Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-11-06
2003-10-07
Maung, Nay (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S440000, C455S446000, C455S067700
Reexamination Certificate
active
06631263
ABSTRACT:
BACKGROUND
Cellular communication systems conventionally consist of a plurality of base stations arranged in a pattern so as to define a plurality of overlapping cells which provide radiocommnunication support in a geographic area. A remote transmitter/receiver unit communicates with the base station of the corresponding cell in which the remote unit resides. This communication typically occurs through a channel assigned to the connection by the system. When the remote unit is mobile, or when the base station is non-stationary (i.e., an orbiting satellite), the remote unit may transition between adjacent cells due to the relative movement between the remote unit and the base station. Absent some intervention by the system, this transitioning would eventually terminate the connection because the received signal strength associated with the signals would diminish to a level where either the base station or remote station cannot adequately receive the other's transmissions to decode information associated therewith. Transitioning between cells can additionally cause a significant degradation in signal quality. This signal quality degradation is typically measured at the mobile station by a quality measure such as bit error rate (BER). Signal quality degradation and termination of communication due to inadequate signal strength represent aspects of the cell transition problem in mobile cellular communications.
A solution to this aspect of the cell transition problem is commonly called “handover.” This conventional technique “hands off” an in-process communication with a remote unit from one base station in a first cell to another base station in another cell. This hand-off process maintains the quality and continuity of the connection and prevents the termination of the call when the mobile station is moving from one cell to another. The hand-off process may be accomplished using a number of system dependent methods. In existing analog systems, for example, a serving base station determines the need for a handoff of a mobile station whose connection it is handling based on periodic measurements of the signal strength and/or signal quality of the uplink voice channel signals received from that mobile station. If the measured signal strength and/or signal quality is below a predetermined level, the serving base station sends a hand-off request to the mobile switching center. The mobile switching center queries neighboring base stations for reports of the previously measured signal strength of signals on the voice channel currently being used by the mobile station. The mobile switching center then selects the target candidate cell associated with the neighboring base station reporting the strongest signal, provided that the reported signal strength is above a threshold, and transmits appropriate commands to that neighboring base station and to the mobile station via the serving base station to implement the handoff on the same or a different traffic channel.
In digital or dual-mode systems, as specified for example in the EIA/TIA IS-136 standard, hand-off from a digital traffic channel may also be implemented using a Mobile-Assisted Handoff (MAHO) procedure. Using this procedure, a mobile station may be ordered by the network to measure and report signal strength and/or other parameters of digital radio channels emitted by the serving base station, as well as those emitted by neighbor base stations. This enables handoff decisions made by the network to be based not only on the measured signal strength and other parameters of the uplink signal received from the mobile station, but also on the downlink signal parameters detected by the mobile station on channels associated with the serving and neighbor base stations.
Additionally, U.S. Pat. No. 5,235,633 (Dennison et al.) provides a method for hand-off that uses mobile station location information instead of the conventional uplink or downlink signal quality information discussed above. In the technique of Dennison, a GPS receiver is used for determining the exact mobile station location. The mobile station relays the GPS location information to the mobile switching center, which then uses this location information, in conjunction with a look-up table that specifies cell boundary points, to determine when hand-off should be initiated and to select the most appropriate target cell for hand-off. In addition to the GPS positioning method of Dennison, various other techniques are known in the art for determining mobile station position, such as, for example, the technique disclosed in European Patent Application EP 0800319A1.
Though not specifically described in Dennison, a number of conventional location techniques can be used to determine cell boundary points or hand-off borders. These techniques use a test mobile that measures signal strength while driving between two base stations (
6
and
7
, FIG.
1
). In one such technique, called handoff based on best server criteria, the handoff border has been reached, and the location can be noted, when the signal strength from the candidate base station
7
is equal to the signal strength from the serving base station
6
. This border can be confirmed by doing the same test while driving in the opposite direction.
In another technique there will be a difference between incoming and outgoing handoff borders, called handoff hysteresis. Hand-off hysteresis is used to avoid oscillating hand-offs which can occur for a mobile driving at the border of two adjoining cells. In a hand-off technique based on hysteresis criteria, the outgoing hand-off border of the cell associated with a first base station is located when the signal strength from a second base station in an adjoining cell is greater than the signal strength from the first base station plus a predefined threshold value:
SS
2
>SS
1
+th
val
Similarly, to determine the incoming hand-off border for the first base station, signal strength is measured while driving in the opposite direction from the second base station to the first base station. The location of the incoming hand-off border is determined when the signal strength from the first base station is greater than the signal strength from the second base station plus a predefined threshold value:
SS
1
>SS
2
+th
val
Using either of the above conventional techniques, a network hand-off border map can be manually constructed from the collected test mobile data.
The hand-off border location monitoring discussed above is conventionally accomplished through performance of drive tests by network operator staff. To perform this monitoring, operator staff drive throughout the network and conduct and record call quality checks and handover positions. A conventional system such as TEMS (Test Mobile System) is used to perform the monitoring. TEMS uses mobiles modified with specialized software for monitoring parameters of the radio environment. Radio environment monitoring is initiated by an operator who connects the modified mobile to a personal computer via a standard RS-232 serial connection. A GPS receiver is also connected to the PC to provide mobile position information. Survey data is then compiled during the monitoring process including data such as the geographic locations associated with signal strengths, bit error rates, interference, dropped calls, or handovers. Post-processing of the data gathered by TEMS is performed in a geographical information system (GIS) that enables the operator to visualize survey data with different colors and symbols that are indicative of status and operation of the mobile.
Network mapping achieved through TEMS surveying, for example, is used for more than just providing a basis for hand-off decisions such as that shown in Dennison. Manually collected hand-off border information is also used for network maintenance and cell planning purposes. Thus, if a survey of hand-off border information indicates hand-off problems in certain areas of the network, network maintenance can be initiated to correct the problem. Additionally, consistent han
Lee John J
Maung Nay
Telefonaktiebolaget LM Ericsson (publ)
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