Technology Today

2013 Issue 1

Interoperability Analysis of RF Systems

Interoperability Analysis of RF Systems

In operational environments, it is imperative that wireless systems maintain performance even in the presence of extreme noise interference originated by other radio frequency (RF) sources in near proximity. To ensure system performance and mission effectiveness, today’s RF engineers must design-for-interoperability and conduct system engineering analyses that will help identify, prevent and mitigate performance shortfalls. A team of Raytheon engineers has developed the Communication System Engineering Toolset (COMSET) simulation environment to facilitate the process of designing for interference and for assessing the interoperability performance of RF systems in dense and complex spectral environments.

COMSET is a simulation environment originally focused on analyzing communication system architectures and their performance in complex environments. The COMSET suite of simulation tools and their associated processes have evolved to address the interoperability needs of RF systems in general, including radar and electronic warfare (EW) systems. While the main benefits of COMSET reside in RF interoperability analysis, the tools are used for diverse purposes, including the prediction of jamming effectiveness on targets and quantifying the impact of new technologies introduced into system architectures.

The Concept of Cosite Interference

During the last three decades, military operations have benefitted from the proliferation of wireless technologies, electromagnetic sensors, and non-kinetic effectors like jammers. Conventional military systems operate in frequencies ranging from HF bands (3-30 MHz) to W-band (75-110 GHz) and in many cases co-exist with civil and commercial wireless services. Global Positioning System (GPS) devices, navigation systems, radar and surveillance sensors, electronic support and electronic attack measures, and communication networks are examples of RF capabilities commonly found in many military platforms. Due to the constraints on some platforms, antennas often must be located in close proximity to each other. When two antennas are near each other, the probability of electromagnetic interactions, or coupling, among RF systems increases. The closer the antennas, the higher the electromagnetic interactions among them can be. This type of electromagnetic interaction is best known as cosite interference. This type of interference is often unintentional and can easily degrade the performance of receiver systems to the point where signals cannot be processed as they get shadowed or buried in the noise. The intentional use of electromagnetic energy to degrade a receiver’s performance is known as jamming. In this case, the energy directed to the receiver is intended to deny the receiver the ability to process any incoming signal or support any kind of service.

To address issues inherent with cosite interference, designing-for-interoperability requires a good understanding of RF system architectures, electromagnetic scattering, signal propagation while traveling through space (better known as signal in space), and electromagnetic phenomenology. Additionally, the designer must understand that during the design process the platform and other nearby RF systems will contribute to signal interactions and receiver degradation. Predicting potential cosite interference interactions up front is important to minimize the need for future design modifications or mitigation measures that may be non-compliant with system specifications.

Figure 1. The COMSET Simulation Environment Architecture has five main modules. The COMSET application provides the user easy connectivity to the various commercial models and simulation modules currently available in the simulation environment. Modules are selected by the user through the GUIs that facilitate the execution of the interoperability analysis process.
Communication Systems Engineering Toolset (COMSET)

Raytheon has developed COMSET to facilitate the interoperability analysis of RF systems in operational environments and to predict a system’s effectiveness throughout its life cycle. Initially, COMSET was developed to fulfill specific needs in the military community by predicting the performance of communication systems operating in complex tactical environments. Since its original conception, the COMSET tools have evolved to support the broader need across Raytheon with regard to modules and processes capable of supporting the analysis of other types of multifunction RF systems in operational scenarios, including EW and radars.

The COMSET simulation environment is MATLAB®-based and includes commercial and customized simulation packages, descriptive system models, graphical user interfaces (GUIs), and visualization tools. COMSET enables the user to (1) derive performance specifications for a given system based on notional concepts of operations; (2) verify and validate component, subsystem, and/or system-level requirements; (3) support the development of platform RF footprints; (4) conduct system interoperability analyses; and (5) apply statistics and probabilistic methods to estimate system performance effectiveness.

