Wavelet Based Detection of Shadow Fading in Wireless Networks

In wireless communications, shadow fading can cause at least 6 dB power loss for 10% of the time [1]. Early detection of shadow fading plays an important part in facilitating the design of adaptive data transmission schemes. We propose an accurate, on-line detection mechanism to detect a receiver entering or leaving shadow regions using simply signal strength measurements. The method is based on wavelet decomposition of signal strength time series into independent fading components. Our measurements indicate that fast fading signals have a scale invariant nature. This scale invariance of fading signals is destroyed when shadow fading, which is log-normal distributed and is independent of fast fading components, is added to the signal. An online detection mechanism is then proposed which exploits this phenomenon. Real measurements of signal strength traces are used to validate the detection mechanism.