Abstract:
Intersystem connection handover is based on transmission quality of a first radio communication system to a base station as determined by a subscriber station. If the transmission quality is lower than a threshold value, a transmission quality determination routine to a base station of the second radio communication system is triggered. The determination routine is continued until a criterion separate from the threshold is fulfilled.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and hereby claims priority to German Application No. 100 109 58.6 filed on 6 Mar. 2000, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for an intersystem handover. In particular, the invention is suitable for use in a mobile radio system or wireless subscriber access system. 
     2. Description of the Related Art 
     In radio communication systems (also called radio access technology—RAT—in the text which follows), for example the second-generation European mobile radio system GSM (global system for mobile communications), information such as, for example, voice, image information or other data are transmitted via a radio interface with the aid of electromagnetic waves. The radio interface relates to a connection between a base transceiver station and a multiplicity of subscriber stations, where the subscriber stations can be, for example, mobile stations or stationary radio stations. Electromagnetic waves are radiated with carrier frequencies which are located within a frequency band provided for the respective system. For future radio access technologies, for example the UMTS (universal mobile telecommunication system) or other third-generation systems, frequencies are provided in a frequency band of approx. 2000 MHz. For the third mobile radio generation UMTS, two modes are provided, one mode being an FDD (frequency division duplex) mode and the other mode being a TDD (time division duplex) mode. These modes are used in different frequency bands and both modes support a so-called CDMA (code division multiple access) subscriber separation method. 
     For descriptions of the second-generation mobile radio system GSM, the book by J. Biala “Mobilfunk und Intelligente Netze” (which may be translated as “Mobile radio and intelligent networks”), Vieweg Verlag, 1995 provides the basic general state of the art. 
     Due to the fact that the second- and third-generation radio access technologies exist in parallel and it is desired to harmonize between them, subscriber stations which have set up a connection in one radio access technology are to be provided with the possibility of handing the connection over to another radio access technology which may support a different transmission mode. This handing over between technologies is also called intersystem handover. 
     At the beginning of the spread of the future UMTS mobile radio system, so-called multimode subscriber stations will support both the GSM standard and the UMTS standard at the least. This is mainly of significance to operators which are implementing both, for example, a full coverage of an entire country by the GSM mobile radio system and an initially locally limited coverage by the UMTS mobile radio system. 
     According to the GSM standard, the subscriber station must observe, for example, the received signal strength indicators (RSSI) of up to 32 neighboring cells and the six neighboring cells offering the best transmission conditions must be periodically signaled to the base transceiver station providing coverage. 
     In parallel with this observation of neighboring cells of the system to which the subscriber station has set up a connection or is in a so-called idle mode, the subscriber station will also have to additionally observe radio cells of one or more radio access technologies available in parallel in future. On the one hand, this disadvantageously increases the energy consumption of the subscriber station and, on the other hand, a certain time for observing all available systems is required in each case. For this reason, it is basically logical if the subscriber station initially only observes radio cells of one system and an observation of another system is only stimulated if the transmission quality to this system drops below a certain threshold value. 
     For the GSM standard, it is presently proposed to expand the so-called control parameters in such a manner that two threshold values QSEARCH and QACCEPT are additionally signaled in the control parameters in the downlink. The complete list of control parameters is given in a table in  FIG. 2 . The proposed threshold values are emphasized in italics. 
     The first threshold value QSEARCH describes a lower value for the quality of a received GSM signal above which no measurements of further systems (alternate RATs) are performed. The second threshold value QACCEPT describes a lower value for the quality of the signal of the alternate RAT below which a handover to the other system is unacceptable. 
       FIG. 4  is used for describing how these additional parameters affect the behavior of a subscriber station. Referring to  FIG. 1  described hereinafter, values determined in the subscriber station for a transmission quality are plotted against time t. The time-variant first signal sig 1  is, for example, the general signaling channel BCCH (broadcast control channel) from a base transceiver station of a GSM mobile radio system, which is transmitted by the base transceiver station with an increased and constant transmit power. The time-variant second signal sig 2  is, for example, a general signaling channel BCH (beacon channel) transmitted by a base station of a UMTS mobile radio system. The time variance is mainly due to a movement of the subscriber station, but a change in the traffic loading within the radio cell within which the subscriber station is located can also lead to fluctuations in the transmission quality. 
     According to the proposal described, the subscriber station begins to observe the second signal sig 2  after the first signal drops below the first threshold value QSEARCH and ends this observation after the first threshold value QSEARCH has subsequently been exceeded etc. In this process, the consequence of dropping below the first threshold value QSEARCH and the second signal sig 2  simultaneously exceeding the second threshold value QACCEPT would be, for example, that a handover to the second system is stimulated. 
     The consequence of this behavior is that a frequent short-time starting and stopping of the observation of the second system would occur, particularly when values of the transmission quality are close to the first threshold value. This wastes precious energy, on the one hand, and, on the other hand, the period of observation is too short for being able to draw reliable conclusions for any handover from the measurements of the transmission quality to the second system. In this context, it must be generally taken into consideration that a single measurement of a radio cell of the second system is not sufficient. Instead, a number of measurements must be performed in order to obtain a reliable result. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of specifying a method which, based on the known method, realizes a more efficient and more reliable method for handover. 