While various methods can be used to predict RF interference, most provide only marginal estimates due to the utilization of low-fidelity models. Unlike other interference prediction environments, COMSET is grounded in physics-based models with simulations taking into account the actual system architecture, the in-band and out-of band RF performance of nearby systems, the physical structure and features of the platform, and the propagation environment.

The analysis engine of the COMSET simulation environment was built on the principles of cosite interference. The concept of signal-to-noise and distortion (SINAD) was implemented as a carrier- (i.e., signal) to-noise (i.e., interference) ratio (CNR) that takes into consideration intrinsic and extrinsic contributions acting independently to degrade the receiver performance — better known as susceptibility to interference.

To accurately estimate the system performance, validated radio models describing the in- and out-of-band transmitter and receiver characteristics are required. External noise sources are estimated or characterized. Estimation of the receiver susceptibility to noise and interference provides insight into how well the system can support services under a set of assumed constraints.

COMSET Architecture and Simulation Environment

Five major sub-applications, or modules, are embedded in the COMSET environment: (1) model generation and RF analysis; (2) propagation analysis; (3) antenna coupling and placement; (4) statistical operational analysis; and (5) GUIs. Figure 1 illustrates the COMSET simulation environment architecture, application and modeling capabilities. An extensive library of radio models is maintained and is easily accessible, facilitating model re-use for analysis. The RF models are created in Agilent’s Advanced Design System® (ADS) environment, and it is relatively easy to import/export models from/to other commercial or customized environments.

Within the RF analysis module, the component and system models describe the physical layer of a radio. The transmitter chain model describes the linear and nonlinear characteristics generated by the RF transmitter distribution subsystem, the exciter and waveforms. High-order intermodulation products provide insight into the non-linear performance of the transmitter. The receiver model, on the other hand, includes a detailed description of the RF receiver distribution subsystems and its components up to the analog-to-digital converter. In the receiver analysis, non-linear effects such as reciprocal mixing products are considered noise contributors. An important output of the RF analysis is the estimation of the receiver susceptibility to interference. Typically, the receiver susceptibility can only be estimated by characterizing the receiver system in a controlled environment. COMSET, on the other hand, provides a virtual laboratory to characterize the receiver’s susceptibility to interference and noise over a wide range of simulated conditions.

A comprehensive library of propagation models is currently available in the COMSET simulation environment. Because some models have been developed for specific applications and environments, a customized user interface enables the analyst to choose a model from the library that can be tailored to best represent the propagation media.

Computational electromagnetic (CEM) techniques are used to estimate coupling coefficients and scattering products, to support near- and far-field analyses, and to produce 2D and 3D antenna patterns. Statistical methods are also implemented to estimate system performance and operational effectiveness.

The COMSET GUIs allow easy access to the various application modules within the tool. All GUIs were created using the MATLAB Graphical User Interface Development Environment (GUIDE) and are written in MATLAB. Pull-down menus provide the user with options to import and export data, invoke models and applications, execute analyses and create reports. Other sub-applications in the COMSET tool can be accessed from the main COMSET GUI application. These include model creation applications, the analysis applications, the propagation model applications, and the multiplatform scenario application.

COMSET Analysis Methodology

The COMSET interoperability analysis methodology is a well structured process supported by menu-driven GUIs that enable the user to proceed through a set of intuitive steps. This process is illustrated in Figure 2. For most cases, the process is a top-down analysis approach with the starting point being the concept of operations (CONOPS) requirements. However, the COMSET environment also offers the user flexibility to conduct bottom-up sensitivity analyses.

Figure 2. The COMSET simulation process for the top-down analysis of system performance in operational environments. The process provides the flexibility for a user to conduct engineering trade-offs in support of requirements derivation and/or validation. The process is also applicable to the mission analysis of existing/new capabilities and new concepts of operation. Multiple requirement verification steps are introduced in the analysis (6, 8A) with alternative paths (7A, 7B and 9A) to allow interoperability requirement optimization.

