1 . A communication device comprising: a signal determiner to determine whether there is a reception signal or not; and a period estimator to estimate an interruption period of interruption of a signal transmitted from a device that is a source of the reception signal, using a determination result from the signal determiner, wherein the period estimator includes a differential operator to calculate a differential value of the determination result, a masking operator to calculate a provisional period of the interruption period using the differential value, to control use of the differential value and of the provisional period based on an internal state representing an operational state of the period estimator, and to output the provisional period to be used, a period calculator to calculate the interruption period using the provisional period output from the masking operator, a signal existing section calculator to calculate a signal existing section referring to a section in which the reception signal exists, using the provisional period output from the masking operator, a periodic timing estimator to estimate periodic timing representing timing of a change, in the determination result, from a section in which the reception signal does not exist to the signal existing section, using the provisional period output from the masking operator and using the signal existing section, and a state determiner to determine the internal state using the interruption period.
2 . The communication device according to claim 1 , wherein the internal state has two states that are a period seeking state being a state in which the interruption period is not identified in the period estimator and a period-identified state being a state in which the interruption period is identified in the period estimator, and the period estimator specifies the period seeking state as an initial state, that a transition condition from the period seeking state to the period-identified state is that the interruption period for a predetermined number of previous cycles falls within a predetermined range including a reference period, where the reference period is a latest interruption period calculated by the period calculator, and that a transition condition from the period-identified state to the period seeking state is that the identified period is not updated even after a time period elapses since an update time of an identified period, where the time period is a predetermined factor times the identified period and the identified period is an interruption period calculated in the period-identified state.
3 . The communication device according to claim 2 , wherein the masking operator does not use the provisional period when the internal state is the period seeking state and the provisional period is out of range from a predetermined minimum period to a predetermined maximum period, and does not use the provisional period when the internal state is the period-identified state and the provisional period is out of a predetermined range including the identified period.
4 . The communication device according to claim 2 , wherein the period estimator includes an averager to average the determination result or an input value using the identified period when the internal state is the period-identified state.
5 . The communication device according to claim 2 , wherein the period estimator includes a smoother to smooth the determination result or an input value based on a predetermined number of times of smoothing.
6 . The communication device according to claim 5 , wherein the smoother executes a smoothing loop L times on the determination result or on the input value at time t, and in processing of an n-th smoothing loop, corrects a state “signal non-existing” at time t to a state “signal existing” in a case in which determination results or input values at time t−n, time t, and time t+1 are respectively “signal existing”, “signal non-existing”, and “signal existing”, and corrects a state “signal existing” at time t to a state “signal non-existing” in a case in which the determination results or the input values at time t−n, time t, and time t+1 are respectively “signal non-existing”, “signal existing”, and “signal non-existing”, where time t has a discretized value of time when the determination result is output from the signal determiner, and the value L is an integer greater than or equal to 1, and the value n is an integer ranging from 1 to L, inclusive.
7 . The communication device according to claim 2 , wherein the period estimator includes a period averager to average the interruption period when the internal state is the period-identified state.
8 . The communication device according to claim 2 , wherein the period estimator includes a periodic timing averager to average the periodic timing when the internal state is the period-identified state.
9 . The communication device according to claim 2 , wherein the period estimator includes a signal existing section averager to average the signal existing section when the internal state is the period-identified state.
10 . The communication device according to claim 1 , wherein the period estimator includes a non-periodicity determiner to generate, based on the determination result, a signal constantly existing flag indicating whether the signal is being continuously received and a signal constantly non-existing flag indicating whether the signal is being continuously unreceived.
11 . The communication device according to claim 1 , comprising: a period signal generator to generate a period signal indicating a period of interruption of the signal using the interruption period, the signal existing section, the periodic timing, and the internal state; and a no-signal weight applier to apply a weight to the signal that is received during a time period determined to be “signal non-existing” in the period signal.
12 . The communication device according to claim 2 , comprising: a transmission controller to provide control to transmit a burst signal having a burst signal length less than a length of the signal existing section when the internal state is the period-identified state.
13 . The communication device according to claim 10 , comprising: a transmission controller to provide control to continuously transmit a signal from the communication device in a case of reception, from the non-periodicity determiner, of a signal constantly existing flag indicating that the signal is being continuously received.
14 . The communication device according to claim 10 , comprising: a transmission controller to provide control to stop transmission of a signal from the communication device in a case of reception, from the non-periodicity determiner, of a signal constantly non-existing flag indicating that the signal is being continuously unreceived.
15 . The communication device according to claim 12 , wherein the transmission controller sets a first time margin before the signal existing section, sets a second time margin after the signal existing section, and determines the burst signal length to be a length of a time zone that is obtained by removing the first time margin and of the second time margin from the signal existing section.
16 . The communication device according to claim 15 , wherein the transmission controller provides control to sequentially transmit the burst signal by time diversity transmission when the burst signal length obtained is less than a minimum transmittable burst length.
17 . The communication device according to claim 15 , wherein the transmission controller allocates a symbol having a higher required received power in a center of the burst signal, and allocates a symbol having a lower required received power in a front half and in a rear half of the burst signal.
18 . A method for predicting interruption comprising: a signal determination of determining whether there is a reception signal or not; and a period estimation of estimating an interruption period of interruption of a signal transmitted from a device that is a source of the reception signal, using a determination result, wherein the period estimation includes calculating a differential value of the determination result, calculating a provisional period of the interruption period using the differential value, controlling use of the differential value and of the provisional period based on an internal state representing an operational state estimating the interruption period, and outputting the provisional period to be used, calculating the interruption period using the provisional period, calculating a signal existing section referring to a section in which the reception signal exists, using the provisional period, estimating periodic timing representing timing of a change, in the determination result, from a section in which the reception signal does not exist to the signal existing section, using the provisional period and using the signal existing section, and determining the internal state using the interruption period.
19 . A control circuit for controlling a communication device, wherein, in estimating an interruption period of interruption of a signal transmitted from a device that is a source of the reception signal, using a determination result indicating whether there is a reception signal or not, the control circuit causes the communication device to perform: calculating a differential value of the determination result, calculating a provisional period of the interruption period using the differential value, controlling use of the differential value and of the provisional period based on an internal state representing an operational state estimating the interruption period, and outputting the provisional period to be used, calculating the interruption period using the provisional period, calculating a signal existing section referring to a section in which the reception signal exists, using the provisional period, estimating periodic timing representing timing of a change, in the determination result, from a section in which the reception signal does not exist to the signal existing section, using the provisional period and using the signal existing section, and determining the internal state using the interruption period.
20 . A non-transitory program recording medium storing therein a program for controlling a communication device, wherein, in estimating an interruption period of interruption of a signal transmitted from a device that is a source of the reception signal, using a determination result indicating whether there is a reception signal or not, the program causes the communication device to execute: calculating a differential value of the determination result, calculating a provisional period of the interruption period using the differential value, controlling use of the differential value and of the provisional period based on an internal state representing an operational state estimating the interruption period, and outputting the provisional period to be used, calculating the interruption period using the provisional period, calculating a signal existing section referring to a section in which the reception signal exists, using the provisional period, estimating periodic timing representing timing of a change, in the determination result, from a section in which the reception signal does not exist to the signal existing section, using the provisional period and using the signal existing section, and determining the internal state using the interruption period.
