Kevin Wanuga

SDC testbed: Software defined communications testbed for wireless radio and optical networking

This paper describes the development of a new Software Defined Communications (SDC) testbed architecture. SDC aims to generalize the area of software defined radio to include propagation media not exclusively limited to radio frequencies (optical, ultrasonic, etc.). This SDC platform leverages existing and custom hardware in combination with reference software applications in order to provide a complete research and development platform. This platform can be used to implement current and future standards that make use of highly demanding communications techniques, including ultrawideband (UWB) radio and free-space optical communications. This paper describes the commercial and custom hardware that is being integrated into the platform, including the baseband hardware and the modular transceiver frontends. Furthermore, the paper describes the software development currently in progress with this platform, including the integration of available open source designs into the platform, and the development of custom IP for scalable OFDM PHY implementations in radio and optical communications. We seek to create a complete research platform for the commercial and academic wireless communities, capable of delivering the highest possible performance and flexibility while providing the necessary development tools and reference designs in order to minimize system learning curve and development cost.

High-data-rate ultrasonic through-metal communication

A link-adaptive frequency division multiplexing (OFDM) ultrasonic physical layer is proposed for high-datarate communications through metal walls. The ultrasonic link allows for communication without physical penetration of the metal barrier. Link-adaptive OFDM mitigates the severe frequency- selective fading of the ultrasonic channel and greatly improves throughput over impulse or narrowband communication systems. Throughput improvements of 300% are demonstrated over current narrowband low-frequency techniques, and show improved spectral efficiency over high-frequency techniques found in the literature.

Application of Adaptive OFDM Bit Loading for High Data Rate Through-Metal Communication

The acoustic through-metal channel is characterized by strong multipath components caused by the echoing of acoustic energy within the channel. Transmission at high data rates is therefore difficult to achieve with traditional single-carrier systems. This paper applies an adaptive bit-loading technique to the transmission of digital signals through metal barriers using ultrasonic signaling. The multi-carrier approach discussed here allows us to mitigate severe frequency selectivity of the through-metal communication link and improve spectral efficiency by exploiting the stationary nature of the channel. Experimental performance of bit loading is examined in an ultrasonic through-metal channel. Our results indicate that non-power-scaled rate adaptive bit loading significantly outperforms non-adaptive modulation. Adaptive bit loading was shown to adhere to a strict BER constraint while increasing data rates by roughly 240% from values of 5 Mbps to approximately 12 Mbps when compared to narrowband modulation techniques.

Echo-Cancellation for Ultrasonic Data Transmission through a Metal Channel

The process control industry has shown great interest in implementation of low cost, low power wireless sensor networks. Such networks are much easier to deploy and reconfigure compared to wired alternatives. In this paper, we describe the use of radio-frequency (RF) based sensor networks in sensing and control applications on naval vessels. In this environment, metal bulkheads (which divide the ship into watertight compartments) and other metallic obstacles can lead to unreliable network connectivity. We propose to address the challenge by augmenting the RF network with ultrasonic data repeaters. The repeaters are designed to pass data from one side of a watertight bulkhead to the other without requiring the bulkhead to be physically penetrated. Through experimentation, we observed that echoes of pulses transmitted through the ultrasonic channel (i.e. the bulkhead) lead to considerable intersymbol interference and hinder high data rate transmission. In this paper, we investigate the nature of this interference and propose a method of mitigating its effect. The method applies a pre-distortion filter to the data which leads to destructive interference that reduces the echo amplitude.

Performance Evaluation of MIMO OFDM Systems in On-Ship Below-Deck Environments

Below-deck compartments on naval vessels provide a challenging environment for wireless networks. The metallic walls of the compartments produce multiple reflections that can degrade signal integrity. Between compartments, the metal bulkheads impede the propagation of electromagnetic waves, limiting network connectivity. Orthogonal frequency-division multiplexing (OFDM) is proposed to mitigate the effects of intersymbol interference (ISI) caused by multiple reflections. Additionally, the use of multiple antennas for channel diversity has shown to improve communications reliability and capacity. Single and multiantenna OFDM physical layers were tested within several below-deck spaces aboard Thomas S. Gates (CG 51), a decommissioned Ticonderoga-class US Navy cruiser. Measurements were taken with four OFDM-based schemes typical of current-generation Wireless Local Area Network (WLAN) technologies. The performance of multiantenna signaling techniques, including 2 × 2 Alamouti space-time coding and 2 × 2 multiple-input-multiple-output spatial multiplexing (MIMO-SM), were compared to the performances of 1 × 2 maximal ratio combining (MRC) and a conventional single-input-single-output (SISO) system. Results indicate that the tested MIMO techniques can approximately double the channel capacity. Throughput as high as 36 Mb/s was achieved in conventional situations where SISO links only admitted rates of 18 Mb/s.

