METHOD FOR ALLOCATING RESSOURCES IN A MOBILE RADIO COMMUNICATION NETWORK, CORRESPONDING TRANSMIHER AND RECEIVER. BACKGROUND OF THE INVENTION The present invention relates to a metliod for allocating resources to transnnitter adapted to send data over a radio link to a receiver in a mobile radio communication network. During data transmission over radio links, it is often necessary to track and retransmit some data portions which have not or erroneously been received due to distortions of the radio signal. ARQ (Automatic Repeat ReQuest) or HARQ (Hybrid Automatic Repeat reQuest) protocols have been designed for this purpose and are applied in most radio communication networks for guarantying the integrity of the data transmitted over radio links. In such protocols, each data entity is labeled with a sequence number. It is the task of the receiver to detect wether all data entities e.g. with increasing and continuous sequence numbers are received. If a sequence number is missing in the flow of received data entities, the receiver generates a feedback message (e.g. NACK : non acknowledgement) containing the sequence number of the missing data entity. Upon reception of such a NACK, the transmitter, which stores all data entities until they are positively acknowledged, retrieves the missing data entity and retransmits it on the radio link. On the contrary, if data are correctly received a positive acknowledgement (e.g ACK) is sent back to the transmitter Figure 1 shows a message flow between a transmitter and a receiver implementing a HARQ mechanism as known in the prior art. A resource is allocated to the transmitter by a scheduler. The transmitter sends user data using this resource to the receiver. This transmission is not successful due e.g. to bad channel conditions. The receiver detects this missing data and sends a NACK message back to the transmitter. Optionally, in case of dynamic resource allocation the transmitter gets a new resource allocation from the scheduler and resends the user data using this new resource to the receiver. In case of static resource allocation, the transmitter repeats the user data on the same resource as previously allocated. This basic mechanism has been adapted by different improvements depending on the requirements and constraints of the different radio communication networks. OFDM based air-interfaces have been more and more used in order to increase the data rate of the transmission on the air interface. Such OFDM systems are based on the fact that the different (orthogonal) sub carriers may be used in parallel to transmit data over the air Interface. In these systems, the time and frequency extensions are used to define the extension of the data entity. Such data entities are so-called Physical Resource blocks PRBs consisting of a set of contiguous sub carriers and having a predefined time extension. Some sets of sub carriers may experience bad radio channel quality over a rather long period of time resulting in that not only the first transmission of user data but also its retransmission on the same set of sub carrier have a rather high probability to not allow for a successful transmission. It has then be suggested, in case of erroneous transmission to allocate resources outside of the resources used for the first transmission. Consequently, ttie resource allocation nnechanism for retransmission may select a RPB preferably having a minimum distance to the erroneous RPB chosen so that the retransmission RPB is shifted by multiples of the radio channel coherence bandwidth in frequency domain or of the radio channel coherence time in the time domain. Such shifts in frequency extension and time extension must be communicated to the transmitter for it to adapt to better channel conditions. Such a feedback is not necessary in' case the transmitter applies a deterministic frequency extension shift known at both the transmitter and the receiver. Such a deterministic scheme is nevertheless not optimal. In case of non deterministic new resource allocation for retransmissions of user data, new frequency extension and/or time extension have to be communicated through a feedback message to the transmitter by the receiver or by any other network entity in charge of resource scheduling. Usually, this entity is in the e-NB (enhanced Node B) for radio communication systems as defined in 3GPP LTE systems. Alternatively, it may be located or in any other central entity of the network. Usually, the n/12 control channel is used for signaling dynamic resource assignment. In case of a 5MHz bandwidth, the capacity of the n/L2 control channel is limited to approximately and not more than 5 to 6 signaling of resource assignments for each transmit directions. Using this mechanism for signaling of resource assignment in case of retransmission would restrict greatly the possibility to signal resource assignment for initial transmissions causing a starvation of new resource allocation in a system experiencing particularly degraded channel conditions. A particular object of the present invention is to provide an improved signaling of the resource assignment for further transmission of data , especially in a multi carrier radio communication network. Another object of the invention is to provide corresponding transmitter and receiver adapted to perform this method. SUMMARY OF THE INVENTION These objects, and others that appear below, are achieved by a method for controlling the retransmission of data erroneously transmitted over a radio link between a transmitter and a receiver in a mobile radio communication network according to claim 1, a receiver according to claim 8 and a transmitter according to claim 10. According to the present invention, upon generation of a feedback message according to an ARQ/HARQ protocol, new frequency and or time extensions are determined for the physical resource block to be used for further transmission of data at said transmitter. An information enabling it to univocally retrieve the new frequency and/or time extension Is included in the feedback message sent to the transmitter. Upon reception of the feedback message modified according to the present invention, the transmitter retrieves the new frequency and/or time extension of the resource allocated for further transmission and sends further data in the newly allocated resource. In a first embodiment, if a positive feedback message is sent 1o the transmitter, the newly allocated resource is used for transmission of new data. I In a second embodiment, if