Mohamed M. Khairy

LMI based design of constrained fuzzy predictive control

Predictive control of nonlinear systems subject to output and input constraints is considered. A fuzzy model is used to predict the future behavior. Two new ideas are proposed here. First, an added constraint on the applied control action is used to ensure the decrease of a quadratic Lyapunov function, and so guarantee Lyapunov exponential stability of the closed-loop system. Second, the feasibility of the finite-horizon optimization problem with the added constraints is ensured based on an off-line solution of a set of LMIs. The novel stability method is compared to the existing methods, such as the techniques based on the end-point constraints (terminal constraint set), and the robust stability techniques based on the small gain theory. The proposed method ensures Lyapunov exponential stability, does not need an auxiliary controller and can be used with any feasible controller parameters. Illustrative examples including the predictive control of a highly nonlinear chemical reactor (CSTR) are discussed.

Efficient FPGA implementation for the IEEE 802.16e interleaver

In this paper, we implement and evaluate a novel design for the hardware of the multi-mode interleaver block used in the OFDMA mode of the IEEE 802.16e (Mobile WiMAX) standard. A new architecture that is both area and delay efficient is introduced. The area and delay efficiency of this new architecture is verified via quantitative comparisons between FPGA implementations of this architecture and classical interleaver designs.

Proposal for a cross-layer coordination framework for next generation wireless systems

Cross-Layer design has been the focus of several recent research efforts. Due to the highly variable nature of the links used in wireless communication systems and the resource-poor nature of the wireless mobile devices, there have been multiple research efforts to improve the performance of the protocol stack by allowing cross-layer interaction in wireless systems. Cross-layer interaction means allowing communication of a layer with any other possibly non-adjacent layer in the protocol stack. Several issues related to the cross-layer design paradigm need to be addressed before it can achieve its promises. One of these issues is to have a well defined framework that manages the interaction between the different layers of the protocol stack, such that the modularity of the stack is preserved while still achieving the flexibility and adaptability which cross-layer design promises. This paper addresses this issue by proposing a cross-layer coordination framework for next generation wireless systems. The proposed framework enables the interaction between non-adjacent layers in a systematic organized way while preserving the modularity of each layer. The proposed framework addresses some of the concerns stated in recent research about cross-layer design. We believe that the existence of such a framework will ease the development of cross-layer design schemes.

LTE and WiMAX: performance and complexity comparison for possible channel estimation techniques

SUMMARY The similarities between the physical layer of LTE and WiMAX systems trigger the design of a unified hardware platform. As a consequence, there exists a great need to design common blocks that can be adapted to support both of them. In this paper, the choice of the channel estimator is studied where the performance and complexity trade-off is investigated to seek a unified channel estimation design that can suit both systems under different channel conditions. The performance of different channel estimation techniques when applied to LTE and WiMAX systems is assessed and compared. Two channel profiles with different delay spreads are considered for urban macrocell and bad urban macrocell channels. Three different Doppler frequencies are assumed to illustrate the effect of users’ mobility. The estimators’ computational complexity is also thoroughly studied.Copyright

A novel frequency offset estimation technique for Mobile WiMAX

In this paper, a novel frequency offset estimation technique for the OFDMA based WiMAX system is proposed. It is perceived that Mobile WiMAX based on the IEEE802.16e standard has an aperiodic preamble, hindering the most efficient frequency offset estimation techniques which are based on utilising the preambles periodicity. In this paper, a novel technique for creating periodicity in the preamble by exploiting its design is proposed and evaluated. The results show that the proposed scheme has a significantly better performance than the currently used techniques. Copyright

Cognitive interference-minimizing code assignment for underlay CDMA networks in asynchronous multipath fading channels

We propose a secondary CDMA network that operates as an underlay for a primary CDMA network where the code assignment for the secondary network is done cognitively. A code assignment criterion that minimizes interference from primary users and existing secondary users is formulated. The primary user codes as well as bit boundaries are unknown to the secondary network. The problem is initially formulated as a constrained optimization problem. Exhaustive search has complexity that is exponential in the spreading factor. The constraints are therefore relaxed to obtain a near-optimal solution with a complexity that is linear in the spreading factor and performance very close to the exhaustive search method. We consider the different cases where the primary users are bit- and chip-synchronous, chip-synchronous but bit-asynchronous, and chip-asynchronous over multi-path fading channels. We propose a method for blind epoch acquisition by processing the covariance matrix of the received signal. In all cases the performance improvement of the interference-minimizing code assignment over random code assignment is significant reaching over 3 dB for coded systems.

System parameter selection for asymmetric underlay CDMA networks with interference-minimizing code asssignment

We propose a framework for designing an underlay secondary CDMA network. The objective is to select the secondary systems bit rate and spreading factor that improve the secondary network throughput given a required bit error rate and an allowable transmit power level acceptable to the primary network. The selection scheme is applied to both the case of random code assignment and an interference-minimizing code assignment (IMCA) scheme. Our simulations reveal that the parameter selection for the system with interference-minimizing code assignment scheme leads to higher system throughput over that achieved with random code assignment. We also show that the IMCA scheme, unlike random code assignment, suffers negligible interference from adjacent bands in asynchronous channels.

Power allocation strategies for Non-Orthogonal Multiple Access

Non-Orthogonal Multiple Access (NOMA) is considered as a promising downlink Multiple Access (MA) scheme for future radio access. In this paper two power allocation strategies for NOMA are proposed. The first strategy is based on channel state information experienced by NOMA users. The other strategy is based on pre-defined QoS per NOMA user. In this paper we develop mathematical models for the proposed strategies. Also we clarify the potential gains of NOMA using proposed power allocation strategies over Orthogonal Multiple Access (OMA). Simulation results showed that NOMA performance using the proposed strategies achieves superior performance compared to that for OMA.

Time domain leakage in DFT-based channel estimation for OFDM systems with guard bands

In this paper, a mathematical analysis of the time domain leakage problem in the discrete Fourier transform DFT based channel estimation technique is presented. The time domain leakage is because of the absence of pilots in the guard band. Several solutions to this problem were previously proposed based on reducing the leakage in the frequency domain. These solutions significantly increase the receiver complexity. In this paper, the root cause of the leakage problem is linked to the time domain and a modification is proposed carrying slight additional complexity over the conventional DFT-based estimator. Performance evaluation is assessed in terms of the bit error rate and the mean square error. The proposed modification significantly decreases the error floor of the conventional DFT-based channel estimation technique. In addition, its performance is comparable to other more complex techniques recently proposed.Copyright

Optimal spectrum assignment for cognitive radio sensor networks under coverage constraint

Cognitive radios emerged as a solution to spectrum scarcity problem. The integration of cognitive radios and wireless sensor networks enables a new paradigm of communication, in which the sensor nodes can avoid heavily-crowded transmission bands by tuning their transmission parameters to less-crowded bands. The authors consider the problem of spectrum assignment for cognitive radio sensor network (CRSN) under coverage, interference, minimum data rate and power budget constraints. A mixed-integer non-linear programming problem formulation that addresses optimal power allocation, channel selection and node scheduling is presented. Following a practical assumption, that any CRSN node can only access one channel for its transmission with the CRSN base station, the problem is transformed to a binary linear programming (BLP) problem. Using the relaxation techniques, the problem is transformed to a linear programming problem that is solvable in polynomial time, and has the same optimal solution of the BLP problem. Hence, the minimum power algorithm that achieves the optimal solution of our problem is proposed. To further reduce the complexity of the solution, three heuristic lower-complexity algorithms are proposed to solve the problem: random, greedy and two-stage (decoupled) algorithms.