RE-SELECTION FOR REAL TIME PACKET DATA NETWORKS

Abstract: There is disclosed a system for re-selection to a neighbor cell while a mobile station is engaged in a packet data session in a cellular packet data network system. The system includes a mobile station control system adapted to transmit a request to change the channel from a serving base station of the packet data session to a new base station, and if the request is accepted, then the mobile station acquires service with the new base station, otherwise continuing communication with the serving base station. A network control system is adapted for the serving base station to communicate with the new base station to selectively accept or reject the request to change channel, and to transmit a response to the mobile station indicating that the request was accepted or rejected, and if the request is accepted to also transmit identification information for a new channel associated with the new base station.

Description: RE-SELECTION FOR REAL TIME PACKET DATA NETWORKS BACKGROUND OF THE INVENTION The present invention generally relates to cellular and wireless communication. More specifically, the invention relates to a method and apparatus for performing a fast channel re-selection in a packet data communication system. Recently, there has been a trend in the telecommunication community to focus more and more on wireless packet data communication rather than circuit switched communication. With the tremendous increase of Internet users, and usage of Internet protocols, it is believed that the packet switched communication will soon become larger than the circuit switched communication that today dominates, e.g., the cellular communication. Cellular communication system manufacturers and operators are therefore looking for solutions to integrate their circuit switched services with wireless packet switched services that can provide reliable and more spectrum efficient connections for packet switched users, e.g., Internet users. This trend has made different types of packet switched communication system evolutions flourish. One of the more well known packet switched cellular systems in the telecommunications community, is the extension of the present GSM (Global System for Mobile Communications) cellular communication system, called GPRS (General Packet Radio Service). GPRS is a packet switched system that uses the same physical carrier structure as the present GSM cellular communication system and is designed to coexist and provide the same coverage as GSM. GPRS radio interface is thus based on a TDMA (Time Division Multiple Access) structured system with 200 kHz carriers divided into eight timeslots with GMSK (Gaussian Minimum Shift Keying) modulation. The multiplexing is such that several users can be allocated on the same timeslot, and use it only when data needs to be transmitted. One user can also be allocated more than one timeslot to increase its throughput of data over the air. The GPRS specification includes a number of different coding schemes to be used dependent on the quality of the radio carrier. With GPRS, data rates well over 100 kbps will be possible. There is also ongoing a development and standardization of a new air interface mode in GSM, which will affect both packet and circuit switched modes. This new air interface mode is called EDGE, Enhanced Data rates for Global Evolution. EDGE's main features are new modulation and coding schemes for both packet switched and circuit switched data communication. In addition to the Gaussian Minimum Shift Keying (GMSK) modulation, which today is used in both GPRS and GSM circuit switched mode, an 8 symbol Phase Shift Keying (8PSK) modulation is introduced. This modulation can provide users with higher data rates than GMSK in good radio environments. The packet data mode with EDGE modulation is called EGPRS (Enhanced GPRS) and the circuit switched data mode is called ECSD, Enhanced Circuit Switched Data. With EGPRS, data rates over 384 kbps will be possible with EDGE. Recent development for another TDMA based cellular system, the cellular communication system compliant to the ANSI/136 standard, below referred to as TDMA/136 has been focused on a packet data system to be integrated with the TDMA/136 circuit switched mode. This packet data system will also be based on the new EDGE technology as defined for the GPRS extension. It will then allow TDMA/136 operators with a packet data mode to provide data rates up to 384 kbps on 200 kHz carriers with GMSK and 8PSK modulation as defined for EGPRS. Two modes of EGPRS will be standardized for use together with TDMA/136 systems, one which relies on time synchronization between base stations in the system and one which does not. These two modes are generally referred to as COMPACT and Classic respectively. While the evolution of cellular packet data communication initially has focused on developing a system that efficiently utilize resources to transfer delay-insensitive data, the focus is now shifting towards delay sensitive transmissions and higher quality of service requirements. The main application discussed is voice. While it is today rather straightforward to establish and keep a circuit switched connection for voice communication, it is more difficult to do the same for a packet switched system, since these systems are traditionally designed for delay-insensitive data. These lower delay demands are easily recognized in, e.g., the way resource allocation and cell- or channel re-selection is performed in, for example, a GPRS/EGPRS system. Re-selection of cell, or rather, from a first base station (referred to as serving base station), to a second base station (referred to as target base station) is required e.g., if the signal level of transmissions to and from the serving base station becomes too degraded. Alternatively, the network can otherwise decide to move the allocation of a mobile station to another base station. As mentioned earlier, the resource allocation is such that several users may be multiplexed on a single transmission resource. This may obviously lead to some delay (and variations of delay), if, e.g., several users allocated on the same resource have data to transmit at the same time. This is in contrast to a circuit switched connection, where a user is a sole owner of a resource, irrespective of whether or not transmission of data actually occur or not. The process