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We now have an initial understanding of the allocation mechanism, let us look at in more details. We may start with DCI format, but it would be more helpful if we first look at couple of important fields of DCI format: Resource allocation type and Modulation & Coding index, IMCS.
There are three types of allocation types: 0, 1, and 2 [36.213:7.1.6].
In Type 0, radio blocks are allocated in groups (RBG) of size P. RBG size is constant for particular bandwidth as shown in table below:
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BW (MHz), NDLRB
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RBG size (P)
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bits (NDLRB/P)
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1
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6
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2
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8
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2
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13
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3
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17
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4
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19
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4
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25
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Type 0 allocation field contains a bitmap wherein nth bit if set shows allocation of nth RBG (RBG 0 at lowest frequency).
In Type 1, resource blocks are divided in groups of size P. These groups are further divided into P subsets. Allocation is indicated with shift (1 bit) and bitmap of size int( NDLRB/P + 1 ) - int( log2P + 1 ) - 1. Example is shown below for 25 resource blocks.
If you notice, even though we have divided RBs in groups, individual RBs can be allocated unlike Type 0. Number of bits required are still same, like in the example of 25 resource blocks, we require 13 bits in Type 0, and in Type 1, we need 1 (subset) + 1 (shift) + 11 (bitmap) = 13 bits !
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