Aliye Ozge Kaya

Coordinated dual-layer beamforming for public safety network: Architecture and algorithms

The revolutionary success of commercial broadband wireless network has spurred significant interest in employing related technologies such as 3GPP long term evolution (LTE) technologies to build a nationwide mobile broadband network for public safety entities. The successful deployment and operation of a public safety network, however, are more challenging than the traditional commercial wireless networks due to the high reliability and security requirements, unpredictable traffic patterns, and fast time-varying user population density. In this paper, we propose a framework of coordinated dual-layer beamforming schemes to address these challenges. The distinct features of the proposed systems are detailed and the architecture and realization are discussed. While a smart antenna is capable of forming multiple beams, it is unlikely to activate all of them simultaneously. We thus develop efficient beam scheduling algorithms that could adapt to the time-varying channel conditions and traffic loads by adaptively adjusting the beam switching sequence. We also reveal that the coordinated dual-layer beamforming technologie not only has the potential of significantly enhancing the system capacity, but also can turn a NP-complete beam scheduling problem into a problem that can be easily solved if network MIMO technologies are employed. It is seen through simulation studies that the proposed schemes could offer multi-fold capacity improvements over more traditional systems equipped with broader antenna patterns (e.g., omnidirectional). Also, while we focus on public safety network in this paper, the proposed coordinated dual-layer beamforming framework can be equally applied to provide mobile access to any hotspot area with dense mobile users, e.g., a conference room or a stadium.

Modeling Three Dimensional Channel Characteristics in Outdoor-to-Indoor LTE Small Cell Environments

The main contribution of the paper is demonstrating a ray tracing framework to extract the three dimensional (3D) channel parameters as input to stochastic models in studying small cell environments. We focus on Long Term Evolution (LTE) small cells, taking as reference outdoor small cells deployments providing services to indoor users. Many stochastic channel models model the channel parameters in two dimensions and ignore the variation in the elevation plane. They are designed to capture the characteristics of macrocellular networks where eNBs are placed hundreds of meters away from each other. In such scenarios the two-dimensional channel characteristics might be sufficient to capture the channel properties. In contrast, small cells are deployed much more closely to each other. The performance of small cells is highly dependent on 3D antenna patterns, the environment specific characteristics such as 3D geometry of the buildings, materials used for building construction and their specific propagation properties. Therefore in small cell environments the site specific modeling of channel parameters in three dimensions is needed. Measurements to determine the channel parameters may not be available for every environment of interest. Ray tracing could close the gap in extracting the 3D site-specific channel parameters pertaining to small cell environments.

On the performance of stadium high density carrier Wi-Fi enabled LTE small cell deployments

Offering good Quality of Experience (QoE) in stadiums poses unprecedented challenges to Wireless Operators, due to extreme traffic conditions. During popular sporting events, there could be tens of thousands of active users present in a relatively small area, downloading/uploading pictures and video clips through smart phone applications. This results in a large traffic density and drives requirements for high capacity, and yet economically feasible solutions for stadiums. Ensuring high capacity required in such open and heavily interfered environments is a daunting task. Small cells offer promising advantages to increase the spectral efficiency of wireless systems, thanks to their small frequency reuse factor. We show through environment simulations that it is possible to serve thousands of users in a stadium using small cell technologies. In particular, Wi-Fi and LTE small cells can be used in stadiums to complement each other to cope with the increasing capacity demand.

Predicting MIMO Performance in Urban Microcells Using Ray Tracing to Characterize the Channel

We describe a method for estimating achievable data rates in urban microcells using multiple-input/multiple-output (MIMO) techniques. Specifically, we use site maps and a versatile ray-tracing tool to compute MIMO gain matrices as a function of terminal location; and we use these matrices to determine achievable rates for various MIMO transmission modes (spatial multiplexing, beamforming, and diversity). Numerical results are generated for specific paths in Boston and Manhattan, though our results are shown to be fairly insensitive to neighborhood or city. We also show that, in urban microcells, data rate prediction using site-specific ray tracing is more informative than using stochastic models; and that adaptive switching among MIMO transmission modes as a terminal moves along its trajectory can help sustain high data rates. A new mode-switching algorithm is proposed that requires switching rates lower than those for the optimal scheme by a factor greater than 10, with little loss in average data rate. We also propose a novel software algorithm for optimally placing microcell bases. For a Manhattan neighborhood of area 0.5 km2, we find that full coverage can be obtained using only 5 bases, and that the highest total throughput is achieved using a frequency reuse factor of 1.

