WO2001030108A1

WO2001030108A1 - Methods of selecting a control channel carrier frequency and related terminals and systems

Info

Publication number: WO2001030108A1
Application number: PCT/US2000/026670
Authority: WO
Inventors: Torbjorn Solve; Carsten Hoirup
Original assignee: Ericsson Inc
Priority date: 1999-10-19
Filing date: 2000-09-28
Publication date: 2001-04-26
Also published as: AU7728400A

Abstract

One of a plurality of control channels is selected at a user terminal in a radio communications system wherein each of the control channels is assigned to a respective geographic area. In particular, one of the control channels providing at least a predetermined level of received power at the user terminal is selected as an initial control channel for the user terminal wherein the initial control channel is assigned to a first geographic area. A list of neighbor control channels is obtained using the initial control channel wherein the neighbor control channels are assigned to geographic areas neighboring the first geographic area, and a preferred control channel is selected from the initial control channel and the neighbor control channels. After selecting one of the control channels, obtaining the list of neighbor control channels, and selecting the preferred control channel, communications service is obtained using the preferred control channel.

Description

METHODS OF SELECTING A CONTROL CHANNEL CARRIER FREQUENCY AND RELATED TERMINALS AND SYSTEMS

FIELD OF THE INVENTION The present invention relates to the field of communications and more particularly to radio communications.

BACKGROUND OF THE INVENTION

In radiotelephone communications systems such as satellite and cellular radiotelephone communications systems, frequency reuse can be provided by dividing the service area into geographic areas generally referred to as cells in terrestrial cellular systems and referred to as spotbeams in satellite radiotelephone systems. In a terrestrial cellular system, a base station can provide service for a respective cell, and a control channel carrier frequency transmitted by the base station provides information relating to that cell allowing a user terminal to establish service within that cell. In a satellite radiotelephone system, a plurality of spot beams can be defined wherein a control channel carrier frequency for each spot beam is transmitted by the satellite to provide information relating to the respective spotbeam allowing a user terminal to establish service within a spot beam. Here, the terms cell and spotbeam will both be used generically to refer to a division of a service area in either a terrestrial cellular radiotelephone system or a satellite radiotelephone system. In a satellite radiotelephone system, for example, a service area for a satellite can be divided into a plurality of overlapping spotbeams, and control/system information can be transmitted by the satellite to each of the spot beams using respective control channel carrier frequencies. More particularly, a satellite according to the Asia Cellular Satellite (ACeS) system can transmit on a control channel carrier frequency for each spotbeam similar to a control channel carrier frequency used for each cell of a GSM terrestrial radiotelephone system. The information transmitted by the satellite on this control channel carrier frequency can be used to provide a location area identification for the respective spotbeam, carrier frequencies for traffic channels for the respective spot beam, synchronization information, and other information used by a user terminal to establish and maintain radiotelephone service within the spot beam. The control channel carrier information can also be used to identify carrier frequencies of control channels for neighboring spotbeams which may be overlapping with, adjacent to, or near the spotbeam.

Before a user terminal can initiate or receive a radiotelephone call, the user terminal must first identify the control channel carrier frequency for a spotbeam within which the radiotelephone is located. Using the information received in the spotbeam, the radiotelephone can then register for service within that spotbeam, and/or receive incoming calls. When a radiotelephone is turned on, however, the spotbeam within which the radiotelephone is located may not be known. Accordingly, it may be necessary to search for a control channel carrier frequency to identify the spotbeam within which the radiotelephone is located. In a satellite radiotelephone system in particular, this search for a control channel carrier frequency may be time consuming because of the signal strengths and delays, thus delaying service.

There have thus been efforts to reduce the time required to select a control channel carrier frequency. For example, the user terminal may include memory to save a list of the control channel carrier frequencies for the last three spotbeams within which service was obtained by the user terminal. The user terminal can thus check these control channel carrier frequencies first thereby increasing the probability of finding the desired control channel carrier frequency quickly. In other words, it is assumed that the user terminal is likely to be in a spotbeam that has been recently used.

