ISI or Inter symbol interference?
Interference in ofdm symbols results in ISI effect on RF channels. This effect is result of multi path degradation. Due to multi-path the previous symbol gets delayed and arrives late. When it reaches the receiver it starts interfering with current good symbol. To protect against ISI the standard employs guard
Next DL reference signals.
Role of reference signals in Downlink:
Each antenna port transmits one reference signal to convey channel response of each antenna for channel estimation at reciver. Primary role of reference signal in downlink is for cell search and cell acquisition.
Mobile Entity uses reference signal for frame timing detection and cell ID identification. During channel estimation, receiver compares received symbols with know reference symbols and estimates channel response. This estimated channel response is used for channel equalization. In other words, required in coherent demodulation.