Following illustration is the 5G/NR channel mapping from MAC layer through PHY layer based on 38.321 and 38.211. In this page, I would try to give you only the big picture of the whole channel structure in NR, I would not get into the details of each type of channel processing separately. It is too big topics to be described in single page. I would write separate pages for channel mapping and channel processing for each different channels.

If you give it just a brief glance, you would think it is same as LTE. In terms of MAC layer mapping (MAC to Transport mapping), I also would say it is same as LTE. However, if you take a little bit of closer look at PHY layer, you would notice some difference from LTE PHY channel and PHY signal. Followings are the list of difference of NR PHY channel and LTE PHY channel.

• NR does not use (has no) CRS (Cell Specific Reference Signal)
• NR PDSCH requires DMRS whereas LTE PDSCH does not use DMRS. This is understandable because NR PDSCH would require its own reference signal (DMRS) since there is no CRS.

NOTE : As mentioned above, the dotted line does not indicate any direct mapping between channels. It indicate 'there is some relationship between the two channels connected by the dotted line, but it is not a direct physical mapping. Followings are more detailed description of each dotted lines.

• PSS, SSS, PBCH : As in LTE, PSS,SSS,PBCH are used and PSS/SSS/PBCH are bundled into a specific area in the downlink resource grid called SSB(SS Block)
• DL-SCH and PDSCH/DMRS : This implies PDSCH/DMRS is required for PHY/MAC scheduling for DL-SCH(PDSCH/PDSCH-DMRS).
• UL-SCH and PUSCH/DMRS : This implies PUSCH/DMRS is required for PHY/MAC scheduling for UL-SCH(PUSCH/PUSCH-DMRS)
• DL-SCH and PUCCH/DMRS : This implies PUCCH/DMRS is required for HARQ Response(ACK/NACK) for DL-SCH(PDSCH/DMRS)

Channel Mapping at MAC Layer

The illustration shown above may show you a little bit detailed picture of MAC process, but it may not be so clear about the channel mapping unless you follow through each lines very carefully. In terms of channel mapping, the tables in 38.321 would be clearer and simple to understand and my illustration to the right would be even more clear and intuitive :).

As you see, most of channels from Logical channel to Transport channel is one-to-one or many-to-one relation, but BCCH case it maps to BCH and DL-SCH.

What does this mean ? Does this mean that a BCCH message maps both to BCH and DL-SCH simultaneously ?

No. It means some BCCH data maps to BCH and some BCCH data maps to DL-SCH.  If you are familiar with LTE, you would know there are largely two types of BCCH in LTE. One is MIB and the others are SIBs. MIB goes through BCCH-BCH path and SIBs go through BCCH-DL SCH path. NR would use the same pattern of channel mapping.

Reference

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