Communication device and communications system

A communication device, comprising:

• a first state-switching control unit that controls switching between a first state in which synchronization for communicating with a first communication partner device is established, a second state in which the communication is started after establishing the synchronization in the first state, a third state in which the communication is intermittently stopped without losing the establishment of the synchronization, and a fourth state in which the communication is stopped for a longer time than the third state and synchronization is reestablished with the first communication partner device when restarting the communication;
• a second state-switching control unit that controls switching between the first state, the second state, the third state, and the fourth state in communication with a second communication partner device;
• a first communication control unit that transmits, upon repeatedly receiving, in a first cycle, a first signal continuously transmitted from the second communication partner device within a first signal interval while the second communication partner device is in the fourth state, the received first signal to the first communication partner device;
• and
• a second communication control unit that transmits, upon receiving a second signal output from the first communication partner device in response to the first signal, the second signal to the second communication partner device, wherein
• the first state-switching control unit transitions from the fourth state to the first state upon transmitting the second signal from the first communication partner device.

A communication device, comprising:

• a state-switching control unit that controls switching between a first state in which synchronization for communicating with a communication partner device is established, a second state in which the communication is performed after establishing the synchronization in the first state, a third state in which the communication is intermittently stopped without losing the establishment of the synchronization, and a fourth state in which the communication is stopped for a longer time than the third state and synchronization is reestablished with the communication partner device when restarting the communication;
• a first communication control unit that repeatedly receives, in a first cycle, a first signal continuously transmitted from the communication partner device within a first signal interval while the communication partner device is in the fourth state;
• and
• a second communication control unit that transmits, to the communication partner device, a second signal synchronized with the first signal received by the first communication control unit.

A communications system, comprising:

• a first communication device;
• and
• a second communication device that alternately transmits/receives information to/from the first communication device within a period assigned by a time division duplex (TDD) communication method, wherein
• the second communication device includes
• a state-switching control unit that controls switching between a first state in which synchronization for communicating with the first communication device is established, a second state in which the communication is performed after establishing the synchronization in the first state, a third state in which the communication is intermittently stopped without losing the establishment of the synchronization, and a fourth state in which the communication is stopped for a longer time than the third state and synchronization is reestablished with the first communication device when restarting the communication,
• a first communication control unit that repeats, in a first cycle, an operation of continuously transmitting a first signal within a first signal interval to the first communication device in the fourth state, and
• a second communication control unit that transmits, upon receiving a second signal output from the first communication device in response to the first signal, a third signal synchronized with the second signal, and transitions from the fourth state to the first state.