**3. Multi-user MIMO system**

The growth in MIMO technology has led to the emergence of new communication systems. We are particularly interested in this chapter in multi-user MIMO (MU-MIMO) ones [11]. MU-MIMO [12] system is often considered in literature as an extension of Space-Division Multiple Access (SDMA). This technology supports multiple connections on a single conventional channel where different users are identified by spatial signatures. SDMA uses spatial multiplexing and enables for higher data rate. This could be achieved by using multiple paths as different channels for carrying data. Another benefit of using the SDMA technique in cellular networks is to mitigate the effect of interference coming from adjacent cells. Traditional communication MIMO systems are usually referred as single-user MIMO systems (SU-MIMOs) or also point-to-point MIMO. Case of MIMO systems, the access point communicates with only one mobile terminal (the user). Both the access point and the mobile terminal are equipped with multiple antennas. In contrast to the single-user case, the access point is able to communicate with several mobile terminals. SU-MIMO and MU-MIMO systems are two possible configurations for multi-user communication systems. We also find other configurations in literature such as MU-MIMO with cooperation where cooperation is

**Figure 4.** Improvement of the BER for MIMO (*NR* × <sup>2</sup>) as a function of receive antennas number

established between base stations [2]. Basic configurations of downlink multi-user MIMO systems are depicted in Figure 5. Figure 5(a) represents the SU-MIMO system where a Base Station (BS) equipped with antennas *Tx*1,..., *TxN* communicates with user *U* which is equipped with *M* antennas *Rx*1,..., *RxM*. In Figure 5(b), the presented MU-MIMO system consists of two base stations *BS*<sup>1</sup> and *BS*<sup>2</sup> each one is equipped with *N* antennas. Generalized MU-MIMO systems may consist of more base stations where the number of antennas could be different. At the receive side, *K* users *U*1,..., *UK* with respectively *M*1,..., *MK* antennas communicate with the transmit base stations. The same communication model is performed for the MU-MIMO with cooperation (Figure 5(c)) where cooperation is established between *BS*<sup>1</sup> and *BS*2.

Once multi-user communication systems are introduced, we explain in the following section the difference between SU-MIMO and MU-MIMO configurations.
