Patent Publication Number: US-2009238151-A1

Title: Method and apparatus for enabling quick paging in telecommunication systems

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The application claims the benefit of U.S. Provisional Patent Application No. 61/037,924 filed on Mar. 19, 2008, the entirety of which is incorporated here by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to radio communication systems and more particularly to paging in such systems. 
     BACKGROUND 
     The Institute of Electrical and Electronic Engineers (IEEE) 802.16 Working Group on Broadband Wireless Access Standards is specifying standards for broadband radio communication systems in wireless metropolitan area networks. The IEEE 802.16 family of specifications is called the Wireless Metropolitan Area Network (WirelessMAN) standard and has been dubbed “WiMAX”, which is short for Worldwide Interoperability for Microwave Access, by an industry group called the WiMAX Forum. The mission of the WiMAX Forum is to promote and certify compatibility and interoperability of products complying with the IEEE 802.16 specifications. 
     The WirelessMAN standard defines aspects of the air interface between a radio transmitter and a radio receiver, including the physical (PHY) layer, and the Medium Access Control (MAC) layer. The WiMAX Forum has defined an architecture for connecting a WiMAX network with other networks, such as networks complying with IEEE 802.11 and cellular networks, and a variety of other aspects of operating a WiMAX network, including address allocation, authentication, etc.  FIGS. 1A ,  1 B show examples of WiMAX networks, and it should be understood that the arrangement of functionalities depicted in  FIGS. 1A ,  1 B can be modified in WiMAX and other communication systems. As depicted in  FIG. 1A , the network  100 A includes base stations (BSs)  102 ,  104 ,  106 ,  108  that respectively transmit and receive radio signals in geographic areas called “cells”, which typically overlap to some extent as shown. Subscriber stations (SSs)  110 ,  112  are located in the cells and exchange radio signals with the BSs according to the WiMAX air interface standard. An SS is typically either a mobile SS (MS) or a fixed SS, and it will be understood that a network can include many cells and many SSs. In  FIG. 1A , the BSs communicate with and are controlled by Access Service Network (ASN) Gateways (G/Ws)  114 ,  116  that also communicate with each other, and with other core network nodes and communication networks (not shown), such as the public switched telephone network and the internet. SSs, such as SSs  110 ,  112 , can be organized into groups for paging, and BSs, such as BSs  102 ,  104 ,  106 ,  108 , can be organized into paging areas as described in more detail below. 
       FIG. 1B  depicts a WiMAX network  100 B that also includes BSs  102 ,  104 ,  106 ,  108  and SSs  110 ,  112  as in the network  100 A. The network  100 B is more decentralized than the network  100 A in that, in  FIG. 1B , the BSs communicate with each other directly through a suitable routing network  118  that also communicates with other core network nodes and communication networks (not shown). 
     According to one mode of IEEE 802.16, the downlink (DL) radio signals transmitted by the BSs are orthogonal frequency division multiple access (OFDMA) signals. In an OFDMA communication system, a data stream to be transmitted by a BS to a SS is portioned among a number of narrowband subcarriers, or tones, that are transmitted in parallel. Different groups of subcarriers can be used at different times for different SSs. Because each subcarrier is narrowband, each subcarrier experiences mainly flat fading, which makes it easier for a SS to demodulate each subcarrier. 
     The DL radio signals and uplink (UL) radio signals transmitted by the SSs are organized as successions of OFDMA frames, which are depicted in  FIGS. 2A ,  2 B according to a time-division duplex (TDD) arrangement in the IEEE 802.16e standard.  FIG. 2B  is a magnification of  FIG. 2A  and shows the format of the DL and UL subframes in more detail than in  FIG. 2A . In  FIGS. 2A ,  2 B, time, i.e., OFDMA symbol number, is shown in the horizontal direction and subchannel logical number, i.e., OFDM subcarrier frequency, is indicated by the vertical direction.  FIG. 2B  shows one complete frame and a portion of a succeeding frame, with each DL subframe including sixteen symbols and each UL subframe including ten symbols, not counting guard symbols. 
