This paper presents the first reliable physical-layer network coding (PNC) system that supports real TCP IP applica- tions 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 pro- totype, 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 s - 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 two-party video conferencing with UDP over RPNC. Experimental results show that the achieved throughput approaches the PHY-layer data rate at high SNR, demonstrating the high efficiency of the RPNC system. Index Terms—Network coding, physical-layer network coding, two-way relay network, implementation, ARQ, TCP IP.
 Physical layer network coding (PNC) has the potential to improve the spectral efficiency of wireless relay communications by utilising the superposition of signal propagation. In the ideal case, PNC can double the throughput of a symmetric relay network when compared to conventional time division multiplexing. However, in practice imperfect channel conditions and hardware can lead to time and frequency offsets at the relay receiver that degrade PNC performance. Physical layer network coding has attracted extensive theoretical interest, although relatively little research has been done in support of deployment to wireless networks where internode synchronization is difficult to achieve. In particular, wireless networks constructed with inexpensive and commercially available software defined radio technology, or more generally, radio front-end samplers connected to internet-based remote processors (i.e., the Internet of Things network) may exhibit large time, frequency, and phase offsets that are difficult to control. In this paper, we define an asynchronous discrete-time model that accounts for these impairments as part of the information transfer between network users and a relay. Derived from this model are maximum likelihood algorithms for relay parameter estimation and a symbol decoder inspired from asynchronous multi-user detection. Additionally, null space-based frequency offset estimation that reduces computational complexity is proposed. Simulation results and the design and performance of a two-user system implemented with the Universal Software Radio Peripheral (USRP) platform and GNU radio are included to demonstrate the proof of concept. Our results indicate that the physical layer network coding technique can be successfully deployed and yields significant benefits even in the presence of impairments found in practical settings. In this paper we consider physical-layer network coding (PLNC) in OFDM-based two-way relaying systems. Practically, a key impairment for the application of PLNC is the carrier frequency offset (CFO) mismatch between the sources and the relay, which can not be compensated completely in the multiple-access (MA) phase. As this CFO mismatch results in inter-carrier interference (ICI) for OFDM transmissions, practical CFO compensation and ICI cancelation strategies are investigated to mitigate the impairment for a-posterior probability (APP) based PLNC decoders at the relay. Furthermore, we perform hardware implementation of the two-way relaying network employing an long term evolution (LTE) near parametrization adapted to PLNC. The APP-based decoding schemes with CFO compensation and ICI cancelation are applied on this real-time transmission platform to verify the analytical results.