  This paper presents the first reliable physical-layer network coding (PNC)
system that supports real TCP/IP applications for the two-way relay network
(TWRN). Theoretically, PNC could boost the throughput of TWRN by a factor of 2
compared with traditional scheduling (TS) in the high signal-to-noise (SNR)
regime. Although there have been many theoretical studies on PNC performance,
there have been relatively few experimental and implementation efforts. Our
earlier PNC prototype, built in 2012, was an offline system that processed
signals offline. For a system that supports real applications, signals must be
processed online in real-time. Our real-time reliable PNC prototype, referred
to as RPNC, solves a number of key challenges to enable the support of real
TCP/IP applications. The enabling components include: 1) a time-slotted system
that achieves us-level synchronization for the PNC system; 2) reduction of PNC
signal processing complexity to meet real-time constraints; 3) an ARQ design
tailored for PNC to ensure reliable packet delivery; 4) an interface to the
application layer. We took on the challenge to implement all the above with
general-purpose processors in PC through an SDR platform rather than ASIC or
FPGA. With all these components, we have successfully demonstrated image
exchange with TCP and twoparty video conferencing with UDP over RPNC.
Experimental results show that the achieved throughput approaches the PHYlayer
data rate at high SNR, demonstrating the high efficiency of the RPNC system.
