Abstract:
Techniques for efficient storage and retrieval of Preferred Roaming Lists are disclosed. In one aspect, PRL entries are stored in two tables. One table contains records that are common to two or more PRL entries. Another table stores any information that is unique to a PRL entry, as well as an indicator of which common record is associated with it. The common record is concatenated with the unique information to generate the uncompressed PRL entry. Various other aspects of the invention are also presented. These aspects have the benefit of reducing the memory requirements for storing a PRL. In addition, time required to download the compressed PRL is reduced.

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §120  
       [0001]     The present Application for Patent is a Continuation and claims priority to patent application Ser. No. 11/009,281 entitled “METHOD AND APPARATUS FOR PREFERRED ROAMING LIST COMPRESSION” filed Dec. 9, 2004, now allowed which is a continuation application of U.S. Pat. No. 6,901,395 entitled “METHOD AND APPARATUS FOR PREFERRED ROAMING LIST COMPRESSION” issued May 31, 2005, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
     
    
     FIELD  
       [0002]     The present invention relates generally to communications, and more specifically to a novel and improved method and apparatus for Preferred Roaming List (PRL) compression.  
       BACKGROUND  
       [0003]     Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.  
         [0004]     A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), (4) the “TIA/EIA-IS-856 CDMA2000 High Rate Packet Data Air Interface Specification” (the IS-856 standard), and (5) some other standards.  
         [0005]     Cellular communication system users commonly have a service agreement with a cellular provider. The system operated by a cellular provider may cover a limited geographical area. When a user travels outside of this geographical area, service may be provided by another system operator, under a roaming agreement. There is often more than one service provider in a particular region, so a user may have a choice as to which service provider to roam with. As cellular communication systems have proliferated, networks of cellular systems have been organized under common service providers, or with contractual agreements between service providers. Roaming fees are minimized or eliminated when a user transfers between systems which are party to such agreements. As such, modern mobile stations often make use of Preferred Roaming Lists (PRLs), which contain information about the preferred systems for roaming and various parameters needed for communication therewith. PRLs may be pre-programmed in a mobile station when service is initiated. Alternatively, PRLs can be programmed with over-the-air data transfers. Such programming is described in “TIA/EIA-683-B Over-the-Air Service Provisioning of Mobile Stations in Spread Spectrum Systems”, a standard compatible with the above named wireless communication systems.  
         [0006]     The list of sectors in a typical PRL can be quite large, and will likely grow larger as more mobile stations are equipped for international roaming. Furthermore, in data communication systems, such as the HDR standard, each sector is assigned an IPv6 (Internet Protocol version 6) address which is 128 bits in length. As the length of the PRL increases, and as the information in each record in the PRL expands, the memory requirements to store the PRL will grow accordingly. Furthermore, over-the-air updates to the PRL will take longer as the PRL size expands. There is therefore a need in the art for efficient storage and retrieval of Preferred Roaming Lists.  
       SUMMARY  
       [0007]     Embodiments disclosed herein address the need for efficient storage and retrieval of Preferred Roaming Lists (PRL). In one aspect, PRL entries are stored in two tables. One table contains records that are common to two or more PRL entries. Another table stores any information that is unique to a PRL entry, as well as an indicator of which common record is associated with it. The common record is concatenated with the unique information to generate the uncompressed PRL entry. Various other aspects of the invention are also presented. These aspects have the benefit of reducing the memory requirements for storing a PRL. In addition, time required to download the compressed PRL is reduced.  
         [0008]     The invention provides methods and system elements that implement various aspects, embodiments, and features of the invention, as described in further detail below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:  
         [0010]      FIG. 1  is a general block diagram of a wireless communication system capable of supporting a number of users;  
         [0011]      FIG. 2  depicts a mobile unit configured in accordance with an embodiment of the present invention;  
         [0012]      FIG. 3  depicts a compressed PRL;  
         [0013]      FIG. 4A  illustrates a process for generating a compressed PRL;  
         [0014]      FIG. 4B  illustrates a method for accessing a subnet from a compressed PRL;  
         [0015]      FIG. 5  depicts a detailed embodiment of a compressed PRL;  
         [0016]      FIG. 6  depicts an embodiment of a method for accessing the compressed PRL of  FIG. 5 ;  
         [0017]      FIG. 7  depicts a procedure for retrieving common information from a subnet table;  
         [0018]      FIG. 8  depicts an alternate detailed embodiment of a compressed PRL;  
         [0019]      FIG. 9  depicts an embodiment of a method for accessing the compressed PRL of  FIG. 8 ;  
         [0020]      FIG. 10  depicts an alternate procedure for retrieving common information from a subnet table; and  
         [0021]      FIG. 11  depicts an exemplary network ID, subnet mask, subnet address, and their inter-relationship.  
