Method of controlling handover, base station and mobile station for use In a communication network Background of the invention The present invention relates to a method of managing mobility in a communication network wherein a mobile station is handed over from a source base station to a tar¬get base station- The present invention further relates to a base station and to a mobile station for use in a communication network. A Radio Access Network (RAN) of, e.g., a Public Land Mobile Network (PLMN) can generally be realised with a centralised or with a decentralised architecture. Using a centralised architecture, a central node (e.g., Radio Network Controller (RNC) in UWTS {Universal Mobile Telecommunications System)) controls all of the base sta¬tions or at least a group of base stations in a communication network. The central node provides functions such as radio resource management, transmits power man¬agement, handover decisions, etc. for all radio cells in its field of responsibility (e.g. Radio Network Subsystem (RNS) in UMTS) in the communication system. In a decentralised architecture with no central node, all of said functions of the centra! node have to be distributed among the base stations of the communication network. in the latter context, a particular problem concerning handover arises if base stations from different manufacturers are used in the RAN: a source base station currently serving a mobile station or user equipment performs a handover decision primarily based on measurement reports comprising quality-related radio link parameters from said user equipment (UE), However, an algorithm used for deriving said handover decision generally is not standardised due to competition among manufacturers. If the UE switches to a base station (target base station) of another manufacturer, then a different algorithm for a handover decision will generally be employed in that base station, in this way, it may happen that the target base station immediately decides on a handover back to the source base station (so-called backward handover) even if the measurement report on radio link quality, which is the basis for the handover de¬cision, Is essentially the same {e.g., UE has not moved). ■ In this particular case, the UE Is swapped between the source base station and the target base station in a "ping-pong" manner. Such a ping-pong effect will produce a large amount of signlling and will influence the services running on the UE in a negative manner, e.g., by Increase of packet loss, service delay, and jitter. In current systems with centralised architecture {e.g. UMTS) similar situations occur, e.g., when the central entity deciding on handover Is changed during a handover. Examples for this are UE involved SRNS Relocations or Inter RAT handovers. In the above-mentioned examples the algorithms used often differ from each other because the nodes hosting these algorithms are of different releases, from different technol¬ogy, or from different vendors. However, in centralised systems interoperability is possible without extensive ping-pong events owing to the fact that the different algo¬rithms have to follow some specifications, operator demands, and a common under¬standing of the desired behaviour by the different vendors. Furthermore, only a small part of the handover processes actually coincide with a change of the handover deci¬sion entity. Contrary to that, in a decentralised architecture every handover is accompanied with a change of the handover decision entity, thus leading to an increased risk of ping-pong events. Object of the invention It is the object of the present invention to provide a method of the above-defined type as well as a base station and a mobile station for use in a communication network which avoid ping-pong effects during handover. Summarv of the Invention According to a first aspect of the present invention the object is achieved by providing a method of the above-defined type, said method comprising: - providing the target base station with a first measurement report on radio link qual¬ity, said first measurement report comprising radio link quality measurements by the mobile station prior to said handover; , - providing the target base station with a second measurement report on radio link quality, said second measurement report comprising radio link quality measure¬ments by the mobile station after said handover; - in case of a proposed handover of the mobile station back to the source base sta¬tion, comparing said first and second measurement reports; - during a predefined time interval, allowing the proposed handover only in case of an overload situation at the target base station and/or in case of differences be¬tween said first and second measurement reports which are above a predefined threshold. According to a second aspect of the present invention, the object Is achieved by pro¬viding a base station for use in a mobile communication network, comprising: - means for receiving, in connection with a handover of a mobile station from a fur¬ther base station in said communication network, a first measurement report on radio link quality by the mobile station from a first entity in said communication network; - means for receiving a second measurement report on radio link quality by the mo¬bile station from the mobile station; ... - means for evaluating at least its own load situation and for providing an evaluation result; - means for comparing said first and second measurement reports and for providing a comparison result; - means for commanding a handover back to said further base station in accor¬dance with said evaluation result and with said comparison result during a prede¬fined time interval only if said evaluation result indicates an overload situation and/or if differences between said first and second measurement reports (MR1, MR2) are above a predefined threshold. According to a third aspect of the present invention the object is achieved by provid¬ing a mobile station for use in a mobile communication network, comprising means for measuring a radio link quality to a base station in said network, further compris¬ing: - means for storing, in connection with a handover, a first measurement report on radio link quality prior to said handover; - means for providing said first measurement report and a second measurement re¬port on radio link quality after said handover to a currently serving base station. Thus, in accordance with a basic idea underlying the present invention, the proposed solution is primarily based on informing the target base station about the basis of de¬cision (measurement report) for the performed handover. After handover from source base station to target base station, the latter is not allowed to command a backward handover, if a new measurement report from the UE to the target base station has not changed substantially with respect to the measurement report which formed the basis for the performed handover. In other words, for backward handover differences between said two measurement reports have to be above a predefined threshold (hysteresis). Otherwise, only in case of an overload in the target base station, a backward handover may be allowed. However, in this case and in a further embodiment of the method in accordance with the present