station pools and resolves the information from the active base stations and sends the resolution of the pooled information over a downlink control channel for evaluation and response by the mobile device.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to wireless communication systems.  
         [0003]     2. Description of the Related Art  
         [0004]     Communication systems, such as wireless systems, are designed to meet various subscriber demands. Service providers continuously seek ways to improve the overall performance of the communication system. In the past, wireless communication systems have been used for voice communications, but more recent technological developments have allowed high speed data transmission as well. As wireless communications become more and more popular for subscribers to obtain data (i.e., e-mail or information from the internet), communication systems should be capable of a higher throughput and be tightly controlled to maintain a high quality of service. Communication is conducted according to any desired communications standard, such as the Universal Mobile Telecommunications Standard (UMTS) or a CDMA standard.  
         [0005]     As is known in the art and shown generally in  FIG. 1 , a wireless communication system  100  serves a service coverage area that is divided into cells  101  having one or more sectors  102 . Base stations  104 ,  104   a  are associated with the sectors  102  in at least one cell  101 . Adaptive modulation and coding allows selection of an appropriate transport format (e.g., modulation and coding) for the current channel conditions seen by the user. There are two directions of data flow in such systems; communications from the base station  104 ,  104   a  to a mobile device  106  are considered to flow in a downlink direction, while communications originating at the mobile device  106  and sent to the base station are considered to flow in an uplink direction. A given base station is considered a serving base station  104   a  if it is currently transmitting data to the mobile device  106 . These base stations, and other base stations  104 ,  104   a  that may be chosen by the mobile device  106  as a candidate for acting as a new serving base station, are collectively known as the “active set” of base stations. Thus, the serving base station  104   a  is considered to be a member of the active set of base stations. Control information, such as information required for power control of transmissions by the mobile device  106 , is transmitted by the entire active set of base stations.  
         [0006]     The CDMA standard, and specifically CDMA200 Revision D, incorporates several additional air-interface control mechanisms that control transmissions from the mobile device  106  on the uplink to a given base station  104 ,  104   a . The control mechanisms allow the base stations  104  in the system  100  to perform various functions with respect to mobile device  106  transmissions. These functions include schedule grants that schedule the mobile device  106  for data transmission (i.e., direct the mobile device  106  to transmit up to a selected data rate and/or power level), rate control commands that indicate any changes in transmission rates of the mobile devices  106 , and acknowledgements that acknowledge successful or failed reception of transmissions from the mobile devices  106 .  
         [0007]     As is known in the art, the mobile device  106  communicates with just the serving base station  104   a  when no other base stations are in the active set. When the mobile device  106  starts communicating concurrently with other base stations (i.e., the active base stations  104 , including the serving base station as well as additional base stations), the mobile device  106  is considered to be in a handoff situation. A signaling message, such as a Universal Handoff Direction Message (UHDM), is sent to the mobile device  106  by one or more of the base stations  104 ,  104   a  to move the mobile device  106  into a handoff state.  
         [0008]     As shown in  FIG. 1 , the mobile device  106  conforming to a CDMA standard, such as the CDMA2000 Revision D standard, receives a scheduling grant from only the serving base station  104   a . However, rate control commands are sent from all active set base stations, while acknowledgements may be sent from some or all of the active base stations  104 , but always the serving base station  104   a.    
         [0009]     Typically, three separate shared downlink control channels at each base station  104 ,  104   a  are used to communicate the schedule grants, rate control commands, and acknowledgements from the base stations  104 ,  104   a  to the mobile devices  106 . Further, a specific sub-channel on each of these channels communicates control information to a given mobile device  106 . The identity of these shared downlink control channels and code or time-slot sub-channels must be communicated to the mobile device  106  for each base station in the active set even if the system  100  does not intended to send scheduling, rate control, and/or acknowledgement information from all of the active base stations  104   104   a . This is because the mobile device  106  or a controller in the system  100  may change the base station acting as the serving base station  104   a  at any time.  
