METHODS AND SYSTEMS FOR ERROR HANDLING IN RADIOCOMMUNICATION SYSTEMS EMPLOYING PRE-PAGING

Abstract: Methods and systems for handling errors associated with pre-paging in radiocommunication networks are described. If radio contact to a terminal is lost after a successful pre-page, the network can detect this event. Then, when the corresponding call delivery message is received, the network can re-page the terminal and re-allocate radio resources if the terminal is available. Additionally, a guard timer can be started when radio resources are first allocated. If the call delivery message is not received prior to expiration of the timer, the radio resources can be deallocated to avoid wasting such resources in the event that the call delivery message is lost in the network.

Description: METHODS AND SYSTEMS FOR ERROR HANDLING IN RADIOCOMMUNICATION SYSTEMS EMPLOYING PRE-PAGING RELATED/PRIORITY APPLICATION This application claims priority from U.S. Provisional Patent Application 60/133,688, filed on May 10, 1999, entitled "Gateway Location Register and Pre- Paging In the Universal Mobile Telecommunication System", the disclosure of which is expressly incorporated here by reference. BACKGROUND The present invention relates to paging techniques in radiocommunication systems and, more specifically, to techniques for handling errors in radiocommunication systems which employ pre-paging. The radiocommunication industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices. One technique which is commonly employed in such radiocommunication systems is referred to as paging. Paging is a process whereby the radiocommunication system locates a terminal for which communication is requested (e.g., termination attempts of calls, delivery of messages, etc.). For example, in many radiocommunication systems, an idle mobile station (i.e., one which has not been assigned a traffic channel) will listen to a paging channel for paging messages that have its assigned address. The listening process may be periodic so that the terminal can power down between listening periods to conserve battery power. However, when a terminal receives a page that has its assigned address, the terminal will acknowledge the page and the process of establishing a connection between that terminal and the radiocommunication system has begun. This process includes providing routing information to the radiocommunication system for the paged terminal so that the message can be properly transmitted to the terminal at its current location. An example of conventional paging techniques will now be described with respect to the GSM radiocommunication system. Paging techniques, among other things, for GSM systems are described, at least in part, in a standards document referred to as GSM TS 09.02 (ETSI ETS 300 974), the disclosure of which is incorporated here by reference. Therein, the Mobile Application Part (MAP) specifies, for GSM networks, a conventional procedure for handling the signalling interaction between the visiting PLMN (VPLMN) and home PLMN (HPLMN) when a connection is to be setup with a terminal. Figure 1A depicts such a conventional process for setting up a connection with a terminal 5. Therein, a Gateway Mobile Switching Center (GMSC) 10 will receive an Initial Address Message (IAM) to terminate a call towards a particular mobile subscriber (message 1). The GMSC 10 then requests a roaming number for the terminal 5 from a home location register (HLR) 20. This enables the GMSC 10 to address the MSC (not shown) which is currently supporting radiocommunications with the terminal 5 in the VPLMN. The GMSC 10 accomplishes this by transmitting a Send Routing Information (SRI) message (message 2) to the HLR 20 of the HPLMN that "owns" the subscriber associated with terminal 5, i.e. , the terminal's home system. Note that the physical relationships between nodes are shown conceptually in Figure 1A; physically implementations may vary. For example, the GMSC can be incorporated within the HPLMN or within an originating PLMN. When the HLR 20 receives an SRI message from a GMSC 10 (which GMSC in not necessarily within HPLMN), the HLR 20 will request a roaming number from the Visited Mobile Switching Center/Visited Location Register (VMSC/VLR) 30 servicing the area in which the terminal 5 is located. Note that, in this example, the VMSC and VLR are illustrated as combined nodes, although in implementation they can be separate nodes. Moreover, throughout this description these nodes may be referenced individually. The request of a roaming number can be accomplished by, for example, sending a Provide Roaming Number (PRN) message to the VMSC/VLR 30 (message 3). The VMSC/VLR 30 in the VPLMN provides the roaming number, which is then forwarded to the GMSC 10 via the HLR 20 using a PRN & SRI messages. The GMSC 10 will then forward the IAM (message 4) to the VMSC/VLR 30. Once the IAM (message 4) has been received by the VMSC/VLR 30, a number of internal messages within the VMSC/VLR node occur (i.e. Send Information For Incoming Call, Page), and as a result, the subscriber's terminal will be paged (message 5). Upon receipt of a paging acknowledgment from the terminal, a traffic channel can be setup between the serving base station (not shown) and the terminal to establish the connection. Figure IB is a flow diagram showing the procedure described above and depicted in Figure 1A. A significant feature of conventional paging techniques is that the paging of terminal 5 does not occur until the VMSC/VLR receives the IAM message, as can be seen in these Figures. Recently another approach to handling connectivity with a terminal has been considered, which approach has been referred to as Paging-Before-Routing or Pre- Paging. In radiocommunication systems employing pre-paging, the VMSC/VLR initiates the paging of a subscriber terminal when the VMSC/VLR receives a request for a