RESELLING FOR DATA NETWORKS BY REAL TIME PACKAGES BACKGROUND OF THE INVENTION The present invention relates generally to cellular and wireless communication. More specifically, the invention relates to a method and apparatus for effecting rapid channel reselection in a packet data communication system. Recently, there has been a trend in the • telecommunications community to focus increasingly on wireless packet data communication instead of circuit-switched communication. With the impetuous increase of Internet users, the use of Internet protocols, it is believed that packet-switched communication will soon be greater than the circuit-switched communication that dominates today, for example, cellular communication. The manufacturers and operators of cellular communication systems 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, for example, Internet users. . This trend has made different types of evolutions of packet-switched communication systems flourish. One of the best-known packet-switched cellular systems in the community of
Telecommunications is the extension of the present GSM cellular communication system (Global System for Mobile Communication), which is known as 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 with GSM and provide the same coverage as GSM. The GPRS radio interface is therefore based on a structured system in accordance with TDMA (Time Division Multiple Access) with 200 kHz carriers divided into 8 time segments with GMSK modulation (Gaussian Minimum Shift Keying) ] The multiplexing is such that several users can be assigned the same time segment, and use it only when data must be transmitted. A user can also be assigned more than a segment of time to increase their production of data in the air. The GPRS specification includes numerous different coding schemes to be used according to the quality of the radio carrier. With GPRS, data rates greater than 100 kbps are possible. There is a constant development and an increasing standardization of the new air interface mode in GSM, which
will affect both packet switched modes and circuit switched modes. This new mode of air interface is known as EDGE, Enhanced Data Rates for Global Evolution [Increased Data Rates for Global Evolution]. The main features of EDGE are new modulation and coding schemes for packet-switched and circuit-switched data communication. In addition to the Gaussian Minimum Shift Keying (GMSK) modulation, which is used today in both GPRS switched mode and GSM circuit switched mode, a phase shift modulation is introduced (Phase Shift Keying) of 8 symbols
(8PSK). This modulation can provide users with higher data rates than GMSK in a good radio environment. The packet data mode with EDGE is known as EGPRS
(enhanced GPRS) [GPRS increased] and the circuit switched data mode is known as ECSD, Enhanced Circuit Switched Data. With EGPRS, data rates greater than 384 kbps are possible with EDGE. Recent development for another cellular system based on TDMA, the cellular communication system that complies with the ANSI / 136 standard, is known below as TDMA / 136 and has been focused on a packet data system
integrate with the switched mode according to TDMA / 136. This packet data system will also be based on the new EDGE technology in accordance with what is defined for the GPRS extension. It will allow TDMA / 136 operators with a packet data mode to provide data at a rate of up to 384 kbps in 200 kHz carriers with GMSK and 8PSK modulation in accordance with what is defined in EGPRS. Two EGPRS modes will be standardized for use in conjunction with TDMA / 136 systems, one that is based on time synchronization between base stations in the system and a system that is not based on that time synchronization. These two modes are generally known as COMPACT and Classic
(Classic) respectively. While the evolution of cellular packet data communication is initially focused on the development of a system that efficiently uses resources to transfer data not sensitive to delay, the focus is now shifting towards delay-sensitive transmissions and toward higher quality of data. service requirements. The main application commented is the voice. While it is simple today to establish and maintain a switched circuit connection for voice communication, it is more difficult to do the same in the case of a packet switched system, since these systems are traditionally designed for data not sensitive to delay. These demands
Low in terms of delay are easily recognized, for example, in the way in which the allocation of resources and the reselection of cells is carried out by channels, for example, a GPRS / EGPRS system. The cell reselection, or preferably, from a first base station. { known as a service base station), up to a second base station (known as a target base station) is required, if the level of transmission signal to the service base station and from the service base station is too degraded. Alternatively, the network may decide otherwise to move the assignments. from one mobile station to another base station. As mentioned above, the allocation of resources is such that several users can be multiplexed into a single transmission resource. This can obviously cause certain delays (and delay variations), if for example several users assigned in the same resource have data to transmit at the same time. This is in contrast to a circuit switched connection where a user is the sole owner of a resource, regardless of whether or not the data transmission actually occurs or not. The process by which the network and a mobile station interact to ensure that the mobile station is always served by the best available channel can be mentioned as reselection.
