Adaptive Subcarrier Nulling: Enabling partial spectrum sharing in wireless LANs

Emerging WLAN standards have been incorporating a variety of channel widths ranging from 5MHz to 160MHz, in order to match the diverse traffic demands on different networks. Unfortunately, the current 802.11 MAC/PHY is not designed for the coexistence of variable-width channels. Overlapping narrowband channels may block an entire wide-band channel, resulting in severe spectrum underutilization and even starvation of WLANs on the wide-band. A similar peril exists when a WLAN partially overlaps its channel with multiple orthogonal WLANs. In this paper, we propose to solve the problem of partial spectrum sharing using Adaptive Subcarrier Nulling (ASN). ASN builds on the 802.11 OFDM PHY, but allows the radios to sense, transmit, detect, and decode packets through spectrum fragments, or subbands. An ASN transmitter can adapt its spectrum usage on a per-packet basis, by nulling the subbands used by neighboring WLANs, and sending packets through the remaining idle subbands. ASN preserves the 802.11 CSMA/CA primitives while allowing users to contend for access to each subband, and can opportunistically exploit the merits of wide-band channels via spectrum aggregation. We have implemented and evaluated ASN on the GNURadio/USRP platform. Our experimental results have shown ASN to achieve detection and decoding performance comparable to the legacy 802.11. Our detailed simulation in ns-2 further shows that ASN substantially improves the efficiency and fairness of spectrum sharing for multi-cell WLANs.