Space-time communication for OFDM with implicit channel feedback

We consider wideband communication (e.g., using orthogonal frequency-division multiplexed (OFDM) systems) over a typical cellular "downlink," in which both the base station and the mobile may have multiple antennas, but the number of antennas at the mobile is assumed to be small. Implicit channel feedback can play a powerful role in such systems, especially for outdoor channels, which typically exhibit narrow spatial spreads. A summary of our findings is as follows: a) Implicit channel feedback regarding the covariance matrix for the downlink space-time channel can be obtained, without any power or bandwidth overhead, by suitably averaging uplink channel measurements across frequency. Since this approach relies on statistical reciprocity, it applies to both time-division duplex (TDD) and frequency-division duplex (FDD) systems. Using such covariance feedback yields significantly better performance at lower complexity than conventional space-time or space-frequency codes, which do not employ feedback; b) We provide guidelines for optimizing antenna spacing in systems with covariance feedback. Theoretical investigation of a hypothetical system with completely controllable channel eigenvalues shows that the optimal number of channel eigenmodes is roughly matched to the (small) number of receive antenna elements. Thus, while antenna elements in conventional systems without feedback should be spaced far apart in order to ensure uncorrelated responses, the optimal antenna spacing with covariance feedback is much smaller, thereby concentrating the channel energy into a small number of eigenmodes.     

Geometric techniques for implicit two-dimensional systems

Geometric tools are developed for two-dimensional (2-D) models in an implicit Fornasini–Marchesini form. In particular, the structural properties of controlled and conditioned invariance are defined and studied. These properties are investigated in terms of quarter-plane causal solutions of the implicit model given compatible boundary conditions. The definitions of controlled and conditioned invariance introduced, along with the corresponding output-nulling and input-containing subspaces, are shown to be richer than the one-dimensional counterparts. The analysis carried out in this paper establishes necessary and sufficient conditions for the solvability of 2-D disturbance decoupling problems and unknown-input observation problems. The conditions obtained are expressed in terms of output-nulling and input-containing subspaces, which can be computed recursively in a finite number of steps.     

一种基于用户隐式反馈的微博信息过滤方法

微博平台的兴起革新了人们的互动方式,给人们获取信息带来了极大便利.然而,在信息超载的环境下,人们需要花费大量的时间从许多冗余的微博信息中寻找自己感兴趣的信息,剔除无用信息.针对该问题,本文设计了一种新的方法对用户的微博信息进行过滤.该方法在传统方法基础上增加用户反馈环节;同时,考虑用户兴趣随时间变化的特点,在进行信息过滤时考虑时间对兴趣度的影响.该方法为微博信息个性化过滤提供了一种新思路.     

A relation of inclusion for implicit linear systems

In [2] a relation of covering for implicit systems has been introduced and studied. It has been shown that this notion is useful when studying the problems of minimal left inverse and minimal observer design. In the present paper we study a slightly more general notion: a relation of inclusion for implicit systems that enjoy the outputuniqueness property. We are chiefly interested in systems of a minimal size including a given system. A particular application we have in mind is a minimal order observer or a minimal left inverse construction. We show that study of including systems may allow us in some cases to reduce the size of an observer or an inverse beyond the one given by minimization under external equivalence. A similar approach has been applied in [2] to minimal covering systems. However, including systems have an important advantage over the covering ones: since they constitute a broader class, they offer us more freedom in picking a minimal one of a desired sort.This research was supported by NSF Grant ECS-8805932 and Grant MEN nr 225 of the Polish Ministry of National Education.     

Combined pulse-modulation scheme for feedback systems

A new form of combined pulse modulator using integral pulse-frequency modulation and pulse-width modulation is proposed in the letter. Using this modulator in a feedback system, a nonlinear discrete equivalent system is established, and a method for computation of state trajectories is indicated. It is also shown that systems having pulse-frequency modulation only or pulse-width modulation only reduce to special cases.     

Sensitivity-function determination for linear feedback systems

For linear feedback systems with distinct eigenvalues, a simple method is presented for evaluating the sensitivity functions as a function of initial conditions without using numerical (or analogue) methods for solving the system sensitivity equations.     

Sensitivity relationships for linear feedback systems

The sensitivity of the s plane pole position of a closed-loop system transfer function to variation of an open-loop system parameter is shown to be the coefficient of a complex frequency function in the response of the closed-loop system to a certain input signal. In many cases of practical interest, this signal is a simple one.     

Output feedback nonlinear control for electro-hydraulic systems

In this paper we present an output feedback nonlinear control for position tracking of electro-hydraulic systems (EHSs). Although previous nonlinear control methods improved the position tracking performance of EHS, all of the methods require full state feedback. However, due to cost and space limitations, it is not always possible to measure the full state of the EHS. The proposed method consists of a high gain observer and a passivity-based controller. The high gain observer is designed to estimate the full state, and the passivity-based control is implemented for position tracking. In order to design the passivity-based controller with the high gain observer, a defined Lyapunov condition guarantee that the origin of the tacking error dynamics is exponentially stable by selecting the controller gain. The stability of the closed-loop is studied using the singular perturbation theorem. The performance of the proposed method is validated through simulations and experiments.     

On Hautus-type conditions for controllability of implicit linear discrete-time systems

Four notions of controllability for general (i.e., possibly nonregular) implicit linear discrete-time systems are considered. Relationships between them are studied. A Hautus-type characterization of all of these notions is also given.This work was performed under the auspices of RP.I.02: Teoria sterowania i optymalizacji ciagych ukadów dynamicznych i procesów dyskretnych.     