invention, information about said overload situation may be provided to the source base station, so that no second-handover from the source base station back to the overloaded target base station will be commanded. In a corresponding embodiment of the base station in accordance with the present invention, the latter comprises means for providing information with respect to said overload situation to said further base station. in an embodiment of the method in accordance with the present invention, the latter comprises blocking handover back to the source base station during a predefined time interval in case of said differences being below said 'predefined threshold, thus further enhancing capability of preventing ping-pong effects. In an embodiment of the method in accordance with the present invention, the latter comprises sending said first measurement report to the target base station prior to said handover and storing said first measurement report at the target base station for comparing with said second measurement report. Said embodiment therefore re¬quires corresponding storing means at the respective base stations. In a corresponding embodiment of the base station in accordance v/ith the present invention, the latter comprises means for storing at least said first measurement re¬port. In another embodiment of the method in accordance with the present invention, the latter comprises storing said first measurement report at the mobile station and send¬ing said first measurement report to the target base station together with said second measurement report. Said embodiment therefore requires corresponding storing means at individual mobile stations. In a further embodiment of the method in accordance with the present invention, the latter comprises sending said first measurement report from the mobile station to the source base station and forwarding said first measurement report to the target base station together with a load report indicating a current load situation at the source base station. Further advantages and characteristics of the presentinvention-can be gathered from the following description of preferred embodiments given by way of example only with reference to the enclosed drawings. Features mentioned above as well as below can be used in accordance with the present invention either individually or in con¬junction. The described embodiments are not to be regarded as an exhaustive enu¬meration but rather as examples with respect to a general concept underlying the present invention. Brief description of the drawings Fig. 1 is a schematic block diagram of a base station in accordance with an embodiment of the present invention for use in a mobile communication network; Fig. 2 is a schematic block diagram of a mobile station in accordance with an embodiment of the present invention for use in a mobile communication system; Fig. 3 is a schematic signalling diagram for illustrating a first embodiment of the method in accordance with the present invention; and Fig, 4 is a schematic signalling diagram for illustrating a second embodiment of the method in accordance with the present invention. Detailed description Fig. 1 shows a schematic block diagram of a base station 1 in accordance with an embodiment of the present invention for use in a mobile communication network. According to the embodiment of Ffg, 1, base station 1 comprises transceiving means 2, evaluating means 3, comparing means 4, handover commanding means 5 includ¬ing timer means 6 and handover decision algorithm 6', first storing means 7, and -optionally - second storing means 8. Within transceiving means 2, base station 1 fur¬ther comprises first means 2.1 for receiving information related to a load situation from a further base station (not shown) similar to base station-1-in-a communication network, second means 2.2 for providing information to its own load situation to a fur¬ther base station, third means 2.3 for receiving measurement reports comprising in¬formation on radio link quality from further entities (e.g., base stations and/or mobile stations; not shown) in said communication network, and fourth means 2.4 for provid¬ing such measurement reports to further base stations in said communication net¬work. Functioning of the individual means of base station 1 will be described in detail below with reference to appended Figs, 3 and 4. In particular, optional storing means 8 in Fig, 1 is needed for implementing the variant of the method in accordance with the present invention as depicted in Fig. 3. An embodiment of base station 1 without said optional storing means 8 can be used in the embodiment of the method in accor¬dance with the present invention as depicted in Fig. 4, Fig. 2 shows a schematic block diagram of a mobile station 9 in accordance with an embodiment of the present invention for use in a mobile communication system. Mo¬bile station 9 (hereinafter also referred to as user equipment, UE) comprises tran-sceiving means 10 including first means 10.1 for providing measurement reports comprising information on radio link quality to at least one base station in a commu¬nication network, such as base station 1 of Fig. 1. Transceiving means 10 further comprises second means 10.2 for receiving command/request messages from a base station, as will become apparent later. Besides transceiving means 10 mobile station 9 further comprises means 11 for measuring said radio link quality, i.e., corre¬sponding radio link parameters, as known to a person skilled in the art, and for pro¬viding said measurement reports. Furthermore, mobile station 9 comprises optional storing means 12, which are intended for use in the embodiment of the method in ac¬cordance with the present invention as depicted in appended Fig. 4. Operation of base station 1 {Fig. 1) and mobile station 9 (Fig. 2) will now be ex¬plained in detail with reference to embodiments of the method In accordance with the present invention as depicted in appended Figs. 3 and 4. Fig. 3 shows a schematic signalling diagram for illustrating a first embodiment of the method in accordance with.the-presentinvent!Qn.-i?eferring=to.Eig-,thereJsd6-.. picted a mobile communication network 13 comprising user equipment 9 and first and second base stations 1.1,1.2, respectively. Hereinafter, the first base station 1.1 will also be referred to as source base station, whereas the second base station 1.2 will also be referred to as target base station. As already stated above with reference to appended Fig. 1, in the embodiment of Fig. 3 said first and second base stations 1.1,1.2 comprise first and second storing means 7.1, 7.2 and 8.1, 8.2, respectively. Hereinafter, said first storing means 7.1, 7.2 will also be referred to as toad report da¬tabase, and said second storing means 8.1, 8.2 will also be referred to as measure¬ment report database with reference to their respective contents. In the following it is assumed that UE 9 is in connected mode, can receive signals from source base station 1.1 and target base station 1.2, and is currently being sen/ed by source base station 1.1. According to the embodiment of Fig, 3, UE 9 transmits (e.g., on demand or periodi¬cally) a measurement report with measurement results of the serving link (with source base station 1.1) and,potential candidate linl