Direct Antenna Modulation for High-Order Phase Shift Keying

An antenna capable of directly phase modulating a radio frequency (RF) carrier is discussed, designed, and measured as both an antenna and a modulator. Access point densification for the Internet of Things will be expensive in part due to the cost and inefficiency of amplifying waveforms with large peak-to-average power ratios for downlink transmission. Directly modulating at the antenna means only a carrier wave has to be amplified, reducing the cost of densification. Here, reconfigurable frequency selective surfaces are suggested as phase modulators. The design process for producing a phase modulating antenna is detailed, and a prototype is fabricated that is capable of up to 8-PSK modulation with 5.3 dB variation in constellation points and a peak gain of 2.3 dB. When implemented in an end-to-end communications system, the antenna exhibits only 1.5 dB drop in performance compared with instrument grade modulation in an additive white Gaussian noise (AWGN) channel.