# Performance Evaluation of the New Compound-Carrier-Modulated Signal for Future Navigation Signals
## 1 Introduction  
Satellite navigation systems and their augmentation systems are fundamental infrastructures for countries, and signal structure design is one of their key links. The navigation signal is precisely a function of the message vehicle and air interface to link up the ground-based infrastructures and users with satellites. Therefore, the signal modulation scheme directly determines the systematic intrinsic performance. The Global Navigation Satellite System (GNSS) and its augmentation systems are undergoing a process of enhancement and diversification. The ever increasing user requirements in terms of accuracy, robustness, integrity and reliability, are stimulating the evolution towards the future advanced GNSS signals. The overall evolutionary trends focus on spectrum compatibility and resistance to propagation and environmental dagration [1]. The construction of GNSSs is increasingly improving and expanding. Meanwhile, regional augmentation systems and space-based augmentation systems are emerging in succession. It is predicted that by 2030 there will be more than 160 satellites and over 400 signals in space [2]. The already intensive frequency resource will be more crowded and the increasing mutual interference will degrade the systematic accuracy and performance. Better spectrum efficiency will become a critical issue in navigation signal design. The positioning accuracy is also a primary concern for signal design since the signal strictly determines the systematic natural accuracy limits. The potential performance of the GNSS receiver can only finitely approach the limits, regardless of any development of the technology. Furthermore, due to the navigation channel characteristics, multipath effects are some of the main error sources [3], so the multipath mitigation capability becomes a key design consideration. In addition, the fusion of navigation service and communication service has become a development direction of the information industry. Composite designs based on the navigation signals and communication signals have turned into a new solution for future advanced GNSSs. Binary phase shift keying (BPSK) modulation was the first modulation scheme adopted by satellite navigation because of its simplicity. Due to the coexistence of ever-increasing space signals and its optimal performance, binary offset carrier (BOC) modulation became the serious candidate [4]. Derivatives of the BOC concept are also utilized by the GNSSs, such as the alternative BOC (AltBOC) for E5 Galileo and B2 Beidou, multiplexed BOC (MBOC) for E1 Galileo and B1 Beidou, time-multiplexed BOC (TMBOC) for L1 GPS. Although these modulation schemes offer significant performance in terms of tracking accuracy, multipath mitigation and jamming reduction, there are still critical issues to be addressed, such as their large spectrum occupancy, high spectral side lobes and implementation complexity [5; 6]. Subsequent innovations for signal design have focused on the continuous phase modulation (CPM), multilevel coded symbol (MCS) and m-PSK BOC modulation [7; 8; 9] approaches, where there is also a trade-off between implementation complexity and signal performance. Multi-carrier signals are widely used in terrestrial communication, and even in satellite communication. The study of multi-carrier signals demonstrates they have efficient spectrum utilization, high out-of-band attenuation properties and multipath mitigation capability. Especially, orthogonal frequency division multiplexing (OFDM), as a representative, has already been the dominant modulation scheme for mobile communications. For example, the fourth generation (4G) mobile communication adopts OFDM as its core technology [10]. The European digital video broadcast-terrestrial (DVB-T) system utilizes OFDM modulation as the air interface [11]. The OFDM-based terrestrial communication signals have become a signal-of-opportunity candidate for positioning services to supplement GNSS [12; 13].