     
    
     DETAILED DESCRIPTION  
       [0022]      FIG. 1  is a diagram of a wireless communication system  100  according to one embodiment that supports a number of users, and which can implement various aspects of the invention. System  100  may be designed to support one or more CDMA standards and/or designs (e.g., the W-CDMA standard, the IS-95 standard, the cdma2000 standard, the IS-856 standard). For simplicity, system  100  is shown to include three base stations  104  in communication with two mobile stations  106 . The base station and its coverage area are often collectively referred to as a “cell”. In IS-95 systems, a cell may include one or more sectors. In the W-CDMA specification, each sector of a base station and the sector&#39;s coverage area is referred to as a cell. As used herein, the term base station can be used interchangeably with the term access point. The term mobile station can be used interchangeably with the terms user equipment (UE), subscriber unit, subscriber station, access terminal, remote terminal, or other corresponding terms known in the art. The term mobile station encompasses fixed wireless applications.  
         [0023]     Depending on the CDMA system being implemented, each mobile station  106  may communicate with one (or possibly more) base stations  104  on the forward link at any given moment, and may communicate with one or more base stations on the reverse link depending on whether or not the mobile station is in soft handoff. The forward link (i.e., downlink) refers to transmission from the base station to the mobile station, and the reverse link (i.e., uplink) refers to transmission from the mobile station to the base station. The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments.  
         [0024]      FIG. 2  shows an embodiment of mobile unit  106 . For clarity, only a subset of the components is shown. Signals are received at antenna  210 , and delivered to receiver  220  where amplification, down-conversion, sampling, and demodulating takes place. Various techniques for receiving CDMA signals are known in the art. In addition, the principles of the present invention apply with equal force to wireless communication systems deploying air interfaces other than those based on CDMA. Receiver  220  is in communication with a central processing unit (CPU)  230 . CPU  230  may be a microprocessor or digital signal processor (DSP), or one of various processors known in the art. CPU  230  communicates with memory  240 , which is shown containing PRL  250 . PRL  250  can be programmed via over-the-air programming in conjunction with antenna  210  and receiver  220 , or the data for the PRL can come in from other inputs to CPU  230 , labeled “alternate input” in  FIG. 2 . CPU  230  is also connected to transmitter  260 , for transmitting messages, data, voice, etc., using any of the techniques for transmission known in the art. Transmitter  260  is connected to antenna  210 , for transmission to a base station, such as base station  104 . Receiver  220  and transmitter  260 , in conjunction with antenna  210 , can be used to communicate with one or more systems identified in PRL  250  when the mobile station is roaming.  
         [0025]     In an IS-856 system, each sector has a unique IPv6 address, which is 128 bits in length. In some instances, a network operator may deploy numerous sectors within a system. The IP addresses of these sectors may differ only slightly (i.e., in the least significant bits) since a large portion of each sector address identifies the carrier. In addition, various parameters associated with each of these sectors may be common among the sectors due to their collocation within the network, such as frequency, PN offset, and the like. As used herein, the term subnet refers to an entry in the PRL associated with a group of sectors. The principles of the present invention apply to the concept of a subnet as defined for Internet Protocol (IP) addresses. However, these principles apply more generally to compression of a PRL regardless of the exact nature of the information stored in each record of the PRL. As such, the term subnet, as used herein, should be construed to refer to any of the myriad possibilities of PRL records.  
         [0026]      FIG. 3  depicts an embodiment of PRL  250 . Recall that PRL  250  is contained in memory  240 . PRL  250  contains two tables, system table  310  and subnet table  320 . System table  310  contains entries corresponding to each record of the PRL. In each system table  310  entry, information unique to that entry will be stored, along with an indicator for accessing corresponding data in subnet table  320 . Subnet table  320  contains records which are shared in common with one or more entries in system table  310 . Thus, rather than including duplicate copies of information in various entries of system table  310 , one common copy is stored in subnet table  320 , and an indicator for accessing that common copy will be stored in each corresponding entry in system table  310 .  