     Each DL frame  200  starts with a preamble signal that includes a known binary signal sent on every third OFDM tone or subcarrier, as depicted by  FIG. 3 . The range of subcarriers shown in  FIG. 3  is numbered 0, 3, 6, . . . ,  1701 , but a preamble can use fewer than that many subcarriers. 
     As seen in  FIGS. 2A ,  2 B, each frame&#39;s preamble is followed by a DL transmission period and then an UL transmission period. According to the standard, the preamble signal is sent in the first OFDM symbol of a frame, which is identified by an index k in  FIG. 2B , and is defined by the segment, i.e., one of the three sets of tones to be used, and a parameter IDCell, which is the transmitting cell&#39;s identification (ID) information. A SS uses the preamble for initial synchronization of its receiver to the BS (the network), and to determine the location of a frame control header (FCH), which is among the first bursts appearing in the DL portion of a frame. A SS also uses the preambles in signals transmitted by neighboring BSs to synchronize to them for purposes of measurement for handover from one cell to another. 
     The FCH gives information on the DL signal parameters, including a DL map message (DL-MAP), which is a medium access control (MAC) message that defines DL allocations for data, and parameters relevant for reception of the signal. The DL-MAP may be followed by an UL map message (UL-MAP), which provides UL allocations for data, and other parameters relevant for transmission of signals from an identified SS. With the assignments in time and frequency from the DL-MAP, an identified SS can receive the data in the particular location. Similarly, it can identify assignments in time and frequency on the UL-MAP, and transmit accordingly.  FIGS. 2A ,  2 B also show a transmit/receive transition gap (TTG) interval and a receive/transmit transition gap (RTG) interval, which are used by the BS and SS to switch from transmit to receive and vice versa. 
       FIG. 2A  also illustrates how a BS pages an SS operating in idle mode, showing the relationship between paging cycles, paging offset, BS paging interval, and OFDMA frames. Only two of the succession of paging cycles are shown in  FIG. 2A . An SS “listens” for a page message from the BS during only a portion of a paging cycle, and the location of that paging interval is determined by a paging offset from the start of the paging cycle. A paging message can span several OFDMA frames, which the SS needs to demodulate to read the entire message. 
     Thus, while a SS is idle, the SS periodically turns on its baseband unit, which includes a fast Fourier transform (FFT) demodulator and decoder, even when there are no paging messages for it and no system configuration changes/updates. The SS first synchronizes with the preamble and reads the FCH, and it then reads the DL-MAP to look for the location and the format of a broadcast connection identifier (CID). If the DL-MAP shows a broadcast CID, the SS demodulates that burst to determine whether there is a BS broadcast paging message (MOB_PAG-ADV). 
     Most of the time, there is no paging message and no action required by the SS, but during each paging interval, the SS has to be fully “awake”, which is to say, its receiver has to be powered up, for a number of OFDMA frames, using electrical power and possibly draining a battery over time. For a BS, periodically sending MOB_PAG-ADV messages that require no action also wastes downlink capacity. In addition to MOB_PAG-ADV messages, changes in channel descriptors or broadcast system updates can trigger an idle SS to stay on for updating the system parameters or reading other coming messages. 
     A “quick” paging mechanism that can reduce the negative effects of the conventional paging mechanism is not specified in current versions of the WiMAX standards. In such a quick paging mechanism, a simple signal would indicate to a group of SSs that a paging signal exists in a subsequently transmitted signal block. Thus far, proposals for quick paging either steal system resources from a system&#39;s available resources, thereby reducing system capacity, or occupy transmit and receive gaps in a TDD version of the system, which could lead to issues of compatibility among different device implementations. 
