In Figure 2 the resource map above is the legacy resource mapping per PRB in one subframe , considering 2 Antenna ports and 2 OFDM symbols control field . In Figure 2 the resource map below is the short TTI resource mapping , considering 2 OFDM symbols used for the control field in order to ensure the backward compatibility . The loss rates ( Llegacy , e . g . 5 % - 50 %) of the PHY layer in short TTI duration are assumed .