For example, the three most recently used control channel carrier frequencies can be saved in user terminal memory. When the user terminal is turned on, a power profile can be sequentially performed for each of the three control channel carrier frequencies most recently used. Power profiling includes measuring the carrier frequency for the averaged power received at the user terminal. The first control channel carrier frequency with a power profile at or above a predetermined level likely to allowed advantaged service is selected. Advantaged service means that the radiotelephone can both receive from the satellite and transmit to the satellite. If none of the carrier frequencies has a profile at or above the predetermined level likely to allow advantaged service, the user terminal can proceed with a search for a control channel carrier frequency of a spotbeam providing disadvantaged service where the user terminal may be unable to transmit to the satellite.

The user terminal can then attempt synchronization with the selected control channel carrier frequency. The synchronization can be performed in two steps using a high margin synchronization channel on the selected control channel carrier frequency. If this synchronization is successful, the user terminal attempts to read synchronization data on a normal margin synchronization channel of the carrier frequency. If the synchronization data is successfully read, the user terminal proceeds to read the location area identification from a broadcast control channel on the control channel carrier frequency. Otherwise, the user terminal can proceed with a search for a carrier frequency providing disadvantaged service.

Once the synchronization data and the location area identification have been obtained, the user terminal can provide communications with the satellite communications system using the selected control channel carrier frequency. This method, however, may not guarantee that the optimum control channel carrier frequency is selected at this point. Accordingly, a user terminal may continue with a process of spotbeam reselection wherein control channel carrier frequencies for neighboring spotbeams are monitored after establishing service. If better service can be provided in a neighboring spotbeam, known spotbeam reselection techniques may take as long as five minutes to select the new control channel carrier frequency and re-establish service.

According to a particular example, the user terminal discussed above may have most recently visited spotbeams A, B, and C in that order, and presently be in a region covered by spotbeams B and D with spotbeam D providing the better service. According to the method discussed above, the user terminal would select spotbeam B after approximately 18 seconds. In some situations, the user terminal may be unable to return transmissions using spotbeam B so that attempts to register with spotbeam B will fail. The user terminal registration information may thus be erased so that the user terminal will be unable to receive pages until the reselection process selects spotbeam D. The reselection process, however, may take up to 5 minutes.

Accordingly, there continues to exist a need in the art for improved methods of selecting control channels in radio communications systems.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide improved methods of obtaining control channels in radio communications systems.

It is another object of the present invention to reduce a time to obtain service in a radio communications system. These and other objects can be provided according to the present invention by selecting an initial control channel corresponding to a first geographic region, and obtaining a list of neighbor control channels for geographic regions neighboring the first geographic region using the initial control channel before attempting to obtain communications service. A preferred control channel is then selected from the initial and neighbor control channels, and communications service is then obtained using the preferred control channel. By selecting the preferred control channel from the initial and neighbor control channels before attempting to obtain communications service, the probability of obtaining satisfactory communications service on the first attempt can be increased.

According to one aspect of the present invention, a method can be provided for selecting one of a plurality of control channels at a user terminal in a radio communications system wherein each of the control channels is assigned to a respective geographic area. In particular, one of the control channels providing at least a predetermined level of received power at the user terminal is selected as an initial control channel for the user terminal wherein the initial control channel is assigned to a first geographic area. A list of neighbor control channels is obtained using the initial control channel wherein the neighbor control channels are assigned to geographic areas neighboring the first geographic area. A preferred control channel is selected from the initial control channel and the neighbor control channels. After selecting one of the control channels, obtaining the list of neighbor control channels, and selecting the preferred control channel, communications service is obtained using the preferred control channel.

More particularly, the step of selecting the preferred control channel can include measuring a received signal strength at the user terminal for each of the initial control channel and the neighbor control channels, and selecting from the initial control channel and the neighbor control channels based on the control channel having the highest received signal strength. In addition, the step of selecting the initial control channel can include selecting the initial control channel from a group of control channels previously used by the user terminal. Moreover, a received power at the user terminal can be sequentially measured for the group of control channels with a first measured control channel in the group having at least the predetermined level of received power being selected as the initial control channel.

