Patent ID: 7634088

Claim:
A radio communications system comprising: a first antenna having a directivity electrically switchable; a second antenna; and first and second radio devices mutually transmitting and receiving a radio wave through a radio transmission path via only said first and second antennas, wherein: said first radio device receives a radio wave or waves from said second radio device while changing a directivity of said first antenna with prescribed patterns to form a plurality of directivities, generates a first receive signal profile indicative of a signal strength of a plurality of radio waves received with the respective ones of the directivities, and generates a first private key based on the generated first receive signal profile; and said second radio device receives a radio wave or waves from said first radio device while changing a directivity of said first antenna with prescribed patterns to form a plurality of directivities, generates a second receive signal profile indicative of a signal strength of a plurality of radio waves received with the respective ones of the directivities, and generates a second private key identical to said first private key based on the generated second receive signal profile.