**3.1. Problem statement and system setup**

As indicated in the state of the art, it can generally be said that most contributions on multiuser transmission schemes focus on particular aspects of the problem and simplify the rest. Usually, when the focus is laid on the PHY layer transmission techniques, practical mechanisms for the channel access and the feedback acquisition are not considered, whereas multiuser MAC schemes often fail to consider PHY layer implementation issues. For example, some schemes optimize resource allocation but ignore feedback mechanisms and others minimize the required feedback but assume a dedicated control channel and a less sophisticated scheduling policy.

This chapter will introduce a multiuser MAC mechanism that handles in a joint manner the processes of channel access, scheduling, channel estimation and feedback acquisition, in conjunction with a low-complexity beamforming technique at the PHY layer. The proposed schemes have been designed in the context of a downlink communication channel in an infrastructure WLAN in which multiple antennas are available at the transmitter side. Without loss of generality, a MISO scenario with single-antenna users has been considered, even though the presented analysis can be also applied to MIMO systems with multiple-antenna users.

The considered setup is illustrated in Figure 2. The proposed schemes can be considered as a downlink transmission phase, initiated by an Access Point (AP) equipped with *nt* antennas (*nt* ≥ 2) in a system with *N* single-antenna users. By exploiting the MIMO/MISO spatial signal processing capabilities and employing an appropriate transmission technique, the AP can serve up to *nt* users at the same frequency and time. Nevertheless, in order to extract multiuser diversity gain, the pool of served users should exceed the number of transmitting antennas (i.e., *N* > *nt*). Transmitting multiple downlink packets simultaneously, however, is feasible only when there is no interference among the selected users, or in a more realistic case, when the interference is relatively low. Hence, the AP must have some knowledge of the channel to select the most appropriate set of users for each transmission. These issues must be handled by the MAC layer in a practical way, as it will be described in detail in the following sections.
