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The number of mobile terminals keeps a sustaining growth, and brings the importance of wireless network to a new level. Wireless networks are supposed to provide higher capacity for broadband services, as well as to be as robust and fast as wired network. The reason for the distinct gap between the two is that in wireless networks, the signal transmitted by a node may reach several other nodes, and a node may receive signals from several other nodes simultaneously. This broadcast nature is traditionally considered harmful, because it may cause interference or even collision. However, the novel concept of network coding turns this drawbacks into a capacity boosting potential advantage. Network coding is a technique where the nodes of a network take several packets and combine them together for transmission instead of simply relaying the packets they receive. In this paper, we proposed a physical-layer network coding scheme which is simple to implement for OFDM system. We apply this scheme in 802.11g system to demonstrate that with modified frame format, intentionally collided messages can be decoded and improve network throughput as well as spectrum efficiency.