Preben Mogensen

A stochastic MIMO radio channel model with experimental validation

Theoretical and experimental studies of multiple-input/multiple-output (MIMO) radio channels are presented. A simple stochastic MIMO model channel has been developed. This model uses the correlation matrices at the mobile station (MS) and base station (BS) so that results of the numerous single-input/multiple-output studies that have been published in the literature can be used as input parameters. The model is simplified to the narrowband channels. The validation of the model is based upon data collected in both picocell and microcell environments. The stochastic model has also been used to investigate the capacity of MIMO radio channels, considering two different power allocation strategies, water filling and uniform and two different antenna topologies, 4/spl times/4 and 2/spl times/4. Space diversity used at both ends of the MIMO radio link is shown to be an efficient technique in picocell environments, achieving capacities within 14 b/s/Hz and 16 b/s/Hz in 80% of the cases for a 4/spl times/4 antenna configuration implementing water filling at a SNR of 20 dB.

A stochastic model of the temporal and azimuthal dispersion seen at the base station in outdoor propagation environments

A simple statistical model of azimuthal and temporal dispersion in mobile radio channels is proposed. The model includes the probability density function (PDF) of the delay and azimuth of the impinging waves as well as their expected power conditioned on the delay and azimuth. The statistical properties are extracted from macrocellular measurements conducted in a variety of urban environments. It is found that in typical urban environments the power azimuth spectrum (PAS) is accurately described by a Laplacian function, while a Gaussian PDF matches the azimuth PDF. Moreover, the power delay spectrum (PDS) and the delay PDF are accurately modeled by an exponential decaying function. In bad urban environments, channel dispersion is better characterized by a multicluster model, where the PAS and PDS are modeled as a sum of Laplacian functions and exponential decaying functions, respectively.

LTE Capacity Compared to the Shannon Bound

In this paper we propose a modification to Shannon capacity bound in order to facilitate accurate benchmarking of UTRAN long term evolution (LTE). The method is generally applicable to wireless communication systems, while we have used LTE air-interface technology as a case study. We introduce an adjusted Shannon capacity formula, where we take into account the system bandwidth efficiency and the SNR efficiency of LTE. Separating these issues, allows for simplified parameter extraction. We show that the bandwidth efficiency can be calculated based on system parameters, while the SNR efficiency is extracted from detailed link level studies including advanced features of MIMO and frequency domain packet scheduling (FDPS). We then use the adjusted Shannon capacity formula combined with G-factor distributions for macro and micro cell scenarios to predict LTE cell spectral efficiency (SE). Such LTE SE predictions are compared to LTE cell SE results generated by system level simulations. The results show an excellent match of less that 5-10% deviation.

CSCW challenges: cooperative design in engineering projects

This paper investigates how to support work and in particular cooperation in large-scale technical projects. The investigation is based on a case study of a specific Danish engineering company and it uncovers challenges to Computer Supported Cooperative Work (CSCW) in this setting. The company is responsible for management and supervision of one of the worlds largest tunnel/bridge construction projects. Our original goal was to determine requirements for CSCW as they unfold in this specific setting as opposed to survey and laboratory investigations. The requirements provide feedback to product development both on specific functionality and as a long term vision for CSCW in such settings. As it turned out, developing our cooperative design techniques in a product development setting also became a major issue. The initial cooperative analysis identified a number of bottlenecks in daily work, where support for cooperation is needed. Examples of bottlenecks are: sharing materials, issuing tasks, and keeping track of task status. Grounded in the analysis, cooperative design workshops based on scenarios of future work situations were established to investigate the potential of different CSCW technologies in this setting. In the workshops, mock-ups and prototypes were used to support end-users in assessing CSCW technologies based on concrete, hands-on experiences. The workshops uncovered several challenges. First, support for sharing materials would require a huge body of diverse materials to be integrated, for example into a hypermedia network. Second, tasks are closely coupled to materials being processed thus a coordination tool should integrate facilities for managing materials. Third, most daily work tasks are event driven and plans change too rapidly for people to register them on a computer. Without meeting these challenges, new CSCW tools are likely to introduce too much overhead to be really useful.

