A Ka Band High Data Rate Transmitter for LEO Satellites Design and System Simulation

This paper addresses the design of a high-throughput Ka-band transmitter for LEO satellites. This article particularly focuses on RF and microwave subsystems and system simulation. The purpose of this transmitter (named KaLLISTO) is to operate the downlink from a LEO satellite to a ground station. High data rates are required to meet the expressed demand from satellite operators to enable much more data volume downlink. The equipment is the latest state-of-the-art qualified COTS based design. This design is perfectly adapted to New-Space programs, where cost effectiveness, performance and reliability are key factors. The allocated band goes from 25.5 to 27GHz Concerning the waveform the DVB-S2 standard is used to obtain flexibility for the modulation (QPSK, 8PSK, 16APSK, 32APSK) and for the coding rate. Traditionally, the X band from 8.025 to 8.4 GHz is used to ensure the downlink but the available bandwidth is relatively limited, only 375MHz. Also, it is important to know that X Band is starting to become saturated due to an increased number of users. This explains the growing interest in Ka Band. The first part of the paper will present an overview of the transmitter architecture based on direct IQ up-conversion. We continue to present the downlink RF link budget in X and in Ka Bands between the transmitter embedded in the satellite in LEO orbit and the ground station. The objective is to highlight the RF key parameters to optimize the use of the highest modulation order as possible thanks to the DVB-S2 standard (QPSK, 8PSK, 16APSK and 32APSK). A comparison between X-Band and Ka-Band on the total bytes transferred during one satellite pass is carried out. To optimize the design of SOE (Satellite-Onboard Electronics ex-Syrlinks) radio frequency transmitters (in X and Ka-band) we set up a system simulation based on the well-known Complex Envelope technique to modelize an RF signal. This simulation approach makes it possible to know the contribution of each transmitter stage (baseband, RF synthesizer, IQ Mixer, power amplifier, filtering, etc.) to the quality of the transmitted RF signal. The quality of the in-band RF signal is characterized by the EVM (Error Vector Magnitude) extracted from the complex constellation signal. We will highlight the statistical characteristics of the modulated signals with CCDF curves for the four modulation schemes of the DVB-S2 standard. A comparison between simulations and measurement on spectrums, ACPR (Adjacent Channel Power Ratio) and EVM at different transmitter stage (Baseband, IQ modulator output, RF output including power amplifier and filtering) will also be described. Finally, the paper will present the consolidated relationship between EVM and Bit Error Rate (BER). This simulation work prepares the introduction of predistortion techniques in our transmitters.