Gagik Mkrtchyan

Doppler Spread Estimation Method for OFDM Signal Using Mean Square of Channel Impulse Response's Time Derivative

Many applications of OFDM systems require Doppler spread estimation. This is quite difficult in multi-path fading channels with no strong direct path. This letter proposes a novel Doppler spread estimation method, which uses the mean square (MS) value of channel impulse responses time derivative. The proposed method is very simple compared with the previously proposed methods. Simulation results show that it allows easy and precise Doppler spread calculation for OFDM by using the channel estimation based on either pilot tones or pilot symbols.

Model Based Prediction of Uplink Multi-Path Fading Channel Response for Pre-Equalization in Mobile MC-CDMA Systems

Multi-carrier code division multiple access (MC-CDMA) has been considered as one of the promising techniques for the next generation of mobile communication systems because of its efficient bandwidth usage, robustness to the multi-path fading and simple channel-sharing scheme. However, MC-CDMA cannot be employed in the uplink communication where the transmitted signal from each user propagates through the different multi-path fading channel, and the received signals are no longer orthogonal at the base station. As a result, bit error rate (BER) performance in the uplink MC-CDMA communication would be strongly degraded due to the occurrence of multi-user interference (MUI). To solve the MUI problem in the uplink MC-CDMA, the pre-equalization method was proposed in which the uplink signal is pre-equalized at the user terminal by using the channel response estimated from the downlink. Although the pre-equalization method is very effective for the stationary uplink channel with fixed users, it is hard to be employed in the time varying fading channel with mobile users, because there is a big difference in the channel responses between downlink and uplink. For the efficient MUI compensation, each user terminal would be required to predict the future channel conditions based on the current observation. This paper proposes a method for model based uplink channel response prediction by employing the spectral decomposition of the downlink channel impulse response. Computer simulation results show that the proposed method can achieve the accurate prediction of channel response for mobile users during the uplink transmission and allows the effective MUI compensation.

Correction of OFDM Signal Form in Time Domain to Reduce ICI Due to the Doppler Spread and Carrier Frequency Offset

This paper analyzes the immediate cause of the ICI in OFDM systems due to the Doppler spread and carrier frequency offset. As shown, ICI occurs because of the using DFT (FFT) and IDFT (IFFT) for signal conversion from time domain into frequency domain, and vice versa, when the sampled signal has limited duration. Proposed method refines the spectral density function of subcarriers, when applied in the transmission side, and improves the characteristics of the DFT as a digital filter, when applied in the receiver side. Simulation includes, working under the same conditions, models of conventional OFDM, PCC-OFDM and proposed method. Results of the simulation show that proposed method provides up to the 10 times less BER than PCC-OFDM and up to the 100 times less BER than conventional OFDM, which allows using of the OFDM in the mobile communication for vehicle speed up to the 500 km/hr.

Prediction of time-varying multipath fading channel for pre-equalization of the signal in uplink mobile MC-CDMA communications

MC-CDMA is hard to be employed for uplink transmission because signals of users propagate through the different multi-path fading channels and lose orthogonality. To solve this problem pre-equalization must be used by users employing the downlink channel response. The pre-equalization is effective for stationary channels with fixed users, but not for time-varying channels, since there is a big difference between downlink and uplink channel responses. For time-varying channels users should predict the uplink response from downlink observation. This paper proposes a method for the prediction by means of the spectral decomposition of downlink response. Computer simulation results show that the proposed method provides accurate prediction for high mobility users during the whole uplink transmission

Evolutional Algorithm Based Learning of Time Varying Multipath Fading Channels for Software Defined Radio

Software defined radio, which uses reconfigurable signal processing devices, requires the determination of multiple unknown parameters to realize the potential capabilities of adaptive communication. Evolutional algorithms are optimal multi dimensional search techniques, and are well known to be effective for parameter determination. This letter proposes an evolutional algorithm for learning the mobile time-varying channel parameters without any specific assumption of scattering distribution. The proposed method is very simple to realize, but can provide precise channel estimation results. Simulations of an OFDM system show that for an example of OFDM communication under the time-varying fading channel, the proposed learning method can achieve the better BER performance.

Dversity acquisition from inter-carrier interference in the frequency division based mobile communication

This paper describes how to achieve a frequency diversity gain from ICI (inter-carrier interference) in the FDM (frequency division multiplexing) based communication. If in the OFDM (orthogonal FDM) the CP (cyclic prefix) is not employed then propagated through the multipath-fading channel subcarriers are not more orthogonal, and in the receiver side along with the ISI (inter-symbol interference) appears also the ICI, which is especially strong in the high mobility case. It was believed that ICI is always the degradation factor, but we show that actually can be got advantage from it. The performance of the proposed scheme was compared with conventional OFDM, and results of computer simulation show that it provides from 10 to 50 times lower BER (bit error rate).