  In multiple-input multiple-output (MIMO) wireless systems with frequency-flat
channels, a single-antenna jammer causes receive interference that is confined
to a one-dimensional subspace. Such a jammer can thus be nulled using linear
spatial filtering at the cost of one degree of freedom. Frequency-selective
channels are often transformed into multiple frequency-flat subcarriers with
orthogonal frequency-division multiplexing (OFDM). We show that when a
single-antenna jammer violates the OFDM protocol by not sending a cyclic
prefix, the interference received on each subcarrier by a multi-antenna
receiver is, in general, not confined to a subspace of dimension one (as a
single-antenna jammer in a frequency-flat scenario would be), but of dimension
L, where L is the jammer's number of channel taps. In MIMO-OFDM systems, a
single-antenna jammer can therefore resemble an L-antenna jammer. Simulations
corroborate our theoretical results. These findings imply that mitigating
jammers with large delay spread through linear spatial filtering is infeasible.
We discuss some (im)possibilities for the way forward.
