Diversity and spatial multiplexing of MIMO amplitude detection receivers

We consider nonlinear MIMO systems that use amplitude-only (envelope) receivers. Such systems are interesting for low-power, low-complexity applications like sensor networks. We study different modulation schemes and the respective diversity order obtained by the maximum likelihood detector in an uncoded system with perfect channel state information. We show that nonlinear MIMO amplitude receivers loose half the available receive diversity when the uncoded transmission rate increases above 1 bit/s/Hz, or when spatial multiplexing is performed.