     A new standard for mobile broadband communication is under development as IEEE 802.16m, which is required to be backward-compatible with products complying with the current WiMAX standards and at the same time should improve performance considerably compared to current WiMAX technology. In developing IEEE 802.16m, a proposal has been made for a quick paging mechanism that is described in IEEE C802.16m07/217, “Wake-up Signal for 802.16m OFDMA Idle Mode” (Nov. 7, 2007). If an SS decodes the quick paging signal correctly, the SS needs to listen to the conventional paging signal; otherwise, the SS can go back to “sleep”, thereby saving its resources, such as battery power. 
     U.S. Provisional Patent Application No. 61/014,471 filed on Dec. 18, 2007, which is now U.S. patent application Ser. No.  12 /______, filed on Dec. ______, 2008, by the current inventors describes using unused subcarriers (i.e., unused system resources) in a preamble signal to send assigned code words for quick paging. The code words assigned to SSs can include unused conventional preamble sequences and orthogonal sequences, such as Walsh-Hadamard (W-H) sequences, or bi-orthogonal sequences, such as W-H sequences and their inverses. Those patent applications are incorporated here by reference. 
     For one example, a W-H code word can be used as the signal for quick paging as described in the patent applications cited above. With a 10-MHz-wide WiMAX channel using an FFT of length 1024 bits, the length of the conventional preamble is 284 bits. Thus, there are 568 unused subcarrier positions that can be used for a quick paging signal, and so a W-H code word of length 512 bits can be used. For a 5-MHz-wide WiMAX channel, the FFT size is 512 bits and the preamble length is 143 bits, and so 286 unused subcarrier positions are available for the quick paging signal, thereby allowing use of a W-H code word of length 256 bits. Other channel bandwidths, such as 8.75 MHz, can be accommodated in a similar manner. Each such quick paging code word can identify a respective group of SSs, and the presence of a code word in a DL signal indicates to the SS(s) to which that code word is assigned that those SS(s) are required to read the full paging message in a subsequent DL signal. 
     In cellular telephone networks using code division multiple access (CDMA), such as CDMA2000 and wideband CDMA (WCDMA) networks, paging groups are predefined by the applicable standards based on mobile station IDs. Similarly, a mapping between quick paging messages and mobile station IDs is also predefined. The cellular telephone architecture is centralized, and so a central node passes registration information about a mobile station to multiple cells in a paging area. Thus, the mobile station can be reached in any cell belonging to the assigned paging area using a quick paging message. Additionally, the mobile station informs the network whenever it enters a new cell that belongs to a different paging area, triggering defined paging area updating procedures. 
     Nevertheless, there is no fixed mapping between SS IDs and the paging groups predefined in a WiMAX network, and there is no fixed mapping between SS IDs and quick paging messages. In addition, the typical WiMAX network architecture (see  FIGS. 1A ,  1 B) lacks a central node, such as a radio network controller or mobile switching center in a cellular telephone network, that is in charge of distributing the necessary information and mappings. 
     SUMMARY 
     This application describes methods and apparatus by which transmitting stations or other network nodes can autonomously assign quick paging code words to receiving stations. Base stations within a paging area exchange information pertaining to the subscriber stations to which such quick paging code words are assigned. The information exchanged by base stations can include a database of subscriber station identities and assigned quick paging code words. 
     In accordance with aspects of this invention, there is provided a method of enabling a transmitting station for quick paging a receiving station in a telecommunication system having a plurality of transmitting stations. The method includes notifying all other transmitting stations in a paging area of quick paging code words associated with the receiving station by the transmitting station. 
     Also in accordance with aspects of this invention, there is provided a transmitting station in a telecommunication system having a plurality of transmitting stations for communicating with receiving stations. The transmitting station includes a memory configured to store identifiers of respective receiving stations and corresponding quick paging code words and information identifying all other transmitting stations in a paging area that includes the transmitting station; and a control unit configured to provide and receive control and other signals and to store and retrieve information from the memory. The code words have been assigned to receiving stations by any transmitting station in a paging area of the transmitting station. 
