Digital wireless sensor server using an adaptive smart-antenna/retrodirective array

A wireless sensor server is developed based on a reconfigurable active smart-antenna/retrodirective array. The system can serve as both a retrodirective array transponder and a smart-antenna receiver simply by changing the frequency of the local oscillator applied to the mixers, enabling it to utilize its hardware best to suit its communication environment. When operating as a direct-conversion receiver, the receiver array successfully demodulates a quaternary phase-shift keying (QPSK) modulated signal with circuit gain of 7 dB, and E_b/N₀ for BER=10^-4 is approximately 12 dB without any error correction. In the retrodirective array mode, the system provides 20 dB circuit gain and 20 dB radio-frequency/intermediate-frequency isolation at the center frequency as well as phase conjugation, exhibiting excellent retrodirectivity. The mixers perform phase conjugation and modulation simultaneously, enabling the transmission of locally stored data. The local data is successfully extracted by an interrogator.