The satellite platform determines “whether the satellite can work properly” and “how long it can work”, while the payload determines “what the satellite can do” and “how well it can be done.” No matter how perfect a satellite’s platform is, if its payload performance is low or fails, it will become a pile of expensive space junk. Communication satellite payload refers to the space instruments and equipment on communication satellites that directly perform communication tasks. Its core goals can be summarized as: achieving the maximum communication capacity, the highest signal quality, the most flexible business deployment and the longest on-orbit lifespan under limited satellite resources (power, bandwidth, weight). A complete communication load system is mainly composed of two major subsystems: the antenna subsystem and the transponder subsystem. The antenna subsystem receives the uplink signal and sends it to the repeater subsystem to process the signal. The antenna subsystem then sends the processed signal as a downlink signal to complete the relay and forwarding of communication signals.
The antenna serves as the gateway for radio waves to enter and exit between the satellite and the ground. Its performance directly determines the coverage, signal strength and frequency reuse capabilities of the satellite. Communication antennas can be roughly divided into global beams, hemispheric beams, regional beams, spot beams, multi-beams, and shaped variable beam antennas. Global beam antennas are mostly implemented using conical horns, and communications within about 1/3 of the world’s regions can be achieved within the coverage of one beam. Early communication satellites, such as my country’s “Dongfanghong-2”, used global beam antennas. Hemispherical and regional beam antennas were later developed due to the wide beams of global beam antennas and the fact that satellite communications are sometimes not necessarily needed in most areas within its coverage area. Hemispheric beam antennas are mostly implemented with reflective surface antennas and are often used in international communication satellites, such as the INTELSAT satellite fixed over the Pacific Ocean. It has two hemisphere beams. The eastern hemisphere beam covers North and South America, and the western hemisphere beam covers eastern Asia and Oceania. Regional beam antennas are implemented through shaped reflectors or array antennas to cover specific geographical areas and can be “tailored” according to the shape of the service area and the communication needs of various places in the area. my country’s “Dongfanghong-2A” and “Dongfanghong-3” communication satellites use regional beam antennas to cover our country’s territory and surrounding areas. The spot beam antenna can be implemented using a reflective surface antenna or a phased array antenna. Its beam width is very narrow and the coverage area is very small. However, the antenna gain is extremely high, so the ground antenna gain and transmission power can be greatly reduced. A communication satellite may have multiple spot beams, which is very important in military communications, and the direction of the spot beams can also be controlled. Multi-beam antennas are the hallmark of modern advanced communication satellites. Through a large antenna array, dozens or even hundreds of spot beams are generated simultaneously to cover the entire service area like a honeycomb. Multi-beam antennas bring two revolutionary advantages. One is frequency reuse. The same frequency can be reused in non-adjacent beams, which increases the total communication capacity of the satellite exponentially. The other is high EIRP. Each point beam has high gain, making the equivalent omnidirectional radiation power from the satellite to the user very high. There are roughly two types of shaped beam antennas, multi-feed shaping and single-feed reflecting surface shaping. Multi-feed shaping is to use multiple feeds to superimpose multiple secondary patterns formed by the same reflection parabola, and rely on the beam forming network to control the phase amplitude of each feed to achieve shaping. It requires a large space and mass to accommodate the feed array and beam forming network. However, if components with adjustable amplitude and phase are added to the network, the ground can achieve remote control of the beam shape and become a remotely controlled variable shaping beam antenna. Single feed reflective surface shaping is achieved by changing the shape of the reflective surface, which is simple and reliable. The feed part is small in size and mass, but generally cannot be changed after it is made.
The transponder subsystem is essentially a wide-band transceiver. It receives weak uplink signals from the antenna, processes them, amplifies them, and sends them back to the ground through the antenna. The signal processing by the transponder subsystem can take many forms, which can be roughly divided into two categories: transparent type and processing type. This is the key to distinguishing traditional satellites from advanced satellites. The transparent transponder only converts the frequency and amplifies the uplink signal received by the antenna. Sometimes it also performs some simple transfer between frequency bands and sends it to the antenna for delivery. It does not perform baseband processing such as demodulation and decoding, so it is “transparent” to the ground system, like a transparent pipe. The signal goes out as it comes up (except for noise and distortion). Transparent transponders generally consist of preamplifiers, local oscillators, mixers, filters, low-power amplifiers, input multiplexers, output multiplexers, high-power amplifiers, etc. The advantages of transparent transponders are mature technology, simple structure, and extremely low delay; the disadvantage is that it amplifies noise and interference “equally”, has poor flexibility, and cannot optimize signals. Processing transponders mainly add demodulators, modulators and signal processing units, which can demodulate, decode, switch, route, and even recode and modulate signals on the satellite. The key components of a processing transponder include preamplifiers, frequency converters, power amplifiers (traveling wave tube amplifiers, solid-state power amplifiers), channelization equipment, etc. The preamplifier is responsible for low-noise amplification of the uplink signal, and the mixer multiplies the signal from the preamplifier and the signal from the local oscillator to achieve frequency conversion. The demodulator demodulates the received modulated microwave signal or intermediate frequency signal into a baseband signal. After processing by the signal processing unit, the modulator modulates the processed baseband signal on the intermediate frequency or microwave carrier. There are roughly three types of baseband signal processing by the signal processing unit. The first is to “regenerate” the digital baseband signal, with the purpose of avoiding the accumulation of noise distortion; the second is to perform signal exchange processing, connecting the uplink signal exchange of different beams or different channels of the same beam to the corresponding downlink channel; the third is to perform transformation and processing of other complex signals, including encryption processing and conversion between different multiple access modes. The processing transponder can dynamically allocate bandwidth and power resources to different beams and users according to business needs, and supports on-board Mesh networks, reducing signal “hops” and reliance on ground gateway stations.
In actual design, the core difficulty is system-level trade-offs and optimization. The resources that satellite platforms can provide are limited. Increasing bandwidth can increase capacity, but requires larger antennas and wider transponders, which increase weight; increasing power can enhance signals, but requires larger solar wings and heavier power systems, and the heat generated by the power amplifier also requires a more complex temperature control system. As the industry continues to pursue cost, efficiency and flexibility, communication satellite payload technology has shifted from the pure pursuit of capacity and power to the deep integration of intelligence, flexibility, integration and efficiency. Integration and modularization are the cornerstones of industrialization, and flexibility and intelligence are the core of the future. In the past, satellites were expensive “customized works of art.” However, in order for the space economy to truly enter people’s lives and detonate the value of the space economy to a greater extent, satellites must be turned into “industrialized products.” Moreover, the introduction of artificial intelligence and machine learning also enables the payload to have the capabilities of intelligent spectrum management, interference suppression and autonomous health management, allowing the satellite to evolve from a “static relay” to an “intelligent node”. The ultimate goal of technology is application. Mega-constellations with LEO satellites as the core are subverting the traditional communications field. They achieve global coverage and low latency through their numerical advantage, and directly serve satellite Internet access. In order to adapt to the booming development trend of the global satellite Internet industry, satellite communication payloads continue to increase communication capacity and application flexibility to meet the future market demand for flexible deployment of satellite capacity. With the further improvement of satellite communication capacity and flexibility, satellite-borne high-speed digital processing capabilities and satellite-ground integrated systems will be further improved. Product system functions will achieve substantial breakthroughs. Problems such as call quality, data transmission rate, and usage costs will be easily solved. The advantages of flexible networking and large coverage areas of communication satellites will be further demonstrated. And as software definition, AI, and commercialization continue to bring new opportunities, human communication technology is also constantly moving towards new peaks.
