Abstract:
A communication system that allows a soft handoff to be completed, even when the communications link between the active base station and the mobile station deteriorates before the mobile station has received the handoff direction message. The mobile station maintains a list of base stations that the mobile station is in communication with, referred to as an “Active Set”. In addition, the mobile station maintains another list of base stations that are proximate to the base stations in the active set. This list is referred to as the “Neighbor Set”. In accordance with the disclosed method and apparatus, the mobile station places a base station in the active set upon including the base station in a pilot strength measurement message (PSMM). The mobile station monitors transmissions from all of the base stations on the active set to receive a handoff direction message (HDM).

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. 10/020,036, entitled “Method for Robust Handoff in Wireless Communication System,” filed Dec. 7, 2001, now allowed, which is a continuation application of U.S. Pat. No. 6,360,100, issued Mar. 19, 2002, entitled “Method for Robust Handoff in Wireless Communication System,” and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates to the field of telecommunications, and more particularly to an improved method and system for handing off communications between cells or sectors of a wireless communication system. 
     2. Background 
     Wireless cellular communications is becoming a convention method for people to communicate with one another. A conventional wireless cellular communication system in accordance with an industry standard commonly known as IS-95B, which is issued by the Telecommunications Industry Association and Electronics Industry Association (TIA/EIA) defines the way in which one type of wireless cellular communications are performed. In accordance with IS-95B, a mobile station (such as a wireless cellular mobile telephone) communicates with other mobile stations, a conventional telephone, or other such communication devices over a communications link that includes at least one cellular base station. The mobile station transmits a radio signal to the cellular base station. The cellular base station establishes a connection to a wired network that might include conventional telephone circuits (commonly known as the public switched telephone network (PSTN)). 
     A mobile station need only establish communication through one base station at a time in order to communicate with a device at the other end of the communications link (i.e., make a “call”). However, as a mobile station moves, the mobile station and the base station may lose the ability to communicate over the radio link. For example, if the mobile station moves outside the range of the base station or if an obstruction comes between the mobile station and the base station, the communications between the mobile and base stations will be interrupted. Therefore, the placement of base stations is planned such that there is an overlap between the coverage areas of each base station. This overlap ensures that a mobile station can contact at least one base station in every geographic point intended to be covered by the system. This is important because if the mobile loses contact with all base stations for any substantial amount of time, the call is “dropped”. Once a call is dropped, the call must be reestablished by the mobile station redialing the call. 
     Due to the substantial overlap between base station coverage areas, a procedure known as “soft handoff ” can be performed. Soft handoff is a process in which the mobile station receives identical signals from both a first and a second base station. A mobile station will preferably enter soft handoff (i.e., signals from a second base station will be received by the mobile station) whenever a second station becomes available. Soft handoff ensures that a call is not dropped as the mobile station moves out of the coverage area of a first base station and into the coverage area of a second base station. 
     One conventional method for performing a soft handoff is illustrated in  FIG. 1 .  FIG. 1  shows a mobile station  101 , a first base station  103 , a second base station  105 , and a mobile switching center (MSC)  107 . In addition, the time sequence of communications between each is illustrated as follows. Arrowheads that terminate on the vertical line  109  that descends from the mobile station  101 , for example, represent signals received by the mobile station  101 . Arrows that terminate without an arrow head (i.e., at originate) at the vertical line  109  represent signals that have been transmitted from the mobile station  101 . Arrows that are closer to the top of the figure represent signals that are transmitted before signals represented by arrows closer to the bottom of the figure. In some instances, an arrow that is above another arrow may represent a signal that is transmitted continuously and thus may be transmitted concurrent with the signal represented by the lower arrow. For example, the traffic signal represented by arrow  111  may continue to be transmitted concurrent with the pilot strength measurement message (PSMM) that is represented by arrow  113 . 
