Interference cancellation is one major multiuser detection strategy for suppressing interference effects and improving system performance. In this paper, a novel blind decision-feedback interference cancellation framework and its implementations with least squares, maximum likelihood and minimum mean squared error criteria are proposed and analyzed. Compared with existing blind multiuser receivers, the proposed framework and schemes require a small number of previous received signals and no subspace separation or sequence estimation. The detection complexity and delay can therefore be low. The proposed schemes can also be implemented in an adaptive fashion. Theoretical analysis as well as computer simulations is provided to demonstrate the performance of the proposed schemes.

Recent research for multiple input, multiple output (MIMO) systems has utilized the linear structure of linear dispersion codes (LDCs) to construct precoders with partial transmitter channel knowledge. In this paper, we build upon a coding structure known as dispersive covariance coding (DCC) derived in R. Hayes and J.J. Caffery (2005) for precoding scenarios in which statistical channel covariance knowledge is present at the transmitter. Previous work on DCCs has focused on special full-rank cases where code parameters were fixed. In this paper, we relax DCC parameter selection and design DCC codes with improved design flexibility such that any desired number of channel modes can be excited by the precoder. Unlike previous full-rank DCCs, we prove that our new DCCs are rank-deficient. Through theory and numerical simulation, we quantify the system's resulting loss in capacity. Furthermore, we develop useful distance metrics which illustrate the relationship between the full-rank and rank-deficient precoders

In this paper, we present an alternative multiuser signal model and discuss its applications for blind multiuser receiver design. We compare the model with the conventional and signal subspace-based signal models as well as their applications in receiver design. The geometric interpretation, bit-error rate, signal processing bounds, etc., of these signal models and blind receivers are compared and discussed. From the analyses, the trade-offs between the complexity and performance of blind multiuser receiver design are presented. Computer simulations are finally provided to demonstrate the performance and theoretic analysis

We present a new precoder designed to utilize channel covariance knowledge at the transmitter to simultaneously achieve high capacity and an optimized error performance. These dispersive covariance codes (DCCs) utilize the linear construction of linear dispersion codes (LDCs) to spread information-bearing symbols in the time and spatial domains. It is shown that the linear framework permits the design of capacity optimal codes with desirable performance properties. Since DCCs utilize only statistical channel information instead of instantaneous estimates, feedback overhead is low, and the simple linear encoder simplifies codebook characterization. The DCCs are constructed to satisfy a capacity optimality criterion in conjunction with a pairwise error probability (PEP) constraint to improve error performance. Through numerical simulation, a two-stage DCC design procedure is shown to utilize statistical channel knowledge to provide significantly improved capacity and bit error rate performance over LDC codes in the presence of channel correlations.

In this paper, we present a joint delay/AOA estimator for CDMA systems in a multiuser environment. The algorithm is applicable to space-time channel estimation for space-time processing systems and to location systems that employ hybrid AOA/TOA technology. The estimator is based on a novel formulation of ESPRIT in which the delays and AOAs of each user can be estimated and automatically paired. The estimator avoids the computational burden of a matching, or pairing, procedure and does not require the use of a Fourier transform to aid the estimation of the delays. Further, the algorithm has lower computational complexity than other joint angle/delay estimators. An asymptotic first order error analysis as well as the Cramer-Rao lower bound (CRLB) are derived. Simulations are presented to show the performance of the estimator.

Techniques for locating wireless subscribers for E-911 emergency services must meet the stringent accuracy requirements set by the FCC. In non-line-of-sight propagation conditions, large measurement errors with unknown statistics cause standard location schemes to perform poorly. In these challenging environments, it is necessary to develop methods that can integrate heterogeneous measurements and utilize all available information to mitigate the errors and improve location accuracy. This paper illustrates a hybrid time and angle of arrival technique that utilizes non-linear constrained optimization to estimate the mobile terminal's location with bounds on the range and angle errors inferred from geometry. Another simple hybrid TOA/AOA algorithm based on a least squares solution of lines of position is also proposed. These algorithms are compared to previous approaches using TOA only and simulation results are provided that show the accuracy gains of the proposed algorithms.

Multiuser detection is a key technology for combating multiple access interference (MAI) in CDMA systems. In this paper, we propose three effective one-shot multiuser semi-blind decorrelating detectors based on least squares (LS), total least squares (TLS) and mixed LS/TLS (MLS) formulations. All of the proposed algorithms are based on the classic decorrelating detector and a new semi-blind signature matrix. Different assumptions regarding the noise in the proposed semi-blind signature matrix lead to different detection algorithms. We also find that the classic decorrelating detector can be regarded as a special case of the proposed LS decorrelating detector. The proposed detection algorithms are simple and direct, where only the timing, signature and amplitude of the desired user are required. In addition, no search or convergence procedure is required. Finally, theoretical analysis and computer simulations are presented to illustrate the performance of the proposed algorithms.

Multiuser transmitter precoding is a technique that has been proposed for combating multiple-access interference (MAI). In this paper, the structure and principles of multiuser transmitter design are reviewed. In addition to multiuser precoding using the minimum mean-square error (MMSE) criterion [B. R. Vojoic and W. M. Jang, October 1998], we present a decorrelating multiuser precoder (DP), an approximate maximum likelihood (AML) multiuser precoder and a maximum asymptotic multiuser efficiency (MAME) multiuser precoder for mitigating MAI in synchronous CDMA systems. In addition, the geometric properties, asymptotic multiuser efficiency (AME) and mutual information rate between the transmitter and receiver are discussed. Finally, computer simulations are provided to demonstrate the performance of the proposed multiuser precoding algorithms.