The step of selecting one of the control channels providing at least a predetermined level of received power at the user terminal can include selecting one of the control channels having at least an average predetermined power level. In addition, the step of obtaining communications service can include camping on the preferred control channel, and/or registering with the radio communications system to receive pages over the preferred control channel. Methods, terminals, and systems according to the present invention can thus provide a more efficient registration process. In particular, by selecting the preferred control channel from the initial and neighbor control channels before attempting to obtain communications services, the probability of a successful first attempt to obtain communications services can be increased. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a satellite communications system according to the present invention.

Figure 2 is a diagram illustrating a plurality of overlapping spotbeams according to the satellite communications system of Figure 1.

Figure 3 is a graphical illustration of control channels on a control channel carrier frequency according to the satellite communications system of Figure 1.

Figure 4 is a block diagram of a user terminal for use in the satellite communications system of Figure 1. Figures 5A and 5B are flow diagrams illustrating methods and/or operations of the user terminal of Figure 4.

DETAILED DESCRIPTION The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

A satellite communications system according to the present invention is illustrated in Figure 1. As shown, a satellite 11 provides radiotelephone communications to user terminals 15 located in a service area divided into a plurality of spotbeams 17. A radio link 19 with a ground controller 21 provides communications with ground based telephone communications systems such as a public switched telephone network 23. In the Asia Cellular Satellite (ACeS) system, for example, a single geostationary satellite can provide service over a plurality of spotbeams with control information being transmitted for each spotbeam over a control channel carrier frequency. Alternately, service for different spotbeams could be provided by different satellites, and/or one or more non-geostationary satellites could be used. An example of an arrangement of spotbeams according to the present invention is illustrated in Figure 2. As shown, a geographic area serviced by the satellite 11 can be divided into a plurality of overlapping spotbeams 17A-J, and control information is transmitted to user terminals in each spotbeam using a respective control channel carrier frequency. Because of the overlap of adjacent spotbeams, different control channel carrier frequencies are generally used for adjacent spotbeams. More generally, frequency reuse patterns for the control channel carrier frequencies are preferably assigned so that interference between different spotbeams using a common control channel carrier frequency is kept below an acceptable level.

An example of information transmitted on a control channel carrier frequency according to the ACeS system is illustrated in Figure 3. In particular, a control channel carrier frequency can be divided into time division multiple access (TDMA) multiframes where each multiframe includes 102 TDMA time frames. Different segments of information can thus be transmitted during different frames of a multiframe. In particular, a multiframe can start with a high margin synchronization channel (HMSCH) HS in a first frame followed by a normal margin synchronization channel (SCH) S in a second frame. The multiframe can include a plurality of broadcast control channels (BCCH) BCCH each transmitted over four consecutive frames, and a high margin control channel (HMCH) HC transmitted over 81 frames. In particular, each broadcast control channel within a multiframe can transmit the same information.

In a first multiframe, the broadcast control channels can be used to transmit a location area identification (LAI) for the spotbeam as discussed in the ACeS specification at section 9.1.34 entitled System Information Type 3 (PS- SAI13100 Rev C.2, page 185, Nov. 16, 1998, ACeS SAIS 04.08). In a second multiframe, the broadcast control channels can be used to transmit a list of control channel carrier frequencies for neighbor spotbeams as discussed in the ACeS specification at section 10.5.2.22 entitled Neighbor Spotbeams Description (PS-SAI13100 Rev C.2, page 298, Nov. 16, 1998, ACeS SAIS 04.08). The broadcast control channels in other multiframes can be used to transmit other types of information. By repeating the broadcast control channel in each multiframe, the probability of receiving the information at a user terminal can be increased.

As shown in Figure 3, the high margin synchronization channel HS and the high margin control channels HC are transmitted at a higher power than the normal margin synchronization channel S and the broadcast control channel BCCH. Accordingly, there is a higher probability of receiving the high margin control and synchronization channels. In particular, the information transmitted over a single broadcast control channel over four time frames is transmitted at a higher power and with a higher level of error detection and correction on the high margin control channel using all 81 frames. A user terminal that is unable to receive a broadcast control channel may be able to receive the same information using the high power control channels. The time required to receive this information using the high power control channels is increased, however, because the information is spread over 81 frames distributed with 3 frames in each multiframe. A user terminal can thus receive the control information over the high power control channels even if the lower power broadcast control channels cannot be received.

