This equation is also known as the two-dimensional Nyquist criterion. A channel that fulfills these requirements is known as underspread. If Eq. (8.9) is not fulfilled, then the channel can only be identified by making specific assumptions e.g., a certain parametric model. Fortunately, the overwhelming majority of wireless channels are underspread: in many cases, even 2rmaxvmax 1 is fulfilled. We will assume that this is fulfilled in the following. This also implies that the channel is slowly time variant (see Chapter 6, so that h(t. r) can be interpreted as the impulse response h(r) that is valid at a certain (fixed) time instant t.l