LTE: Why padding is required in MAC layer.

Question

How LTE MAC treats MBMS differently compare to the normal user data. What different actions are taken for the MBMS specific data in the MAC layer ?

Answer 1

Hi,

When you don't have enough data to send in one mac TB then MAC layer will do padding and sends to UE.

Comments

what do you mean by don't have enough data, how much data is needed to send to UE.If you don't have enough data why do you add padding because it wastes the resource.

Answer 2

Hi rohit,

enough data in the sense minimum RB to schedule is 1RB, so if you have data below 1 RB then we should make the data to fit in one RB. in this case MAC will do padding to make it 1Rb or any other nearby round offs then it will send as a TB.

Like say e.g RLC has data of 2900 bytes in its buffer, when MAC schedules, it gives grant of 15Rb because 14RB < 2900 < 15RB , say e.g in 15RB you can send 3000 bytes of data then MAC has to do 100bytes of padding.

Comments

Hi jaganathan
i m not satisfied with your ex.. its difficult me to understand can you please explain in details .... as i know 1rb = 84 RE ,
15 RB =   1260 RE Then what you mean by 3000 bytes in 15rb
how to map 3000 byte on 15 RB
please explain

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Hi,
Dont confuse with RE's,  3000 is in-terms of bytes,  Each RB will have some capacity of data to send based on the MCS.  so in my e.g 15 RB can send 3000 bytes of data.

1 RB will have 72 RE's and if the modulation type is BPSK then each RE can carry 2bits data. so one RB can carry 144bits of data. then in 1000 RB you can send 1440 bits of data in one ms.