David A. Taggart

Communication systems modeling techniques

Modem simulation and modeling is frequently used to design and understand communications systems from both a theoretical and practical point of view. This can result in very significant cost savings, speed up the design process and add another dimension into the reliability of the system design. Nevertheless. the simulation of advanced communications systems is a non-trivial task and the errors introduced due to the very nature of the digital simulation process must he properly modeled, where possible avoided, and their impact on the simulation results must be properly ascertained. The purpose of this paper is to discuss, demonstrate, and give examples of the many important considerations and potential pitfalls in the modeling and simulation of communications systems. For example, it is sometimes possible and expeditious to model the communications systems at a baseband frequency resulting in orders of magnitude reduction in sampling rate and thereby speeding up the simulation process and reducing requirements on the computing resources. This is demonstrated in the paper by simulating a P C W M Communication system. The paper also discusses other important DSP concepts including zone aliasing when simulating nonlinear devices, spectral analysis techniques, and frequency binning effects.

Usage of commercial satellite systems for homeland security communications

Since the World Trade Center and Pentagon attacks on September 11th, homeland security has been redirected to account for potential terrorist attacks and other national disasters. The difficulty of many of the first responders (such as local fire, police, and search and rescue departments) to communicate with themselves and other federal agencies confirmed the need for an interoperable and flexible communication infrastruchke. In a disaster situation, such as that experienced after the September 11th attacks, the communication links used by emergency response agencies can be greatly damaged or even destroyed. Since many of these agencies must make timecritical decisions, there may not be enough time for communication links to be restored using conventional mobile ground or airborne nodes. Communication volume and connectivity during and immediately following a major event is typically very chaotic and can involve multiple systems as a function of time or distance from the focal point of activity. Nominally it is quite likely the communication requirements in the immediate vicinity of an event will be extensive, and the need to communicate beyond those boundaries will evolve with time as information is obtained and support is needed from other entities. Providing additional communication links from the immediate vicinity to regional and extended reach zones could be accomplished with access to commercial satellite networks. In this paper, the application of commercial satellite systems to aid and supplement communications for homeland security is investigated.

Impact of phase noise on the performance of the QPSK modulated signal

This paper presents the impact of phase noise on the performance of a communication link with QPSK digital modulation. For example, how phase noise affects bit error rate is demonstrated with detailed simulations. Also, this paper illustrates how phase noise affects the receiver signal constellation diagram and employs a model for simulating the phase noise. More importantly, this paper shows clearly both by simulation and analysis that a band limiting filter such as a root raised cosine (RRC) filter, which is commonly used in digital communication systems, has little effect on the receiver phase noise probability density function, the phase noise variance, and the probability of bit error. Furthermore, this paper emphasizes certain intuitive aspects of phase noise, such as its impact on the signal constellation diagram showing that phase noise can result in crossover of the decision boundaries in the signal space and thus cause symbol errors even in the absence of the receiver additive noise. Significantly, this intuitive result is also verified by bit error results obtained by both simulations and analysis. 12

Performance analysis of a command destruct subsystem of the spacelift range system

The paper presents an analysis of a typical command destruct subsystem (CDS) of the U.S. Air Force Spacelift Range System. The command destruct system sends a signal comprising a sequence of pairs of tone signal pulses to the space vehicle, in case the space vehicle significantly deviates from its intended trajectory and results in safety concerns. By necessity, due to the safety-critical nature of such a system, the command link for the CDS is designed for relatively high signal-to-noise ratio (SNR) and it is generally considered that high detection performance can be achieved due to the high SNR. However, a closer analysis shows the presence of some non-ideal factors which can potentially limit the achievable performance of the CDS system independent of the SNR. More specifically the paper analyzes the impact of narrowband tone filters present in a typical CDS receiver on the detection performance of the system. Another important source of possible performance degradation is the spill over effect of the FFT processor.

Tone interference effects on the performance of QPSK modulation in communication

In contemporary communication satellite systems, a digital processing unit on board the satellite can be employed to filter, channelize, and switch the uplink communication signals. 1 2 This processing is performed subsequent to the radio frequency (RF) to intermediate frequency (IF) downconversion of the uplinked signals. Then, an analog-to-digital converter (ADC) is employed to convert the analog signals to digital signals. This conversion process can lead to spurious responses, which can act like tone interference to the uplinked modulated signals, including commonly employed quadrature phase-shift keying (QPSK) modulated signals. Additionally, there are a wide variety of other spurs that can be created by the satellite communication payload. All of these spurs can adversely affect bit error rate (BER) performance. This paper discusses the adverse effect such spurious responses have on QPSK modulated signals, and the spurs are modeled as tone interference. Results are presented, including BER curves, for the following: (1) the spur offset in frequency from the center frequency of the QPSK signal, (2) false lock of the receiver onto the spur frequency and phase, and (3) the addition of convolutional coding to the receiver and the improvement in BER that results.

