Adaptive MIMO Research Articles

A MIMO System With Multifunctional Reconfigurable Antennas

A multiple-input-multiple-output (MIMO) system equipped with a new class of antenna arrays, henceforth referred to as multifunction reconfigurable antenna arrays (MRAAs), is investigated. The elements of MRAA, i.e., multifunction reconfigurable antennas (MRAs) presented in this work are capable of dynamically changing the sense of polarization of the radiated field thereby providing two reconfigurable modes of operation, i.e., polarization diversity and space diversity. The transmission signaling scheme can also be switched between transmit diversity (TD) and spatial multiplexing (SM). The results show that the reconfigurable modes of operation of an MRAA used in conjunction with adaptive space-time modulation techniques provide additional degrees of freedom to the current adaptive MIMO systems, resulting in more robust system in terms of quality, capacity and reliability. A performance gain up to 30 dB is possible with the proposed system over conventional fixed antenna MIMO systems depending on the channel conditions

Adaptive MIMO decision feedback equalization for receivers with time-varying channels

In an attempt to reduce the computational complexity of vertical Bell Labs layered space time (V-BLAST) processing with time-varying channels, an efficient adaptive receiver is developed based on the generalized decision feedback equalizer (GDFE) architecture. The proposed receiver updates the filter weight vectors and detection order using a recursive least squares (RLS)-based time- and order-update algorithm. The convergence of the algorithm is examined by analysis and simulation, and it is shown that the proposed adaptive technique is considerably simpler to implement than a V-BLAST processor with channel tracking, yet the performances are almost comparable.

A Reconfigurable Spiral Antenna for Adaptive MIMO Systems

We present a reconfigurable spiral antenna for use in adaptive MIMO systems. The antenna is capable of changing the sense of polarization of the radiated field. It is fabricated by using an RF-MEMS technology compatible with microwave laminate substrates developed within the author's group. The proposed antenna structure is built on a number of rectangular-shaped bent metallic strips interconnected to each other with RF-MEMS actuators. Two senses of polarization, RHCP and LHCP, are achieved by configuring the physical structure of the antenna, that is, by changing the winding sense of the spiral, through judicious activation of MEM actuators. The fabrication process for the monolithic integration of MEM actuators with bent microstrip pixels on RO4003-FR4 microwave laminate substrate is described. The measured and calculated radiation and impedance characteristics of the antenna are given. The operating frequency of the presented antenna design can easily be adjusted to be compatible with popular IEEE networking standards such as 802.11a.

Multivariable adaptive control of instabilities arising in jet engines

We consider a class of MIMO LTI models with uncertain resonant modes and time delays, which are common in control of instabilities arising in jet engines. With uncertain delays preventing the use of model reference adaptive control, we develop an adaptive MIMO pole placement scheme for the system. We use indirect adaptation, estimating a small number of physical parameters from a nonlinearly parametrized plant. To address the highly noisy environment in jet engines we introduce the deadzone in the adaptation law and present simulations that successfully stabilize the system in the presence of noise and severe actuator saturation.

Adaptive active control of flexible structures subjected to rigid body displacements

When flexible structures are subjected to rigid body displacements, their dynamic behavior presents a coupling between rigid body displacements and flexible modes. Moreover, if these structures are articulated, their inertia characteristics can vary considerably through time and cause this dynamic behavior to become highly nonlinear. The paper presents the principle of a new adaptive controller MIMO. making it possible to render nearly constant the dynamic behavior of multi-articulated flexible structures in spite of changes in the geometry of their masses. The principle of this controller is based on the operation in parallel of a number of finite Gaussian quadratic linear controllers calculated before simulations, for the operating points crossed during the motion. The global multivariable command generated by the adaptive controller is the sum of the commands generated by the linear controllers, weighted by functions of rigid body displacements to allow the smoothing of the control actions on the structure. Adaptive control is first applied to a rigid bi-articulated structure, then to mono-articulated and bi-articulated flexible structures equipped with piezoelectric sensors and piezoelectric and electromechanical actuators. After temporal optimization, the results of the proposed adaptive control are compared with those of a linear controller. This is done by using two methods, chosen according to whether the structures are stressed by disturbances or by tracking. In the case of regulation, the performances of this controller highlight on the one hand, the near-constant dynamic behavior of the controlled structure by eliminating instabilities, in spite of the considerable changes of kinetics and, on the other hand, clearly improved performances when compared to a fixed linear controller. In the case of a set point tracking, performances in terms of rapidity are also appreciably improved compared to those of a linear controller, while guaranteeing stability, significant damping and absolute precision without residual strains when the commands are performed.

