Multiuser detection for Rate-½ OSTBC wih four transmit antennas

In this paper we consider a multiuser detection scheme where K users, each with N_T transmit antennas, transmit their symbols simultaneaously using space-time block coding (STBC). Previously, it was shown that a receiver can completely separate signals of K users, where each user has two antenna and uses Alamouti coding, using a number of receive antennas equals to or greater than the number of users (i.e., N_R ≥ K). Also, it was shown that if there are two users employing rate-1/2 complex orthogonal STBC (OSTBC) the receiver can separate their signals. However, there is no work investigating the scenario when number of user is greater than two and each user has more than two transmit antennas. In this paper we show that if the K users are equipped with four transmit antennas and use rate-1/2 complex OSTBC, the receiver can completely separate the signals of all the users provided N_R ≥ K/2. For example, in the case of four users the receiver can separate all the signals using two antennas and provides diversity order of two. Numerical results are provided for the uncorrelated Rayleigh fading channel.