MAC CE (MAC Control Element)
The purpose and function of NR MAC CE is almost same as LTE MAC CE.
When we say 'communication between UE and Network', we normally think about only signaling message (RRC or NAS message). When I say 'communication' in this case, it means 'control command exchange' between UE and network, not the data traffic.
In UMTS case, it is true that only RRC and NAS message functions as communication between UE and Network, but in LTE and NR a special type of communication is introduced working at MAC Layer. It implies that there are special MAC structure that carries special control information. These special MAC structure carrying the control information is called 'MAC CE', which means 'MAC Control Element'. Motivation for this kind of communication would be obvious. It would be much faster comparing to RRC layer or NAS layer communication.
This special MAC structure is implemented as a special bit string in LCID field of MAC Header (Refer to LTE MAC page for the details of MAC header).
There are several MAC CE in downlink MAC and also several MAC CE in uplink MAC. Following table from 38.213 shows the LCID types of MAC header. The parts marked in red rectangle is LCID representing various MAC CE. These are relatively small set of MAC CE defined in 38.321 Rel 15 - 6.2.1. You may see this gets longer as new release comes out as you have seen in LTE.
As in LTE, the structure and functionality of MAC CE would be an area that is evolving continuously as new 3GPP release comes out. In this section, I will consollidates MAC CE LCID structure from different releases so that we can keep track of the structure and functional evolution of MAC CE.
LCID: The Logical Channel ID field identifies the logical channel instance of the corresponding MAC SDU or the type of the corresponding MAC CE or padding as described in 38.321-Tables 6.2.1-1, 6.2.1-1c and 6.2.1-2 for the DL-SCH and UL-SCH respectively. There is one LCID field per MAC subheader. The size of the LCID field is 6 bits. If the LCID field is set to 34, one additional octet is present in the MAC subheader containing the eLCID field and follow the octet containing LCID field. If the LCID field is set to 33, two additional octets are present in the MAC subheader containing the eLCID field and these two additional octets follow the octet containing LCID field.
eLCID: The extended Logical Channel ID field identifies the logical channel instance of the corresponding MAC SDU or the type of the corresponding MAC CE as described in 38.321-tables 6.2.1-1a, 6.2.1-1b, 6.2.1-2a and 6.2.1-2b for the DL-SCH and UL-SCH respectively. The size of the eLCID field is either 8 bits or 16 bits.
Followings are the list of the MAC CEs specified in 38.321 as of now. Since each of these items is a pretty big top, I will create a separate page for each of these items.