 deployments where 240, and 480-KHz SCS is used in >6 GHz deployments. Synchronization channels support 15/30 KHz SCS (<6 GHz) or 120/240 KHz (>6 GHz) (Fig. 2-28). 5G also defines a so-called mini slot which is a sequence of 2, 4, or 7 symbols that are a smaller unit of scheduling used in URLLC systems, but that is not covered in-depth here. From MAC Transport Block CRC Attachment Code Block Segmentation Forward Error Correction Encoded CB Encoded CB Encoded CB Rate Matching Code Block Concatenation FIGURE 2-30 Channel coding and associated bit processing (3GPP 38.212). 74 Open Radio Access Network (O-RAN) Systems Architecture and Design In the frequency axis, subcarriers are grouped into PRB which is a group of 12 subcarriers as shown in Fig. 2-29. A slot can be allocated to be all-downlink, all-uplink or a combined uplink + downlink slot (mixed slot), where the mixed slot configuration can be either static or dynamic (unique for each slot). The slot format is indicated through the Slot Format Indicator (SFI) which can be static or semistatic (configured through the RRC) or dynamic (configured through the DCI carried in the control channel). The Physical Layer processing chain is split into two pieces: channel coding (covered in 3GPP 38.212) and physical channel mapping and modulation (covered in 3GPP 38.211). Channel coding (3GPP 38.212) Channel coding transforms the MAC TB into a set of bits that can be mapped into the OFDM frame (channel) that is transmitted over the air. As shown in Fig. 2-30, this includes error detection (CRC) to be able to detect any errors in the wireless transmission, channel coding to add redundant information to reduce the impact of channel imperfections, and rate matching to reduce channel coding overhead and match the encoded TB size to the available space on the OFDM frame, as shown for downlink/uplink shared channels: In a little bit more detail: Code block segmentation-this breaks the TB (or MAC PDU) as delivered by the MAC layer into smaller code blocks (CBs) with discrete size 