roaming number (e.g., an MSRN) from the HLR, instead of waiting until the IAM is forwarded as occurs in the conventional paging technique described above. Furthermore, the roaming number is not sent to the HLR until a page response is received from the subscriber terminal, indicating that the terminal is currently available, which differs from the technique described above wherein the VMSC/VLR returns a roaming number promptly upon request by the HLR. This pre-paging procedure has at least one significant advantage as compared with the conventional paging technique of Figures 1A and IB. If the subscriber terminal does not respond to the page, such that the call cannot be completed (which can happen quite often), no switching and transmission resources between the GMSC 10 and the VMSC/VLR 30 are unnecessarily reserved, as would be the case with the conventional GSM procedure described above. Hence, pre-paging techniques save switching and transmission resources in the destination and transit networks. Figure 2A depicts conventional procedures associated with an exemplary pre-paging technique for handling connectivity. As shown therein, the GMSC 10 receives an Initial Address Message (IAM) to terminate the call towards a mobile terminal 5 (message 1). The GMSC 10 then requests the roaming number by sending a SRI to the HLR (message 2). The HLR will request the roaming number from the VLR by sending a PRN (message 3). Up to this point, the procedure illustrated in FIG. 2A is the same as the GSM procedures illustrated in FIGS. 1A and IB. However, before the VMSC/VLR 30 in the VPLMN provides the roaming number, the VMSC/VLR 30 will pre-page (i.e., page the terminal even though the VMSC/VLR doesn't yet have the IAM message) pre-page) the terminal 5 to ensure the availability of the subscriber within its service area (message 4). Only after successfully pre-paging the subscriber does the VMSC/VLR 30 provide a roaming number that is forwarded to the GMSC 10 via the HLR 20 (i.e., by the PRN & SRI response). Moreover, only after successfully pre-paging the subscriber are radio resources allocated to the expected call, e.g., a signalling channel for the mobile to respond to the paging request and, optionally, a traffic channel over which the call can be connected. Then after receiving the response to the SRI message, the GMSC 10 is able to forward the IAM (message 5) to the VMSC/VLR 30. An information flow diagram relating to the procedure of Figure 2 A is illustrated in Figure 2B. The procedure illustrated in Figures 2A and 2B reflect the pre-paging feature under normal conditions, that is, when all the network nodes involved in the process operate normally. However, network failures can occur during the pre- paging process, which failures can have detrimental effects on the radiocommunication system's performance. For example, consider the situation where, after the terminal 5 has been paged and the roaming number has been provided by the VMSC/VLR 30, radio contact with the subscriber terminal 5 is lost. When the IAM message subsequently arrives at the VMSC/VLR 30, the VMSC will not be able to correlate the IAM message with the terminal because the radio resources have been released. As another example of a network failure which can occur during pre-paging, consider the situation where, after the terminal 5 has been paged and the roaming number has been provided by the VMSC/VLR 30, the IAM message that is sent by the GMSC 10 is lost during routing between the GMSC 10 and the VMSC/VLR 30 due to network congestion. In this case, the radio resources remain allocated for the expected call even though the IAM message has been lost in the network (and therefore the call cannot be setup), which situation results in lower system capacity since resources are being unnecessarily reserved. Accordingly, it would be desirable to provide pre-paging techniques in radiocommunication systems which are able to retain the benefits of conventional pre-paging techniques (e.g., avoiding the usage of network resources to forward the IAM message when the terminal is not available) while also handling these types of fault or error conditions in the network. SUMMARY According to exemplary embodiments of the present invention, error procedures have been developed by Applicants to avoid problems that may arise after a subscriber in a VPLMN has been pre-paged. For example, according to a first exemplary embodiment of the present invention, the problem of radio contact with the subscriber terminal being lost after pre-paging is addressed. According to this exemplary embodiment, a loss of radio contact is detected and noted by the system. Upon receipt of the call delivery message, the terminal is paged again and, if available, radio resources are again allocated thereto. This second set of radio resources is then correlated with the received call initiation message and the call is set-up. According to another exemplary embodiment of the present invention, the problem of continuing to reserve radio resources with the terminal after paging while awaiting an IAM message that has been lost is addressed. According to this exemplary embodiment, a guard timer is initiated after radio resources are allocated in response to a successful pre-page. If the guard timer times out before the call initiation message is received, then the radio resources are deallocated. BRIEF DESCRIPTION OF THE DRAWINGS These, and other, features, objects and advantages of the present invention will become more readily apparent to those skilled in the art upon reading the following detailed description, in conjunction with the drawings in which: Figure 1A is a signalling diagram of a conventional paging technique for establishing a connection with a terminal; Figure IB is an information flow diagram corresponding to the conventional paging technique illustrated in Figure 1A; Figure 