Multifunctional reconfigurable MEMS integrated antennas for adaptive MIMO systems

Multi-input multi-output systems with associated technologies such as smart antennas and adaptive coding and modulation techniques enhance channel capacity, diversity, and robustness of wireless communications as has been proven by many recent research results both in theory and experiments. This article focuses on the antenna aspect of MIMO systems. In particular, we emphasize the important role of the reconfigurable antenna and its links with space-time coding techniques that can be employed for further exploitation of the theoretical performance of MIMO wireless systems. The advantages of the reconfigurable antenna compared to the traditional smart antenna are discussed. Establishment of reconfigurable antennas requires novel radio frequency microelectromechanical systems technology, which has recently been developed in the authors' group. We briefly introduce this technology with emphasis on its distinct advantages over existing silicon-based MEMS technologies for reconfigurable antennas. A reconfigurable antenna design that can change its operating frequency and radiation/polarization characteristics is described. Finally, we present the experimental and theoretical results from impedance and radiation performance characterization for different antenna configurations.

Adaptive MIMO OFDM receivers: Implementation impairments and complexity issues

MIMO OFDM communications is a promising choice for future high data rate wireless systems. However, the commercial deployment of MIMO OFDM systems faces some challenges, including those arising from front-end analog implementation impairments and complex receiver structures. The impairments are caused by the analog processing of the received radio frequency (RF) signal and they cannot be e±ciently nor entirely eliminated in the analog domain. This paper illustrates the e®ect of In-phase and Quadrature-phase (IQ) imbalances on system performance. Receiver algorithms are described that compensate for IQ imbalances. The structure of space-time block codes, along with the assumed structure of the distortion models, are exploited to design reduced-complexity adaptive receivers that are robust to IQ imbalances.

Design of adaptive modulation using imperfect CSI in MIMO systems

A design of variable-power variable-rate adaptive modulation using imperfect channel state information (CSI) in multiple-input-multiple-output (MIMO) systems is proposed. Analysis and simulation results show that this design provides a good trade-off between the spectral efficiency and bit error rate performance in an adaptive way. Hence, it makes the adaptive modulation MIMO system much more robust to CSI imperfections than a system designed under a perfect CSI assumption.

Efficient adaptive transmission technique for LDPC coded OFDM cellular systems using multiple antennas

An adaptive MIMO transmission scheme using QAM and LDPC code is proposed for an OFDM cellular system employing FDD. It is shown that the proposed scheme can provide up to 2–3 dB gain over the conventional scheme at the expense of only six more bits in feedback information.

Adaptive MIMO Backstepping Controller for High-Performance DC Motor Field Weakening

A novel field-weakening technique has been investigated for separately excited DC motor systems that exhibit considerable nonlinearities. Based on proper selection of a Lyapunov function, an adaptive multi-input multi-output backstepping field-weakening control law has been derived for speed tracking. Because of the decoupling effect, this controller achieves speed tracking at any desired set point of operation. Computer tests are carried out for a specific motor to demonstrate the effectiveness of the proposed controller. It has been shown that the controller provides the globally robust speed control performance, while the motor system is operated over a wide dynamic regime of field weakening, even under the presence of uncertainties in both parameters and load torque.

Special issue: Adaptive antennas and MIMO systems

Special issue: Adaptive antennas and MIMO systems

Adaptive modulation and MIMO coding for broadband wireless data networks

Link adaptation techniques, where the modulation, coding rate, and/or other signal transmission parameters are dynamically adapted to the changing channel conditions, have emerged as powerful tools for increasing the data rate and spectral efficiency of wireless data-centric networks. While there has been significant progress on understanding the theoretical aspects of time adaptation in LA protocols, new challenges surface when dynamic transmission techniques are employed in broadband wireless networks with multiple signaling dimensions. Those additional dimensions are mainly frequency, especially in multicarrier systems, and space in multiple-antenna systems, particularly multiarray multiple-input multiple-output communication systems. We give an overview of the challenges and promises of link adaptation in future broadband wireless networks. We suggest guidelines to help in the design of robust, complexity/cost-effective algorithms for these future wireless networks.

Towards Robust Adaptive Least-Squares Parameter Estimation with Internal Feedback

The new concepts of the ‘covariance matrix normalization’ and the ‘cascade’ structure of the adaptive least-squares estimator are shown to generalize and extend the use of internal information feedback in various robustness/alertness-oriented modifications to the standard ALS estimation algorithm. In the cascade estimation structure it is possible to ‘naturally’ stabilize, rather than maximize, the information matrix so that the covariance windup and blowup are effectively eliminated and the celebrated square root update of the covariance matrix is no longer needed Consequently, a new, ‘single-loop/cascade’ ALS MIMO estimation algorithm, enabling to effectively track both slow and jump parameter variations, is presented. The algorithm is coupled with a new, simple but robust predictive control scheme, offering a new adaptive MIMO control strategy.