An example of a user terminal according to the present invention is illustrated in Figure 4. As shown, the user terminal 15 includes a transceiver 31 , a processor 33, a memory 35, a user interface 37, and an antenna 39. The transceiver 31 transmits and receives radio communications over the radio link 19 under the control of the processor 33. The processor 33 receives communications to be transmitted from the user interface 37 and provides received communications through the user interface 37. The user interface, for example, can include a microphone and/or a keypad to receive sound and/or alpha-numeric information for transmission over the radio link. The user interface can also include a speaker and/or a display to reproduce communications received over the radio link. A memory 35 can be used to store information helpful in establishing communications. The memory, for example, can be used to store a list of control channel carrier frequencies for the three spotbeams within which the user terminal has most recently communicated.

As will be understood by those having skill in the art, the user terminal can be a satellite radiotelephone, a terrestrial radiotelephone, or a dual mode radiotelephone providing both terrestrial and satellite radio communications. While the user terminal is discussed herein as being a radiotelephone, the user terminal can alternately be a computing device and/or a data processing device such as a personal digital assistant or a laptop computer providing wireless data communications. The user terminal can also combine radiotelephone and computing/processing functionalities. In other words, a user terminal according to the present invention can be any communications device providing wireless voice and/or data communications.

Methods and systems for selecting a control channel carrier frequency according to the present invention will now be discussed with reference to the flow diagrams of Figures 5A and 5B. When the user terminal is turned on, for example, the user terminal will select a control channel carrier frequency for the spotbeam within which the user terminal is located. In particular, the user terminal can store a predetermined number of control channel carrier frequencies for the most recently used spotbeams, in memory 35. Three control channel carrier frequencies, for example, can be stored in the memory. Because the user terminal is most likely to be in a spotbeam in which it has been used before, one of the stored control channel carrier frequencies is most likely to be the control channel carrier frequency for the spotbeam in which the user terminal is currently located. As shown at block 101 , the processor 33 retrieves the first control channel carrier frequency from the memory 35 wherein the first control channel carrier frequency is preferably the most recently used frequency. A power profile is performed on the first control channel carrier frequency at block 103. The power profile can include measuring an average energy received over the frequency at the mobile terminal. If the power profile step indicates that the mobile terminal can likely obtain advantaged service using the first control channel carrier frequency at block 105, coarse and fine synchronization for the first control channel carrier frequency is performed at blocks 107 and 109. Otherwise the processor loops back through blocks 111 and 115 to obtain a power profile for the second control channel carrier frequency stored in memory. Coarse and fine synchronization steps are then performed at blocks 107 and 109 if one of the control channel carrier frequencies stored in memory is likely to provide advantaged communications. Here, advantaged communications means that the mobile terminal is likely to be able to receive both high margin synchronization and control channels as well as normal margin synchronization and broadcast control channels and also transmit back to the satellite thus completing a return link.

If the power profile steps indicate that none of the control channel carrier frequencies is likely to provide advantaged communications, the mobile terminal can proceed with steps to select a control channel carrier frequency providing disadvantaged communications at block 117. Here, disadvantaged communications means that the mobile terminal selects a control channel carrier frequency over which high power frames can be received but over which the user terminal may be unable to receive normal power frames or transmit back to the satellite. Because the mobile terminal is able to receive control information (including neighbor lists) using the high margin control channels, the mobile terminal may be able to eventually locate a control channel carrier frequency of sufficient strength to establish communications.

