  We investigate techniques for designing modulation/coding schemes for the
wireless two-way relaying channel. The relay is assumed to have perfect channel
state information, but the transmitters are assumed to have no channel state
information. We consider physical layer network coding based on multilevel
coding techniques. Our multilevel coding framework is inspired by the
compute-and-forward relaying protocol. Indeed, we show that the framework
developed here naturally facilitates decoding of linear combinations of
codewords for forwarding by the relay node. We develop our framework with
general modulation formats in mind, but numerical results are presented for the
case where each node transmits using the QPSK constellation with gray labeling.
We focus our discussion on the rates at which the relay may reliably decode
linear combinations of codewords transmitted from the end nodes.
