  We present results for the fluctuations of the displacement of a tracer
particle on a planar lattice pulled by a step force in the presence of
impenetrable, immobile obstacles. The fluctuations perpendicular to the applied
force are evaluated exactly in first order of the obstacle density for
arbitrarily strong pulling and all times. The complex time-dependent behavior
is analyzed in terms of the diffusion coefficient, local exponent, and the
non-Skellam parameter, which quantifies deviations from the dynamics on the
lattice in the absence of obstacles. The non-Skellam parameter along the force
is analyzed in terms of an asymptotic model and reveals a power-law growth for
intermediate times.
