Mohammad Shikh-Bahaei

Interference cancellation in W-CDMA cellular structures using statistical processing

A statistical processing method is considered for interference suppression in W-CDMA systems. The major first-order statistic of interference, namely, the pdf function is estimated by use of higher-order statistics (HOS) of the received samples. This estimate, in turn, leads to deriving the best non-linear processor of the samples for enhanced detection performance of the desired user. A mathematical model for the process is presented and the relative asymptotic efficiency of the detector with respect to the conventional linear detector is computed. Some numerical results are also provided to show the achievable enhancement by the proposed nonlinear processing as applied to UMTS-based receivers.

Multi-User Time-Frequency Downlink Scheduling and Resource Allocation for LTE Cellular Systems

In this paper, new adaptive scheduling and resource allocation algorithms for frequency-selective time- varying downlink channels are proposed. The proposed methods are particularly useful in LTE-based systems. Analytical adaptive power optimization formulae have been obtained in order to maximize channel spectral efficiency for continuous and discrete rate schemes. The formulae are derived for single-user scenario with a limited number of time- frequency grids. Optimum rate adaptation in each grid is first considered and subsequently extended to a multi-user case while maintaining the fairness among all the users. Further, three different algorithms are proposed for resource scheduling according to the obtained analytical results. Finally, simulation results confirm that the proposed dynamic time and frequency sub-carrier allocation and scheduling algorithms provide fairness and improve system capacity.

Cross-Layer Resource Allocation for Video Streaming Over OFDMA Cognitive Radio Networks

In this paper, a cross-layer resource allocation algorithm is proposed in the context of orthogonal frequency division multiple access (OFDMA)-based cognitive radio (CR) video application systems. User video quality and channel awareness are incorporated in the design, towards an optimal subcarrier and power allocation scheme, subject to minimum secondary receiver (SRx) video quality and primary receiver (PRx) interference threshold constraints. First, the relationship between PRx interference margin and power limits at the secondary transmitter (STx) is analytically derived. Then, to provide PRx with satisfactory quality of service (QoS), we propose a new probabilistic approach to mitigate the total imposed interference of the STx on PRx, considering imperfect STx to PRx channel state information (CSI). The effect of PRx interference limit violation probability, interference threshold, and error variance of imperfect CSI are evaluated through mathematical analysis and computer simulations. Results indicate that significant improvement in SRx total video quality is achieved through our proposed quality-aware (QA) algorithm over other state-of-the-art non-quality-aware (NQA) designs in the literature. The enhanced performance was obtained whilst guaranteeing SRx minimum quality and PRx prescribed QoS constraints.

Variable rate and variable power MQAM system based on bayesian bit error rate and channel estimation techniques

The impact of inaccurate channel state information at the transmitter for a variable rate variable power multilevel quadrature amplitude modulation (VRVP-MQAM) system over a Rayleigh flat-fading channel is investigated. A system model is proposed with rate and power adaptation based on the estimates of instantaneous signal-to-noise ratio (SNR) and bit error rate (BER). A pilot symbol assisted modulation scheme is used for SNR estimation. The BER estimator is derived using a maximum a posteriori approach and a simplified closed-form solution is obtained as a function of only the second order statistical characterization of the channel state imperfection. Based on the proposed system model, rate and power adaptation is derived for the optimization of spectral efficiency subject to an average power constraint and an instantaneous BER requirement. The performance of the VRVP-MQAM system under imperfect channel state information (CSI) is evaluated. We show that the proposed VRVP-MQAM system that employs optimal solutions based on the statistical characterization of CSI imperfection achieves a higher spectral efficiency as compared to an ideal CSI assumption based method.

Optimum Power and Rate Adaptation for MQAM in Rayleigh Flat Fading With Imperfect Channel Estimation

We derive an optimum power and rate adaptation for maximizing the spectral efficiency based on an imperfect channel estimate and subject to average power and instantaneous bit error rate (BER) constraints for multilevel quadrature amplitude modulation (MQAM) over Rayleigh flat-fading channels. The optimal solution is derived for continuous- and discrete-rate adaptation and is expressed in terms of a specific bounded function that is the solution of a nonlinear equation and cannot be expressed in a closed mathematical form. The optimum power adaptation for the continuous rate is shown to be a generalization of water pouring for that function. It is also shown that the conventional water-pouring (with bias) strategy for power adaptation in the continuous-rate condition is a suboptimum solution of the general optimization problem, and it tends to the optimal solution as the correlation coefficient between the true channel gain and its estimate tends to one. We also show that less than 1 dB power is lost by using discrete-rate MQAM with only six different signal constellations compared to the continuous-rate adaptation with an unconstrained constellation size.