Signal processing principles for systems analysis and design

The author indicates, for the benefit of system designers, how and to what extent independently available information-or generic characteristics derived from the signal itself-can be used to enhance the extraction of wanted signals in the presence of noise or interference. To bring out the commonalities in apparently different problems or methods, he uses the `state-space' concept, common to the dimensions of time, frequency, antenna aperture and image space, and treats signal processing as a sequence of transformations subject to a set of limiting constraints     

Zero forcing based sphere decoder for generalized spatial modulation systems

To reduce the number of radio frequency (RF) chains in multiple input multiple output (MIMO) systems, generalized spatial modulation (GSM) techniques have been proposed in the literature. In this paper, we propose a zero‐forcing (ZF)‐based detector, which performs an initial pruning of the search tree that will be considered as the initial condition in a sphere decoding (SD) algorithm. The proposed method significantly reduces the computational complexity of GSM systems while achieving a near maximum likelihood (ML) performance. We analyze the performance of the proposed method and provide an analytic performance difference between the proposed method and the ML detector. Simulation results show that the performance of the proposed method is very close to that of the ML detector, while achieving a significant computational complexity reduction in comparison with the conventional SD method, in terms of the number of visited nodes. We also present some simulations to assess the accuracy of our theoretical results.     

Reducing feedback for opportunistic scheduling in wireless systems

We study reducing feedback overhead of users' channel state information required for opportunistic scheduling at a base station while minimizing the throughput penalty incurred due to reduced feedback. We first propose a simple contention based scheme known as 'static splitting' for best effort traffic. The idea is to divide users into static groups, with users that belong to a group, and have their currently supported rate above a threshold, contending to send their current feedback to the base station. This is combined with maximum quantile scheduling -scheduling a user whose current rate is high relative to its distribution, to obtain thresholds that are independent of users' rate distributions even when they are heterogeneous, allowing off-line optimization of thresholds. Next we develop the insight that for a traffic mixture of best effort and real-time traffic one has to combine contention and polling to reduce feedback while providing quality of service. We propose a scheme based on this insight. Under this scheme we prove a lower bound on the service seen by a real-time when users' channel capacities are fast fading. Furthermore, we propose a heuristic modification that is able to exploit a larger fraction of opportunism. Simulation results illustrate the performance advantage of the proposed schemes.     

Generalized feedback detection for spatial multiplexing multi-antenna systems

We present a unified detection framework for spatial multiplexing multiple-input multiple-output (MIMO) systems by generalizing Heller's classical feedback decoding algorithm for convolutional codes. The resulting generalized feedback detector (GFD) is characterized by three parameters: window size, step size and branch factor. Many existing MIMO detectors are turned out to be special cases of the GFD. Moreover, different parameter choices can provide various performance-complexity tradeoffs. The connection between MIMO detectors and tree search algorithms is also established. To reduce redundant computations in the GFD, a shared computation technique is proposed by using a tree data structure. Using a union bound based analysis of the symbol error rates, the diversity order and signal-to-noise ratio (SNR) gain are derived analytically as functions of the three parameters; for example, the diversity order of the GFD varies between 1 and N. The complexity of the GFD varies between those of the maximum-likelihood (ML) detector and the zero-forcing decision feedback detector (ZF-DFD). Extensive computer simulation results are also provided.     

Optimized signaling for MIMO interference systems with feedback

The system mutual information of a multiple-input multiple-output (MIMO) system with multiple users which mutually interfere is considered. Perfect channel state information is assumed to be known to both transmitters and receivers. Asymptotic performance analysis shows that the system mutual information changes behavior as the interference becomes sufficiently strong. In particular, beamforming is the optimum signaling for all users when the interference is large. We propose several numerical approaches to decide the covariance matrices of the transmitted signals and compare their performance in terms of the system mutual information. We model the system as a noncooperative game and perform iterative water-filling to find the Nash equilibrium distributively. A centralized global approach and a distributed iterative approach based on the gradient projection method are also proposed. Numerical results show that all proposed approaches give better performance than the standard signaling, which is optimum for the case without interference. Both the global and the iterative gradient projection methods are shown to outperform the Nash equilibrium significantly.     

Limited feedback unitary precoding for spatial multiplexing systems

Multiple-input multiple-output (MIMO) wireless systems use antenna arrays at both the transmitter and receiver to provide communication links with substantial diversity and capacity. Spatial multiplexing is a common space-time modulation technique for MIMO communication systems where independent information streams are sent over different transmit antennas. Unfortunately, spatial multiplexing is sensitive to ill-conditioning of the channel matrix. Precoding can improve the resilience of spatial multiplexing at the expense of full channel knowledge at the transmitter-which is often not realistic. This correspondence proposes a quantized precoding system where the optimal precoder is chosen from a finite codebook known to both receiver and transmitter. The index of the optimal precoder is conveyed from the receiver to the transmitter over a low-delay feedback link. Criteria are presented for selecting the optimal precoding matrix based on the error rate and mutual information for different receiver designs. Codebook design criteria are proposed for each selection criterion by minimizing a bound on the average distortion assuming a Rayleigh-fading matrix channel. The design criteria are shown to be equivalent to packing subspaces in the Grassmann manifold using the projection two-norm and Fubini-Study distances. Simulation results show that the proposed system outperforms antenna subset selection and performs close to optimal unitary precoding with a minimal amount of feedback.     

Numerical analytical simulation technique for nonlinear feedback systems

A simulation method for nonlinear inertial feedback systems has been proposed. It involves the presenting of each functional block of the system in the form of a set of nonlinear transfer functions. Nonlinear transfer functions of the system as a whole are determined by using the differential Taylor transformations.