Abstract:
The present invention relates to a method for transmission of control information from a primary station to at least one secondary station, comprising the step of transmitting with a first set of control information parameters control information corresponding to data transmission with a first set of data parameters and transmitting with a second set of control information parameters control information corresponding to data transmission with a second set of data parameters, wherein at least one control information parameter has a difference in value between the two sets of control information parameters, said difference in value having a dependency on the difference between the value of a data parameter in the first set of data parameters and the value of the data parameter in the second set of data parameters, said data parameter being a different parameter from the at least one control information parameter.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of signalling control information in a telecommunication network. 
         [0002]    This invention is, for example, relevant for mobile systems, like Universal Mobile Telecommunication Network (UMTS), GSM or similar. The invention could also be applied to any other communication systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a conventional mobile telecommunication system, like UMTS, the primary station like a base station (or NodeB) and the secondary stations, like the mobile stations communicate together by means of a plurality of channels. Mainly, data channels are used for transmitting data from a primary station to the secondary stations, and control channels are used for giving information on the operation of the communication systems. 
         [0004]    Usually, when the primary station wishes to change a transmission parameter for data, like increasing the transmission power of the signals transmitted on the data channels, the primary station applies the same increase in power for the control channel. 
         [0005]    However, this leads to an increase of the interference for all the secondary stations, and is not always the most adequate parameter amendment to be carried out. 
         [0006]    More specifically, in systems where it is possible for the secondary stations to apply specific beamforming for data transmissions (in UMTS LTE), the range of coverage for the data transmissions increases for the beamforming transmissions compared to other transmissions in normal mode. However, if the control signalling indicating the data transmission parameters (inter alia, the time-frequency resources and MCS (modulation and coding scheme)) is unreliable, the increase in coverage for data will not be advantageous as the secondary station would be unable to receive sufficient information to know how to decode the data (even though if it had that information the SNR would be sufficient for decoding). 
         [0007]    One solution would be to apply beamforming to the control channel also. However, in systems such as LTE which have a common control channel containing control signalling for multiple secondary stations, beamforming cannot easily be used for the transmission of the control channel, as the pilot symbols embedded in the control channel are common for all secondary stations—i.e. they are used as a phase reference for decoding the control signalling by all secondary stations, irrespective of whether the data transmissions to a particular secondary station use beamforming or not. Consequently, the pilot symbols in the control channel cannot use beamforming (otherwise they would not provide a correct phase reference for the secondary stations whose control channels do not use beamforming), and therefore if beamforming were applied to the control signalling for some secondary stations, there would be no suitable beamformed phase reference for the control signalling for those secondary stations. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an object of the invention to propose a method for transmission of control information which permits to reduce the interference by using different transmission parameters from the data transmission. 
         [0009]    Another object of the invention is to propose a method of transmission of control information to a plurality of secondary stations, where the data transmissions to some of the secondary stations is operated by beamforming. 
         [0010]    To this end, in accordance with the invention a method is proposed for transmission of control information from a primary station to at least one secondary station, comprising the step of transmitting with a first set of control information parameters control information corresponding to data transmission with a first set of data parameters and transmitting with a second set of control information parameters control information corresponding to data transmission with a second set of data parameters, wherein at least one control information parameter has a difference in value between the two sets of control information parameters, said difference in value having a dependency on the difference between the value of a data parameter in the first set of data parameters and the value of the data parameter in the second set of data parameters, said data parameter being a different parameter from the at least one control information parameter. 
         [0011]    As a consequence, if a data parameter is changed, it is possible to choose a change in a different control parameter, to prevent an increase of the interference arising from transmission of the control channel. For instance, when the transmission power is increased for data transmission with a secondary station, the corresponding parameter change in the signalling of control information may be a decrease in the code rate for the information to be signalled to the considered secondary station. 
         [0012]    In particular, the invention has the benefit that by linking the change in control parameter to the change in data parameter in a coordinated way, unnecessary interference or wastage of transmission capacity are avoided compared to the case when parameter changes are uncoordinated. 
