ttenuation with distance, etc. The knowledge of CSI at the transmitter and/or the receiver makes it possible to adapt data transmission to current channel condi- tions, which is crucial for achieving reliable and robust communication with high data rates in multi-antenna systems. The CSI is often required to be estimated at the receiver, and usu- ally quantized and fed back to the transmitter. The downlink channel can be estimated from uplink reference signals in TDD systems under certain conditions due to reciprocity. New Radio Access Physical Layer Aspects (Part 2) 497 In general, the transmitter and receiver can observe different CSI. There are two types of CSI, that is, instantaneous CSI and statistical CSI. In instantaneous CSI or short-term CSI the current channel conditions are known, which can be interpreted as knowing the impulse response of a digital filter. This provides an opportunity to adapt the transmit signal to the impulse response and thereby to optimize the received signal for spatial multiplexing or to achieve low bit-error-rates. In statistical CSI or long-term CSI, the statistical characteristics or statistics of the channel are known. The latter information may include the type of fading distribution, the average channel gain, the line-of-sight component, and the spatial correla- tion. Similar to the instantaneous CSI, this information can be used for optimization of transmission parameters. The CSI estimation accuracy is practically limited by how fast the channel conditions are varying. In fast-fading channels where the channel conditions may vary rapidly during transmission of a single information symbol, only statistical CSI is reasonable. On the other hand, in slow-fading scenarios, the instantaneous CSI can be estimated with reasonable precision and used for transmission adaptation for a period of time before becoming obsolete. In practical scenarios, the available CSI is often manifested as instantaneous CSI with some estimation/quantization error combined with some statistical infor