  Spatial filtering based on multiple-input multiple-output (MIMO) processing
is a powerful method for jammer mitigation. In principle, a MIMO receiver can
null the interference of a single-antenna jammer at the cost of only one degree
of freedom - if the number of receive antennas is large, communication
performance is barely affected. In this paper, we show that the potential for
MIMO jammer mitigation based on the digital outputs of finite-resolution
analog-to-digital converters (ADCs) is fundamentally worse: Strong jammers will
either cause the ADCs to saturate (when the ADCs' quantization range is small)
or drown legitimate communication signals in quantization noise (when the ADCs'
quantization range is large). We provide a fundamental bound on the mutual
information between the quantized receive signal and the legitimate transmit
signal. Our bound shows that, for any fixed ADC resolution, the mutual
information tends to zero as the jammer power tends to infinity. Our bound also
confirms the intuition that for every 6.02 dB increase in jamming power, the
ADC resolution must be increased by 1 bit in order to prevent the mutual
information from vanishing.
