Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-06-02
2001-03-13
Maung, Nay (Department: 2744)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S432300, C455S436000, C370S320000, C370S331000
Reexamination Certificate
active
06201969
ABSTRACT:
TECHNICAL FIELD
This invention relates to methods and apparatus for controlling handoff in Cellular Wireless Telecommunications Systems.
PROBLEM
In recent years, one of the most popular types of cellular wireless systems is one based on the use of code division multiple access (CDMA) communications between the cellular mobile station and a base station. The advantage of the CDMA approach is that more voice channels can be accommodated in a given band of radio frequencies than is the case with other types of cellular systems.
All cellular systems that serve mobile stations must be prepared to accommodate a situation wherein a mobile station moves out of the range of the base station that served that mobile station and into the range of another base station. Under these circumstances, the mobile station is “handed off” to the other base station. In the case of CDMA, it has been found desirable to use a soft handoff which includes a period of time when the mobile station is served by two or more base stations, during which time both the mobile station and the mobile switching center select the strongest signal arriving from the base stations. Current systems have arrangements which allow up to six base stations to serve a call simultaneously; this is particularly useful in a situation wherein the mobile station is in hilly terrain, or is driving among many buildings and wherein the buildings or hills interfere with transmission from ones of the base stations serving the call.
A primary base station is usually assigned to control the call, and to analyze measurement information received from the mobile station in order to determine which base stations should be added to serve the call, and which base stations may be dropped from serving the call. The primary base station makes this decision based on measurement information that it receives from the mobile station; the mobile station measures the strength of all neighboring base stations which are serving the call, or which may be called upon to serve the call, as signals from other base stations become stronger and/or as the signals from the serving base stations become weaker.
Each base station maintains a list of neighboring base stations that are potential candidates for serving a call currently being served by that base station. This list contains the identities, network addresses, and other characteristics of each neighboring base station. The base station sends a list of the neighboring base station identities to the mobile station, instructing it to periodically measure the signal strength of each of those neighboring base stations. When the mobile station reports signal strength measurements to the base station, the base station uses the other information in the neighbor list to make a handoff decision and to perform the handoff. When multiple base stations are serving a call, the primary base station needs a full list of neighboring base stations from each of the base stations involved in the call, as well as its own list. When a base station is added to serve the call, the primary base station merge the lists of neighboring base stations from each of the base stations serving the call into a single list, and sends this new combined list to the mobile station. Likewise, when a base station is dropped from serving the call, the primary base station removes the dropped base station's list of neighbors from consideration, and sends a new list to the mobile station containing only the neighboring base stations of those base stations which are still serving the call.
The base stations communicate with each other via data links connected to the mobile switching center serving the base stations. If base stations need to communicate with other base stations served by another mobile switching center, they are interconnected using the data links between the base stations and their respective serving mobile switching centers, plus another data link interconnecting the mobile switching centers.
Each base station's neighbor list is administered by technician input via the mobile switching center. The neighbor list may be changed from time to time, as experience is obtained in recognizing which base stations may serve a mobile station in the primary area served by a base station, as system parameters are adjusted to improve system performance, as additional base stations are added to a system to accommodate system growth, and as traffic or usage patterns in an area change. A primary base station needs to have access to an up-to-date copy of the neighbor lists of all other base stations involved in a call in order to inform the mobile station of the combined list of neighbors to measure, and in order to make handoff decisions. One possible method of making other base stations' neighbor list data available to a primary base station is to download to each base station a copy of the neighbor lists from each of its neighboring base stations. With this method, whenever one base station's neighbor list data was changed by technician input, the MSC would have to ensure that a copy of the updated list was downloaded to all other base stations which needed it. This method has several shortcomings. Since three or more base stations may serve a call at one time, a primary base station may need a neighbor list from another base station which is not its direct neighbor, but is a neighbor of one of its neighbors. This makes it difficult to accurately identify the complete set of neighbor lists that a given base station may need to have. Maintenance of copies of each base station's neighbor list at many neighboring base stations is also complex, especially when neighboring base stations are in different MSCs. An alternate approach, which overcomes these problems, has been implemented by the prior art. In the prior art, when a base station is added to a call, the added base stations transmits its neighbor list to the primary base station with each acknowledgment of a handoff request message. Thus for a given call the primary base station will always receive an up-to-date copy of the neighbor list data from each other base station serving the call.
A problem of the prior art in CDMA cellular telecommunication systems is that the data traffic over the data links interconnecting base stations with mobile switching centers is very high, especially when there are many hand offs occurring during the course of many of the calls.
SOLUTION
Applicant has recognized that much of the traffic that loads the data links is traffic used to provide neighbor list information to base stations involved in a call. The primary base station needs a full list of its own neighboring base stations, and the neighboring stations of all other base stations currently serving the call. In the prior art, this information is provided by transmitting a neighbor list with each acknowledgment of a handoff request message.
The above problem is substantially alleviated, and an advance is made over the prior art in accordance with Applicant's invention wherein each base station builds up a data base of the neighboring cells for other base stations to which it has handed off calls, or from which it has been handed calls, so that if the data base is up to date, it is not necessary to transmit the identity of the neighboring base stations on each call. The validity of the neighbor list of any particular base station is specified by a time stamp which accompanied the most recent update of the neighbor list for that base station. When making a handoff request, the primary base station transmits the time stamp in its handoff request message; if that time stamp is in fact the time stamp of the most recent update of the neighbor list of the base station receiving the handoff request, then that base station need not send a neighbor list for this call. If the time stamp does not match the time stamp of the base station receiving the handoff request message, then that base station transmits its own (home base station) neighbor list, an updated list, back to the primary b
Lucent Technologies - Inc.
Maung Nay
Ulrich Werner
LandOfFree
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