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.

I. Introduction

Wireless systems continue to boost their performance and enhance their security at the physical (PHY) layer by employing various techniques, including MIMO and artificial noise for improved transmission security. However, in these wireless systems, the preamble is never utilized for conveying user-generated bits at the PHY layer; a feature that can facilitate new functionalities related to enhanced performance and/or security. In a wireless system, the frame preamble is a special and often publicly known signal, prepended to the (modulated) PHY-layer header at the transmitter (Tx) and used by the receiver (Rx) to perform several PHY-layer functions. These functions include frame detection, frequency offset (FO) estimation, channel state information (CSI) estimation, dynamic range estimation (used for automatic gain control (AGC) convergence), etc. In this paper, we consider the frame preamble in OFDM-based WiFi systems (802.11a/n/ac). The duration of this preamble in 802.11a [1] is and can be up to in 802.11n/ac MIMO systems [2], [3], irrespective of the size and transmission rate of the PHY-layer payload. So depending on the payload size and the transmission rate, the preamble in 802.11a systems can take up to 50% of the frame duration, as shown in Table I, and even more in 802.11n/ac MIMO systems. The preamble in these systems is under-utilized because it is assumed to be constant and is not designed to carry any user-generated bits.TABLE I Percentage of the Preamble Duration in a Frame for Different PHY-Layer Payload Sizes and Data Rates (802.11a Systems)

Packet type	Size (bytes)	6 Mbps	18 Mbps	36 Mbps	54 Mbps
TCP ACK	72	13.8%	30.8%	44.4%	50%
WLAN MTU	2324	0.5%	1.5%	3.0%	4.3%