A Direct Antenna Modulator With Beam Steering Capability Based on Space-Time-Coding Arrays

We propose an RF front-end architecture with simultaneous direct antenna modulation and beam steering capability based on space-time-coding (STC) arrays. In an STC array, digital codes control the state of each array element. The element state determines the amplitude and phase of the radiated wave, and in a time scale over which the state of the elements is changed by a few successive codes, the frequency harmonics are generated. We first introduce a vector representation of bits in a code and their effects on the resultant harmonic wavefront. We show that by choosing proper codes with a minimum of two repetitions, one can modulate data on the phase and amplitude of a desired far-field harmonic where proof-of-concept simulations are given for a 16-APSK digital modulation scheme. We investigate the data rate, spectral efficiency, and the limitation of switching speed in our proposed scheme. Moreover, it is shown that the harmonic beam patterns of an STC array can be easily steered by a consecutive shifting of bits in the code without affecting the transmitted data symbols. Through cosimulation of digital design and coded antenna array, we calculate the constellation of the QPSK digital modulation scheme for a steered beam and study its angular dependency. Finally, we provide an upper bound for the efficiency of STC arrays.