The MAC PDU (Protocol Data Unit) format in LTE varies depending on the specific transmission scenario, such as DL-SCH (Downlink Shared Channel), UL-SCH (Uplink Shared Channel), or control signaling. Here is a general overview of the MAC PDU format:
MAC Header:
Mac Header consists of multiple subleaders.
Below are the MAC header components.
- RNTI (Radio Network Temporary Identifier): Identifies the UE or group of UEs associated with the MAC PDU.
- MCS (Modulation and Coding Scheme): Specifies the modulation and coding scheme to be used for the transmission.
- HARQ Process Information: Indicates the HARQ process number and related information for error detection and correction.
- Transport Block Size Information: Specifies the size of the transport blocks within the MAC PDU.
Sub header:
MAC header consists of multiple subheaders
- One subheader for each control element, MAC SDU, or padding
- Each subheader is 1, 2, or 3 bytes in length
Control element, fixed or variable length payload
- PDU Type: Indicates the type of MAC PDU, such as a DL-SCH PDU, UL-SCH PDU, or control PDU.
- Length: Specifies the length of the MAC PDU in bits or bytes.
- Control Information: Contains additional control information specific to the PDU type, such as channel information or special flags.
In Above figure we can see subheader of L filed 7 and 15 bits.
Where,
R=Reserved bit (1 bit) set to 0.
E=Extension (1 bit) defines if more fields are present in sub-header
LCID= Length is 5 bits
F= Format (1 bit)
L= Length 7/15 bits (size of SDU)