MIMO Technologies IN 5G NEW RADIO

ports have been specifi ed to support the transmission of up-to 8 data layers in single user (SU) MIMO [3, 4]. Th e multiuser (MU) MIMO has also been supported dynamically with SU-MIMO over frequency domain in LTE release-10 with limited number of data layers or streams. Recently, MIMO with a large number of transmit antennas at the base station, a.k.a. massive MIMO, is introduced, which signifi cantly increases system throughput via MU-MIMO transmissions given a large number of degrees of freedom (DoF) at the transmit side [10]. Practically, considering the form factor limitation at the base station, 3D massive-MIMO systems employing a two dimensional 2D antenna array exploits the DoF at both the elevation domain and the azimuth domain. Th e 3D massive MIMO is considered as full-dimensional MIMO (FD-MIMO) in 3GPP LTE-Advanced systems. [13] With the attractive performance gain from massive MIMO, MIMO continues drawing great interest and attention in the standardization discussions for the next generation, i.e., the 5G cellular systems [1].2 On the other hand, the high frequency millimeter wave (mmWave) will be considered as the carrier in 5G new radio to provide Giga bit-per-second (bps) data transmission rates due to large bandwidth available in this frequency regime. For mmWave with a high carrier frequency, the signal experiences a much larger path loss than that of low carrier frequency of several Giga-Hertz (GHz) or less. However, in mmWave communication, it is feasible In this article, we review the multiantenna, or multiple-input multipleoutput (MIMO) technologies for the fi ft h generation (5G) cellular systems. MIMO has been adopted in the fourth generation (4G) long term evolution (LTE) cellular systems to improve the throughput and reliability. Recently, with a large scale transmit antenna array, or so called massive MIMO, the cell throughput and reliability can be further enhanced for both low frequency sub-6GHz and high frequency millimeter wave (mmWave) transmissions. Th erefore, MIMO still plays an important role in the next-generation cellular systems for a wide range of carrier frequency. Th e 3rd generation partnership project (3GPP) has initiated standardization activities for MIMO technologies in 5G new radio. In this paper, we review the MIMO-related items that have been considered in 3GPP RAN1 group focusing on the physical layer specifi cation.1 The next generation, or the fi ft h generation (5G), cellular network will provide much higher peak data rate, larger data volume per unit area, lower latency, larger number of connected devices, higher mobility, higher reliability, and better energy effi ciency than current 4G LTE systems [1]. One of the key enabling techniques is the multi-antenna or MIMO technology. MIMO technologies have played a vital role in the 4G LTE systems [8]. With multiple transmit and receive antennas, the diverse channel among diff erent transceiver antennas can be exploited to provide the spatial multiplexing and diversity gain over single antenna systems and, consequently, improve the data rate and/or the reliability of wireless links. During the evolution of 4G, MIMO have been extensively studied. Enhanced MIMO technologies and features have been included in several specifi cation releases in 3GPP [1]. In LTE release-10, the reference signals for up-to 8 antenna