A MIMO (multiple input multiple output) system corresponds to a wireless communication system using multiple transmission antennas and multiple reception antennas. The MIMO system minimizes a fading impact occurring on a radio channel using a diversity scheme and can enhance throughput by simultaneously transmitting a plurality of streams using spatial multiplexing. In case of the SM (spatial multiplexing) scheme, when the number of transmission antennas corresponds to Nt and the number of reception antennas corresponds to Nr, the maximum number of transmittable streams corresponds to min (Nt, Nr). In particular, it is already known that inclination of communication capacity is shown as min (Nt, Nr) in high SNR. Since the communication capacity corresponds to maximum throughput capable of being logically transmitted on a given channel, if the number of transmission antennas and the number of reception antennas are increasing at the same time, the communication capacity is also increasing.
A massive MIMO system including the huge number of transmission and reception antennas is receiving attention as one of technologies constructing 5G. Many theses and experiments assume the MIMO system as a single base station (including a distributed antenna system) equipped with a plurality of antennas and a plurality of user equipments equipped with a single antenna. In this case, although a user equipment is equipped with a single antenna, since a plurality of the user equipments are receiving a service from a single base station at the same time, a channel between the base station and all of a plurality of the user equipments can be comprehended as MIMO. If the number of all user equipments is defined as K, the aforementioned inclination of the communication capacity in the high SNR can be represented by min (Nt, K).
Meanwhile, when a base station including the logically infinite number of transmission antennas transmits data to a plurality of user equipments, an optimal transmission algorithm of the base station corresponds to an MRT (maximal ratio transmission) algorithm. Meanwhile, when a base station receives data transmitted to the base station by a plurality of user equipments, an optimal reception algorithm of the base station corresponds to an MRC (maximal ratio combining) algorithm. Since the MRT and the MRC do not consider interference, performance degradation may occur when the base station is equipped with the finite number of antennas. Yet, if the base station is equipped with the infinite number of antennas, since the interference is gone, the MRT and the MRC may become an optimal solution.
Since a base station can make a beam to be thin (sharp) via antenna beamforming, the base station can concentrate energy on a specific user equipment. By doing so, identical information can be delivered using smaller power. On the contrary, since the aforementioned method does not interfere neighboring different user equipments, it may become a method capable of minimizing performance degradation of a system due to interference.