Multipath resistance of spread spectrum systems
The key idea in spread-spectrum communications is to use much more bandwidth than the minimum required for the signal being sent.

Multipath effects occur when the sent signal $s(t)$ and a delayed, attenuated version of it interfere at the receiver:
$$
r(t) = s(t) + \alpha \cdot s(t-\tau)
$$

If $s(t)$ is really narrow band (a sinusoid), and $\alpha = 1$, then 
$$
r(t) = \sin(\omega t) + \sin(\omega(t+\tau))
$$
and if $\omega\tau = \pi$ you can get perfect nulling:
$$
r(t) = \sin(\omega t) + \sin(\omega t + \pi) = 0
$$

If, however, $s(t)$ is not a simple sinusoid, then perfect nulling can be eliminated:
$$
r(t) = [\sin(\omega t) + \sin(\omega_2 t) ] + [\sin(\omega(t+\tau)) + \sin(\omega_2 (t+\tau)) ]
$$
even if  $\omega\tau = \pi$, if $\omega_2$ is chosen well, then $\omega_2\tau \ne \pi$ so there is still some signal to receive.