Hi sripathi,
DL RB allocation has 3 types.
RAT 0 : This is interms of RBG, ENB will send RBG bitmap to UE , like ********** , which ever bit is set corresponding RBG is scedhuled.
RAT 1 : This is also a bitmap allocation but it hasw one more field called RB shift(0 to 2), based on this valure divide the whole BW, and bitmap will tell which RB's scheduled in those RBG.
RAT 2 : This is continuous allocation , like start RB and total no of RN will be given.
In UL allocation only one type that is RAT 2, There is no other RAT available for UL.