Defense Notices

​Stretch processing with the use of a wideband LFM transmit waveform is a commonly used technique, and its popularity is in large part due to the large time-bandwidth product that provides fine range resolution capabilities for applications that require it. It allows pulse compression of echoes at a much lower sampling bandwidth without sacrificing any range resolution. Previously, this technique has been restrictive in terms of waveform diversity because the literature shows that the LFM is the only type of waveform that will result in a tone after stretch processing. However, there are also many examples in the literature that demonstrate an ability to compensate for distortions from an ideal LFM waveform structure caused by various hardware components in the transmitter and receiver. This idea of compensating for variations is borrowed here, and the use of nonlinear FM (NLFM) waveforms is proposed to facilitate more variety in wideband waveforms that are usable with stretch processing. A compensation transform that permits the use of these proposed NLFM waveforms replaces the final fast Fourier transform (FFT) stage of the stretch processing configuration, but the rest of the RF receive chain remains the same. This modification to the receive processing structure makes possible the use of waveform diversity for legacy radar systems that already employ stretch processing. Similarly, using the same concept of compensating for distortions to the LFM structure along with the notion that a Fourier transform is essentially the matched filter bank for an LFM waveform mixed with an LFM reference, a least-squares based mismatched filtering (MMF) scheme is proposed. This MMF could likewise be used to replace the final FFT stage, and can also facilitate the application of NLFM waveforms to legacy radar systems.     The efficacy of these filtering approaches (compensation transform and least-squares based MMF) are demonstrated in simulation and experimentally using open-air measurements and are applied to different scenarios of NLFM waveform to assess the results and provide a means of comparison between the two techniques.

Rising the yield of wheat crops is essential to meet the projected future demands of consumption and it is expected that most yield increases will be associated to improvements in biomass accumulation. Cultivars with canopy architectures that focus the light interception where photosynthetic-capacity is greater achieve larger biomass accumulation rates. Identifying varieties with improved traits could be performed with modern breeding methods, such as genomic-selection, which depend on genotype-phenotype mappings. Developing a non-destructive sensor with the capability of efficiently phenotyping wheat-canopy architecture parameters, such as height and vertical distribution of projected-leaf-area-density, would be useful for developing architecture-related genotype-phenotype maps of wheat cultivars. In this presentation, new scattering analysis tools and a new 2-18 GHz radar system are presented for efficiently phenotyping the architecture of wheat canopies.
The radar system presented was designed with the objective to measure the RCS profile of wheat canopies at close range. The frequency range (2-18 GHz), topology (Frequency-modulated-continuous-wave) and other radar parameters were chosen to meet that goal. Phase noise of self-interference signals is the main source of coherent and incoherent noise in FMCW radars. A new comprehensive noise analysis is presented, which predicts the power-spectral-density of the noise at the output of FMCW radars,
including those related to phase noise. The new 2-18 GHz chirp generator is based on a phase-locked-loop that was designed with large loop bandwidth to suppress the phase noise of the chirp. Additionally, the radar RF front-end was designed to achieve low levels of LO-leakage and antenna feed-through, which are the main self-interference signals of FMCW radars.
In addition to the radar system, a new efficient radar simulator was developed to predict the RCS waveforms collected from wheat canopies over the 2-18 GHz frequency range. The coherent radar simulator is based on novel geometric and fully-polarimetric scattering models of wheat canopies. The scattering models of wheat canopies, leaves with arbitrary orientation and curvature, stems and heads were validated using a full-wave commercial simulator and measurements. The radar simulator was used to derive retrieval algorithms of canopy height and projected-leaf-area-density from RCS profiles, which were tested with field-collected measurements.

Sea ice in polar regions is typically covered with a layer of snow. The thermal insulation properties and high albedo of the snow cover insulates the sea ice beneath it, maintaining low temperatures and limiting ice melt, and thus affecting sea ice thickness and growth rates. Remote sensing of snow cover thickness plays a major role in understanding the mass balance of sea ice, inter-annual variability of snow depth, and other factors which directly impact climate change. The Center for Remote Sensing of Ice Sheets (CReSIS) at the University of Kansas has developed an ultra-wide band FMCW Snow Radar used to measure snow thickness and map internal layers of polar firn. The radar’s deployment on high-endurance, fixed-wing aircraft makes it difficult to track the surface from these platforms, due to turbulence and a limited range window. In this thesis, an automated onboard real-time surface tracker for the snow radar is presented to detect the snow surface elevation from the aircraft and track changes in the surface elevation. For an FMCW radar to have a long-range (high altitude) capability, a reference chirp delaying ability is a necessity to maintain a relatively constant beat frequency. Currently, the radar uses a filter bank to bandpass the received IF signal and store the spectral power in each band by utilizing different Nyquist zones. During airborne missions in polar regions with the radar, the operator has to manually switch the filter banks one by one as the aircraft elevation from the surface increases. The work done in this thesis aims at eliminating the manual switching operation and providing the radar with surface detection, chirp delay, and a constant beat frequency feedback loop in order to enhance its long range capability and ensure autonomous operation.​

