Radio channel (wireless) including channels that are highly regarded and have been used. These channels are affected by the environment and change. Multipath propagation leads to rapid fluctuations of the phase and amplitude of the signal. In contrast to these channels, using Orthogonal Frequency Division Multiplexing (OFDM) technique is much better and more efficient. In order to access the basic information in the receiver, correct estimation impulse response is the main criterion. The current channel estimation methods can be classified into two categories. The first one is based on the pilots and the second one is not based on using pilot. A channel estimation method, the original channel estimation techniques using statistical information that are very useful and satisfying. This method provides our initial favorable data according to statistical information channel. In this paper presents, several common methods such as Wiener filter, Kalman filter and MMSE for channel estimation in OFDM system are evaluated and compared.    تفاصيل المقالة

Equalization is a kind of the preferred methods for increasing the efficiency of modern digital communication systems. The paper contribution lies in using a modified version of Sliced Mul-ti-Modulus Algorithm (S-MMA) and genetic algorithm (GA) for updating per-tone equaliza-tions taps in the OFDM modulation using too long complicated fading channel, present of dif-ferent noises and ISI impairment. For more efficiency, it is assumed that the channel impulse response is longer than the cyclic prefix length and as a result, the system will be more efficient but at the expense of the high ISI impairment. Both mathematical analysis and computer simu-lations demonstrate the better performance of the new proposed application of the S-MMA and GA are compared to commonly used equalizer methods such as the LMS, RLS and multi modu-lus algorithm (MMA) in the channels with AWGN, burst noises and ISI impairment simultane-ously. Therefore, the new modified S-MMA equalization and GA are good candidates for high speed and real-time applications such as DAB, WiFi, WiMAX, and DVB systems. تفاصيل المقالة

In this paper, an asymptotic estimating technique to detect direct sequence spread spectrum (DSSS) signals using modified frequency domain fourth-order cumulant is proposed. Based on 2-D slices fourth-order cumulant, an efficient algorithm is proposed to solve the problem of complex computation and huge processing time. The method of fourth-order cumulant in frequency domain can achieve detection probability above 50% at SNR -25dB. In this paper, we strongly decrease processing time and complexity of computations that it makes this method appropriate for employing in the DSSS detection systems. This method only requires to store a row vector while ignoring all the low and diagonal elements and therefore the required memory is highly reduced. In addition, as it uses two-line search strategies (predictor and corrector) to obtain a new iterative point, the spectral properties of the diagonal updating scheme is improved, and rapid convergence property is gained. Also, this method estimates carrier frequency and symbol rate simultaneously in the online processing. Simulation results and computation analysis indicate that processing time of the proposed method is decreased about 30% with respect to the commonly used fourth-order cumulant methods. تفاصيل المقالة