COMMUNICATION SYSTEM

1 . A communication system comprising:

plural receiving nodes; and

a sending node configured to send simultaneous broadcast data to the plural receiving nodes,

wherein

the sending node comprises:

a sending unit configured to send the simultaneous broadcast data to the plural receiving nodes by use of at least one of plural subcarriers preliminarily prepared;

a storage unit configured to store transmission path condition information representing a condition of a transmission path for each of the receiving nodes;

a node grouping unit configured to perform a grouping process of referring to the transmission path condition information, and classifying the plural receiving nodes into plural destination groups based on a classification criterion related to the condition of the transmission path, and creating a destination group set defined by the plural destination groups; and

a subcarrier selection unit configured to refer to the transmission path condition information and select, from the plural subcarriers, a subcarrier or subcarriers to send the simultaneous broadcast data to the receiving node belonging to the destination group, for each of the destination groups of the destination group set, and

the sending unit is configured to send the simultaneous broadcast data to each of the plural destination groups of the destination group set by use of the subcarrier or subcarriers selected by the subcarrier selection unit in turn.

2 . The communication system as set forth in claim 1 , wherein

the condition of the transmission path indicates for each subcarrier whether or not the subcarrier is an effective subcarrier available for communication with the receiving node.

3 . The communication system as set forth in claim 2 , wherein

the node grouping unit is configured to, in the grouping process, perform:

a selection process of selecting the effective subcarrier of the receiving node having the least number of effective subcarriers, as a priority subcarrier;

a calculation process of calculating the number of priority subcarriers included in the effective subcarriers of the receiving node, for each receiving node;

a judgment process of judging whether or not the number of priority subcarriers is not less than a grouping threshold, for each receiving node; and

a classification process of classifying the receiving nodes having the number of priority subcarriers not less than the grouping threshold into the destination group which is different from the destination group including the receiving nodes having the number of priority subcarriers less than the grouping threshold.

4 . The communication system as set forth in claim 3 , wherein

the grouping threshold is a constant value.

5 . The communication system as set forth in claim 3 , wherein

the node grouping unit is configured to:

create the plural destination group sets by performing the plural grouping processes having the mutually different grouping thresholds; and

select one from the plural destination group sets according to a predetermined condition.

6 . The communication system as set forth in claim 3 , wherein

the node grouping unit is configured to perform multiple separation processes defined by a combination of the selection process, the calculation process, the judgment process, and the classification process to create the destination group set defined by the three or more destination groups, and

the node grouping unit is configured to, in the classification process, create a first destination group with the number of priority subcarriers not less than the grouping threshold and a second destination group with the number of priority subcarriers less than the grouping threshold, and

the node grouping unit is configured to perform a first round of the separation processes with regard to all of the receiving nodes, and perform a second or subsequent rounds of the separation processes with regard to the second destination group created through the previous separation process.

7 . The communication system as set forth in claim 2 , wherein

the node grouping unit in configured to, in the grouping process, perform:

a selection process of selecting the effective subcarriers of the receiving node having the least number of effective subcarriers, as a priority subcarrier;

a calculation process of calculating the number of priority subcarriers included in the effective subcarriers of the receiving node, for each receiving node;

a judgment process of judging whether or not the number of priority subcarriers is not less than a grouping threshold, for each receiving node; and

a classification process in which the node grouping unit judges whether or not plural boundary receiving nodes each defined as the receiving node having the least number of priority subcarriers out of the receiving nodes having the number of priority subcarriers not less than the grouping threshold exist, and, upon concluding that the plural boundary receiving nodes exist, creates the plural destination group sets based on a combination of the plural boundary receiving nodes and the receiving node having the number of priority subcarriers greater than the number of priority subcarriers of the boundary receiving nodes.

8 . The communication system as set forth in claim 7 , wherein

the node grouping unit is configured to, in the classification process, classify the boundary receiving node(s) having the less number of effective subcarriers out of the plural boundary receiving nodes into the same destination group that includes the receiving nodes having the number of priority subcarriers greater than that of the boundary receiving node, in priority to the boundary receiving nodes having the greater number of effective subcarriers.

9 . The communication system as set forth in claim 2 , wherein

the subcarrier selection unit is configured to classify the plural subcarriers into shared subcarriers and unshared subcarriers, for each of the destination groups of the destination group set, and

the shared subcarrier is defined as subcarriers serving as the effective subcarrier to all of the receiving nodes included in the destination group, and

the unshared subcarrier is defined as the subcarrier serving as the effective subcarrier to not all but at least one of the receiving nodes included in the destination group, and

the subcarrier selection unit is configured to create a subcarrier set for each of the destination groups of the destination group set, and

the subcarrier set includes at least one of the shared subcarrier and an unshared subcarrier set, and the unshared subcarrier set is defined as a group of the plural unshared subcarriers selected so as to allow all of the receiving nodes belonging to the destination group to receive the simultaneous broadcast data, and

the sending unit is configured to send the simultaneous broadcast data by use of the subcarrier set created by the subcarrier selection unit, for each of the destination groups of the destination group set.

