Adaptive Concurrent Equalization Applied to Multicarrier OFDM Systems

Abstract

Receivers for wireless Orthogonal Frequency Division Multiplexing (OFDM) systems usually perform the channel estimation based on pilot carriers in known positions of the channel spectrum. Interpolation is applied between pilot carriers to determine the channel transfer function in all carrier frequencies. Channel variations along time are compensated by means of interpolation between successive channel estimates on the same carrier frequency. However, not rarely do fast channel variations exceed the time interpolator capability, as is the case for mobile operation. In this article we propose a new technique based on concurrent de- convolution, as a mean to further increase the time interpolator capability. Concurrent deconvolution, also known as concurrent equalization, is based on the concurrent operation of two stochastic gradient time-domain algorithms. One gradient-based algorithm minimizes a cost function that measures the received signal energy dispersion and the other minimizes the Euclidean distance between the received digital modulation symbols and the ones in the reference constellation which are assigned to each OFDM sub-channel. Results show that the proposed technique, when subsequently applied to the time interpolation stage, improves the system robustness for fast varying channels. The whole channel compensation computational cost is increased by the cost of a two coefficient multirate adaptive FIR filter, added only to those subcarriers at the FFT output to which a pilot sequence has not been assigned.