     Also in accordance with aspects of this invention, there is provided a computer-readable medium having stored therein instructions that, when executed, cause the computer to carry out a method of enabling a transmitting station for quick paging a receiving station in a telecommunication system having a plurality of transmitting stations. The method includes notifying all other transmitting stations in a paging area of quick paging code words associated with the receiving station by the transmitting station. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The several features, objects, and advantages of this invention will be understood by reading this description in conjunction with the drawings, in which: 
         FIGS. 1A ,  1 B depict examples of telecommunication networks; 
         FIGS. 2A ,  2 B depict downlink and uplink signals organized as successions of frames; 
         FIG. 3  depicts an arrangement of subcarriers for preamble signals; 
         FIG. 4  is a flow chart of method of enabling quick paging in a telecommunication system; and 
         FIG. 5  is a block diagram of a transmitting station in a telecommunication network. 
     
    
    
     DETAILED DESCRIPTION 
     This description focusses on radio communication systems according to the WiMAX standards, but the artisan will understand that the invention in general covers other wireless communication systems. In WiMAX, a quick paging mechanism can use code words assigned to SSs or groups of SSs and sent as quick paging signals similar to the preambles of DL frames over unused frequency resources to signal the SSs. As an example of that mechanism, the quick paging code word(s) assigned to a particular SS can be unused preamble sequences and/or a subset of W-H sequences, and the respective code word(s) are masked by a cell-specific pseudorandom-noise (PN) sequence. A SS determines whether a quick paging code word assigned to it was sent by a BS, and based on that determination, the SS can decide to receive a full conventional paging signal in a subsequent OFDMA frame. 
     The inventors have recognized that whether different BSs use the same method or different methods of assigning quick paging code words to SSs, it is advantageous for an SS simply to assume that the same quick paging code words are used by all BSs in a paging area. To enable such operation by the SSs in a paging area covered by several BSs, those several BSs share information about the quick paging code words assigned to the various SSs known to the BSs. 
       FIG. 4  is a flow chart of a method of enabling quick paging in a telecommunication system, such as a WiMAX system. For example, let BS  108  have assigned SS  110  quick paging code words called c 1 , c 2 , and c 3  by any method desired, even an arbitrary one (step  402 ). Such an assignment would advantageously have been done when BS  108  was the preferred base station for SS  110 . In step  404 , the BS  108  notifies the other BSs, e.g., BSs  102 ,  104 ,  106 , in the same paging area of the code words it has assigned. The notification can communicate the code word assignment information to the other BSs in any suitable way, e.g., by messages sent to the other BSs through ASN G/Ws, such as ASN G/Ws  114 ,  116 , or through a routing network, such as routing network  118 , or through any other suitable communication channel. For example, the code word assignment messages can be sent according to a suitably defined protocol, such as the Simple Network Management Protocol (SNMP), which is defined in RFC 3411-RFC 3418 promulgated by the Internet Engineering Task Force (IETF). The artisan will understand that other protocols can be used. As an alternative, the BS  108  can send the code word assignment information using a multicast message, for which the other BSs  102 , 104 ,  106  in its paging area are members of a multicast group. 
     In step  406 , the BS  108  receives similar notifications from all other transmitting stations in the paging area, e.g., BSs  102 ,  104 ,  106 , of quick paging code words associated with receiving stations by the other transmitting stations. If the SS  110  moves and selects another BS, e.g., BS  106 , as its preferred BS, the SS  110  need not seek to determine quick paging code words assigned to it by BS  106 . Instead, the SS  110  needs only to read the broadcast information transmitted by the BS  106  and thereby determine the PN sequence specific to BS  106 . It will be understood that the identification of the paging area to which a BS belongs is typically known to the SS by information broadcast by the BS. If a SS moves into the coverage area of a BS in a different paging area, the SS typically “reconnects” to the network to inform it of the SS&#39;s change of paging area. If a SS moves into the coverage area of a BS in the same paging area, the SS might not perform such reconnection. 
