1. Field of the Invention
Aspects of the present invention relate to Multiple-Input Multiple-Output (MIMO) communication technology that transmits signals via a plurality of antennas, and more particularly, to a MIMO communication system and method that more effectively embodies a communication system based on channel quality information (CQI) fed back from a user terminal in a MIMO communication technology.
2. Description of the Related Art
According to multiple antenna technology, a data rate can be increased with multiple transmitting and/or receiving antennas, without adding frequencies and without increasing power. The multiple antenna technology transmits signals via multiple antennas to thereby obtain a diversity effect.
However, when transmitting a plurality of data streams, interference may occur between channels. Accordingly, a complex receiver is used to reduce the interference between the channels. In this case, precoding may effectively reduce the interference between the channels. Generally, a precoding scheme can be classified into a linear preceding scheme and a non-linear precoding scheme. Currently, research on the linear precoding schemes (such as a zero-forcing (ZF) beamforming scheme, a Per-User Unitary Rate Control (PU2RC) scheme, etc.) is ongoing.
In a conventional Multiple-Input Multiple-Output (MIMO) communication system, each of a plurality of user terminals feeds back channel quality information (CQI) to a base station. The base station performs various types of operations (such as selecting a beamforming vector) based on the received CQI. Each of the user terminals generates the CQI when a transmission rank is 1 and feeds back the generated CQI to the base station.
When a plurality of users exists or when the transmission rank is greater than 2, many problems may occur when the base station uses the CQI as is. That is, when the plurality of users exists or when the transmission rank is greater than 2, interference may exist between user terminals. Accordingly, the CQI may be inaccurate.
The ZF beamforming scheme and the PU2RC scheme each have advantages and disadvantages based on a respective radio environment. For example, the PU2RC scheme selects a set of at least one user terminal from the plurality of user terminals based on a sum of data rates and the like, and allocates a unitary preceding matrix to the user terminal that belongs to the selected set. When a large number of user terminals exist, the PU2RC scheme is generally effective. Conversely, when a small number of user terminals exist, there may be a wasted beamforming vector. As a result, the PU2RC scheme may be ineffective while the ZF beamforming scheme may be more effective. Accordingly, there is a need for a MIMO communication system that can flexibly select between the ZF beamforming scheme and the PU2RC scheme based on a radio environment.
In other words, there is a need for a MIMO communication system and method that enables a base station to obtain accurate CQI and flexibly determines a beamforming mode to thereby effectively cope with a changing radio environment.