Analysis of cyclic prefix length effect on ISI limitation in OFDM system over a Rayleigh-fading multipath

In this work, the influence of the cyclic prefix on the performance of the OFDM system is studied. We worked out an OFDM transceiver using a 16 QAM modulation scheme, a comparison of the BER for various lengths of the cyclic prefix has been achieved, and the influence of the noise introduced in the channel has been highlighted, for both a Gaussian and Rayleigh noise. The simulation was carried out on MATLAB where the curves of the BER for various lengths of the cyclic prefix are given and compared. We also adopted as a metric the QAM constellation to show the dispersion of the carriers as a consequence of the transmission channel, the mitigation of this effect by the CP is noticeable

Optimal signal processing for next-generation communication systems

University of Technology Sydney. Faculty of Engineering and Information Technology.In order to meet consumers’ exponentially growing demand, the next-generation wireless communication systems are aimed at achieving high data rate, low latency, large device density, and superior energy efficiency (EE). In this dissertation, we address problems in different scenarios to ensure superior performance of next-generation wireless communication systems. We first consider the pilot sequence design for orthogonal frequency-division multiplexing (OFDM) systems in a high mobility environment. The design of pilot sequence to minimize the mean squared error (MSE) of the channel estimate is proposed under a linear minimum mean squared error (LMMSE) estimator for average path gains. Due to the existence of interferences of pilot and data subcarriers, the MSE of the LMMSE estimator is a nonconvex function. Then, the MSE of the LMMSE estimator is transformed to a concave quadratic function and we develop a path-following optimization procedure, which improves the MSE in every iteration and it quickly converges at least to its local optimal solution. The developed path-following procedure can also be adapted to design pilot sequences for the least-square (LS) and maximum-likelihood (ML) estimators. The second part of research is devoted to the optimal design of training sequences for channel estimation in large-scale multiple-input multiple-output (MIMO) OFDM systems. Under the criterion of minimizing the MSE of the channel estimate, the optimal design of training sequences for such systems poses a truly large-scale optimization problem, to which existing optimization solvers are not applicable. We develop a fast convex programming (FCP) procedure to find its global optimal solution. In each iteration of the proposed FCP procedure, a solution is found in a scalable and closed form. The singularity and ill-conditionedness of the channel correlation matrices are also exploited to improve the computation efficiency. Furthermore, we also examine the design of reduced-length training sequences and develop a successive quadratic programming (SQP) procedure to find the solutions. Thirdly, we consider the joint design of user power allocation and relay beamforming in relaying communications, in which multiple pairs of single-antenna users exchange information with each other via multiple-antenna relays in two time slots. The aim is to maximize the system’s EE subject to quality-of-service (QoS) constraints in terms of exchange throughput requirements. The QoS constraints are nonconvex with many nonlinear cross-terms, so finding a feasible point is already computationally challenging. The sum throughput appears in the numerator while the total consumption power appears in the denominator of the EE objective function. The former is a nonconcave function and the latter is a nonconvex function, making fractional programming useless for EE optimization. Nevertheless, efficient iterations of low complexity to obtain its optimized solutions are developed. Finally, we consider MIMO multicell networks, where the base stations (BSs) are full-duplex transceivers, while uplink users and downlink users are equipped with multiple antennas and operate in a half-duplex mode. The problem of interest is to design linear precoders for BSs and users to optimize the network’s energy efficiency. Given that the EE objective is not a ratio of concave and convex functions, the commonly-used Dinkelbach-type algorithms are not applicable. We develop a low-complexity path-following algorithm that only invokes one simple convex quadratic program at each iteration, which converges at least to the local optimum

Indexed-channel estimation under frequency and time-selective fading channels in high-mobility systems

Index modulation (IM) techniques have been employed in different communication systems to improve bandwidth efficiency by carrying additional information bits. In high-mobility communication systems and under both time-selective and frequency-selective fading channels with Doppler spread, channel variations can be tracked by employing pilot-aided channel estimation with minimum mean-squared error estimation. However, inserting pilot symbols among information symbols reduces the system's spectral efficiency in pilot-aided channel estimation schemes. We propose pilot-aided channel estimation with zero-pilot symbols and an energy detection scheme to tackle this issue. Part of the information bit-stream is conveyed by the indices of zero-pilot symbols leading to an increase in the system's spectral efficiency. We used an energy detector at the receiver to detect the transmitted zero-pilot symbols. This paper examines the impacts of diversity order on the zero-pilot symbol detection error probability and the mean-squared of error estimation. The impacts of pilot symbols number and the zero-pilot symbol number on the mean-squared error of the minimum mean-squared error (MMSE) estimator and the system error performance are also investigated in this paper

Pilot Optimization for Estimation of High-Mobility OFDM Channels

© 2017 IEEE. Obtaining channel state information is very crucial for realizing high-performance high-rate wireless communications. For an orthogonal frequency-division multiplexing (OFDM) system operating in a high-mobility environment such as in high-speed trains, a sequence of pilot samples is inserted in each OFDM symbol to track the fast-varying channel responses. For such a high-mobility environment, the design of pilot sequence to minimize the mean squared error (MSE) of the channel estimate under a linear minimum mean squared error (LMMSE) estimator poses a difficult polynomial fractional optimization problem. In this paper, we develop a path-following optimization procedure, which improves the MSE in every iteration and quickly converges at least to its locally-optimal solution. Each iterative solution is given in a closed form with very low computational complexity. The developed path-following procedure can also be adapted to design pilot sequences for the least-square and maximum-likelihood estimators. Extensive simulation results demonstrate the effectiveness and superior performance of the proposed solutions and algorithms when compared to the state-of-The-Art algorithms in the literature