There are 3 resource allocation schemes in LTE namely Resource allocation type 0, 1 and 2.
Resource allocation 0:
Under this resource allocation scheme, the whole system bandwidth(RBs) arranged into RBG.
Number of RBs within RBG depend on system bandwidth. 1.4MHz has 1 RB per RBG and 20MHz has 4 RBs per RBG. In 20MHz deployment 100RBs kept into 25 RBG. 25 bitmap is used to represent the system resources. In the 25 bitmap string, 1 at the particular location represents that RBG is allocated and 0 does not. This is under physical resource allocation where UE can directly map to physical resources.
Resource allocation 1:
Under this resource allocation scheme, All RBGs kept into subset. Contiguous RBGs never become part of RBG subset. This is also under physical resource allocation but provide much flexibility in such a way that individual RB inside a RBG information is provided to UE rather than just RBG .
Resource allocation 2:
This is called virtual resource allocation scheme. There are two types of virtual resource allocation defined.
a) Localized Virtual Resource Allocation:
There is 1to1 mapping of virtual resource blocks and physical resource blocks.
RIV value in side DCI is used to find the start RB and # of RBs.
# of RB = RIV / Total # of RBs in system + 1;
Start RB = RIV % Total # of RBs in system.
b) Distributed Virtual Resource Allocation:
The virtual resource blocks number mapped to physical resource blocks number according to some rule. This provides inter-slot hopping.
I don't have much knowledge about frequency diversity that's why I can't explain much about Distributed Virtual Resource Allocation. Can some please elaborate more about Distributed Virtual resource allocation.