Carl B. Dietrich

Spatial, polarization, and pattern diversity for wireless handheld terminals

This paper examines the antenna diversity configurations that improve the performance in handheld radios. Experiments using spatial, polarization, and pattern diversity were conducted for both line-of-sight (LOS) and obstructed outdoor and indoor multipath channels that experienced Ricean fading. Antenna separation, polarization, and pattern were varied independently to the extent possible. Envelope correlation, power imbalance, and diversity gain were calculated from the measurements. Diversity performance is measured by diversity gain, which is the difference in signal-to-noise ratio (SNR) between the output of a diversity combiner and the signal on a single branch, measured at a given probability level. Diversity gain increases with decreasing envelope correlation between the antenna diversity branches. However, diversity gain decreases as the power imbalance between diversity branches increases because a branch that has a weak signal has only a small contribution to the combined signal. Diversity gain values of 7-9 dB at the 99% reliability level were achieved in non-line-of-sight (NLOS) channels for all diversity configurations even with very small antenna spacings. The use of polarization diversity reduced polarization mismatches, improving SNR by up to 12 dB even in LOS channels.

Smart antennas in wireless communications: base-station diversity and handset beamforming

We review diversity and smart antenna research applied to both base-stations and terminals. To illustrate the performance gains possible, the paper describes research being conducted by the Smart Antenna Group at Virginia Tech, in both smart base-stations and smart handheld terminals.

Performance advantages of distributed antennas in indoor wireless communication systems

With the smaller physical scale of distributing the radio frequency signal in buildings, distributed antenna systems, which may be too expensive in outdoor systems, become affordable. This paper explores quantitatively and qualitatively the advantages of distributed antenna systems over single point source antenna systems, and develops theorems comparing the two systems based on a two slope path loss model.<<ETX>>

Wireless distributed computing: a survey of research challenges

Recent advancements in radio technology provide great flexibility and enhanced capabilities in executing wireless services. One of these capabilities that can provide significant advantages over traditional approaches is the concept of collaborative computing in wireless networks. With collaborative radio nodes, multiple independent radio nodes operate together to form a wireless distributed computing (WDC) network with significantly increased performance, operating efficiency, and abilities over a single node. WDC exploits wireless connectivity to share processing- intensive tasks among multiple devices. The goals are to reduce per-node and network resource requirements, and enable complex applications not otherwise possible, e.g., image processing in a network of small form factor radio nodes. As discussed in this article, WDC research aims to quantify the benefits of distributed processing over local processing, extend traditional distributed computing (DC) approaches to allow operation in dynamic radio environments, and meet design and implementation challenges unique to WDC with the help of recently available enabling technologies, such as software radios and cognitive radios.

Open-source SCA-based core framework and rapid development tools enable software-defined radio education and research

This article describes the Open Source SCA Implementation::Embedded project, an open source software development kit designed for rapid prototyping of software defined radios consistent with the software communications architecture. The SCA is a product of the American militarys SDR acquisition program and has played a large role in SDR development in the military and in the wireless industry. OSSIE was designed for use in wireless communications curricula and research efforts, so it is easy to learn and illustrative of software engineering, programming, and communications engineering concepts important in industrial practice today. OSSIE includes a core framework (i.e., common services enumerated in the SCA). It also includes graphical user interface-oriented tools that are easily learned and free to download and use. The tools auto-generate SCA-specific component source code and supporting files, leaving the developer to provide signal processing functionality. In addition, visualization tools for debugging and a growing library of SDR software components are available. Discussed herein are examples of SDRs designed using OSSIE, including embedded applications. OSSIE enables easy transition from concepts to practice in SDR design for communications engineers who may not have a strong software background.

Understanding the software communications architecture

The Software Communications Architecture is an open architecture developed by the U.S. Department of Defense to standardize the development of software-defined radio, improve communication systems interoperability, and reduce development and deployment costs. The SCA facilitates software reuse and technology insertion by abstracting radio applications from the supporting platform and defining a common operational environment across platforms. The SCA relies on commercial standards, classic software engineering principles, and software design patterns. While some SCA design choices are controversial and tightly tied to the specific needs for which it was developed, the basic design principles of software reuse and abstraction are sound and necessary if SDR is to achieve its full potential. Some of the techniques and concepts used in the SCA may be foreign to a communications engineer, and can result in confusion and long learning curves. The understanding of these concepts is of great relevance for communications engineers independent of any opinion about the SCA itself. This tutorial is aimed at educating communication engineers on these software engineering principles and describing how the SCA applies them to achieve its goals. We describe the different interfaces of the SCA that provide a framework for the implementation of SDR. The tutorial provides introductory material to understand the basic operation of the SCA as implemented in the Open-Source SCA Implementation::Embedded developed by Wireless @ Virginia Tech.

