Performance Evaluation of Downlink Multi-Beam Massive MIMO with Simple Transmission Scheme at Both Base and Terminal Stations  
[PERSON]  
1 Faculty of Engineering, Niigata University, Ikarashi 2-nocho 8050, Nishi-ku Niigata 950-2181, Japan; [EMAIL_ADDRESS] (K.N. ); [EMAIL_ADDRESS] (R.T.)  
[PERSON]  
1 Faculty of Engineering, Niigata University, Ikarashi 2-nocho 8050, Nishi-ku Niigata 950-2181, Japan; [EMAIL_ADDRESS] (K.N. ); [EMAIL_ADDRESS] (R.T.)  
[PERSON]  
1 Faculty of Engineering, Niigata University, Ikarashi 2-nocho 8050, Nishi-ku Niigata 950-2181, Japan; [EMAIL_ADDRESS] (K.N. ); [EMAIL_ADDRESS] (R.T.)  
[PERSON]  
2 Faculty of Engineering, Niigata University, Ikarashi 2-nocho 8050, Nishi-ku Niigata 950-2181, Japan; [EMAIL_ADDRESS] (K.N. ); [EMAIL_ADDRESS] (R.T.)  
Mathematically, 4-1  
feedback from the UTs [3]. As a counter-measure for this, implicit beamforming (IBF) has been proposed [13]. However, even if IBF is implemented in the massive MIMO systems, the CSI estimation itself will be a large overhead when considering short packet communications such as that in wireless local area network (LAN) systems [3]. The authors proposed an analog-digital hybrid massive MIMO configuration that eliminates CSI estimation, in a previous work [14]. In conventional analog-digital hybrid massive MIMO configurations [15; 16], a beam scanning is required for user tracking in each sub-array, and overheads are incurred in determining the analog weight values. In the proposed configuration, unlike conventional analog-digital hybrid massive MIMO configurations, analog multi-beams are created and several beams with large received powers are selected in the analog part without the overhead for the beamforming [14]. Owing to the selected narrow beams, interference signals can be mitigated, and the residual interference can be cancelled using the constant modulus algorithm (CMA) [17], which does not require training signals in the digital part. Via computer simulations, the basic performance and effectiveness of the uplink channel is evaluated, for the proposed method [14; 18]. Previous studies regarding multi-beam massive MIMO mainly focused on the uplink channel [14; 18]. However, in the next-generation wireless communication systems, including the latest wireless communication systems, the data rate improvement in the downlink channel is essential, and massive MIMO is mainly expected to improve the transmission rate in the downlink. In this paper, we propose a transmission beamforming method that uses multi-beam massive MIMO configuration and control method in the uplink channel; the proposed method does not require CSI in the downlink. It is necessary to consider the control of the analog part and the digital part in the downlink respectively, because the proposed configuration employs the analog-digital hybrid method, unlike conventional digital beamforming (DBF). In this paper, first, we verify the effectiveness of the analog part using analog weight which is selected in the uplink channel alone. Owing to the selected narrow beams, it is shown that the interference signals can be mitigated. However, when the angle of arrivals (AoAs) are close among users, we show that the effect of the analog part is not sufficient. In order to solve this problem, we propose using the CMA for the UT side as well as the elimination of the CSI estimation at the BS. In addition, we propose a method combining the least-square CMA (LS-CMA) and the steepest-descent CMA (SD-CMA) methods in order to utilize CMA stably, with realistic data sizes even for high-order modulation schemes. Finally, the effectiveness of the proposed method is verified using the evaluation of the signal to interference plus noise power ratio (SINR) and the bit rate by the bit error rate (BER). The remainder of this paper is organized as follows. In Section 2, we describe the principle of analog multi-beam massive MIMO with CMA in the digital part. In Section 3, we show the proposed configuration and propose the control method for the downlink.