     If BS  106  wants to page SS  110 , BS  106  simply sends (step  408 ) a quick paging message using one of the code words assigned to the SS  110  by the BS  108 . In accordance with the patent applications cited and incorporated by reference above, the BS  106  can send such a quick paging message at a predetermined time offset earlier than a conventional full paging message. 
     It will be noted that the BS  106  need not know that the SS  110  has selected BS  106  as its preferred base station to use the quick paging mechanism. Such a situation can occur when a mobile SS performs a cell reselection while it is in idle mode. It will also be noted that as part of quick paging the SS  110  (step  408 ), the BS  106  can use its cell-specific PN sequence to mask the quick paging code word sent to the SS  110  in the quick paging message. 
     As described above, the communication of assigned quick paging code words among BSs in a paging area can be carried out in many ways. For example, BSs in a paging area can communicate with each other through a central entity, such as an ASN G/W, that can receive assignment information from multiple base stations and re-send collated information to all base stations within a paging area. For yet another example, the BSs in a paging area can directly communicate with the other BSs in the paging area, such as through a suitable routing network, and exchange information about quick paging code word assignments. In the latter example, a BS can either transmit the assignment information to individually addressed BSs, or the BS can transmit the assignment information to all other BSs when the paging area is defined by a suitable multicast address, which enables all base stations in the paging area to receive a single message that is sent. The direct communication between BSs can occur according to a suitably defined protocol, such as SNMP, which can operate over conventional TCP/IP (transmission control protocol/internet protocol). Alternatively, a suitably defined message can be sent over well-known multicast schemes/protocols used over IP-based networks. 
       FIG. 5  is a block diagram of a portion of the BS  102 , which is typical of other BSs  104 ,  106 ,  108  and such transmitting nodes or stations in a WiMAX OFDMA network  100 , that can communicate with other BSs in a paging area for the methods described above. It will be appreciated that the functional blocks depicted in  FIG. 5  can be combined and re-arranged in a variety of equivalent ways, and that many of the functions can be performed by one or more suitably programmed digital signal processors and other known electronic circuits. 
     The BS  102  is operated by a control processor  502 , which typically and advantageously is a suitably programmed digital signal processor. The control processor  502  typically provides and receives control and other signals from various devices in the BS  102 . For simplicity in  FIG. 5 , the control processor  502  is shown exchanging information with a suitable memory  503 , which is a repository of SS IDs and corresponding quick paging code words that have been assigned by other BSs in the paging area of the transmitting station  400  and that have been assigned by the station  500 . The memory can also store information identifying other BSs in the same paging area and a station-specific PN sequence for masking quick paging code words transmitted. The artisan will understand that the PN sequence can alternatively be computed by the control processor  502  or a suitable PN sequence generator. 
     Such information is provided to a quick paging code word generator  504 , which uses the information to sort SSs into paging groups and paging groups into super paging groups, and to assign code words. Typically, the transmitting station  500  would sort those SSs for which the transmitting station  500  is the serving node or preferred cell. The code word generator  504  also generates selected quick paging code words for transmission to one or more SSs and paging groups as described above. For that purpose, the generator  504  can produce suitable code word sequences, such as W-H sequences, or retrieve unused conventional preamble sequences from a preamble sequence memory  505 . The quick paging code word generator  504  can also be configured to mask the selected quick paging code words by combining those code words with a PN sequence. 
     It will be understood that although the generator  504  is depicted in  FIG. 5  as part of the control processor  502 , this is not necessary; the generator  504  as well as one or more other devices depicted as part of the processor  502  can be implemented by dedicated programmed processors or other suitable logic configured to perform their functions. 
     A preamble generator  506  also retrieves stored conventional preamble sequences from the memory  505  that are then used for producing the conventional preamble portion of the DL signal transmitted by the BS  102 . 
