Beam Training Technique for Millimeter-Wave Cellular Systems Using Retrodirective Arrays

Beam training in millimeter-wave (mmWave) cellular systems requires a long processing time that is proportional to the product of the number of transmitting and receiving beams. In this paper, we propose a beam training technique that can significantly reduce the beam training time in mmWave cellular systems, using a retrodirective directional array (RDA). In the proposed technique, the beam sweeping operations required at the base station (BS) and mobile station (MS) are significantly reduced owing to the use of the RDA, which automatically returns a signal in the direction along which it originated. A preamble sequence design technique for beam training is proposed to identify the BS, MS, and beams simultaneously transmitted from the BS/MS, using the Zadoff-Chu sequence. The ambiguity condition and detection algorithms are derived so that we can uniquely identify the parameters for beam alignment in asynchronous environments with symbol timing offset (STO) and carrier frequency offset (CFO). Simulations show that the proposed algorithm can correctly detect the parameters for beam alignment in mmWave cellular systems with RDA in asynchronous environments. Moreover, the proposed technique can significantly reduce the period required for beam training, compared with the conventional techniques.