Tien M. Nguyen

Modeling of HPA and HPA linearization through a predistorter: Global Broadcasting Service applications

This paper presents a technique to linearize the high power amplifier (HPA) through a predistorter (PD). The characteristics of the PD circuit are derived based on the extension of Salehs model for HPA and a simple linear-log model. Numerical results are shown for Global Broadcasting Service (GBS) applications.

Satellite payload architectures for wideband communications systems

Future communications satellites will employ broadband communications techniques. In order to provide for a planning of their development and conceptual design, various satellite payload architectures need to be investigated. Of particular interest, the architectures discussed in this paper include no-processing, partial-processing, and full-processing satellite payloads. Functional characteristics and features associated with each of these architectures are described in detail. Typical link budget calculations are presented for comparison. In addition, this paper presents a brief discussion on the issue of implementing the onboard channelization and switching circuitry, comparing the use of analog and digital components.

War-gaming application for future space systems acquisition

Recently the U.S. Department of Defense (DOD) released the Defense Innovation Initiative (DII) [1] to focus DOD on five key aspects; Aspect #1: Recruit talented and innovative people, Aspect #2: Reinvigorate war-gaming, Aspect #3: Initiate long-range research and development programs, Aspect #4: Make DOD practices more innovative, and Aspect #5: Advance technology and new operational concepts. Per DII instruction, this paper concentrates on Aspect #2 and Aspect #4 by reinvigorating the war-gaming effort with a focus on an innovative approach for developing the optimum Program and Technical Baselines (PTBs) and their corresponding optimum acquisition strategies for acquiring future space systems. The paper describes a unified approach for applying the war-gaming concept for future DOD acquisition of space systems. The proposed approach includes a Unified Game-based Acquisition Framework (UGAF) and an Advanced Game-Based Mathematical Framework (AGMF) using Bayesian war-gaming engines to optimize PTB solutions and select the corresponding optimum acquisition strategies for acquiring a space system. The framework defines the action space for all players with a complete description of the elements associated with the games, including Department of Defense Acquisition Authority (DAA), stakeholders, warfighters, and potential contractors, War-Gaming Engines (WGEs) played by DAA, WGEs played by Contractor (KTR), and the players’ Payoff and Cost functions (PCFs). The AGMF presented here addresses both complete and incomplete information cases. The proposed framework provides a recipe for the DAA and USAF-Space and Missile Systems Center (SMC) to acquire future space systems optimally.

Performance of MPSK and 16QAM in the satellite communication environment

This paper presents computer simulation results on the performance of quadrature phase shift keying (QPSK), 8-ary phase shift keying (8PSK), and 16 quadrature amplitude modulation (16QAM) schemes in a satellite communication environment employing frequency division multiple accessing (FDMA). The end-to-end communication system, which includes the modulator, klystron amplifier, satellite transponder, carrier and timing recovery loops, and the channel filters, is modeled using the cadence signal processing work system (SPW) tool. The satellite transponder components modeled in SPW include filters, up/down converters, digital channelizer filters, and high-power amplifiers (HPAs). Measured data is used to characterize the transmitter klystron and the transponder HPA. The performance of the filtered signal of interest is evaluated in the presence of filtered QPSK, 8PSK, and 16QAM signals in adjacent channels. Due to the complex waveforms, the nonlinear channel model, and the multi-user scenario involved in the simulations, it is very difficult to develop an analytical expression to accurately predict the received signal power level. Therefore, a power calibration process is developed to determine the received signal power level, required to compute the bit energy to noise spectral density ratio (E/sub b//N/sub o/) used in any specific SPW simulation run. Simulation results of the paper show that the bit error rate (BER) performance of the simulated communication system is significantly impacted by the presence of the adjacent channel interference (ACI) and the intermodulation (IM) noise.

