  This paper investigates the capacity of a wireless two way relay channel in
which two end nodes exchange information via a relay node. The capacity is
defined in the information-theoretic sense as the maximum information exchange
rate between the two end nodes. We give an upper bound of the capacity by
applying the cut-set theorem. We prove that this upper bound can be approached
in low SNR region using "separated" multiple access for uplinks from the end
nodes to the relay in which the data from the end nodes are individually
decoded at the relay; and network-coding broadcast for downlinks from the relay
to the end nodes in which the relay mixes the information from end nodes before
forwarding. We further prove that the capacity is approachable in high SNR
region using physical-layer network coding (PNC) multiple access for uplinks,
and network-coding broadcast for downlinks. From our proof and observations, we
conjecture that the upper bound may be achieved with PNC in all SNR regions.
