# Precursor ISI - causality violation?

Today, in my lecture on intersymbol interference (ISI), there was a discussion on postcursor ISI, and precursor ISI. Postcursor ISI is caused by the past bits, whereas, precursor ISI is caused by the future bits on the present bit. I could not understand how precursor ISI can occur. I was told that it is because of the delay of the channel. What does it mean? Isn't there a causality violation? What am I missing?

Can someone please explain? Thanks.

## 1 Answer

Every symbol in the channel will be affected by ISI from previous symbols and next symbols. To understand this you must notice that there's a propagation delay from transmitter(TX) to receiver(RX). That means that it is possible for several symbols to be present in the channel at the same time before the first of them actually arrives at the receiver. When a symbol actually arrives at the receiver it will already have ISI from the symbols that were before and after it and shared that propagation time. Only the first symbol of the transmission (if the channel is free of previous symbols from perhaps some other packet, etc) will not have ISI from previous symbols. If we assume that the propagation time between TX and RX lasts, for instance what it takes to the TX to send 5 symbols then it is correct to assume that the 5 of them will interfere each other in more or less proportion because the are all present during the same time window in the channel. At the RX the first symbol will exhibit ISI from others which it hasn't received yet but were present in the channel.

You shouldn't analyze this as Oh, I'm receiving ISI from the future! Because the time t = 0 is not when the receiver received the first symbol but when the transmitter send it which gives you the whole propagation time to be causal.