An Optimized and Payload Achievable Multiplexing Design Technique for GNSS Signals
Jiayi Zhang, Zheng Yao, Junjie Ma and Mingquan Lu, Tsinghua University, China
Constant envelope multiplexing (CEM) is typically used in combining multiple global navigation satellite system (GNSS) signals for efficient transmission on payload. As GNSSs evolve, it has been a trend in the main navigation systems that the number of signals to be multiplexed increases, modulations and frequencies of signals become diverse. When multiplexing these signals with currently CEM techniques in the signal generator on payload, it requires more bandwidth and complexity compared with that of the legacy signals. Limited by the current available bandwidth and non-ideal transmission characteristics of hardware on satellites, distortion is introduced to the CEM signal which degrades the pseudoranging accuracy at users.
Existing methods to reduce the impact of payload non-ideality in engineering are to characterize and compensate the transmission characteristics with a digital predistortion unit . However, the wideband and multi-component CEM signal exaggerates the distortion caused by imperfect implementation on payload. It results in that the predistortion methods should be more complex to apply an accurate compensating model, which can hardly be accepted on satellite. Also, additional performance degradation of signal is introduced by predistortion. Therefore, distortion still exists in the predistorted signal, especially shown in a fluctuant amplitude before high power amplifier (HPA), which is one of the main source of non-linear effects. It is also shown in a diverse group delays of multiple signal components, which restricts the potential jointly processing of the CEM signal.
To solve this problem, we consider the hardware characteristics and theoretical signal as a whole, and propose a new methodology of GNSS CEM design which aims at optimizing the overall performance of payload transmitted signal. It is conducted as follows.
First, a CEM signal is analytically expressed with a linear combination of signals and inter-modulation terms with different amplitudes and bandwidths. Second, transmission characteristics before HPA can be collected and modeled as a pre-HPA filter, whose amplitude /phase response affects differently on the multiple signal components of a CEM signal. In this case, a primary object is to minimize the fluctuation of envelope of the filtered signal, thus reduce the HPA distortion caused by inconstant input amplitude. Further, reconstruction of the CEM design is conducted considering the filtering effects to every signal components, thus compensates the inconsistency of pseudoranging at individually receiving of CEM signal components.
By applying the proposed method, compensation of non-ideality of payload is achieved in the theoretical design of CEM signal and represented as a lookup table (LUT), which is a general form of CEM scheme for signal generator. Therefore, it reduces the complexity to implement the predistortion unit, as well as promotes the quality of signal after transmission.
 M. Rapisarda, P. Angeletti, and E. Casini, “A simulation framework for the assessment of navigation payload non-idealities,” 2007.