James Yoh

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.

Survey of Ka-band satellites for wideband communications

The interim generation of wideband communications satellites will be the wideband gapfiller satellites (WGS) which will preserve and augment the legacy missions of the Defense Satellite Communications System (DSCS) and the Global Broadcast Service (GBS), and add the new so-called two-way Ka. In order to provide for conceptual design of these satellites and plan for their acquisition, a survey has been conducted on currently operational and planned communications satellites that operate in the Ka-band. Our search has been performed to identify articles in the open literature on the description of the Ka-band satellites. Of particular interest are the salient features that pertain to the communications subsystems of these satellites. These features include frequencies, data rates, waveforms and frame structures. Data was obtained for the communications satellites from Italy, Germany, Japan and the European Space Agency (ESA), as well as some satellites from the United States (US). Many US Ka-band communications satellites are currently being developed. Unfortunately, information on waveforms and frame structures is not readily available in open literature on all of them. This article gives brief descriptions of these satellites. Feature parameters are presented in tabular format for comparison among different systems.

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.

Turnaround command effects on USB and SGLS satellite downlinks

Many satellites using commercial off-the-shelf USB and SGLS transponders such as the L3-Com CXS 2000 do not have turnaround command (TAC) suppression in their downlinks. Without TAC suppression, these satellites exhibit greater downlink service modulation losses for carrier, ranging, and telemetry. Depending on the selected uplink command modulation index and the turnaround ratio, these additional modulation losses could vary from 0.1 dB to 3 dB (for command mod indices less than 1 radian). They are due to the allocation of downlink power to the TAC, and partly to an increase in intermodulation (IM) power. For an uplink command modulation index of 0.3 radians, our calculations for both USB and SGLS signals show that the loss of downlink power to TAC and IM is less than 10%. However, when the uplink command modulation index is increased to a nominal operating value of 1.0 radian, the loss of downlink service power to TAC and IM becomes 40%. This large loss of downlink power to the TAC and IM increases the modulation losses for other downlink services, which could result in denial of services such as telemetry and ranging to ground users with small antennas. This paper shows that suppression of turnaround command will result in lower TAC and IM losses, which will in turn improve downlink services with higher link margins. These more robust downlink margins permit the use of a smaller and cheaper high-power amplifier (HPA) in the satellite transmitter12.

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.

War-gaming application for future space systems acquisition: MATLAB implementation of war-gaming acquisition models and simulation results

The paper describes the MATLAB (MathWorks) programs that were developed during the REU workshop1 to implement The Aerospace Corporation developed Unified Game-based Acquisition Framework and Advanced Game - based Mathematical Framework (UGAF-AGMF) and its associated War-Gaming Engine (WGE) models. Each game can be played from the perspectives of the Department of Defense Acquisition Authority (DAA) or of an individual contractor (KTR). The programs also implement Aerospace’s optimum “Program and Technical Baseline (PTB) and associated acquisition” strategy that combines low Total Ownership Cost (TOC) with innovative designs while still meeting warfighter needs. The paper also describes the Bayesian Acquisition War-Gaming approach using Monte Carlo simulations, a numerical analysis technique to account for uncertainty in decision making, which simulate the PTB development and acquisition processes and will detail the procedure of the implementation and the interactions between the games.

DIVERSITY AND COMBINING TECHNIQUES FOR 2G AND 3G PCS SYSTEMS

This paper provides an overview of the diversity and combining techniques for the second (2G) and third (3G) generation Personal Communication Systems (PCS). The 2G system employs path diversity with maximum combining ratio combining technique. On the other hand, the 3G system uses antenna diversity along with path diversity and the maximal ratio combining (MRC) technique. Both of these systems use convolutional code to combat channel errors. The paper also presents simulation results describing the performance of the diversity and combining techniques for a 2G PCS system.