     As shown in  FIG. 1 , the traffic signal  111  is initially transmitted between the mobile station  101  and the base station  103 . The traffic that is transmitted from the mobile station  101  to the base station  103  is then sent on to the MSC  107  by the base station  103 . Likewise, traffic that originates at the MSC  107  is sent to the base station  103 . This traffic is then transmitted from the base station  103  to the mobile station  101 . When the mobile station  101  detects a pilot from the second base station  105  with sufficient power, the mobile station  101  transmits a PSMM to the first base station  103  indicating the pilot strength of all the pilots that are currently being received at a signal level that is above a predetermined threshold. In the case shown in  FIG. 1 , the PSMM indicates that the mobile station  101  is receiving pilot signals that are above the predetermined threshold from both the first base station  103  and the second base station  105 . This PSMM is then transmitted from the first base station  103  to the MSC  107 , as represented by arrow  115 . The MSC  107  responds to the receipt of this PSMM by requesting the second base station  105  to allocate resources to establishing a communication link between the second base station  105  and the mobile station  101 , as represented by the block  116 . In addition, the MSC  107  generates a handoff direction message (HDM). The HDM is transmitted from MSC  107  to the first base station  103 , represented by the arrow  117 , after a time delay, represented by the arrow  119 . The HDM message is then transmitted from the first base station  103  to the mobile station  101 , represented by the arrow  121 . The HDM indicates to the mobile station  101  that a request has been made for the second base station  105  to allocate resources to establishing a communications path between the second base station  105  and the mobile station  101 . 
     The mobile station  101  responds to the HDM by adding the second base station  105  to the “Active Set” in the mobile station  101  and transmitting a handoff completion message (HDM) to both the first base station  103  and the second base station  105 , represented by the arrows  123 ,  125 . Both the first and second base stations  103 ,  105  transmit the HCM to the MSC  107 , represented by the arrows  127 ,  129 . The active set in the mobile station  101  indicates which base stations are actively in communication with the mobile station  101 . Traffic will then be transmitted from the MSC  107  to the mobile station  101  through both the first and second base stations  103 ,  105 . 
     This procedure works well in most cases. However, in some cases, the pilot transmitted from the second base station  105  is received by the mobile station shortly before the signals received from the first base station  103  can no longer be received by the mobile station  101 . If the time delay between receipt of the PSMM  115  and the transmission of the HDM from the first base station  103  is such that the communication link between the mobile station  101  and the first base station  103  deteriorates before the HDM can be received from the first base station  103  by the mobile station  101 , then the call will drop. 
     SUMMARY OF THE INVENTION 
     The disclosed method and apparatus allows a soft handoff to be completed, even when the communications link between the active base station and the mobile station deteriorates before the mobile station has received the handoff direction message. The mobile station maintains a list of base stations that the mobile station is in communication with, referred to as an “Active Set”. In addition, the mobile station maintains another list of base stations that are proximate to the base stations in the active set. This list is referred to as the “Neighbor Set”. A memory within the mobile station includes information that would allow the mobile station to demodulate information transmitted from those base stations on the neighbor set. In accordance with the disclosed method and apparatus, the mobile station places a base station in the active set upon including the base station in a pilot strength measurement message (PSMM). Alternatively, the mobile station places a base station in the active set upon detecting that the signals transmitted from that base station are being received at a signal strength that is greater than a predetermined threshold. 
     The mobile station will monitor transmissions from all of the base stations on the active set. When a PSMM transmitted from the mobile station is received by the base station, the base station will transmit the PSMM to the mobile switching center (MSC). The MSC then requests each of the base stations indicated in the PSMM to allocate resources to the mobile station and to send a handoff direction message (HDM). Accordingly, even if the communication with the base station through which the mobile is currently receiving traffic fails before that base station has successfully transmitted the HDM to the mobile station, the mobile station will receive the HDM from each of the other base stations that were indicated in the PSMM sent by the mobile station. Since each of these base stations will be included in the mobile station&#39;s active set, the mobile station will monitor the communications from each such base station and thus receive the HDM. 
     The details of the preferred and alternative embodiments of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of one method for performing soft handoffs in accordance with the prior art. 
         FIG. 2  is an illustration of a mobile station in accordance with one embodiment of the disclosed apparatus. 
         FIG. 3  is an illustration of a base station in accordance with one embodiment of the disclosed apparatus. 
         FIG. 4  is an illustration of a mobile switching center (MSC) in accordance with one embodiment of the disclosed apparatus. 
         FIG. 5  is an illustration of a communication system that includes mobile stations, the base stations, and an MSC. 