a negative feedbacl< message is sent to the transmitter, the newly allocated resource is used for the retransmission of the erroneously transmitted data. The method according to the present invention presents the advantage to combine the signaling of a feedbacl< message (ACK, NACK) with the signaling of a new resource allocated for the further transmission of data, Another advantage of the present invention consists in not affecting the capacity of resource assignments for initial transmissions as performed on the L1/L2 control channel. Thus, the resource assignment for initial transmissions is not at ail dependent on the channel quality since this capacity remains constant regardless of the number of retransmissions having to be scheduled. Further advantageous features of the invention are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Other characteristics and advantages of the Invention will appear on reading the following description of a preferred embodiment given by way of non-limiting illustrations, and from the accompanying drawings, in which: - Figure 1 shows a message flow between a transmitter and a receiver implementing a HARQ mechanism as known in the prior art; - Figure 2 details a message flow between a terminal and a base station when data are sent in the uplink direction using a retransmission control according to the present invention; - Figure 3 details a message flow between a terminal and a base station when data are sent in the downlink direction using a retransmission control according to the present invention; - Figure 4 shows an example for the structure of a modified feedback message according to the present invention; - Figure 5 shows an embodiment of a base station according to the present invention; - Figure 6 shows an embodiment of a terminal according to the present invention. DETAILED DESCRIPTION OF THE INVENTION Figure 1 has already been described in connection with prior art. Figure 2 details a message flow between a terminal and a base station when data are sent in the uplink direction using a retransmission control according to the present invention. Step 21 consists in assigning a resource (or PRB) to the terminal UE needing to send uplink data. This resource assignment is preferably performed at the base station enhanced Node B as foreseen for 3GPP LIE systems. The entity performing the scheduling is nevertheless not necessarily the base station. It will be understood by those skilled in the art that the scheduling may as well be performed by any other central entity of the network. Step 22 consists in sending by the terminal UE user data in the uplink using the allocated resource. The uplink transmission is distorted on the air interface and not properly received at the base station. Step 23 consists in detecting the erroneous transmission at the base station eNB and generating a scheduling decision for the retransmission of the erroneous data. For this purpose, a new resource is determined according to a method which is not part of the present invention. Some criteria which may be taken into account could be to select the resource by guarantying a minimum distance between the frequency extension of the erroneous Physical Resource Blocl< and the Physical Resource Block for retronsnnission are nnultiples of the radio channel coherence bandwidth. A Physical Resource Block should be understood as a resource that can be allocated to a user. Ressource and RPB are used with the same meaning in the context of this invention. Preferably, a PRB consists in an OFDM system of M sub-carriers in the frequency domain that can either be consecutive or non consecutive. In the time domain, a PRB has an extension of N consecutive time symbols (e.g. OFDM symbols). It will be understood by those skilled in the art that the invention may apply if the PRB has only a frequency extension or only a time extension or a mix of both time and frequency extension. Again, the base station eNB itself may implement the module for determining the resource for the retransmission or any other central entity in the network which may then communicate the characteristics of the selected resource to the base station in an appropriate message. Step 24 consists according to the present invention to generate at the base station eNB and send to the terminal UE a modified NACK message comprising as well the usual information indicating that the user data were not correctly received and an information related to a new frequency extension and/or time extension for the resource to be used for retransmission. Step 25 consists in sending the retransmission of the user data in the new allocated resource as indicated in the NAGK message. The method has been here described in the case a NACK message is sent bacl< to the transmitter, the information regarding a new allocated resource included in the modified NACK feedback message being used for retransmission of the erroneously received data. It will be understood by those skilled in the art that a similar process can be applied in case an ACK message is sent back to the transmitter. In such a case a modified ACK message containing information related to new resource the transmitter has to use for further data transmission can be generated. This remark applies to the uplink direction as well as for the downlink direction. Figure 3 details a message flow between a terminal and a base station when data are sent in the downlink direction using a retransmission control according to the present invention. Step 31 consists in assigning a resource to send downlink data. This resource assignment is preferably performed at the base station (enhanced Node B as foreseen for 3GPP LTE systems). The entity performing the scheduling is nevertheless not necessarily the base station. It will be understood by those skilled in the art that the scheduling may as well be performed by any other central entity of the network. The resource assignment is then announced to the terminal. Step 32 consists in sending by the base station eNB user data in the downlink using the allocated resource. The downlink transmission is distorted on the air interface and not properly received at the terminal UE. Step 33 consists in detecting the erroneous transmission at the terminal UE and generating a scheduling recommendation for the retransmission of tlie erroneous data. For this purpose, a new resource is determined according to a method which is not part of the present invention. Some criteria which may be taken into account could be to mal