Radio Link Control

In LTE, the RLC layer can operate in acknowledged mode (AM), unacknowledged mode (UM), or

transparent mode (TM), and is responsible for

error correction through Automatic Repeat reQuest (ARQ)

concatenation, segmentation and reassembly of RLC SDUs in order to generate RLC PDUs of appropriate size from the incoming RLC SDUs

re‐segmentation of RLC data PDUs, if these do not fit to the actual transport blocks; and

reordering of RLC data PDUs, duplicate detection and RLC SDU discard, RLC re‐establishment,

and protocol error detection.

For 5G, the main challenge or design goal w.r.t. RLC is to increase the processing efficiency, reduce

overhead and duplicate functions, better segregate real‐time and non‐real‐time functions, and enable

the support of lower layers with mixed numerologies. 3GPP has already agreed that

multiple parallel logical channels can be configured with different characteristics and priorities,

e.g., involving different numerologies and transmit time interval (TTI) lengths

gNBs should have means to control which logical channels the UE may map to which numerology

and/or TTIs with variable duration; and

concatenation of RLC PDUs is performed in the MAC layer instead of the RLC layer. This allows

the precomputation of RLC and MAC headers for faster processing and higher data rates, and the

parallelization of the PHY encoding and MAC PDU construction. Further, it allows ARQ to be

fully decoupled from real‐time processing, and helps to avoid the duplication of concatenation‐like

operations in RLC and MAC.