erwise, the gNB may not receive the UE correctly and will not use the right beam direction in the downlink for the PRACH response. Compared to LTE, the UE needs to select correct resources in time, frequency, and additionally in the spatial domain. The UE will use a power ramping procedure if it does not receive the response for the PRACH preamble, and the UE will then transmit the preamble with a higher trans- mit power at the next suitable resource. The gNB will use the beam direction based on PRACH detection until further measurements are received from the UE for possible beam adjustment. Downlink User Data Transmission The user data in the downlink direction is carried on the PDSCH. As indicated in con- nection with the frame structure, the allocation period in the time domain can have dif- ferent values depending on the subcarrier spacing, and the mini-slot structure can also be used. The PDCCH informs the device about the downlink allocation and other impor- tant parameters like modulation and the coding format being used, what resources are actually allocated as well as HARQ information (which HARQ process and whether the transmission is a new packet or not). The modulation alternatives used in the downlink direction are QPSK, 16-QAM, 64-QAM, and 256-QAM. For the users in the cell edge area, one naturally uses either QPSK or 16-QAM, while users closer to the cell center may use also 64-QAM or even 256-QAM. 6 5G Physical Layer The channel encoding chain has the following functionality: Cyclic redundancy check (CRC) attachment, which enables detection of the erro- neous decoding in the receiver side. Low density parity check (LDPC) encoding for low complexity and high performance of user data coding. See the later channel coding section for more details. Rate matching to fit for the available downlink resource. Code block concatenation (Figure 6.18). After the encoding, the following operations take place: Scrambling. This is different from ciphering, as the aim is to ensure that sufficient variation exists 