Minimum resource unit that the LTE scheduler can allocate to a UE in OFDMA is a RB, Specifically, from MAC layer perspective, an RB in the time domain is one TTI and one subband in the frequency domain
(180 kHz). The LTE scheduler is invoked for every TTI to allocate the resource blocks to the UEs following specific priority metrics. For the FD version, the LTE scheduler distinguishes the resources along both the frequency and time scales; hence, the minimum resource unit to allocate is a RB within a TTI.
In contrast, the TD version of LTE scheduler assigns all RBs of the wideband to one UE in the current
TTI. So smallest unit of allocation is actually the RBG rather than RB.
Hope this clears your doubt.