         [0010]     One possible method to communicate the scheduling, rate control, and/or acknowledgement channel and sub-channel identities is to communicate the channel and sub-channel identities to the mobile device by including this information in the UHDM. This method is enabled by the standard. Setting the identities of the forward control channels of each active base station  104 ,  104   a  a priori via the UHDM when the mobile device  106  moves into the handoff state requires each base station  104 ,  104   a  communicating with the mobile device  106  to reserve some or all of its capacity on the control channels for communication on the downlink. Further, sending the control channel identity information via a message, such as the UHDM, imposes a substantial cost in down link power in addition to forcing each base station  104 ,  104   a  to reserve downlink Walsh codes, bit positions, etc.  
         [0011]     For mobile devices  106  in the handoff state, each active set base station  104 ,  104   a  may send control information that is the same as or different from control information from other base stations  104 ,  104   a  receiving communications from the same mobile device  106 . For example, one of the active base stations  104 ,  104   a  may send a positive acknowledgement when it has successfully received a transmission from the mobile device  106  while at the same time another active base station  104 ,  104   a  may respond with a negative acknowledgement. Similarly, one base station  104 ,  104   a  may ask the mobile device  106  to increase its transmission rate while at the same time another base station  104 ,  104   a  may ask the mobile device  106  to decrease its transmission rate. The mobile device  106  evaluates the information from the multiple base stations and sets its response (e.g., transmission rate, re-transmissions, etc.) based on this evaluation.  
         [0012]     Not all of the base stations  104 ,  104   a , however, that are transmitting over the shared downlink control channels know the quality of their respective downlinks to the mobile device  106 . This is because the mobile device  106  typically provides downlink quality reports only for a single base station sector  102  (i.e., the serving sector). As a result, other base station sectors  102  do not have any associated downlink quality reports that would be used to allocate appropriate power levels on their downlink control channels to the mobile device.  
         [0013]     There is a desire for a method that allows control information from multiple base stations to be transmitted to a single mobile device without excessively reducing resources available for other traffic between the base stations and the mobile device.  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention is directed to a method for transmitting control information from multiple base stations to a mobile device in a wireless communication system without consuming excessive resources, either over the air or on the wireless network infrastructure. Generally, the invention may reduce resource consumption by sending information corresponding to one or more active base stations through at least a single serving base station. The information includes, for example, schedule grant(s), rate control command(s), and acknowledgement(s) from the base stations as well as the identity of the channel(s) and sub-channel(s) on which they may be transmitted to the mobile device. The serving base station pools and resolves the information that would ordinarily have been sent individually by each of the active base station(s) to the mobile device. The serving base station may then send the resolution of the pooled information to the mobile device.  
         [0015]     By transmitting control information from some or all of the active base stations through one serving base station rather than through the individual active base stations, the invention allows information from multiple base stations to reach the mobile device while minimizing the power and bandwidth needed to transmit this information. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a representative diagram of a wireless communication system; and  
         [0017]      FIG. 2  is a representative diagram of the wireless communication system illustrating one example of information transmitted in the system according to one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1  is an illustrative example of the wireless communication system  100  in which the inventive method can be executed. As explained above, the system  100  is divided into cells  101  having multiple sectors  102  corresponding with different geographic areas, with each base station  104  being associated with one or more sectors  102 . The base station  104  and the mobile device  106  can distinguish among the sectors  102  via any known metric implemented by, for example, a baseband processing application specific integrated circuit (ASIC)  105  in the base station  104 . For example, each sector  102  may be associated with a particular Walsh symbol (also called a “Walsh cover”), which would be used by the base station  104  and the mobile device  106  to identify each sector  102 .  
         [0019]     The system  100  may also include a central base station controller  110  that can communicate with all of the base stations  104  in the system  100  and handle data corresponding to multiple sectors  102  and/or multiple base stations  104 . For purposes of the detailed description below, the base stations  104  that are either chosen or may be chosen by the mobile device  106  to transmit data at a given time are called “active base stations,” while the base station  104  that is currently transmitting data to the mobile device  106  is considered to be a “serving base station”  104   a . Note that the serving base station  104   a  is considered to be one of the active base stations  104  as well.  
         [0020]     To control the data transmissions made by the mobile device  106 , the current serving base station  104   a  sends the scheduling grants to the mobile device  106 . Additionally, rate control and acknowledgement information are potentially conducted by some or all of the active base stations  104 ,  104   a . As noted above, each active base station  104 ,  104   a  may communicate a schedule grant (if it is a serving base station  104   a ), rate control command, and/or acknowledgement to the mobile device  106  via shared downlink control channels.  
