Optical Physical-layer Network Coding over Fiber-Wireless Zhixin Liu (1) , Lu Lu (2) , Lizhao You (2) , Chun-Kit Chan (1) , and Soung-Chang Liew (1) (1) Deptof Information Engineering, The Chinese University of Hong Kong. zxliu@ie.cuhk.edu.hk (2) Institute of Network Coding, The Chinese University of Hong Kong. lulu@inc.cuhk.edu.hk Abstract We propose and experimentally demonstrate the first optical physical-layer network coding (OPNC) prototype to boost throughput in an OFDM fiber-wireless network. Our technique does not require symbol-level synchronization and only requires moderate modifications of the packet preamble in IEEE 802.11 standard specification. Introduction Hybrid fiber-wireless (FiWi) network is a promising technology for providing ubiquitous high-capacity connectivity for mobile terminals, wireless local area networks (WLANs), and fixed wireless access services 1 . The access point (AP) in a FiWi network only serves as an analog front-end that directly converts the wireless signals into optical signals without demodulation. An issue is how best to transmit these wireless- overlaid optical signals to their destinations. Recently, physical-layer network coding (PNC) has attracted much attention in the wireless communication community. It emulates full-duplex transmission in a two-way relay channel (TWRC) by allowing two communicating users in a star-topology network to transmit signals to each other simultaneously 2 . Doing so greatly improves the network throughput. With a similar network topology, the throughtput of FiWi networks can also be improved with PNC. Conventional PNC is realized by electronic logic operations at the relay (i.e., PNC router 3 ), as shown in Fig. 1(a). However, in FiWi, the signals arriving at the PNC router are optical signals, and optical logic operations 4 require stringent synchronization and complicated optical signal processing. Recently, we have successfully demonstrated an optical physical- layer network coding system 5 named OPNC. In OPNC, the optical signals from the two communicating users are coupled together. The superimposed optical signals are then broadcasted to the two users. Each of the users then obtains the signal from the other user by subtracting its own signal from the superimposed signals. In this paper, we incorporate the key concept of OPNC in a FiWi network. Unlike our previous demonstration, our current prototype adopts the Orthogonal Frequency Division Multiplexing (OFDM) frame format as specified in the widely adopted 802.11n WLAN standards. Binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK) are assumed in our system. To the best of our konwlege, this is the first feasibility demonstration of OFDM OPNC in FiWi networks. Notably, our OFDM OPNC scheme does not require symbol-level synchronization of the OFDM packets, and only requires moderate modifications of the packet preamble in 802.11 OFDM physical layer specifications (PHY). Principle of Operation Consider a four-node network as depicted in Fig. 1(a). Four APs are connected through a single router in a star topology. Suppose that node A (AP A ) and node B (AP B ) want to exchange two packets with each other. Fig. 1(b) compares traditional scheduling and PNC scheduling for this purpose. In traditional scheduling, node A sends its packet P A to the PNC router in the first time slot. The PNC router then forwards it to node B in the second timeslot. Two more time slots are needed for the delivery of packet P B from node B to node A in a similar fashion. A total of four timeslots are needed. PNC scheduling can achieve the same goal with two timeslots. In the first time slot, nodes A and B transmit their packets simultaneously to the PNC router. Let   and   denote the signals of packets P A and P B , respectively. These signals are superimposed on each other to form       , the arithmetic sum of the two signals. The router then broadcasts   to both (a) (b) Fig. 1: (a) Proposed system architecture. (b) Comparison of the scheduling approach Mo.3.F.3.pdf