Effects of indoor environments on the coherence bandwidth and the beamformer patterns of a null steering smart antenna for infrastructure Wireless Local Area Networks

The effect of the dimensions of an arbitrary indoor environment on the coherence bandwidth of the wireless wideband channel as well as its impact on a smart antenna beamformer patterns is investigated. A deterministic ray-tracing channel modelling tool is first used to evaluate and compare the Power-Delay-Angle Profile (PDAP) for two arbitrary indoor architectures of different dimensions as well as other parameters of interest like the delay spread, the angular spread and approximately estimating the coherence bandwidth. The coherence bandwidth is also evaluated using the frequency autocorrelation function and compared with the approximate calculation. The frequency dependence of the weights and patterns of a null steering beamformer, serving four simultaneous users, are compared for both architectures. A weight interpolation algorithm is then proposed targeting infrastructure Wireless Local Area Network (WLAN) applications in indoor scenarios following the IEEE802.11a or HIPERLAN/2 standards. It is shown that for a small indoor environment with a large coherence bandwidth, the beamformer weights can be estimated two or three times over a band of 52 OFDM subcarriers and interpolated for the rest of the subcarriers. For a large indoor environment with a small coherence bandwidth the weights need to be estimated every two or three subcarriers.