On Side Lobe Power Level in Wideband Massive Antenna Array Using Phase Shifters and True Time Delay Units

MasterBeamforming, as a signal processing technique applied to an antenna array, was mainly focused on forming the shape of a beam in signal transmission for radar. Nowadays, it is recognized as the technique of controlling the magnitude and phase of a signal, or delaying the signal either in transmission or in reception. Its application is not limited only to radar systems but extends to wireless communication systems. Since this technique enables us to transmit the signal to an intended direction, or to receive the signal from an intended target, it is referred to as spatial filtering because it performs filtering in the spatial domain. So far, the most popular form of a beamformer has been a phased array equipped with phase shifter units. However, this array does not form a beam well in transmitting and receiving wideband signals because of the so-called beam squint phenomenon that makes each frequency component of a phase-shifted signal leans towards different directions. This phenomenon can be mitigated by using an array with phase shifters and true-time delay units, which is called a hybrid beamformer in this thesis. The main contribution of this thesis is to quantify the strength of an emerging side lobe compared to an adjacent main lobe when beamforming is performed by the hybrid beamformer. With the hybrid beamformer, the power of the main lobe can be smaller than that of the side lobe depending on the numbers of phase shift units and true-time delay units. In addition, the receiver in an intended direction can suffer a severe reduction of the received signal power. This problem may become more critical when a massive antenna array is deployed for space-division multiplexing or multiple access. It is shown that the detrimental effect can be mitigated from the step of designing the hybrid beamformer, especially by carefully choosing the numbers of phase shifters and true-time delay units. The proposed approach is first to derive the frequency response from the mathematical model of the hybrid beamformer, then to find the period and the magnitude of grating lobes, and finally to estimate the angle and power of a side lobe by using the period and the power of the grating lobes. This approach leads us to a simple and accurate estimation of the difference between the power of main lobe and the power of side lobe as a function of the frequency and the numbers of units. The results can be tabulated and used to design the hybrid beamformer having tolerable side lobe power