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 deconvolution, 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.

Receivers for wireless Orthogonal Frequency Division Multiplexing (OFDM)

This paper proposes a new blind approach for time synchronization of orthogonal frequency division multiplexing (OFDM) receivers (RX). It is largely known that the OFDM technique has been successfully applied to a wide variety of digital communications systems over the past several years — IEEE 802.16 WiMax, 3GPP-LTE, IEEE 802.22, DVB T/H, ISDB-T, to name a few. We focus on the synchronization for the ISDB-T digital television system, currently adopted by several South American countries. The proposed approach uses the coarse synchronization to estimate the initial time reference and then, the fine synchronization keeps tracking the transmitter (TX) time reference. The innovation on the proposed approach regards to the closed loop control stabilization of the fine synchronization. It uses a smith predictor and a differential estimator, which estimates the difference between TX and RX clock frequencies. The proposed method allows the RX to track the TX time reference with high precision ([Formula: see text] sample fraction). Thus, the carriers phase rotation issue due to incorrect time reference is minimized, and it does not affect the proper RX IQ symbols demodulation process. The RX internal time reference is adjusted based on pilot symbols, called scattered pilots (SPs) in the context of the ISDB-T standard, which are inserted in the frequency domain at the inverse fast Fourier transform (IFFT) input in the TX. The averaged progressive phase rotation of the received SPs at the fast Fourier transform (FFT) output is used to compute the time misalignment. This misalignment is used to adjust the RX fine time synchronism. Furthermore, the proposed method has been implemented in an ISDB-T RX. The FPGA-based receiver has been evaluated over several multipath, Doppler and AWGN channel models.

Receivers for wireless Orthogonal Frequency Division Multiplexing (OFDM)