Abstract:
A method for assessing reception quality of a common transmission by a set of receivers comprising deriving a reception quality for different combinations of the set, and assessing each against a threshold which varies according to the number of receivers in the respective combination. A single assessment of quality can be produced which takes into account multiple different receivers, and multiple different combinations of such receivers. Such assessment allows transmission power and/or the number of active receivers to be set according to a given reception criteria.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119(a)-(d) of United Kingdom Application No. 1208626.0, filed on May 16, 2012 and entitled “A method and device for encoding and decoding a video signal”. The above cited patent application is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to assessment of reception quality in a receiver arrangement having multiple inputs and providing a single output. The invention is particularly, but not exclusively, concerned with the setting or adjusting of transmission power according to an assessment of reception quality in a wireless network system. 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention finds particular application in the wireless transmission of uncompressed High Definition (HD) video or image data for applications requiring low Bit Error Rate (BER) and low latency transmission. More specifically, this invention has been conceived in consideration of a 60 GHz wireless network system using a single moving emitter and several fixed receivers, said receivers being connected to a system controller device. 
         [0004]    A wireless network system using the millimeter wave frequency band (60 GHz) is well adapted to the transmission of uncompressed HD video or image data. An advantageous characteristic of a wireless network using 60 GHz frequency band is a large available bandwidth. This large bandwidth allows very high data rate transmission (&gt;3 Gbps). 
         [0005]    Another characteristic of a wireless network using 60 GHz frequency band is its sensibility to masking phenomenon. Some static or moving obstacles such as objects, structures, people etc. can interrupt or degrade the communication path and cause transmission errors. 
         [0006]    To mitigate against transmission errors, a 60 GHz wireless network system is proposed which uses a multi-reception technique to create spatial diversity and a Multi-Reception Error Correcting Code to reach the BER and low latency. The 60 GHz wireless network system proposed forms a network cell. Practically, several network cells can be used simultaneously within the same area. For example, these networks cells can be used in an industrial environment. 
         [0007]    The use of several network cells simultaneously in the same vicinity can lead to Radio Frequency (RF) interference problems in between adjacent network cells. Moreover, a network cell used in an industrial environment is typically surrounded by a lot of metallic surfaces. These metallic surfaces create a 60 GHz wireless channel with a lot of multi-paths which can disturb the radio communications. 
         [0008]    To mitigate the RF interference problems in between adjacent network cells and to mitigate the multi-path within a network cell, a solution is to manage the transmission power of the emitter node appropriately. 
         [0009]    US20040259584A1 proposes a transmission power control method in a point to point wireless communication system. In this document the transmission power is controlled to match a reception quality target. The signal reception quality is measured on the receiver side (measured Signal-to-Interference-Ratio) and compared to a target reception quality (target Signal-to-Interference-Ratio). A Transmission Power Control (TPC) is generated or adjusted accordingly. 
         [0010]    It is an object of certain aspects of the present invention to provide an improved transmission power control method for a single emitter, multiple receiver wireless communication system, to mitigate against RF interference between adjacent network cells, while maintaining appropriate reception quality. 
         [0011]    It is further an object of certain aspects of the invention to provide an improved method for assessing reception quality. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    Accordingly, in a first aspect the present invention provides a method for assessing reception quality of a common transmission by a set of receivers, said method comprising deriving a reception quality value for each of a plurality of different combinations of said set, assessing each determined value against a threshold which varies according to the number of receivers in the respective combination, and determining whether at least one combination exceeds said threshold. 
         [0013]    In this way, a single assessment of quality can be produced which takes into account multiple different receivers capable of receiving the same data, and multiple different combinations of such receivers. The method is particularly advantageous in allowing combinations of different numbers of receivers to be combined in a single assessment, by adapting the threshold value accordingly. Consideration of such combinations in parallel in this way allows transmission power and the number of active receivers to be set simultaneously according to a given reception criteria, as will be explained below. 
         [0014]    In a preferred embodiment, the result of the determination is used to control the transmission power of said transmission. Either the result or a signal generated in response to the result is communicated to the transmission source to effect such a change for example. 
         [0015]    Control of transmission power may be performed iteratively, by continuously assessing reception quality and adjusting transmission power in response to the assessment until a satisfactory result is achieved. The method may involve iteratively increasing transmission power by a fixed amount in response to a negative determination, until a positive determination is achieved, or conversely iteratively decreasing transmission power by a fixed amount in response to a positive determination until a negative determination is achieved. In each of these two cases the transmission power will typically be set at a lower and upper initial value respectively. In the case of iteratively decreasing power, the power setting of the penultimate iteration is typically selected. 
         [0016]    It should be noted that a reception quality value can be a positive quality assessment (such as SNR for example) where a higher numerical value represents a higher quality, or a negative quality assessment (such as an error rate for example), where a higher numerical value represent a lower quality. 
