Patent ID: 7151437

Claim:
A communication system, comprising: a first communication apparatus; and a second communication apparatus associated with a unique data value formed of a sequence of bits; said first communication apparatus including: transmitting means for transmitting a first signal from the first communication apparatus to the second communication apparatus using an electromagnetic wave of a first operational frequency, the first signal being a command, for transmitting a further signal from the first communication apparatus to the second communication apparatus using the electromagnetic wave of the first operational frequency after said transmitting of the first signal, and for transmitting a sequence of check pulses to said second communication apparatus concurrent with said transmitting of the further signal using an electromagnetic wave of a second operational frequency whereby a given one of the sequence of check pulses is transmitted at a predetermined time interval after the transmitting of its immediately preceding check pulse; said second communication apparatus including: using means for using the received further signal to generate internal power in response to receiving the first signal, generating means for generating an associated surface acoustic wave whenever one of the sequence of check pulses is received, determining means for determining whether to respond to a particular one of the sequence of check pulses based on a value of a given one of the sequence of bits, said determining being carried out for each one of the sequence of bits, and responding means for reflecting the surface acoustic wave when said determining means determines that said second communication apparatus is to respond to the associated check pulse, and for allowing the surface acoustic wave to pass when said determining means determines that said second communication apparatus is not to respond to the associated check pulse; said first communication apparatus further including: determining means for determining the values of each one of the sequence of bits by detecting, for each one of the sequence of transmitted check pulses, whether there is a response from said second communication apparatus.