     The code word generator  504  provides information about the quick paging code words and/or the appropriate quick paging code words to a multiplexer  507 , which also receives the conventional preamble generated by the generator  506 . The multiplexer  507  combines the information or code words with the preamble and other data in a DL frame or subframe to be transmitted. The combined information stream produced by the multiplexer  507  is converted by a suitable OFDM modulator  508  into modulation symbols that are provided to an OFDM radio transmitter  509 , which impresses the modulation symbols on suitable subcarrier signals. The modulated subcarrier signals are transmitted through a suitable antenna  510 . 
     As described above, the BS  102  is responsive to a request by the network to reach a SS or group of SSs by transmitting the quick paging code word(s) associated with the SS(s). In  FIG. 5 , such a request is shown as provided through an ASN gateway  114  to control processor  502  and generator  504 . In response to the request, the generator  504  retrieves the code word(s) or code word ID(s) associated with the desired SS(s) from the memory  503 , and generates the appropriate quick paging signal for transmission by the BS  102 . For example, the control processor  502  receives the IDs of SSs that need to be paged, accesses the database  503  in which the identities of the PGs and SPGs and their corresponding SSs are stored, and produces the identity of the SPG to be paged. The quick paging signal generator  504  then outputs the appropriate quick paging code word. 
     This invention enables a receiving station such as a SS to save significant power by giving the SS the ability to receive a unified quick paging message across multiple cells, avoiding a need to receive quick paging assignments in each cell. It also enables transmitting stations such as base stations in a decentralized network to exchange relevant information in order for an SS to realize the power savings. 
     It will be appreciated that procedures described above are carried out repetitively as necessary, for example, to respond to the time-varying nature of communication signals exchanged by transmitters and receivers. Descriptions and examples of principles, aspects, and embodiments of this invention are intended to encompass both structural and functional equivalents, and it is intended that such equivalents include both currently known functional equivalents as well as functional equivalents developed in the future, regardless of structure. The artisan will also appreciate that block diagrams can represent conceptual views of illustrative circuitry embodying the principles of the technology and that flow charts, state transition diagrams, pseudocode, and the like represent processes which may be substantially represented in a computer-readable medium and so executed by a computer or programmable processor, whether or not such computer or processor is explicitly shown. 
     To facilitate understanding, many aspects of this invention are described in terms of sequences of actions that can be performed by, for example, elements of a programmable computer system. It will be recognized that various actions could be performed by specialized circuits (e.g., discrete logic gates interconnected to perform a specialized function or application-specific integrated circuits), by program instructions executed by one or more processors, or by a combination of both. Wireless transceivers implementing embodiments of this invention can be included in, for example, mobile telephones, pagers, headsets, laptop computers and other mobile terminals, base stations, and the like. 
     Moreover, this invention can additionally be considered to be embodied entirely within any form of computer-readable storage medium having stored therein an appropriate set of instructions for use by or in connection with an instruction-execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch instructions from a medium and execute the instructions. As used here, a “computer-readable medium” can be any means that can contain, store, communicate, or transport the program for use by or in connection with the instruction-execution system, apparatus, or device. The computer-readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device. More specific examples (a non-exhaustive list) of the computer-readable medium include an electrical connection having one or more wires, a portable computer diskette, a random-access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), and an optical fiber. 
     Thus, the invention may be embodied in many different forms, not all of which are described above, and all such forms are contemplated to be within the scope of the invention. For each of the various aspects of the invention, any such form may be referred to as “logic configured to” perform a described action, or alternatively as “logic that” performs a described action. 
     It is emphasized that the terms “comprises” and “comprising”, when used in this application, specify the presence of stated features, integers, steps, or components and do not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. 
     The particular embodiments described above are merely illustrative and should not be considered restrictive in any way. The scope of the invention is determined by the following claims, and all variations and equivalents that fall within the range of the claims are intended to be embraced therein.