         [0017]    This will be evident to the skilled reader, and determining whether or not such a value exceeds a quality threshold will be interpreted accordingly, including a case where a numerical value less than said threshold results in said (quality) threshold being exceeded. 
         [0018]    For each combination of receivers, copies of the same data but received via different receivers are preferably combined, and in one example reconstructed data blocks are preferably formed from combinations of sub blocks of copies of the same data block from respective receivers. Other possible combination techniques include a majority decision data combination for example, which can be performed on a bit by bit basis. 
         [0019]    A reception quality value is typically determined by assessing the number and/or distribution of transmission errors in respective copies of data received via the different receivers, and hence transmission paths. Forming different combinations of copies of data in parallel contributes towards this, as described above. 
         [0020]    In certain embodiments, a reception quality value is determined by means of an error correction code, preferably, but not necessarily, the false check rate (FCR) of a combination of receivers. 
         [0021]    As noted above, this aspect is advantageous in allowing combinations of different numbers of receivers to be combined in a single assessment. This provides further adaptability in terms of distribution between channels. 
         [0022]    The use of an adaptive threshold enables combinations of different numbers of receivers to be considered simultaneously in a meaningful manner. Preferably a parameter is used which can be expressed as a function of bit error rate (BER), and more preferably different functions can be provided, or a variation made to the same function to account for different numbers of receivers. In this way, equivalence of a common target BER between combinations of different numbers of receivers can be maintained. 
         [0023]    At each thresholding or comparison step, a threshold value can be calculated dynamically, or can be stored in advance and retrieved form a memory for example. 
         [0024]    According to a further aspect the invention provides a system controller for assessing reception quality of a common transmission by a set of receivers comprising an input unit configured to receive from each of a set of receivers data corresponding to said common transmission; a data processing unit configured to combine, for each of a plurality of different combinations of receivers, data from the respective receivers, and calculate a quality parameter for each said combination based on said combined data; a reference unit configured to provide a reference value which varies according to the number of receivers in a respective combination; and an output unit configured to determine if at least one calculated quality parameter exceeds the relevant reference value, and to output an indication of reception quality for said set, based on said determination. 
         [0025]    According to a still further aspect of the invention, there is provided a wireless network system comprising a transmitter node, a plurality of receiver nodes, and a system controller as set out above. 
         [0026]    The invention also provides a computer program and a computer program product for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein, and a computer readable medium having stored thereon a program for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein. 
         [0027]    The invention extends to methods, apparatus and/or use substantially as herein described with reference to the accompanying drawings. Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, features of method aspects may be applied to apparatus aspects, and vice versa. Furthermore, features implemented in hardware may generally be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings in which: 
           [0029]      FIG. 1  shows an arrangement of network cells, each cell having a transmitter node and multiple receiver nodes; 
           [0030]      FIG. 2  shows the detailed architecture of a cell of  FIG. 1 ; 
           [0031]      FIG. 3  illustrates a multi-rx ECC module and the FCR quality parameter of embodiments of the invention; 
           [0032]      FIG. 4  represents a method of determining transmission power using a multi-rx quality assessment; 
           [0033]      FIG. 5  represents a derivative of the method of  FIG. 4 ; 
           [0034]      FIG. 6  shows a relationship obtained by simulation between FCR and BER for different numbers of receivers. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]      FIG. 1  illustrates several network cells  10   a ,  10   b  and  10   c  that are placed close to each other in an industrial environment, for example in a factory. The networks cells  10   a ,  10   b  and  10   c  are identical, so only the network cell  10   a  will be described below. 
         [0036]    In the network cell  10   a , the node  12  is called emitter node, the nodes  13 ,  14 ,  15  are called receivers nodes and the node  16  is called system controller node. These nodes names correspond to the direction of the main communication, called downlink communication, which takes place within the network cell  10   a . This main downlink communication is used for wireless transmission of HD video or image data for example, from emitter node  12  to system controller node  16  through the receiver nodes  13 ,  14  and  15 . This main downlink communication used around 90% of the total bandwidth of the network cell  10   a  in this embodiment. 
         [0037]    A secondary communication, called uplink communication, takes place within the network cell  10   a  from the wireless controller node  16  to the receiver nodes  13 ,  14  or  15 , or from the wireless controller  16  to the emitter node  12  through one or all receiver nodes  13 ,  14  and  15 . This secondary uplink communication is used for transmission of control/command data from system controller  16  to receiver nodes  13 ,  14  or  15  or from system controller node  16  to emitter node  12  through one or all receiver nodes  13 ,  14  and  15 ; as first example, this secondary uplink communication is used to put a receiver node in standby mode; as second example, this secondary uplink communication is used to update the transmission power of the emitter node  12 . This secondary uplink communication used around 10% of the total bandwidth of the network cell  10   a  in this embodiment. 
