Showing papers by "Sheetal Kalyani published in 2004"

Quantised decision based gradient descent algorithm for fast fading OFDM channels

Abstract: For very fast fading OFDM channels robust channel tracking is needed. We propose a quantised decision based gradient descent algorithm (QDGDA) which exploits the fact that the fading in mobile environments is correlated across time. A single tap NLMS tracker is run across time for each subcarrier in an OFDM symbol to track the channel. To make the algorithm less sensitive to error propagation, the gradient estimate has to be continuously and correctly updated, (almost) irrespective of any decision error. To obtain reliable gradient information for the NLMS algorithm at time instant n, we use a weighted combination of the gradient at time n and time n-1. These time varying weights are a function of reliability of the decision made on data symbols.

Spectrally weighted frequency domain equalizer for mobile OFDM

Abstract: Orthogonal FDM (OFDM) systems are inherently sensitive to fast fading channels. In this paper, tracking techniques applicable for the frequency domain equalization (FDE) of a fast fading (mobile) OFDM system are proposed. The fact that OFDM systems are over parameterized in the frequency domain (compared to the time domain where the maximum impulse response possible is designed to be less than cyclic prefix) is exploited to derive the spectrally weighted frequency domain equalizer (SWFDE). An error weighted channel tracking and estimation technique is proposed which exploits the diversity due to constrained impulse response length. The proposed SWFDE is particularly useful for narrowband OFDM systems and also implicitly reflects guard tone positions in deriving the weights. For medium to high normalized Doppler fading the SWFDE technique gives a 2 dB gain in error rate performance when compared with a conventional least squares FDE.