Scheduling acquisition attempts of service providing systems

Details Scheduling acquisition attempts of service providing systems Scheduling acquisition attempts of service providing systems

2001-10-30

2004-07-20

Gary, Erika (Department: 2681)

Telecommunications

Radiotelephone system

Zoned or cellular telephone system

C455S552100, C455S432100, C455S435100, C455S168100

Reexamination Certificate

active

06766169

ABSTRACT:

BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for scheduling acquisition attempts of one or more service providing systems in a subscriber station capable of operation with a plurality of communication systems.
II. Related Art
As mobile communication systems become more prevalent in society, the demands for greater and more sophisticated service have grown. To meet the capacity needs of mobile communication systems, techniques of multiple access to a limited communication resource have been developed. The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems.
The use of CDMA techniques in a multiple access communication system is known in the art and is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, also assigned to the assignee of the present invention.
In order for service to be provided using a subscriber station, the subscriber station must first acquire a service providing system. Such service providing systems can be identified, for example, by mode (for example, CDMA, TDMA, GSM, etc.) and frequency. Many wireless subscriber stations (also known as wireless communications devices, user terminals, or mobile stations) include tables and/or lists that are used by a subscriber station when it is attempting to obtain service. For example, a subscriber station can maintain a preferred roaming list (PRL) that is maintained in such a manner that the subscriber station can readily determine which systems (preferred or negative) cover common geographical regions (that is, areas of common radio coverage). Moreover, the PRL can rank systems from most desirable to least desirable. Accordingly, subscriber stations can prioritize their attempts to acquire service such that they attempt to acquire service on more desirable systems before attempting to acquire service on less desirable systems.
Subscriber stations can also include tables or lists of the N (for example, N=10) most recently used systems (that is, the systems that were most recently used to provide service). Such a table or list is typically referred to as a most recently used (MRU) table. A subscriber station can, for example, attempt initial system acquisition (for example, after a power up) using the MRU table.
Subscriber stations typically use one or more of the above discussed tables (and/or other similar types of tables) to determine the order in which to attempt to acquire systems. For example, after power up, a subscriber station may attempt to acquire a system based on an order specified by an MRU table and/or a PRL. Similarly, after a call is dropped (for example, due to loss of signal or signal fading), a subscriber station may attempt to acquire (including re-acquire) a system based on an order specified by the MRU table and/or the PRL. More specifically, upon detecting specific events, such as power up or system loss, a subscriber station may be programmed such that the first system the subscriber station attempts to acquire (specifically in this case, re-acquire) is the last system upon which service was successfully provided (that is, the most recently used system). An example of another event that can be detected is the loss of a paging channel (for example, due to loss of signal or signal fading) while the subscriber station is in idle mode.
For example, after a subscriber station is powered up, the subscriber station first attempts to acquire service using the mode and frequency of the system upon which service was last provided when the subscriber station was last powered on (that is, prior to power down). In other words, the subscriber station first attempts to re-acquire the most recently used system before attempting to acquire one or more other systems. In this example, the acquisition of the most recently used system is attempted because it is likely that the subscriber station has remained within the same geographic region, and thus, that service can again be provided using the most recently used system.
Similarly, after a subscriber station drops a call due to loss of signal or signal fading, the subscriber station immediately attempts to re-acquire service using the mode and frequency of the system upon which service was being provided when the call was dropped. In this example, the re-acquisition of the most recently used system is attempted because it is likely that the cause of the system loss is only temporary, and thus, that service can again be provided using the most recently used system. For example, a subscriber station may lose system acquisition because the user of the subscriber station travels on an elevator for a few seconds, travels behind a service blocking building for a few seconds, or travels in an underground tunnel for a few minutes. In each of these examples, successful re-acquisition of the most recently used system is likely once the user, for example, exits the elevator, is no longer behind the building, or exits the underground tunnel.
Some prior subscriber stations, designed by the assignee of the present invention, are adapted to alternate between the most recently used system and other systems when attempting to acquire (including re-acquire) a system. More specifically, the subscriber station's first attempt is to acquire service on the most recently used system, the second attempt is to acquire an alternative system, the third attempt to acquire the most recently used system, the fourth attempt is to acquire another alternative system, the fifth attempt to acquire the most recently used system, the sixth attempt is to acquire a further alternative system, and so on. The order in which the subscriber station attempts to acquire service on the alternative systems can be based on various algorithms that are known in the art. Once all alternative systems are attempted in accordance with one of such algorithms, the above described alternating with the most recently used system is stopped and acquisition is attempted based only on an algorithm that does not give such repeated high priority to the most recently used system. Even though the above described method of alternating between the most recently used system and alternative systems (referred to hereafter as the “alternating method”) has proved useful, this alternating method has some drawbacks.
First, since the alternating method is based on an alternating sequence scheme, it is dependent on the processing speed of the subscriber station's hardware. More specifically, if a subscriber station operates at a high speed, the subscriber station may alternate between the most recently used system and all of the hypothesized available alternative systems in only a few seconds. As mentioned above, the alternating method stops after unsuccessful attempts to acquire all the hypothesized available alternative systems. Thus, the subscriber station may stop giving the most recently used system repeated high priority earlier than it should. For example, while the subscriber station is located in an elevator, the subscriber station may complete the alternating method and stop giving the most recently used s

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