SYSTEM FOR TRACKING BASE STATIONS IN A WIRELESS COMMUNICATION NETWORK

Abstract: A wireless communication network having a plurality of base stations (4) for servicing one or more mobile stations (6) is disclosed. Each base station (4) maintains a record of other neighboring base stations (4) to support "handoffs" between neighboring base stations (4) when a mobile station (6) moves from a cell of a first base station (4) to a cell of a second neighboring base station (4). The mobile stations (6) transmit reports to base stations (4) indicating the acquisition of pilot signals from other base stations (4). Base stations (4) maintain and update records of other neighboring base stations (4) based upon the reports from the mobile stations (6).

Description: SYSTEM FOR TRACKING BASE STATIONS IN A WIRELESS COMMUNICATION NETWORK BACKGROUND I. Field of the Invention: The description herein relates to wireless communication systems. In particular, the description relates to systems including base stations which provide communication services to mobile stations. II. Related Art: A wireless communication network typically includes a plurality of base stations dispersed over a geographical area which provide communication services to subscriber mobile stations such as in a mobile cellular telephone communication network. Base stations can typically provide service to one or more mobile stations located within a geographical area surrounding the base station known as a "cell." As mobile stations move geographically in the network, the mobile stations may be served by different base stations along the movement path. Such movement of the mobile stations through the network places the mobile station in and out of cells associated with particular base stations. When a mobile station transitions from one cell to another, the mobile station may no longer be able to receive services from the base station of the previous cell and require service from the base station of the new cell. This transition is facilitated through a "handoff" procedure using techniques known to those of ordinary skill in the art of mobile communications. As part of the handoff procedure, the mobile stations typically use information which identifies its "neighboring" base stations. A neighboring base station of a mobile station is typically defined as a base station which is capable of providing service to the mobile station at the mobile stations current location, in a current radio frequency (RF) environment. Thus, a mobile station may transition from receiving services from neighboring base stations by transitioning from the cell of a first base station into the cell of a second base station. The second base station then resumes providing services to the mobile station following the handoff procedure. A mobile station typically identifies neighboring base stations through the detection of "pilot" signals transmitted by base stations which are capable of providing service to the mobile station. Thus, the mobile station searches for pilot signals transmitted by potential neighboring base stations. A pilot signal from a base station is typically spread according to a code associated with the base station. The mobile stations may detect the pilot signal of a neighboring base station by despreading the pilot signal of the neighboring base station with the code associated with the base station. Searching for pilot signals from potential neighboring base stations at the mobile station from a very large number of base stations in the network is computationally intensive. To prioritize searching for neighboring base stations among a large number of potentially neighboring base stations in the network, and therefore intelligently allocate the computation resources at the mobile station for searching for pilot signals, the base stations typically transmit information to the mobile stations which is indicative of members in a "neighbor set" or "neighbor list." The neighbor set is to include base stations which may be capable of supporting handoffs. Base stations typically include a memory which maintains data representative of the identification of neighboring base stations in the neighbor set. The neighbor sets are typically pre-programmed at the individual base stations when the wireless communication network is established. As base stations are added or removed from the network, these memories in the base stations are typically manually re-programmed to indicate the addition or removal of base stations from neighbor sets. There is a need for a less cumbersome and more convenient method and system for establishing and maintaining neighbor sets at the memories of the base stations. SUMMARY An object of an embodiment of the present invention is directed to improving the flexibility in establishing base stations in a wireless communication network. It is another object of an embodiment of the present invention to provide a method of dynamically informing base stations of a wireless communication network as to the existence of neighboring base stations. It is another object of an embodiment of the present invention to simplify the integration of new base station into a wireless communication network which includes one or more existing base stations. It is another object of an embodiment of the present invention to simplify the removal of a base station of a wireless communication network. It is yet another object of an embodiment of the present invention to provide an efficient system and method for dynamically maintaining information in base stations of a wireless communication network regarding the existence of neighboring base stations. Briefly, an embodiment of the present invention is directed to a wireless communication network which includes a first base station having a memory for storing data representative of identities of base stations in a neighbor set. A second base station transmits a pilot signal which includes information