Fast Beam Alignment through Simultaneous Beam Steering and Power Spectrum Estimation Using a Frequency Scanning Array

In this work we demonstrate that by using a frequency scanning (FS) uniform linear array (ULA) at the base station (BS) of a cellular network, it is possible to estimate the angle of departure (AoD) for all user equipments (UEs) in parallel. Frequency scanning antenna arrays are able to simultaneously excite a broad angular domain by delaying a wideband signal at each antenna element by a fixed time in the order of the reciprocal of the bandwidth fB. Thus, we are able to obtain a bijective mapping of conventional temporal baseband frequencies f and the broadside angles of the antenna array. Such an additional high frequency hardware component at the BS enables a UE to estimate its preferred AoD through a simple spectrum analysis, as each temporal frequency can be demapped to its corresponding spatial frequency or AoD. Based on this, a downlink beam probing can be defined which simultaneously finds optimal AoDs for each UE, thereby reducing the typically large timing overhead of beam alignment, and enabling millimeter wave (mmWave) communication in scenarios suffering from short coherence times due to high mobility and high spatial resolution. In this paper, we further demonstrate that by oversampling the received power spectrum and exploiting the correlation between samples, the AoD estimation can be refined using a linear minimum mean square error estimator. Without affecting time efficiency, we are able to achieve AoD estimates with a mean error of less than 3° even for small antenna geometries with 8 antenna elements and SNR = 0dB.