Method for reducing control channel scan time

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METHOD FOR REDUCING CONTROL
CHANNEL SCAN TIME

FIELD OF INVENTION

The present invention relates generally to the communications field and, in particular, to a method for reducing the amount of time and resources required for a mobile unit to find an appropriate communications control channel in wireless communications systems.

BACKGROUND OF THE INVENTION

Wireless communication systems, such as cellular telephone systems, have proved to be very popular. Such systems typically include numerous mobile units (e.g., cellular telephones), a plurality of base stations at fixed locations, and one or more switching centers connecting the wireless communication systems to other communications systems, such as the land line public switched telephone network.

It is common for such systems to employ a large number of discrete communication channels for communicating voice and data from one location to another and for controlling system functions. It is also common for there to be multiple communications service providers in competition with each other for each type of communication service. Further, within a given geographical area, it is typical for the various communication channels to be allocated to the different competitors such that only one service provider is responsible for providing service on any given channel.

To facilitate communications channel allocation, the spectrum of channels may be divided into several channel bands, each containing a plurality of communications channels. Thereafter, an entire channel band is typically allocated to a particular service provider for a given geographic region. Thus, in a certain region, service provider Alpha may be allocated channel bands A and C, while service provider Beta may be allocated channel band B, and service provider Gamma all other channel bands. The particular details of the channel band allocations are well known in the industry.

During use, it is common for the mobile unit to initiate a band scan in order to locate an appropriate control channel. The band scan is a process by which a mobile unit searches through channels in a channel band, looking for an appropriate control channel to camp on. A band scan could be initiated for a wide variety of reasons, including at powerup, at call release, or when a control channel is lost. In some instances, multiple band scans are run back-to-back on different channel bands in what is called a wide-band scan. Because the band scanning process takes time and otherwise utilizes system resources including mobile unit power, it is very desirable for the band scan or wide-band scan to happen as quickly as possible.

SUMMARY OF THE INVENTION

The present invention is an improved method for a mobile unit to locate appropriate control channels in a wireless communications system, such as a cellular telephone system, operating on a communications spectrum having channel bands. The method is an improved band scan method which includes storing a list of acceptable service providers in the mobile unit, receiving service provider information identifying the service provider for a particular control channel, receiving minimum acceptable signal strength information for the control channel, determining the acceptability of the service provider by comparing the

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received service provider information against the acceptable service provider list, and, if the service provider is acceptable, comparing the received signal strength for the control channel against the minimum acceptable level. If the

5 control channel is suitable, i.e. has sufficient received signal strength, and is allocated to an acceptable service provider, the mobile unit camps on the control channel. If the control channel is allocated to an unacceptable service provider, the mobile unit stores the received service provider information,

10 the channel number, and the channel band and then returns to the band scan invoking procedure. By checking the service provider information early, the method allows for the checking of received signal strength against the minimum acceptable level to be deferred or avoided. Thus, the method

15 reduces the time required to locate appropriate control channels in some situations and otherwise increases the efficiency of control channel scanning. The method is particularly useful in IS-136 cellular telephone systems.

20 BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified representation of a cellular telephone wireless communications system.

FIG. 2 is an example of a typical communications spec

, trum organization showing channel bands.

25 6 6

FIG. 3 is a simplified logic flow diagram of the prior art. FIG. 3Ais a simplified logic flow diagram of the prior art showing a channel pointer trace process.

FIG. 4 is a simplified logic flow diagram of the present 30 invention.

FIG. 4A is a simplified logic flow diagram of the present invention showing a modified channel pointer trace process.

DETAILED DESCRIPTION OF THE 35 PREFERRED EMBODIMENT

A wireless communications system 10 typically includes

numerous mobile units 12 (e.g., cellular telephones), a

plurality of base stations 14, and one or more switching

4Q centers 16 connecting the wireless communications system

10 to other communications systems 18, such as the public

switched telephone network (see FIG. 1). Typically, the

mobile units 12 communicate with the base stations 14 using

radio links 13. Likewise, the base stations 14 typically

, communicate with the switching center 16 via radio links 15. 45 6

The radio links 13 typically include numerous discrete

communications channels 20 on one or more communications spectrums 50 (see FIG. 2). A communications channel 20 is typically a pair of frequencies, one transmit and one

50 receive, used by a mobile unit 12 to communicate within a wireless communications system 10. Some communications channels 20 are for control functions (control channels) while others are for voice and data communications (traffic channels). Examples of control functions include locating a

55 particular mobile unit 12, establishing a new communications session, and transferring oversight of a communications session from one base station 14 to another.

A wireless communications system 10 may utilize more than one communications spectrum 50. For instance, one

go spectrum 50 may be in the 800-900 MHz range while another may be in the 1850-2000 MHz range. For simplicity, the following description will discuss only one spectrum 50, but it is to be understood that the invention also includes the use of multiple spectrums 50.

65 A plurality of communications service providers typically compete in providing wireless communications service on a particular communications spectrum 50. It is customary for 3

these different service providers to be allocated portions of the communications spectrum 50 called "channel bands" 30. FIG. 2 shows a communications spectrum 50 divided into six channel bands (A-F) 30, each comprised of a plurality of communications channels 20. As shown in FIG. 2, each 5 communications channel 20 in the spectrum 50 has a channel number and is composed of a specific mobile unit-base station frequency pair. Also as shown in FIG. 2, each communications channel 20 is assigned to a specific channel band 30. 10

Each channel band 30 typically includes both control channels and traffic channels, with the particular assignment of a given communications channel 20 to these categories possibly changing from service provider to service provider and/or from area to area. In addition, different channel bands :5 30 may include different quantities of control channels and traffic channels.

