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MCS to Itbs

Transport Block size table

Edit: for some context

I'm working on an LTE project and was wondering if anyone more knowledgeable than I would be able to answer this question. Given the first table, we can see the relationship between MCS (related to channel quality) and TBS (Transport Block Size) index, and Modulation order which isn't too important right now. Looking at MCS 8 for example, we'll take the TBS index on the right of the first table, go to our second table, and than we can clearly see how many bits we can fit into a certain amount of Resource Blocks. At MCS 8, we see 1 Resource Block (RB) can fit up to 120 bits, and 2 may fit up to 256. This table is supposed to have some relationship such that: If you halve the MCS and double the amount of resource blocks, you should have some equivalent amount of bits.

This means if your channel quality degrades to half (for example sake we'll keep it simple) all you must do is give each user one extra RB to satisfy whatever allocation they have.

A real example: starting at MCS 8, at 1 RB we have 120 bits. If we drop to MCS 4 (half of 8) we will have to send twice as many RBs (two RBs), which we can confirm in the table as being 120 bits (equivalent to MCS 8 at 1 RB).

My question is why the heck is MCS 6, 1 RB 328 bits? It doesn't make too much sense to me for a lower quality channel than MCS 7, 8, 9... to have more bits in the first RB. Also this breaks the general trend that I just described. Does anyone know why this is happening? Also, how is it possible to have more bits in one resource block than two? Are these imaginary bits that dissapear somehow?

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    \$\begingroup\$ You should be more explicit on what you're talking about. Not everybody know what is MCS/RB/... and your post has been flagged as unclear already. I voted to leave it open, but it may still get closed due to lack of context. \$\endgroup\$
    – dim
    Commented Jan 24, 2017 at 10:46
  • \$\begingroup\$ @dim I just added some context, hopefully this makes my question a bit more clear. \$\endgroup\$
    – blurb
    Commented Jan 24, 2017 at 18:59
  • \$\begingroup\$ Let's hope so. Honestly, I can't judge or answer, this is not my field at all. I just looked at your post because it was flagged, thought "WTF is this question"? Made a bit of research and finally realized it is certainly a legitimate question. But now I see there are 4 close votes... \$\endgroup\$
    – dim
    Commented Jan 24, 2017 at 19:37
  • \$\begingroup\$ @dim I'm not sure where else I would put such a question, and the fact that it hasn't been answered anywhere obvious is very strange. I can close it if you want, but I'd appreciate some alternative location for it as everyone seems to recommend communications questions go to this SE \$\endgroup\$
    – blurb
    Commented Jan 24, 2017 at 21:38
  • \$\begingroup\$ I don't want it closed. In fact, I tried to save your question. I am just saying I see 4 other people have voted to close. I doubt there is a better place, but once again, it's not my domain. I don't know. \$\endgroup\$
    – dim
    Commented Jan 24, 2017 at 21:56

1 Answer 1

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I found an answer in the text of

Long Term Evolution In Bullets, 2nd Edition

towards the back of the book (page 575).

It states:

TBS index 6 for a single allocated Resource Block specifies a transport block size of 328 bits. Table 387 indicates that TBS index 6 uses QPSK as a modulation scheme so the capacity of a single Resource Block allocation over a subframe is (12*13-6) = 300 bits, when assuming a single OFDMA symbol is allocated to the PDCCH and the cell specific Reference Signal is transmitted for a single antenna port. Thus, the transport block size of 328 bits cannot be fully accommodated by the Resource Block allocation. The transport block size of 328 bits has been specified for the purposes of Voice over IP (VoIP) users allocated QPSK towards cell edge. The use of a single Resource Block helps to maximise system capacity. TTI bundling can be used to transfer the complete set of 328 bits by applying a different puncturing pattern for each transmission within the TTI bundle.

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