Active Flutter Suppression Using Multi-Input/Multi-Output Adaptive Least Mean Square Control

The feasibility of an adaptive feedforward multi-input/multi-output (MIMO) controller for active flutter suppression has been investigated in this study. An unswept flexible wing structure is modeled by a multidegree-of-freedom finite element representation with beam elements for bending and rod elements for torsion. Control action is provided by one or more flaps attached to the trailing edge of the wing and extending along a fraction of the wingspan. Time-domain unsteady aerodynamics have been used to generate the air forces acting on the wing model. The adaptive feedforward MIMO controller has been designed based on the filtered-X least mean square (LMS) algorithm. The controller structure includes an on-line adaptive system identification that provides the LMS controller with a reasonably accurate model of the plant. The controller is capable of tracking time-varying parameters in the plant and providing effective control. The wing model in closed loop exhibits highly damped responses at airspeeds where the open-loop responses are destructive. Simulations with the flexible wing model in a time-varying airstream show a 53% increase over its corresponding open-loop flutter airspeed. The ability of the MIMO LMS controller to suppress instabilities in a wing with abruptly changing parameters (sudden onset of flutter caused by the release of stores) has also been studied. In the examples studied, it is found that adaptation is rapid enough to successfully control flutter at accelerations in the airstream of up to 9 ft/s 2 . An increase in the number of flaps provides an enhancement in the overall control authority, but results in a decrease in the convergence speed of the controller, resulting in downgraded performance. The optimum number of control flaps for the model studied is found to be four, extending along 40% of the wingspan from the tip.

An Adaptive MIMO VSC Algorithm, and its Application to the Control of an Underwater Vehicle

Discrete-time Variable Structure Control (VSC) for MIMO systems is addressed in this note. Two control techniques, consisting in VSC laws cascaded to a Generalized Minimum Variance (GMV) controller, are presented. The former algorithm refers to a completely known system, while the latter has been designed to deal with parameter uncertainties in the plant. The presented algorithms have been applied to the problem of position and orientation control of an underwater Remotely Operated Vehicle (ROV). Resulting performances have been tested by vsimulation, modelling the ROV with a nonlinear continuous-time equations system. Parameter variations have been applied to the plant parameters, and the presence of measurement noise has been considered.

Multiplexed and distributed control of automated welding

Although modern sensor technology and control algorithms have enabled in-process regulation of arc welding, classical single-torch actuation methods provide only a few welding conditions that can be modulated in real time to control multiple weld geometry characteristics. To decouple the process dynamics and simultaneously control thermal characteristics of the weld, multiple virtual heat inputs are implemented by rapid periodic reciprocation (timesharing) of the single torch on the weld surface. Dynamic analytical, numerical and linearized experimental process models are developed for the design of adaptive MIMO control systems of both geometrical and thermal characteristics, and their performance is tested in rejecting disturbances and following setpoint changes. To maximize the range of achievable weld features, a continuous heat distribution and temperature monitoring on the entire weld surface is finally adopted. The necessary vector-scanning trajectories of the torch are regulated in real time by a distributed-parameter control strategy, integrated to the weld design software for flexibility in production.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparison of low-earth-orbit satellite attitude controllers submitted to controllability constraints

A rule-based fuzzy controller is presented and compared with an adaptive MIMO LQR controller in a low-Earth-orbit small satellite attitude control system. The attitude is passively gravity gradient stabilized and actively three-axis magnetorquer controlled. This method insures an Earth-pointing satellite making use of a nondepletable and nonmechanical means of control. A realistic simulation environment, using a nonlinear satellite dynamic model with linear attitude estimators plus sensor measurement noise and external disturbance torques, was used to evaluate the different control techniques.

Performance of an adaptive MIMO controller for a multiple-element ultrasound hyperthermia system.

A prototype adaptive automatic control algorithm was implemented to regulate temperatures measured at several points in a tumour by adjusting the power applied to several ultrasound transducers. The goal was to control the temperatures under the elements of a mosaic applicator individually without any priori knowledge of which probes are under which elements. The control algorithm was devised for clinical applications where the position of each probe with respect to the heat sources is difficult to determine precisely. Instead, the program 'learns' the relationship between the inputs (power levels) and the outputs (temperatures) automatically. Based on the observed transfer function relating the power at m sources to the temperatures n probes, where n and m are not necessarily the same, a new method was used to implement a feedback controller. This method simplifies the design of the controller for a multiple-input/multiple-output (MIMO) system, while taking into account the coupling that may exist between the various elements of the system. As a result of using an adaptive scheme, the regulator continuously tracks changes in the system, such as blood flow variations or patient motion, by modifying its control parameters. The algorithm performance has been tested in simulations as well as experiments in dog thigh and a perfused kidney model.

Non-linear adaptive control of fermentation processes utilizing a priori modelling knowledge

This paper describes both SISO and MIMO adaptive versions of non-linear generic model control (GMC) applied to a baker's yeast fermentation. A mechanistic non-linear model is used whose structure can represent a wide range of different fermentation systems and operating conditions. This a priori representation of process knowledge is then combined with a simple and effective adaptation scheme to yield optimal flexibility of the model. Only a few parameters which appear linearly in the model need to be estimated on-line, resulting in very fast and potentially offset-free tracking of the process by the model-based controller. Simulations demonstrate that an adaptive MIMO version of GMC is superior to the corresponding non-adaptive version when faced with modelling errors. Experiments with a bench-scale yeast fermentation system demonstrate for the SISO case the applicability of adaptive non-linear control methods to an actual process and compare adaptive GMC performance with that of a conventional PI controller.