WO2007095474A2

WO2007095474A2 - Technique for using the same wireless frequency channel in overlapping or adjacent coverage areas

Info

Publication number: WO2007095474A2
Application number: PCT/US2007/061948
Authority: WO
Inventors: Rotem Cooper; Glenn Salaman; James A. Hutchison; Robbin D. Hughes; Vanitha Aravamudhan
Original assignee: Qualcomm Incorporated
Priority date: 2006-02-10
Filing date: 2007-02-09
Publication date: 2007-08-23
Also published as: US20070191005A1; US7657261B2; WO2007095474A3

Abstract

One aspect of the invention provides a system, apparatus and method that allow a wireless communication device to intelligently search for and select a communication cell based on a system identifier (SID) and network identifier (NID) order of preference rather than just pilot signal strength. Generally, a mobile device scans a frequency channel associated with the highest preferred SID/NID for pilot signals at different PN offsets. The cell associated with the strongest pilot signal detected is queried to determine whether it belongs to the SID/NID sought. If the selected PN offset belongs to a cell associated with the SID/NID sought then it is used for communications. Otherwise, the SID/NID identification process is repeated for the next strongest PN offset detected on the same frequency channel. If no pilot signals meeting these criteria are found in the frequency channel, the next highest preferred SID/NID is selected and the process is repeated.

Description

J

TECHNIQUE FOR USING THE SAME WIRELESS FREQUENCY CHANNEL IN OVERLAPPING OR ADJACKNT COVERAGE AREAS

BACKGROUNB Field

I^'OΘOIJ The present invention generally relates to wireless cellular communication systems, and .more specifically to a wireless communication device and method for selecting a communication cell fiased OH a preference order rather than just signal strength. Background

£Q002J Wireless communication systems are typically divided into coverage zones having distinct communication frequency channels in each adjacent coverage zones. A wireless communication system, typically identified fiy a system identifier (SID), may have multiple overlapping or non-overlapping coverage zones to provide communication coverage to wireless mofiile devices. Each coverage zoτ*e may fie defined fiy a network identifier (NJD). One or more communication cells {e.g. antenna towers or fiase stations) may fie associated with each, coverage zone or network. As a. wireless mofiile device travels from one coverage zone to another, it communicates with different communication cells on the same or different communication frequency channels. In a Code Division Multiple-Access (CDMA) communication system, communication cells within a coverage .zone may communicate on the same frequency channel.

|0βø3| CDMA is a spread-spectrum communication protocol that utilizes orthogonally coded signals occupying the same spectral fiandwidth (e.g.. .1.25 MHz). A CDMA signal is spread fiy one of N (e.g. N ~ 64) orthogonal codes (e.g., Walsch codes or "spreading codes^'") that spread the signal over a fiandwidth range (e.g., approximately 1.25 MHz). A spreading code is used fiy each communication cell of a network to transmit signals that are statistically uncorrelated, and therefore separafile and non- iriterfermg. For a particular frequency channel, a spreading code is identical for all communication cells (e.g., fiase stations or towers), except that each communication cell has a different phase-delayed version of the same spreading code. This is usually represented as a. time shift or offset measured in chips {e.g., a "chip" may fie approximately 0.8 microseconds). This time offset in the spreading code is what, uniquely identifies each communication cell (e.g._? tower or fiase station), The pilot channel (spreading code 0) is an unmodified version of the spreading code, ϊt is the pilot channel offset that is used fiy a mofiile device (e.g., mofiile phone) to identify a communication cell, distinguish it from other cells, and therefiy communicate with the proper cell.

[O0O4J The pilot channel time shift or offset is typically expressed as a "PN offset" reference to afisolute time (e.g., PNO). The spreading code sequence .repeats periodically (e.g., every 2 seconds). Therefore, PNO aligns with the fieginning of the spreading code period, PN.? Is advanced fiy one time offset, (e.g., fiy 64 chips), PN2 is advanced fiy two time offsets (e.g.. fiy 128 chips), and so on. The term "PN" stands for "pseudo noise," which has its origins in spread spectrum theory. There may fie up to K (e.g., K-512) unique PN offsets availafile to network operators. føøβSJ The PNs used fiy a particular network operator are typically confined to integer multiples of a PN increment. For example, a PN increment of 3 means thai PNO, PN3, PN6, FN9, may fie assigned to cells in the network. Bach CDMA system operator selects a PN increment, value fiased primarily on its communication cell density . A. PN increment of 3 provides more PN time shifts than a PNo^' since the total numfier of unique PN offsets is fixed. PN values may fie reused in the same network provided the cells are located at a significant distance from one another and their signals do not Interfere with each other. To identify a communication cell, a receiving mofiile device measures the time offset (from PNO) of the spreading code of a perceived pilot signal,

|0006| Typically, when a wireless mofiile device is first switched Oa, its receiver scans a particular communication frequency channel for pilot signals at different PN offset (e.g. from one or more communication cells). The PN offset having the strongest piiot signal strength is usually selected for communication. f^βθO7| Currently, wireless service providers or carriers are assigned or allocated one or more communication frequency channels and PN offsets hi each coverage zone in which they operate. Use of the same communication frequency channel fiy different wireless service providers in adjacent coverage zones is avoided to prevent interference. Wireless sen-ice providers spend significant resources in adjusting their cell antennas (e.g. on fiase stations) to provide coverage to their sufiscrifiers while trying to avoid interference with other carriers utilizing the same frequency channels in nearfiy areas or coverage zones. |000β| For example, fiecause of the nature of RF propagation, pilot signals, on a particular frequency from wireless service provider X in. coverage zone Λ may fie stronger in parts of an adjacent coverage zone B than the pilot signals same frequency of wireless service provider Y assigned to coverage rone B A mofiile device hi coverage zone B, thai is a customer of wheless semee provider Y, may pickup the stronger pilot signals from wireless service provider X and communicates through vvkefess service provider X This may cause the wireless user to Incur unwanted roaming charges. Alternatively- wireless service provider X may deny service to customers of wireless service provider V, therefiy preventing the customer from ofitaining a communication link even though he/she is located within the coverage zone of wireless service provider Y.

