Anders Ranheim

Mutual coupling effects on the capacity of multielement antenna systems

A study of the capacity of multiple element antenna systems is presented, with particular emphasis on the effect that mutual coupling between the antenna elements has on the capacity. The results presented here show, contrary to some earlier claims, that correlation between different channel coefficients as a function of antenna spacing, can in fact decrease when the mutual coupling effect is accounted for. As a consequence, capacity also improves. A realistic channel model is used to perform simulations to support these claims.

A decoupled approach to adaptive signal separation using an antenna array

This paper considers a new approach to the problem of adaptive separation of digitally modulated signals arriving at an antenna array. The basic idea is to decouple the estimation procedure, treating one signal at a time to be the signal of interest. The method uses an iterative scheme similar to the well-documented iterative least squares with projection (ILSP) algorithm, exploiting the finite alphabet (FA) property of the signals to obtain signal estimates. The proposed algorithm is analyzed in terms of consistency and uniqueness properties, and a statistical performance analysis is carried out. The analysis together with simulation results indicate that a significant saving in computational complexity is accomplished relative to ILSP, while essentially having similar performance.

Decoupled blind symbol estimation using an antenna array

The problem of separating superimposed digitally modulated signals using an array of antennas is considered. The proposed method exploits the finite alphabet structure to demodulate one signal at the time, resulting in a computationally efficient solution. The resulting signal estimates are shown to be exact in the noise-free case. In noisy scenarios, the performance is comparable with that of the iterative least squares approach, which demodulates all signals simultaneously at a higher computational cost.

Performance of decoupled direction finding based on blind signal separation

A number of methods for blindly separating superimposed digitally modulated signals arriving at an antenna array have been proposed. These techniques are efficient at the up-link (mobile to base) in a mobile communication system. However, for solving the base-to-mobile beamforming problem it may be necessary to also estimate the directions-of-arrival (DOAs) of the various signal paths. We present an optimal decoupled DOA estimation procedure based on information from the blind separation algorithm. Its performance is evaluated in the presence of spatially correlated noise and array modeling errors. The proposed technique has computational advantages as compared to traditional DOA estimation, because the different signal waveforms are treated in a separated fashion. Yet, the decoupled approach is shown to be substantially less sensitive to modeling errors and interference.

An iterative receiver algorithm for space-time encoded signals

An iterative receiver is proposed for a wireless communication system employing multiple transmit and receive antennas. The transmitted symbol sequences are space-time encoded. By exploiting the inherent structure in the space-time encoded data sequence, the receiver is able to significantly improve the initial estimate of the unknown channel leading to significant performance gains in terms of the bit-error-rate (BER) as a function of the signal-to-noise ratio (SNR). Computer simulations demonstrate the efficacy of the scheme in single-user and multi-user environments.

Joint symbol detection and channel parameter estimation in asynchronous DS-CDMA systems

A method for jointly estimating the time delay and complex gain parameters, as well as detecting the transmitted symbols in an asynchronous multipath DS-CDMA system, is presented. A short training sequence is used to obtain a coarse estimate of the channel parameters, which is consequently used to detect the symbols. By exploiting structure in the digitally modulated signals, the method iterates to (i) improve the estimate of the channel parameters and (ii) reduce the probability of incorrect detection. The methods efficacy is demonstrated by numerical simulations.

Time delay estimation for multipath CDMA-systems based on a fast minimization technique for subspace fitting

A low complexity algorithm for parameter estimation in a block fading multipath DS-CDMA system is presented. The main contribution is a novel technique for minimizing a subspace fitting criterion which is obtained as a large sample approximation of the maximum likelihood (ML) estimator. The minimization procedure is based on an approximation of the exact criterion function, allowing a direct analytic solution. For the acquisition phase, an initialization procedure similar to alternating projections is employed which exhibits remarkable global convergence properties. The efficacy of the proposed method is demonstrated by means of numerical simulations.

A decoupled WLS approach to DS-CDMA multiuser detection

A new algorithm for multiuser detection in a DS-CDMA system is presented. Based on an algorithm originally developed in the array-processing context, it minimizes a weighted least-squares criterion, and detects one signal at a time. A performance analysis in terms of the bit-error rate is carried out, and demonstrates that the proposed method is asymptotically identical to the LMMSE-estimator, but without assuming or requiring any knowledge of the signal amplitudes. Only the desired users signature sequence and timing is required. Both a batch and a recursive formulation of the algorithm is described and simulation results are included in addition to the analysis, to highlight the comparison to other methods.

A recent subspace-based approach to joint detection and time-delay estimation in DS-CDMA systems

This paper presents a novel method for jointly estimating the time-delay parameters and detecting the transmitted symbols in a DS-CDMA system. A short training sequence is used to get an initial estimate of the time-delay, which is consequently used to detect the symbols. The method then iterates while exploiting the signal structure, to improve the performance. Simulation results are presented to compare the algorithm with the decorrelating criterion and the matched-filter receiver in terms of bit-error rate, and with the MUSIC algorithm and the sliding correlator in terms of the variance of the time-delay estimates.

Improved iterative least squares RAKE-combiner using beamspace transformations

Antenna arrays can be used in mobile communication systems to increase capacity and performance. However, as the number of antenna elements grows, the computational burden increases significantly. To reduce the total computational cost, a beamspace transformation is derived, based on the statistical model of the array signal. Although the transformation derived here is applicable to many algorithms, it is matched to the decoupled weighted iterative least squares with projections RAKE-combiner algorithm. Not only is the computational cost reduced, the overall performance is improved by using an appropriate beamspace transformation.