why LTE doesn't use DC subcarrier in the downlink? I know LTE doesn't use the DC subcarrier for OFDM/OFDMA modulation for the downlink, but no author gives a detailed explanation to this stuff. 

Can you tell me the reason behind it?

Thank you!
The DC subcarrier is normally omitted because in a zero-IF receiver it overlaps with DC offsets in the hardware which are very difficult to remove.