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MAC

 

As of Jul 2017, not much details are defined in 38.321 (MAC Specification). It would be because 38.211, 38.212 is not finalized yet. However, you can find some overall description on NR in the specification.

High level MAC functionality and its interaction with lower and higher layer can be summarized as in the following diagram.

 

 

 

At high level view, NR MAC function/operation is very similar to LTE MAC function/operation. For your reference, I put the NR MAC function and LTE MAC function side-by-side as below.

 

 

5G/NR(38.321)

LTE(36.321)

Services

provided to upper layers

  • data transfer
  • radio resource allocation
  • data transfer
  • radio resource allocation

MAC Functions

  • mapping between logical channels and transport channels
  • multiplexing of MAC SDUs
  • demultiplexing of MAC SDUs
  • scheduling information reporting;
  • error correction through HARQ;
  • logical channel prioritisation.
  • mapping between logical channels and transport channels;
  • multiplexing of MAC SDUs
  • demultiplexing of MAC SDUs
  • scheduling information reporting
  • error correction through HARQ
  • priority handling between UEs
  • priority handling between logical channels of one MAC entity
  • Logical Channel prioritisation
  • transport format selection
  • radio resource selection for SL

Services

expected from physical layer

  • data transfer services;
  • signalling of HARQ feedback;
  • signalling of Scheduling Request;
  • measurements (e.g. Channel Quality Indication (CQI)).
  • data transfer services;
  • signalling of HARQ feedback;
  • signalling of Scheduling Request;
  • measurements (e.g. Channel Quality Indication (CQI)).

 

 

There is another way of describing MAC functionality. It is to describe about each separate procedures that are performed by MAC as summarized below. This is the way I like to describe MAC layer. When MAC and PHY layer specification gets finalized, I would create separate pages for each of these process.

 

Procedure Name

Description

Random Access Procedure Get the initial uplink grant and perform synchronization to network
DL-SCH data transfer (NW) Do everything needed to perform DL Data Transfer (DCI-Scheduling, HARQ etc)
UL-SCH data scheduling (NW) Schedule UL data transmission by sending DCI X (UL Grant)
UL-SCH data transfer (UE) Do everything needed to perform UL Data Transfer (DCI X decoding, HARQ, multiplexing and assembly)
SR-Scheduling Request (UE) Send the request to Network to get a UL Grant
DRX - Discontinous Reception Control UE's PDCCH monitoring activity in special pattern mainly to save energy consumption
SPS - Semi Persisent Scheduling Scheduling DL/UL transmission in special pattern to reduce scheduling overhead
PCH Reception Monitoring Paging message in special period
BCH Reception Get basic information on cell (MIB and SFN)

 

 

Channel Mapping at MAC Layer

 

The illustration shown above may show you a little bit detailed picture of MAC process, but it may not be so clear about the channel mapping unless you follow through each lines very carefully. In terms of channel mapping, the tables in 38.321 would be clearer and simple to understand and my illustration to the right would be even more clear and intuitive :).

 

 

As you see, most of channels from Logical channel to Transport channel is one-to-one or many-to-one relation, but BCCH case it maps to BCH and DL-SCH.

What does this mean ? Does this mean that a BCCH message maps both to BCH and DL-SCH simultaneously ?

No. It means some BCCH data maps to BCH and some BCCH data maps to DL-SCH.  If you are familiar with LTE, you would know there are largely two types of BCCH in LTE. One is MIB and the others are SIBs. MIB goes through BCCH-BCH path and SIBs go through BCCH-DL SCH path. NR would use the same pattern of channel mapping.

 

Refer to Channel Mapping page to see how this mapping embedded into the mapping with wider scope.

 

 

Reference

 

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