Receiver, transmitter, radio communication system, and radio communication method

A receiver comprising:

• a reception unit to sequentially receive modulated signals resulting from a spread spectrum from a transmitter;
• and
• a demodulation unit to demodulate a first signal received by the reception unit by performing despreading using a first spreading code, the first signal including information for identifying a second spreading code with a longer code period than the first spreading code, identify the second spreading code on a basis of information obtained from the first signal, and demodulate a second signal received after the first signal by the reception unit by performing despreading using the second spreading code,
• wherein
• the signal received by the reception unit is a signal of frames each including a frame number for identification in a header,
• the first signal further includes information indicating a frame number corresponding to a time when the transmitter has switched from the first spreading code to the second spreading code, and
• each time a signal is received by the reception unit, the demodulation unit compares a frame number included in a header of a frame obtained from the received signal with the frame number indicated by the information obtained from the first signal, and switches from the first spreading code to the second spreading code when the frame numbers match with each other.

A receiver comprising:

• a reception unit to sequentially receive modulated signals resulting from a spread spectrum from a transmitter;
• and
• a demodulation unit to demodulate a first signal received by the reception unit by performing despreading using a first spreading code, the first signal including information for identifying a second spreading code with a longer code period than the first spreading code, identify the second spreading code on a basis of information obtained from the first signal, and demodulate a second signal received after the first signal by the reception unit by performing despreading using the second spreading code,
• wherein
• the information included in the first signal is information identifying a common encryption key held by the receiver and the transmitter, and
• the second spreading code is a spreading code constituted by a combination of a plurality of kinds of first spreading codes in an order identified by data encrypted using the encryption key,
• wherein the demodulation unit monitors correlations values each obtained by a correlation process, and before demodulating the second signal, the demodulation unit adjusts an amplitude of the second signal depending on detected change in a correlation value.

A radio communication system comprising:

• a transmitter to modulate a first signal by performing spread spectrum using a first spreading code, the first signal including information for identifying a second spreading code with a code period longer than the first spreading code, modulate a second signal to be transmitted after the first signal by performing spread spectrum using the second spreading code, and sequentially transmit modulated signals;
• and
• a receiver comprising a reception unit to sequentially receive modulated signals resulting from a spread spectrum from a transmitter;
• and
• a demodulation unit to demodulate a first signal received by the reception unit by performing despreading using a first spreading code, the first signal including information for identifying a second spreading code with a longer code period than the first spreading code, identify the second spreading code on a basis of information obtained from the first signal, and demodulate a second signal received after the first signal by the reception unit by performing despreading using the second spreading code,
• wherein the demodulation unit performs, on a signal received by the reception unit, a first correlation process using a first code replica corresponding to the first spreading code and a second correlation process using a second code replica corresponding to the second spreading code, detects a time when the transmitter has switched from the first spreading code to the second spreading code on a basis of a first correlation value obtained by the first correlation process and a second correlation value obtained by the second correlation process, and switches from the first spreading code to the second spreading code at a time corresponding to the detected time,
• the receiver sequentially receiving signals modulated by the transmitter from the transmitter, by the reception unit.

A transmitter comprising:

• a modulation unit to modulate a first signal by performing spread spectrum using a first spreading code, the first signal including information for a receiver to identify a second spreading code with a longer code period than the first spreading code, and demodulate a second signal to be transmitted after the first signal by performing spread spectrum using the second spreading code;
• and
• a transmission unit to sequentially transmit signals modulated by the modulation unit to the receiver,
• wherein the first signal further includes information indicating a time when the modulation unit switches from the first spreading code to the second spreading code,
• wherein the modulation unit switches from the first spreading code to the second spreading code at:
• (i) a beginning of any one of code periods of the first spreading code, or
• (ii) a beginning of any of a plurality of bit strings each having a fixed length and which constitute a signal of data modulated by the modulation unit.

A radio communication method comprising:

• by a transmitter, modulating a first signal by performing spread spectrum using a first spreading code, the first signal including information for identifying a second spreading code with a code period longer than the first spreading code, modulating a second signal to be transmitted after the first signal by performing spread spectrum using the second spreading code, and sequentially transmitting modulated signals;
• and
• by a receiver, sequentially receiving signals modulated by the transmitter from the transmitter, demodulating the first signal by performing despreading using the first spreading code, identifying the second spreading code on a basis of information obtained from the first signal, and demodulating the second signal by performing despreading using the second spreading code,
• wherein the receiver performs, on a signal received, a first correlation process using a first code replica corresponding to the first spreading code and a second correlation process using a second code replica corresponding to the second spreading code, detects a time when the transmitter has switched from the first spreading code to the second spreading code on a basis of a first correlation value obtained by the first correlation process and a second correlation value obtained by the second correlation process, and switches from the first spreading code to the second spreading code at a time corresponding to the detected time.