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A high-resolution channel sounding technique has been used to investigate the cross-polarization of electromagnetic waves in the 5-6 GHz band. Experiments were performed in two non-line-of-sight indoor locations, and it was found that there is a strong dependency of the cross-polarization of multipath components on the elevation angle-of-arrival. For a vertically polarized transmitting antenna, clusters of co-polarized multipath components were confined predominantly to the region around the horizontal plane that contains the virtual line-of-sight between transmitter and receiver. In contrast, cross-polarized signals were detected for a variety of elevation angles with considerable power levels. The surroundings of the receiver were identified as the principal source of depolarized signals. In addition, time dispersion analysis of the multipath signals led to the determination of the AoAs where there is a strong correspondence between co- and cross-polarized signals as a consequence of the partial depolarization of MPCs. This work supports the exploitation of the joint space and polarization diversities in indoor propagation scenarios to improve the system performance. Index Terms--Electric field measurement, indoor propagation, multipath channels, polarization.

The application of genetic algorithm (GA) techniques to the problem of overloaded arrays, in which the number of transmitted narrowband signals is greater than the number of receiver array elements, is explored. A new receiver algorithm is presented, which achieves nearly optimal performance but requires significantly less complexity than the maximum-likelihood joint detection (MLJD) receiver. It uses GA techniques with soft-biased initialization, which is efficiently generated using spatial filtering, providing dramatic convergence improvements compared with other initialization schemes. Simulations using both idealized channel models and measured channel responses are used to investigate the impact of spatial correlation and imperfect channel state information (CSI). Index Terms--Array signal processing, genetic algorithms (GAs), multiuser detection.

Tests for the wide-sense stationarity (WSS) of multivariate random processes are developed and applied to measured multiple-input-multiple-output (MIMO) radio channel responses. Evolutionary spectral density functions are used to estimate the time-dependent power spectral density, and analysis of variance procedures are applied to determine whether the first and second moments of the data series exhibit statistically significant changes over short intervals. It is found that while the assumption of WSS is met in 40%-70% of half-second intervals for a single element at the transmitter and receiver, only 2%-12% of intervals with two elements at the transmitter and receiver are jointly WSS; furthermore, the occurrence of non-WSS intervals is not dependent on the antenna separation. The analyses also show that WSS intervals are interspersed with long non-WSS intervals. Index Terms--Multipath channels, multiple-input-multipleoutput (MIMO) systems, radio propagation mesurements, statistical analysis, wide-sense stationarity.

A reduced-complexity time-domain equalization scheme for wideband turbo-multiple-input multiple-output (turbo-MIMO) systems is presented. This scheme, called iterative trellis search equalization, is based on a modified version of the M-BahlCocke--Jelinek--Raviv (M-BCJR) algorithm, applied to a suitably chosen trellis representation of the wideband MIMO channel process. Exploiting the properties of quadrature amplitude modulation (QAM) signal constellations with block-partitionable labels, this modified M-BCJR algorithm has complexity per bit that is independent of the constellation size, and polynomial in the number of transmit antennas and channel memory. Results from computer simulations show that the new scheme successfully mitigates intersymbol interference even if only a very small fraction of trellis state transitions is considered. It is also demonstrated that asynchronous transmission of the spatially multiplexed symbol streams can result in considerable performance improvement compared to synchronous MIMO systems. Index Terms--Complexity reduction, iterative detection/ decoding, multiple-input multiple-output (MIMO) systems, trellis searching.

This paper presents a reduced-complexity soft-input soft-output detection scheme, called iterative tree search detection, for multiple-input multiple-output wireless communication systems employing turbo processing at the receiver. In this scheme, a reduced search space is selected with the aid of the M-algorithm, and QAM signal constellations with block partitionable labels are used in order to make the detection complexity per bit almost independent of the modulation order, as well as asymptotically linear in the number of transmit antennas. Results from computer simulations are presented which demonstrate the capability of the scheme to approach optimal performance at considerably reduced complexity. Index Terms--Complexity reduction, iterative detection/ decoding, multiple-input multiple-output (MIMO) systems, tree searching.