         [0021]     The mobile device  106  has the option of switching between the active base stations  104  to select which base station  104  will act as the serving base station  104   a  from which it will receive data. When this happens, there needs to be a transfer of the channel or sub-channel over which the mobile device  106  receives the scheduling, acknowledgement and rate control messages from the selected serving base station  104   a . To do this, the mobile device  106  must know which channel or sub-channel to check to look for this information. Thus, as noted above, the identity of these shared downlink control channels must be communicated to the mobile device  106  even if the system  100  does not intended to send scheduling, rate control, and/or acknowledgement information from all of the active base stations  104  because the mobile device  106  or the controller  110  may change the base station  104  that acts as the serving base station  104   a.    
         [0022]     Rather than having each base station  104 ,  104   a  individually send the shared channel identities via the UHDM at the time the mobile device enters into soft handoff, the first part of the invention involves sending the control channel identity information from the active base stations  104  to the serving base station  104   a  instead of the mobile device at the time the mobile indicates an intention to switch the serving sector. The serving base station  104   a  then pools and resolves the received information together and sends the resolved information through any available control channel to the mobile device  106  ( FIG. 2 ).  
         [0023]     One example of resolving information, in a case where only the serving sector sends the rate control and acknowledgement commands, is when the serving sector selects the sub-channel identities for the sector the mobile device  106  has indicated it will switch to; in this case, the serving sector sends only those sub-channel identities to the mobile device  106 . In this scheme, only the serving base station  104   a  needs to reserve and consume resources to transmit this information to the mobile device  106 . Further, only the sub-channels that are in use are reserved, and only at the time when the mobile device  106  indicates a switch.  
         [0024]     The second aspect of this invention relates to the routing of the actual rate control and acknowledgement information to the mobile device  106  via the serving base station  104   a . The non-serving base stations  104  compute this control information but do not transmit the control information directly to the mobile device  106 .  
         [0025]     More particularly, the mobile device  106  in the handover state sends packet data transmissions to the active base stations  104 , which includes the serving base station  104   a  from which it is currently receiving data. In this particular example, the active base stations  104 ,  104   a  send positive acknowledgements and rate reduction commands, but any information in response to the mobile device  106  transmission is possible. Instead of sending the acknowledgement and rate control command directly to the mobile device  106 , however, the non-serving active base stations  104  send them to the serving base station  104   a . Thus, the active base stations  104  do not send any information via their respective downlink control channels to the mobile device  106 .  
         [0026]     The serving base station  104   a  then pools the information it receives from the active base station  104 . As part of the pooling process, the serving base station  104   a  may evaluate the information it receives from the non-serving active base stations  104  as well as the information it generates itself and resolve the pooled information through any rules specified by the standard used by the system  100 .  
         [0027]     In one example, the serving base station  104   a  may receive acknowledgements from multiple base stations  104  where one or more of the multiple base stations  104  each send a positive acknowledgement (indicating successful receipt of a transmission from the mobile device  106 ) and the remaining base stations  104  each send a negative acknowledgement (indicating unsuccessful receipt of the transmission). Note that a negative acknowledgement may not necessarily be sent in every case; instead, the negative acknowledgement may be inferred at the serving base station  104   a  by the absence of an indication from the remaining base stations  104 .  
         [0028]     Pooling the information from the active base stations  104  into the serving base station  104   a  also allows the system  100  to take advantage of soft handoff gains, that is, the gain achieved when a non-serving active base station  104  receives a transmission that is not received successfully by the serving base station  104   a . In such a case, one or more of active base stations  104  that successfully received the mobile device transmission (and thereby would send a positive acknowledgement) sends the received transmission to the serving base station  104   a . The serving base station  104   a  then sends a positive acknowledgement to the mobile device  106  instead of a negative acknowledgement. This is possible because the mobile device  106  only needs one base station  104  in the group of active base stations  104 ,  104   a  to receive its information accurately for proper communication. In other words, the serving base station  104   a  can send a positive acknowledgement to the mobile device  106  whenever any one of the active base stations  104 , including itself, wishes to send a positive acknowledgement. Using a similar approach, the transmission rate of the mobile device  106  may be lowered by the serving base station  104   a  if either any one of the non-serving active base stations  104  or the serving base station  104   a  itself requires a reduction in mobile transmission rate or power level.  