Detailed analysis of the impact of the distortion due to nonlinear amplifiers on BER performance

This paper presents a detailed analysis of the distortion effects of the power amplifier nonlinearity on the bit error rate (BER) performance of a band limited digitally modulated signal present at the amplifier input. Previous analyses of such distortion effects are mainly concerned with the total distortion power present at the amplifier output and in some cases present results on the amplifier output power spectral density. However, the power spectral density of the total signal in the fundamental zone at the amplifier output does not lend itself to the BER analysis. In the case where the total distortion power is known, one may assume that the distortion is independent of the signal of interest and has a Gaussian probability density function and thereby add the distortion power to the receiver noise power in order to obtain the BER. While such a simplified approach may be valid in some cases, such assumptions do not apply to the case of a single band limited digital signal at the amplifier input, which is the case of interest here. In fact this paper shows that in this case the distortion is neither Gaussian nor independent of the signal. Thus the BER analysis in such a case becomes relatively difficult. This paper presents a semi-analytical approach to the evaluation of the BER in the presence of both the amplifier distortion and the receiver noise. It is shown in the paper that the conditional probability density function of the amplifier distortion can be modeled by a sum of Gaussian distributions whereby the BER in the presence of both the amplifier distortion and receiver noise can be expressed in terms of a weighted sum of Q-functions.

Nonlinear amplifier noise product ratio modeling and simulation

A noise product ratio (NPR) test and/or simulation is a traditional way of examining the nonlinearity of devices such as high-power amplifiers (HPAs). The advantage of NPR testing and/or simulation includes the ability to use a noise source and filters to obtain the band of noise with the appropriate notch for test purposes, which is typically more easily accomplished than performing BER testing and/or simulation for the specific modulated carriers that are anticipated to go through the HPA. Additionally, the types of signals anticipated to go through the HPA may be quite variable, and not all possibilities can be practically tested and/or simulated using conventional methods. NPR testing helps preclude the need for testing every possible signal combination. In this paper the authors discuss a simulation and modeling technique for determining NPR for a nonlinear amplifier with arbitrary AM/AM and AM/PM transfer curves. While NPR tests are typically done at radio frequency (RF), the model chosen is a baseband model. Specific examples of the transfer curves are chosen to demonstrate the utility and versatility of the simulation and modeling technique. Importantly, the approach described in this paper has been validated by detailed RF lab measurements.

Dynamic proximity communication link analysis tool for orbiting satellites and ground assets on Mars

A dynamic proximity link analysis tool developed by The Aerospace Corporation is described in this paper. Given appropriate link budget parameters, the tool can return the data throughput for proximity links from ground assets to communications satellites. In order to determine the data throughput, the tool calculates the link margin as a function of time and determines the variable data rate(s) achievable as a function of time. Given any time period, the tool is then able to compute the total data throughput based on the data rate computations. Given a spacecrafts trajectory, the location of the ground asset, antenna gain patterns, and standard link budget parameters (i.e., transmitter power, carrier frequency, etc.), the tool can compute link closure duration, variable data rate(s) achievable, and the data throughput. As a demonstration of the tools capability, several examples of proximity links between a Mars ground asset and an orbiting relay satellite will be shown

Dynamic link analysis tool for a telemetry downlink system

A model is described in this paper for dynamic S-band telemetry link analysis for the spacelift range system (SLRS). From an operational standpoint, this tool determines telemetry link closure of a launch vehicle to different ground terminal sites. This model allows the users to react accordingly to any anomalies and to perform subsequent corrective actions. One application of the tool was to determine antenna characteristics required for the support of the SLRS missions. The application of the model is of considerable importance to the Air Force, Navy, and the nation. Specifically, the softwares ability to analyze the various launches is critical to the National Defense and the National Space program of the United States. The Aerospace Corporation developed the dynamic link analysis (DLA) tool for SLRS S-band telemetry. This tool determines the telemetry link margin during the flight of a launch vehicle as monitored by the SLRS. The model takes into account several dynamic effects in calculating the link budget including transmit antenna pattern, space loss, beam spreading, polarization diversity, rain attenuation, and plume attenuation. The trajectory of the launch vehicle in addition to its orientation is also taken into account. As a result, a clock and a cone angle are calculated in order to compute the dynamic antenna gain. Also, an aspect angle is calculated to compute the impact of plume attenuation. A slant range is computed at each instant in time to calculate the corresponding space loss. Finally, a link margin is determined and plotted versus time. Different aspects of the analytical model with respect to the waveform, transmitter, receiver, plume effects, antenna pattern, and dynamics of a launch vehicle for the SLRS S-band telemetry system are presented in this paper. The results of several analysis tools are presented for different launch vehicles and their associated flight trajectories. This sophisticated tool provides a new, powerful, and useful capability to perform trade studies and to analyze link closure for generalized launch vehicle S-band telemetry to its appropriate range ground sites.