  We study the degrees of freedom (DoF) of MIMO two-way X relay channels.
Previous work studied the case $N < 2M$, where $N$ and $M$ denote the number of
antennas at the relay and each source, respectively, and showed that the
maximum DoF of $2N$ is achievable when $N \leq \lfloor\frac{8M}{5}\rfloor$ by
applying signal alignment (SA) for network coding and interference cancelation.
This work considers the case $N>2M$ where the performance is limited by the
number of antennas at each source node and conventional SA is not feasible. We
propose a \textit{generalized signal alignment} (GSA) based transmission
scheme. The key is to let the signals to be exchanged between every source node
align in a transformed subspace, rather than the direct subspace, at the relay
so as to form network-coded signals. This is realized by jointly designing the
precoding matrices at all source nodes and the processing matrix at the relay.
Moreover, the aligned subspaces are orthogonal to each other. By applying the
GSA, we show that the DoF upper bound $4M$ is achievable when $M \leq
\lfloor\frac{2N}{5}\rfloor$ ($M$ is even) or $M \leq
\lfloor\frac{2N-1}{5}\rfloor$ ($M$ is odd). Numerical results also demonstrate
that our proposed transmission scheme is feasible and effective.