Coarse synchronization at block 107 is used to provide coarse time and frequency correction with respect to TDMA multiframes of the control channel carrier frequency selected at block 105. As shown in Figure 3, each multiframe can begin with a high margin synchronization channel HS followed by a normal margin synchronization channel S. In addition, high margin control channels HC can be provided at predetermined intervals throughout each multiframe. Using the known intervals between the high margin synchronization and control channels, the processor can roughly identify the beginning point of each multiframe. The coarse synchronization can then proceed by monitoring 4 double wide bursts (4X2X156bits) at approximately the beginning of a multiframe to provide a coarse synchronization with respect to the beginning of each TDMA multiframe within ±2 bits and to the carrier frequency within ±1 kHz. After course synchronization, a fine synchronization is then performed at block 109. In particular, the fine synchronization can proceed by monitoring 4 single wide bursts (4X156bits) at approximately the beginning of a multiframe to provide a fine synchronization with respect to the beginning of each TDMA multiframe within ±1/2bits and to the carrier frequency with ±100Hz. If the mobile terminal is unable to obtain either coarse or fine synchronization at block 119, a search for a disadvantaged carrier is performed at block 117.

If coarse and fine synchronization are obtained at block 119, the data of the normal margin synchronization channel S is read at block 121. If the normal margin synchronization channel S is successfully read at block 123, the mobile terminal proceeds to read the normal margin broadcast control channel BCCH at block 125. Otherwise, a search for a disadvantaged carrier can be performed at block 117.

In particular, a broadcast control message (a type 2 message according to section 10.5.2.22 of the ACeS specification, for example) including control channel carrier frequencies for neighbor spotbeams is read at block 125. If the neighbor spotbeam information is successfully received at block 127, a list of carrier frequencies is created at block 129 including the serving control channel carrier frequency obtained at block 105 and the neighbor control channel carrier frequencies obtained at block 125. Otherwise, a search for a disadvantaged carrier can be performed at block 117. A first control channel carrier frequency is then selected at block 131 and measured to obtain a received signal strength indication (RSSI) at block 133. Each of the control channel carrier frequencies in the list is then measured to obtain a respective received signal strength indication (RSSI) as shown at blocks 135 and 137. In particular, the received signal strength indications can be provided by measuring only periods of active transmission for the control channel carrier frequencies in the list. In contrast, power profiling as discussed above with reference to block 103 may include measuring an average energy on a carrier frequency over 1 or more multiframes. Other measures of energy, power, and/or signal strength, however, may be used at blocks 103 and 133. For example, a power profile could be performed at block 133 instead of obtaining a received signal strength indication.

At block 137, the control channel carrier frequency with the highest received signal strength indication is selected, and the normal margin broadcast control channel BCCH for this control channel carrier frequency is read at block 139. In particular, a broadcast control channel message including a location area indication for the selected control channel carrier frequency is read. If the location area indication is successfully read at block 141, the mobile terminal proceeds to obtain communications service at block 143 using the control channel carrier frequency selected at block 137. Otherwise, the mobile terminal reverts to the servicing control channel carrier frequency selected at block 105 and reads the broadcast control channel including the location area indication at block 145. If the location area indication is successfully read at block 147, the mobile terminal proceeds to obtain service using servicing control channel carrier frequency selected at block 105. Otherwise, the mobile terminal proceeds with a search for a disadvantaged carrier at block 117. Please note that block 117 is provided a second time in Figure 5B for ease of illustration. It will be understood that block 117 of Figure 5B is the same as block 117 of Figure 5A.

By obtaining service, the user terminal is thus able to transmit and receive communications with the communications system. In particular, the user terminal may register with the communications system to indicate the spotbeam within which the user terminal is located, and to indicate the corresponding control channel carrier frequency over which the user terminal can receive pages. The user terminal can also "camp on" the control channel carrier frequency for the spotbeam within which the user terminal is located to receive pages from the communications system. In summary, the methods, terminals, and systems of the present invention obtain a list of neighbor control channel carrier frequencies from a servicing control channel carrier frequency, and compare signal strengths of the servicing and neighbor control channel carrier frequencies before obtaining service. Accordingly, a strongest of the servicing and neighbor control channel carrier frequencies can be selected before obtaining service thus saving time in the event that the servicing control channel carrier frequency selected at block 105 is not the optimum control channel carrier frequency.

