Patent Literature 1: JP 2008-312053 A
A vehicle is mounted with ECUs (Electronic Control Units), sensors, actuators, and the like. Communications are needed between the ECUs, between the ECUs and the sensors, or between the ECUs and the actuators for sharing data, inputting detection values or operating states, or outputting operation instructions. Further, a power line communication (PLC) system is recently studied so as to perform communications and transfer of power via a transmission cable. The PLC system performs modulation communications using a contactless coupler at a branch between the transmission cable and a node.
The transmission cable includes a plurality of branches; this favors an OFDM communication (Orthogonal Frequency Division Multiplexing) providing a multipass resistance. The case of using the OFDM communication needs to prevent an inter symbol interference by setting a guard interval to be longer than a delay spread of reflected waves. For example, a communication system mounted in a vehicle is variable in the number of channels since a node is permitted to be attached or detached to a transmission cable in a dealer-installed option. That is, the topology in the communication bus is caused to change; this changes a delay spread of reflected waves in the system. The system thus needs to expect a worst value in the delay spread and previously lengthen a guard interval sufficiently. However, the lengthened guard interval may pose redundancy that increases an electricity consumption and decreases a transmission efficiency.
Patent Literature 1 describes a control method to adjust a guard interval to be an optimum length so as to perform data communications efficiently between a master apparatus and slave apparatuses. In this control method, a slave apparatus sets a length of the guard interval to an initial value based on the notice information from the master apparatus. Then, when not receiving normally a response from the slave apparatus, the master apparatus repeatedly performs a process that increases the length of the guard interval step by step until the master apparatus can receive the reception successfully.
The control method in Patent Literature 1 causes the master apparatus to execute a step-by-step adjustment by repeating a lengthening process of lengthening a guard interval with respect to each of the slave apparatuses; this increases a time necessary for determining the guard interval lengths of all the slave apparatuses as the number of slave apparatuses increases. For example, suppose a case that the number of slave apparatuses is N and the number of repetitions of the lengthening process for each slave apparatus is k. In this case, a time necessary for determining the guard interval lengths of all the slave apparatuses needs N×k times a time of the lengthening process for lengthening the guard interval with respect to each slave apparatus.