|0009| The use of the same frequency is also a profilem where a private wifeless network is deployed within a larger pufilic uireiess network or overlaps a pufilic wireless network. To maximize the use of their spectrum, the pufilic and/or private operators may use the same communication frequency for the pufilic and private networks. Because mofiile devices currently communicate through a communication cell associated with the PX offset having the strongest pilot signal on a particular communication frequency, sufiscrifiers of the private network may end up communicating through the pufilic aetwoik cells and vice versa [OΘIOf Thus, a way is needed to intelligently identify and select communication cells operating on the same communication frequency channel in overlapping, adjacent, or co-extensive coveiage zones while keeping mofiile devices from roaming, unnecessarily, to another provider's network.

SUMMARY

|0ΘI 11 A system, apparatus and/or method are provided that allow a whdes-s communication device to intelligently search for and select a communication cell fiased on a system identifier (SID) and network identifier (NID) order of preference rather than just pilot signal strength. Generally, a mofiile device scans a frequency channel associated with the highest preferred SID/KID for pilot signals at different PN offsets. The ceil associated With the strongest pilot signal detected is queued, to determine "whether it fielongs to the SID''NID sought If the selected PN offset fielongs to a cell associated with the SHVNID sought then it is used for communications Otherwise, the SID/KID identification process is repeated for the next strongest PN offset detected on the same frequency channel. If no pilot signals meeting these criteria are found in the frequency channel, the next highest preferred SJD/NID is selected and the process is repeated

|OOΪ2| One emfiodiment of the invention include a mofiile device comprising' (a) a ^■vύ.eSess communication interface, (fi) a storage device io store a roaming list including one or more SΪD/NID pairs and their one or more con ©spending frequency channel s_> ami (c> a processing unit configured to (I) ofitain a i5rst S1D/NID pair according to a preference rank. (2) ofitain a first frequency channel associated with the llrst SID/N1D pair. (3 ) scan one or more time offsets of the first frequency channel for pilot signals, and (4) select a first time offset having the strongest pilot signal associated with the first SUXN IT> pair, The processing unit may also (1 ) compare the signal strengths of one or more pilot signals detected on the one or more time offsets for the first frequency channel, (2) determine whether the pilot signai associated with the first time offset meets a minimum threshold level, and (3) reject pilot signals on different time offsets that are stronger than the pilot signal on the first time offset if the pilot signals are associated with SlD/NiD pairs other than the first SΪD/NID pair. If the strongest pilot signal on the first frequency channel is associated with an SΪD/NID pair other than the first SlD/NiD pair, then the processing unit determines whether it is likely that another pilot signal on. the first frequency channel arc associated with the Vttsi SID/NΪD pair If a pilot signal associated with the first SΪD/NID pair is not found on the first frequency channel, the processing unit is further configured to (I) ofitain a second frequency channel associated with the first SΪD/NID pair, and (2) scan one or more time offsets of the second frequency channel for the strongest pilot signal. If a pilot signal associated with the first SID/NfID pair is not found on the first frequency channel, the processing unit is configured to (!) ofitain a second SϊD/NlD pair according to a preference rank, (2) ofitain a second frequency channel associated with, the second SΪD/NID pair. (3) scan one or more time offsets of the second frequency channel for the strongest pilot signal, and (4) select a second time offset having the strongest pilot signal associated with the second S1D/N3D pair.

