Robust channel estimation in wireless LANs for mobile environments

The recently developed wireless LAN systems, HIPERLAN/2 and IEEE 802.11a, operate in the 5 GHz frequency range and were originally designed for stationary terminals in slowly time-varying environments. Hence, pilot symbols for channel estimation are only provided at the beginning of a burst (in the preamble), since the channel behaviour is assumed not to change significantly over the burst duration. In order to support new scenarios with higher terminal mobility, it is essential to track the changes of the channel transfer function over time. We focus on techniques for a robust channel estimation, mainly based on enhanced pilots (EP). Three different channel tracking schemes are developed and applied to the wireless LAN systems: (1) use of scattered pilots; (2) use of midambles; (3) an additional decision-directed technique, combined with the midamble concept. The performance of the different methods is studied and compared quantitatively. As a result, suitable channel tracking techniques for different terminal speeds and data rates are proposed.