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Emerging techniques such as Wi-Fi direct makes the objective of always-on be true. People can easily chat and share files with nearby friends even without AP (Access Point) or cellular coverage. In this paper, we focus on the channel efficiency issue of AP-free Wi-Fi networks, which can be easily constructed in the subway, in a high-speed railway, or when camping in the wild. Today IEEE 802.11 DCF is the most commonly used MAC protocol for Wi-Fi networks, however, due to the control messages and backoff time, channel efficiency in high data rate networks can be extremely low. To solve this, we pro-pose CD-MAC, which allows control messag-es to be transmitted with data packets concur-rently, and thus eliminates the overheads of backoff and explicit contention. To maintain the reception reliability, we redesign the con-trol messages and use signal detection in PHY instead of bits decoding to detect them. In MAC layer, CD-MAC is built upon our Cor-relation Detection based PHY. We have imple-mented and evaluated CD-MAC using USRP N210. Evaluation results show that CD-MAC can achieve over 95.5％ channel efficiency and provide throughput gains of up to 80％, 60％, and 29.1％ compared with DCF, 802.11ec, and back2F, respectively.