  We consider the $K$-cell multiple-input multiple-output (MIMO) interfering
multiple-access channel (IMAC) with time-invariant channel coefficients, where
each cell consists of a base station (BS) with $M$ antennas and $N$ users
having $L$ antennas each. In this paper, we propose two opportunistic
interference alignment (OIA) techniques utilizing multiple transmit antennas at
each user: antenna selection-based OIA and singular value decomposition
(SVD)-based OIA. Their performance is analyzed in terms of \textit{user scaling
law} required to achieve $KS$ degrees-of-freedom (DoF), where $S(\le M)$
denotes the number of simultaneously transmitting users per cell. We assume
that each selected user transmits a single data stream at each time-slot. It is
shown that the antenna selection-based OIA does not fundamentally change the
user scaling condition if $L$ is fixed, compared with the single-input
multiple-output (SIMO) IMAC case, which is given by $\text{SNR}^{(K 1)S}$,
where SNR denotes the signal-to-noise ratio. In addition, we show that the
SVD-based OIA can greatly reduce the user scaling condition to
$\text{SNR}^{(K-1)S-L+1}$ through optimizing a weight vector at each user.
Simulation results validate the derived scaling laws of the proposed OIA
techniques. The sum-rate performance of the proposed OIA techniques is compared
with the conventional techniques in MIMO IMAC channels and it is shown that the
proposed OIA techniques outperform the conventional techniques.