         [0038]    The emitter node  12  is connected to an HD video or image data source device  11  through a wired interface  128 . The source device  11  can be an HD digital camera, an HD digital camcorder or another such device. The wireless emitter node  12  processes the HD video or image data, and sends the processed data wirelessly through its antenna  12   a . The emitter node  12  can send the processed data from several different positions within the area  20 . 
         [0039]    The data sent by the emitter node  12  is received by the receiver nodes  13 ,  14  and  15  respectively through their antennas  13   a ,  14   a  and  15   a . The receiver nodes  13 ,  14  and  15  are located in different positions to create spatial diversity. In this embodiment, 3 receiver nodes are used but other configurations could be used, for example configurations with 2, 4, 5 or 6 receiver nodes could be used. 
         [0040]    Items  18  and  19  represent obstacles that can be positioned between emitter node  12  and receiver nodes  13 ,  14  and  15 . These obstacles can be metallic objects, humans, structures etc. Depending on the position of the emitter  12  within the area  20  and depending on the position of the obstacles  18  and  19 , one or more line of sight communication path between the emitter node  12  and the receiver nodes  13 ,  14  and  15  can be disrupted or blocked. As a result, the receiver nodes  13 ,  14  and  15  may have different reception quality, i.e. different BER. 
         [0041]    Receiver nodes  13 ,  14  and  15  process the data received from the emitter node  12  and send the processed data to the system controller  16  respectively through the wired interfaces  137 ,  147  and  157 . The system controller  16  receives the 3 received copies representing the same original data from the receiver nodes  13 ,  14  and  15 . The system controller node  16  includes a Multi-rx Error Correction Code (ECC) module and performs an assessment of reception quality (to be described in greater detail below) which in turn enables an appropriate (minimum) RF transmission power to be used by the emitter node  12 . Additionally or alternatively, the minimum number of required receivers among the receivers  13 ,  14  and  15  can also be determined in certain embodiments. The system controller  16  then sends the decoded HD video or image data to the sink device  17  through a wire interface  168 . The sink device  17  can be an HD video/image display, a Personal Computer or other device. 
         [0042]      FIG. 2  shows the functional block diagrams of emitter node  12 , receiver nodes  13 ,  14  and  15  and system controller  16 . The operation of the emitter node  12  is described below, firstly in case of downlink communication and secondly in case of uplink communication. 
         [0043]    In case of downlink communication, the emitter node  12  is connected to an HD video or image source device  11  (shown in  FIG. 1 ) through a wire interface  128 . The wire interface  128  can be an HDMI interface, a Camera Link interface or other. The source device  11  (shown in  FIG. 1 ) is connected to the module  127  of the node  12  via the wire interface  128 . The module  127  is the Application layer module of the emitter node  12 . The module  127  retrieves the HD video or image content received from source device  11  (shown in  FIG. 1 ) and formats these HD video or image data to be processed by MAC layer module  126 . The formatted data are then sent to the MAC layer module  126 . 
         [0044]    The MAC module  126  receives the formatted data sent by the Application layer  127 . The MAC module  126  builds the MAC data packets by adding header data to the received formatted data and by adding ECC redundancy bits. 
         [0045]    For example, the addition of ECC redundancy bits consists of performing CRC encoding on portions of data. For example, each 32 Bytes of data, the MAC module  126  computes and adds a 4 Bytes CRC to form an encoded data block. A MAC data packet consists of several encoded data blocks. Then the MAC data packets are sent to the Channel Coding module  125 . 
         [0046]    The Channel Coding module  125  receives the MAC data packets and performs channel encoding function. For example, the module  125  encodes the MAC data packets using a Reed Solomon (216/224) encoder and a convolutive encoder (2/3). The output of Channel Coding module  125  is connected to the RF transceiver module  124 . The RF transceiver  124  receives the MAC data packets after channel encoding by the module  125 . Then the RF transceiver  124  builds the radio packets by modulating the received data and by adding a preamble pattern. Then the RF transceiver  124  performs the remaining functions needed for the transmission of radio packets on the 60 GHz radio channel through the antenna  12   a.    
         [0047]    In case of uplink communication, the emitter node  12  receives radio packets from receiver nodes  13 ,  14  or  15 . These radio packets embed control/command data sent by the system controller node  16  and forwarded by one or several receiver nodes  13 ,  14  or  15 . 
         [0048]    In the emitter node  12 , the RF transceiver  124  performs the function needed for the reception of radio packets on the 60 GHz radio channel through the antenna  12   a . After the reception of radio packets, the RF transceiver  124  removes the preamble pattern from the radio packet and demodulates the received data. The demodulated data are then sent to the channel decoding module  125 . The channel decoding module  125  receives the demodulated data and performs channel decoding. For example, the module  125  decodes the demodulated data using a Viterbi decoder (2/3) and a Reed Solomon (216/224) decoder. Then, the Channel Decoding module  125  sends the retrieved MAC data packets to the MAC module  126 . 
