Rui Zhang Ying-Chang Liang Chin Choy Chai Shuguang Cui Optimal beamforming for two-way multi-antenna relay channel with analogue network coding. 699-712 2009 27 IEEE Journal on Selected Areas in Communications 5 http://dx.doi.org/10.1109/JSAC.2009.090611 db/journals/jsac/jsac27.html#ZhangLCC09