Abstract
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 between pilot carriers is applied 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, the fast channel variations exceed the time interpolator capability, as is the case for mobile operation. In this article we present a new channel compensation technique based on the concurrent operation of two stochastic gradient timedomain algorithms, one which minimizes a cost function that measures the received signal energy dispersion and other which minimizes the Euclidean distance between the received digital modulation symbols and the ones in the reference constellation assigned to each OFDM sub-channel. Results show that the new technique advantageously improves the system robustness to fast channel variations since, with a low computational cost, it dramatically reduces the demodulator symbol error rate even when the receiver is operating in an intense dynamic multipath scenario.
Highlights
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The Concurrent Equalizer (CE) is a blind deconvolution algorithm based on the principle of the concurrent operation between the direct decision (DD) equalizer and the constant modulus (CMA) equalizer [2]
The minimization of the Euclidean distance based DD cost function only takes place when the minimization of the energy dispersion based CMA cost function is judged to have achieved a successful adjustment with high certainty
Summary
Where 6Q L is the IQ symbol applied to the QWK carrier of the LWK OFDM symbol at the transmitter and 1 Q L is the respective additive channel noise. Q′ to which a pilot symbol is assigned, the estimation of +Ö Q′ L for a particular time index L is performed by a time direction interpolation filter with impulse response given. The CE performs a further adjustment on the channel transfer function estimates obtained from a linear interpolation procedure between two adjacent pilot symbols in frequency direction. As in the Wiener filter case, since the +Ö Q′ L estimates from (16) are obtained for all frequencies Q′ to which a pilot symbol is assigned, they can be used with any given time index L as pilot symbol references in the subsequent frequency domain linear interpolation:. Notice that +Ö Q L given by (17) yields the estimates of the channel transfer function for all frequencies and time indexes of the OFDM frame represented in the grid.
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