For a circuit switched connection that is currently the common connection for voice, the reselection is carried out as smoothly as possible in order to optimize the quality of service. The network decides that a reselection must be made and ensures that a target base station is ready to act as a new service base station for the user at the same time that the user is asked to terminate a connection to the base station of current service. The network is based on measurement reports sent from the mobile phone to make the decision. In systems switched by circuits and during an active connection, this method is commonly known as an "assisted mobile transfer". In a packet switched system, for example GPRS, the reselection is designed in such a way that a mobile station can autonomously determine when a reselection occurs. The mobile station leaves a continuous connection to a service base station and establishes (via control signaling to the target base station) a new connection and requests transmission resources to a target base station. In the case of circuit switching, there is a larger amount of "preparation7" before the transmission itself: a target base station is identified and resources are allocated before base station switching occurs.
packages, where an autonomous reselection is used, no network preparation occurs. This may cause some delay before the allocation of a resource in the target base station. These interruptions are of little importance or have no relevance if the user, for example, is involved in a browsing session on the Internet, however, a user involved in a delay-sensitive application, for example, voice, may find this unacceptable situation The prior art, EP 0 589 552 A, discloses an example of a packet switched system that uses a method for transferring nodes in order to maintain the connectivity of these nodes in a wireless local area network. There is therefore a need for a mechanism to ensure a "transfer type" reselection also in packet data systems, if the voice over the packet data is to be comparable in quality to circuit switched voice connections. current SUMMARY OF THE INVENTION In one aspect of the present invention, a method and apparatus for rapid reselection in a cellular packet communication system is provided. The method and apparatus ensures that a reselection of these base stations, from a service base station to a target base station, can be performed without introducing delay.
for signaling and allocation of transmission resources to the target base station. This will substantially decrease the time to accelerate the reselection and thereby shorten the time between the suspension of communication with a mobile station in a service base station and the resumption of communication with the user equipment in a target base station. In one aspect of the present invention, rapid reselection is initiated in a mobile station by transmitting a reselection command to a service base station. The reselection request includes at least one parameter, at least sufficient for the service base station to identify a target base station. The service base station (or a network node that controls the service base station) then sends signals to the target base station (or a network node that controls the target base station) to request a reservation of transmission resources for the mobile station. The target base station responds to the service base station, with the result of the request by transmitting at least one parameter. The response to the request can be "acknowledgment of receipt", which means that the transmission resources are reserved, or "no acknowledgment of receipt" which means that the transmission resources are not reserved. It is handled later
appropriately the result of the request through the service base station and transferred to the mobile station. In another aspect of the present invention, a time advance value is determined in a mobile station operating in a system where base station sites are synchronized in time. The time advance value, which indicates the time difference corresponding to the propagation delay between a base station and a mobile station, is determined in the mobile station before any communication with the target base station. The determination of the time advance value for the target base station is effected only by the mobile station, by estimating the difference in time alignment of transmissions originating from the service base station and those that originate from the target base station. The foregoing and other features and advantages of the present invention are explained in detail below with reference to the illustrative examples shown in the accompanying drawings. Those of ordinary skill in the art will note that the described modalities are provided for purposes of illustration and that numerous equivalent modalities are contemplated. BRIEF DESCRIPTION OF THE DRAWINGS
Characteristics, objects and advantages of the present invention will be apparent to those skilled in the art upon reading the following detailed description wherein reference will be made to the accompanying figures wherein: Figure 1 illustrates a cellular pattern of an exemplary cellular communication system of data by packages; Figure 2 illustrates a flow chart of a reselection process in accordance with that implemented in a mobile station, in accordance with an exemplary embodiment of the present invention; Figure 3 illustrates a flow diagram of a reselection process implemented in a network, in accordance with an exemplary embodiment of the present invention; and Figure 4 illustrates a general presentation of GPRS nodes present in an exemplary GPRS system. DETAILED DESCRIPTION The present invention will be described below with reference to a cellular packet data communication system based on GPRS and extensions thereof, in accordance with that briefly described in the background.
