# Evaluation of Multi-Beam Massive MIMO Considering MAC Layer Using IEEE802.11 ac and FDD-LTE
## 2 Massive MIMO Transmission and Proposed System  
The bandwidth required by 5G systems is achieved using frequencies of 20 GHz or higher. Massive MIMO utilizes BF technology to resolve propagation loss, which is one of the more serious problems encountered under high-frequency bands. Typically, a technique known as digital BF (DBF) is adopted, as shown on the right-hand side of Figure 1. In this technique, weight values are calculated through digital signal processing. However, the massive MIMO systems that apply DBF experience problems with respect to power consumption and implementation cost [17; 18; 1].  
The left-hand side of Figure 1 shows the configuration of a typical hybrid BF massive MIMO with sub-arrays [19]. In the figure, \(N_{L}\) and \(N_{K}\) indicate the number of elements and receivers in a sub-array, respectively. This configuration produces a hybrid analog-digital BF that tracks a desired signal through analog control while removing interference using digital signal processing, and it has attracted significant interest [20; 21; 22]. Massive MIMO is known to be highly effective in addressing the power consumption and implementation cost problems owing to its hybridization of analog and digital approaches.  
Multi-beam massive MIMO eliminates the requirement for CSI estimation by forming numerous narrow directional beams and spatially separating the signals of each UT. As a result, the synchronization between a BS and a UT, which is required for CSI estimation, becomes unnecessary. Under standard MU-MIMO, UTs must transmit signals to a BS simultaneously to employ CSI feedback prior to communication. As multi-beam massive MIMO does not require CSI feedback, a UT can communicate with a BS at any time.  
Figure 2 shows the configuration of analog multi-beam massive MIMO under uplink communication, along with its 16 multi-beam pattern. The proposed system generates \(N\) orthogonal multiple beams in the analog component and uses a Butler matrix circuit to achieve multi-beam formation [23; 24]. The butler matrix circuit, which was introduced in [24], forms an orthogonal beam using a Fast Fourier Transform (FFT) on a spatially arranged array. This system is also effective from the viewpoint of the pilot contamination problem in the multi-cell scenario because of the signals by multiple UTs can be received without the CSI estimation by the multi-beam circuit.  
Figure 1: Uplink configurations of typical analog–digital hybrid (**left**) and full-digital (**right**) beamforming (BF) massive multiple-input multiple-output (MIMO) systems.  
Figure 2: Uplink configuration of multi-beam massive MIMO in uplink and 16 multi-beam pattern.  
The proposed method performs beam selection using an amplitude detector and a processor. Through beam selection, it is possible to spatially separate a specific UT using beam directivity. However, interference signals from the other UTs, which are also received by side lobes, cannot be rejected through only the multi-beam forming network. A method that uses only amplitude information, such as amplitude correlation, must be used to achieve beam selection with low interference power.  
The system developed in this study applies robust independent component analysis (ICA) using a blind adaptive algorithm [16; 25]. ICA is a blind signal processing technique that is commonly used in image processing and medicine. In this technique, an observed random vector is decomposed into statistically independent variables [16]. ICA does not require CSI because it utilizes only received signals. The use of ICA allows for a hybrid configuration for efficient transmission in massive MIMO systems in which multi-beams are applied to analog components and a blind algorithm is applied to digital components.