Quantification of Main Lobe Squint in Wideband Massive Antenna Array

Abstract

Phased array can be used to form a very narrow beam in desired direction. As the name suggests, the principle behind this is to linearly control the phase difference of input signals feeding the array. Because of this feature, phased array is widely used in various areas including military and private applications such as radar and wireless communication. Though it is appropriate for narrowband signal transmission, manifestation of beam squint occurs when used for wideband signal transmission. Beam squint is a phenomenon in which the direction of the main lobe of the transmitted beam is frequency dependent. In other words, with differing frequencies, the direction of the main lobe is different. Therefore, various researches have been done to solve beam squint phenomenon in wideband signal transmission using phased array. Array with combination of phase shift (PS) units and true time delay (TTD) units is one of the methods that can significantly reduce the extent of beam squint. In this paper, we investigate the main lobe squint arising in wideband signal transmission using uniform linear array with combination of PS units and TTD units in conjunction with mmWave and massive MIMO. Assuming the linear time invariance of the system, first we derived the frequency response of the aggregate system composed of the uniform linear array with combination of PS units and TTD units and the transmission channel. Using the obtained frequency response, we derived that the extent of main lobe squint of the uniform linear phased array is unrelated to the number of transmit antennas. In addition to that, we showed that array with all TTD units does not cause main lobe squint. Also, by comparing the 3 dB beam width with the extent of main lobe squint of uniform linear phased array, we showed that the main lobe squint generated cannot be ignored for massive MIMO case. So, if we consider spatial division multiplexing and spatial division multiple access using wideband phased array for massive MIMO, the problem of main lobe squint has to be solved. Finally, as a solution to the problem of the main lobe squint, we showed that the extent of main lobe squint can be reduced by increasing the number of TTD units by considering a particular massive MIMO uniform linear array with combination of PS units and TTD units.