A stochastic multiple-input-multiple-output radio channel model for evaluation of space-time coding algorithms

A simple framework for Monte Carlo simulations of a multiple-input-multiple-output radio channel is proposed. The derived model includes the partial correlation between the paths in the channel, as well as fast fading and time dispersion. The only input parameters required for the model are the shape of the power delay spectrum and the spatial correlation functions at the transmit and receive end. Thus, the required parameters are available in the open literature for a large variety of environments. It is furthermore demonstrated that the Shannon capacity of the channel is highly dependent on the considered environment.

Spatial channel characteristics in outdoor environments and their impact on BS antenna system performance

This paper present measurement results obtained with a testbed equipped with an array antenna. The investigations focus on the power azimuth spectrum of the channel in urban and rural areas, for various base station antenna heights. The power azimuth spectrum is well modeled with a Laplacian function. The local azimuth spread is found to range from 1/spl deg/ to 25/spl deg/ depending on the environment and antenna height. The azimuth spread significantly increases as the antenna height is reduced. An analytical expression for the spatial correlation function is derived based on the Laplacian model. Finally, the power azimuth spectrums impact on a conventional beamformer ability to suppress interfering users is investigated.

Experimental analysis of the joint statistical properties of azimuth spread, delay spread, and shadow fading

Empirical results characterizing the joint statistical properties of the local azimuth spread (AS), the local delay spread (DS), and the shadow (slow) fading component are presented. Measurement data from typical urban, bad urban, and suburban (SU) environments have been analyzed. It is found that a log-normal distribution accurately fits the distribution function of all the investigated parameters. The spatial autocorrelation function of both AS, DS, and shadow fading can be modeled with an exponential decay function. However, for SU environments the spatial autocorrelation function is better characterized by a composite of two exponential decaying functions. A positive cross correlation is found between the AS and the DS, while both parameters are negatively correlated with shadow fading. All essential parameters required for the implementation of a simulation model considering the joint statistical properties of the AS, DS, and shadow fading are provided.

From antenna spacings to theoretical capacities - guidelines for simulating MIMO systems

Capacity increases promised by multiple-input multiple-output (MIMO) systems mostly depend on the spatial correlation properties of the radio channel. The paper investigates the connection between these properties and the capacity figures. It first derives the correlation coefficient between two antenna elements as a function of their spacing, the power azimuth spectrum (PAS), the azimuth spread (AS) and the mean angle of incidence of the waves, for three different types of PAS, namely uniform, truncated Gaussian and truncated Laplacian. With the help of the established relations, correlated flat-fading MIMO channels are generated, whose capacity performance and effective degrees of freedom (EDOF) are investigated for two power allocation schemes, water-filling and uniform. The impact of channel estimation errors is also evaluated.

Autonomous component carrier selection: interference management in local area environments for LTE-advanced

Low-power base stations such as femtocells are one of the candidates for high-data-rate provisioning in local areas, such as residences, apartment complexes, business offices, and outdoor hotspot scenarios. Unfortunately, the benefits are not without new challenges in terms of interference management and efficient system operation. Due to the expected large number of user-deployed cells, centralized network planning becomes impractical, and new scalable alternatives must be sought. In this article we propose a fully distributed and scalable solution to the interference management problem in local areas, basing our study case on LTE-Advanced. We present extensive network simulation results to demonstrate that a simple and robust interference management scheme, called autonomous component carrier selection, allows each cell to select the most attractive frequency configuration; improving the experience of all users and not just the few best ones, while overall cell capacity is not compromised.

Carrier load balancing and packet scheduling for multi-carrier systems

Abstract-In this paper we focus on resource allocation for next generation wireless communication systems with aggregation of multiple Component Carriers (CCs), i.e., how to assign the CCs to each user, and how to multiplex multiple users in each CC. We first investigate two carrier load balancing methods for allocating the CCs to the users- Round Robin (RR) and Mobile Hashing (MH) balancing by means of a simple theoretical formulation, as well as system level simulations. At Layer-2 we propose a simple cross-CC packet scheduling algorithm that improves the coverage performance and the resource allocation fairness among users, as compared to independent scheduling per CC. The Long Term Evolution (LTE)-Advanced is selected for the case study of a multi-carrier system. In such a system, RR provides better performance than MH balancing, and the proposed simple scheduling algorithm is shown to be effective in providing up to 90% coverage gain with no loss of the overall cell throughput, as compared to independent scheduling per CC.