F.J. Martinez-Lopez

Quality-of-Service Analysis for Linear Multiuser Detectors in the Uplink of a Wireless Network

In any wireless network is important to measure the quality-of-service (QoS) at the mobile units. Based on this information, the network will relocate their resources in order to achieve the performance specifications. Hence this objective motivates this paper, where analytical evaluations of the signal-to-noise plus interference ratio (SNIR) are carried out for each active user in a wireless communication system. In particular, this work studies the performance of linear detectors for direct-sequence code division multiple access (DS-CDMA) systems at different loading levels using singular value decomposition (SVD) techniques in the uplink channel. The QoS in terms of the SNIR are compared for the matched filter (MF), decorrelator, projector and minimum-mean-squared error (MMSE) detectors. In the analytical evaluations, two techniques are proposed to quantify the SNIR according to the multiple-access interference (MAI) and noise representation. The first one is based on an average of the MAI and noise information of all mobile users at the base station. The second approach relies on computing explicitly the interference for each active user. Simulation results show a performance comparison between both approaches, and a Monte-Carlo numerical evaluation in terms of the SNIR and the number of active users.

Variable speed evaluation of a model-based fault diagnosis scheme for induction motor drives

In the last decades, fault diagnosis of voltage source inverters has received a lot attention since their application to variable speed drives has become an industrial standard. Recently, some methods have been suggested for open-circuit faults in the switching devices of voltage source inverters. However, most of these methods might not work properly for different working conditions of the motor drive. In this paper, the evaluation of a robust model-based fault detection and isolation scheme for actuator faults in induction motor drives is presented. Residuals are synthesized by applying a differential geometry approach, and they are generated by using a bank of proportional-integral observers. Departing from previous results (Espinoza-Trejo and Campos-Delgado, 2008), the diagnosability and decoupling conditions for actuator faults are satisfied in the induction motor model. In this way, residuals are decoupled from the load torque and operating conditions, and simultaneous faults can be addressed. Finally, a variable voltage and frequency evaluation of this new model-based diagnosis scheme is presented. The variable speed evaluation is verified experimentally in a test-bench of 1 HP under single and simultaneous faults, and by considering three reference frequencies.

A family of driving forces to suppress chaos in jerk equations: Laplace domain design

A family of driving forces is discussed in the context of chaos suppression in the Laplace domain. This idea can be attained by increasing the order of the polynomial in the expressions of the driving force to account for the robustness and/or the performance of the closed loop. The motivation arises from the fact that chaotic systems can be controlled by increasing the order of the Laplace controllers even to track arbitrary orbits. However, a larger order in the driving forces can induce an undesirable frequency response, and the control efforts can result in either peaking or large energy accumulation. We overcame these problems by showing that considering the frequency response (interpreted by norms), the closed-loop execution can be improved by designing the feedback suppressor in the Laplace domain. In this manner, the stabilization of the chaotic behavior in jerk-like systems is achieved experimentally. Jerk systems are particularly sensitive to control performance (and robustness issues) because the acceleration time-derivative is involved in their models. Thus, jerky systems are especially helped by a robust control design.

Robust FDI Scheme in Power Actuators for Induction Motor

Abstract In this paper a robust model-based FDI scheme for actuator faults in induction motor (IM) drives is presented. A voltage source inverter is studied as power actuator for the induction motor. Residuals are synthesized by applying a differential geometry approach. In order to obtain a robust observer-based FDI scheme a bank of PI-observers is used, which according to the fault time profile allows to identify the faults. Departing from the results in Espinoza et al. [2008], the diagnosability and decoupling conditions for actuator faults are satisfied in the IM model. In this way, residuals are decoupled from the load torque and operating conditions, and simultaneous faults can be addressed. Hence, in order to isolate faults related to switching devices, it is suggested a directional residual evaluation. Hence 3 residuals are constructed to isolate the faulty switches. The ideas presented in the paper are verified experimentally in a test-bench of 1 HP induction motor under single and simultaneous faults.

A Suboptimal LQ Power Control Algorithm for a CDMA Wireless System

In todays wireless systems, power control is an important problem which is related to the quality-of-service (QoS) and battery utilization at the mobile units. This problem is studied in this work for the uplink of a direct-sequence code- division multiple-access communication (DS-CDMA) system. By a proper selection of the error function, the resulting linear decoupled feedback loops can be controlled efficiently following a distributed power control approach, and these closed-loops are independent of the users channel variations. A suboptimal LQ strategy is suggested and derived for power update. This control law has a variable structure that depends on the transmission roundtrip delay in the feedback system, which is a key parameter to establish closed-loop stability. It is demonstrated that in the limit of some synthesis parameters, the LQ-controller tends to a Dead-Beat response, improving the time response but sacrificing robustness. Meanwhile, the LQ-controller can be adjusted to increase its robustness. Simulation results are presented to compare the control algorithms using a standard single-step power correction approach.

Dead-Beat and LQ-optimal power control algorithms in the uplink of wireless systems

Power control is an important problem in todays wireless systems, which is related to the battery utilization in the mobile units. In this work, this problem is addressed using a distributed approach. The uplink of a direct-sequence code-division multiple-access communication (DS-CDMA) system is studied, and through a proper selection of the error function, the nonlinear coupling among the active users is transformed to individual loops, where the power references incorporate the information of the remaining users in the cell. It is concluded that the uplink channel variations do not destroy the stability of the feedback structures. However, the delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. An LQ-optimal power control strategy is derived and compared to a dead-beat approach. In this sense, the dead-beat algorithm is sensitive to the roundtrip delays estimation, but the LQ-optimal control can be adjusted to be robust to this estimation error. However, there is a severe compromise between robustness and performance. But through an appropriate selection of the LQ weight parameter, an stable closed-loop system can be always guaranteed independently on the uncertainty in the estimation of the roundtrip delay. Simulation results are presented to compare the control algorithms using a standard single-step power correction approach.

Distributed Power Control Performance in DS-CDMA Systems with Adaptive Quantization

One practical constraint imposed on closed-loop power control algorithms for a direct-sequence code-division multiple-access communication (DS-CDMA) system is the limited amount of feedback information. This paper proposes a closed-loop power control scheme with adaptive quantization on the basis of the number of power command bits. Adaptive quantization is used on the hypothesis that the quantization sensitivity can be defined as a function of the standard deviation of the reference error signal, while maintaining the feedback number of bits constant. Simulation results are presented to illustrate the performance of a distributed power control (DPC) scheme with adaptive quantization, using a classic proportional integral derivative (PID) power update. Moreover, the performance of the DPC algorithm with fixed quantization is also presented. Finally, both schemes are compared with the standard single-step power correction scheme.

Distributed Power Control Algorithms in the Uplink of Wireless Systerns

In this paper, the problem of power control in wireless systems using a distributed approach is analyzed. The uplink of a direct-sequence code-division multiple-access communication (DS-CDMA) system is studied in this work. The power control strategies are derived using classical design approaches (PID and dead-beat). It is concluded that the uplink channel variations do not destroy the stability of the feedback structures. However, the delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. As a result, the control laws have to be selected according with the expected transmission delays in order to maximize the resulting performance. Simulation results are presented to compare the control algorithms using a standard single-step power correction approach.