Mobile ad hoc networks (MANETs) consist of mobile nodes that can communicate with each other through wireless links without the help of any infrastructure. The dynamic topology of MANETs poses a significant challenge for the design of routing protocols. Many routing protocols have been developed to discover routes in MANETs through different mechanisms such as source routing and link state routing. In this thesis, we present a comprehensive performance analysis of several prominent MANET routing protocols. The protocols studied are Destination Sequenced Distance Vector protocol (DSDV), Optimized Link State Routing protocol (OLSR), Ad hoc On-demand Distance Vector protocol (AODV), and Dynamic Source Routing (DSR). We evaluate their performance on metrics such as packet delivery ratio, end-to-end delay, and routing overhead through simulations in different scenarios with ns-3. These scenarios involve different node density, node velocity, and mobility models including Steady-State Random Waypoint, Gauss-Markov, and Lévy Walk. We believe this study will be helpful for the understanding of mobile routing dynamics, the improvement of current MANET routing protocols, and the development of new protocols.

One of the most important reliability concerns for Solid-State Transformers (SST) is related to high voltage side switch and grid faults. High voltage stress on the switches, together with the fact that most modern SST topologies comprise a large number of power switches in the high voltage side, contribute to a higher probability of a switch fault occurrence. Furthermore, high voltage grid faults that result in unbalanced operating conditions in SSTs can lead to more dire consequences in regards to safety and reliability in comparison to traditional transformers. This work proposes a new SST topology in conjunction with a fault-tolerant operation strategy that can fully restore operation of the proposed SST in case of the two mentioned fault scenarios. Also, the proposed SST is a new topology to generate three-phase voltages from two-phase voltages, and it is designed to increase the lifetime of the proposed SST.

Nowadays, wireless communications are becoming so tightly integrated in our daily lives, especially with the global spread of laptops, tablets and smartphones. This has paved the way to dramatically increasing wireless network dimensions in terms of subscribers and amount of flowing data. The two important fundamental requirements for the future 5G wireless networks are abilities to support high data traffic and exceedingly low latency. A likely candidate to fulfill these requirements is multi-cell multi-user multi-input multiple-output (MIMO); also termed as coordinated multipoint (CoMP) transmission and reception. In order to achieve the highest possible performance of this aforementioned candidate technology, a properly designed resource allocation algorithm is needed. By designing a resource allocation algorithm which maximizes the network throughput, this technology is able to manage the exponential growth of wireless network dimensions. Moreover, with the rapidly growing data traffic, interference has become a major limitation in wireless networks. To deal with this issue and in order to manage the interference in the wireless network systems, various interference mitigation techniques have been introduced among which interference alignment (IA) has been shown to significantly improve the network performance. However, how to practically use IA to mitigate inter-cell interference in a downlink multi-cell multi-user MIMO networks still remains an open problem. To address the above listed problems, in this dissertation we improve the performance of wireless networks, in terms of spectral efficiency, by developing new algorithms and protocols that can efficiently mitigate the interference and allocate the resources. In particular, we will focus on designing new beamforming algorithms in downlink multi-cell multi-user MIMO networks. Furthermore, we mathematically analyze the performance improvement of multi-user MIMO networks employing proposed techniques. Fundamental relationships between network parameters and the network performance will be revealed, which will provide guidance on the wireless networks design. Finally, the results of theoretical study will be demonstrated using MATLAB.​

Even though fiber optics communication is providing a high bandwidth channel to achieve high speed data transmission, there is still a need for higher spectral efficiency, faster data processing speeds while reduced resource requirements due to ever increasing data and media traffic. Various multilevel modulation and demodulation techniques are used to improve spectral efficiency. Although, spectral efficiency is improved, there are other challenges that arise while doing so such as requirement for high speed electronics, receiver sensitivity, chromatic dispersion, operational flexibility etc. Here, we investigate multilevel modulation techniques to improve spectral efficiency while reducing the resource requirements.

We demonstrated a digital-analog hybrid subcarrier multiplexing (SCM) technique which can reduce the requirement of high speed electronics such as ADC and DAC, while providing wideband capability, high spectral efficiency, operational flexibility and controllable data-rate granularity.