         [0049]    The MAC module  126  processes the received MAC data packets to retrieve the control/command data sent by the system controller node  16 . Then, the control/command data are stored within internal registers of the MAC module  126  to be further processed by the CPU  122 . An example of control/command data sent by the system controller node  16  to the emitter node  12  is the RF transmission power value to be used by the emitter node  12 . 
         [0050]    The CPU module  122  of the emitter node  12  is connected to a ROM  120  and a RAM  121 . The ROM  120  contains a software program which can be used, when executed by the CPU  122  (using the RAM  121 ), to implement certain aspects of the present invention. The RAM  121  is used for the execution by the CPU  122  of the above-mentioned software program and for the processing of the different tasks performed by the CPU  122 . 
         [0051]    The CPU  122  is connected to the modules  127 ,  126 ,  125  and  124  via a bi-directional address/data bus  123 . Amongst other things, this connection permits to the CPU  122  to initialize and configure the modules  127 ,  126 ,  125  and  124  at system start-up. This connection also permits to the CPU  122  to read the updated value of RF transmission power stored within internal registers of MAC module  126 . Then the CPU  122  provides this updated RF transmission power value to the RF transceiver  124  to be used for the next transmission of radio packets. 
         [0052]    The operation of the receiver nodes  13 ,  14  and  15  is described below, firstly in case of downlink communication and secondly in case of uplink communication. In case of downlink communication, the receiver nodes  13 ,  14  and  15  receive the radio packets sent by the emitter node  12  respectively through their antenna  13   a ,  14   a  and  15   a . The receiver nodes  13 ,  14  and  15  have the same functional bloc diagram, as a result only receiver node  13  will be described here. 
         [0053]    In the receiver node  13 , the RF transceiver  134  performs the function needed for the reception of radio packets on the 60 GHz radio channel through the antenna  13   a . After the reception of radio packets, the RF transceiver  134  removes the preamble pattern from the radio packet and demodulates the received data. The demodulated data are then sent to the channel decoding module  135 . The channel decoding module  135  receives the demodulated data and performs channel decoding function. For example, the module  135  decodes the demodulated data using a Viterbi decoder (2/3) and a Reed Solomon (216/224) decoder. Then the channel decoding module  135  sends the retrieved MAC data packets to the Cable Interface module  136 . 
         [0054]    The Cable Interface module  136  receives the MAC data packets from the Channel Decoding module  135 . The module  136  formats the MAC data packets to transmit them to the system controller  16  via the wire link  137 . The wire link  137  is typically a serial wire link able to support data rate up to several Gbps. 
         [0055]    In case of uplink communication, the receiver node  13  receives MAC data packets from the system controller  16  through the wire link  137 . These MAC data packets embed control/command data either for the emitter node  12  or for the receiver node  13 . The cable interface module  136  of the receiver node  13  receives the MAC data packets sent by the system controller node  16 . 
         [0056]    When the received MAC data packets embed control/command data for the emitter node  12 , the cable interface module  136  provides the MAC data packet to the channel coding module  135 . The channel coding module  135  receives the MAC data packets and performs channel encoding function. For example, the module  135  encodes the MAC data packets using a Reed Solomon (216/224) encoder and a convolutive encoder (2/3). The output of channel coding module  135  is connected to the RF transceiver module  134 . The RF transceiver  134  receives the MAC data packet after channel encoding by the module  135 . Then the RF transceiver  134  builds the radio packets by modulating the received data and by adding a preamble pattern. Then the RF transceiver  134  performs the remaining functions needed for the transmission of radio packets on the 60 GHz radio channel through the antenna  13   a.    
         [0057]    When the received MAC data packets embed control/command data for the receiver node  13 , the cable interface module  136  processes the received MAC data packets to retrieve the control/command data sent by the system controller node  16 . Then, the control/command data are stored within internal registers of the cable interface module  136  to be further processing by the CPU  132 . An example of control/command data sent by the system controller node  16  to the receiver node  13  is the standby mode information. 
         [0058]    The CPU module  132  of the receiver node  13  is connected to a ROM  130  and a RAM  131 . The ROM  130  contains a software program which can be used, when executed by the CPU  132  (using the RAM  131 ), to implement the present invention. The RAM  131  is used for the execution by the CPU  132  of the above-mentioned software program and for the processing of the different tasks performed by the CPU  132 . The CPU  132  is connected to the modules  136 ,  135  and  134  via a bi-directional address/data bus  133 . Amongst other things, this connection permits to the CPU  132  to initialize and configure the modules  136 ,  135  and  134  at system start-up. 
