Linear least squares (LLS) estimation is a suboptimum but low-complexity localization method based on measurements of location-related parameters. It has been proved that selection of the reference anchor influences the LLS localization accuracy. In addition, hybridization of different types of measurements can fix the deficiencies of one type of measurements. In this paper, we proposed a new reference selection criterion for the hybrid TOA/RSS LLS localization technique (called H-LLSRS), which considers both measured ranges and the information about their coarse variances. Moreover, we consider a general scenario that variances of range measurements are different, and derive a weighted LLS (WLLS) estimator for hybrid TOA/RSS localization according to the information about the accurate ranging variances and the correlations among the observations. Simulation results show that if the RSS-based ranging variances are considerably larger than the TOA-based ranging variances, the H-LLS-RS localization technique yields better accuracy than the conventional LLS localization techniques. Furthermore, reference selection has no effect on the accuracy of WLLS localization technique.

This paper considers the existence of sidelobe emission in OFDM based rich-Femtocell systems. By modeling the interference from the arbitrary-distributed Femtocells as shot-noise process, the paper analyzes the effect of sidelobe interference on the system performance metrics such as transmission capacity (TC) and area spectral efficiency (ASE). The numerical results calculated for three different OFDM schemes show that, under the circumstance of very high Femto deployment intensity, the rigorous constraints on first-tier, namely the Macrocell outage probability and the reduction of first-tier ASE, prohibit the Femtocells from excessive subband access. Thus the out-of-band emissions contribute significantly to the system performance loss. Further analysis has shown that poor channel condition can even worsen the situation, making the existence of sidelobe interference not ignorable. Among the three schemes, OQAM-OFDM is proved to achieve the best performance. Although OQAM-OFDM has higher requirements of implementation, it is reasonable to be considered as one of the candidate transmission schemes for future Femtocell applications.

Orthogonal frequency division multiplexing (OFDM) signals are sensitive to the mismatching errors between the in-phase (I) and quadrature (Q) branches in the up and down-conversions. These mismatching errors including phase and amplitude mismatches make the signal constellation expand and rotate, which severely degrades the performance of OFDM systems. In this paper, a method of signal design for OFDM communication system is proposed in order to mitigate the impact of IQ mismatching errors. A simple quasi-symmetric conjugate (QSC) structure of signal in frequency domain designed at the transmitter side can completely remove the self-interference caused by IQ mismatching errors. At the receiver, the estimation and the compensation for IQ mismatch are not necessary. Analytic and simulation results can show the efficiency of QSC method in IQ mismatch mitigation compared to other techniques such as Adjacent Symbol Repetition (ASR), Adjacent Conjugate Symbol Repetition (ACSR), Symmetric Symbol Repetition (SSR), or Symmetric Conjugate Symbol Repetition (SCSR), respectively.

In order to coexist with existing legacy wireless systems, the transmit power spectral density (PSD) of ultra-wideband (UWB) impulse radio systems is limited. The coverage range of the UWB systems is then confined within a few meters. One possible way to achieve a greater coverage is a short-range multihop relaying strategy. This paper proposes a two-hop UWB relay system using multiple detect-and-forward relays, each of which is equipped with multiple antennas. Each multiple-antenna relay performs spatial diversity reception with partial-Rake scheme in the first hop, and decides whether or not to perform spatial transmit diversity with partial-pre-Rake scheme in the second hop. This leads not only to low complexity at the receiver, but also to reduced error propagation. Moreover, we derive the outage probability of the proposed relay system based on the IEEE 802.15.3a channel model. The effects of using multiple relay antennas, relays, and Rake/pre-Rake fingers on the outage performance are demonstrated. The tradeoffs among the numbers of relay antennas, relays, and Rake/pre-Rake fingers are also investigated. Numerical results show that it is best to use multiple relay antennas to improve the outage performance of the proposed system. Furthermore, in general, increasing the number of relays is more beneficial than increasing the number of Rake/pre-Rake fingers.

An antenna selection scheme for multi-input multi-output (MIMO) ultra-wide band (UWB) communication system with time reversal (TR) is investigated in this paper. The system exploits spatial multiplexing (SM) scheme to achieve high data rate. In order to cope with long delay spread of the UWB channel, time reversal technique is adopted. TR can mitigate not only the interference intersymbol (ISI) but also multi stream interference (MSI) caused by transmitting several data streams simultaneously. Antenna selection algorithm allows to reduce the number of transmit antenna by using the channel state information (CSI), which is already available for time reversal. Simulation results show that the algorithm can considerably improve the BER performance of the system when the number of diversity branch is not so large.

