Single-Antenna Jammers in MIMO-OFDM Can Resemble Multi-Antenna Jammers

TL;DR

This paper reveals that in MIMO-OFDM systems, a single-antenna jammer with delay spread can mimic a multi-antenna jammer, complicating mitigation efforts using linear spatial filtering.

Contribution

The study demonstrates that a single-antenna jammer violating OFDM protocol can appear as a multi-antenna jammer in frequency-selective channels, challenging existing mitigation strategies.

Findings

Single-antenna jammers can resemble multi-antenna jammers in OFDM systems.

Mitigating large delay spread jammers with linear filtering is infeasible.

Theoretical results are supported by simulations.

Abstract

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.