code length used for different combinations of payload length $G$ and number of information bits $A$ for both uplink and downlink channels; segmentation for PUCCH is identified by area darkening.

Message segmentation {#subsec:seg}
--------------------

Given message length $A$ and payload length $G$, the information may be decomposed in two blocks and encoded separately. Segmentation is activated by flag $I_{seg}$, and in particular it may be activated for PUCCH and PUSCH UCIs ($I_{seg} = 1$) while it is always bypassed for PBCH payloads and PDCCH DCIs ($I_{seg} = 0$). Segmentation is necessary for the uplink when message or payload lengths are longer than maximum mother polar code length and shortening or puncturing is used as rate matching mechanism; this is not necessary for the downlink since the message length is always shorter than mother polar code length and repetition is used for large payload sizes. When code parameters satisfy the condition $(A \geq 1013) \vee (A \geq 360 \wedge G \geq 1088)$, segmentation is activated. In this case, the message is divided into two parts of length $A' = \lceil A/2 \rceil$; if $A$ is odd, the first message is composed by the first $\lfloor A/2 \rfloor$ bits with the addition 