  An extension of Proper Orthogonal Decomposition is applied to the wall layer
of a turbulent channel flow (Re {\tau} = 590), so that empirical eigenfunctions
are defined in both space and time. Due to the statistical symmetries of the
flow, the igenfunctions are associated with individual wavenumbers and
frequencies. Self-similarity of the dominant eigenfunctions, consistent with
wall-attached structures transferring energy into the core region, is
established. The most energetic modes are characterized by a fundamental time
scale in the range 200-300 viscous wall units. The full spatio-temporal
decomposition provides a natural measure of the convection velocity of
structures, with a characteristic value of 12 u {\tau} in the wall layer.
Finally, we show that the energy budget can be split into specific
contributions for each mode, which provides a closed-form expression for
nonlinear effects.