         [0027]     Consider the following example as illustrated in  FIG. 4A . The proposed IS-856 system record comprises, among other fields, a network ID and a subnet mask length, m. In an exemplary embodiment, PRL  250  comprises a plurality of these system records. The network ID is a 128-bit value. A subnet mask can be formed using a subnet mask length m by concatenating m ones with 128-m zeros.  FIG. 11  depicts this example. When network ID  1100  is bit-wise ANDed with subnet mask  1110 , subnet address  1120  ( FIG. 4A, 440 ) is the result. All the sectors within a subnet will share a common subnet address, and will be distinguished using the 128-m least significant bits. The size of the subnet is limited by the number of bits assigned to distinguish the sectors within it. Performing the operation shown in  FIG. 11  on the network ID, included in the system record, for all sectors in a subnet will yield an identical result for each subnet address. So, the subnet identifies a group of sectors. It is expected that the most significant k bits of the subnets associated with a particular wireless operator is to be the same. Therefore, the upper k bits of the subnet address can be stored once in subnet table  320 , and the lower 128-k bits can be stored for each record in the system table  310 . Note that m is the length of the subnet ( FIG. 4A, 450 ), whereas k is the length of the common part of the subnet that is to be factored out.  
         [0028]     System table  310  and subnet table  320  will be detailed more fully below in the descriptions of various embodiments deploying them. Note that these tables, and the PRL, are shown as discrete entities for clarity only. While, in an alternative embodiment, each table could be housed in a discrete memory, a more common embodiment will have system table  310  and subnet table  320 , which make up PRL  250 , as a subspace of a common memory element  240 .  
         [0029]      FIG. 4B  depicts an embodiment of a method for accessing a PRL, such as PRL  250 . In step  410 , a record from the system table  310  is retrieved, which corresponds to an entry in the PRL  250 . The record will contain any information that is unique to the entry, as well as an indicator of common information, if any. A variety of techniques for indexing, storing, and accessing the common information can be employed, examples of which are detailed in embodiments described below. In step  420 , a common portion of the subnet is retrieved from a subnet table  320 , if there is a common portion corresponding to the record. In step  430 , the common portion is concatenated with the unique portion to form the complete subnet record.  
         [0030]     The creation of a system table  310  and a subnet table  320  from a PRL can be accomplished by reversing the steps depicted in  FIG. 4B . The details of partitioning and indexing will depend on the procedure chosen, examples of which are detailed below. The resultant system table  310  and subnet table  320  form a compressed PRL  250 . Thus, the time required to transmit the compressed PRL to mobile station  106  is reduced, whether the transmission occurs via a wired connection or is updated over the air, as described in IS-683.  
         [0031]     An exemplary embodiment of a method for compressing a PRL comprises the following steps: First, data that is common to two or more subnets is factored out and stored in a subnet table ( FIG. 4A, 470 ). The remaining data, not factored out, is stored in the symbol table, with an indicator for accessing the associated common information in the system table ( FIG. 4A, 480 ). As described in an example above, one convenient way to factor out common information is to look for common characteristics, such as shared subnet address, frequency, and the like.  
         [0032]     It may be that, in some cases, a larger common portion can be factored out of a first subset of PRL records, but a smaller common sub-portion of that common portion can be factored out of a second, larger subset of PRL records. One example of this may occur in a system that allows subnets within subnets. In such a system, records corresponding to one subnet within a larger subnet will share a large part of their network address in common. Another set of records, corresponding to a different subnet within the same larger subnet, will similarly share a large part of their network address in common. However, all of the records in both subnets, within the larger subnet, will still have in common the portion of the network address identifying the larger subnet, although the common portion will be smaller than the common portion of their individual subnets.  
