d coded bits at the output of polar encoder is written into a length-N circular buffer in an order that is predefined for a given value of N. As shown in Fig. 4.17, to obtain M coded bits for transmission, puncturing is realized by select- ing bits from position N - M to position N - 1 from the circular buffer, shortening is real- ized by selecting bits from position 0 to position M - 1 from the circular buffer, and repetition is realized by selecting all bits from the circular buffer, and additionally repeating M - N consecutive bits from the circular buffer [7]. For each CORESET, there is an associated CCE-to-REG mapping based on the REG bundle (see Fig. 4.14). A REG bundle is a set of REGs across which the device can assume the 446 Chapter 4 precoding is constant. This property can be exploited to improve the channel estimation per- formance, which is similar to PRB bundling for PDSCH. The CCE-to-REG mapping can be either interleaved or non-interleaved, depending on the characteristics of the transmission channel, that is, frequency-flat or frequency-selective fading channel. There is only one CCE-to-REG mapping for a given CORESET; however, since the mapping is a property of the CORESET, multiple CORESETs can be configured with different mappings. The CCE- to-REG mapping for a CORESET can be interleaved or non-interleaved, configured by the RRC parameter ace-REG-MappingType in the ControlResourceSet information element and is described by REG bundles. The REG bundle i is defined as REGs {iLREG, iLREG + 1, iLREG + LREG - - 1 where LREG is the size of the REG bundle and REG CORESET = NCORESET the number of REGs in the CORESET. The jth CCE consists of REG bundles {(6j/LREG), (6j/LREG + 1), (6j/LREG + 6/LREG - 1)}, where P(.) denotes an interleaving function. In case of non-interleaved CCE-to-REG mapping LREG = 6 and (j)=j, whereas in the case of interleaved CCE-to-REG mapping LREG = {2,6} for NCORESET symb and LREGE & N CORESET,6} for NCCORESET symb E {2,3} where LREG is configured by the RRC parameter reg-Bundl