Interference Suppression in the Wigner Distribution Using Fractional Fourier Transformation and Signal Synthesis

A novel method for the suppression of cross terms in the time-frequency domain is introduced. First, interference is identified using a fractional Fourier transform-based technique. Then, auto-terms are detected, synthesized, and subtracted from the original signal. The process is repeated until all signal components are extracted. Finally, the Wigner distributions of pure auto-terms are superimposed to yield a high readability representation

A Low-Complexity Analytical Modeling for Cross-Layer Adaptive Error Protection in Video Over WLAN

We find a low-complicity and accurate model to solve the problem of optimizing MAC-layer transmission of real-time video over wireless local area networks (WLANs) using cross-layer techniques. The objective in this problem is to obtain the optimal MAC retry limit in order to minimize the total packet loss rate. First, the accuracy of Fluid and M/M/1/K analytical models is examined. Then we derive a closed-form expression for service time in WLAN MAC transmission, and will use this in mathematical formulation of our optimization problem based on M/G/1 model. Subsequently we introduce an approximate and simple formula for MAC-layer service time, which leads to the M/M/1 model. Compared with M/G/1, we particularly show that our M/M/1-based model provides a low-complexity and yet quite accurate means for analyzing MAC transmission process in WLAN. Using our M/M/1 model-based analysis, we derive closed-form formulas for the packet overflow drop rate and optimum retry-limit. These closed-form expressions can be effectively invoked for analyzing adaptive retry-limit algorithms. Simulation results (network simulator-2) will verify the accuracy of our analytical models.

Joint Optimization Of “Transmission Rate” and “Outer-Loop SNR Target” Adaptation Over Fading Channels

Joint optimization of signal-to-noise ratio (SNR) target and transmission rate adaptation is examined for multilevel quadrature amplitude modulation (MQAM) over flat-fading channels, to maximize the spectral efficiency subject to an average transmit power constraint. We propose an adaptive transmission scheme in which the outer-loop SNR target and data rate are adapted to bit-error rate (BER), where total or truncated channel-inversion strategies are exploited for the (fast) inner-loop power control. We obtain the optimal solutions for both continuous and discrete rate adaptation, and consider cases where diversity combining is performed in the receiver. We show that by using this BER-based adaptive scheme, spectral efficiency can be improved compared with optimal SNR-based variable-rate variable-power MQAM. We also show that for continuous rate adaptation, the optimal SNR target monotonically increases with BER, while it descends within a BER range with constant rate

Design of Near-Allpass Strictly Stable Minimal-Phase Real-Valued Rational IIR Filters

In this brief, a near-allpass strictly stable minimal-phase real-valued rational infinite-impulse response filter is designed so that the maximum absolute phase error is minimized subject to a specification on the maximum absolute allpass error. This problem is actually a minimax nonsmooth optimization problem subject to both linear and quadratic functional inequality constraints. To solve this problem, the nonsmooth cost function is first approximated by a smooth function, and then our previous proposed method is employed for solving the problem. Computer numerical simulation result shows that the designed filter satisfies all functional inequality constraints and achieves a small maximum absolute phase error.

Spectral Efficiency of Adaptive MQAM/OFDM Systems With CFO Over Fading Channels

In this correspondence, we investigate the impact of power and continuous-rate adaptation using variable-power M-ary quadrature amplitude modulation (MQAM) on spectral efficiency of orthogonal frequency-division multiplexing (OFDM) systems subject to carrier frequency offset (CFO) in Rayleigh fading channels with perfect and imperfect channel information at the transmitter. Convex optimization-based adaptation schemes to maximize the spectral efficiency subject to average power and instantaneous bit error rate (BER) constraints are used in this correspondence. Adaptive MQAM/OFDM systems under CFO with perfect channel information at the transmitter are shown to have substantial gain in spectral efficiency over their nonadaptive counterparts. Theoretical and simulation results suggest that adaptive modulation can also improve the spectral efficiency of OFDM systems under CFO with imperfect channel estimates in comparison with nonadaptive modulation, provided the correlation between the true and estimated channels is above 0.5. Furthermore, we have shown that the outage power threshold in our adaptive scheme increases with increasing CFO values and/or decreasing correlation between the true and estimated channels.