         [0029]     Note that in some cases a non-serving active base station  104  may experience a reporting delay when reporting its acknowledgement of the serving base station  104   a  and report too late for the serving base station  104   a  to alter its acknowledgement (e.g., from a negative acknowledgement to a positive one). If this occurs, retransmission of a given data packet by the mobile device  106  may be dictated by the success or failure in decoding the transmission in the serving base station  104   a . The positive acknowledgement from the non-serving active base station(s)  104 , however, is still helpful to the serving base station  104   a  because the delayed positive acknowledgement can still be used by the serving base station  104  to send a positive acknowledgement to the mobile device  106  as long as the active base station  104  is still receiving retransmissions from the mobile device  106  for the same data packet. Even if the serving base station  104   a  receives the positive acknowledgement from the non-serving active base station  104  too late to indicate a positive acknowledgement to the mobile device  106 , the serving base station  104   a  can still use this information to prevent a later request for an unnecessary retransmission of the data packet to the mobile device  106 . A similar approach to handling reporting delays can be used with respect to rate control instructions from the non-serving active base stations  104 ; these instructions can be used by the serving base station  104   a  to reduce the granted rate (and therefore power) to the mobile device  106 .  
         [0030]     When the mobile device  106  indicates its intention to switch to a new serving base station, the current serving base station  104   a  serving the mobile device  106  tells the mobile device  106  which channels or sub-channels the mobile device  106  needs to monitor to obtain the acknowledgement and/or rate control information from the new serving base station it targeted. The intention to switch may be detected by any of the active base stations  104  and is relayed back to the central controller  110 . The central controller  110  notifies all of the active base stations  104  regarding the intention to switch, causing them to start sending their acknowledgement and/or rate control information to the new serving base station targeted by the mobile device for consolidation. When the mobile device  106  finally switches to the new serving base station, the mobile device  106  receives the pooled information from the active base stations  104  from the new serving base station.  
         [0031]     In one example, the active base stations  104  do not need to send both positive and negative acknowledgements and/or both rate increases and rate decreases to the serving base station  104 . Instead, the serving base station  104   a  only needs to know when one of the active base stations  104  wishes to send a positive acknowledgement or requires a rate decrease (i.e., the serving base station  104   a  does not need to be notified if the base stations  104  wishes to send a negative acknowledgement or requires a rate increase because this information is superfluous and does not warrant a change in any part of the system  100 . By limiting information sent to the serving base station  104   a  in this manner, the invention further reduces the power and bandwidth needed to provide information from the active base stations  104  to the serving base station  104   a.    
         [0032]     By pooling control information from multiple base stations  104  into the serving base station  104   a  and using only the serving base station  104   a  to send the control information to the mobile device  106 , the invention reduces both bandwidth and power resources needed to transmit information corresponding to multiple base stations  104 . Moreover, limiting the information transmitted to the serving base station  104   a  for pooling reduces backhaul bandwidth usage.  
         [0033]     While the particular invention has been described with reference to illustrative embodiments, this description is not meant to be construed in a limiting sense. It is understood that although the present invention has been described, various modifications of the illustrative embodiments, as well as additional embodiments of the invention, will be apparent to one of ordinary skill in the art upon reference to this description without departing from the spirit of the invention, as recited in the claims appended hereto. Consequently, this method, system and portions thereof and of the described method and system may be implemented in different locations, such as network elements, the wireless unit, the base station, a base station controller, a mobile switching center and/or radar system. Moreover, processing circuitry required to implement and use the described system may be implemented in application specific integrated circuits, software-driven processing circuitry, firmware, programmable logic devices, hardware, discrete components or arrangements of the above components as would be understood by one of ordinary skill in the art with the benefit of this disclosure. Those skilled in the art will readily recognize that these and various other modifications, arrangements and methods can be made to the present invention without strictly following the exemplary applications illustrated and described herein and without departing from the spirit and scope of the present invention. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.