Adaptive time synchronization for VHT wireless LAN

Abstract

IEEE 802.11ac is an upcoming standard for wireless LAN physical layer and MAC layer for very high throughput (VHT) communication with targeted throughput greater than 1 Gbps [1] below 6 GHz band. The IEEE 802.11ac (also termed as VHT wireless LAN) physical layer is based on orthogonal frequency division multiplexing (OFDM) modulation and signal processing with multiple antennas at transmitter and receiver (MIMO). A device compliant to IEEE 802.11ac standard should be backward compatible to wireless LAN standards IEEE 802.11a and IEEE 802.11n. The VHT Physical layer supports legacy frame format, high throughput (HT) mixed frame format, HT green-field frame format and VHT mixed format [2, 3]. The VHT wireless LAN receiver has to synchronize for any of these frame formats. The OFDM modulation is sensitive to timing synchronization in multipath fading channel. The cyclic shift diversity (CSD) used in VHT wireless LAN introduces pseudo multipath at the receiver and causes error in the symbol boundary estimate. Hence, there is a need for a robust and adaptive time synchronization that suits for all frame formats in VHT wireless LAN. Without the accurate time synchronization, there will be inter symbol interference (ISI) and inter carrier interference (ICI) in the demodulated data, and cause many bit errors in the decoded data. In this paper, we propose a robust and adaptive timing synchronization technique for VHT Wireless LAN device compliant to IEEE 802.11ac standard. We will show the need for the adaptive window length in time synchronization theoretically. The proposed method works well for all frame formats and BW options of the IEEE 802.11ac standard. We will give simulation results to show the usefulness of the method.