Non-line-of-sight propagation in wireless localization systems leads to a challenging problem for the estimation of a mobile position. A hybrid idea has been considered as a promising approach to increase estimation accuracy. Existing methods, however, are formulated using separate measurements, necessitating different observation quantities, e.g. received signal and received signal strength, thus making the parameter estimation cumbersome. In this paper, we propose a new hybrid wireless geolocation model requiring only one observation quantity, namely the received signal. The attenuation model for transmit power from base stations is explored herein to capture propagation features in the received signal model. Fortunately, it also provides a more realistic approach to wireless geolocation. To investigate geolocation accuracy, the Cramer-Rao bound (CRB) is derived for the estimation error of the mobile position. For any value of path loss exponent, the obtained result provides a generalized form of the CRB for the usual time delay case. In small cells, e.g. office building picocells, numerical examples illustrate that the accuracy of mobile position estimation exploring path loss is improved comparing with that provided by the usual time delay method.

The mobile position estimation using time-of-arrival (ToA) is considered for the wireless NLoS geolocation exploring a signal strength(SS) based on path loss. As exploited the path attenuation, a hybrid SS-ToA approach indicates a performance improvement compared with the usual ToA method. To realize this prospect, it calls for an estimator to determine the mobile position from the time delay. In this paper, we show that the use of line-of-sight (LoS) time delay provides the same performance as that given by using both the LoS and non-LoS (NLoS) time delays. We then propose least squares (LS), weighted least squares (WLS) and maximum likelihood (ML) to estimate the mobile position. Theoretical performance of the LS and WLS is analyzed. It reveals that for different LoS time delay error variances, the LS error variance is larger than the WLS error variance, which is equal to the Cramer-Rao bound (CRB). Numerical results illustrate that the time delay performance analysis is accurate when the time delay estimate is close to its true value, i.e. for small time delay error variance. For high SNR and large effective bandwidth, the LS cannot provide the performance compared with the CRB, whereas the WLS and ML are statistically efficient.

Ultra-wide-band (UWB) technology combined with multiple transmit and receive antennas (MIMO) is a viable way to achieve data rates of more than 1 Gb/s for wireless communications. UWB is typically applied to short-range and therefore mainly indoor communications in environments characterized usually by dense multipath propagation. For this type of environment, MIMO systems allow for a substantial increase of spectral efficiency by exploiting the inherent array gain and spatial multiplexing gain of the systems. In this paper, we provide a brief overview for UWB-MIMO wireless technology. The overview covers channel capacity, space-time coding (STC), and beamforming. It is shown that the spectral efficiency is increased logarithmically and linearly,respectively, for single transmit and multiple receive antennas (SIMO) and MIMO systems. For multiple transmit and single receive antenna (MISO) systems, a threshold for the data transmission rate exists such that the spatial multiplexing gain can be obtained if the data rate is lower than this threshold, but it is not beneficial to deploy multiple transmit antennas if the required data rate is higher than the threshold. Two STC schemes for UWB-MIMO are briefly discussed, and their performance comparison is presented. A discussion about antenna selection is also presented, and the performance comparison between antenna selection and equal gain combiner is provided showing the diversity gain for some scenarios. For the beamforming, it is shown that the optimal beamformer is obtained if all the weighting filters in each antenna branch are identical. About the optimal beamformer, it is found that the amplitude of the side lobe is independent of the ray incidence angle, and the amplitude of the main lobe is increased by a fold of the element number in the array. Three kinds of beam patterns are defined, and the beamwidth of the main lobe is given. Experimental results based on an offline testbed are provided to verify some analytical results presented in this paper. Since UWB-MIMO is still in its research infancy, the aim of this paper is to present some first results on spatial multiplexing, STC, and beamforming to illustrate the potential of UWB-MIMO.

In this paper, we present detect-and-forward relaying approaches for a coherent ultra-wideband (UWB) detection (specifically selective-Rake reception) system and for a noncoherent UWB detection (specifically differential transmittedreference, DTR) system in order to achieve greater coverage in multipath fading channels. Multiple-antenna relay system are also proposed to further enhance the overall system performance. The corresponding bit error rate (BER) performance is evaluated theoretically and via simulations. The effect of some design factors on this performance is investigated. Analytical and simulation results verify the performance improvement of the proposed relay systems over the direct transmission systems.

