Wireless channel interpretation
I will just add and not repeat what the other posters have said. 

If you transmit at a carrier frequency \$f_c\$ and the relative velocity between TX and RX is \$\Delta v(t)\$ then the perceived frequency deviation at the receiver at time \$t\$ (call this \$\Delta f(t)\$) will be

$$
\Delta f(t) = \frac{\Delta v(t)}{c} f_{\text{TX}} = \frac{\Delta v(t)}{c} f_c = \frac{||\Delta v(t)||}{\lambda} \cos \theta(t) = f_{\text{max}} \cos \theta (t)
$$

$$
f(t) = f_c + \Delta f(t)
$$ 

This change in receive frequency is usually called frequency dispersion and because it makes the channel sensitive to time variable \$t\$ it is said to cause time selectivity. Note that because \$\cos \theta (t) \$ will be in the interval \$[-1,1]\$, the maximum deviation is clearly \$\pm f_{\text{max}}\$, the quantity \$f_{\text{max}}\$ is the maximum doppler shift.

Now we are looking to find a particular time \$T_c\$ that would characterise how fast our channel is changing. In order words we want to find the time interval \$T_c\$ in which our channel is relatively flat so we are able to say "our channel only changes after every \$T_c\$ seconds". We want to find maximum \$T_c\$ such that:

$$
h\big(f(t)\big) \approx h\big(f(t + T_c)\big)\\
$$

or 

$$
 h\big(t\big) \approx h\big(t + T_c \big)
$$

In order to find \$T_c\$ we first need to properly define what we mean by a channel change. If we say two channels \$h_1\$ and \$h_2\$ are different what do we actually mean? We can consider two channel responses \$h(t)\$ and \$h(t + \Delta t)\$ similar if their correlation coefficient is at least \$X\$. Where \$X\$ can be defined for any value in (0,1) but is usually taken to be either \$\frac{1}{\sqrt{2}}\$ or \$\frac{1}{2}\$.

So given the speed \$(||v(t)||)\$ and arrival angle function \$(\theta (t))\$ we can compute \$T_c\$. Now, given that our wireless system is supposed to be able to support a maximum speed of say \$v_{\text{max}}\$ we usually assume \$\theta (t)\$ to be uniformly distributed between \$[0,2\pi]\$ as this is the worst case scenario. All these design variables will give us a doppler spectrum (i.e. distribution of \$f(t)\$) that looks like this:



Using the design parameters defined above and taking \$X = 0.5\$ then \$T_c\$ will be \$\approx 9/(16 \pi f_{\max})\$. This is the coherence time value that is usually reported so this is the expression you could use to estimate your coherence time.

Note that the analysis was done by taking the worst case scenario of having \$\theta (t)\$ uniformly distributed between \$[0,2\pi]\$. In practice, the spectrum will most likely be not as severe as that and the spectrum will typically be something like (image is from Introduction to Space-Time Wireless Communications by Paulraj):