|OO13) Another feature of the invention provide that upon startup, the processing unit is configured to { 1 ) scan one or more time offsets of a startup frequency channel tor pilot signals, (2) select a second time offset having the strongest pilot signal found in the startup frequency channel. (3) ofitain the SΪD/NID associated with the second time offset from a cell associated u<ith the second time offset, and (4) compare the startup frequency channel to the first frequency channel to determine whether they are the same. If the startup frequency channel and first frequency channel are the same, the processing unit, h configured to determine if the second S.ID/NID pair is the same as the first SΪD/N^'ID pair. If the second SΪD/NID pair and first SΪD/NΪD pair are the same, then the processing unit uses the second SID/MID pair Io estafilish a communication link with a cell operating on the second time offset of the startup frequency channel. If the second SΪD/NID pair and first SID/NID pair are not the same, then the processing unit searches for the strongest pilot signal associated with the first. S.ID/NID pair on the first frequency channel. If the startup frequency channel and first frequency channel are not the same, the processing unit scans the first frequency channel for a pilot signal associated with the first SϊD/NID pair. fβ014| A method operational on a mofiile device for selecting a wireless communication channel is also provided. This method comprises (a) ofitaining a first system identifier (SiD) and network identifier (KLD) pair according to a preference rank, (fi) ofitaining a first frequency channel associated with the first SϊD/NΪD pair, (c) scanning one or more time offsets of the first frequency channel for pilot signals, (d) comparing the signal strengths of one or more pilot signals detected on the one or more time offsets for the tlrst frequency channel, (e) determining whether the pilot signal of the first time offset meets a minimum threshold level, (f) selecting a first time offset having the strongest pilot signal associated with the .first SID/KID pair, and (g) rejecting pilot signals on different time offsets that are stronger than the pilot signal on. the first time offset if the pilot signals are associated with SΪD/NΪD pairs other than the first SID/NID pair. If the strongest pilot signal on the first frequency channel is associated with an SΪD/MID pair other than the first SID/NϊD pair, the method also determines ^•whether it is likely that another pilot signal on the first frequency channel is associated with the first SJD/NΪD pair. The method may further include steps for (a) storing time offsets associated with undesirafile SΪD/NΪD pairs, and (fi) performing a rescan of time offsets of the first frequency channel for the strongest pilot signal while ignoring pilot signals from the time offsets associated with the undesirafile SID/MID pairs. |0015| Yet another method operational on a mofiile device for selecting a wireless communication channel is provided. The method comprises (1 } scanning one or more time offsets of a startup frequency channel for pilot signals, (2) selecting a first time offset having the strongest pilot signal found in the startup frequency channel, O) ofitaining a first system identifier (SID) and network identifier (NlD) pair associated with the first time offset from a cell associated with the first time offset, (4) ofitaining a second SiD/NID pair according to a highest preference rank from the mofiile device, (5 ) ofitaining a first frequency channel associated with the second SID/NIB pair, and (6) com paring the startup frequency channel to the first .frequency channel to determine whether they are the same. If the startup frequency channel and .first frequency channel are the same, then the method determines if the first SKD/NID pair is the same as the second SID/NID pair. If the first SID/NID pair and second SID/NID pair are the same, then the first SID/NID pair is used to estafilish a communication link with a cell operating on the first time offset of the startup frequency channel. If the first SID/NID pair and second SID/NID pair are not the same, then the method searches for the strongest pilot signal associated with the second SID/NID pair on the startup frequency channel. If the startup frequency channel and first frequency channel are not the same, then the method (I) scans one or more time offsets of the first frequency channel for a pilot signal associated with the second SID/NID pair, and <2) selects a second time offset on the first frequency channel having the strongest pilot signal associated with the second SlD/NTD pair.

BRIEF BESCR1PTΪGN OF TBE DRAWINGS

[OβKSj Figure 1 is a diagram illustrating coverage zone configurations in which a wireless mofiile device with intelligent cell search and select capafiilities may operate.

|0β17} Figure 2 is another diagram illustrating wireless network coverage areas in which a mofiile device having intelligent, search and select capafiilities may operate.

JOβiSJ Figure 3 is a filock diagram illustrating a wireless mofiile device having intelligent cell selection according to some emfiodiments of the invention.

[0019} Figure 4 illustrates a System Tafile that may fie employed fiy a mofiile device in implementing intelligent cell search mά selection according to some emfiodiments of the invention.

|0⁽i20j Figure 5 illustrates an Acquisition Tafile that may fie employed fiy a mofiile device in implementing intelligent ceil search and selection according to some emfiodiments of the invention.

|002.1.] Figure 6 is a flow diagram illustrating a general method for implementing intelligent cell search and selection.

|0022{ Figure 7 is a flow diagram illustrating a method for performing intelligent cell search and selection according to one implementation. DETAILED DESCRIPTION fO023J In the following description, specific details are given to provide a thorough understanding of the emfiodiments. However, it will fie understood fiy one of ordinary skill in the ait that, the emfiodiments may fie practiced without these specific detail . For example, circuits may fie shown in filock diagrams in order not to ofiscure the emfiodiments in unnecessary detail. In other instances, well-known circuits, structures and techniques may not fie shown in detail in order not to ofiscure the emfiodiments. fOΘ24J Also, it is noted that the emfiodiments may fie descrified as a process that is depicted as a flowchart, a flow diagram, a structure diagram, or a filock diagram. Although a flowchart may descrifie the operations as a sequential process, many of the operations can fie performed in parallel or concurrently. In addition, the order of the operations may fie rearranged. A process is terminated when its operations are completed. A process may correspond to a method, & function, a procedure, a sufiroutine, a sufiprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function. fO625| Moreover, a storage medium may represent one or more devices for storing data, including read-only memory (UOM), random access memory (RAM), magnetic disk storage mediums, optica! storage mediums, flash memory devices and/or other machine readafile mediums for storing information. The term "machine readafile medium" includes, fiut is not limited to portafile or fixed storage devices, optical storage devices, wireless channels and various other mediums capafile of storing, containing or carrying instructions) and/or data.