         [0033]     A variety of techniques for factoring fall within the scope of the present invention. One technique is to apply a multi-pass factoring step, which calculates and compares various compression results (accounting for multiple factoring options), selecting the best compression result. Another technique is to extend the two-table example to allow nested tables. For example, if subnets within subnets are available in the system, then subnet tables can be equipped with indicators to locate common elements within a sub-subnet table. Yet another technique is to store more than one indicator in a record in the system table, each of the indicators identifying a separate entry in the subnet table. Those of skill in the art will recognize how to deploy various combinations of the techniques disclosed herein to accommodate various system configurations.  
         [0034]      FIG. 5  depicts an exemplary embodiment of compressed PRL  250 . It comprises system table  310  and subnet table  320 . Each table contains records identified by a system record field and an associated field length, in bits. System table  310  comprises N records,  510 A- 510 N, corresponding to N entries in the PRL. Subnet table  320  comprises M common records,  550 A- 550 M, which are associated with various of the N system table records,  510 A- 510 N. System table records  510 A- 510 N comprise the fields SUBNET_TAG  520 A- 520 N, SUBNET_RESIDUAL_LENGTH  530 A- 530 N, and SUBNET_RESIDUAL  540 A- 540 N. Subnet table records  550 A- 550 M comprise the fields SUBNET_TAG  560 A- 560 M, SUBNET_COMMON_LENGTH  570 A- 570 M, and SUBNET_COMMON  580 A- 580 M, respectively.  
         [0035]     In this embodiment, each SUBNET_TAG  520 A- 520 N and  560 A- 560 M is eight bits in length. A system table SUBNET_TAG  520 A- 520 N corresponds to at most one subnet table SUBNET_TAG  560 A- 550 M. A value of zero in a system table SUBNET_TAG indicates that none of the subnet table records  550 A- 550 M correspond with that system table record. For non-zero values, the system table SUBNET_TAG identifies one subnet table record with the corresponding SUBNET_TAG value.  
         [0036]     The arrows shown in  FIG. 5  depict exemplary mappings. For example, system table records  510 A and  510 K both correspond with subnet table record  550 M. Thus, SUBNET_TAG  520 A, SUBNET_TAG  520 K, and SUBNET_TAG  560 M are identical. When retrieving the common information for either of system table records  510 A or  510 K, subnet table record  550 M is identified by the SUBNET_TAG value  560 M. Then, SUBNET_COMMON_LENGTH  570 M, a seven-bit field in this example, identifies the length of the common information, contained in SUBNET_COMMON  580 M. The SUBNET_COMMON_LENGTH field  570 M may indicate the length of SUBNET_COMMON  580 M in any unit of data length—bits or bytes are typically convenient measures. The amount of data contained in SUBNET_COMMON  580 M as delineated by SUBNET_COMMON_LENGTH  570 M can then be retrieved from subnet table  320  for association with the system table record, in this example  510 A or  510 K. Similarly, subnet table record  550 A is associated with system table record  510 N.  
         [0037]     In this embodiment, SUBNET_RESIDUAL_LENGTH  530 A- 530 N is a seven-bit field which indicates the length of SUBNET_RESIDUAL  540 A- 540 N. SUBNET_RESIDUAL is the unique information associated with each system table record  510 A- 510 N.  
         [0038]      FIG. 6  depicts an exemplary embodiment of a procedure for accessing a PRL  250 , such as that shown in  FIG. 5 . In step  610 , retrieve SUBNET_TAG from the system table  310 . Proceed to decision block  620  to test if SUBNET_TAG is equal to zero. If it is zero, there is no common element to be retrieved from the subnet table  320 . Proceed to step  630 , and retrieve SUBNET_RESIDUAL from the system table. The subnet is identified completely by SUBNET_RESIDUAL.  
         [0039]     If, in decision block  620 , SUBNET_TAG is not equal to zero, proceed to step  640  and retrieve SUBNET_COMMON corresponding to SUBNET_TAG from the subnet table  320 . Proceed to step  650  and retrieve SUBNET_RESIDUAL from system table  310 . Proceed to step  660 . Concatenate SUBNET_COMMON with SUBNET_RESIDUAL to identify the subnet.  
         [0040]      FIG. 7  is a more detailed embodiment of step  640 . In step  710 , locate SUBNET_TAG in subnet table  320 . In step  720 , retrieve SUBNET_COMMON_LENGTH to determine how much common data to retrieve. Proceed to step  730  to retrieve the amount of data, from subnet table  320 , as specified in SUBNET_COMMON_LENGTH.  