With conventional Quadrature Phase Shift Keying (QPSK), to achieve maximum spectral efficiency, we need high spectral efficient Nyquist filters which takes high FPGA resources for digital signal processing (DSP). Hence, we investigated Quadrature Duobinary (QDB) modulation as a solution to reduce the FPGA resources required for DSP while achieving spectral efficiency of 2bits/s/Hz. Currently we are investigating all analog single sideband (SSB) complex field modulated direct detection system. Here, we are trying to achieve higher spectral efficiency by using QDB modulation scheme in comparison to QPSK while avoiding signal-signal beat interference (SSBI) by providing a guard-band based approach.

In coherent detection systems, the MLSE receiver could be implemented using Viterbi algorithm. However, in case of direct detection systems due to square law detection the noise in the received signal is not Gaussian anymore. This leads to requirement of channel behavior estimation for the implementation of MLSE receiver in direct detection systems. Recently, Kramers-Kronig receiver has attracted great deal of attention. We are working on utilizing Kramers-Kronig receiver to implement MLSE receiver for direct detection system without the need for channel estimation.

Trends toward large-scale integration and the high-power application of green energy resources necessitate the advent of efficient power converter topologies, multilevel converters. Multilevel inverters are effective solutions for high power and medium voltage DC-to-AC conversion due to their higher efficiency, provision of system redundancy, and generation of near-sinusoidal output voltage waveform. Among many proposed multilevel topologies, the neutral-point-clamped (NPC), flying capacitor (FC), and cascaded H-bridge (CHB) converters are the most well-known classical multilevel topologies. For generation of output voltages with more than five levels, the number of required diodes and capacitors in NPC and FC increases rapidly. Also, these two topologies suffer from a significant capacitor voltage balancing problem. CHBs also require bulky multi-winding transformers to realize several isolated dc sources. Recently, modular multilevel converter (MMC) has become increasingly attractive due to its modularity, high efficiency, excellent output voltage waveform, and no need for separate dc sources. To improve the harmonic profile of the output voltage, there is the need to increase the number of output voltage levels. However, this would require increasing the number of submodules (SMs) and power semi-conductor devices and their associated gate driver and protection circuitry, resulting in the overall multilevel converter to be complex and expensive. Fewer efforts have been devoted to proposing MMC-based multilevel topologies focusing on reduced part count. This work will investigate new medium-voltage high-power MMC-based multilevel inverter with reduced component numbers while using conventional half-bridge SM structure.

The second part of this work is on improving control of MMC-based high-power DC-DC converters. Medium-voltage DC (MVDC) grids have been the focus of numerous research studies in recent years due to their increasing applications in rapidly growing grid-connected renewable energy systems, such as wind, solar and wave farms. MMC-based DC-DC converters are employed for collecting power from offshore wind and wave farms. Among various developed high-power DC-DC converter topologies, MMC-based DC-DC converter with medium-frequency (MF) transformer is a valuable topology due to its numerous advantages. Specifically, they offer a significant reduction in the size of the MMC arm capacitors along with the ac-link transformer and arm inductors due to the ac-link transformer operating at medium frequencies. As such, this work focuses on improving the control of isolated MMMC-based DC-DC converters. Conventionally, the active power is controlled by phase shifts between the primary side and secondary side of transformers.  Through this work, adding degree of freedom is investigated by considering the amplitude ratio index of MMC leg as a single control parameter. From the derived analytical formulas, this will lead to operating points where the same active power is transferrable but current stress is reduced. Subsequently, longer lifetimes of the high-frequency transformer and power switches are expected.

The specific goals of this work are, (1) Investigating new topology of MMC-based inverter that generate the same peak-to-peak output voltage and voltage levels as conventional MMC but require fewer components. (2) Improving control of isolated MMC-based DC-DC converters to reduce the current stress of the switches and transformer while delivering same power.

Owing a mobile phone has come to be regarded as a necessity in today’s world. Smart phone is an effective way to locate a person anywhere in this world. Android is an open source software stack with the largest number of users. Hence, this application is developed in Android. LocTrac is an Android application used to track the location of the user. During the time of emergencies or accidents, a person may not be in a situation to let others know about his/her location. LocTrac is an application which automatically send the user’s location to registered contacts so that they can track him/her down. In this application we initially register few contacts as guardians, when the user doesn’t answer the call, his/her location is automatically sent to the registered contacts. This application also uses sensors to capture the phone movement and send the location. Timer, alarm, emergency call are other features of this application.

Enormous amount of data is being generated due to advancement in technology. The basic question of discovering knowledge from the data generated is still pertinent. Data mining guides us in discovering patterns or rules. Various techniques are applied to find the error rate on testing data sets based on rules generated from stratified training data sets. In this project, using the k-Fold Cross Validation approach, we vary the number of folds the training data set is divided into, stratify the folds, and find the error rates on testing data sets for each ‘k’. For every data set in each k, experiment is repeated certain number of times such that there is a random testing data set each time. We observed that as the value of k increases, the error rate starts getting stabilized, and there is a stage when error rate doesn't increase even if we increase the number of folds.