  In this paper we present low-complexity blind multi-input multi-output (MIMO)
adaptive linear multiuser receivers for direct sequence code division multiple
access (DS-CDMA) systems using multiple transmit antennas and space-time block
codes (STBC) in multipath channels. A space-time code-constrained constant
modulus (CCM) design criterion based on constrained optimization techniques and
low-complexity stochastic gradient (SG) adaptive algorithms are developed for
estimating the parameters of the space-time linear receivers. The receivers are
designed by exploiting the unique structure imposed by both spreading codes and
STBC. A blind space-time channel estimation scheme for STBC systems based on a
subspace approach is also proposed along with an efficient SG algorithm.
Simulation results for a downlink scenario assess the receiver structures and
algorithms and show that the proposed schemes achieve excellent performance,
outperforming existing methods.