2A is a signalling diagram of a pre-paging technique for establishing a connection with a terminal, in which exemplary embodiments of the present invention can be employed; Figure 2B is an information flow diagram corresponding to the pre-paging technique of Figure 2A; Figure 3 is a block diagram of an exemplary base station and mobile station which can be used to implement the present invention; Figure 4 is a flow diagram representing a first exemplary embodiment of the present invention for handling a loss of allocated radio resources after prepaging; Figure 5 is a flow chart depicting an exemplary method according to the first exemplary embodiment; Figure 6 is a flow diagram representing a second exemplary embodiment of the present invention for handling a loss of a call delivery message after prepaging; and Figure 7 is a flow chart depicting an exemplary method according to the second exemplary embodiment. DETAILED DESCRIPTION In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, and circuits are omitted so as not to obscure the description of the present invention. The exemplary radio communication systems discussed herein are described as being implemented according to the GSM specifications using the time division multiple access (TDMA) protocol, in which communication between the base station and the mobile terminals is performed over a number of time slots. However, those skilled in the art will appreciate that the concepts disclosed herein find use in systems operating in accordance with other protocols, including, but not limited to, frequency division multiple access (FDMA), code division multiple access (CDMA), or some hybrid of any of the above protocols. For context, Figure 3 is provided as a generalized example of a conventional cellular radio communication system 100 in which the present invention can be implemented. The radio communication system 100 includes a plurality of radio base stations (BS) 30a-30n communicate with a plurality of mobile terminals, only one of which, mobile station (MS) 5, is shown here. Communication from a base station 30a to a mobile station 5 is typically referred to as downlink communication, whereas communication from a mobile terminal to the base station is referred to as uplink communication. The base stations 30a-30n are connected to a mobile switching center (MSC)/visitor location register (VLR) 50. Among other tasks, the MSC/VLR 50 coordinates the activities of the base stations, such as during the handoff of a mobile station from one base station to another. The MSC/VLR, in turn, can be connected to a public switched telephone network (not shown). Each base station 30a-30n will have a plurality of tranceivers 35 used to transmit information on RF carriers associated with traffic and signalling channels. Each mobile station, such as mobile station 5, has a transceiver 10 for communicating with base station 30a over the air interface, as well as various I/O devices represented by block 20, e.g. , a display, speaker, microphone, keypad, etc. When establishing a connection between the radiocommunication system (represented in Figure 3 by base station 30a) and the mobile station 5 using pre- paging techniques according to the present invention, the various potential error conditions described above are addressed. First, an exemplary embodiment of the present invention addresses the problem associated with losing radio contact with the subscriber terminal after successful pre-paging as illustrated in Figure 4 and 5. First, as step 500 a call is initiated, e.g. , using the IAM, SRI and PRN signals described above. In this context, call initiation refers to call delivery or establishment of a call terminated in e.g., a cellular phone. Then, the terminal is pre-paged at step 502. As mentioned above, once the mobile station 5 has been pre-paged, it is allocated a first set of radio resources at step 504, e.g. , it is assigned a signalling channel over which it awaits the transmission of call setup information, e.g. , a channel assignment message. If the MSC/VLR 30 receives the IAM message while the mobile station is in radio contact with the system using the first set of radio resources (steps 506 and 508 along paths "No" and "Yes", respectively), then a call can be set-up using those resources (step 510). Alternatively, the mobile station 5 may not be able to listen to system using the first set of resources, e.g., if it moves out of range of base station 30a which has been assigned to transmit the call setup information to mobile station 5 over the signalling channel assigned during the pre-paging process. Once radio contact is lost, the MSC/VLR 50 notes this event (e.g., by setting a flag associated with the mobile station's identity number at step 512) and releases all of the radio resources that were previously allocated for the call to be setup (e.g. , the signalling channel and the traffic channel(s)) at step 514. This release of resources is indicated in Figure 4 by the termination of the thickened black lines extending vertically from the pre-paging block joining the mobile station 5 and the MSC/VLR. When an IAM message associated with the previously paged subscriber arrives at the MSC/VLR (step 516), the MSC/VLR notes that the radio connection established with the terminal identified in the IAM message has been lost, e.g., by the status of the flag set in memory. A second paging of mobile station 5 is then performed at step 518 to ensure the availability of that terminal within the MSC/VLR's service area. If the subscriber is still available, a second set of radio resources (which may or may not be the same as the first set of radio resources) is assigned to support the call and the MSC/VLR correlates the incoming IAM message with the newly allocated radio resources to complete the call set-up process. Another problem associated with conventional pre-paging