whereby the network and a mobile station interact so as to ensure the mobile station is always served by the best possible channel available can be referred to as re-selection. For a circuit switched connection, which today is the common connection for voice, the re-selection is made in as smooth a manner as possible in order to maximize the quality of service. The network decides that a re-selection shall be made and secures that a target base station is ready to act as a new serving base station to the user, at the same instant as the user is commanded to terminate a connection with the current serving base station. The network relies on measurement reports sent from the mobile to make the decision. In circuit switched systems and during an active connection, one usually call this procedure a " mobile assisted handover". In a packet switched system like GPRS, re-selection is designed such that an MS may autonomously determine when re-selection shall occur. The MS leaves an ongoing connection to a serving base station and establish (by way of control signalling to the target base station) a new connection and request for transmission resources to a target base station. In the circuit switched case, there is thus a larger amount of "preparation" before the handover actually takes place: A target base station is identified, and resources are allocated before the base station switch occurs. In a packet switched system, where autonomous re-selection is used, no preparations occur in the network. This may cause some delay before a resource may be allocated in the target base station. These interruptions are of little or no significance if a user, e.g., is involved in a browsing session on the Internet, however a user involved in a delay- sensitive application like voice, may find this unacceptable. There is thus need for a mechanism to secure a "handover-like" re-selection also in packet data systems, should the voice over packet data ever going to be comparable in quality to the present circuit switched voice connections. SUMMARY OF THE INVENTION In one aspect of the present invention, a method and apparatus is provided for fast re-selection in a cellular packet communication system. The method and apparatus ensures that a re-selection of base stations, from a serving to a target base station, may be performed without introducing delay for signalling and allocation of transmission resources to the target base station. This will substantially decrease the time to expedite the re-selection and thereby shorten the time between suspending communication with a mobile station in a serving base station and resuming communication with the user equipment in a target base station. In one aspect of the present invention, a fast re-selection is initiated in a mobile station by transmitting a re-selection command to a serving base station. The re-selection request includes at least one parameter, at least sufficient for the serving base station to identify a target base station. The serving base station (or a network node controlling the serving base station) then signals to the target base station (or a network node controlling the target base station) to request a reservation of transmission resources for the mobile station. The target base station respond to the serving base station, with the result of the request by transmitting at least one parameter. The response to the request may be "acknowledge", meaning that transmission resources are reserved, or "not acknowledge" meaning that transmission resources are not reserved. The result of the request is then acted upon appropriately by the serving base station and transferred to the mobile station. In another aspect of the present invention, parameters to include in a re- selection command message transmitted from a mobile station to a serving base station may be determined by the network by way of broadcast messages or associated control signalling messages, transmitted on an allocated transmission resource. In yet another aspect of the present invention, a timing advance value is determined in a mobile station operating in a system where base station sites are time synchronized. The timing advance value, indicating the timing difference corresponding to the propagation delay between a base station and a mobile station, is determined in the mobile station prior to any communication with the target base station. The determination of timing advance value for the target base station is performed solely by the mobile station, by estimating the difference in time alignment of transmissions originating from the serving base station and those originating from the target base station. The above described and other features and advantages of the invention are explained in detail in hereinafter with reference to the illustrative examples shown in the accompanying drawings. Those of ordinary skill in the art will appreciate that the described embodiments are provided for purposes of illustration and that numerous equivalent embodiments are contemplated therein. BRIEF DESCRIPTION OF THE DRAWINGS Features, objects and advantages of the present invention will become apparent to those skilled in the art by reading the following detailed description where references will be made to the appended figures in which; Figure 1 illustrates a cellular pattern in of an exemplary cellular packet data communication system; Figure 2 illustrates a flow chart of a re-selection process as implemented in a mobile station, according to an exemplary embodiment of the present invention; Figure 3 illustrates a flow chart of a re-selection process implemented in a network, according to an exemplary embodiment of the present invention; and Figure 4 illustrates an overview of GPRS nodes present in an exemplary GPRS system. DETAILED DESCRIPTION The present invention will now be described making references to a GPRS based cellular packet data communication system and extensions thereof, as briefly described in the background. In a GPRS communication system, physical channels on which communication can occur are divided into timeslots on a radio frequency carrier. Each carrier frequency is divided into eight timeslots, or eight physical channels. Eight consecutive timeslots form a GSM frame. The timeslots (TS0-TS7) refers to both up- and downlink timeslots. Figure 1 illustrates a simple cellular reuse pattern. A mobile station, MS, (10) is