Propagation effects on the wideband gapfiller communication link

This paper investigates the propagation effects on the wideband gapfiller satellite communication link. The atmospheric loss and rain attenuation are estimated using the current empirical atmospheric gaseous and rain models. In addition, the contributions on system noise temperature due to rain and gaseous attenuation are also estimated. The attenuation results are presented for a range of link availability at X-band uplink/Ka-band downlink, X-band uplink/X-band downlink, and Ka-band uplink/Ka-band downlink. Further, the results of well-known rain models, i.e., Crane (1980), ITU-R (1997), and COMSAT/DAH (1997), are compared.

Impact of baseband filtering on the SGLS waveform

This paper investigates the effects of the baseband filtering on the data performance of the Space-Ground Link Subsystem (SGLS) uplink. The uplink SGLS waveform investigated in this paper includes data, command, and ranging signals. The performance of the data channel will be evaluated analytically for ideal conditions, namely, perfect carrier/sub-carrier tracking, frequency synchronization, timing recovery, and filtering. For imperfect conditions, a computer simulation model has been developed to examine the impacts of baseband filtering on the power spectral density (PSD) of the uplink signal and bit error rate (BER) performance of the data channel. It was found that (1) the baseband filter improves the out-of-band attenuation by about 35 dB, and (2) the additional bit signal-to-noise (SNR) degradation due to baseband filtering is about 0.5 dB. In addition, to optimize BER performance when inserting the baseband filter, it is necessary to adjust the effective modulation indices.

On the power spectral density of SGLS and USB waveforms

Most of the DoD satellites utilize the Space-to-Ground Link Subsystem (SGLS) for tracking, telemetry, and command (TT and C) controls. The standard SGLS waveform uses a phase modulation (PM) scheme on the combined PCM/FSK/AM signal and pseudo-random noise (PRN) square wave signal for command and ranging, respectively. The SGLS uplink operates in the 1755-1850 MHz (L-band), while the downlink operates in the 2200-2290 MHz (S-band). On the other hand, the non-DoD satellites for other various governmental agencies primarily use Unified S-Band (USB) for their TT and C links. Two waveforms, PCM/PSK/PM and PCM/PM/Bi-Phase, are mainly used in the USE for command along with either PRN squarewave or sinusoidal tone for ranging. Due to a high demand of utilizing the L-band for the mobile satellite services, the government has been thinking of migrating its SGLS uplink from L-band to USB to avoid the increasing radio frequency interference (RFI) from/to other mobile satellite systems. Such a migration raises a concern with respect to interference between the immigrated and existing links. To study the RFI impact, the power spectrum density (PSD) of both SGLS and USE waveforms and their respective bandwidth efficiency need to be studied. This paper derives the PSD for the SGLS and USB waveforms and compares their respective occupied bandwidths.

RFI modeling of satellite communications

This paper describes the development of an analytical model for predicting the interference experienced by a generic satellite communications receiving station due to an interfering satellite. In these initial studies, both satellites, the desired as well as the interfering satellite, are considered to be in circular orbits. Furthermore, the unknown parameters in the model are assumed to be random, and the mean of the radio frequency interference (RFI) duration, time between RFI and fraction of time there is RFI are also statistically estimated.

Modeling and simulation analyses of satellite radio frequency interference - Part I: Modeling and simulation

The objective of this paper is four-fold: (1) to develop mathematical models for uplink and downlink radio frequency interferences for three-dimensional elliptical-orbit satellites; (2) to formulate geometric criteria for determining all possible interference scenarios; (3) to simulate developed models with a sample communication network system; and (4) to compute the interference durations. Mathematical framework and numerical results will be presented. The model will be used to simulate and extract statistical information from the analyses of the simulation to predict the mean RFI duration, i.e., the time between RFI occurrences, and the fraction of time that RFI occurs. These parameters are essential to communication system designs such as frequency and orbit planning, assigning, scheduling.

War-gaming application for future space systems acquisition part 1: program and technical baseline war-gaming modeling and simulation approaches

This paper describes static Bayesian game models with “Pure” and “Mixed” games for the development of an optimum Program and Technical Baseline (PTB) solution for affordable acquisition of future space systems. The paper discusses System Engineering (SE) frameworks and analytical and simulation modeling approaches for developing the optimum PTB solutions from both the government and contractor perspectives.