         FIG. 6  is an illustration of the flow of messages between the mobile station, the base station X, the base station Y, and the MSC in accordance with the disclosed method and apparatus. 
         FIG. 7  is a flow chart that indicates the procedure performed by the mobile station in accordance with the disclosed method and apparatus. 
         FIG. 8  is a flow chart that indicates the procedure performed by an MSC in accordance with the disclosed method and apparatus. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout this description, the preferred embodiment and examples disclosed should be considered as exemplars, rather than as limitations on the present invention. 
       FIG. 2  is an illustration of a mobile station  200  in accordance with one embodiment of the disclosed apparatus. As shown in  FIG. 2 , the mobile station  200  has a front end  201 , a signal processor  203 , a general purpose processor  205 , and a memory  207 . The memory includes an area  209  in which an “Active Set” is stored, and an area  211  in which a “Neighbor Set” is stored. The function of each of the components of the mobile station  200  will be described below. 
       FIG. 3  is an illustration of a base station  300  in accordance with one embodiment of the disclosed apparatus. As shown in  FIG. 3 , the base station has a front end  301 , a signal processor  303 , a general purpose processor  305 , a memory  307 , and a communication interface  308 . The function of each of the components of the base station  300  will be described below. 
       FIG. 4  is an illustration of a mobile switching center (MSC)  400  in accordance with one embodiment of the disclosed apparatus. As shown in  FIG. 3 , the base station has a communication interface  401 , a general purpose processor  403 , and a memory  405 . The function of each of the components of the MSC  400  will be described below. 
       FIG. 5  is an illustration of a communication system that includes mobile stations  200 , the base stations  300 , and an MSC  400 . It should be noted that the number of mobile stations, base stations, and MSCs that are shown in  FIG. 5  is selected merely to make it convenient to describe the disclosed method and apparatus. However, in systems that incorporate the disclosed method and apparatus, there will typically be a greater number of each of these components. Nonetheless, the relationship between these components will be essentially as shown in  FIG. 5 . 
     As shown in  FIG. 5 , the mobile stations  200  (such as cellular telephones) each move about within a system of base stations  300 . Each base station is in communication with an MSC  400 . When a call to or from a mobile station  200  is established, the mobile station communicates with the base station  300 . The base station relays the “traffic” to the MSC  400 . For the purpose of this description, the traffic is that portion of the information that is sent from the mobile station  200  to the base station and which is intended for the device at the other end of the call. The MSC  400  in turn typically sends the traffic to the device at the other end of the call through a land based system, such as the public switched telephone network (PSTN) or over the internet. However, it should be noted that in an alternative embodiment of the disclosed method and apparatus, the MSC  400  may transmit the traffic via an air link, such as a satellite uplink, line of sight microwave link, or other such radio connection. Therefore, it should be understood that there is no limitation to the manner in which the traffic is communicated from the MSC to another device which is at the other end of the call. 
     In one embodiment of the disclosed method and apparatus, the base stations  300  are arranged to transmit information into three sectors  501 ,  502 ,  503 . In  FIG. 5 , one mobile station  200 A is in a sector  501 A of a first base station  300 A, and is concurrently in a sector  502 B of a second base station  300 B. Therefore, the mobile station  200 A will receive a pilot signal from both the base station  300 A and the base station  300 B. If the mobile station  200 A was initially in the sector  501 A of the base station  300 A, but sufficiently far away from the base station  300 B that the pilot signal being transmitted from base station  300 B was not above a predetermined threshold, referred to hereafter as the “Active Pilot Threshold”, then the following process would occur in accordance with the disclosed method and apparatus. 
     When the pilot transmitted by the base station  300 B is first received by the mobile station  200 A at a power level that is above the Active Pilot Threshold, then the mobile station  200 A will generate a Pilot Strength Measurement Message (PSMM). Referring to  FIG. 2 , the pilots from both the base station  300 A and the base station  300 B will be received by the front end  201  in the mobile station  200 A. The signals will preferably be digitized in the front end  201  and the digital representation of the signals coupled to the signal processor  203 . The signal processor  203  will determine the signal strength of the pilot signals in conventional fashion. The values of the signal strength of each pilot will then be coupled the general purpose processor  205  to determine whether each pilot is above the Active Pilot Threshold. In addition, a determination will be made as to whether each pilot that is currently being received at a signal strength above the Active Pilot Threshold is currently in the Active Set  209  stored in memory  207 . If a pilot is being received at a signal level that is above the Active Pilot Threshold, but is not in the Active Set, then a PSMM will be generated by the general purpose processor  205 . 