In contrast, known techniques of selecting control channel carrier frequencies may initially attempt to establish service using a carrier frequency selected using power profiling techniques on carrier frequencies stored in memory without first checking neighbor lists of carrier frequencies. In these known techniques, subsequent spotbeam reselection techniques performed after obtaining or attempting to obtain service may take as long as five minutes to locate a preferred control channel carrier frequency for a neighboring spotbeam. In addition, if a user terminal is unable to establish communications using the carrier frequency selected using power profiling techniques (i.e. the mobile terminal is unable to close the return link), the registration may fail and the mobile terminal may be unable to receive pages until reselection takes place. In an example according to the present invention, a user terminal may be located in an overlap region covered by spotbeams 17A and 17B of Figure 2, such that the user terminal can close the return link (transmit) to the satellite using spotbeam 17A, but with the user terminal being unable to close the return link to the satellite using spotbeam 17B. Moreover, the user terminal may have stored in memory the control channel carrier frequencies for spotbeams 17B, 17E, and 17F as the control channel carrier frequencies most recently used to obtain service. According to this example, the user terminal should select the control channel carrier frequency 17B at block 105 as the initial control channel carrier frequency. Assuming that the initial control channel carrier frequency can be synchronized, that the normal margin synchronization channel can be read, and the list of neighbor control channel carrier frequencies can be read, a carrier list will be created at block 129 including the control channel carrier frequencies for spotbeams 17A, 17B, 17C, 17D, and 17F (the control channel carrier frequency selected at block 105 and the carrier frequencies for the adjacent spotbeams). After obtaining received signal strength indications for each of the carrier frequencies, the control channel carrier frequency for spotbeam 17A should be selected at block 137, and service established using the control channel carrier frequency for spotbeam 17A at block 143. Because signal strengths for each of the control channel carrier frequencies corresponding to the initial and neighboring spotbeams is measured prior to establishing service, service is initially established with spotbeam 17A thus saving time.

In contrast, known techniques may first attempt to establish service using the control channel carrier frequency corresponding to spotbeam 17B. If the mobile terminal is unable to close the return link with the satellite or base station, however, the registration information for the user terminal may be erased so the user terminal is unable to receive pages until a spotbeam reselection procedure selects the control channel carrier frequency for spotbeam 17A. As discussed above, the known spotbeam reselection procedure may take as long as 5 minutes to select the control channel carrier frequency for spotbeam 17A thus delaying communications. The methods, terminals, and systems of the present invention can thus reduce the time required to establish service.

The present invention may be embodied as methods or devices. In addition, the invention may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining both hardware and software aspects. The present invention has been described in part with respect to the block diagram of Figure 4 and the flow diagrams of Figures 5A and 5B. It will be understood that each block of the illustrations, and combinations of blocks, can be implemented by computer program instructions. These program instructions, which may represent steps, may be provided to a processor to produce a machine. Accordingly, blocks of the diagrams support combinations of means for performing the specified functions in combinations of steps for performing the specified functions. It will be understood that each block of the illustrations, and combinations of blocks, can be implemented by special purpose hardware-based systems which perform the specified functions or steps, or combinations of special purpose hardware and computer instructions.

In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims

THAT WHICH IS CLAIMED:

1. A method for selecting one of a plurality of control channels at a user terminal in a radio communications system wherein each of the control channels is assigned to a respective geographic area, the method comprising the steps of: selecting one of the control channels providing at least a predetermined level of received power at the user terminal as an initial control channel for the user terminal wherein the initial control channel is assigned to a first geographic area; obtaining a list of neighbor control channels using the initial control channel wherein the neighbor control channels are assigned to geographic areas neighboring the first geographic area; selecting a preferred control channel from the initial control channel and the neighbor control channels; and after selecting one of the control channels, obtaining the list of neighbor control channels, and selecting the preferred control channel, obtaining communications service using the preferred control channel.

2. A method according to Claim 1 wherein the step of selecting the preferred control channel comprises: measuring a received signal strength at the user terminal for each of the initial control channel and the neighbor control channels; and selecting from the initial control channel and the neighbor control channels based on the control channel having the highest received signal strength.

3. A method according to Claim 1 wherein the step of selecting the initial control channel comprises selecting the initial control channel from a group of control channels previously used by the user terminal.

