DL HARQ MCS Adaptation while keeping TBS same in LTE

4G LTE
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1

Hi Experts,
I am currently working on DL Adaptive HARQ feature, and I have a doubt.

As per the specifications, for DL retransmission, I can use the reserved MCS for retransmission (which specifies the modulation order to use).

So, my question is that can we re-transmit same TB Size with different modulation order within same number of RBs?
If yes, how will the same set of RBs can accomodate a TB of large size (lets say 28 MCS,100PRB, so TBS 75376 bits) within the same set of PRBs with modulation order QPSK.

To clarify the question, my doubt is how am I supposed to fit a TB which was first transmitted using 64 QAM, now within the same resource blocks as first transmission with modulation order 16 QAM or QPSK? There will be cases that the same number of resources wont be able to hold the same TB with reduced modulation order.

2

If you decrease MCS index but keep the same TB size, the number of RBs will increase…

3

How much MCS reduced during retransmission without changing number of resource block?

Ideally TB size shouldn’t be reduced drastically means MCS should not change too much in this case.

4

From (MCS 28) 64QAM to (MCS 29) QPSK.