     The PSMM will be transmitted to the MSC  400  over the connection  501  between the base stations and the MSC  400  (see  FIG. 5 ). The PSMM will identify each of the pilots that are currently being received at a signal level that is greater than the Active Pilot Threshold. 
     In addition, the general purpose processor  205  within the mobile station  200  will add each pilot that is being received by the mobile station  200  to the Active Set  209 . In one embodiment of the disclosed method and apparatus, the general purpose processor  205  will determine how many pilots are currently in the Active Set. If the Active Set includes more than a desired number of pilots, then the general purpose processor  205  selects the desired number of pilots from among all of the pilots that were presented in the PSMM from the mobile station  200 . The decision as to which pilots to include in the Active Set is preferably made by selecting those pilots that were received by the mobile station  200  at the strongest signal levels. 
     Once a pilot is included in the Active Set  209 , the mobile station  200  will demodulate the traffic channel that is being transmitted from the base station associated with that pilot. The information that is required to demodulate the traffic channel for each of the pilots in the Neighbor Set is stored together with the Neighbor Set. A pilot that is to be included in the Active Set should be a neighbor to one of the active pilots (i.e., one of the pilots in the Active Set). Therefore, the information necessary to demodulate the traffic channel of any pilot that is received at a pilot signal strength that is greater than the Active Pilot Threshold should be available to the mobile station  200 . In one embodiment of the disclosed method and apparatus, the information that is stored in the Neighbor Set  211  is provided by one or more of the base stations associated with the pilots in the Active Set. 
     Once the mobile station  200 A transmits the PSMM to the base station  300 A, the base station  300 A relays the PSMM to the MSC  400 . Referring to  FIG. 3 , the base station  300  receives the PSMM on either the reverse traffic channel or a control channel that is time multiplexed, code multiplexed, or otherwise distinguished from the traffic and pilot channels. The PSMM is received by the base station via the front end  301 . The PSMM is digitized in the front end  301  and provided to the signal processor  303  for demodulation. The signal processor  303  demodulates the signal and provides the content of the signal to the general purpose processor  305  for transmission to the MSC  400  via the communication interface  308 . 
     Referring to  FIG. 4 , the MSC  400  receives the content of the PSMM from the base station via the communication interface  401  within the MSC  400 . The content of the PSMM is then coupled to the general purpose processor  403 . The general purpose processor  403  within the MSC  400  generates an HDM. The HDM is a message that indicates which base stations  300  will be transmitting a forward traffic channel to the mobile station  200 A. Since the MSC  400  preferably has the ability to select one or more base stations to transmit traffic, the HDM is essential to inform the mobile station  200 A which of the base stations  300  identified by the pilots in the Active Set will truly be transmitting traffic. 
     The HDM is coupled back to the communication interface  401  within the MSC  400  for transmission to each of the base stations  300  indicated in the PSMM. The HDM is received within each of the base stations  300  by the communication interface  308 . The HDM is then coupled to the general purpose processor  305  within each base station  300 . Each general purpose processor  305  couples the HDM to the mobile station  200 A that sent the PSMM. The mobile station  200 A receives the HDM from at least the base station  300 B, even if the signals transmitted on the forward traffic channel by base station  300 A are no longer strong enough to be received by the mobile station  200 A. 
     It should be understood that even though the disclosed method and apparatus is described as using a PSMM and HDM (terms that are well known in the industry), only the functions that are described herein are relevant to the disclosed method and apparatus. Therefore, if an industry standard PSMM or HDM has other functions, formats, or characteristics which are not referenced in this disclosure, then they are not to be considered as part of the disclosed method and apparatus. In effect, any message format may be used to indicate to the base stations  300  which pilots have been received at levels above the Active Pilot Threshold. Likewise, any message format may be used to indicate to the mobile station  200 A which base stations will be transmitting traffic to that mobile station  200 A. 
