How many Re is one PRB?

The RE is the smallest physical channel unit, each uniquely identifiable by its subcarrier index k and symbol index l within the PRB. A PRB spans 12 subcarriers over 7 OFDM symbols and in total 12 × 7 = 84 REs .

Abstract: In LTE (Long Term Evolution) systems, the PRB (Physical Resource Block ) usage ratio is considered importantly to manage the QoS (Quality of Service) of LTE traffic. As the PRB usage ratio increases, the resource may not be allocated in a timely and reliable manner to the users of the cell.

Physical Resource Block . A E-UTRA PRB (Physical Resource Block) is defined as consisting of 12 consecutive subcarriers for one slot (0.5 ms). A PRB is the smallest element of resource allocation assigned by the eNB scheduler.