In this paper, we investigate how various kinds of design parameters in IEEE 802.11n wireless networks affect the system throughput. It is focused on the design of the optimal packet length from the viewpoint of cross-layer interaction between the MAC and PHY. The following results are obtained: I) The optimal packet length in logarithmic scale, expressed as log Lp,opt, can be finely approximated by a second-order polynomial of the signal-to-noise ratio (SNR); 2) The coefficients of the aforementioned approximation function depend on the data transmission rate and other parameters specified in the protocol, but it does not depend on the type of the MAC access mechanisms; 3) The number of active nodes has little effect on Lp,opt, especially when the SNR is low; 4) The system throughput is sensitive to the packet length Lpwhen the SNR is low, while it is insensitive to Lpin a considerably large neighborhood of Lp,optwhen the SNR is high; and 5) Aggregation can always increase the system throughput. Particularly, when the SNR is low, the increasing rate of the system throughput with the number of subframes is remarkably high. Therefore, in the low SNR regime, making the packet length adaptive to the channel SNR and increasing the number of subframes are essential to improve the system performance.

In this paper, we aim at developing an Hinfinapproach, from control-theoretic viewpoint, to the design of an active queue management (AQM) based congestion controller for wireless networks supporting Internet protocol. We study networks in which the backbone is a traditional wired network supporting Internet TCP, while end user access is via wireless way. First, a dynamic model for the congestion control problem of wireless networks is built up, which enables the application of modern control theory on time-delay systems to this problem. Second, an Hinfindesign approach for general time-delay systems is presented. Finally, the proposed approach is applied to the congestion controller design of wireless networks, yielding an effective and systematic way for the design problem. Simulation results are provided to illustrate the design procedure and the effectiveness of the proposed method

The issue for designing robust adaptive stabilizing controllers for nonlinear systems in Takagi-Sugeno fuzzy model with both parameter uncertainties and external disturbances is studied in this paper. It is assumed that the parameter uncertainties are norm-bounded and may be of some structure properties and that the external disturbances satisfy matching conditions and, besides, are also norm-bounded, but the bounds of the external disturbances are not necessarily known. Two adaptive controllers are developed based on linear matrix inequality technique and it is shown that the controllers can guarantee the state variables of the closed loop system to converge, globally, uniformly and exponentially, to a ball in the state space with any pre-specified convergence rate. Furthermore, the radius of the ball can also be designed to be as small as desired by tuning the controller parameters. The effectiveness of our approach is verified by its application in the control of a continuous stirred tank reactor.

The output tracking control problem for nonlinear systems in the presence of both parameter perturbations and external disturbances is studied. Our approach is based on the Takagi-Sugeno (T-S) fuzzy modeling method and a variable structure control (VSC) technique. Therefore, the systems considered are not and need not be in the triangular and parametric strict-feedback form, which are prevalent among adaptive model following control for nonlinear systems, or in the normal form, which pervades almost all existing results in neuro-fuzzy model following control approach. We first study the problem of stabilization of T-S fuzzy systems by using a VSC technique. A method for the design of a switching surface based on linear matrix inequalities is developed and a stabilizing controller based on a reaching law concept in the presence of both parameter perturbations and external disturbances is proposed. Then, the method is extended to design controllers for output tracking of T-S fuzzy nonlinear systems in two cases, i.e. systems which possess the so-called strong passive subsystems and strong stable zero dynamics, respectively. Finally, illustrative examples are presented to demonstrate the whole design procedure from the original nonlinear systems to their fuzzification and finally to the realization of the desired controllers. Simulation results show that the goal of output tracking can be achieved by the proposed controllers.

The design problem of proportional and proportional-plus-integral (PI) controllers for nonlinear systems is studied. First, the Takagi-Sugeno (T-S) fuzzy model with parameter uncertainties is used to approximate the nonlinear systems. Then a numerically tractable algorithm based on the technique of iterative linear matrix inequalities is developed to design a proportional (static output feedback) controller for the robust stabilization of the system in T-S fuzzy model. Next, we transform the problem of PI controller design to that of proportional controller design for an augmented system and thus bring the solution of the former problem into the configuration of the developed algorithm. Finally, the proposed method is applied to the design of robust stabilizing controllers for the excitation control of power systems. Simulation results show that the transient stability can be improved by using a fuzzy PI controller when large faults appear in the system, compared to the conventional PI controller designed by using linearization method around the steady state.