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.
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). The goal of this paper is to show how 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. PNC can yield higher capacity than straight-forward network coding when applied to wireless networks. 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. PNC opens up a whole new research area because of its implications and new design requirements for the physical, MAC, and network layers of ad hoc wireless stations. The resolution of the many outstanding but interesting issues in PNC may lead to a revolutionary new paradigm for wireless ad hoc networking. The applicability of analog network coding (ANC) to a wireless network is constrained by several limitations: 1) some ANC schemes demand fine-grained frame-level synchronization, which cannot be practically achieved in a wireless network; 2) others support only a specific type of modulation or require equal frame size in concurrent transmissions. In this paper, a new ANC scheme, called restriction-free analog network coding (RANC), is developed to eliminate the above limitations. It incorporates several function blocks, including frame boundary detection, joint channel estimation, waveform recovery, circular channel estimation, and frequency offset estimation, to support random concurrent transmissions with arbitrary frame sizes in a wireless network with various linear modulation schemes. To demonstrate the distinguished features of RANC, two network applications are studied. In the first application, RANC is applied to support a new relaying scheme called multi-way relaying, which significantly improves the spectrum efficiency as compared to two-way relaying. In the second application, RANC enables random-access-based ANC in an ad hoc network where flow compensation can be gracefully exploited to further improve the throughput performance. RANC and its network applications are implemented and evaluated on universal software radio peripheral (USRP) software radio platforms. Extensive experiments confirm that all function blocks of RANC work effectively without being constrained by the above limitations. The overall performance of RANC is shown to approach the ideal case of interference-free communications. The results of experiments in a real network setup demonstrate that RANC significantly outperforms existing ANC schemes and achieves constraint-free ANC in wireless networks. Traditionally, interference is considered harmful. Wireless networks strive to avoid scheduling multiple transmissions at the same time in order to prevent interference. This paper adopts the opposite approach; it encourages strategically picked senders to interfere. Instead of forwarding packets, routers forward the interfering signals. The destination leverages network-level information to cancel the interference and recover the signal destined to it. The result is analog network coding because it mixes signals not bits. So, what if wireless routers forward signals instead of packets? Theoretically, such an approach doubles the capacity of the canonical 2-way relay network. Surprisingly, it is also practical. We implement our design using software radios and show that it achieves significantly higher throughput than both traditional wireless routing and prior work on wireless network coding.