Optimization of the performance of multicarrier transmission over a linear dispersive channel is presented. The optimum data and power assignment to the subcarriers are derived for both the conventional error probability criterion, and a new criterion based on the normalized mean-square error. The assignments and algorithms hold for channels where performance is degraded by additive noise, intersymbol and interchannel interference. Lower bounds on throughput are derived and are used to compare multicarrier performance with conventional single-carrier quadrature amplitude modulation (QAM) with both linear and decision feedback equalization. It is seen that multicarrier transmission can provide a significant improvement at low and intermediate channel signal-to-noise ratios. As an example, the optimization is applied to the high-speed digital subscriber loop, and multicarrier transmission is demonstrated to be superior to decision feedback equalized single-carrier QAM. Index Terms - Muiticarrier transmission, channel equalization, high-speed digital subscriber loop.

Frame synchronization is traditionally achieved by inserting a given sequence periodically into the data stream. Here, a new method of synchronization is introduced in which the synchronization sequence is not contiguous, but is interleaved with the data symbols. The properties of these distributed sequences are investigated, and requirements are specified for detectibility of these sequences when inserted in random data. Methods for constructing and searching for good sequences are presented. It is demonstrated that this new method reduces the probability of false synchronization over traditional methods in the presence of additive white Gaussian noise. Index Terms--Correlation, frame synchronization, Gaussian channels, maximum-likelihood detection, sequences, signal detection, synchronization.

For channels which suffer predominantly from additive noise and intersymbol interference, the decision-feedback equalizer has provided a relatively simple solution for reducing the effects of interfering symbols at the input to the decision device. In this paper, a technique is developed that enables fast, accurate calculation of the error performance of decision-feedback equalization for a number of channel models. The method is to calculate the n-step transition probability for an associated Markov process and then use this transition probability as an approximation to the stationary probability distribution. For systems with finite memory, it is proved that the method converges. If the signal-to-noise ratio (SNR) is high and the signal amplitude is more than twice the worst-case interference, it is shown that the convergence is rapid. Numerical results indicate that the convergence is rapid enough to make this an efficient method of calculation, even for channels for which the interference does not fully satisfy this condition. Two examples are given here, but the technique has been tested on most of the examples that have been presented in the literature. The method yields results in closer agreement with simulation results than previous results obtained using bounding techniques, especially at low to moderate SNR's and requires less computation. Index Terms--Decision-feedback equalization, intersymbol interference.

In this paper, we develop a cross-layer analytical framework for studying the performance of reliable relaying schemes in multihop cooperative diversity systems. A generalized signaling model over independent nonidentical Nakagami-m fading channels is assumed, and mathematical expressions for end-to-end throughput rate, spectral efficiency, and latency distribution are derived. The proposed framework can capture the effect of correlated packet errors caused by cooperative retransmission attempts and, thus, adequately evaluate the network performance. Using the framework, the performance of automatic-repeat-request-based relaying schemes is demonstrated over a range of network conditions and system parameters. It is shown that tradeoffs are necessary among network robustness, throughput rate, spectral efficiency, and latency variability. [ABSTRACT FROM AUTHOR]

This paper presents a method to model and simulate time-varying narrowband multiple-input-multiple-out (MIMO) channels based on observations from measured data. The data were obtained in a vehicular urban environment, with a fixed transmitter array and a mobile receiver array. The measured channel response matrices were decomposed to isolate the coupling from the transmitter eigenbasis to the received eigenbasis, as in the Weichselberger model. These complex coupling elements have been characterized and seen to comprise directional components that may be Ricean or Rayleigh fading. The Rayleigh fading directional components can be well modeled using the von Mises probability density function, which is parameterized for the time-varying model using the measured data. The model has been validated by comparing the mutual information and eigenstructure autocorrelation characteristics of its output with those of the measured data. The statistical nature of the model means that different realizations can be generated, each representative of the originating data. [ABSTRACT FROM AUTHOR]

This correspondence considers a wireless open-spectrum communication system with N orthogonal narrowband tones and K users who compete to maximize their individual data rates. The correspondence proposes a robust version of the iterative water-filling algorithm (IWFA) that takes into account potential inaccuracies in the noise-plus-interference levels available at the transmitters. It is shown that, in comparison with the conventional version, the robust IWFA (RIWFA) can lead the users to behave in a less greedy fashion and to collaborate unintentionally in order to increase the network sum-rate. In particular, using an appropriately chosen RIWFA parameter, we show that RIWFA yields almost the same sum-rate as price-based IWFA (PIWFA) which requires collaboration of users. [ABSTRACT FROM AUTHOR]