4. A method according to Claim 3 wherein a received power at the user terminal is sequentially measured for the group of control channels with a first measured control channel in the group having at least the predetermined level of received power being selected as the initial control channel.

5. A method according to Claim 1 wherein the step of selecting one of the control channels providing at least a predetermined level of received power at the user terminal comprises selecting one of the control channels having at least an average predetermined power level.

6. A method according to Claim 1 wherein the step of obtaining communications service comprises camping on the preferred control channel.

7. A method according to Claim 1 wherein the step of obtaining communications service comprises registering with the radio communications system to receive pages over the preferred control channel.

8. A user terminal providing communications in a radio communications system wherein each of a plurality of control channels is assigned to a respective geographic area, the user terminal comprising: a transceiver that transmits and receives communications to and from the radio communications system; and a processor coupled with the transceiver wherein the processor selects one of the control channels providing at least a predetermined level of received power at the user terminal as an initial control channel for the user terminal wherein the initial control channel is assigned to a first geographic area, wherein the processor obtains a list of neighbor control channels from the initial control channel wherein the neighbor control channels are assigned to geographic areas neighboring the first geographic area, wherein the processor selects a preferred control channel from the initial control channel and the neighbor control channels, and wherein the processor obtains communications service using the preferred control channel after selecting one of the control channels, obtaining the list of neighbor control channels, and selecting the preferred control channel.

9. A user terminal according to Claim 8 wherein selection of the preferred control channel comprises measuring a received signal strength for each of the initial control channel and the neighbor control channels, and selecting from the initial control channel and the neighbor control channels based on the control channel having the highest received signal strength.

10. A user terminal according to Claim 8 wherein selection of the initial control channel comprises the processor selecting the initial control channel from a group of control channels previously used by the user terminal.

11. A user terminal according to Claim 10 wherein a received power at the user terminal is sequentially measured for the group of control channels with a first measured control channel in the group having at least the predetermined level of received power being selected as the initial control channel.

12. A user terminal according to Claim 8 wherein selection of one of the control channels providing at least a predetermined level of received power at the user terminal comprises selecting one of the control channels having at least an average predetermined power level on the carrier frequency for the control channel.

13. A user terminal according to Claim 8 wherein obtaining communications service comprises camping on the preferred control channel.

14. A user terminal according to Claim 8 wherein obtaining communications service comprises registering with the radio communications system to receive pages over the preferred control channel.

15. A user terminal providing communications in a radio communications system wherein each of a plurality of control channels is assigned to a respective geographic area, the user terminal comprising: means for selecting one of the control channels providing at least a predetermined level of received power at the user terminal as an initial control channel for the user terminal wherein the initial control channel is assigned to a first geographic area; means for obtaining a list of neighbor control channels from the initial control channel wherein the neighbor control channels are assigned to geographic areas neighboring the first geographic area; means for selecting a preferred control channel from the initial control channel and the neighbor control channels; and means for obtaining communications service using the preferred control channel after selecting one of the control channels, obtaining the list of neighbor control channels, and selecting the preferred control channel.

16. A user terminal according to Claim 15 wherein the means for selecting the preferred control channel comprises: means for measuring a received signal strength at the user terminal for each of the initial control channel and the neighbor control channels; and means for selecting from the initial control channel and the neighbor control channels based on the control channel having the highest received signal strength.

17. A user terminal according to Claim 15 wherein the means for selecting the initial control channel comprises means for selecting the initial control channels from a group of control channels previously used by the user terminal.

18. A user terminal according to Claim 17 wherein a received power at the user terminal is sequentially measured for the group of control channels with a first measured control channel in the group having at least the predetermined level of received power being selected as the initial control channel.

19. A user terminal according to Claim 15 wherein the means for selecting one of the control channels providing at least a predetermined level of received power at the user terminal comprises means for selecting one of the control channels having at least an average predetermined power level on the carrier frequency for the control channel.

20. A user terminal according to Claim 15 wherein the means for obtaining communications service comprises means for camping on the preferred control channel.

21. A user terminal according to Claim 15 wherein the means for obtaining communications service comprises means for registering with the radio communications system to receive pages over the preferred control channel.