In a GPRS communication system, physical channels in which a communication can occur are divided into time segments in a radiofrequency carrier. Each carrier frequency is divided into 8 time segments, or 8 physical channels. Each time segment
consecutive form a GSM box. The time segments (TS0-TS7) refer to both ascending and descending time segments. Figure 1 illustrates a simple cell reuse pattern. A mobile station MS, (10) is illustrated in communication with a base station (12). The mobile station (10) is assigned a transmission resource while it is busy in transmission and reception. In the figure, the transmission resource in the coverage area where the mobile station (10) is located is a frequency indicated as F2. The mobile station communicates on the frequency F2 until it is triggered to effect a new selection of a channel resource. This can be, for example, when the mobile station (10) is traveling to a distant location from the service base station (12) such that there is a degradation of the quality of the signal, for example, due to at a too important loss of propagation. In such cases, the cellular structure of the system usually provides better communication quality if the base station communicates with another base station (14), through another frequency, F3. In Figure 1, the signal quality from a target base station '(14) can provide adequate signal quality conditions for communication and the MS (10) must therefore perform a reselection to the base station
(14) when the service base station (12) is no longer sufficient. Both during an active communication station and in an inactive mode, the mobile station monitors the neighboring base stations for identification purposes by reading the broadcast base station identity code (BSIC), as well as for the purpose of monitoring signal levels. received. The base stations to be monitored or the physical channels to be monitored are indicated in messages broadcast from the service base station. These measurements are usually reported continuously to a network node in a circuit switched system, for mobile assisted transfer purposes
(MAHO), but not necessarily in a packet switched system for reselection purposes. In one aspect of the present invention, measurements of the strength of the signal made in neighboring base station transmissions do not have to be reported, the mobile station is responsible for the determination when a reselection should occur. Thus, in comparison with circuit switched systems where measurement reports are repeatedly transmitted to a service base station, measurement reports of this type are not transmitted in the rapid reselection method of the present invention. This saves considerable uplink resources that can be
used for another communication. A mobile station busy in the transmission can determine when a reselection should occur, and initiate this reselection by sending a reselection request to the service base station. This reselection request may include information as to which base station has identified the mobile station as the target base station. Additional information may also be transmitted, for example, signal strength values for the service as well as target base station, or other indications that justify the reselection request. The service base station receives a reselection request and sends the request, possibly through one or more network nodes, to the target base station. Possibly, the service base station processes the request and, for example, adds or removes information, for example, the present assignment and the quality requirement of the transmission. This does not necessarily have to be included in the reselection request transmitted from the mobile station since the information is also present in the service base station. The target base station receives the request and either allocates resources for the mobile station (if resources are available) and accepts the reselection request or does not allocate resources, that is, does not accept
give service to the mobile station. A response is then transmitted from the target base station to the service base station, indicating whether the mobile station can begin its communication through the target base station or node. If accepted, the response includes a message of "acknowledgment" and if it is not accepted, a message of "non-acknowledgment" is sent to the mobile station through transmission resources with the service base station. If it is recognized, the response to the mobile station also indicates the allocation information, ie, in which time segment (s) the communication starts in the new cell. This reselection can terminate by a recognition of the mobile station to the service base station indicating that the reselection is made or will be made. The service base station sends this indication to the target base station to alert that the mobile station will perform the reselection. Typically, this information may also include a certain indication of time, when re-selection will actually occur. If the reselection request is not recognized at the target base station, the response to the mobile station may include information indicating that additional reselection attempts may not occur to the same target base station for a certain period of time. With reference now to Figure 2, the
procedure for rapid reselection in a control system of the mobile station. Figure 2 shows the procedure for a mobile station active in a data session, ie transmission or reception or is ready to transmit and receive data (20). The mobile station continuously monitors the strength of the signal (21) from other base station transmissions in accordance with the list of neighboring stations broadcast by the service base station. When the quality of the signal in the communication between the mobile station and the service base station is degraded, the mobile station determines whether there is another base station to perform a reselection (22). If this is not the case, then the mobile station is still in communication with the service base station. If another available base station exists, the mobile station sends a reselection request to the service base station 23. The mobile station receives a response (24) to the reselection request from the service base station. Depending on the result of the reselection request, the mobile station either proceeds with communication with the current service base station (no acknowledgment), (25) or reselect (26) in accordance with the indication (recognition). ) and follows the transmission with the target base station (27). The target base station is then the new service base station and the
Reselection then ends (28). Typically, in the step of following the transmission with the service base station (25), there is an update of a penalty indication preventing the mobile station from attempting to reselect the same target base station for a certain period of time. Referring now to Figure 3, the procedure for rapid reselection at the service base station and target base station or one or more network nodes controlling the base stations is shown. In Figure 3, the procedure is shown when a service base station is active in communication with a mobile station