illustrated as communicating with a base station (12) The mobile station (10) is, while occupied in transmission and reception, allocated a transmission resource. In the figure, the transmission resource in the coverage area the MS (10) is located in, is a frequency denoted F2. The MS will communicate over the frequency F2 until triggered to do a re-selection of a channel resource. This may for example be when the MS (10) is moving around to a distant location from the serving base station (12) such that the signal quality degrades, e.g., because of too high path loss. In such cases, the cellular structure of the system usually provides better communication quality if the MS communicates with another base station (14), over another frequency, F3. In Figure 1, the signal quality from a target base station (14) may provide adequate signal quality conditions for communication and the MS (10) should thus perform a re-selection to base station (14) when the serving base station (12) no longer suffice. Both during an active communication session and in idle mode, the mobile station monitors neighbor base stations, both for identification purposes by reading the base station identity code (BSIC) broadcast, but also for purposes of monitoring received signal levels. The base stations to monitor, or rather, the physical channels to monitor are indicated in broadcast messages from the serving base station. These measurements are usually continuously reported to a network node in a circuit switched system, for mobile assisted handover (MAHO) purposes, but not necessarily in a packet switched system for re-selection purposes. In one aspect of the present invention, the signal strength measurements made on neighbor base station transmissions need not be reported, the mobile station is responsible in determining when re-selection should occur. Thus, compared to circuit switched systems, where measurement reports are repeatedly transmitted to a serving base station, no such measurement reports are transmitted in the inventive method of fast re-selection. This will save considerable uplink resources, that can be used for other communication. A mobile station occupied in transmission may determine itself when re- selection should occur, and initiate this by sending a re-selection request to the serving base station. This re-selection request may include information about which base station the mobile station has identified as a target base station. Additional information may also be transmitted, e.g., signal strength values for the serving as well as target base station, or other indications justifying the re-selection request. The serving base station receives the re-selection request and forward the request, possibly via one or several network nodes, to the target base station. Possibly, the serving base station process the request and e.g., add or delete information like the present allocation and quality requirement of transmission. This is something that not necessarily has to be included in the re-selection request transmitted from the mobile station, since the information is present also in the serving base station. The target base station receives the request and, either allocate resources for the mobile station (if resources are available) and accept the re-selection request, or does not allocate resources, i.e., does not accept to serve the mobile station. A response is then transmitted from the target base station to the serving base station, indicating whether the mobile station may start communication through the target base station or not. If accepted, the response include an "acknowledge" message and if not accepted, a "not acknowledge" message is forwarded to the mobile station through transmission resources with the serving base station. If acknowledged, the response to the mobile station also indicates allocation information, i.e., what timeslot/s to start communication over in the new cell. The re-selection may be completed by an acknowledgement from the mobile station to the serving base station indicating the re-selection is/will be performed. The serving base station forward this indication to the target base station, to alert that the mobile station will perform the re-selection. Typically, this information may also include a certain time indication, when the re-selection actually will occur. If the re-selection request is not acknowledged in the target base station, the response to the mobile station may include information indicating that no further re-selection attempts to the same target base station may occur for a certain period of time. Referring now to Figure 2, wherein the procedure for fast re-selection in a control system of the mobile station is shown. In Figure 2 is depicted the procedure for a mobile station active in a data session, i.e., transmitting or receiving or is ready to transmit and receive data (20). The mobile station continuously monitors signal strength (21) from other base station transmissions according to the neighbour list broadcast by the serving base station. When signal quality in the communication between the mobile station and the serving base station becomes degraded, the mobile station determines if there is another base station to do a re-selection to (22). If not, then the MS continues communication with the serving base station. If there is, the mobile station sends a re-selection request to the serving base station, 23. The mobile station receives a response (24) to the re-selection request from the serving base station. Dependent upon the result of the re-selection request the mobile station either continues communication with the current serving base station (not acknowledge), (25) or perform a re-selection (26) according to the indication (acknowledge) and continue transmission with the target base station (27). The target base station is then the new serving base station and the re-selection is then finalized (28). Typically in the step of continuing transmission with the serving base station (25) there is an update of a penalty indication that prohibits the mobile station to attempt reselect to the same target base station for a certain amount of time. Referring now to Figure 3, wherein the procedure for fast re-selection in the serving and target base station or one or several network nodes controlling the base stations is shown. In figure 3 is depicted the procedure when a