In　this　paper,　we　study　the　problem　of　joint　resource　allocation　in　the　two-way　relay　system,　where　a　pair　of　multiantenna　users　wish　to　exchange　information　via　a　multi-antenna　amplify-and-forward　relay　under　orthogonal　frequency-division　multiplexing　(OFDM)　modulation.　We　formulate　a　sum-rate　maximization　problem　subject　to　a　limited　power　constraint　for　each　individual　user　and　relay.　Our　resource　allocation　strategy　aims　at　finding　the　best　pairing　scheme　and　optimal　power　allocation　over　subchannels　in　frequency　and　space　domains.　This　turns　out　to　be　a　mixed　integer　programming　problem.　We　then　derive　an　asymptotically　optimal　solution　though　the　Lagrange　dual　decomposition　approach.　Finally,　simulation　results　are　provided　to　demonstrate　the　performance　gain　of　the　proposed　algorithms.