2D Pre-Beamforming Control Method via Analog Radio Over Fiber for Wireless Communication Systems With Massive Planar Array Antenna

To rapidly deploy large capacity wireless communication services using high frequency bands, a system is being considered that use radio-over-fiber (RoF) technology to separate the wireless base station function into a central station (CS) with signal processing functions and remote radio units (RRU) with antenna functions. In this system, remote beamforming, in which the CS performs beamforming control, is desirable for simplifying the RRU. We have proposed a fixed wavelength allocation beamforming scheme and a pre-beamforming (pre-BF) control method as a means of remote beamforming suitable for high frequency band RoF systems. However, when these methods are applied to RoF systems with a massive planar array antenna, the beam may not be formed properly. In this paper, the performance of applying these methods to massive planar array antennas is evaluated in simulations. This evaluation shows that the phase spacing irregularities due to the wavelength spacing expanding as the number of antenna elements increase causes BF gain reducing by beam misshaping and directional shifts. To solve this problem, we propose a two-dimensional (2D) pre-BF control method, which extends the pre-BF control method to two dimensions. The proposed method compensates for the phase spacing irregularities due to the wavelength spacing expanding. Thus, high accuracy beamforming with the fixed wavelength allocation beamforming scheme can be realized even for planar array antennas. We show that the 2D pre-BF control method can conduct high accuracy beamforming by reducing the degradation from the ideal beamforming gain to about 0.2 dB with a 100-element planar array antenna.