         [0059]    This connection also permits to the CPU  132  to read the standby mode information stored within internal registers of cable interface module  136 . Depending on the value of the standby mode information, the CPU  132  will either put the receiver node  13  in standby mode or output the receiver node  13  from standby mode. 
         [0060]    The operation of the system controller node  16  is described below, firstly in case of downlink communication and secondly in case of uplink communication. In case of downlink communication, the system controller  16  receives 3 copies of each formatted MAC data packet from the 3 receiver nodes  13 ,  14  and  15  respectively via the wire link  137 ,  147  and  157 . The first copy of formatted MAC data packet is transmitted by the receiver node  13  and received by the system controller  16  through its Cable Interface module  164   a . The Cable Interface module  164   a  processes the received data and sends the first copy of MAC data packet to the module  165 . The second copy of formatted MAC data packet is transmitted by the receiver node  14  and received by the system controller  16  through its Cable Interface module  164   b . The Cable Interface module  164   b  processes the received data and sends the second copy of MAC data packet to the module  165 . The third copy of formatted MAC data packet is transmitted by the receiver node  15  and received by the system controller  16  through its Cable Interface module  164   c . The Cable Interface module  164   c  processes the received data and sends the third copy of MAC data packet to the module  165 . 
         [0061]    The module  165  is a Multi-rx ECC module. This module applies a Multi-Rx error correcting code on the 3 copies received from the receiver nodes  13 ,  14  and  15  and typically allows the BER required by the application to be met. The computation of a quality parameter for each combination of receivers is also performed by this module in the present example. These quality parameters can in turn be used to determine the minimum RF transmission power to be used by the emitter node  12 , and/or in some embodiment the smallest number of required receivers among the receivers  13 ,  14  and  15 . 
         [0062]    An example of a Multi-rx ECC module  165  and its operation is described in greater detail below in relation to  FIG. 3 . 
         [0063]    After decoding, the module  165  selects the first reconstructed copies that satisfied the CRC check and provides it to the MAC module  166 . The MAC module  166  receives the reconstructed copies outputted by the module  165  and re-constructs each MAC data packet. Then the MAC module  166  retrieves the HD video or image data by removing the header information attached to the MAC data packets. Next, the MAC module  166  provides the HD video or image data to the Application layer module  167 . 
         [0064]    The Application layer  167  receives the HD video or image data from the MAC layer  166  and re-builds the HD video or image content. The HD video or image content is then sent to the sink device  17  (shown in  FIG. 1 ) through the wire interface  168 . The wire interface  168  can be an HDMI interface, a Camera Link interface or else. 
         [0065]    In case of uplink communication, the system controller node  16  sends control/command data either to the emitter node  12  through one or all receiver nodes  13 ,  14  or  15 , or to the receiver nodes  13 ,  14  or  15 . 
         [0066]    As part of one operating method, the MAC module  166  builds MAC data packets which contain updated value of RF transmission power and sends these MAC data packets to the emitter node  12  through one or all receiver nodes  13 ,  14  or  15 . This task is done iteratively by the MAC module  166  up to the determination of minimum RF transmission power value to be used by the emitter node  12 . Additionally or alternatively, the combination of minimum number of receiver nodes necessary to reach the BER required by the application can be determined. Then the MAC module  166  builds and sends MAC data packets which contain control/command data to put in standby mode the receiver nodes out of the determined combination. 
         [0067]    The CPU module  162  of the system controller  16  is connected to a ROM  160  and a RAM  161 . The ROM  160  contains a software program which can be used, when executed by the CPU  162  (using the RAM  161 ), to implement the present invention. The RAM  161  is used for the execution by the CPU  162  of the above-mentioned software program and for the processing of the different tasks performed by the CPU  162 . The CPU  162  is connected to the modules  167 ,  166 ,  165 ,  164   a ,  164   b ,  164   c  via a bi-directional address/data bus  163 . This connection permits also to the CPU  162  to initialize and configure the modules  167 ,  166 ,  165 ,  164   a ,  164   b ,  164   c  at system start-up. 
         [0068]      FIG. 3  describes the Multi-rx ECC module and the FCR quality parameter used in a preferred embodiment of the invention. The operation of the preferred Multi-rx ECC module is described below. 
         [0069]    The preferred Multi-rx ECC module  165  is described here by considering 3 receivers (N1=3) and a division of copies of an encoded data block into 2 sub-blocks (m=2). These values, N1=3 and m=2, are not limitative and other values can be used. 