Wideband air-to-ground radio channel measurements using an antenna array at 2 GHz for low-altitude operations

Wideband measurements were performed using a direct-sequence spread-spectrum measurement system in the air-to-ground radio environment to characterize propagation between an airborne transmitter and a ground-based receiving antenna array at a center frequency of 2.05 GHz. The transmitter was flown along constant-radius arcs at low altitudes around the receiver location to obtain measurement results for 7.5, 15, 22.5, and 30 degree elevation angles. An 80 megachip per second (Mcps) modulating PN sequence was transmitted by the airborne station. The receiver was located in a campus environment of four- to six-story buildings and rolling terrain. The receiver used a four-element antenna array and sampled received signals at 1 gigasample per second (Gsps) per channel. Power-delay profiles that approximated channel impulse responses were used to measure magnitude, phase, and delay of multipath signal components received at each element. Characterization parameters produced from the measurements include RMS delay spread, excess delay spread, multipath fading CDFs, antenna diversity gain, and gain achieved through spatial-temporal combining. The measurements presented here support analysis of wireless systems for intentional transmissions, such as data communications between ground nodes and low-altitude aircraft. In addition, these measurements support investigations into interference from ground sources to low-altitude aircraft (e.g., on instrument approaches) or interception of signals originating from ground sources.

Software-defined radio: a new paradigm for integrated curriculum delivery

Software-defined radio is a rapidly developing field that is driving the development of and innovation in communications technology, and promises to significantly impact all communications sectors. Entities developing these SDR systems require a trained workforce that has been prepared with the mindset, knowledge, skills, and tools required to address both the system (breadth) and technical (depth) aspects of SDR systems. Developing SDRs necessarily involves a collection of disciplines including, but not limited to, electromagnetics, radio-frequency engineering, communications, digital signal processing, embedded systems, computer programming, and systems engineering. Whereas electrical engineering and computer science and engineering curricula at the university level may include courses in all of these areas, a students typical curriculum does not; nor does it usually involve the integration of all these topics. However, SDR can be employed as an integrative construct that facilitates systems thinking and cross-domain learning via peers. In this article, we present several significant educational efforts across six U.S. universities that have developed integrated curricula in SDR, most including a significant laboratory component.

On the Co-Existence of TD-LTE and Radar Over 3.5 GHz Band: An Experimental Study

This letter presents a pioneering study based on a series of experiments on the operation of commercial Time-Division Long-Term Evolution (TD-LTE) systems in the presence of pulsed interfering signals in the 3550-3650 MHz band. TD-LTE operations were carried out in channels overlapping and adjacent to the high power SPN-43 radar with various frequency offsets between the two systems to evaluate the susceptibility of LTE to a high power interfering signal. Our results demonstrate that LTE communication using low antenna heights was not adversely affected by the pulsed interfering signal operating on adjacent frequencies irrespective of the distance of interfering transmitter. Performance was degraded only for very close distances (1-2 km) of overlapping frequencies of interfering transmitter.

Full length article: Dynamic reconfiguration of software defined radios using standard architectures

The software defined radio (SDR) has opened the doors for levels of radio reconfiguration not possible through the use of more traditional radio design approaches. While most radios allow variation of parameters such as carrier frequency, an SDR enables large-scale reconfiguration (e.g., changing to a different protocol type or MAC). In this research, we explore automated, dynamic large-scale radio reconfiguration through the implementation and characterization of three alternative reconfigurable radio designs. These implementations seek to quantify the impacts of implementing large-scale radio reconfiguration through SDR application management enabled by the Software Communications Architecture (SCA) and similar SDR architectures. The SCA, a widely used SDR standard developed by the US Department of Defense, primarily supports static configurations with infrequent reconfiguration. However, we present a novel approach that uses features of the SCA and its enabling technologies to achieve rapid dynamic reconfiguration of SDRs. The act of radio reconfiguration through SCA application management is found to momentarily decrease the radio throughput. Although some overhead is experienced in the demonstration systems, the methods discussed achieve a high degree of flexibility through the use of standard SDR architectures.