LTE MAC Header Structure

How LTE MAC header is encoded? What are different header elements in LTE MAC header?

LTE MAC PDU is a bit string but octet aligned (i.e multiple of 8 bits). A MAC PDU header consists of one or more subheaders. Each subheader corresponds to either a MAC SDU, MAC control element or padding.

MAC PDU header can have these six elements R/R/E/LCID/F/L. The last subheader in MAC PDU and subheaders for MAC control elements consists solely of four elements R/R/E/LCID. A MAC PDU subheader corresponding to padding consists of the four header fields R/R/E/LCID.

Here are some of the guidelines followed when multiple subheaders need to be present in a single MAC PDU

• MAC PDU subheaders have the same order as MAC SDUs, MAC control elements and padding
• MAC control elements always placed before MAC SDU.
• Padding occurs at the end of MAC PDU except when single byte or two bytes padding required. When single-byte or two-byte padding is required, one or two MAC PDU subheaders corresponding to padding are placed at the beginning of the MAC PDU before any other MAC PDU subheader.

A maximum of one MAC PDU can be transmitted per TB per UE. A maximum of one MCH MAC PDU can be transmitted per TTI.