OFDM pilot allocation block type and comb type
In both cases, you need to interpolate the channel between the pilots you've got.
Both cases are typically suboptimal, since they'd only work perfectly for (a) actual block-fading (which is a convenient model, but doesn't look like reality) or (b) for a channel that is perfectly interpolatable from just a few points of observation in frequency (but that would imply you have designed an OFDM system with too many subcarrier, and that has other downsides). 

You'd usually need both, because over the course of a longer data transmission, the channel might change, so there's need for regularly estimating more than just a few subcarriers. And because you cannot just guess the subcarrier state from a pilot some (up to) $\frac Q2$ subcarriers away, in general.

Modern systems tend to solve this by having something like (a) at the beginning of a frame, and then distributing pilots in a pseudorandom, or in a diagonal way throughout the frame, so that you get updates on the channel behaviour over time. Due to having updated your channel state info underway, you can space your pilot blocks further apart (i.e. increase $Q$), thus not reducing your data symbol rate by a lot.

LTE does that. There's a lot of resources you can find when searching for "LTE pilots", but linking to any of these here is a bit futile, since I don't know which background you're coming from.