Fine Grained Channel Access in Wireless LAN

Modern communication technologies have been driven up the physical layer (PHY) data rate in wireless local area networks (WLAN), to hundreds of Mbps in current 802.11n and over Gbps in the near future. However, due to the disproportional increase of media access control (MAC) overhead compared to the useful data transmission time as the PHY data rate goes up, the throughput efficiency in these high-data-rate WLANs goes down rapidly. This requires a fundamental change to the current media access control (MAC) protocol design, which has always been allocating the whole channel as one unit. This paper argues that, in a high-data-rate WLAN, the whole channel width should be divided into proper size subchannels commensurate with PHY data rate and typical frame size. When OFDM is used to eliminate the interchannel guard-bands, the fundamental challenge is how to coordinate the transmissions as WLAN is distributed and asynchronous in nature. We proposes FICA, a fine-grained channel access method to address the inefficiency issue in current WLAN. FICA addresses this challenge with two novel techniques. First, FICA designs a new PHY architecture based on OFDM that retains orthogonality among subchannels solely relying on the coordination mechanisms in existing WLAN i.e. carrier-sensing and broadcasting. Second, FICA employs a frequency-domain contention that uses physical layer RTS/CTS signaling and frequency domain backoff to efficiently coordinate the subchannel access. We have implemented FICA, both MAC and PHY using software radio. Our experiments demonstrate the feasibility of FICA design. And our simulation results show FICA can improve the efficiency ratio of WLAN by up to 400% compared to existing 802.11.