Channel Mapping, MAC PDU, SubHeader and LCID with respect to 5G and LTE

Channel Mapping in 5G NR:

• Logical Channels: These channels carry data between the Radio Link Control (RLC) and Medium Access Control (MAC) layers. They are categorized as control channels (CCH) or traffic channels.
• Transport Channels: These channels carry data between the MAC and Physical (PHY) layers. They are used for actual radio transmission.
• Mapping: In most cases, there is a one-to-one or many-to-one mapping between logical channels and transport channels. This means that a logical channel maps to a single transport channel or multiple logical channels can map to a single transport channel.
• BCCH Exception: The Broadcast Control Channel (BCCH) is an exception. It maps to two different transport channels:
• BCH: Broadcast Channel (carries Master Information Block, MIB)
• DL-SCH: Downlink Shared Channel (carries System Information Blocks, SIBs)
• Data Separation: This means that BCCH data is split into two categories:

• MIB: Essential cell information transmitted on BCH for guaranteed reception, even in weak signal conditions.
• SIBs: Non-essential cell information transmitted on DL-SCH for more flexible and efficient resource allocation.

Therefore, a BCCH message does not map to both BCH and DL-SCH simultaneously. The type of information in the message determines the channel it uses.

Exploring MAC PDU and Header Contrasts: NR vs LTE

• In LTE, the MAC subheaders are always at the start of a MAC PDU.
• But in NR, they’re right before the data they belong to.
• So, in LTE, the MAC subheaders and data are in different parts.
• While in NR, they’re together, side by side.
• This makes NR’s setup more compact and easier to handle.

A MAC sub-header for 5G-NR

In both NR and LTE, the header structure for MAC subheaders looks alike.

• However, in NR, there’s no ‘E’ field unlike in LTE.
• NR doesn’t need the ‘E’ field because each subheader is right before the data it belongs to.
• Also, in NR, you won’t find situations where multiple headers are next to each other.

Differences between the MAC LCID and MAC eLCID

The major differences between the MAC LCID and MAC eLCID lie in their size, purpose, and flexibility:

1. Size:

• LCID: 6 bits long, allowing for 64 possible values
• eLCID: 8 or 16 bits long, depending on the value of LCID:
• 8 bits when LCID is set to 34
• 16 bits when LCID is set to 33
• This offers significantly more range than LCID

2. Purpose:

• LCID: Primarily identifies the logical channel instance of a MAC Service Data Unit (SDU) or the type of a MAC Control Element (CE).
• eLCID: Used to further extend the range of possible logical channel identities when the 6-bit LCID isn’t sufficient.

3. Flexibility:

• LCID: Limited in the number of channels it can identify due to its size.
• eLCID: Provides greater flexibility by offering significantly more unique logical channel identifiers through its larger size.

Additional points:

• eLCID is not present in every MAC subheader. It’s only included when LCID is set to specific values (33 or 34).
• Both LCID and eLCID are defined in the 3GPP standard 38.321.