1. Asvin, G., Modi, H. & Patel, S. K. 5G technology of mobile communication: a survey. In IEEE International Conference on Intelligent System and Signal Processing, 288–292 (2013).
2. Hayashi, H., Hitko, D. A. & Sodini, C. G. Four-element planar Butler matrix using half-wavelength open stubs. IEEE Microw. Wirel. Compon. Lett. 12, 73–75 (2002).
3. Butler, J. & Lowe, R. Beam-forming matrix simplifies design of electronically scanned antennas. Electron. Des. 9, 170–173 (1961).
4. Kalam, S. V. & Rathi, A. B. Optimum design of 4x4 symmetrically structured butler matrix. Int. J. Sci. Res. Eng. Technol. 5, 31–34 (2016).
5. Sahu, B. Design and Implementation of 4x4 Butler Matrix (Indian Institute of Science, Bangalore).