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.

翻译：在多输入多输出（MIMO）无线系统中，当信道为频率平坦衰落时，单天线干扰机产生的接收干扰局限于一个一维子空间。因此，此类干扰可通过线性空间滤波以消耗一个自由度为代价得到消除。频率选择性信道通常借助正交频分复用（OFDM）技术转换为多个频率平坦子载波。本文证明：当单天线干扰机违反OFDM协议（即不发送循环前缀）时，多天线接收机在各子载波上收到的干扰一般不再局限于一维子空间（如频率平坦场景下单天线干扰机的情况），而是局限于维度为L的子空间，其中L为干扰机的信道抽头数。因此在MIMO-OFDM系统中，单天线干扰机可等效为L天线干扰机。仿真结果验证了我们的理论分析。这些发现表明，通过线性空间滤波来抑制具有大时延扩展的干扰机是不可行的。本文进一步探讨了未来研究的一些可行与不可行方向。