The process starts with the identification of operational and performance requirements for a given capability. The user then develops the platform-level model that provides the physical description of the platform (i.e., airplane, ground vehicle, maritime ship, etc.) in which the RF systems resides. This model includes detailed descriptions of the placement of the antenna ports. Each RF system within the platform is then represented by a radio model that describes the electrical and electromagnetic performance of the system. Each radio model has three main components: (1) antenna, (2) RF distribution system for both transmitter and receiver chains, and (3) the analog receiver and exciter subsystems.

Creating the Radio System Models (Step 4) has two parts. The first is to create the communication model, which provides the linkage between the radio transmitter and the receivers. The second set is to develop a model description of the signal propagation and its interactions with the physical environment. The system performance in a non-interfering environment is estimated (Steps 5 and 6), providing a rough order of magnitude assessment of the validity of the system requirements. The second part of the analysis takes into account coupling and interference sources and potential electromagnetic interactions among systems in a given platform. During this step (7A), the analyst estimates the contributions to the expected system degradation as a result of non-intentional interference from other systems on the platform and within the operational environment.

The COMSET modeling approach links the radio, platform and propagation models to perform the interference analysis. The radio model includes a full description of transmit and receive functions and the antenna patterns of a given RF system. At the platform level, antenna coupling models are used to describe the potential electromagnetic interference (cosite interference) between close-proximity antennas. Multiple communication channels are established to describe the linkage between remote radios and platforms in a complex operational scenario. Finally, propagation models are implemented to describe signal propagation and attenuation due to the physical environment. The propagation models take into consideration environmental variables such as weather conditions, platform altitude, reflection and scattering phenomena, and obstruction density (e.g., urban canyons, heavy vegetation).

This process is repeated for every system and every platform under consideration in the analysis until an operational picture of the RF environment is fully synthesized.

Each receiver channel responds differently to interference. The susceptibility of a receiver is a combination of the receiver’s response to all active emitters, antenna coupling and environmental noise. An important step in the interference analysis is to estimate the antenna coupling. Figure 3 illustrates an antenna coupling analysis in which seven antenna ports were considered on a platform under evaluation. In this case, 15 primary interference pairs needed to be considered to determine the net receiver performance degradation due to antenna coupling (Figure 3, left side). The next step in the process is to estimate the overall receiver susceptibility to interference on a platform or in an operational environment. CNR is used as a measure of the receiver susceptibility. CNR is frequency dependent and will vary from system to system in a given platform. Figure 3 right side illustrates a result of a receiver susceptibility analysis. In this case, a system pair is analyzed and the receiver degradation is estimated when both systems are fully operational in the platform. The CNR estimates are used in a link analysis to estimate the quality of service of a communication channel in a highly dense spectral environment. Methodical analyses are conducted by reviewing all possible interfering mechanisms until performance estimates show acceptable link margins consistent with the operational requirements. During the process, the user has the flexibility to isolate specific cases and apply design of experiments methodologies to perform further analysis as needed.

COMSET Utility

COMSET capabilities have been effective in simulation and analysis of RF interoperability in complex operational scenarios. COMSET enables engineers to (1) derive performance specifications for a given system based on candidate concepts of operation; (2) verify and validate component, sub-system, and/ or system-level requirements; (3) support development of a platform’s RF footprint; (4) conduct system interoperability analyses; and (5) apply statistics and probabilistic methods to estimate system performance effectiveness. The COMSET analysis methods, when used up front during the design phase, allow the user to design for interoperability. In addition, the interoperability and operational effectiveness analyses have proven to be a powerful tool for preventing, identifying and/or resolving issues in systems operating in a complex environments throughout the life-cycle of a program. COMSET has been used in support of multiple efforts across Raytheon, and has received multiple accreditations and awards.

Edgar J. Martinez, Ph.D.
Contributors: William Davis, John VanPatten, Mark Beals, L. Paul Drury,
Anthony McDowell, Kelvin Quinn and Jacob Kim

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