serving base station is active in communication with a mobile station (30). A serving base station is receiving a reselect request (31) and forwards this to a target base station indicated in the request (32) possibly after some processing of data included in the request, or addition of data to the request that is stored about the communication in the base station. The serving base station receives a response to the request from the target base station (33). If the re-selection is accepted by the target base station (36), the serving base station may communicate this to a network node controlling the communication flow to different base stations (37). The acknowledge message is also transmitted to the mobile station ( 38) such that it can perform the re-selection. If the re- selection is not accepted (24), the serving base station will send this reject result to the mobile station. Possibly, in the transmissions, the serving base station may process the messages transmitted to or from the mobile, network nodes and target base station, by adding or extracting information. Alternatively the serving base station may itself, based upon information about the cause for re-selection request from the mobile station, acknowledge or not acknowledge the request. Referring now to Figure 4, illustrating a number of system nodes in an exemplary packet data communication system according to GPRS . In other exemplary systems, additional nodes may occur, or some node may be absent. In Figure 1, an MS (42) may communicate with a serving base station (43). The serving base station (43) is the base station receiving a re-selection request. Of course, more base stations and MSs are usually present in packet data systems, as illustrated in Figure 1. The serving base station may be connected to a Base Station Control node, BSC (44), which in turn is connected to a Serving GPRS Support Node, SGSN (45), serving one or several BSCs. The SGSN is typically the node controlling the packet flow to and from the different base stations, via the BSCs. Another GPRS support node is a Gateway GPRS Support Node (46) connected to e.g., Internet or other external networks (not illustrated). In Figure 4, a control unit (47) is illustrated. It is exemplary located in the base station (43), but could alternatively be located in other network nodes as well, e.g., BSC or SGSN. Control functionality may also be split between different nodes, however, for simplicity it is located in one node in figure 4. The control unit will in this exemplary system control allocation in base stations as well as transmission flows and re-selection. Thus, the control unit in a serving base station receive and possibly process a re-selection request from a mobile station, before forwarding it to a target base station (48). A target base station is illustrate in dotted lines (48) In this exemplary system, the target base station is served by the same BSC (44) Alternatively a target base station that are served by another BSC (not illustrated) may be considered. The control unit in the target base station (48) receives the re- selection request, process it and send back a response to the serving base station (43). The serving base station (43) also, if re-selection is acknowledged, transmit an indication to the SGSN (45) or BSC (44) node (or both) to redirect traffic to the new, target base station. According to yet another aspect of the present invention an MS may be engaged in communication in a system where base station sites are time synchronized One such typical system is the packet data system referred to as COMPACT. This is also based on EGPRS technique, however, to be able to allow a bandwidth limited deployment, the system is time synchronized. The principles used in COMPACT are explained in US Patent Application No 09/263, 950, "High Speed Data Communication System and Method" to Mazur et al., hereby incorporated by reference. An MS which reports to a network via its serving base station that it suggests a change of base stations also estimates the distance in terms of propagation time to the target base station. Usually this is done by transmitting a shortened burst, an access burst, to the target base station. The access burst is received in the target base station and then a response is transmitted to the MS, indicating the propagation delay time. The mobile station then takes this into account in following communication to the target base station. This response is herein referred to as timing advance value. The timing advance value is coded as a certain number of modulation symbol durations. Since the base stations in a COMPACT system are synchronized the MS may determine the distance to the target base station without transmitting the access burst. The distance is found through comparison of time difference between the start of a time slot transmission from the serving base station and the start of a timeslot transmission from the target base station. These two time instants and the timing advance value the MS use with the serving base station is enough to calculate the new timing advance value like, for the target base station: TAnew =TOLD - 2(tmsold-tmsnew) Where TAnew is the timing advance value to the target base station, TOLD is the timing advance value to the serving base station, tmsold is the, in the mobile station, perceived start of a downlink timeslot from the serving base station, tmsnew is the perceived start of a downlink timeslot from the target base station, where tmsold and tmsnew are measured in modulation symbol periods or fractions thereof. With this method, the mobile station does not have to use random access resources to determine its distance to the target base station. Instead, it may start communication with the target base station channel immediately when the serving base station has informed the mobile station about the allocation in the target base station. Although the present invention has been described with examples from a packet switched communication system compliant to the GPRS/GSM specifications, it should be noted that the solutions presented is equally well applicable to any other packet switched data communication system with the same or similar structure and functionality. The specific embodiments should therefore be considered exemplary rather than limiting the scope of the invention. The invention should rather be defined by the following claims.

