Exploiting Frame Preamble Waveforms to Support New Physical-Layer Functions in OFDM-Based 802.11 Systems

The frame preamble in current WiFi systems is designed to facilitate various PHY-layer functions, including frequency offset estimation and frame detection. However, this preamble is typically fixed and is never used to convey any user-specific bits. Embedding information into the preamble opens the door for several new PHY-layer applications. For example, the PHY header no longer needs to be transmitted at a known (lowest) rate if this rate can be announced earlier in the preamble. A full-duplex transmitter can use the embedded information to inform other devices of its current operation mode (e.g., transmit/receive versus transmit/sense), obviating the need for additional control packets. In security applications, a PHY-layer sender identifier can be embedded in the preamble to facilitate PHY-level encryption. However, modifying the standard preamble to embed user information may disrupt the operation of 802.11a/n/ac devices. In this paper, we propose P-modulation, a method that enables an OFDM-based 802.11 transmitter to embed up to 19 user-specific bits in the frame preamble while maintaining the highest reliability required by the system. The proposed P-modulation is also backward-compatible with legacy receivers. Our analysis and USRP-based experimental results confirm the practicality of the scheme. Our scheme further provides insights into designing time-varying preambles for future wireless systems.