**4. Selection criteria with multiple users**

To maximize the performance, if *M* = 1, the user who can have the minimum PEP is chosen for a given MIMO detector. In Section 3, a few user selection criteria are derived, depending on the types of actually employed MIMO detectors. Note that only one user is selected (i.e., *M* = 1) in Section 3. To extend the user selection criteria to the case of *M* > 1 here and in the consecutive sections, we consider the combinatorial and greedy user selection criteria.

#### **4.1. ML and MMSE selection criteria**

For a given *M* > 1, the set of the users who can access the channel can be found using the MDist or ME user selection criterion as follows:

$$\mathcal{K}\_{\text{MDist}} = \arg\max\_{\mathcal{K}} \mathcal{S}\left(\mathbf{H}\_{\mathcal{K}}\right) \tag{22}$$

or

$$\mathcal{K}\_{\text{ME}} = \arg\max\_{\mathcal{K}} \lambda\_{\text{min}} \left( \mathbf{H}\_{\mathcal{K}}^{\text{H}} \mathbf{H}\_{\mathcal{K}} \right) \tag{23}$$

respectively. If the ML detector is employed, the MDist user selection criterion can be used to choose the *M* users who can have the lowest BER, whereas the ME criterion is used to choose the *M* users with the highest worst SNR (i.e., max-min SNR).
