Wireless communication system and parent node search method

1. A wireless communication system comprising a parent node as a host communication device and a child node as a subordinate communication device,

wherein the parent node includes:

a parent node search signal receiver receiving a parent node search signal transmitted from the child node;

a measurement device measuring, upon receiving the parent node search signal, an intensity of wireless link with the child node transmitting the parent node search signal;

a responder returning a response signal to the child node as a sender of the parent node search signal after waiting for a response wait time in accordance with the intensity of the wireless link; and

a response wait time determination device determining the response wait time in accordance with the intensity of the wireless link so that the response wait time becomes shorter with an increase in the intensity of the wireless link and the response wait time becomes longer with a decrease in the intensity of the wireless link,

wherein the child node includes:

a transmitter transmitting the parent node search signal;

a response signal receiver receiving the response signal; and

a parent node selector selecting a parent node for routine communication with the child node in accordance with a wait time from transmission of the parent node search signal to reception of the response signal, and

wherein the parent node selector of the child node selects a parent node returning the response signal first.

2. The wireless communication system according to claim 1 , wherein

the responder of the parent node returns a response signal to the child node as a sender of the parent node search signal while minimizing the response wait time when the intensity of the wireless link is a threshold or higher, and finishes processing without returning the response signal when the wireless link is weaker than the threshold, and

the parent node selector of the child node selects a parent node returning the response signal first.

3. The wireless communication system according to claim 2 , wherein the threshold is a specified value.

4. The wireless communication system according to claim 2 , wherein

the transmitter of the child node transmits the threshold together with the parent node search signal to the parent node, and

the child node further comprises a threshold change device that, in the case of a failure in receiving a response signal from the parent node after transmission of the parent node search signal, changes the threshold to a value smaller than a current value and makes the transmitter retransmit the parent node search signal and the changed threshold.

5. The wireless communication system according to claim 1 , wherein the measurement device measures a RSSI as an index indicating the intensity of the wireless link.

6. A parent node search method, comprising the steps of:

a transmission step where a child node as a subordinate communication device transmits a parent node search signal;

a parent node search signal reception step where a parent node as a host communication device receives the parent node search signal transmitted from the child node;

a measurement step where, when the parent node receives the parent node search signal, the parent node measures an intensity of wireless link with the child node transmitting the parent node search signal;

a response wait time determination step where the parent node determines a response wait time in accordance with the intensity of the wireless link so that the response wait time becomes shorter with an increase in the intensity of the wireless link and the response wait time becomes longer with a decrease in the intensity of the wireless link;

a response step where the parent node returns a response signal to the child node as a sender of the parent node search signal after waiting for a response wait time in accordance with the intensity of the wireless link;

a response signal reception step where the child node receives the response signal; and

a parent node selection step where the child node selects a parent node for routine communication with the child node in accordance with a wait time from transmission of the parent node search signal to reception of the response signal and where the child node selects a parent node returning the response signal first.

7. The parent node search method according to claim 6 , wherein

in the response step, a response signal is returned to the child node as a sender of the parent node search signal while minimizing the response wait time when the intensity of the wireless link is a threshold or higher, and processing is finished without returning the response signal when the wireless link is weaker than the threshold, and

in the selection step, the child node selects a parent node returning the response signal first.

8. The parent node search method according to claim 7 , wherein the threshold is a specified value.

9. The parent node search method according to claim 7 , wherein in the transmission step, the threshold together with the parent node search signal are transmitted to the parent node, wherein

the parent node search method further comprises a threshold change step where, in the case of a failure in receiving a response signal from the parent node after transmission of the parent node search signal, the child node changes the threshold to a value smaller than a current value and retransmits the parent node search signal and the changed threshold.

10. The parent node search method according to claim 6 , wherein in the measurement step, a RSSI is measured as an index indicating the intensity of the wireless link.