         [0013]    In an embodiment of this invention, it is further proposed a method for transmission of control information from a primary station to at least one secondary station, comprising the step of transmitting control information with a set of parameters, said set of parameters has a dependency on a data transmission mode to the at least one secondary station, wherein the data transmission mode is one of a beamforming transmission mode and a normal mode. 
         [0014]    Then, it permits to obtain the same increase of range obtained by using beamforming on the data channel, but without using beamforming in the control channel. 
         [0015]    The present invention also relates to a secondary station comprising means for carrying out the method in accordance with the invention. 
         [0016]    The present invention also relates to a primary station comprising means for carrying out the method in accordance with the invention. 
         [0017]    These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein: 
           [0019]      FIG. 1  is a block diagram representing a network comprising a primary station and a secondary station in accordance with the invention; 
           [0020]      FIG. 2  is a representation of the coverages corresponding to each mode of transmission in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The present invention relates to a system of communication  300  as depicted in  FIG. 1 , comprising a primary station  100 , like a base station or an evolved Node B (eNodeB), and at least one secondary station  200  like a mobile station or a User Equipment (also denoted UE). 
         [0022]    The radio system  300  may comprise a plurality of the primary stations  100  and/or a plurality of secondary stations  200 . The primary station  100  comprises a transmitter means  110  and a receiving means  120 . An output of the transmitter means  110  and an input of the receiving means  120  are coupled to an antenna array  130 , comprising a plurality of antennas, by a coupling means  140 , which may be for example a circulator or a changeover switch. Coupled to the transmitter means  110  and receiving means  120  is a control means  150 , which may be for example a processor. The secondary station  200  comprises a transmitter means  210  and a receiving means  220 . An output of the transmitter means  210  and an input of the receiving means  220  are coupled to an antenna array  230 , which may comprise a plurality of antennas, by a coupling means  240 , which may be for example a circulator or a changeover switch. Coupled to the transmitter means  210  and receiving means  220  is a control means  250 , which may be for example a processor. Transmission from the primary radio station  100  to the secondary station  200  takes place on a first set of channels  160  and transmission from the secondary radio station  200  to the first radio station  100  takes place on a second set of channels  260 . 
         [0023]    In a first embodiment of the invention, a primary station may transmit data to a secondary station pursuant to several different modes. In each mode, a set of a plurality of transmission data parameters is chosen. When the primary station wishes to change from a first mode to another mode, for instance to reach a higher throughput, or to increase the range of the transmission, at least one of the data parameters is changed. 
         [0024]    In several variants of the invention, the amended data parameter may comprise at least one of:
       a beamforming mode or a number of antennas, or a number of data streams,   a transmission power,   a code rate,   a modulation scheme,   a transmit diversity scheme,   a number of spatial layers,   a precoding matrix,   a number of subcarriers,   a frequency band,   a spreading factor.       
 
         [0035]    Similarly, the control information is transmitted with different sets of control parameters, each set corresponding to a data transmission mode. When the primary station switches from a first mode to the second mode, a modification of transmission is carried out on at least one data parameter, and a corresponding modification of at least one control parameter is automatically carried out as well. However, in accordance with the invention, the amended control parameter is of a different type from the type of the amended data parameter. 
         [0036]    For instance, the switching from a first mode to a second mode may comprise an increase of the number of subcarriers of the data signal, and an increase of the code rate of the control information. 
         [0037]    The selection of the second set of control parameters may be done in a clever way, if the second set of data parameters is selected such that it changes the communication range of the data transmission compared to the first set of data parameters. This is the case for instance if the transmission power is increased (whole coverage increased) or if a beamforming is applied (range increased in a single direction). It is then advantageous if the second set of control information parameters is selected such that it also changes the communication range of the control information compared to the first set of control information parameters. In such a case, the range of the control information can be increased if the range of the data transmission is also increased due to the switching from one mode to another. The control parameter values may be selected, or determined so that the change in communication range of the control information corresponds approximately to the change in communication range of the data transmission. 