representative of an identity of the second base station. A mobile station acquires the pilot signal and extracts the information representative of the identity of the second base station. If the identity of the second base station extracted at the mobile station is not included in the neighbor set, the data representative of the identities of the base stations in the neighbor set is updated to include the identity of the second base station. Detecting neighboring base stations at the mobile station eliminates the need for complicated pre-programming neighbor sets in base stations of a wireless communication network when new base stations are added. When a new base station is added, a subscriber mobile station acquires the pilot signal of the new base station. If the newly acquired base station is not in the neighbor list, there is a flaw in the neighbor list. The subscriber mobile station preferably informs the surrounding base stations of the flaw by transmitting the code associated with the pilot signal of the newly acquired base station to the surrounding base stations. Also, this system allows a base station to remove particular base stations from its neighbor set if no mobile station has identified the particular base station as being a neighbor for a period of time. BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows the topology of a wireless communication which includes base stations and mobile stations according to an embodiment of the present invention. Figure 2 shows a schematic diagram illustrating an architecture of processing hardware in a base station according to the embodiment of Figure 1. Figure 3 shows a schematic diagram illustrating an architecture of processing hardware in a mobile station according to the embodiment of Figure 1. DETAILED DESCRIPTION According to an embodiment of the present invention, base stations in a wireless communication network update and maintain records which identify "neighboring" base stations in response to messages received from mobile stations in the network. Each mobile station acquires pilot signals transmitted by base stations to identify which base stations are to be included as neighboring base stations in records of a particular base station which is currently serving the mobile station. The mobile station transmits messages having information identifying base stations neighboring to the base station which currently serves the mobile station. Upon receipt of these messages, the base station which is currently serving the mobile station updates record identifying its neighboring base stations. According to an embodiment, the mobile station receives a transmission from base station currently serving the mobile station which includes a neighbor list. The mobile station maintains a copy of the received neighbor list and searches for pilot signals associated with base stations in the neighbor list and for pilot signals of base stations which are not in the neighbor list. If the mobile station acquires a pilot signal of a base station which is not in the neighbor list, the mobile station transmits an indication to the base station currently serving the mobile station that there is a flaw in the neighbor list. This indication may be in the form of a data packet alerting the base station as to the deficiency in the neighbor list and identifying the code of the pilot associated with the base station which is not in the neighbor list. Upon receipt of the data packet, the base station currently serving the mobile station adds the newly acquired base station to its neighbor list. Figure 1 shows a topographical diagram of a wireless communication network which includes a plurality of base stations 4 and a plurality of mobile stations 6. As shown in Figure 1, the wireless communication network includes a plurality of cells 2. The mobile stations 6 within a cell 2 are serviced by a corresponding base station 4. Figure 1 shows that the cells 2 are adjacent but not overlapping. It is understood, however, that adjacent cells may overlap such that a mobile station within the overlapping region may be serviced by either base station 6 corresponding to the respective overlapping cells 2, or by both of the base stations. A mobile station in one cell 2 serviced by a corresponding base station 4 may travel to a different cell 2 to be serviced by a different base station 4. As an example, mobile station 6h is shown as being in cell 2c and serviced by base station 4c as indicated by the solid arrow connecting the base station 4c to the mobile station 6h. Since the mobile station 6h is traveling in a direction toward a base station 4d, the mobile station 6h will eventually travel out of the cell 2c and into the 2d. At a certain point, the base stations 4c and 4d, along with the mobile station 6h will coordinate a " handoff" such that the responsibilities for servicing the mobile station 6h will transition from the base station 4c to the base station 4d. Base stations 4c and 4d are therefore neighboring base stations. According to an embodiment, each of the base stations 4 maintains data in a memory which is representative of a "neighbor set" including information representative of the identity of one or more neighboring base stations. Referring to Figure 1, base station 4d has as its neighboring base stations 4a, 4b, 4c, 4e, 4f and 4g as the cell 2d corresponding to the base station 4d borders on the respective cells 2x of these neighboring base stations. The neighbor set, stored in the memory at base station 4d, may then potentially include the identity of each of these six neighboring mobile stations 4x. Similarly, base station 4b has base stations 4a, 4d and 4e as its neighboring base stations. The neighbor set stored in the memory at base station 4b may then potentially have information identifying base stations 4a, 4d and 4e. Each base station 4 services mobile stations 6 within its cell 2c by transmitting to the mobile station 6 on a forward