         [0070]    The Multi-rx ECC module  165  receives the 3 copies  30 ,  40  and  50  of a MAC data packet respectively from the 3 receiver nodes  13 ,  14  and  15 . The MAC data packet consists of several encoded data blocks. The copy  30  of the MAC data packet consists of several encoded data blocks identical in format to the encoded data block  31 . The encoded data block  31  is made of a data part  32  and a CRC part  33 . For example the size of the data part  32  is 32 Bytes and the size of CRC part  33  is 4 Bytes. Copies of the MAC data packet  40  and  50  have a similar structure as illustrated. 
         [0071]    The 3 copies of each encoded data block within a MAC data packet are inputted and processed in parallel within the Multi-rx ECC module  165 . 
         [0072]    Firstly, the module  165  performs a division of each copy of an encoded data block into 2 sub-blocks. The copy  31  of an encoded data block is divided into 2 sub-blocks  31   a  and  31   b  of equal size in this embodiment. The copy  41  of an encoded data block is divided into 2 sub-blocks  41   a  and  41   b  of equal size. The copy  51  of an encoded data block is divided into 2 sub-blocks  51   a  and  51   b  of equal size. In alternative embodiments different numbers of sub blocks, and unequal size sub blocks may be used. 
         [0073]    The cross  60  within the sub-block  31   a  of copy  31  of an encoded data block represents an error introduced during the wireless transmission between the emitter node  12  (shown in  FIG. 1 ) and the receiver node  13  (shown in  FIG. 1 ). As a result, the copy  31  cannot be decoded successfully as received from receiver node  13 . The cross  61  within the sub-block  41   b  of copy  41  of an encoded data block represents an error introduced during the wireless transmission between the emitter node  12  (shown in  FIG. 1 ) and the receiver node  14  (shown in  FIG. 1 ). As a result, the copy  41  cannot be decoded successfully as received from receiver node  14 . The cross  62  within the sub-blocks  51   a  of copy  51  of an encoded data block represents errors introduced during the wireless transmission between the emitter node  12  (shown in  FIG. 1 ) and the receiver node  15  (shown in  FIG. 1 ). As a result, the copy  51  cannot be decoded successfully as received from receiver node  15 . 
         [0074]    Secondly, the module  165  concatenates of the various sub-blocks to build N1 m =3 2 =9 reconstructed data blocks. Each reconstructed data block is therefore formed from a group of sub blocks of data corresponding to the same original data (ie copies of the same data) but received via different receivers. The first reconstructed data block is built by concatenating sub-blocks  31   a  and  31   b , the second reconstructed data block is built by concatenating sub-blocks  31   a  and  41   b , the third reconstructed data block is built by concatenating sub-blocks  31   a  and  51   b , and so on until all combinations are accounted for. 
         [0075]    The module  165  then performs in parallel a CRC check on each of the 9 reconstructed data blocks. Finally, the module  165  selects a reconstructed data block that satisfied the CRC check and provides it to the MAC module  166  (shown in  FIG. 2 ). 
         [0076]    The False Check Rate (FCR) is an example of a metric indicative of the quality of the decoding, i.e. indicative of the quality of the point to multi-point communication, of the preferred Multi-rx ECC module  165 . The FCR metric is defined as the ratio between the number of false CRC checks and the total number of checks at the output of the preferred Multi-rx ECC module  165 . 
         [0077]    Therefore, for any one given combination of receivers (eg receiver  14  and receiver  15 ) N m  combinations of sub blocks are possible, resulting in N m  reconstructed blocks (4 possible reconstructed blocks in the present example). The FCR for this combination (or receivers) can then be determined by considering the number (out of 4 in this case) of reconstructed blocks which give rise to a false CRC check. 
         [0078]    In reference to the  FIG. 3  describing the preferred Multi-rx ECC module  165 , a FCR metric can be computed for each combination of N1−x (with x ε [0 to N1−1]) receivers. In this example, 7 combinations of receivers are possible: 1 combination of 3 receivers exists. This combination associates the receiver nodes  13 ,  14  and  15 . 3 combinations of 2 receivers exist. These combinations associate respectively the receiver nodes  13  and  14 , the receiver nodes  14  and  15 , the receiver nodes  13  and  15 . 3 combinations of 1 receiver exist. These combinations associate respectively the receiver node  13 , the receiver node  14 , the receiver node  15 . 
