Difference between 5G NR MAC PDU and LTE MAC PDU:
From the above image we can infer below points:
1. A MAC PDU in LTE has a single header that has all the information for decoding the entire PDU.
2. A MAC PDU in 5G NR has one or more sub headers and each sub header providing information to decode corresponding MAC PDU.
This helps to reduce the latency in 5G NR.
MAC sends/receives Control Elements with its own sub header.
1. In LTE RLC layer does the concatenation of RLC PDU.
2. In 5G NR RLC does not do concatenation. An RLC data PDU contain only one RLC SDU. Multiplexing/demultiplexing of RLC PDU will be done at MAC.
Different types of MAC PDU 5G NR
There are different types of MAC PDU but has general structure:
1. Downlink MAC PDU
2. Uplink MAC PDU
3. Sidelink Channel PDU
4. Random Access Procedure PDU [Discussed in next chapter]
A MAC PDU consists of one or more MAC subPDU. Each MAC subPDU consists of one of the following:
-> A MAC subheader only (including padding);
-> A MAC subheader and a MAC SDU;
-> A MAC subheader and a MAC CE;
-> A MAC subheader and padding.
Each MAC subheader corresponds to either a MAC SDU, a MAC CE, or padding.
Below shows an example of DL and UL MAC PDU
1. For DL PDU, that required subPDU assembly CEs take precedence over SDU, SDU takes precedence over padding.
2. For UL PDU, that required subPDU assembly SDUs take precedence over CEs, CEs takes precedence over padding.
A MAC PDU will have one or more MAC subPDU.
A MAC subPDU always start with a subheader. A sub-header is followed by a MAC SDU, a MAC CE or padding.
If a MAC subPDU doesnt fill a TB, a MAC subPDU with padding is included.
All MAC SDU, CE and sub-header are aligned in multiple of 8 bits.
Below are the mail files of 5G NR MAC subheader
R : Reserved: It is set to 0 as it can be used for further use.
F : Format: It indicated the size of Length field L. If F = 0 L is 8 else L is 16 bits.
L: Length: It indicates the length in bytes of either a MAC SDU or variable sized CE.
LCID: Logical Channel ID: It is a 6-bit field that identifies the logical channel carried in a MAC subPDU a specific CE or padding.
eLCID: Extended Logical Channel ID: This extends the range of LCID field.
Below table shows the LCID field in MAC Subheader based on 38.321
Text and Image source TS 38.321