     The method according to the invention has the advantageous effect that, due to the additional criterion, the determination of the transmission quality to a base transceiver station of the second system is performed until there is a reliable finding about the appropriateness of a handover to the second system. 
     According to the invention, a time interval or a decision hysteresis is used as the criterion separate from the threshold value. 
     The method according to the invention described is used in a particularly advantageous manner in a radio access technology implemented as a mobile radio system or wireless subscriber access system. 
     Exemplary embodiments of the invention will be explained in greater detail with reference to the attached drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a block diagram of two neighboring radio access technologies, 
         FIG. 2  is a table with known control parameters, 
         FIG. 3  is a table according to  FIG. 2  containing additional control parameters according to the invention, 
         FIG. 4  is an exemplary timing diagram for the method according to the prior art, and 
         FIG. 5  is an exemplary timing diagram for the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  shows in each case sections from two mobile radio systems RAT 1 , RAT 2  as examples of radio access technologies. A mobile radio system typically consists of a multiplicity of mobile switching centers MSC or UMSC (mobile switching center or UMTS mobile switching center), respectively, which belong to a switching subsystem (SSS) and are networked together or, respectively, establish access to a landline network, and of in each case one or more base station subsystems (BSS) connected to these mobile switching centers MSC, UMSC. A base station subsystem BSS, in turn, exhibits at least one BSC (base station controller) or RNC (radio network controller) facility for assigning radio engineering resources and at least one base transceiver station BTS or NB (node B), connected thereto. A base transceiver station BTS, NB can set up connections to subscriber stations UE (user equipment) such as, e.g. mobile stations or other mobile and stationary terminals via a radio interface. Each base transceiver station BTS, NB forms at least one radio cell Z. As a rule, the size of the radio cell is determined by the range of a general signaling channel BCH (beacon channel) or BCCH (broadcast control channel) which is transmitted by the base transceiver stations BTS, NB with a transmitting power which is in each case higher than the traffic channels. In the case of sectorization or of hierarchical cell structures, it is also possible for each base transceiver station BTS, NB to cover a number of radio cells. The functions of this structure can be transferred to other radio access technologies in which the invention can be used, particularly for subscriber access networks with wireless subscriber access. 
     The example of  FIG. 1  shows a subscriber station UE which is constructed as a mobile station and which is located in a radio cell Z of the first mobile radio system RAT 1  which supports a GSM standard, and in a radio cell Z of a second mobile radio system RAT 2  which supports a UMTS standard. The subscriber station UE has set up a connection to the base transceiver station BTS, shown by way of example, of the first mobile radio system RAT 1 . During the call, the subscriber station UE periodically evaluates transmission conditions of the radio interface to surrounding base transceiver stations such as, for example, the specified base transceiver station NB of the first mobile radio system RAT 1 , for example, in the case of a deterioration of the transmission quality to the base transceiver station BTS of the second mobile radio system RAT 2  or in the case of the additional setting-up of a service, in order to stimulate a handover to the base transceiver station NB of the first mobile radio system RAT 1 . 
     If the transmission quality to the first mobile radio system RAT 1  becomes generally poorer, for example due to the fact that the subscriber station UE is moving away from the radio coverage area of the first mobile radio system RAT 1 , an additional described observation of radio cells of a parallel mobile radio system must be stimulated in order to perform a handover to this system, if necessary. 
     The same method is also used, for example, in the case of hierarchical network structures when a connection between different hierarchical levels, for example from a microcell into a macrocell which operate in a different frequency band in each case, is handed over. 
     The method according to the invention can also be extended to a handover between radio access technologies utilizing the same transmission method. This can occur, for example, in the case of a handover between a GSM 900 system and a GSM 1800 system of one or two operators. 
       FIG. 3  shows, by way of example, a table according to  FIG. 2 , containing additional control parameters according to the invention (in italics). These parameters are, on the one hand, a hysteresis QSEARCH_HYST and, on the other hand, a time interval QSEARCH_TIME. Each of these parameters by itself already solves the problem of the prior art as described. Combining the two parameters, however, very advantageously provides for very high flexibility for the operator due to the fact that the parameters can be individually adapted. 
     The hysteresis QSEARCH_HYST defines a virtually additional threshold value above the threshold value QSEARCH as can be seen from  FIG. 2 . The width of the hysteresis QSEARCH_HYST can be selected by the operator, for example, individually for a radio cell and in dependence on environmental conditions. 
     If the quality of the first signal sig 1  drops below the threshold value QSEARCH, the subscriber station UE begins to observe the quality of the second signal sig 2 . The observation is continued until the quality of the first signal sig 1  exceeds the upper limit of the hysteresis band. 
     The time interval QSEARCH_TIME defines a period within which the subscriber station UE determines at least the quality of the second signal sig 2 . In combination with the hysteresis QSEARCH_HYST, the time interval QSEARCH_TIME is only interrupted if the first signal sig 1  exceeds the upper limit of the hysteresis band. 
     The time interval can be individually adapted but it should be ensured in every case that it at least provides sufficient time for determining a reliable value with respect to the transmission quality. The values found are advantageously averaged over the time interval and the averaged value is compared with the threshold value QSEARCH. 
     The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.