Claims: CLAIMS I CLAIM: 1. The method of re-selection to a neighbor cell for a mobile station communicating in a cellular packet data network, comprising the steps of: transmitting a request by the mobile station to change channel from a serving base station to a new base station; the serving base station communicating with the new base station to selectively accept or reject the request to change channel; the serving base station transmitting a response to the mobile station indicating that the request was accepted or rejected, and if the request is accepted also transmitting identification information for a new channel associated with the new base station; and if the request is accepted, then the mobile station acquiring service on the new channel, otherwise continuing communication with the serving base station. 2. The method of claim 1 wherein the mobile station periodically monitors signal strength of adjacent cells while active on a packet data channel with the serving base station. 3. The method of claim 2 further comprising developing the request by the mobile station if signal strength of a neighbor cell base station is stronger than signal strength of the serving base station. 4. The method of claim 1 wherein the step of the serving base station communicating with the new base station comprises the serving base station sending a change channel allocate message to the new base station, and if accepted the new base station sending a change channel allocate acknowledge message to the serving base station along with the identification information indicating relevant allocation parameters. 5. The method of claim 1 further comprising the step of the serving base station transmitting a change channel indication message to a network support node, the support node thereafter transmitting packet data destined for the mobile station to the new base station. 6. The method of claim 1 further comprising the step of the serving base station transmitting a change channel indication message to a network support node, the support node thereafter transmitting packet data destined for the mobile station to both the serving base station and the new base station. 7. The method of claim 1 further comprising the step of determining time alignment by the mobile station to use with the new channel prior to acquiring service on the new channel. 8. The method of claim 7 further comprising the step of determining time alignment by the mobile station by comparing downlink time difference between the serving base station and the new base station. 9. The method of claim 1 further comprising the step of periodically monitoring signal strength by the mobile station of adjacent cells identified in a stored re- selection list while active on a packet data channel with the serving base station. 10. The method of claim 9 further comprising the step of removing a particular cell from the re-selection list if a request to change channel to the particular cell is rejected. 11. In a cellular packet data network system, a system for re-selection to a neighbor cell while a mobile station is engaged in a packet data session, comprising: a mobile station having a mobile station control system, said mobile station control system transmitting a request to change channel from a serving base station of the packet data session to a new base station, and if the request is accepted, then the mobile station acquires service with the new base station, otherwise continuing communication with the serving base station; and a network control system coupled to the serving base station and the new base station, said network control system communicating with the new base station to selectively accept or reject the request to change channel, said network control system further transmitting a response to the mobile station indicating that the request was accepted or rejected, and if the request is accepted also transmitting identification information for a new channel associated with the new base station. 12. The system of claim 11 wherein the mobile station control system periodically monitors signal strength of adjacent cells while engaged in the packet data session with the serving base station. 13. The system of claim 12 wherein the mobile station control system develops the request if signal strength of a neighbor cell base station is stronger than signal strength of the serving base station. 14. The system of claim 11 wherein the network control system sends a change channel allocate message from the serving base station to the new base station, and if accepted the new base station sends via the network control system a change channel allocate acknowledge message to the serving base station along with the identification information indicating relevant allocation parameters 15. The system of claim 11 wherein the network control system transmits a change channel indication message to a network support node, the support node thereafter transmitting packet data destined for the mobile station to the new base station. 16. The system of claim 11 wherein the network control system transmits a change channel indication message from a serving base station to a network support node, the support node thereafter transmitting packet data destined for the mobile station to both the serving base station and the new base station. 17. The system of claim 11 wherein the mobile station control system determines time alignment to use with the new channel prior to acquiring service on the new channel. 18. The system of claim 17 wherein the mobile station control system determines time alignment by comparing downlink time difference between the serving base station and the new base station. 19. The system of claim 11 wherein the mobile station control system periodically monitors signal strength of adjacent cells identified in a stored re-selection list while active on a packet data channel with the serving base station. 20. The system of claim 19 wherein the mobile station control system further comprises removing a particular cell from the re-selection list if a request to change channel to the particular cell is rejected.

Inventors: DIACHINA JOHN, LINDHEIMER CHRISTOFER, MOLNO JOHAN, RYDNELL GUNNAR, DIACHINA, JOHN, LINDHEIMER, CHRISTOFER, MOLNO, JOHAN, RYDNELL, GUNNAR
Applicants: ERICSSON INC, ERICSSON INC.
Application year: 2000
Main classification: H04Q7
IPC codes: H04J3, H04L12, H04W36