The spectrum 50 of FIG. 2 represents the 1900 MHz spectrum of the Telecommunication Industry Association's interim standard IS-136 and is illustrative of a common 20 method of structuring a communications spectrum 50. The spectrum structure of FIG. 2 forms the basis for the description herein; however, other communications spectrum 50 organizations are possible. The present invention is intended to work with any communications spectrum 50 structure that 25 utilizes a band or grouping approach with predetermined assignment of communication channels 20 to bands or groups where there is only one service provider per channel band 30.

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It is also common to further subdivide the channel bands 30 into sub-bands, where each sub-band contains one or more communications channels 20. For purposes of illustration only, this description will assume that each channel band 30 of FIG. 2 is divided into 2.5 MHz sub-bands. In this 3J arrangement, each sub-band contains approximately eightythree communications channels 20.

The channel bands 30 are allocated as units to various service providers in a given area such that only one service provider is responsible for providing service on all the 40 communications channels 20 in any given channel band 30 in a given area. Thus, service provider Alpha in a certain region may be allocated channel bands A and C, while service provider Beta may be allocated channel band B, and service provider Gamma all other channel bands 30. 45

For a particular mobile unit 12, the various service providers may be divided into three broad classifications: preferred, swing, and disfavored. The preferred classification includes service providers that are most desirable for communications. Such service providers are known as 50 "acceptable" service providers. The disfavored classification includes service providers that should be used for communications only in emergencies and/or if no acceptable service providers are available. Such service providers are known as "unacceptable" service providers. The swing classification 55 includes service providers that can be either "acceptable" or "unacceptable" depending on various settings known in the art.

It is common in wireless communications systems 10 for mobile units 12 to contain a programmable Intelligent 60 Roaming Database (IRDB). The IRDB contains information that the mobile unit 12 uses during scanning operations. This information typically includes a listing of preferred service provider identification codes, a listing of disfavored service provider identification codes, a listing of swing service 65 provider identification codes, a band search list, and various other control information. The IRDB is commonly stored in

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semi-permanent memory in the mobile unit 12. That is, the IRDB is stored in the mobile unit 12 even while the mobile unit 12 is not in use, but the contents of the IRDB can be changed by the wireless communications system 10, possibly through over-the-air programming, or by an authorized technician. The mobile unit 12 user is typically not able to program the IRDB independently.

In practice, the IRDB is usually supplemented by the Number Assignment Module (NAM). The NAM is programmable memory containing specific information about a mobile unit 12 such as the electronic serial number, phone number, and the like. Like the IRDB, the NAM is semipermanent memory. However, it should be noted that the electronic serial number portion of the NAM is typically only programmable by the mobile unit's 12 manufacturer due to "cloning" concerns. The NAM also contains the identification code for the mobile unit's 12 home service provider. For simplicity of discussion, the NAM will be considered part of the IRDB.

From time to time in wireless communications systems 10, the mobile unit 12 may initiate a band scan procedure. Examples of events which may trigger a band scan include call release without a valid control channel 20 assignment, radio link failure, and power-up. A band scan is a search through a channel band 30 for a suitable control channel 20, i.e., one having adequate signal characteristics; preferably, the suitable control channel 20 is allocated to an "acceptable" service provider. Ideally, a band scan results in the mobile unit 12 registering with an "acceptable" service provider as quickly as possible. In some instances, multiple band scans are run back-to-back on different channel bands 30 in what is called a wide-band scan. In a wide-band scan, it is typical for the mobile unit 12 to search through channel bands 30, usually in the order specified by the band search list of the IRDB, until a suitable control channel 20 is located for an "acceptable" service provider. In addition, the mobile unit 12 stores information about "unacceptable" control channels 20 encountered during the wide-band scan in case no "acceptable" service providers are encountered by the conclusion of the wide-band scan. In such a situation, the mobile unit 12 will typically run a procedure known as a "rescan" in an attempt to camp on a suitable control channel 20 of an "unacceptable" service provider, preferably based on some preferential ranking of "unacceptable" service providers known in the art. The method of the present invention is not intended to be used during the rescan process.

In a broad sense, the band scan is accomplished by scanning channel sub-bands, searching for control channels 20 of "acceptable" service providers with adequate signal characteristics. In this process, the service provider identification code which is broadcast on a control channel 20 is compared against the IRDB to determine whether the service provider is "acceptable" or "unacceptable."

Referring to FIG. 3, the prior art band scan process for scanning a channel band 30 is shown. The band scan process includes the main process shown in FIG. 3 and the channel pointer trace process shown in FIG. 3A. For the main process, the sub-band is first scanned to identify the two strongest channels from a signal strength perspective (box 100). The two strongest channels are identified (box 100) and checked in order of signal strength (box 110). The Received Signal Strength (RSS) of the channel 20 is checked against a pre-determined threshold (box 120). If the channel 20 does not have a RSS above the threshold and the other channel 20 has not been checked, the next channel 20 is checked (box 110). If the last channel 20 does not have a