(30) A service base station is receiving a reselection request (31) and sends said request to a target base station indicated in the request (32) possibly after certain data processing included in the request, or the addition of data to the request that is stored in the communication at the base station. The service base station receives a response to the request of the objective base station (33). If the reselection is accepted by the objective base station
(36), the service base station can communicate this to a network node that controls the communication flow to different base stations (37). The acknowledgment message is also transmitted to the mobile station
(38) in such a way that it can carry out the reselection. If the reselection (24) is not accepted, the service base station sends this rejection result to the mobile station. Possibly, in the transmissions, the service base station can process the messages transmitted to the mobile station, network nodes and target base station or from the mobile station, network nodes and target base station, by adding or information extraction. Alternatively, the service base station itself, based on information about the cause of the reselection request of the mobile station, can recognize or not recognize the request. Referring now to Figure 4, numerous system nodes are illustrated in an exemplary packet data communication system in accordance with GPRS. In other exemplary systems, additional nodes may occur, or some nodes may be absent. In Figure 4, a mobile station (42) can communicate with a service base station (43). The service base station (43) is the base station that receives a reselection request. Obviously, more base stations and mobile stations are usually present in packet data systems, as illustrated in Figure 1. The service base station can be connected to a base station control node, BSC (44) , which in turn is connected to a
service GPRS support node, SGSN (45), which serves one or more BSCs. The SGSN is typically the node that controls the flow of packets to the different base stations and from the different base stations, through the BSCs. Another GPRS support node is a gate GPRS support node (46) connected for example to the Internet or to other external networks (not illustrated). In Figure 4, a control up to (47) is illustrated. In this example it is found in the base station (43) but could alternatively be found in other network nodes as well, for example, BSC or SGSN. A control functionality can also be divided between different nodes, however for simplicity it is found in a node in figure 4. The control unit, in this example system, will control the allocation in base stations as well as transmission of flows and reselection. Thus, the control unit in a service base station receives and possibly processes a reselection request from a mobile station, before sending said request to a target base station (48). An objective base station is illustrated in dotted lines (48). In this example system, the target base station is served by the same BSC (44). Alternatively, a target base station that is served by another BSC (not illustrated) can be considered. The control unit in the target base station (48)
receives the reselection request, processes it and sends a response to the service base station (43). The service base station (43) also, if the reselection is recognized, transmits an indication to the SGSN (45) or BSC (44) node (or both) to redirect traffic to the new target base station. In accordance with another aspect of the present invention, a mobile station may be in communication in a system where base station sites are synchronized over time. A typical system of this type is the packet data system known as COMPACT. This system is also based on the EGPRS technique, however, in order to allow limited deployment of bandwidth, the system is synchronized over time. The principles used in CGMPACT are explained in US Patent Application No. 09 / 263,950, "High Speed Data Communication System and Method" of Mazur et al., Incorporated herein. by reference. A mobile station that reports to a network through its service base station suggesting a change of base station also estimates the distance in terms of propagation time to the target base station. This is usually done by transmitting a shortened burst, an access burst, to the base station
objective. The access burst is received at the target base station and then a response is transmitted to the mobile station, indicating the propagation delay time. The mobile station then takes this into account in subsequent communication to the target base station. This response is known here as the timing advance value. The timing advance value is encoded as a certain duration number of modulation symbols. Since the base stations in a CQMPACT system are synchronized, the mobile station can determine the distance to the target base station without transmitting the access burst. The distance is found by comparing the time difference between the start of a time segment transmission from the service base station and the start of a time segment transmission from the base station to the base station. objective. These two instants and the timing advance value used by the mobile station with the service base station are sufficient to calculate the new timing advance value, for the target base station, as follows: TAnew = TOLD - 2 (tmsold-tms? Ew) where TAnew is the timing advance value towards the target base station, TOLD is the advance value of
timing towards the service base station, tmsold is the perceived start in the mobile station of a downlink time segment from the service base station, tmsnew is the perceived start of a downlink time segment to from the target base station, where tmsold and tmsnew are measured in periods of modulation symbol or fractions thereof. With this method, the mobile station does not have to use random access resources to determine its distance from the target base station. On the contrary, communication with the target base station channel can begin immediately when the service base station has informed the mobile station as to the assignment at the target base station. Although the present invention has been described with examples of a packet-switched communication system that complies with the GPRS / GSM specifications, it should be understood that the presented solutions are also applicable to other packet-switched data communication systems with the same structure and functionality or with similar structure and functionality. The specific modalities must therefore be considered as examples and not be considered as limiting the scope of the invention. The invention will be defined by the following claims.