transmission link and receiving transmissions from the mobile stations 6 on a reverse transmission link. One such communication system may be based upon a code division multiple access (CDMA) system conforming to the "TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System," hereinafter referred to as the "IS-95 Standard." Of course embodiments of the present invention are also applicable to other mobile wireless communication systems in which base stations provide communication services to mobile stations on a forward transmission link and the mobile stations transmit control information to the base stations on a reverse transmission link. One such alternative embodiment is described in U.S. Patent Application Serial No. 08/963,386, entitled "Method and Apparatus for High Rate Packet Data Transmission" assigned to Qualcomm, Inc., incorporated herein by reference. Figure 2 illustrates the processing architecture of base stations in a wireless communication network according to an embodiment. Base station architecture 100 includes a plurality of base station controllers (BSC) 104 which provide the processing resources of the base stations 4 shown in Figure 1. Each of the BSCs 104 are coupled to a master system controller (MSC) 102 which then communicates with a network 106 such as a public switch telephone network (PSTN) for facilitating voice communication or an Internet gateway for data communications. The MSC 102 controls communication among the BSCs 104, and manages the communication traffic between the individual BSCs 104 and the network 106. The MSC 102 communicates with the network 106 through a telephone interface in that the MSC 102 is, in effect, a node in the network 106. Figure 2 shows the processing architecture of a BSC 104a which includes a central bus 124 coupling the processing resources of the BSC 104 to the MSC 102. The BSC 104a also includes a general purpose processor 122, memory 118, RF front end 114 and antenna 116. The RF front end 114 and antenna 116 provide a bi-directional transmission interface between the BSC 104a and individual mobile stations. Thus, all forward and reverse traffic is facilitated by the RF front end 114 and the antenna 116. Figure 3 shows a processing architecture 200 of a mobile station 6. An RF front end 214 and antenna 216 receive transmissions from the base stations 4 on the forward transmission link and transmit to select base stations 4 on the reverse transmission link. A central bus (not shown) enables communication among a user interface (not shown), a general purpose processor 222 and a memory 218. The forward link of a base station transmits a "pilot signal" which is included with transmission data and includes information identifying the transmitting base station. The pilot signal transmitted in the forward link enables mobile station 6 to perform initial acquisition, phase recovery, timing recovery, and ratio combining. The mobile station 6 may also use the pilot signal for measuring a signal-to-noise-and-interference ratio (C/I). Such a transmission of a pilot channel in the forward transmission link is described in the aforementioned U.S. Patent Application Serial No. 08/963,386, with reference to Figure 3A, incorporated herein by reference. The pilot signal transmitted by a particular base station is spread with the code associated with the transmitting base station. According to an embodiment, each of the base stations 4 have an associated code for spreading its respective pilot channel which is orthogonal to all other pilot codes for spreading the signals of the other base stations 6 in the network. According to an embodiment, the pilot signal is transmitted in a manner described in the aforementioned U.S. Patent Application Serial No. 08/963,386, with reference to Figure 4B thereof, incorporated herein by reference. Each base station 4 broadcasts its pilot signal to all mobile stations 6 within range of acquiring the pilot signal. A particular mobile station 6h may acquire the pilot signal of more than one base station 4. For example, Figure 1 shows the mobile station 6 straddling the cells 2c and 2d. The mobile station 6 is therefore capable of acquiring the pilot signals transmitted by each of base stations 4c and 4d. Similarly, mobile station 6e is located within a triangle formed by base stations 4b, 4d and 4e. Mobile station 6e is capable of acquiring the pilot signals transmitted by each of base stations 4b, 4d and 4e. Upon acquiring the pilot signal from a base station 4, a mobile station 6 can identify the base station 4 transmitting the pilot signal by despreading the pilot signal using the pilot spreading code assigned to the base station 4. Therefore, when a mobile station 6 acquires the pilot signal of a particular base station 4, the mobile station 6 determines that it is possible in the current RF environment for the base station 4 to provide service to the mobile station 6. For example, mobile station 6e acquires the pilot signal transmitted by the base stations 4b, 4d and 4e. Mobile station 6e, therefore, determines that it is possible to receive data services from either of these three base stations 4. Further, the mobile station 6e determines that the base stations 4b and 4d should be neighbors of the base station 4e. The mobile stations 6 preferably report detections of neighboring base stations to base stations currently servicing the mobile stations in a message transmitted in the reverse communication link. In an embodiment of the present invention implemented in a system conforming with the IS-95 Standard, such a message is preferably included in a pilot strength measurement message (PSMM) transmitted by mobile stations in such systems. Here, the PSMM preferably includes at least the identity of base stations 4 corresponding to the acquired pilot signals. The base station 4 receiving the PSMM can then update the neighbor set in its memory. Of course this message reporting the acquisition of