  We introduce an opportunistic interference mitigation (OIM) protocol, where a
user scheduling strategy is utilized in $K$-cell uplink networks with
time-invariant channel coefficients and base stations (BSs) having $M$
antennas. Each BS opportunistically selects a set of users who generate the
minimum interference to the other BSs. Two OIM protocols are shown according to
the number $S$ of simultaneously transmitting users per cell: opportunistic
interference nulling (OIN) and opportunistic interference alignment (OIA).
Then, their performance is analyzed in terms of degrees-of-freedom (DoFs). As
our main result, it is shown that $KM$ DoFs are achievable under the OIN
protocol with $M$ selected users per cell, if the total number $N$ of users in
a cell scales at least as $\text{SNR}^{(K-1)M}$. Similarly, it turns out that
the OIA scheme with $S$($<M$) selected users achieves $KS$ DoFs, if $N$ scales
faster than $\text{SNR}^{(K-1)S}$. These results indicate that there exists a
trade-off between the achievable DoFs and the minimum required $N$. By deriving
the corresponding upper bound on the DoFs, it is shown that the OIN scheme is
DoF optimal. Finally, numerical evaluation, a two-step scheduling method, and
the extension to multi-carrier scenarios are shown.