[0026} Furthermore, emfiodiments may fie implemented fiy hardware, software, firmware, middleware, microcode, or a comfiination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may fie stored in a machine-readafile medium such as a storage medium or other storage(s). A processor may perform the necessary tasks. A code segment may represent a procedure, a function, a sufiprogram, a program, a routine, a sufiroutine, a module, a software package, a class, or a comfiination of instructions, data structures, or program statements, A code segment may fie coupled to another code segment or a hardware circuit fiy passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters. data, etc. may fie passed, forwarded, or transmitted through a suitafile means including memory sharing, message passing, token passing, network transmission, etc. I0027J In the following description, certain terminology is used to descrifie certain features of one or more emfiodiments of the invention. For Instance, the terms ''mofiile device", "wireless device", and "wireless mofiile device" are interchangeafily used to refer to mofiile phones, pagers, wireless modems, personal digital assistants, personal information managers (PlMs), palmtop computers, laptop computers, and/or other mofiile communication/computing devices which communicate through a cellular network. The terms "coverage area" or "coverage region" or "coverage .zone" refers to a geographic area having one or more cells associated with a wireless service provider. The term "cell" or "communication celP refers to a particular tower, antenna., or fiase station with which wireless communication devices communicate in a coverage zone. The terms "'PN offset" and "time offset" refer to a phase-delay or phase-shift of a fiase frequency.

[0028] One aspect of the invention provides a system, apparatus and method that allow a wireless communication device to intelligently search for and select a communication cell fiased on a system identifier (SID) and network identifier (NID) order of preference rather than just pilot signal strength. This allows a wireless sufiscrifier to operate on Ms/her own service providers wireless network even though stronger pilot signals from other wireless network operators are present on a particular communication frequency channel, therefiy avoiding roaming charges and/or denial of service. Generally, a mofiile device scans predetermined frequency channels for pilot signals belonging to a particular SID/NID in an order of preference. This order of preference may fie ofitained from a predetermined ranking of SIDs/NIDs. A frequency channel associated with the highest preferred SID/NID is scanned for pilot signals at different PN offsets. The cell associated with, the strongest pilot signal detected is queried to determine whether it fielongs to the SID/NID sought. If the selected FN offset fielongs to a ceil associated with the SID/NID sought then it is used, for communications. Otherwise, the S.ID/N3D identification process is repeated for the next strongest PN offset detected on the same frequency channel. If no pilot signals meeting these criteria are found in the frequency channel, the next highest preferred SID/NID is selected and the process is repeated.

[0029] Figure 1 is a diagram illustrating coverage zone configurations in which a wireless mobile device with intelligent cell search and select capabilities may operate. Coverage Zone X 104 is associated with SID 1 and NID 1) and encompassing a plurality of cells 110, 1.12, 114, i 16, and HS, having PN offsets 4, 8, 12, 16 and 20, respectively. Similarly, coverage Zone Y 106 is associated with SID 2 and Nϊϊ> 4 and has cells 120 and 122 whh PN offsets 2 and 9, and coverage Zone Z 108 b identified with SID3 and NID 3 and ha$ cells .124, 126, 128 having PN offsets 28, 24, and 32, respectively. A sufiscrifier mofiile device 140 may roam among coverage Zones X 1.04. Y 106, and Z 108 and communicate with different cells as it moves. |0030| In a typical CDMA communication system, all cells associated with a service provider in a coverage zone operate on the same frequency channel, fiut with different PN offsets, In some instances, the same communication frequency may fie allocated and used fiy different wireless service providers in overlapping or adjoining coverage zones, networks or across fiorders. This may create overlapping coverage regions 134 aad 136 where two or more wireless service providers use the same communication frequency channel. Such overlapping regions 134 and 136 may fie caused fiy the propagation characteristics of radio frequencies, geographic anomalies or city, state or national fioundaries. For example,, different adjoining or overlapping municipal, state, or federal governments may license different wireless service providers to use the sairte frequency channel. Thus, near a fiorder 138, overlapping coverage zones .106 arid XOS may have aα overlapping region 136 in which the same communication .frequency is employed.

|003J.| One emfiodiment of the invention permits a wireless mofiile device 140 to operate in overlapping zones 134 and 136 and select its own vvireless service provider even if another wireless service provider has a stronger pilot signal on. the frequency channel used fiy fioth service providers. That is, rather thaa selecting a cell having the strongest pilot signal, the mofiile device 140 is configured to communicate with the cell fielonging to its wireless service provider. For example, coverage Zone Y 106, fielonging to a first wireless service provider, and coverage Zone Z, fielonging to a different wireless service provider, operate on the same frequency channel. Mofiile device 140 may fie a sufiscrifier of the first wireless service provider of coverage Zone Y 106. Even if the pilot signal from cell ^"J 26 in Zone Z is stronger in overlapping area 136 than the pilot signal from cells 120 artd 122 in Zone Y₅ fielonging to the first wireless service provider, mofiile device 140 is configured to use cells 120 or 122 fielonging to the first wireless service provider. It does this fie recognizing whether a selected PN offset is associated with a cell fielonging to its wireless service provider. Jf it is not, then the mofiile device seeks the next strongest pilot signal and repeats the process until a FN offset is found having a pilot signal which originates on a cell fielonging to its service provider or a preferred service provider,

