Channel Simulation and Test Solutions for Satellite Communications

In satellite communications, data transmission rate is an important indicator of performance. Recent studies have found that the average power consumption of satellites has increased by 350%, the design life has increased from 10 years to nearly 14 years, and the appearance and complexity of satellites have increased by 2 to 5 times. Shin's new generation Internet Protocol (IP) satellite iPSTAR has a total capacity of 40Gbps, which is said to be 40 times the capacity of existing satellites. The downlink data rate of each spot beam of Hughes Network Systems' SPACEWAY is 800Mbps. Modern information technology requires wider satellite transmission bandwidth.

The advanced broadband communications satellites being developed today are expensive to design, build and deploy. Satellite designers maximize the payload capacity of their satellites to maximize profits once they are operational.

For satellite engineers, this means that because advanced communications satellites are required to maximize data rates and capacity, designers have little margin for error. Failure to analyze or account for any relevant parameters or real-world effects can result in reduced equipment performance and reduced returns. The opposite is over-designing, where all unknown factors are considered or designed for the worst case, resulting in excessive parameter margins, which adds unnecessary cost, components, weight, and complexity.

Figure 1 shows the overall schematic diagram of a traditional "bent pipe" satellite downlink, which includes the main components, but the satellite part only shows a transponder image. In fact, a satellite may carry 30 to 50 transponders.

Figure 1: Schematic diagram of the “bent pipe” satellite downlink.

AAZSZSZEach component has non-ideal performance that can degrade the overall function of the system. The main and most fundamental reason for degrading the overall performance of the system is thermal noise. The carrier-to-noise ratio (CNR) or signal-to-noise ratio (SNR) in the channel can be set by the carrier power. Shannon's law defines the maximum channel capacity (number of bits) per second for a specific signal-to-noise ratio and bandwidth, but this maximum value is never reached in actual operation. To ensure the best system design, all real impairments in the channel must be accurately simulated.

Simulation test solution

Comprehensive and realistic communication channel simulation is a requirement for design and verification. In response to the development of satellite communication services, designers need more effective simulation test equipment with broadband simulation and test functions to truly simulate environmental damage factors in the channel and the impact of various components in the channel on the system. In this way, designers can effectively control design margins during the design phase while meeting cost, reliability and capacity requirements.

Currently, satellite system designers can use several auxiliary tools in the design process. There are three main types of design simulation and testing tools: computer modeling and simulation tools, manufacturing and launching test satellites, and satellite channel simulators.

Computer modeling and simulation tools allow designers to model multiple channel impairments and consider the overall performance of the system. These tools can complete basic design and perform performance evaluation. Such software is available for desktops and advanced workstations. Even with the fastest processors, it takes a lot of computing time to get accurate results, which limits the number of simulation objects and the comprehensive impairment models that can be analyzed. These models may also not correctly reflect the actual situation.

In addition, some companies or units have built and launched experimental satellites to test new communication technologies and concepts with real hardware and software in real environments. These satellites include ACT, Artemis, Kopernikus, N-Star, Superbird, and Italsat F1. These satellites are expensive to build, but they provide a valuable experimental environment for the sophisticated technologies and defined methods of today's broadband satellite design. Satellites are at the top of the entire design pyramid, and their performance is determined by design, so there is little room to change technical specifications as needed, and it is also difficult to test their impact on the overall performance of the system by changing the damage mechanism.

Satellite channel simulators combine the test environment with the simulation environment to provide users with the accuracy and real-time performance they need. The Celerity CS80000 Wideband Channel Simulator (BCS) series produced by Aeroflex is a good example. These simulators create real channels with impairment mechanisms in a controlled, accurate, and repeatable manner in the laboratory. Because these channels are real-time systems with wideband RF inputs and outputs, real hardware terminals can be used during testing. Compared with software models, this real-time testing can run more test objects, thereby enabling more detailed testing.

Their features include: stable, repeatable simulation with defined, controlled impairment mechanisms; real-time and full-bandwidth channels to support real hardware and fast test times; worst-case simulation that can include any comprehensive impairment model; and minimizing costly testing or real satellite testing time with lab equipment.

Figure 2: Block diagram of a two-channel BCS.

The value of Aeroflex channel simulator lies in: very realistic simulation; high bandwidth; high dynamic range (14 b ADC/DAC w/ 36 MHz BW); low phase defect and support for dynamic delay update; modular design and easy upgrade; rich real environment damage selection, including dynamic Doppler, dynamic delay, thermal noise, path loss, in-band interference, adjacent channel interference, channel loading, phase noise , Rayleigh fading, rain loss, group delay distortion, amplitude fluctuation in passband, nonlinear gain, etc.

Other development test tools include the Broadband Signal Generator (BSG) and Broadband Signal Analyzer (BSA). These are not channel simulators, but can create realistic satellite signals and environments with impairments, and record and analyze the performance of the signal channel.

Introduction to Aeroflex Broadband Channel Simulator

Aeroflex Broadband Channel Simulator (BCS) creates accurate and repeatable channels for high-speed communication systems, point-to-point microwave and satellite communications, providing the widest bandwidth channel simulation and connection simulation, and the most comprehensive interference signal simulation. Real-time simulation provides bandwidth up to 500MHz, time delay up to 10 seconds, and can provide 1 to 4 independent channel simulations at the same time. BCS can add interference signals to the input signal in real time to simulate the interference of the actual transmission channel. The interference signals provided include: dynamic delay (short time delay from ground to GEO satellite return), dynamic Doppler, accurate phase noise, Ricean and Rayleigh fading, additive white Gaussian noise, flat fading, rain fading, additive CW and modulated interference signals, passband amplitude and phase odd changes. Frequency converters for related or independent channels can be used to control the frequency of input/output signals. Satellite simulation software package, using industry-standard TLE components, can generate 24 hours of Doppler and delay simulation files. Aeroflex's channel simulation has been successfully used in multiple satellite projects to simulate the round-trip path of signals, allowing comprehensive testing of satellites and ground stations before launch.