information symbols are transmitted simultaneously on independent streams using the same spreading code but from different transmit antennas at the transmitter . these simultaneous data stream could be intended for the same user or for different users . each stream of data can belong to a different signal constellation and use a different channel code . at the receiver , a number of receive antennas equal to at least the number of multiple data streams is used to separate the different data streams . we consider two different cases . the first one when no transmit diversity is used and the second when transmit diversity with space - time block coding is used .