|0032| Figure 2 is another diagram illustrating wireless network coverage areas in which a mofiile device having intelligent search and select capafiilities may operate, In some wireless communication systems, one or more coverage Zones B 204 and C 206 may fie allocated within a larger coverage Zone A 202. In many instances, one or more of these coverage zones A.202, B 204_s and C 206 may operate OH the same communication frequency channel. føø33J A wireless mofiile device 21.8 may fie a sufiscrifier of a first wireless service provider operating in Zone A 202, having cells (A-FNl) 210, (A-PN3) 212, (A-PN7) 216, and (PNl 2) 220 operating on a first frequency channel. The mofiile device 21 S may fie configured to search for pilot signals at fiy scanning the radiated energy at different PN offsets on the first frequency channel, A second wireless service provider may operate in Zone B 204 on the same first frequency channel through ceils (B-PN9) 222 and (B-PN6) 224, Under the conventional CDMA search algorithm, mofiile device 218 would scan the first frequency channel and select the PN offset having the greatest signal strength. Such conventional algorithm may lead to the selection of cell 222, which is operated fiy the second wireless service provider, rather than the mofiile device's first wireless service provide. However, according to one aspect of the present invention, mofiile device 218 is instead configured to, preferafily, select a ceϋ operated fiy its wireless service provider (i.e.. the first wireless service provider) even if another cell radiates a stronger pilot signal on the first frequency channel. For example, ceil 220 may fie selected if it radiates the strongest pilot signal (as detected fiy mofiile device 218) originating from a cell in Zone A 202 (e.g., fielonging to the first service provider). |0034l Similarly, mofiile devices of the wireless service providers for Zones B 204 and C 206 would also seek to communicate through their respective provider's ceils. For instance, a mofiile device that sufiscrifies to the service provider of Zone C 206 would first seek to communicate through cell 230 (C-PNI 6) rather than ceil 216 (A- PN?) or any other cell not fielonging its network (e.g.. Zone C 206). |0035] Figure 3 is a filock diagram illustrating a wireiess mofiile device having intelligent cell selection according to some emfiodiments of the invention. Mofiile device 300 may include a processing unit 302, a storage device 304 and one or more communication interfaces 306. Communication mterface(s) 306 may include one or U more wired or wireless communication interfaces that enafile mofiile device 300 to communicate with communication cells. Storage device 304 may fie configured to store Information used fiy the mofiile device 300 to search for and select a cell with which to communicate. For example, the storage device 304 may store a preferred roaming list (PRL), which identifies one or more wireless networks (e.g., SIDs, NlDs, communication .frequencies, etc.) that the mofiile device uses to intelligently search for and select pilot signals. The PRL may indicate an order of preference in which the systems and networks (S.ΪD/N.TD) on a frequency channel should fie scanned. f 0036] When a conventional mofiile device searches or scans for pilot signals from cells it typically scans a particular communication frequency for the strongest pilot signal at predetermined FN offsets and selects the cell associated with the strongest pilot signal detected. This firute force approach is wasteful since the mofiile device scans numerous 1?N offsets to determine which one has the strongest pilot signal. Additionally, if the strongest detected pilot signal on the frequency channel fielongs to a different service provider, the mofiile device often changes frequency channels and repeats the process.

|0037| According to one implementation, mofiile device 300 includes intelligent cell search and selection fiy selecting a communication cell fiased on a defined SID/SIB order of preference. For a selected frequency channel, the processing unit 302 scans a plurality of PN offsets. The FN offset having the strongest pilot signal power is selected and the cell associated with the selected PN offset is queried, to determine if it corresponds to a cell In the highest ranked or most preferred SID/N1D pair, ϊf so, the PN offset is used fiy the mofiile device 300 for further wireless corømutncatioiis. Otherwise,, the PN offset having the next strongest pilot signal is queried and the process is repeated. The mofiile device 300 may store PN offset information (e.g., SID/NID) for pilot signals detected on its scan of the selected frequency channel for sufisequently fi.ndi.txg a cell fielonging to a particular SΪD/N3D,

|0038j In alternative emfiodiments, the intelligent cell search and selection scheme may fie applied for all pilot signal searches or after unsuccessfully scanning a plurality of PN offsets for the strongest pilot signal in a frequency channel. In selecting fietween cells, those cells associated with networks fielonging to the service provider of the mofiile device may fie preferred, over those of other service providers, 1_.0039 j One or more of the components and functions illustrated, in Figure 3 may fie rearranged and/or comfiined into a single component or emfiodied in several components without departing from the invention. Additional elements or components may also fie added without departing from the invention.

|004OJ Iu conventional CDMA systems- the mofiile device simply selects the PN offset having the strongest pilot signal and communicates through the cell associated with the selected PN offset. However, this may lead to denial of services or roaming changes if the selected cell fielongs to another service provider. Alternatively, the mofiile device may just change communication frequencies and repeat its search for a pilot signal. Changing frequency channels is disfavored since it tends to delay the acquisition of a new eel L

