the circular buffer. Thus by selecting different RVs, different sets of coded bits representing the same set of informa- tion bits can be generated, which is used when implementing HARQ with incremental redundancy. The starting points in the circular buffer (RVO,RV1,RV2,RV3) are defined such that both RV0 and RV3 codes are self-decodable which means that they include the systematic bits under typical conditions. The RV index of the incremental redundancy HARQ in NR is derived differently compared to LTE. Unlike LTE that RV index positions are sequentially incremented in the circular buffer, in NR, if rvid = 0, 1, 2, 3 denotes the RV number for the current transmission, the rate matching output bit sequence {eklk = 0, 1, , E - 1} is generated as ek = d(ko+j)mod Ncb where ko is given by Table 4.9 according to the value of rvid and the LDPC base graph lifting factor [7,14]. 492 Chapter 4 Transport block (TB) Code block segmentation Code block (CB) Code block (CB) Code block (CB) Code block (CB) Code block grouping Code block group 0 Code block group 1 Code block group n Figure 4.38 CBG-based retransmissions [32]. It is possible to perform HARQ retransmissions with a code block granularity. In that case the information included in the DCI would determine the code block group (CBG) which is (re)transmitted, and information about handling the CBGs for soft-buffer/HARQ combining purposes (see Fig. 4.38). The NR supports code block group-based transmission with single or multi-bit HARQ-ACK feedback. For the case of CBG-based retransmission, HARQ- ACK multiplexing is supported. The motivation for CBG-based retransmission is to improve the spectrum efficiency because if CBG-based retransmission is configured, the HARQ feedback is provided per CBG and only the erroneously received code block groups are retransmitted. This can consume less radio resources than retransmitting the entire TB. If the retransmission is caused due to low SNR then combining in the soft-buffer would improve the decoding quality during retransmission