For Resource Allocation Type 0 which is the most common resource allocation type, there is a rules for DL_RB setting
This rule is derived from the Resource Block Group(RBG) size for each bandwidth, and the RBG size for system bandwidth is shown in TS 36.213 Table 18.104.22.168-1. (Number of DL RB should should be the multiples of RBG for the corresponding system bandwidth)
< 36.213 - Table 22.214.171.124-1: Type 0 resource allocation RBG size vs. Downlink System Bandwidth >
* Reference for additional restriction and details :
TS 36.104 Table 5.6.1 for DL (Number of RBs for each System BW),
< Table 5.6-1 Transmission bandwidth configuration NRB in E-UTRA channel bandwidths >
TS 36.211 Section 5.3.3 for UL. Number of UL RB should meet the following requirement.
Possible Number of RBs for UL : (Due to the rules stated in TS 36.211 Section 5.3.3. You can only have following numbers which can be used for UL)