techniques, i.e. , how to handle the loss of the IAM message in the network after pre-paging is addressed by the exemplary embodiments of the present invention illustrated in Figures 6 and 7. This problem was described above, but can also be easily seen in the flowchart of Figure 5 by noting the loops created by the "No" paths out of decision blocks 508 and 516. As before, the system initiates a call (step 700) and pre-pages the terminal (step 702). Once a successful pre-page has occurred, the MSC/VLR will allocate radio resources (e.g., a signalling channel and, possibly, a traffic channel) at step 704. Then, a guard timer is started at step 706. This guard timer can have a countdown (or up) time which is related to the amount of time within which the GMSC should have received the indication that the pre-page was successful and the resulting IAM message should have arrived at the MSC/VLR. For example, this time period can be on the order of 20 ms, although those skilled in the art will appreciate that the specific amount of guard time can be varied. Once the guard timer has been started, the MSC/VLR 30 will look for receipt of the IAM message associated with the terminal that has radio resources allocated based on the successful pre-page as indicated by step 708. If received, the MSC/VLR will proceed to set-up the call, as indicated by step 710. Otherwise, if the guard timer expires before the IAM message is received (the "Yes" path from decision block 712), then the MSC/VLR will release the allocated resources at step 714. Initiating a guard timer in the VMSC to ensure the timely release of radio resources when network failures occur as described above provide a number of advantages. For instance, a UMTS network that employs a pre-paging feature can provide robust capability even in failure situations that might occur during the pre- page procedure. The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. For example, the embodiments of Figures 5 and 7 can be readily combined. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims.

Claims: WHAT IS CLAIMED IS: 1. A method for communicating with a terminal in a radiocommunication system comprising the steps of: a) initiating a call terminating with the terminal; b) pre-paging the terminal to determine an availability of the terminal; c) allocating a first set of radio resources for said call if the terminal is available; d) detecting a loss of radio contact with said terminal; e) forwarding a call delivery message to a first node covering a service area in which the terminal is located; f) paging said terminal for a second time after said call delivery message is received; and g) allocating a second set of radio resources to the call if the terminal is determined to be available to handle the call responsive to said paging step (f). 2. The method of claim 1, wherein said steps of allocating said first and second sets of radio resources further comprise the step of: allocating a signalling channel to said terminal. 3. The method of claim 1, wherein said steps of allocating said first and second sets of radio resources further comprises the step of: allocating a traffic channel to said terminal. 4. The method of claim 1, further comprising the step of: storing an indicator of said loss of radio contact in said first node. 5. The method of claim 1, further comprising the step of: providing, to a second node which generates said call delivery message, a roaming number for said terminal after said terminal is determined to be available in response to said pre-paging step (b) . 6. The method of claim 5, wherein said first node is a visited mobile switching center (VMSC) and said second node is a gateway mobile switching center (GMSC). 7. A method for establishing a connection with a terminal in a radiocommunication system comprising the steps of: a) pre-paging the terminal to determine its availability; b) selectively allocating a first radio channel to said terminal based on a result of said pre-paging step; c) paging said terminal for a second time due to said detected loss of said allocated radio resources; and d) allocating a second radio channel to the call if the terminal is determined to be available to handle the call responsive to said paging step (f). 8. A method for communicating with a terminal in a radiocommunication system comprising the steps of: a) initiating a call terminating with the terminal; b) pre-paging the terminal to determine the availability of the terminal; c) allocating radio resources to said terminal if said terminal is available; d) initiating a guard timer; e) sending a call initiation signal to a node supporting communications with said terminal; f) determining whether said guard timer expires before the call initiation signal is received by said node; and g) releasing the radio resources previously allocated if it is determined that the call initiation signal has not been received before an expiration of the guard timer. 9. The method of claim 8, wherein said guard timer elapses after 20 milliseconds. 10. A method for resource handling associated with paging a terminal in a radiocommunication system comprising the steps of: a) allocating radio resources to said terminal after a successful pre-page; b) starting a guard timer responsive to step a); and c) deallocating said radio resources if a call delivery message is not received before said guard timer times out. 11. The method of claim 10, further comprising the steps of: pre-paging said terminal; and sending, to a node which generates said call delivery message, a roaming number for said terminal after said terminal is determined to be available in response to said pre-paging step.

Inventors: BURGALETA-SALINAS ALBERTO, HU YUN CHAO, BURGALETA-SALINAS, ALBERTO, HU, YUN, CHAO
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 2000
Main classification: H04Q7
IPC codes: H04W68