LTE: Downlink Resource Allocation Type 0

In Resource Allocation Type 0, Resource Block assignment information includes a bitmap indicating a set of Resource Block Groups (RBGs) to the scheduled UE. An RBG is a set of consecutive PRBs. 

In RA Type 0, the signaling overhead is reduced since the bitmap is defined on a group of RBs. The size of RBG (P) is a function of system bandwidth as shown in the table below:

The total number of RBGs (N_RBG) for downlink system bandwidth N_RB^DL­­ is given by N_RBG = ⎾N_RB^DL­­/P⏋where ⎿N_RB^DL­­/P⏌of the RBGs are of size P and if N_RB^DL­­ mod P  > 0 then 

one of the RBGs is of  size N_RB^DL­­ –  P.⎿N_RB^DL­­/P⏌.  

The bitmap is of size N_RBG bits with one bitmap bit per RBG such that each RBG is addressable

An RBG is allocated to the UE if the corresponding bit value in the bitmap is 1, the RBG is not allocated to the UE otherwise

From the above table it could be noted that for smaller bandwidths (N_RB^DL ≤ 10) the value of P = 1 (RBG size = 1) which means that each RB can be addressed with a bit in the bitmap.

The RBG size (P) is increased with system bandwidth as smaller values of P would require more number of bits to address the entire bandwidth

Example:

Let us consider N_RB^DL = 15. From the above table, P = 2, so total number of RBGs N_RBG = 8 out of which 7 RBGs are of size 2 RBs and one RBG is of size 1 RB. The total number of bits required for resource allocation is equal to N_RBG which is 8 in this case. The bitmap is done as shown in the below:

In this example if the resource allocation information is 11000001, this means RBGs 1, 2 and 8 (5 RBs in total) are allocated and the remaining RBGs are not allocated to the UE.


Reference: 3PGP TS 36.213