         [0038]    In the preceding example, the illustrated method is particularly suitable when the first mode is a normal mode (for example a single antenna transmission) and the second mode is a data beamforming mode. This beamforming mode leads to an increase of the data communication range at least in one direction. 
         [0039]    To reduce the interference, and to avoid disturbance of the pilot symbols embedded in the control channels due to phase shifts, the control information is not beamformed, but is for instance transmitted with a reduced code rate. Thus, the control transmission is more robust to interference and may be decoded in a wider area. 
         [0040]    This example is illustrated on  FIG. 2 , where the base station  100  operates firstly in a first mode, which is not a beamforming mode, but a normal mode. In this mode, the data and signalling coverage is noted as  10 , and permits to reach mobile stations  201  and  202 . However, to reach mobile station  203 , the coverage must be changed. Then, the base station  100  starts transmitting with a beamforming mode, where the transmission beam is directed towards the mobile station  203 . In such a case, the data coverage is noted as  20  and permits to reach the secondary station  203 . Moreover, since the control signal for the secondary station  203  cannot be beamformed as explained before, the base station  100  transmits this control signal by using a lower rate, or using a lower frequency band, permitting to increase the coverage. In such a case, the control signal coverage noted  30  permits to reach the secondary station  203 . Moreover, by reducing the code rate rather than increasing the transmission power of the control signal, this avoids an interference increase. 
         [0041]    In this example, the code rate in the control signal is chosen for the second mode so that the range of the control signal is greater than or equal to the range of the data signal for the second mode. 
         [0042]    In an example of this embodiment, the control information is coded in messages of a size being chosen from a set of predetermined control message sizes. Indeed, in current proposals for LTE, the control signalling for each secondary station is coded into one of a set of available control channel message sizes. Then, reducing the code rate of control signalling for a secondary station whose data transmission was beamformed could comprise employing a code rate such that the number of coded bits could be mapped into a larger message size than the signalling information for a secondary station whose data transmission was not beamformed. 
         [0043]    In various examples of the invention, the modified control parameter may be one the following:
       a modulation order;   a transmission power;   a reduced code rate.       
 
         [0047]    According to the present invention, one or more transmission parameters of the control signalling corresponding to a data transmission which uses beamforming are adjusted with respect to the transmission parameters of control signalling corresponding to data transmissions which do not use beamforming. 
         [0048]    This would enable the coverage of the control channel to be increased to substantially match that of the data channel. Advantageously, the adjustment in transmission parameters is designed to give a substantially similar increase in coverage to that which would be obtained by the application of beamforming. The adjustment of transmission parameters could for example comprise one or more of the following:
       reducing code rate;   reducing modulation order;   increasing transmission power.
 
In a further embodiment, a base station can transmit in two frequency bands (for example 900 MHz and 2.1 GHz). Due to differences in radio propagation the coverage is greater for the lower frequency band. Therefore, to maintain coverage, beamforming is applied to transmissions in the higher frequency band. If the control channel is transmitted from the base station in the lower frequency band, then the data channel may be transmitted in the same frequency band, or in a higher frequency band. In this case in accordance with the invention, beamforming is applied to the data channel transmitted in the higher frequency band. If the control channel is transmitted in the higher frequency band then one or more transmission parameters of the control channel are adjusted in order to maintain similar coverage to that achieved with the low frequency band, or the high frequency band with beamforming.
       
 
         [0052]    It is to be noted that the present invention is not limited to mobile communication systems such as the LTE of UMTS, but it is possibly also applicable to other standards such as evolutions of WiMAX or cdma2000. 
         [0053]    Indeed, the present invention is not limited to mobile telecommunications systems described herein as an example, but could be extended to any other communication systems. 
         [0054]    In the present specification and claims the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, the word “comprising” does not exclude the presence of other elements or steps than those listed. 
         [0055]    The invention has been largely described in relation to signalling and data transmissions made by a network entity to a mobile terminal, but it will be understood that the invention applies equally to opposite or other allocations of roles to nodes. 
         [0056]    From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features, which are already known in the art of radio communication and the art of transmitter power control and which may be used instead of or in addition to features already described herein.