10 . The communication system as set forth in claim 2 , wherein

the sending node further includes a sending period calculation unit, and

the node grouping unit is configured to:

create the plural destination group sets by performing the plural grouping processes having the mutually different classification criteria; and

select one from the plural destination group sets according to a predetermined condition, and

the sending period calculation unit is configured to calculate an accumulated sending period necessary to send the simultaneous broadcast data to all of the receiving nodes, for each of the destination group sets, and

the predetermined condition is that the accumulated sending period is the shortest.

11 . The communication system as set forth in claim 9 , wherein

the sending node further includes a sending period calculation unit, and

the node grouping unit is configured to:

create the plural destination group sets by performing the plural grouping processes having the mutually different classification criteria; and

select one from the plural destination group sets according to a predetermined condition, and

the subcarrier selection unit is configured to create the subcarrier set for each of the destination groups with regard to each of the plural destination group sets created by the node grouping unit, and

the sending period calculation unit is configured to:

calculate a carrier number defined as the sum of the number of shared subcarriers and the number of unshared subcarrier sets with regard to the subcarrier set, for each of the destination group; and

calculate an accumulated sending period based on the carrier number, for each of the destination group sets, and

the accumulated sending period is defined as the time necessary to send the simultaneous broadcast data to all of the receiving nodes, and

the predetermined condition is that the accumulated sending period is the shortest.

12 . The communication system as set forth in claim 1 , wherein

the sending node further includes a sending period calculation unit and a modulation method selection unit, and

the sending unit is configured to send the simultaneous broadcast data by using one of the plural modulation methods, and

the storage unit is configured to store the transmission path condition information for each of the modulation methods, and

the node grouping unit is configured to:

create the plural destination group sets by performing the plural grouping processes having the mutually different classification criteria, for each of the modulation methods; and

select one from the plural destination group sets according to a predetermined condition, and

the sending period calculation unit is configured to calculate an accumulated sending period necessary to send the simultaneous broadcast data to all of the receiving nodes, for each of the destination group sets, and

the predetermined condition is that the accumulated sending period is the shortest, and

the modulation method selection unit is configured to select the modulation method according to the destination group set selected by the node grouping unit; and

the sending unit is configured to send the simultaneous broadcast data by use of the modulation method selected by the modulation method selection unit.

13 . The communication system as set forth in claim 9 , wherein

the sending node further includes a sending period calculation unit and a modulation method selection unit, and

the sending unit is configured to send the simultaneous broadcast data by using one selected from plural modulation methods, and

the storage unit is configured to store the transmission path condition information for each of the modulation methods, and

the node grouping unit is configured to:

create the plural destination group sets by performing the plural grouping processes having the mutually different classification criteria, for each of the modulation methods; and

select one from the plural destination group sets according to a predetermined condition, and

the subcarrier selection unit is configured to create the subcarrier set for each of the destination groups with regard to each of the plural destination group sets created by the node grouping unit, and

the sending period calculation unit is configured to:

calculate a carrier number defined as the sum of the number of shared subcarriers and the number of unshared subcarrier sets included in the subcarrier set, for each of the destination group of the destination group set; and

calculate an accumulated sending period based on the carrier number, for each of the destination group sets, and

the accumulated sending period is defined as the time necessary to send the simultaneous broadcast data to all of the receiving nodes, and

the predetermined condition is that the accumulated sending period is the shortest,

the modulation method selection unit is configured to select the modulation method according to the destination group set selected by the node grouping unit; and

the sending unit is configured to send the simultaneous broadcast data by use of the modulation method selected by the modulation method selection unit.

14 . The communication system as set forth in claim 11 , wherein

the sending unit is configured to send a simultaneous broadcast data to the next destination group after a lapse of a predetermined interval period after sending the simultaneous broadcast data to the previous destination group, and the sending period calculation unit is configured to:

calculate a sending period necessary to send the simultaneous broadcast data to all of the receiving nodes belonging to the destination group, for each of the destination groups of the destination group; and

calculate the accumulated sending period of the destination group set by adding the sum of the sending periods of the respective plural destination groups of the destination group set to the total of the interval periods.

15 . The communication system as set forth in claim 1 , wherein:

the sending unit is configured to send destination group information together with the simultaneous broadcast data; and

the destination group information includes destination information indicative of the destination group selected as a destination of the simultaneous broadcast data; and

the receiving node is configured to, upon receiving the simultaneous broadcast data and the destination group information, judge whether or not the receiving node belongs to the destination group indicated by the destination information included in the received destination group information, and to, upon concluding that the receiving node belongs to the destination group indicated by the destination information, accept the simultaneous broadcast data, and to, upon concluding that the receiving node does not belong to the destination group indicated by the destination information, discard the simultaneous broadcast data.