Multipath Tracking Research Articles

Adaptive feedforward/feedback architectures for multiuser detection in high data rate wireless CDMA networks

We consider the design and performance of nonlinear minimum mean-square-error multiuser detectors for direct sequence code-division multiple-access (CDMA) networks. With multiple users transmitting asynchronously at high data rates over multipath fading channels, the detectors contend with both multiple-access interference (MAI) and intersymbol interference (ISI). The cyclostationarity of the MAI and ISI is exploited through a feedforward filter (FFF), which processes samples at the output of parallel chip-matched filters, and a feedback filter (FBF), which processes detected symbols. By altering the connectivity of the FFF and FBF, we define four architectures based on fully connected (FC) and nonconnected (NC) filters. Increased connectivity of the FFF gives each user access to more samples of the received signal, while increased connectivity of the FBF provides each user access to previous decisions of other users. We consider three methods for specifying the FFF sampling and propose a nonuniform FFF sampling scheme based on multipath ray tracking that can offer improved performance relative to uniform FFF sampling. For the FC architecture, we capitalize on the sharing of filter contents among users by deriving a multiuser recursive least squares (RLS) algorithm and direct matrix inversion approach, which determine the coefficients more efficiently than single-user algorithms. We estimate the uncoded bit-error rate (BER) of the feedforward/feedback detectors for CDMA systems with varying levels of power control and timing control for multipath channels with quasi-static Rayleigh fading. Simulations of packet-based QPSK transmission validate the theoretical BER analysis and demonstrate that the multiuser RLS adapted detectors train in several hundred symbols and avoid severe error propagation during data transmission mode.

Reducing multipath tracking errors in spread-spectrum ranging systems

Analysis of coherent and noncoherent delay lock loops shows that a smaller early-late spacing than the commonly used one chip time reduces multipath tracking errors considerably. The results are especially interesting for spreadspectrum ranging systems such as GPS.

Data retrieval from bottom instrumentation using acoustic link

Abstract Bottom instrumentation is being used to collect diverse oceanographic, seismic, and geodetic data, often over prolong periods of time. Such data can be retrieved periodically using an acoustic communication link. Typical but conflicting requirements of such a link are high transmission rates, reliable transmission over long distances, and low power consumption of bottom transmitter. In this article a steerable narrow‐beam acoustic link is suggested as the best feasible approach offering improvement over broad‐beam systems. Various beam configurations are considered and compared in terms of their ability to reject ambient noise and multipath interference. A vertically suspended linear receiving array is postulated as the best solution to multipath suppression and beam tracking.

Multipath Tracking Errors in Elevation-Scanning and Monopulse Radars

The tracking performance of elevation- scanning and monopulse radars in the presence of multipath propagation are compared. The key difference between these two generic types of radars is the way they respond to moving targets. There are no significant differences between their responses to pure specular multipath, nor to diffuse multipath for targets on radial courses. However, they are found to respond quite differently to the diffuse com ponent for low-altitude crossing targets. For these conditions the tracking errors for elevation-scanning radars may be several times those for monopulse radars.

High Range-Resolution Monopulse Tracking Radar

High range-resolution monopulse (HRRM) tracking radar which maintains wide instantaneous bandwidth through both range and angle error sensing channels provides range, azimuth, elevation, and amplitude for each resolved part of the target. The three-dimensional target detail can be used to improve and extend radar performance in several ways: for improved precision of target location, for target classification and recognition, to counter repeater-type ECM, to improve low-angle multipath tracking, to resolve multiple targets, as a miss-distance measurement capability, and for improved tracking in chaff and clutter. These have been demonstrated qualitatively except for the ECCM to repeater ECM and low-altitude tracking improvement. Initial results from an experimental HRRM radar with 3-ns pulse length show resolution of aircraft into its major parts and precise location of each resolved part accurately in range and angle. Realtime closed-loop tracking is performed on aircraft in flight using high-speed sampled, digitized, and processed HRRM range and angle video data. Clutter rejection capability is also demonstrated.

Further Results on Multipath Angle Error Reduction Using Multiple-Target Methods

The two-target technique proposed by the author in an earlier paper [1] for reducing radar multipath angle tracking errors has been simulated on a digital computer assuming an actual closed-loop system. When tracking with noise, the technique provides angle error performance which compares quite favorably with the expected performance given in [1] Furthermore, the large bias errors usually encountered in normal monopulse systems at low elevation angles are removed. Results of typical tracks are given, both for the method of [1], and for a modified version of the method which applies primarily to shipboard radar systems. Some results on loss of lock are also presented.