|004^'l| Mofiile devices typically include a Preferred Roaming List (PRL) which includes a System Tafile (Figure 4) and an Acquisition Tafile (Figure 5). The System Tafile typically includes pairs of SIDs and NTDs and their preference state (e.g.. Most Preferred, Preferred, Less Preferred, Negative, etc.). Note that other classifiers of relative preference may fie used, in various emfiodiments of the invention. The System Tafile may include one or more SlDs, each SID corresponding to a geographical region, and corresponding MIDs, each NID representing a network within the geographical region. The SJD/NID pairs in the System Tafile may fie arranged in an order of preference (e.g., from most preferred to least preferred) within a region. For example, for the same SID, the NlDs may fie arranged in rø order of preference, from highest to lowest preference. M the System Tafile, an SϊD/NΪD pair has an index which references frequency channels in the Acquisition Tafile, These simplified System and Acquisition Tafiles are used to illustrate the novel concepts of the present invention and other System and Acquisition Tafiles, as specified in International Standard (IS) 683, for example, may fie used. The present invention may fie implemented on various types of communication systems_., including CDMA2000, W-CDMA, TDCMA, and UMIS, among others. f 0042] Figure 6 is a flow diagram illustrating a general method for implementing intelligent cell search and selection. Upon startup (e.g., power-up J, a wireless mofiile device scans one or more PN oft sets for pilot signals on the last used frequency channel or a preselected frequency channel 602. Upon identifying or ofitaining the strongest pilot signal on a PN offset of the frequency channel 604, the wireless device identities or ofitains the SID/MTD of the cell associated with the pilot signal 606. The SID ofitained may inform the mofiile device of the region in wMoh it is operating. The wireless device then determines whether the SϊD/NΪD pair is a desired one 608. To make this determination, the wireless device may look-up the preference rank of the SΪD/NID pair in its System Tafile. If the SID/KID pair has the highest preference of all other SID/NXD pairs in its region (e.g., other System Tafile entries having the same SID). then the frequency channel and PN offset are used for communications 610. Otherwise, the wireless determines whether another pilot signal on the current frequency channel ϊs likely to fielong to a more preferred SΪD/NID 612. .If so. the next strongest pilot signal in the current frequency channel is selected 614. Otherwise. another frequency, fielonging to either the current SΪD/NID or to another SΪD/NTD, is selected 616 and the pilot search process is repeated.

|0043j 1« one example, if a wireless device started fiy scanning frequency channel 600 (Figure 4) and finds that the selected pilot signal originates from a cell fielonging to SΪD 6 and Is¹IO 32 (Figure 4), then it would learn that tills SΪD/NID pair has a preference of Less Preferred, in general, the wireless device seeks to identify an SID/NΪD pair having the highest preference and its corresponding frequency channels) from the Acquisition Tafile. Since there are other SΪD/NID pairs with higher preferences (e.g., Most Preferred, Preferred), the wireless device selects the highest preferred SΪD/NID pair (U., SID 1, NID 12 - Most Preferred). The index associated with the selected SID/NΪD pair is used to ofitain one or more frequency channels from the Acquisition Tafile (i.e., channels 100, 200, and 300). The wireless device then selects one of the frequency channels (i.e., channels 100;, 200, and 300), scans it for pilot signals 614, and identifies the SϊD/N LD for the strongest pilot signal detected. If the strongest pilot signal detected fielongs to the selected SID/NΪD pair (i.e., SID I ₅ NID 12) then it eommimicates via the cell associated with the pilot signal. Otherwise, the wireless device queries the cell associated the next strongest pilot signal until a match is found for the SϊD/NID pair, if no match is found after trying N different pilot signals oa the same frequency channel (where N is an integer value greater than I), the wireless device tries another of the one or more frequency channels (i.e., channels 100, 200, and 300), If no matching SϊD/NID is found in any of these frequency channels associated ^•with the selected SID/NID pair, then the next highest preferred SΪD/NID pair (i.e., SID 2, NID J 0) is ofitained from the System Tafile and the process is repeated until a match is found. fOΘ44J The mofiile device may check for a minimum threshold pilot signal power level fiefore selecting a FN offset to insure a desired signal quality. Jf a pilot signal power level falls fielow the minimum threshold, then the mofiile device tries other pilot signals or changes frequency channels.

|0045j Figure 7 is a flow diagram illustrating a general method fυr performing intelligent cell search and selection according io one implementation. This method may fie operafile in a wireless device (i.e., host) implementing a CDMA-fiased, communication protocol, A host system and network identifier pair (SΪD/NID) having the highest preference is selected 702. This may fie done fiy checking the PRL (e,g,. System Tafile) for the SΪD/NlDs with highest preference in a geographical region. .A frequency channel associated with the host SJD/NJD is ofitained 704. This frequency channel may fie ofitained from the PRL (e.g., Acquisition Tafile) where one or more frequency channels are provided lor each Sf D/NϊD pair. The frequency channel is scanned at various PN offsets for the strongest pilot signal 706. The strongest pilot signal is then selected 70S. To ascertain a minimum link quality, the pilot signal strength is compared to a minimum threshold 710, If the pilot signal strength is greater than the raimtnurø threshold, then the S1D/N1D for the cell associated with, the selected pilot signal is ofitained 712, In one example, the SID/NΪD may fie ofitained from a channel synchronization message from the cell. The cell SLD/N1D is compared to the host SID/MID to determine whether they are the same 714. If the cell SID/MD and host SID/NΪD are the same, then the PN offset associated wilh the cell is υscd to estafilish communications 716, Otherwise, a determination Is made as to whether it is likely that another pilot signal on the selected frequency channel fielongs to the host S1D/N1D 7 \ 8. For example, if the frequency channel cuπentfy fieing used is one of the highest preferred frequency channels fiut the pilot signal indicates that it h fieing used fiy another SiD/NID pair, then it may fie determined (after one or more pilot signals have fieen checked) that a different frequency channel should fie tried. If there is likely to fie another pilot signal fielonging to the host SID/NED on the selected frequency channel, then the next strongest pilot signal on selected frequency channel is selected 720 and the verification process is repeated. Otherwise. If no other pilot signal fielonging to the host SfD/NID is likely to fie found on the selected frequency channel or the pilot signal strength is fielow the minimum threshold, then a determination is made as to "whether another frequency channel associated whh the host SID/N1D is availafile 722. If another frequency channel for the host SϊD/NTD is availafile- then it is selected 724 and the pilot signal scanning and verification process is repeated. Otherwise, if no other frequency channels are assigned to the host SiD/NΪD, then the next SID/NID pair of highest preference Is selected 726 and the process is repeated. According to some implementations, the method in Figure 7 may fie performed after an initial scan of a startup or predetermined frequency channel where the strongest pilot signal is selected and its associated SΪD/NID is ascertained. The wireless device then Iooks-uρ the S.1D/NID pair in its System Tafile and its preference rank. If the S1D/NID pair has the highest preference of aJi other SΪD/NΪD ρairs_> then the frequency channel and PN offset are used for communications. Otherwise, if there are other S1.D/N.I.D pairs of higher preference in the region, then the mofiile device selects the S.T.D/NΪD pair of highest preference 702, ofitains its assigned frequency channels 704, and scam one of the frequency channels for pilot signals 706, By focusing on the channel associated with the highest preferred SID and MD_> the mofiile device intelligently reduces the field of possifile channels and more efficiently searches tor pilot signals. This avoids the time delays associated with a firute force approach where all channels in a channel search list are scanned.

