Study of spread spectrum multiple access systems for satellite communications with overlay on current services

The feasibility of using spread spectrum techniques to provide a low-cost multiple access system for a very large number of low data terminals was investigated. Two applications of spread spectrum technology to very small aperture terminal (VSAT) satellite communication networks are presented. Two spread spectrum multiple access systems which use a form of noncoherent M-ary FSK (MFSK) as the primary modulation are described and the throughput analyzed. The analysis considers such factors as satellite power constraints and adjacent satellite interference. Also considered is the effect of on-board processing on the multiple access efficiency and the feasibility of overlaying low data rate spread spectrum signals on existing satellite traffic as a form of frequency reuse is investigated. The use of chirp is examined for spread spectrum communications. In a chirp communication system, each data bit is converted into one or more up or down sweeps of frequency, which spread the RF energy across a broad range of frequencies. Several different forms of chirp communication systems are considered, and a multiple-chirp coded system is proposed for overlay service. The mutual interference problem is examined in detail and a performance analysis undertaken for the case of a chirp data channel overlaid on a video channel

Study of spread spectrum multiple access systems for satellite communications with overlay on current services: Executive summary

Two different methods of generating spread spectrum signals for an overlay service are discussed, and the data rate and efficiency which can be achieved while maintaining low interference with existing traffic are examined

Advances in structure elucidation of small molecules using mass spectrometry

The structural elucidation of small molecules using mass spectrometry plays an important role in modern life sciences and bioanalytical approaches. This review covers different soft and hard ionization techniques and figures of merit for modern mass spectrometers, such as mass resolving power, mass accuracy, isotopic abundance accuracy, accurate mass multiple-stage MS(n) capability, as well as hybrid mass spectrometric and orthogonal chromatographic approaches. The latter part discusses mass spectral data handling strategies, which includes background and noise subtraction, adduct formation and detection, charge state determination, accurate mass measurements, elemental composition determinations, and complex data-dependent setups with ion maps and ion trees. The importance of mass spectral library search algorithms for tandem mass spectra and multiple-stage MS(n) mass spectra as well as mass spectral tree libraries that combine multiple-stage mass spectra are outlined. The successive chapter discusses mass spectral fragmentation pathways, biotransformation reactions and drug metabolism studies, the mass spectral simulation and generation of in silico mass spectra, expert systems for mass spectral interpretation, and the use of computational chemistry to explain gas-phase phenomena. A single chapter discusses data handling for hyphenated approaches including mass spectral deconvolution for clean mass spectra, cheminformatics approaches and structure retention relationships, and retention index predictions for gas and liquid chromatography. The last section reviews the current state of electronic data sharing of mass spectra and discusses the importance of software development for the advancement of structure elucidation of small molecules

Severe Performance Effects of Combined Jamming Interferences and Rayleigh Fading with Mitigation using Joint Complex Spreading and MIMO

The article of record as published may be located at http://dx.doi.org/10.13189/ujcn.2013.010402In communications systems under jamming interference, it is customary to employ spread spectrum techniques to combat performance degradation. In systems with fading, spatial diversity techniques such as MIMO schemes are usually used to mitigate the problem. Here, we analyze and show the extremely severe effects on BER performance of the combined jamming signals and Rayleigh fading of a system undergoing both conditions at the same time. Then, we analyze and derive the performance of systems employing joint MIMO and complex spreading schemes to combat both Rayleigh fading and jamming and note the significant mitigation improvements

Error probabilities of fast frequency-hopped FSK with self-normalization combining in a fading channel with partial-band interference

An error probability analysis is performed for a binary orthogonal frequency-shift-keying (FSK) receiver employing fast frequency-hopped spread-spectrum waveforms transmitted over a frequency-nonselective slowly fading chan­nel with partial-band interference. Diversity is performed using multiple hops per data bit. A nonlinear combination procedure referred to as self-normalization combining is employed by the receiver to minimize partial-band interference effects. Each diversity reception is assumed to fade independently according to a Rician process. The partial-band interference is modeled as a Gaussian process. Thermal noise is also included in the analysis. Diversity is found to completely negate degradation of the self-normalized receiver caused by partial-band interference regardless of the strength of the direct signal component; although, for signals with a large bit energy-to-interference noise density ratio and a very strong direct component, nonlinear combining losses dominate receiver performance and negate any enhancement obtained with diversity if the level of thermal noise is too high. In addition, diversity offers definite receiver performance improvement when the direct signal component is weak. The self-normalized receiver is very sensitive to fading channels, evincing a significant performance degradation as compared with its nonfaded performance. For severe channel fading, the performance of a conventional noncoherent binary FSK receiver with diversity is generally either equivalent or superior to the performance of the self-normalized receiver with the same order of diversity

Error probabilities of fast frequency-hopped MFSK with noise-normalization combining in a fading channel with partial-band interference

An error probability analysis is performed for an M-ary orthogonal frequency-shift keying (MFSK) communication system employing fast frequency-hopped (FFH) spread spectrum waveforms transmitted over a frequency-nonselective, slowly fading channel with partial-band interference. Diversity is obtained using multiple hops per data bit. A procedure referred to as noise­ normalization combining is employed by the system receiver to minimize partial-band interference effects. Each diversity reception is assumed to fade independently according to a Rician process. The partial-band interference is modeled as a Gaussian process. Thermal noise is also included in the analysis. Forward error correction coding is applied using convolutional codes and Reed-Solomon codes. Diversity is found to dramatically reduce the degradation of the noise-normalization receiver caused by partial-band interference regardless of the strength of the direct signal component. In addition, diversity offers significant performance improvement when channel fading is strong. Further significant performance improvement is obtained for higher modulation orders ( M > 2). Uncoded receiver performance with a diversity of four is roughly comparable to coded receiver performance with no diversity for ratios of bit energy-to-interference noise density in the range between roughly 12 and 22 dB. Substantial improvements in receiver performance are obtained by combining diversity, higher modulation orders, and coding

The Military Applications of MEO and ICO Commercial Satellite Systems

At the dawn of the 21st century several low and medium earth orbit commercial satellite constellations will be operational and they will be able to provide high bandwidth global communications in voice, data, and multimedia services for mobile consumers and also "users in the move." This research evolves as a continuation of previous studies (on Iridium, Globalstar, Teledesic, and Odyssey), and considers the ICO as well as the Teledesic and GBS systems in an effort to provide a comprehensive model architecture. This model accommodates narrowband, wideband, and broadcast requirements, respectively, of the U.S. MILSATCOM in addition to the communication needs of a model UN peacekeeping mission. The application of these systems to U.S. MILSATCOM coincides perfectly with the U.S. defense doctrine of a CONUS-based military with the capability of rapid global power projection to respond to crises throughout the global arena. Instead of investing heavily in new satellite systems, the U.S. military services can use the forthcoming commercial LEO and MEO systems to meet the information requirements of tactical commanders.Prepared for: The Institute for Joint Warfare AnalysisNo 00399WRDD181Approved for public release; distribution is unlimited

Digital Satellite Communications

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