         [0079]    To compute a FCR metric for each combination of receivers, it is necessary to establish different combinations of reconstructed data blocks corresponding to the different combination of received copies, i.e. of receivers. The combination of reconstructed data blocks corresponding to the combination of 3 receivers ( 13 , 14 , 15 ) gathers the 9 reconstructed data blocks ( 31   a ; 31   b ), ( 31   a ; 41   b ), ( 31   a ; 51   b ), ( 41   a ; 41   b ), ( 41   a ; 31   b ), ( 41   a ; 51   b ), ( 51   a ; 51   b ), ( 51   a ; 31   b ) and ( 51   a ; 41   b ). The combination of reconstructed data blocks corresponding to the combination of 2 receivers ( 13 , 14 ) gathers the 4 reconstructed data blocks ( 31   a ; 31   b ), ( 31   a ; 41   b ), ( 41   a ; 41   b ) and ( 41   a ; 31   b ). The combination of reconstructed data blocks corresponding to the combination of 2 receivers ( 14 , 15 ) gathers the 4 reconstructed data blocks ( 41   a ; 41   b ), ( 41   a ; 51   b ), ( 51   a ; 51   b ) and ( 51   a ; 41   b ). The combination of reconstructed data blocks corresponding to the combination of 2 receivers ( 13 , 15 ) gathers the 4 reconstructed data blocks ( 31   a ; 31   b ), ( 31   a ; 51   b ), ( 51   a ; 51   b ) and ( 51   a ; 31   b ). The combination of reconstructed data blocks corresponding to the combination of 1 receiver ( 13 ) gathers the reconstructed data blocks ( 31   a ; 31   b ). The combination of reconstructed data blocks corresponding to the combination of 1 receiver ( 14 ) gathers the reconstructed data blocks ( 41   a ; 41   b ). The combination of reconstructed data blocks corresponding to the combination of 1 receiver ( 15 ) gathers the reconstructed data blocks ( 51   a ; 51   b ). 
         [0080]    Then for a given combination of reconstructed data blocks, the FCR metric is computed by calculating the ratio between the number of false CRC check and the total number of check for said combination at the output of the preferred Multi-rx ECC module. 
         [0081]    For example in  FIG. 3 , for the combination of reconstructed data blocks corresponding to the combination of 3 receivers ( 13 , 14 , 15 ), the instantaneous value of FCR metric is 7/9. Another example on  FIG. 3 , for the combination of reconstructed data blocks corresponding to the combination of 2 receivers ( 14 , 15 ), the instantaneous value of FCR metric is ¾. 
         [0082]      FIG. 4  represents the steps of an algorithm corresponding to a preferred embodiment of the present invention. For example this algorithm can be launched by the system controller node  16  when the emitter node  12  moves to a new position within the area  20 . 
         [0083]    In step  551  the system controller node  16  starts the algorithm. 
         [0084]    In step  552  the system controller node  16  initializes the RF transmission power of emitter node  12  (Ptx) to the maximum RF transmission power (Ptx_max), it also initializes the variable N1 to the maximum number of receivers (Nreceiver_max). 
         [0085]    In step  553  the system controller node  16  applies the preferred Multi-rx Error Correcting Code on the N1 received copies from the N1 receiver nodes  13 ,  14  and  15 . For each received copy, the preferred Multi-rx ECC module performs a division into m sub-blocks. Then it concatenates the various sub-blocks to build N1 m  reconstructed copies. Next, the preferred Multi-rx ECC module performs in parallel a CRC check on each reconstructed copy. 
         [0086]    Then, the system controller node  16  computes a quality parameter, called False Check Rate (FCR), for different combinations of reconstructed copies corresponding to the different combinations of N1−x (with x ε [0 to N1−1]) received copies. The number of FCR metrics to compute is determined by the following equation: 
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         [0087]    For a given combination, the FCR metric is the ratio between the number of false CRC check and the total number of check for said combination at the output of the preferred Multi-rx ECC module. 
         [0088]    In step  555  each computed FCR metric is compared to its target quality parameter (Target_threshold), said target quality parameter being determined in accordance with the number of receivers used in the combination. The target threshold may be stored in memory, as a look up table for example, and may be determined as described in more detail in relation to  FIG. 6  below. 
         [0089]    The step  553  and  555  are performed iteratively for different values of RF transmission power as long as at least one computed FCR is lower than its target quality parameter. In this embodiment, the quality parameter is the FCR metric. For this particular FCR metric, the lower is its value, the higher is the reception quality. 
         [0090]    If at least one FCR metric computed in step  553  is lower than its target quality parameter, the algorithm goes to the step  554 . In the step  554 , the system controller node  16  decreases the current RF transmission power (Ptx) by a predetermined value (ΔP) and sends the updated RF transmission power value to the emitter node  12 . Then the algorithm loops to the step  553 . 
         [0091]    If none of the FCR metrics computed in step  553  is lower than its target quality parameter, the algorithm goes to the step  556 . In step  556 , the system controller node  16  determines and sends the minimum RF transmission power (Ptx_min) to be used by the emitter node  12 . In step  556  the system controller node  16  optionally also determines the combination using the minimum number of receivers (N2) which FCR metric is lower than its target quality parameter. 
         [0092]    In step  557 , where a minimum number of receivers has been determined, the system controller node  16  sends control/command data to put in standby mode the receiver nodes outside the combination determined in step  557 . In step  558  the algorithm is stopped. 
