  Efficient heterogeneous communication technologies are critical components to
provide flawless connectivity in smart cities. The proliferation of wireless
technologies, services and communication devices has created the need for green
and spectrally efficient communication technologies. Physical- layer network
coding (PNC) is now well-known as a potential candidate for delay-sensitive and
spectrally efficient communication applications, especially in bidirectional
relaying, and is therefore well-suited for smart city applications. In this
paper, we provide a brief introduction to PNC and the associated distance
shortening phenomenon which occurs at the relay. We discuss the issues with
existing schemes that mitigate the deleterious effect of distance shortening,
and we propose simple and effective solutions based on the use of multiple
antenna systems. Simulation results confirm that full diversity order can be
achieved in a PNC system by using antenna selection schemes based on the
Euclidean distance metric.
