Simulation Model for a Frequency-Selective Land Mobile Satellite Communication Channel

This paper investigates a three-state simulation model for a frequency-selective land mobile satellite communication (LMSC) channel. Aside from ionospheric effects, the propagation channels for LMSC systems are also characterized by wideband effects due to multipath fading which makes the channels time-variant and exhibit frequency-selective distortion. Hence, an adequate knowledge and modelling of the propagation channel is necessary for the design and performance evaluation of the LMSC systems. A three-state simulation model for a frequency-selective LMSC channel, which is a combination of Rayleigh, Rician and Loo fading processes, is developed. The propagation characteristics of the proposed LMSC channel model are presented, and comparisons are made with the Rayleigh, Rician and Loo fading channels using bit error rate (BER) as the figure of merit. The simulation results show that the degree of fading experienced by the LMSC link depends on the length of time the mobile terminal is in a particular state or location, depending on the assumed probability of occurrence of each fading process; and it is observed from the BER results that the propagation impairment of the LMSC fading channel is relatively lower than that of Rayleigh and Loo fading channels but higher than the Rician fading channel. Keywords: mobile, multipath fading, propagation channel, satellite communication, wideband.

Investigating the Convergence and Bit Error Rate of Adaptive Algorithms over Time Varying Rayleigh Fading Channel

The fastest growing segment of the communication industry is the mobile wireless communication system. However, the systems faced a lot of challenges such as delay in the propagation of signals due to time-varying channel and effect of high speed transmission over Rayleigh fading which result into Inter-Symbol Interference (ISI) distortion. Least Mean Square (LMS) and Normalized Least Mean Square (NLMS) have been previously used to adapt the system using the step size, and Eigen value. In this paper, the adaptive Algorithms over a time-varying channel were compared using convergence level, Bit Error Rate (BER), and Mean Square Error (MSE). The system model consists of bits to symbol converter, 16-QAM modulator and Raised Cosine transmit filter, all at the transmitter, time-varying Rayleigh fading with Additive White Gaussian Noise added, and at the receiver are Raised Cosine Receive filter, 16-QAM demodulator, then each of the Adaptive LMS and NLMS filters which received delay from the Random integer generator, and the integer/symbol to bit converter at the output. The system model was simulated using MATLAB/SIMULINK software package. The algorithms were evaluated using convergence MSE at SNR of 10, 20 and 30dB over different number of iterations to determine the convergence rate, constellation diagram and BER. The results obtained showed that the flat convergence level of LMS and NLMS at SNR of 10dB are obtained with 300 and 200 iterations respectively, while 200 and 150 iterations are obtained at SNR of 20 and at SNR 30, the convergence level are obtained at 150 and 100 iterations respectively. BER values of 0.1598 and 0.0858 are obtained for LMS and NLMS respectively. Therefore, LMS algorithm took more iterations than NLMS algorithm to achieve the same error, and also lower BER value of NLMS is also in agreement with the result. Keywords: Convergence, MSE, LMS algorithm, NLMS algorithm, Intersymbol interference (ISI)

Diversity Combiner in Adaptive Modulation over Fast and Frequency Selective Environment

Signal over fast and frequency selective channel suffers from Doppler and delay effects due to propagation mechanism such as reflection, refraction and diffraction resulting in poor quality reception. Maximal Ratio Combining (MRC) and Adaptive Modulation are some of the techniques previously used to address this problem, but each of these techniques suffers from signal fading and interference distortion as result of weak signal and delay spread respectively. Therefore, an Adaptive modulation technique which incorporates MRC is developed over fast and frequency selective Rayleigh fading channel. The system model in this study employed 10,000 bits randomly generated, gray encoded and modulated with M-ary Phase Shift Keying (M-PSK). The signals were filtered using square root raised cosine filter and then transmitted over fast and frequency selective Rayleigh fading channel. At the receiver, two paths at 100km/hr and 200km/hr were combined using MRC, the channel was estimated using Received Signal Strength Indicator (RSSI) to change the constellation size of the modulation in accordance with the severity of fading. The process was simulated using MATLAB software package. The performance of the proposed system was evaluated using Bit Error Rate (BER) at mobile speeds of 100km/hr and 200km/hr. At Signal to Noise Ratio (SNR) of 10dB, the BER values of 0.0003, 0.0013, 0.0686, and 0.3009 were obtained for conventional MRC with BPSK, QPSK, 8PSK, and 16PSK signaling scheme respectively as against 0.0011 for adaptive MPSK at a mobile speed of 100km/hr while at 200km/hr, the BER values of 0.0134, 0.0161, 0.1947, 0.4116 were obtained using MRC with BPSK, QPSK, 8PSK and 16PSK respectively as against 0.0134 for adaptive MPSK. In conclusion, adaptive modulation incorporating MRC gave the best result due to lower BER values obtained at all SNR considered. The effect of fast and frequency selective Rayleigh channel has been reduced at high speed. Keywords: Adaptive Modulation, Maximal Ratio Combining , M-PSK, Rayleigh Environment, Bit Error           Rate (BER), Signal to Noise Ratio (SNR)

Comparative Analysis of CMA and MMSE in MIMO-OFDM System

Channel estimation is one of the techniques used to achieve high data rates and low bit error rates in wireless communications. In wireless communication system, where Multiple Input Multiple Output - Orthogonal Frequency Division Multiplexing (MIMO-OFDM) exists, the effect of channel causes the received signal to be distorted which necessitates the receiver to have an insight of the channel known as the channel estimation. However, most of the existing techniques such as Least Square Error (LSE), Minimum Mean Square Error (MMSE) and Best Linear Unbiased Estimation Algorithm (BLUE) employ pilot symbols. High errors are observed in addition to computational complexity and in the platform other than MIMO-OFDM. In this paper, performances of Constant Modulus Algorithm (CMA) and MMSE are evaluated, and compared with each other in the 3x3 MIMO-OFDM systems. The system model for 3x3 MIMO-OFDM system incorporating each of CMA and MMSE consists of a transmitter, frequency selective channel and the receiver. 1000 bits are generated randomly and served as input signal. Three antennas configurations at the input of the frequency selective channel radiate the signal. The three antennas at the output of the channel receive the radiated power, processed by appropriate signal processing techniques. Each of MMSE and CMA techniques is performed at SNR of 5, 10 and 15dB. The system model is simulated using MATLAB 7.2 application package and evaluated using Mean Square Error (MSE) and convergence value. The results obtained show that CMA gives lower error than the MMSE and converges faster. Therefore, the study has shown the significant reduction in computational complexity and can be used by wireless design. Keywords: Constant Modulus Algorithm, Orthogonality, Channel Estimation, Multiple Antenna, Cyclic Prefix