pilot signals may be transmitted in any other control channel of a reverse transmission link. For example, in an embodiment of the present invention implemented in a system such as that described in the aforementioned U.S. Patent Appl. Ser. No. 08/963,386, this message can be transmitted in a traffic channel request (TCR) which a mobile station 6 transmits to the base station 4 which is currently providing data services to the mobile station 6. In this embodiment, a mobile station 6 will transmit the TCR to a base station 4 in a data channel of the reverse link when either 1) the mobile station 6 is not in a call and is requesting the base station to be placed in a call, or 2) the mobile station 6 detects certain changes in the RF environment. Regardless of the type of reverse link message used, each mobile station 6 preferably includes data in the reverse link message while in a call (e.g., as part of a PSMM or TCR) for identifying base stations which should be included in the neighbor set of the base station currently providing service to the mobile station 6. Alternatively, each mobile station 6 may transmit equivalent data when not in a call (e.g., in an access channel). That is, the mobile station 6 receives and acquires the pilot signal of one or more base stations during a previous time slot. Determining the identity of those base stations 4 transmitting the acquired pilot signals, the mobile station 6 can include information in a message on the reverse link which is representative of the identities of these base stations which should be included in the neighbor set of the base station which is currently providing service to the mobile station 6. The base station 4 receives the reverse link message and then extracts the information representative of the other base stations 4 which should be included in the neighbor set receiving the reverse link message (as determined by the mobile station 6 transmitting the reverse link message). The general purpose processor 122 of the associated BSC 104 (Figure 2) receiving the reverse link message accesses data representative of the base stations which are known to be in the neighbor set and stored in the memory 118. The general purpose processor 122 then correlates the data extracted from the received reverse link message (representative of the identification of base stations which should be in the neighbor set) with the neighbor set data stored in the memory 118. If the identification of any base station extracted from the reverse link message does not match with a corresponding base station in the neighbor set data of the memory, the general purpose processor 122 updates the neighbor set data in the memory 118 to include the non-matching neighboring base station 4. By determining its neighboring base stations from signals provided by mobile stations 6, the base stations 4 can determine its neighbors dynamically without the need for special programming of the memory storing the data representative of the neighbor set. When a new base station 4 is added to the wireless communication network, mobile stations will receive its pilot signal and notify the existing base stations 4 of its new neighboring base station. The neighbor sets maintained at these existing base stations may then be updated to include the information identifying the newly added base station 4. In another embodiment of the present invention, the members of the neighbor sets maintained at the base stations 4 are aged over time and removed from the neighbor set if it is determined that these base stations should no longer be considered as neighbors. This is useful in, for example, updating the neighbor sets to reflect that a base station has been removed from the network or is inoperable. Also, this embodiment is useful in adapting to changes in the RF environment which causes changes in the size of cells 2, affecting whether base stations are neighboring or not. Here, for each base station member of a neighbor set maintained at the memory of a base station 4, the base station 4 maintains a record of when that particular base station in the neighbor set was last identified as a neighbor in a message received on the reverse link (e.g., base stations identified as neighbors through the acquisition of a pilot signal at a mobile station and subsequent identification in a TCR or PSMM message transmitted to the base station 4a). According to an embodiment, upon receipt of a reverse link message indicating the acquisition of a pilot signal of a particular base station in the neighbor set, the base station 4 receiving the reverse link message initiates or resets a timer. When a set number of time slots elapses, increasing the value of the timer to a threshold, the base station 4 determines that the member base station in the neighbor set should be removed. The removed member base station may then be re-added to the neighbor set at some time in the future upon receipt of a reverse link message indicating the acquisition of its pilot at the mobile station transmitting the reverse link message as discussed above. By aging the members of the neighbor sets maintained at a base station memory, the need for reprogramming these memories upon removal or the unavailability of a base station from the network is eliminated. While there has been illustrated and described what are presently considered to be the preferred embodiments of the present invention, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from the true scope of the invention. Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims. What is claimed is:

