  A main distinguishing feature of a wireless network compared with a wired
network is its broadcast nature, in which the signal transmitted by a node may
reach several other nodes, and a node may receive signals from several other
nodes simultaneously. Rather than a blessing, this feature is treated more as
an interference-inducing nuisance in most wireless networks today (e.g., IEEE
802.11). This paper shows that the concept of network coding can be applied at
the physical layer to turn the broadcast property into a capacity-boosting
advantage in wireless ad hoc networks. Specifically, we propose a
physical-layer network coding (PNC) scheme to coordinate transmissions among
nodes. In contrast to straightforward network coding which performs coding
arithmetic on digital bit streams after they have been received, PNC makes use
of the additive nature of simultaneously arriving electromagnetic (EM) waves
for equivalent coding operation. And in doing so, PNC can potentially achieve
100% and 50% throughput increases compared with traditional transmission and
straightforward network coding, respectively, in multi-hop networks. More
specifically, the information-theoretic capacity of PNC is almost double that
of traditional transmission in the SNR region of practical interest (higher
than 0dB). We believe this is a first paper that ventures into EM-wave-based
network coding at the physical layer and demonstrates its potential for
boosting network capacity.
