Channel state information is information about the current value of H, a mathematical value which represents a signal channel. It forms part of the signal model in Wireless communications, the full equation of which is
where R is the received signal, X is the transmitted signal, N is the noise, and H is the channel. The values R, X, N, H are usually not constant. The system usually needs to have some information regarding H to figure out what was sent from the transmitter or to enhance system performance, such as increasing transmission speed. The information can be the current value of H, or the covariance of H. This type of information is generally referred as channel state information (CSI) and is usually estimated. Usually the current value of H is referred as short-term CSI, while the higher order statistics of H are referred as long-term CSI.