Circulant wired MIMO structures

MIMO has developed increased spectral efficiency in wireless systems where there is rich multipath in the propagation channel. MIMO has also been applied to multi-conductor channels where the cross-talk between the individual conductors limits cable capacity. Here, the theoretical capacity of two structures is compared: completely random complex Gaussian (i.i.d.) and circulant (feasible for cables). The comparison is extended to a practicable capacity using QAM modulations. It is shown that for large SNRs, where there is enough power to use all the eigenchannels, circulant structures show higher capacity. For small SNRs, the water-filling eigenchannel cut-off feature means that the completely random channel case has the higher capacity. The more similar pdfs of the eigenvalues of the circulant is the mechanism for higher capacity for large SNRs. This is confirmed by the theorem of maximum capacity for the parallel channels