OFDM Times of Arrival of multipath Assume i have received a known analog OFDM symbol (such the Short-Training-Field (STF) in WiFi systems), call it $s(t)$ with a known DFT per subcarrier, in the presence of multipath and absence of noise.  

$$r(t) = \sum\limits_{i=1}^p \gamma_i s(t-\tau_i)$$
where $\gamma_i$ and $\tau_i$ are complex gains and the delays of the multipath, respectively. Without loss of generality, let $\tau_1 &gt; \ldots &gt; \tau_p$.

My question is simply, what is $\tau_1$? I could imagine what $\tau_2 \ldots \tau_p$ are. They are just relative delays with respect to $\tau_1$. But i don't seem to quite grasp how to quantify or understand what $\tau_1$ means. Is it absolute ? Is it with respect to something ? 

Any suggestion is much appreciated.

Thanks
I don't fully understand your question, since you correctly state


  $\tau_i$ are delays of the multipath


therefore, $\tau_1$ is simply the delay incurred by the shortest path.

And no, $\tau_{i\ge2}$ are not relative delays; look at the formula: you just add up different independent paths with different delays; every path has its own delay, which is totally independent from $\tau_1$.