LTE MAC PDU

Regular MAC PDU (UL SCH/DL SCH/MCH, except for transparent transmission of MAC and RAR)

This type of MAC PDU structure can include four components: a MAC header (variable size), 0 or more MAC SDU (variable size), 0 or more MAC CE (control unit) or padding (padding) Information, optional). The MAC SDU, MAC CE, or padding information are all payloads.

一个MAC头包含一个或多个MAC子头，每个子头对应着MAC SDU或MAC CE或padding类型的净负荷。

The logical channel identifier (LCID) of the MAC subheader is used to distinguish the type of payload.

The "L" in each Sub Header represents the length, which can be 7 bits or 15 bits. Which case is marked by "F", and the length of "L" corresponds to the corresponding MAC SDU or MAC CE The length (number of bytes)。除了最后一个子头和固定大小的MAC CE的情况外，每个子头都存在“L”字段。

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According to the LCID distinction, there are eight types of MAC CE, namely:

➢Cache Status Report (BSR)

➢ Residential Radio Network Temporary Identity (C-RNTI)

➢ Discontinuous receiving command (DRX command)

➢UE Contention Resolution Identity

➢Timing Advance Command

➢Power Headroom

➢MCH Scheduling Information

➢SCELL activation or deactivation (Activation/Deactivation).

Other LCIDs represent MAC SDU or padding.

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Transparent MAC PDU

The feature of this type of MAC PDU is that there is no MAC header, that is, all its information is MAC SDU, and the byte length must be adjusted to the size of the transmission block TB, such as transmitting BCCH and PCCH information, BCCH information can be mapped to BCH or DL-SCH transmission Channel, and then mapped to PBCH or PDSCH physical channel; and PCCH is mapped to PCH transmission channel, and then mapped to PDSCH physical channel.

Random access response MAC PDU

MAC RAR, used for random access response messages, that is, message2.

This type of MAC PDU structure can include three components: a MAC header (variable in size), 0 or more MAC RAR or padding (padding information, optional). The MAC SDU, MAC CE, or padding information are all payloads.

The MAC header can be composed of two types of sub-headers (note: these two types of sub-headers are mutually exclusive at the same time), and the type difference is determined by the type field "T".T=0即指示接下来呈现的是随机接入回退指示---”BI”，T=1期指示接下来呈现的是随机接入前导码标识---RAPID。

T = 0
BI (Backoff Indicator) specifies the time range that the UE needs to wait before retransmitting the preamble (for the value range, see section 7.2 of 36.321). If the UE does not receive the RAR within the RAR time window, or if there is no preamble in the received RAR that matches its own, it is considered that this RAR reception fails. At this time, the UE needs to wait for a period of time before initiating random access. The waiting time is a random value selected from 0 to the waiting time interval specified by BI. (Note: If the conflict resolution fails in step 4, there will also be such a fallback mechanism)

It is worth noting that the time that the UE needs to wait before resending the preamble specified by the BI may conflict with the physical layer timing described above. (How to choose the subframe for sending the preamble depends on the implementation of the UE, and there is no answer in the agreement!

The value of BI reflects the load situation of the cell from the side. If there are more UEs connected, the value can be set larger; if there are few UEs connected, the value can be set smaller.

T = 1 is set to "1"
according to the “E”position in the MAC subheader, which means whether there is an E/T/RAPID field for random access in the following fields, that is, the same MAC RAR message can serve multiple users (Different RAPID)
If a random access response MAC subheader is of RAPID type, then its corresponding payload structure is a fixed length: 6 bytes.

RAPIDIt is the abbreviation of Random Access Preamble IDentifier, and is the preamble index obtained when the eNodeB detects the preamble. If the UE finds that the value is the same as the index used when it sends the preamble, it is considered that the corresponding RAR has been successfully received.

11-bit is Timing advance commandused to specify the time adjustment required for UE uplink synchronization. (I will not describe it in detail here. If possible, I will introduce the uplink synchronization in the future. If you are interested, please see Section 5.2 of 36.213)

The 20-bit UL grant specifies the uplink resources allocated to msg3. When there is uplink data transmission, for example, conflicts need to be resolved, the grant allocated by the eNodeB in the RAR cannot be less than 56 bits.

In the contention-based random access response , if msg3 carries CCCH MAC SDU information (involving layer three messages), then before decoding msg4, the PDCCH information must be decoded with the TC-RNTI address (the effective value of TC-RNTI is 0 0001) ~0 FFF3); and if msg3 carries C-RNTI MAC CE information (purely MAC layer, does not involve layer three messages), then the PDCCH information must be decoded with the C-RNTI address before msg4 is decoded, instead of TC -RNTI, that is, the mobile phone ignores the TC-RNTI address allocated in msg2.

If it is based on non-competition random access , then the base station responds to RAR, TC-RNTI is set to 0*0000, which means that it is invalid TC-RNTI, just padding in the RAR structure, the mobile phone only needs to follow the RAPID information in the RAR, And match the RAPID in the sent msg1, that is, the random access process is considered to be successfully completed, and then the mobile phone applies the C-RNTI specified in the previous step msg0 for this random access.

Regarding the scheduling of RAR messages, the UE must use RA-RNTI as the address to decode PDCCH scheduling information, which represents the time-frequency information for random access. The RA-RNTI calculation formula is

$RA-RNTI=1+t_{id}+10\ast f_{id}$

In China Mobile LTE, because the conventional uplink and downlink subframe ratio is set to SA2, when the PRACH configuration index is 3, then no matter what the situation is random access, the value of RA-RNTI is 3, and when the PRACH configuration index is 4 At this time, the value of RA-RNTI is 8.

The payloads in these three types of MAC PDUs are MAC SDU, MAC CE (control unit), MAC RAR, or padding. Among these payloads, only MAC SDU comes from higher layers, that is, data from the RLC layer. The rest are It is added in this layer of MAC.

Reference link:
1 https://blog.csdn.net/duanzw102/article/details/51547720
2 https://baijiahao.baidu.com/s?id=1620644779711288858&wfr=spider&for=pc