  This paper introduces a new and unified bit error rates performance analysis
of space-time block codes (STBC) deployed in wireless systems with spatial
diversity in generalized shadowed fading and noise scenarios. Specifically, we
derive a simple and a very accurate approximate expressions for the average
error rates of coherent modulation schemes in generalized $\eta-\mu$ and
$\kappa-\mu$ shadowed fading channels with multiple input multiple output
(MIMO) systems. The noise in the network is assumed to be modeled using the
additive white generalized Gaussian noise (AWGGN), which encompasses the
classical Laplacian and the Gaussian noise environments as special cases. The
derived results obviate the need to re-derive the error rates for MIMO STBC
systems under many multipath fading and noise conditions while avoiding any
special functions with high computational complexity. Published results from
the literature, as well as numerical evaluations, corroborate the accuracy of
our derived generalized expressions.
