The two-user multi-input, multi-output (MIMO) interference and cognitiveradio channels are studied under the assumption of no channel state informationat the transmitter (CSIT) from the degrees of freedom (DoF) region perspective.With $M_i$ and $N_i$ denoting the number of antennas at transmitter $i$ andreceiver $i$ respectively, the DoF regions of the MIMO interference channelwere recently characterized by Huang et al., Zhu and Guo, and by the authors ofthis paper for all values of numbers of antennas except when $min(M_1,N_1) >N_2 > M_2$ (or $min(M_2,N_2) > N_1 > M_1$). This latter case was solved morerecently by Zhu and Guo who provided a tight outer-bound. Here, a simpler andmore widely applicable proof of that outer-bound is given based on the idea ofinterference localization. Using it, the DoF region is also established for theclass of MIMO cognitive radio channels when $min(M_1+M_2,N_1) > N_2 > M_2$(with the second transmitter cognitive) -- the only class for which the innerand outer bounds previously obtained by the authors were not tight -- therebycompleting the DoF region characterization of the general 2-user MIMO cognitiveradio channel as well.
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