Nikolaos D. Skentos

Adaptive Antenna Subarray Formation for MIMO Systems

MIMO systems with reduced hardware complexity have attracted researchers attention due to their high efficiency and low cost. Sub-optimum algorithms for antenna subset selection have been intensively studied in the literature. In this paper we present a new technique to maximize the capacity of multiple antenna wireless systems with reduced available RF chains. The technique is based on the adaptive formation of subarrays, i.e. the grouping of antenna elements and the application of appropriate element weights. The elements of each subarray and their weights are dynamically selected by an evolutionary optimization technique using the link capacity as a cost function

Selecting array configurations for MIMO systems: an evolutionary computation approach

This paper presents an antenna selection method for multiple-input multiple-output wireless systems. By exploitation of the channel transfer matrix, the antenna selection criterion is the maximization of the instantaneous capacity achieved using a specific number of transmitting and receiving antenna array elements. For each environment, the proposed method applies a genetic algorithm which seeks the most advantageous subset of antenna elements. The results are based on measured and simulated channels and show that the proposed method selects array configurations that yield superior performance compared to the arrays usually employed. Furthermore, comparative analysis results are presented, with respect to a state-of-the-art algorithm.

Capacity results from short range fixed MIMO measurements at 5.2 GHz in urban propagation environment

In this paper, wideband MIMO channel measurements conducted in Athens, Greece are described. Short range scenarios with fixed transmitter and receiver, in urban like environments under LOS propagation conditions have been measured, using an 8/spl times/8 vector channel sounder operating at 5.2 GHz. Based on the measured MIMO channel matrices, results of mean and outage normalized capacity calculations are presented. These results correspond to systems with various numbers of antenna elements and various ULA antenna configurations.

MIMO Channel Characterization for Short Range Fixed Wireless Propagation Environments

This paper reports results from wideband MIMO measurements performed in short range fixed wireless environments at 5.2 GHz. The objective is to provide MIMO channel characterization results for the measured environments and contribute to the limited available similar studies. Two kinds of propagation scenarios are investigated, rooftop to rooftop and street to rooftop, at three different sites always under LOS propagation conditions. The analysis of measurement data is performed in the context of non physical modeling, providing insight into the statistics of the measured channels. In particular, the slow time varying nature of the channel is studied and the narrow Doppler spectrum shape is approximated. Furthermore, frequency correlation results are obtained and the typical delay dispersion measures are extracted. Then, the antenna correlation is studied and the error of the Kronecker product approximation is evaluated. Finally, capacity results are provided and the channel measurements are characterized in terms of spatial multiplexing quality and multipath richness through condition number analysis.

Pattern diversity for single RF user terminals in multiuser environments

Although the utilization of multiple antennas enhances significantly the performance of multiuser systems, it is prohibitive for mobile devices due to size and power restrictions. In order to alleviate this problem we propose an alternative mobile architecture, built on compact antenna structures with fast beamforming capabilities. It is shown that the proposed architecture combined with a simple training procedure, enhances the average throughput of a multiuser system. The performance is compared with that of user devices with one or three antenna elements employing maximal ratio combining.

MIMO channel characterization results from short range rooftop to rooftop wideband measurements

This paper presents results from fixed outdoor wideband multiple input multiple output (MIMO) channel measurements performed at 5.2 GHz. Rooftop to rooftop, short range environments, under line of sight (LoS) propagation conditions were examined using 8-element uniform linear arrays. The scope is to provide MIMO channel characterization results beneficial for either stochastic or physical channel description. In particular, delay dispersion measures, Doppler spectrum, Rice factor and spatial fading correlation results are reported. Furthermore, with the use of a two dimensional parameter estimation algorithm, multipath components parameters are extracted and indicative double directional power spectra are provided.

Experimental multipath component characteristics for short range urban propagation environments

This paper presents results on the parameters of multipath components extracted from wideband measured Multiple Input Multiple Output (MIMO) channel matrices. Experimental data were collected with wideband channel sounding measurements at 5.2GHz in a microcell urban environment, under line of sight propagation conditions. The double directional and propagation delay parameter values of the dominant multipaths are estimated using the multidimensional parameter estimation algorithm. The results presented provide information in the context of parametric MIMO channel characterisation, refer to the number of identified waves and focus on the path weights characteristics. Additionally, the extracted multipath estimates are used to synthesise MIMO matrices that are compared to the directly measured data.

Multipath Parameter Results for Short Range Urban Propagation Environments

This paper presents results on the parameters of multipath components extracted from wideband measured MIMO channel matrices. Experimental data were collected with channel sounding measurements at 5.2 GHz in an urban environment, under LoS propagation conditions, for short range fixed wireless scenarios. The multidimensional Unitary ESPRIT algorithm was used to estimate the double directional angular and delay characteristics of the propagating waves. The results presented refer to the number of identified waves, their power, the channels angular spectra and aim to provide information in the context of parametric MIMO channel characterization. Additionally, based on the parameter estimates, MIMO matrices are synthesized and their capacity performance is evaluated in comparison to the directly measured data

An Enhanced Embedded-Pilot Channel Estimation Architecture for MIMO MC-CDMA Systems

In this work an enhanced semi-blind channel estimator is presented for multicarrier CDMA systems with multiple transmit and receive antennas. The novel scheme is based on embedded pilots that are characterized as virtual users and their number is determined in a dynamic fashion according to the propagation channel conditions. The superimposed code-multiplexed pilots are used initially for channel estimation before the despreading operation, thus achieving per subcarrier estimation using the known unique multi-layer pattern of the virtual users. Afterwards, the estimation is enhanced by using the despread pilots. Furthermore, an iterative scheme is incorporated in channel estimation process minimizing the effect of multiuser interference and operates either in interference cancellation, or in virtual user mode. The proposed architecture is evaluated for spatial diversity systems (space- frequency block coded) with two transmit and up to four receive antennas operating in realistic propagation channels for various modulation and coding schemes. The results demonstrate a significant performance improvement especially as the channel is dominated by its time variant characteristics. The dynamic assignment of spreading codes in the payload and the pilots offers to the system realistic flexibility and adaptivity in demanding channels achieving concurrently maximization of the real overall system throughput, while maintaining low overhead and complexity burden.