On the in-building performance and feasibility of LTE small cells with beamforming capabilities

We disseminate a set of small cells field trial experiments conducted at 2.6 GHz and focused on coverage/capacity within an office building. LTE pico cells deployed in indoor, as well as LTE small cells deployed in outdoor are considered. The latter relies on small emission power levels coupled with intelligent ways of generating transmission beams with various directivity levels by means of adaptive antenna arrays. We show through measurements that the antenna directivity is well preserved within a building; by employing low power small cells with directive antennas one can establish wireless coverage for a large area of the office building. We demonstrate the possible densification of pico cells deployed in indoor, leading to a capacity solution. Furthermore, we calibrate and validate an analytical three-dimensional (3D) performance prediction framework against field measurements and use it to quantify the angular dispersion which indicates the preservation of the antenna directivity.

On the feasibility of outdoor-to-indoor LTE small cell deployments: Field trial experiments and performance prediction

Small cells offer promising advantages to increase the spectral efficiency of wireless systems, thanks to their small frequency reuse factor. In complement to the macro cellular network, they can provide more uniform Quality of Experience (QoE) across the entire geographical area of interest. This paper focuses on performance of Long Term Evolution (LTE) small cells, taking as reference outdoor small cells deployments providing services to indoor users. The main contribution of the paper is demonstrating the performance benefits of small cells for outdoor to indoor coverage through a set of field trial experiments that were recently conducted on-air at 2.6 GHz. Furthermore, we introduce an analytical three dimensional (3D) performance prediction framework, which we calibrate and validate against field measurements. In particular, this framework can determine detailed performance levels at any point of interest within a building and allows for fast and easy what-if scenarios testing. It can be used to create rules of thumb for deploying small cells, and can be applied to large scale small cell deployments.

A new path loss modeling approach for in-building wireless networks

Stochastic models for path loss are most often of the form PL = A + B log(d), where A and B are empirical constants derived from data via least-squares fitting, and d is the path distance. For in-building environments, however, many investigators have noted the added effects of transmission through interior walls and floors. Here, we represent this effect by a third term, which is exponentially related to log(d), and we model its impact on path loss. The database we use is obtained using a well-validated ray-tracing tool, which we apply to single floors of four very distinct office buildings. The context is an ad hoc wireless network, wherein both the transmit and receive locations can be anywhere on the floor. The resulting model consists of the median path loss, involving three fitting constants A, B and C; and a log-normal variation about the median, with its standard deviation being a function of distance. The structure and details of the model are shown to be remarkably similar across the four distinct buildings studied.

On the Wireless Channel Characteristics of Outdoor-to-Indoor LTE Small Cells

We demonstrate a ray tracing framework to extract the 3-D channel parameters as input to stochastic models in studying small cell environments, and investigate the dependence of the channel parameters on environmental characteristics, antenna patterns, and frequency. We focus on long-term evolution small cells, taking as reference outdoor small cell deployments providing services to indoor users. We present the measurement results proving the feasibility of outdoor deployment of small cells for indoor coverage and validate the ray tracing framework with the available measurements. Many stochastic channel models treat the channel parameters in two dimensions to capture the characteristics of macro cellular networks, where base stations are placed hundreds of meters away from each other. In contrast, small cells are deployed much more closely to each other. The performance of small cells is highly dependent on the 3-D environment characteristics and antenna patterns. Therefore, the site specific modeling of channel parameters in three dimensions is quintessential for small cell environments. We demonstrate ray tracing as an efficient technique to extract the 3-D site-specific channel parameters pertaining to small cell environments, and to calibrate the stochastic models used with site specific channel parameters.

Dynamic path selection in 5G multi-RAT wireless networks

Emerging 5G networks will not only offer higher link rates, but also integrate a variety of Radio Access Technologies (RATs) in order to provide ultra-reliable broadband access to a wide range of applications with high throughput and low latency requirements. SDN-enabled dynamic path selection is of critical importance in exploiting the collective transmission resources in such heterogeneous multi-RAT environments and delivering excellent user performance. In the present paper we propose the ‘best-rate’ path selection algorithm for multi-RAT networks with various types of traffic flows. The best-rate algorithm accounts for the radio conditions and performance requirements of individual flows as well as the load conditions at the various access points. We analytically establish that the rates received by the various flows under the best-rate path selection, in conjunction with local fair resource sharing at the individual access points, are close to globally Proportional Fair. Detailed simulation experiments demonstrate that the best-rate algorithm achieves significant gains in terms of user-perceived throughput performance over various baseline policies.