Recently, MAC protocols using directional antennas for wireless ad hoc networks that are based on and similar to IEEE 802.11 type WLAN have been proposed. These protocols, however, are unable to attain substantial performance improvements because they do not enable the nodes to perform multiple simultaneous transmissions/receptions. In this paper, we propose a MAC layer protocol that exploits space division multiple access, thus using the property of directional reception to receive more than one packet from spatially separated transmitter nodes (equipped with smart antenna systems). Our simulation results show that drastic throughput improvements may be achieved through this scheme.

Due to unpredictable mobility patterns, nodes in an ad hoc wireless network can get separated into partitions and reconnected a number of times. Whenever a larger partition divides into two smaller ones, there is a critical link that fails and thus divides the partition into two. We propose an approach that utilizes network survivability concepts to detect the critical links in an ad hoc wireless network. Once the critical link has been detected, we propose two ways to delay or avoid their failure: change the trajectory of one or both nodes forming the critical link; and bring in another node to reinforce the link. It is assumed that the nodes know their location and periodically update their neighbors with their current locations. We have implemented these techniques in the network simulator ns-2. The simulation experiments show that the approach can significantly improve the network performance and reduce the degree of partitioning.

We examine a high-resolution signal estimation algorithm which can simultaneously estimate the spatial direction /spl theta//sub i/ and temporal frequency f/sub i/ of each source by extending the MUSIC algorithm to use space-time information obtained from one spatial sub-array and one temporal sub-array. Since the dimension of the autocorrelation matrix is flexible (due to the use of the time dimension) and can be chosen larger than the number of sources regardless of the number of array elements, the algorithm can perform joint estimation with only two array elements. Mathematical analysis is used to determine the complexity and compute the Cramer-Rao lower bound (CRLB). Computer simulation results are presented to demonstrate the algorithm's performance.

We present a novel approach for reducing the effect of non-line-of-sight (NLOS) error that inhibits accurate location of wireless mobile stations (MSs). The location algorithm is developed based on the distribution of times of arrival (TOAs) generated from multipath scattering models that have been published in the literature. By matching the statistics of the measured TOAs of several multipath arrivals with those produced by the scattering models, estimates of the line-of-sight (LOS) TOAs between each base station (BS) and the MS are obtained. The LOS TOAs can then be used in any traditional TOA location algorithm. The algorithm shows a significant improvement in location accuracy for a TOA system in NLOS environment.

We present an algorithm for mitigating the non-line-of-sight (NLOS) error that corrupts time of arrival (TOA), or range, measurements. The method uses the available data regarding the ranges and the base station (BS) layout, in conjunction with cell geometry, to adjust the measured ranges to near their true line-of-sight (LOS) values. The range adjustments are obtained by scaling the NLOS ranges using scale factors that are estimated from a constrained nonlinear optimization problem.

The knowledge of wireless ATM cell headers in the uplink data bursts can be used to improve the performance of the concatenated FEC system. This paper proposes a protocol aided error control scheme for wireless ATM. The proposed scheme achieves the performance improvement of about 0.2-1.0 dB at a BER of 10/sup -3/ in the fading channels with respect to the conventional concatenated FEC.

This paper presents a joint channel coefficient and time-delay tracking technique for code-division multiple-access (CDMA) systems. Due to the highly nonlinear nature of time delay estimation, an iterative nonlinear filtering algorithm, called the "unscented filter" (UF), is employed. The UF can provide a better alternative to nonlinear filtering than the conventional extended Kalman filter (EKF) since it avoids errors associated with linearization. The Cramer-Rao lower bound is derived for the estimator, and computer simulations show that it provides a more viable means for tracking time-varying amplitudes and delays in CDMA communication systems than estimators based on the EKF.

This paper presents a performance analysis of the noncoherent delay lock loop (NC-DLL) in the presence of multiple-access interference (MAI). A renewal theory approach is applied along with the more familiar linear theory. The effect of the MAI on performance is evaluated in terms of the mean-time-to-lose-lock and the root-mean-square (RMS) tracking error. The analysis is made tractable by approximating the aperiodic cross-correlation function between two pseudonoise (PN) code sequences. The results are compared with those which assume that the MAI can be approximated as Gaussian noise. As expected, the results indicate a drastic decrease in performance as the number of users in a system is increased. The effect of the near-far ratio, PN code sequence length, system load, and signal-to-noise ratio on the performance of the NC-DLL in MAI are also examined.

A radio location method for code-division multiple-access (CDMA) microcellular networks is investigated. The method discussed focuses on the use of time difference of arrival (TDOA) estimates for deriving the position of a mobile unit in a CDMA system, where the transmitted signal is received by several base stations (BSs). The TDOA estimates are obtained by using a delay-lack loop. A similar method is used in the global position system (GPS), but its performance in an urban microcellular propagation environment has not been previously studied. Simulation results are given for different radio propagation environments.

This paper discusses the use of mobile cellular radio networks for providing vehicle location information to intelligent vehicle/highway system (IVHS) services. We review various automatic vehicle location (AVL) techniques in context to their application in a direct-sequence code-division multiple-access (DS/CDMA) system, in particular, the EIA/TIA/IS-95 standard proposed by Qualcomm Inc. A new matched filter location technique which is suited to the CDMA environment is discussed along with simulated error statistics and recommendations for improvement. We also consider network loading effects in terms of Signaling System 7 (SS7).