The UE shall interpret the resource allocation field depending on the PDCCH DCI format detected. A resource allocation field in each PDCCH includes two parts, a resource allocation header field and information consisting of the actual resource block assignment. PDCCH DCI formats 1, 2, 2A and 2B with type 0 and PDCCH DCI formats 1, 2, 2A and 2B with type 1 resource allocation have the same format and are distinguished from each other via the single bit resource allocation header field which exists depending on the downlink system bandwidth where type 0 is indicated by 0 value and type 1 is indicated otherwise. PDCCH with DCI format 1A, 1B, 1C and 1D have a type 2 resource allocation while PDCCH with DCI format 1, 2, 2A and 2B have type 0 or type 1 resource allocation. PDCCH DCI formats with a type 2 resource allocation do not have a resource allocation header field. A UE shall discard PDSCH resource allocation in the corresponding PDCCH if consistent control information is not detected.
Type 0 Resource Allocation(RAT0)
In RAT0 allocation happens in terms of RBG( Resource block group ).
1 RBG = n consecutive RB's in frequency domain
where n depends on system bandwidth or the total number of RB's in the system for one subframe.
So total number of RBG's in the system will be :-
Number of RBG = Total number of RB's / n
If Total number of RB's % n > 0 , then
Number of RBG = Number of RBG +1 with last RBG containing the remaining RB's
We are working on 10 Mhz, so total number of RB's will be 50.
From above table value of n will be 3 for 50 RB's in system. So total number of RBG's will be
Number of RBG = 50 / 3 = 16
As 50 % 3 > 0
So Total number of RBG = 16 +1 =17
And the last RBG will contain only 2 RB's
Type1 Resource Allocation( RAT1 )
In PDCCH Resource Allocation Header field represents the resource allocation type used. For RAT1 header field value will be 1.
Type1 Allocation also uses the bitmap fundamental. But as we know the limitations of Type0 that allocation happens in terms of RBG's which can lead to the wastage of RB's as explained above. So to provide more granularity in the resource allocation Type1 came into picture.
In Type1 one extra field is added which is subset to provide the granularity and frequency diversity in the system. Allocation happens in terms of RB's belonging to the same subset.
Each Subset p will contain every nth RBG starting from RBG p.
n = Number of RB's in a RBG from the above table.
Suppose system bandwidth is 20 Mhz.
Total number of RB's =100
n = 4
Number of subsets = 4 (From table)
As total number of subsets are 4,
p = 0 to 3