is smaller than the number of transmit antennas. Thus, MMSE estimation can both decrease the estimation error and shorten the required training sequence. It needs however additionally the knowledge of the channel correlation matrix and noise correlation matrix . In absence of an accurate knowledge of these correlation matrices, robust choices need to be made to avoid MSE degradation. Data-aided versus blind estimation In a data-aided approach, the channel estimation is based on some known data, which is known both at the transmitter and at the receiver, such as training sequences or pilot data. In a blind approach, the estimation is based only on the received data, without any known transmitted sequence. The tradeoff is the accuracy versus the overhead. A data-aided approach requires more bandwidth or it has a higher overhead than a blind approach, but it can achieve a better channel estimation accuracy than a blind estimator. See also Channel sounding Weblinks Atheros CSI Tool Linux 802.11n CSI Tool References Wireless Information theory Radio resource management Telecommunication theory