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

• Published in: IEEE communications letters Vol. 27; no. 11; pp. 3103 - 3107
• Main Authors: Marti, Gian, Studer, Christoph
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.11.2023; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Antennas; Channels; Cyclic prefix; Frequency division multiplexing; Interference; jammer mitigation; Jammers; Jamming; MIMO; MIMO communication; OFDM; Orthogonal Frequency Division Multiplexing; Spatial filtering; Subcarriers; Symbols; Transmitters
• ISSN: 1089-7798; 1558-2558
• DOI: 10.1109/LCOMM.2023.3319818

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 <inline-formula> <tex-math notation="LaTeX">L </tex-math></inline-formula>, where <inline-formula> <tex-math notation="LaTeX">L </tex-math></inline-formula> is the jammer's number of channel taps. In MIMO-OFDM systems, a single-antenna jammer can therefore resemble an <inline-formula> <tex-math notation="LaTeX">L </tex-math></inline-formula>-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.