nas, the finer the spatial precision. A generic massive MIMO system operates in TDD mode, where the uplink and downlink transmis- sions take place on the same frequency resource but are separated in time. The physical propagation channels are reciprocal, meaning that the channel responses are theoretically the same in both directions, which can be utilized in TDD operation. In practice, the trans- ceiver hardware is not reciprocal, thus transceiver calibration is required to exploit the channel reciprocity. Since uplink-downlink hardware mismatches only slowly and slightly change over time, they can be mitigated by simple calibration methods even with- out extra reference transceivers by relying on mutual coupling between antennas in the array. There are several reasons for the suitability of the TDD mode for massive MIMO which include the following [16,52]: The base station needs to know the CSI to process the antennas coherently. Favorable propagation means that the channel matrix between the base station antenna array and the users is well-conditioned. In a massive MIMO system, under some conditions, the favorable propagation property holds due to the law of large numbers. In other words, the propagation is said to be favorable when users are mutually orthogonal in some practical sense. New Radio Access Physical Layer Aspects (Part 2) 583 The uplink channel estimation overhead is proportional to the number of terminals and independent of the number of antennas, making the scheme scalable with respect to the number of antennas. Furthermore, basic estimation theory indicates that the estimation quality (per antenna) cannot be reduced by adding more antennas at the base station. In fact the estimation quality improves with the number of antennas, if there is a known correlation structure between the channel responses over the array. The data transmission in massive MIMO is based on linear processing at the gNB. In the uplink, the gNB has NRX observations of the multiple access channel from the Nuser term- inals. 