  We study multi-stream transmission in the $K \times N \times K$ channel with
interfering relay nodes, consisting of $K$ multi-antenna source--destination
(S--D) pairs and $N$ single-antenna half-duplex relay nodes between the S--D
pairs. We propose a new achievable scheme operating with partial effective
channel gain, termed multi-stream opportunistic network decoupling (MS-OND),
which achieves the optimal degrees of freedom (DoF) under a certain relay
scaling law. Our protocol is built upon the conventional OND that leads to
virtual full-duplex mode with one data stream transmission per S--D pair,
generalizing the idea of OND to multi-stream scenarios by leveraging relay
selection and interference management. Specifically, two subsets of relay nodes
are opportunistically selected using alternate relaying in terms of producing
or receiving the minimum total interference level. For interference management,
each source node sends $S \,(1 \le S \le M)$ data streams to selected relay
nodes with random beamforming for the first hop, while each destination node
receives its desired $S$ streams from the selected relay nodes via
opportunistic interference alignment for the second hop, where $M$ is the
number of antennas at each source or destination node. Our analytical results
are validated by numerical evaluation.
