On the importance of using a cyclic prefix or null guard in block transmissions over FIR channels

Considers one aspect of the problem of how to design the linear precoder in a linearly precoded block transmission system operating over an unknown finite impulse response (FIR) channel. It is argued that it is sensible to restrict attention to only those precoders which introduce a cyclic prefix if single blocks are transmitted in isolation (such as in a wireless packet network), or to restrict attention to only those precoders which introduce a null guard between each block if multiple blocks are transmitted consecutively. In both cases, the reason is to ensure that the channel can be accurately inverted at the receiver regardless of the phase of the channel. Connections between using a null guard and using a cyclic prefix are also drawn; using a null guard can be interpreted as being a special case of using a cyclic prefix.