         [0093]    This algorithm is not limitative and some variants may exist. For example, a variant of this algorithm could be to initialize the RF transmission power to a minimum and then to increase it by a predetermined value. 
         [0094]      FIG. 5  illustrates a variation of the above method. According to the method shown in  FIG. 5 , transmission power and the minimum number of receivers are determined sequentially in separate iterative loops. 
         [0095]    In step  501  the system controller node  16  starts the algorithm. In step  502  the system controller node  16  initializes the RF transmission power of emitter node  12  (Ptx) to the maximum RF transmission power (Ptx_max), it also initializes the variable N1 to the maximum number of receivers (Nreceiver_max). 
         [0096]    In step  503  the system controller node  16  applies the Multi-rx Error Correcting Code on the combination of the N1 received copies from the N1 receiver nodes  13 ,  14  and  15 . Next, a quality parameter, indicative of the quality of the decoding, is computed for the established combination. Then the computed quality parameter is compared to a target quality parameter (Target_threshold), said target quality parameter being determined for N1 receivers. 
         [0097]    If the computed quality parameter is lower than the target quality parameter, the algorithm goes to the step  504 . The step  504  corresponds to an error case. In this case, the system controller node is not able to recover the original copy of data sent by the emitter node  12 . If the computed quality parameter is higher than the target quality parameter, the algorithm goes to the step  505 . In step  505  the system controller node  16  decreases the current RF transmission power (Ptx) by a predetermined value (ΔP) and sends the updated RF transmission power value to the emitter node  12 . 
         [0098]    Then the algorithm goes to the step  506 . The step  506  is similar to the step  503  described previously. The step  506  is performed iteratively for different values of RF transmission power as long as the computed quality parameter is higher than the target quality parameter. If the quality parameter computed in step  506  is higher than the target quality parameter, the algorithm loops to the step  505 . If the quality parameter computed in step  506  is lower than the target quality parameter, the algorithm goes to the step  507 . In step  507 , the system controller node  16  determines and sends the minimum RF transmission power (Ptx_min) to be used by the emitter node  12 . This RF transmission power (Ptx_min) will be used by the emitter node  12  for the remaining steps of this algorithm. 
         [0099]    In step  508  the system controller node  16  applies the Multi-rx Error Correcting Code on the various combinations of N1−x (with x ε [1 to N1−1]) received copies from the N1−x receiver nodes. Next, a quality parameter, indicative of the quality of the decoding, is computed for each of the established combination. In step  510  each computed quality parameter is compared to its target quality parameter (Target_threshold), said target quality parameter being determined in accordance with the number of N1−x receivers used in the combination, as discussed below in greater detail. 
         [0100]    If at least one quality parameter computed in step  508  is higher than its target quality parameter, the algorithm goes to the step  509 . In the step  509 , the variable x is increased by one. Then the algorithm loops to the step  508 . If none of the quality parameters computed in step  508  is higher than its target quality parameter, the algorithm goes to the step  511 . In step  511  the system controller node  16  determines the combination using the minimum number of receivers (N2) which quality parameter is higher than its target quality parameter. 
         [0101]    In step  512  the system controller node  16  sends control/command data to put in standby the receiver nodes outside the combination determined in step  511 . In step  513  the algorithm is stopped. 
         [0102]    This algorithm is not limitative and some variants may exist. For example, a variant of this algorithm could be to initialize the RF transmission power to a minimum and then to increase it by a predetermined value. 
         [0103]      FIG. 6  illustrates a relationship between false check rate (FCR) and bit error rate (BER). Such a relationship can be obtained by simulation, and represents the curves BER=f(FCR) for different number of receivers for the preferred embodiment of the invention. 
         [0104]    The y-axis  601  indicates the Bit Error Rate (BER), the x-axis  602  indicates the False Check Rate (FCR). The curve  603  is the simulation result curve obtained for a number of 3 receivers. The curve  604  is the simulation result curve obtained for a number of 2 receivers. A possible target Bit Error Rate required by the application is indicated in  605 . 
         [0105]    The FCR threshold for 3 receivers corresponding to the target BER  605  is indicated in  606 , as a value of 0.24. This FCR threshold  606  is used in the algorithm described in relation to  FIG. 4  for example for the test of the computed FCR metric corresponding to the combination of 3 receivers. 
         [0106]    The FCR threshold for 2 receivers corresponding to the target BER  605  is indicated in  607 , as a value of 0.35. This FCR threshold  607  is used in the algorithm described in relation to  FIG. 4  for example for the test of the computed FCR metrics corresponding to a combination of 2 receivers. 
         [0107]    In embodiments of the invention the relevant threshold values for respective numbers of receivers can be stored, or alternatively the functions can be stored and appropriate values determined by calculation in response to a target BER. 
         [0108]    It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention. 
         [0109]    Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.