Claims: CLAIMS 1. In a wireless communication network including a first base station, the first base station including a memory for storing data representative of identities of base stations in a neighbor set, and a second base station transmitting a pilot signal including information representative of an identity of the second base station, a method for updating the data representative of the identities of base stations in the neighbor set including: at a mobile subscriber station, acquiring the pilot signal transmitted by the second base station to extract the information representative of the identity of the second base station; comparing the extracted information representative of the identity of the second base station with the information representative of the identities of the base stations in the neighbor set to determine whether the neighbor set includes the identity of the second base station; and if the identity of the second base station is not included in the neighbor set, updating the data representative of the indentities of the base stations in the neighbor set to include the second base station. 2. The method of claim 1, the method further comprising transmitting a message from the mobile subscriber station to the first base station in a control channel of a reverse transmission link. 3. The method of claim 1, wherein the wireless communication network further includes base station transmitters associated with the base stations in the neighbor set and one or more mobile stations, the method further including: transmitting a pilot signal from each of the base station transmitters including information representative of the identity of the base station associated with the base station transmitter; detecting an absence of an acquisition by any of the mobile stations of the pilot signal transmitted from a particular base station transmitter for a set duration of time; and upon detection of the absence, disassociating the identity of the base station associated with the particular base station transmitter from the neighbor set in the memory of the first base station. 4. The method of claim 1, the method further including: transmitting the extracted information representative of the identity of the second base station from the mobile station to the first base station; and comparing the extracted information representative of the identity of the second base station with the data representative of identities of base stations in the neighbor set to determine a match. 5. The method of claim 1, the method further including: transmitting data representative of the data representative of the identities of the base stations in the neighbor set to the mobile station; at the mobile station, associating the extracted information representative of the identity of the second base station with the data representative of the identities of the base stations in the neighbor set to determine a match; and if the identity of the second base station does not match with an identity of any base station in the neighbor set, transmitting a signal from the mobile station to the first base station to initiate inclusion of the identity of the second base station in the neighbor set. 6. A wireless communication network comprising: a first base station including a memory for storing data representative of identities of base stations in a neighbor set; a second base station which transmits a pilot signal including information representative of an identity of the second base station; a mobile station for acquiring the pilot signal of the second base station to extract the information representative of the identity of the second base station; logic for comparing the extracted information representative of the identity of the second base station with the data representative of the identities of the base stations in the neighbor set to determine whether the identity of the second base station is included in the neighbor set; and logic for updating the data representative of the identities of base stations in the neighbor set to include the identity of the second base station if not included in the neighbor set. 7. The wireless communication network of claim 6, wherein the mobile station includes circuitry for transmitting messages to the first base station in a control channel of a reverse transmission link. 8. The wireless communication network of claim 6, the wireless communication network further including: a plurality of base stations, each of the plurality of base stations transmitting a pilot signal including information representative of the identity of the base station associated with the base station transmitter; a plurality of mobile subscriber stations, each of the plurality of mobile stations acquiring the pilot signal of at least one of the base stations; and logic for detecting an absence of an acquisition of the pilot signal of a particular one of the plurality of base stations at each of the mobile stations for a set duration of time and disassociating the particular base station from the neighbor set in the memory of the first base station. 9. The wireless communication network of claim 6, wherein the mobile station transmits the extracted information representative of the identity of the second base station to the first base station, and wherein the logic for comparing and the logic for updating are implemented in circuitry at the first base station. 10. The wireless communication network of claim 6, wherein the first base station transmits a signal including the data representative of the identities of the base stations in the neighbor set to the mobile station, and wherein the mobile station further includes: logic for associating the extracted information representative of the identity of the second base station with the data representative of the identities of the base stations in the neighbor set to determine a match; and circuitry for transmitting a signal to the first base station to initiate inclusion of the identity of the second base station in the neighbor set if the identity of the second base station does not match with an identity of any base station in the neighbor set.

Inventors: KIMBALL ROBERT H, KIMBALL, ROBERT, H.
Applicants: QUALCOMM INC, QUALCOMM INCORPORATED
Application year: 2000
Main classification: H04Q7
IPC codes: H04W36, H04W88, H04W92