       FIG. 6  is an illustration of the flow of messages between the mobile station  200 A, the base station  300 A, the base station  300 B, and the MSC  400  in accordance with the disclosed method and apparatus. As shown in  FIG. 6 , a traffic channel is initially established between the mobile station  200 A and the base station  300 A. When the mobile station  200 A detects the pilot from base station  300 A which is above the Active Pilot Threshold, the mobile station  200 A transmits a PSMM to the base station  300 A. The PSMM indicates that the mobile station  200 A is currently receiving the pilots from both the base station  300 A and the base station  300 B at levels that are greater than the Active Pilot Threshold. This is indicated in  FIG. 6  by the “X” and “Y” in parenthesis following the “PSMM”. The PSMM is relayed by the base station  300 A to the MSC  400 . The MSC  400  communicates with the base station  300 B to request resources be allocated by base station  300 B to support a traffic channel to and from the mobile station  200 A. The MSC  400  then generates and transmits to both the base stations  300 A,  300 B an HDM indicating that both the base stations  300 A,  300 B will be establishing traffic channels to the mobile station. The mobile station  200 A then generates and transmits a handoff completion message HDC. The HDC is received by the base station  300 A and relayed to the MSC  400 . The HDC indicates to the MSC  400  that the mobile has successfully received the HDM. 
       FIG. 7  is a flow chart that indicates the procedure performed by the mobile station in accordance with the disclosed method and apparatus. In accordance with the method shown in  FIG. 7 , the mobile station  200 A determines whether any pilots are being received at levels above the Active Pilot Threshold (STEP  701 ). If any pilots are being received at levels above the Active Pilot Threshold, then the mobile station  200 A determines whether each such pilot is in the Active Set  209  (STEP  703 ). If at least one of these pilots is not in the Active Set  209 , then a PSMM is generated and transmitted to the base stations with which the mobile station currently has an established traffic channel (i.e., those base stations  300  associated with pilots that are currently in the Active Set) (STEP  705 ). 
     Next, the mobile station  200 A places each of the pilots that were received at levels above the Active Pilot Threshold in the Active Set  209  (STEP  707 ). After placing all of these pilots in the Active Set, the mobile station  200 A then monitors the transmissions from each of the base stations associated with pilots in the Active Set in an attempt to receive an HDM (STEP  709 ). Once an HDM is received, the mobile station  200 A generates and transmits an HCM indicating that the handoff has been completed (STEP  711 ). The mobile station  200 A then begins to transmit and receive over the traffic channels to and from each of the base stations indicated in the HDM (STEP  713 ). 
       FIG. 8  is a flow chart that indicates the procedure performed by an MSC in accordance with the disclosed method and apparatus. In accordance with the method shown in  FIG. 8 , the MSC  400  awaits receipt of a PSMM from the mobile station  200 A (STEP  801 ). Upon receipt of the PSMM, the MSC  400  requests that each of the base stations associated with a pilot identified in the PSMM allocate resources to the mobile station  200 A (STEP  803 ). Alternatively, the MSC  400  only contacts those base station that do not already have a traffic channel to and from the base station  200 A. In accordance with one method, upon receiving confirmation that the resources are allocated, the MSC  400  generates and transmits an HDM that indicates which base stations have resources currently allocated to the base station  200   a  (STEP  805 ). Alternatively, the HDM only identifies those base stations that have allocated resources in response to the PSMM, and not those that already had resources allocated previous to the receipt of the PSMM. The HDM is preferably transmitted to each of the base stations indicated by the PSMM. In an alternative method, the HDM is transmitted only to those base stations that are identified in the HDM (i.e., those base stations that have successfully allocated resources to the base station  200 A). In one method, the HDM is only transmitted to base stations that were recently added to the Active Set. 
     The MSC  400  then waits for an HCM to be received (STEP  807 ). The HCM indicates that the handoff is complete. The HCM may be received from the mobile station  200 A through all, or only through some, of the base stations that are currently in the Active Set  209 . 
     Upon receipt of the HCM, the MSC  400  begins to route traffic through each of the base stations  300  that were identified in the HCM (STEP  809 ).