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.
Lu, Wang, Liew and Zhang @cite_9 presented the first PNC implementation based on OFDM, and evaluated the PHY-layer SNR BER performance through experiments. @cite_23 also considered an OFDM-based PNC system, and focused on the problem of carrier frequency offset (CFO) mismatch between the end nodes and the relay. The proposed iterative algorithm was evaluated through FPGA implementation and experiments. The works by Chen, Haley and Nguyen @cite_20 and @cite_25 both implemented a single-carrier asynchronous PNC system on USRP, and studied signal processing issues (e.g., CFO estimation and compensation) at the relay. In contrast, our paper here focuses on system-level implementation issues. Specifically a focus of ours is on the design of the MAC layer and the ARQ protocol for TWRN, and we address reliability and real-time challenges for PNC systems on compute-bound GPP-based SDR platform.
@cite_9 @cite_20 @cite_25 @cite_23
2293741284 2066938847 2203527323 2087187421
Abstract This paper presents the first implementation of a two-way relay network based on the principle of physical-layer network coding (PNC). To date, only a simplified version of PNC, called analog network coding (ANC), has been successfully implemented. The advantage of ANC is that it is simple to implement; the disadvantage, on the other hand, is that the relay amplifies the noise along with the signal before forwarding the signal. PNC systems in which the relay performs XOR or other denoising PNC mappings of the received signal have the potential for significantly better performance. However, the implementation of such PNC systems poses many challenges. For example, the relay in a PNC system must be able to deal with symbol and carrier-phase asynchronies of the simultaneous signals received from multiple nodes, and the relay must perform channel estimation before detecting the signals. We investigate a PNC implementation in the frequency domain, referred to as FPNC, to tackle these challenges. FPNC is based on OFDM. In FPNC, XOR mapping is performed on the OFDM samples in each subcarrier rather than on the samples in the time domain. We implement FPNC on the universal soft radio peripheral (USRP) platform. Our implementation requires only moderate modifications of the packet preamble design of 802.11a g OFDM PHY. With the help of the cyclic prefix (CP) in OFDM, symbol asynchrony and the multi-path fading effects can be dealt with simultaneously in a similar fashion. Our experimental results show that symbol-synchronous and symbol-asynchronous FPNC have essentially the same BER performance, for both channel-coded and non-channel-coded FPNC systems. This paper presents the first real-time physical-layer network coding (PNC) prototype for the two-way relay wireless channel (TWRC). Theoretically, PNC could boost the throughput of TWRC 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 few experimental and implementation efforts. We built the first prototype of PNC about a year ago. It was, however, an offline system in which an offline PNC decoder was used at the relay. For a real-time PNC system, there are many additional challenges, including the needs for tighter coordination of the transmissions by the two end nodes, fast real-time PNC decoding at the relay, and a PNC-compatible retransmission scheme (i.e., an ARQ protocol) to ensure reliability of packet delivery. In this paper, we describe a real-time PNC prototype, referred to as RPNC, that provides practical solutions to these challenges. Indoor environment experimental results show that RPNC boosts the throughput of TWRC by a factor of 2 compared with TS, as predicted theoretically. RPNC prototype provides an interface to the application layer, with which we demonstrate the exchange of two image data files between the two end nodes. 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. The design of a reliable physical-layer network coding (PNC) scheme for practical fading two-way relay channels is a challenging task. This is because the signals transmitted by two users arrive at the relay with varied amplitudes and a relative carrier-phase offset, which will impair the performance of PNC. This paper studies a linear PNC scheme for fading two-way relay channels where the transmitters lack the channel state information. In this scheme, the relay computes and broadcast some finite-set integer combinations of two users' messages. The coefficients for the integer combinations used at the relay are carefully designed to minimize the error probability. This scheme can be viewed as a practical embodiment of the compute-and-forward concept. We develop a new LPNC design criterion called minimum set-distance maximization. Using this criterion, we derive an explicit expression for the optimized integer coefficients that minimizes the error probability of LPNC. The optimized integer coefficients turn out to resemble the fading channel coefficients. We further derive a closed-form expression on the average error probability performance over a complex-valued Rayleigh fading two-way relay channel, which shows that our designed LPNC scheme approaches the optimal error performance at a high SNR. Numerical results show that our designed LPNC outperforms existing schemes by more than 5 dB at a medium-to-high SNR regime.