Ran Protocol Stack Part-2: MAC , PHY

Read the first part here: https://blog.magmaindia.org/category/5g-ran-protocol/

4. MAC (Medium Access Control)

These are the functions of Mac :-

1. Logical-channel multiplexing

2. Hybrid-ARQ retransmission

3. Scheduling

4. Multiplexing/Demultiplexing for carrier aggregation

Multiplexing Logical Channel

MAC layer provides the services to the RLC in the form of logical channels.

MAC layer uses services from PHY layer in the form of transport channel.

a) Logical Channel: It defined by the “types of information” it carries and its generally classified as control channel (used for the transmission and configuration information necessary for operating in our system and as a traffic channel (used for the user data).

b) Transport Channel: It is defined by “how and with what characteristic information” is transmitted over the radio interface. This includes paging, broadcast or shared channel.

Logical Channel have:-
a) Broadcast Control Channel (BCCH)
b) Paging Control Channel (PCCH)
c) Common Control Channel (CCCH)
d) Dedicated Control Channel (DCCH)
e) Dedicated Traffic Control Channel (DTCH)

Transport Channel have:-
a) Broadcast Channel (BCH)
b) Paging Channel (PCH)
c) Downlink Shared Channel (DL-SCH)
d) Uplink Shared Channel (UL-SCH)
e) Random Access Channel (RACH)

Carrier Aggregation

In this, the data are distributed over multiple cells not just at one cell.

Hybrid automatic repeat request (hybrid ARQ or HARQ) is a combination of high-rate forward error correction (FEC) and automatic repeat request (ARQ) error-control.

The main mechanism responsible for retransmission in MAC layer is HARQ.

It is hybrid such as, it checks error message through channeling and retransmission of message if error occurs.

Error Correction are done on = PHY layer
Retransmission are done on = MAC layer

In this , one block is transmitted and wait for the “No Negative Acknowlegement (No NAK)” and then transmit another block otherwise retransmit same block until it get feedback as No NAK.

If there is large transport block , it was then divided into small CODE BLOCKS during the channel coding process in the Physical Layer.

Suppose during the transmit , the transport block become erroneous. Now we no need to retransmit whole Transport block instead we need to find out which code block is erroneous . So for that we grouped the code block which is known as CBG(Code Block Group).


Suppose there are three different user connected to base station. Now they have different services and priority to allocate data.

Now the Scheduler comes into the game, it takes frequency resources and for each time instant lets say onee slot for a given time instant of a scheduling interval. It schedules ‘where and how much data’ the different users can use and when this is successfull in the next and subsequent time slot , the scheduler keeps updating for each slot, what is the best allocation of resources between different users. This is the function of scheduler.

Now, officially Scheduler is the part of MAC-Layer in NR. It is better to accept scheduler as a separate entity because it interacts with different layers in order to carry out its functions and also controls its different layers in order to carry out its functions.

On What basis does scheduler makes it decision :-
a. Channel Conditions
b. Buffer Status
c. Data Flows Priority

5. PHY (Physical Layer)

So far, we have contructed transport block which includes both data and control element.

NOTE: So now , the job of physical layer is to transmit the transport block over the wireless channel as efficiently as possible.

PHYSICAL-Layer does this by Modulation, Coding, Multi-Antenna processing and by mapping the signals to the appropriate physical time and frequency resources.

The physical channel mainly handles the mapping of the transport channel to the physical channel.

Physical-Channel is defien by ‘set of time-frequency resources used for transmission of a particular transport channel’.

It includes DCI (downlink control information) which provides the devices with necessary information for proper reception and decoding of downlink data.

It aslo includes UCI (uplink control information) which provides the scheduler on the hybrid ARQ protocol with the information about the situation at the device .

Different types of physical channel:-

  1. PBCH – This carries the system information that the device can use to access the network.
  2. PDSCH – This is the main channel for data transmission to the devices for paging information and also for delivering part of system information.
  3. PDCCH – This is used for downlink control information like scheduling decision.
  4. PUSCH – This is the main channel for data transmission from device side.
  5. PUCCH – This is used by the user equipment for hybrid ARQ acknowledgement indicating whether you received a transport block successfully or unsuccessfully.
  6. PRACH – This is used for random Access Procedures

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