Multiplex Modulation Techniques for a Navigation Constellation Simulator
Philipp Neumaier, Markus Bochenko, Andreas Blumenschein, Nikolas Hansen, Josef Lex, Sowmyashree Lakshmaiah
Nowadays, GNSS signals offer different services on the same signal frequency band. This allows for at least three components to be multiplexed to the same signal. The system designer is facing the challenge to combine multiple signal components to become a constant envelope composite signal with the highest combination efficiency. Several equivalent mathematical descriptions of this signal type are known. In addition, there are various algorithms to find the optimal solution
related to the minimization of the undesired inter-modulation products. The objective of this paper is to analyze different mathematical representations of the multiplex scheme and identify the most suitable one. The goal is to find a hardware efficient solution which can handle all current state-of-art and future multiplexing
schemes. The latter part is fundamental to use it in a Navigation Constellation Simulator with the aim to support all existing and future GNSS signals with this multiplex schema. The evaluatio of the optimal multiplex schemes is an ongoing process and it is probable that the solution is based on numeric simulations. A simulator can contribute to the verification process in regards to non ideal signals.
The modernized and next-generation GNSS system signals have to deal with various types of signals. Due to the high costs of the GNSS satellite and its limitations e.g. payload size, weight and power limitation, the signals are multiplexed to a composite signal with quasi-constant envelope. Therefore, the signals are sharing a common up-converter, amplifier-chain, and antenna aperture which reduces costs. Ideal CEM modulation prevents AM/AM and AM/PM distortions when the high-power amplifier operates into saturation. There is one main disadvantage: Most of the advanced CEM techniques introduce IM products in order to obtain a constant envelope. As a result a part of the transmitted signal cannot be used for Position, Velocity, and Time (PVT). Nowadays there are various methods with advantages and disadvantages. This paper describes a multiplex technique for a NCS which supports all existing multiplexing techniques and the corresponding GNSS signals. Moreover, the NCS has the flexibility to deal with all future multiplexing techniques and its signals for a fast prototyping and evaluation phase. The existing CEM techniques are based on perfect orthogonal based signals. In reality we have non ideal signals and a bias can be introduced in the code tracking. Using the NCS during signal prototyping can help the user to develop a deeper understanding of the possible distortions and its influence to the GNSS receiver. The NCS is described with emphasis on the hardware implementation of the CEM techniques. The advantages of this platform are demonstrated and some measurements results of the state-of-art signals in order to outline their multiplex technique: e.g Galileo E1, E5 and E6 and GPS L1, L2 and L5.