         [0041]      FIG. 8  depicts another exemplary embodiment of compressed PRL  250 . This embodiment uses an index into the subnet table instead of a subnet tag. It also comprises a system table  310  and a subnet table  320 . As before, each table contains records identified by a system record field and an associated field length, in bits. System table  310  comprises N records,  510 A- 510 N, corresponding to N entries in the PRL. Subnet table  320  comprises M common records,  550 A- 550 M, which are associated with various of the N system table records,  510 - 510 N. However, in this alternative embodiment, system table records  510 A- 510 N comprise the fields SUBNET_LSB_LENGTH  820 A- 820 N, SUBNET_LSB  830 A- 830 N, and SUBNET_COMMON_OFFSET  840 A- 840 N. Subnet table records  550 A- 550 M comprise the fields SUBNET_COMMON_LENGTH  840 A- 840 M, and SUBNET_COMMON  850 A- 850 M. In contrast to the embodiment of  FIG. 5 , note that SUBNET_TAG is not a field in either system table  310  or subnet table  320 .  
         [0042]     In this embodiment, SUBNET_LSB_LENGTH  810 A- 810 N performs substantially the same function as SUBNET_RESIDUAL_LENGTH  530 A- 530 N. It is a seven-bit field which indicates the length of SUBNET_LSB  820 A- 820 N, a field which performs substantially the same function as SUBNET_RESIDUAL  540 A- 540 N. SUBNET_LSB is the unique information associated with each system table record  510 A- 510 N.  
         [0043]     In this embodiment, each SUBNET_COMMON_OFFSET  840 A- 840 N is an index into subnet table  320 , the index in this example is 12 bits in length. Each SUBNET_COMMON_OFFSET  830 A- 830 N corresponds to at most one subnet table record  550 A- 550 M. A value of zero in a SUBNET_COMMON_OFFSET indicates that none of the subnet table records  550 A- 550 M corresponds with that system table record.  
         [0044]     The arrows shown in  FIG. 8  depict exemplary mappings. For example, system table records  510 A and  510 K both correspond with subnet table record  550 M. Thus, SUBNET_COMMON_OFFSET  830 A and  830 N are identical, and point to subnet table record  550 M. Then, SUBNET_COMMON_LENGTH  840 M, a four-bit field in this example, identifies the length, in bytes, of the common information, contained in SUBNET_COMMON  850 M. The amount of data contained in SUBNET_COMMON  850 M as delineated by SUBNET_COMMON_LENGTH  840 M can then be retrieved from subnet table  320  for association with the system table record, in this example  510 A or  510 K. Similarly, subnet table record  550 A is associated with system table record  510 N.  
         [0045]      FIG. 9  depicts an exemplary embodiment of a procedure for accessing a PRL  250 , such as that shown in  FIG. 8 . In step  910 , retrieve a record from system table  310 . Proceed to step  920  to retrieve the SUBNET_COMMON from the subnet table corresponding to SUBNET_COMMON_OFFSET contained in the system table record. A SUBNET_COMMON_OFFSET of zero means that no common information is to be retrieved. Proceed to step  930 , concatenate SUBNET_COMMON with SUBNET_LSB to identify the subnet.  
         [0046]      FIG. 10  is a more detailed embodiment of step  920 . In step  1010 , the process accesses SUBNET_COMMON_LENGTH from the subnet table with the pointer SUBNET_COMMON_OFFSET. Proceed to step  1020 . In Step  1020 , access SUBNET_COMMON by retrieving a number of bytes specified by the value of SUBNET_COMMON_LENGTH.  
         [0047]     Another alternative, not shown, is to nest both the subnet table and the system table in one table. In this alternative, the first occurrence of a common record is included in the record with which it is associated. A tag and/or common record length field may be inserted prior to the common record. Subsequent records in the table, which are associated with the common record, can simply include a pointer or tag, depending on the implementation chosen, to indicate the previously stored common record is to be accessed.  
         [0048]     It should be noted that in all the embodiments described above, method steps can be interchanged without departing from the scope of the invention.  
         [0049]     Those of skill in the art will understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.  
         [0050]     Those of skill will further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.  
         [0051]     The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.  
         [0052]     The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.  
         [0053]     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.