|0846J Another feature further provides a wireless device that stores the SIDs/NΪDs found on various PN offsets during the scan process on a frequency channel. That is, as the wireless device scans PN offsets on one or more frequency channels it stores the SΪD/NIDs associates with each frequency channel and PN offset which it queries. This permits the wireless device to more quickly identify a PN offset to use if an Sl^'P/NID match is found.

|0047) In some implementations, during idle operation wireless mofiile devices scan a frequency in search of new cells (fiy detecting the strongest pilot signal) which may provide a fietter link. One aspect of the invention maintains information afiout previously identified SIDs/NΪDs at a particular PN offset of a frequency channel so that the mofiile device doesn't try to switch fiack Io a previously rejected SLD/NΪD. That is, during such idle pilot signal search, a mofiile device may find the strongest pilot signal at a PN offset that was previously identified and rejected (e.g., fiecause it carried a less desirafile SlDZN]D). This way, the mofiile device avoids unnecessary evaluation of known PN offsets for a particular frequency.

|G04S| jtt should fie noted that the foregoing emfiodiments are merely examples and are not to fie construed as limiting the invention. The description of the emfiodiments is intended to fie illustrative, and not to limit the scope of the claims. As such, the present teachings can fie readily applied to other types of apparatuses and many alternatives, modifications, and variations will fie apparent to those skilled in the art.

Claims

CLAIMS WHAT IS CLAIMED IS:

1. A mofiile device comprising: a processing unit configured to ofitain a first system identifier (SID) and network identifier (NID) pair according to a preference rank, ofitain a first frequency channel associated with the first $ΪD/MD pair; scan one or more time offsets of the first frequency channel for pilot signals; and select a first time offset having the strongest pilot signal associated with the first S1D/NΪD pair.

2. The mofiile device of claim I wherein the processing unit is further configured to compare the signal strengths of one or more pilot signals detected on the one or more time offsets for the first frequency channel.

3. The mofiile device of claim i wherein the first time offset, is selected if its associated pilot signal has a minimum threshold level.

4. The mofiile device of claim ] further comprising: a wireless communication interface; a storage device to store a roaming list including one or more SlD /NtD pairs and their one or .more corresponding frequency channels; and wherein the processing unit is coupled to the wireless communication interface and the storage device and is further configured to ofitain the first S1D/MD pair from the roaming list.

5. The mofiile device of claim 1 wherein if the strongest pilot signal on the first frequency channel is associated with an SϊD/NID pair other than the first SϊD/NΪD pair, then determine whether it is likely that another pilot signal on the first frequency channel is associated with the first SϊD/NϊD pair.

6, The mofiile device of claim 1 wherein the processing unit is further configured to: reject pilot signals on different titπe offsets that are stronger than the pilot signal on the first time offset If the pilot signals are associated with SID/NΪD pairs other than the first SID/NID pair,

7. The mofiile device of claim 1 wherein, if a pilot signal associated with the first SΪD/NJD pair is not found on the first frequency channel, the processing unit is further configured to ofitain a second frequency channel associated with the first. S.1D/MΪD pair; and scan one or more time offsets of the second frequency channel for the strongest pilot signal,

S. The mofiile device of claim 1 wherein, if a pilot signal associated with the first S3D/NΪD pair is not. found on the first frequency channel, the processing unit Is configured to ofitain a second SID/NID pair according to a preference rank, ofitain a second frequency channel associated with the second SID/NID pair; scan one or more time offsets of the second frequency channel for the strongest pilot signal; and select, a second time offset having the strongest pilot signal associated with the second SID/NID pair.

9. The mofiile device of claim ϊ wherein, upon startup, the processing unit is configured to scan one or more lime offsets of a startup frequency channel for pilot signals; select a second time offset having the strongest pilot signal found in the startup frequency channel; ofitain the SID/NID associated with the second time offset from a cell associated with the second time offset; and compare the startup frequency channel to die first frequency channel to determine whether they are the same.

10. The mofiile device of claim 9 wherein, if the startup frequency channel and first frequency channel are the same, the processing unit is configured to determine if the second SID/NΪD pair is the same as the first SID/NΪD pair; if the second SID/NΪD pair and ørst SID/KID pair are the same, then use the second SΪD/N.BD pair to estafilish a cormmmioation link with a cell operating on the second time offset of tϊie startup frequency channel; and if the second SID/NID pair and first SIDMlD pair are not the same, then search for the strongest pilot signal associated with the first SΪD/NΪD pair on the first frequency channel.