Bit-Loaded PAPR Reduction for High-Data-Rate Through-Metal Control Network Applications

Data transmission through metallic structures is commonly required in industrial control applications. In a number of these applications, mechanically penetrating the structure to pass cables and establish a wired communication link is either impossible or undesirable. Examples of such structures include metal bulkheads, pressure vessels, or pipelines. Ultrasonic signaling has been proposed as a solution for through-metal data transfer without penetrating the structure. The reverberant nature of the through-metal channel, however, can lead to significant intersymbol interference, limiting the data rate achievable by conventional single-carrier communication techniques. In this paper, we describe a through-metal communication technique that exploits the slow-varying nature of the ultrasonic channel to implement an orthogonal-frequency-division-multiplexing-based rate-adaptive peak-to-average power ratio (PAPR) reduction algorithm. Measurements of the proposed adaptive algorithm have demonstrated transmitted throughput rates of up to 14 Mbps while reducing PAPR by up to 3 dB and maintaining a bit error rate of 10- 5 at average transmit powers of roughly 6 dBm. This enhancement provides the required throughput and error rate to support high-rate network applications in otherwise data-limited environments.

A MATLAB platform for characterizing MIMO-OFDM communications with software-defined radios

A new MATLAB-based, wireless measurement platform using an existing software-defined radio architecture is presented. It augments IEEE 802.11g MIMO-OFDM physical layer schemes with new designs such as Maximal Ratio Combining, Alamouti coding, and Spatial Multiplexing. The platform provides a series of metrics, including channel capacity, Error Vector Magnitude (EVM), and Post-Processing Signal-to-Noise Ratio (PP-SNR) to characterize link and network performance. The software implementation and test protocol of the platform are presented with a validation study demonstrating its application.

Performance of Reconfigurable Antennas in a Below-Decks Environment

Reconfigurable antennas have been proposed for mitigating multipath interference and increasing channel capacity in wireless networks. The majority of studies which have investigated these claims do so either in simulation or through the use of software defined radios in lab or office environments. There has been little work in quantifying experimentally the performance gains of a reconfigurable antenna in highly metallic environments representative of military and industrial applications. This letter quantifies the performance gains provided by the use of electrically reconfigurable antennas in the place of omnidirectional antennas given varying environmental and system configurations. Wireless measurements for various 802.11-like physical layers were performed in a set of multideck, coupled compartments aboard Thomas S. Gates (CG 51), a decommissioned Ticonderoga-class U.S. Navy cruiser. The reconfigurable antennas were observed to provide higher channel capacities than omnidirectional antennas. When the bulkhead doors connecting the coupled compartments were left open, the postprocessing signal to noise ratio (PP-SNRs) of signals received from reconfigurable antennas were up to 4 dB higher than what was observed when using omnidirectional antennas. As the compartments became less electrically coupled (i.e., bulkhead doors were closed), the benefit provided via the antenna pattern diversity of the reconfigurable antennas diminished.

An Empirical Study on the Performance of Wireless OFDM Communications in Highly Reverberant Environments

Reverberation chambers are closed reflective spaces that can emulate highly reverberant electromagnetic environments. The electromagnetic environment is primarily determined by the size of the cavity, effective conductivity, and leakage via apertures. In this effort, we investigate the performance of wireless OFDM communications in relation to the latter two by controlling the loading of a reverberation chamber and the effective aperture into a coupled cavity. A software defined radio measurement platform was used to assess the communication performance through a selection of link-level metrics including error vector magnitude, post processing signal-to-noise ratio, and throughput. The degradation of link quality is quantified for increasingly diffuse environments, as well as the improvement when leveraging a maximal ratio combining receiver diversity scheme. The link quality was found to improve in both the reverberation chamber and the coupled cavity for larger effective apertures. This result was analyzed using a time-dependent model for RF propagation in coupled cavities.

Reconfigurable antennas in highly multipath environments

Highly reflective environments, such as ships, aircraft, and industrial warehouses, can be challenging for wireless communications. This effort investigates the use of transmitter-side reconfigurable antennas to mitigate the effect of multipath interference in such environments. Software-defined radios were used to transmit and receive IEEE 802.11g OFDM packets in “tuned” reverberation chambers at the Naval Surface Warfare Center in Dahlgren, VA. The reconfigurable antenna improved signal integrity by 4.9 dB over a conventional, omnidirectional antenna for SISO, and by a smaller margin for MRC. The reconfigurable antenna achieved greater capacity than the omnidirectional antenna with 16% improvement. It is concluded that reconfigurable antennas are well suited for use with access points and other stationary network infrastructure to facilitate the design and mobility of receivers in highly reflective environments.