Amir Salman Avestimehr M. Amin Khajehnejad Aydin Sezgin Babak Hassibi Capacity region of the deterministic multi-pair bi-directional relay network http://arxiv.org/abs/0903.2653 2009 CoRR abs/0903.2653 db/journals/corr/corr0903.html#abs-0903-2653 informal publication