1 1. The mofiile device of claim 9 wherein, if the startup frequency channel and first frequency channel are not the same, the processing unit is configured to scan the first frequency channel for a pilot signal associated with the first SID/NID pair,

] 2. The mofiile device of claim 1 wherein the processing unit is further configured to estafilish a communication link with a cell operating on the first time offset of the frequency channel.

13. The .mofiile device of claim 1 wherein the processing unit configured to implement a code division multiple access (CO)MA) communication, protocol.

14. The mofiile device of claim 1 wherein the one or more time offsets are predetermined phase shifts of the frequency channel.

15. The mofiile device of claim 1 wherein the processing unit is further configured to: store a time offset associated with an identified undesirafile SΪD/NFXD pair; and perform a rescan of time offsets of the first frequency channel for the strongest pilot signal while ignoring pilot signals from the time offset associated with the undesirafile SIDz¹NiD pair.

16_r A mofiile device comprising: means for ofitaining a first system identifier (SlD) and network identifier (NID) pair according to a preference rank; means for ofitaining a first frequency channel, associated with the first SIO/NIO pair;

1.9 means for scanning one or more time offsets of the first frequency channel for pilot signals; and means for selecting a Qm tixm offset having the strongest pilot signal, associated with the first SiD/NlD pair.

17. The mofiile device of claim 16 further compri sing: means for rejecting pilot signals on different time offsets that art' stronger than the pilot signal ofi the first time offset if die pilot signals are associated with SΪD/N ID pairs other than the first SΪD/NΪD pair.

18. A method operational on a mofiile device for selecting a. wireless communication channel and time offset, comprising; ofitaining a first system identifier (SID) and network identifier (NID) pair according to a preference rank; ofitaining a first frequency channel associated with the first SID/NID pair; scanning one or more time offsets of the first frequency channel for pilot signals; and selecting a first time offset, having the strongest pilot signal associated with the first SID/NID pair.

19. The method of d aim 18 further compri sing: comparing the signal strengths of one or more pilot signals detected on the one or more time offsets for the first frequency channel.

20. The method of claim 1 S further comprising: determining whether the pilot signal of the first time offset .meets a minimum threshold level.

21. The method of claim 18 further comprising: ofitaining the first SϊD/MD pair from a roaming list that specifies a preference rank of SϊD/NTCD pairs.

22. The method of claim 18 further comprising: determining whether it is KkeJy that another pilot signal on the first frequency channel is associated with the first. SID/NID pair, if the strongest pilot, signal on the first frequency channel Is associated with an SΪD/Nl^'D pair other than the first SΪD/NΪD pair,

23. The method of claim 3 S further comprising: rejecting pilot signals on different time offsets that are stronger than the pilot signal on the first time offset if the pilot signals are associated wllli S1D/NΪD pairs other than the first SΪD/NTD pair.

24. The method of claim 18 further comprising: storing time offsets associated xvith undesirafile STD/NΪD pairs; and performing a rescan of time offsets of the first frequency channel for the strongest pilot signal while ignoring pilot signals from the time offsets associated with the undesirafile SΪD/NΪD pairs.

25. A method operational on a mofiile device for selecting a wireless communication channel ami tima offset, comprising: scanning one or more time offsets of a startup frequency channel for pilot signals; selecting a first time offsei having the strongest pilot signai found in the startup frequency channel; ofitaining a first system identifier (SID) and network identifier (NID) pair associated with the first time offset from a cell associated with the first time offset; ofitaining a second SϊD/NϊD pair according to a highest preference rank from the mofiile device; ofitaining a first frequency channel associated with the second SID/NID pair; and comparing the startup frequency channel to the first frequency channel to determine whether they are the same.

26. The method of claim 25 wherein, if the startup frequency channel and first frequency channel are the same, then further determining if the first. SID/NID pair is the same as the second SID/NID pair; if the first SID/NID pair and second SID/NID pair are the same, then using the first SID/NUD pair to estafilish a communication link with a eel! operating on the first time offset of the startup frequency channel; and if the first SΪD/NID pair and second SID/NID pair are not the same, then searching for the strongest' pilot signal associated with the second SL^'D/NID pair on the startup frequency channel.

27. The method of claim 25 wherein_* if the startup frequency channel and first frequency channel are not the same, then further scanning one or more time offsets of the first frequency channel for a pilot signal associated with the second SID/NID pair; and selecting a second time offset on the first frequency channel having the strongest pilot signal associated with the second SIDy¹NID pair.

28. A machine-readafile medium comprising instructions executafile fiy a processor on a mofiile device for intelligently selecting a wireless communication channel and time offset, which when executed fiy a processor, causes the processor to perform operations comprising: ofitaining a first system identifier (SΪD) and network identifier (NID) pair according to a preference rank; ofitaining a first frequency channel associated with the first SID/KID pair; scanning one or more time offsets of the first frequency channel for pilot signals; and selecting a first time offset having the strongest pilot signal associated with the first SID/NID pair.

29. The machine-readafile medium of claim 28 further including instructions that causes the processor to perform operations comprising storing time offsets associated with undesirafile SΪD/NID pairs; and performing & rescan of time offsets of the first frequency channel for the strongest pilot signal while ignoring pilot signals from the tune offsets associated with the undesirafile ST.D/N.ΪD pairs.