allowed 3GPP to focus on the design of puncturing and shortening mechanism while leaving the repetition scheme as simple as possible. As a result, a circular buffer rate matching [@circ_buff_polar] scheme has been adopted incorporating all three techniques in an elegant and simple way.

Assistant bits design
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Assistant bits can be broadly defined as additional bits that help the decoding of the polar code, usually improving its error-correction performance. Their introduction dates back to the conception of SCL decoding, for which it has been noticed that the introduction of an outer CRC code improved the error-correction performance of the code [@tal_list; @CRC_aid]. In general, it has been proven that the minimum distance of polar codes can be dramaticaly improved by adding an outer code [@PC_conc], and SCL decoders can fully exploit this improved code spectrum [@PC_const_list]. Parity-checks (PC) have emerged as an alternative to CRC due to their simplicity and flexibility [@PC_conc; @dynFroz]. Introducing PC bits in the middle of the decoding instead of a single CRC check at the end makes it easier to tune the polar code spectrum; bit-channels corresponding to minimum weight rows are the best candidates to host parity checks [@PC_min_ham]. 3GPP decided to adopt 