## 3 New Radio Link-Level Simulator
### Transmission Chain  
A detailed description of the main blocks is provided as follows. _Transport Block generator_: Data are generated as a sequence of bits. Depending on the transport size (A) and the modulation and coding scheme (MCS), in particular, on the code rate (R), the LPDC base graph selection is performed:  
* if \(A\leq 292\), or if \(A\leq 3824\) and \(R\leq 0.67\), or if \(R\leq 0.25\), LDPC base graph 2 is used;
* otherwise, LDPC base graph 1 is used. Input bits are then segmented into different blocks; the CRC parity bits are calculated on and attached to each block. Depending on the LPDC base graph, the maximum dimension of blocks can change. The lowest order information bit is mapped to the most significant bit of the transport block (i.e., parity bits are found at the end of the block). **LDPC encoder**: LPDC coding is applied to the bit sequence for a given code block. \(N\) encoded bits are obtained as output, where \(N=66\,Z_{c}\) for LDPC base graph 1 and \(N=50\,Z_{c}\) for LDPC base graph 2 (\(Z_{c}\) value can be found in subclause 5.2.2 of TR 38.212 [42]). Figure 1: New Radio link-level simulator scheme. Rate-matching blockA rate-matching block is required to adapt the size of the transport block after LDPC coding to the size of the allocated resources that can be used for PDSCH transmission. PDSCH coded bits must be rate-matched around the resource elements occupied by the CSI-RS and other reference signals, otherwise the coding rate may exceed the unit value (i.e., the REs in the slot available for PDSCH are not sufficient to transmit the information bits). In practice, it is expected that the gNB scales down the MCS in the slots where the CSI-RS transmission causes an LDPC coding rate higher than about 0.95. This approach is also modelled in the link simulator. The throughput calculation must consider this effect, re-scaling the formula accordingly. Scrambling operationBefore modulation, the codewords (up to two) are scrambled, as in subclause 7.3.1 of [42], resulting in a block of scrambled bits. QAM modulationThe block of scrambled bits for each codeword are QAM-modulated using one of the modulation schemes (selected by the MCS) from table 7.3.1.2-1 [42], resulting in a block of complex-valued modulation symbols. Layer and subcarrier mappingThe modulated symbols are mapped to one or more layers (if multi-layer transmission is enabled), by splitting the codewords (one or two) among them. Then, the symbols are inserted into the actual resource elements of the physical resource block, through a predefined look-up table, based on the 3 GPP resource allocation procedure. MIMO processingThe precoding matrix is applied to the PDSCH channel, i.e., sub-carriers containing data. If PMI reporting is enabled, the precoding matrix \(F_{j}\) is selected by the transmitter at the gNB as the \(j\)-th element of the codebook, where \(j\) is the PMI index reported by the UE. The digital beam-forming scheme implemented in the simulator is wideband; that is, the same beam-forming matrix \(F_{j}\) is applied to all the PRBs. If, instead, the channel estimate is given through feedback by the UE, (e.g., SRS transmission in uplink), downlink beam-forming schemes based on the channel matrix can be applied exploiting the downlink/uplink channel reciprocity. CSI-RS mappingThe transmission of CSI-RS signals in the link-level simulator is modelled in a realistic way. The CSI-RS signals are generated and mapped in the time frequency-grid and configuration parameters set the number of ports and the transmission period, denoted as _CSI-RS period_, which can take the set of values defined in the 3 GPP TS 38.331 [45] specifications. The CSI-RS signals are transmitted once every [4; 5; 8; 10; 16; 20; 32; 40; 64; 80; 160; 320; 640] slots. OFDM modulatorThe OFDM modulation is performed through the IFFT Operation. Before the IFFT, an IFFT shift must be applied to properly translate the spectrum into the positive frequency domain. The IFFT must be applied on an OFDM symbol-basis.