# Design of Tracking, Telemetry, Command (TT&C) and Data Transmission Integrated Signal in TDD Mode
## 1 Introduction  
In traditional satellite network systems, the responsibilities of the intersatellite link (ISL) are divided into two independent functions, TT&C and data transmission; that is, the data transmission link and TT&C link are undertaken by two independent sets of equipment [1; 2]. Refs. [3; 4; 5] introduce TT&C's typical methods and related technologies for the intersatellite, including ranging, Doppler frequency measurement, and angle measurement. The two completely independent functions not only have complex structures but also occupy different frequency bands. However, as the number of low earth orbit (LEO) satellites increases, such as OneWeb, Starlink, Telesat, etc., the lack of frequency resources is one of the most important restrictions in the spacecraft and aircraft networks [6; 7]. In addition, the TT&C services adopt the FDD mode to realize high-precision measurement. Under standard operating conditions, the FDD mode will produce problems such as a complex frequency distribution and increased requirements for TT&C equipment. Due to the lack of spectrum resources, it is difficult to obtain the paired spectrum needed in the FDD mode [8; 9]. Ref. [10] adopts space-based TT&C technology to solve the problem of resources dissatisfaction of ground-based TT&C system. However, this method is limited by the number of ground stations. Refs. [11; 12; 13; 14] place the TT&C and data transmission services in the same frequency band to alleviate the problem. An unbalanced signal system with a low information rate was adopted in [11]. In the paper, the in-phase (I) channel transmitted data, and the quadrature (Q) channel transmitted the TT&C signal. Due to the emergence of the unified spread spectrum TT&C system, a TT&C and data transmission system based on code division multiple access (CDMA) was proposed [12; 13]. Ref. [14] inserts the ranging pseudo-noise (PN) code into the high-speed data transmission channel to achieve an integrated signal. However, the signal combination of these methods is complex, which increases the complexity of the modulation and demodulation process. In sum, the existing methods seeking to improve the frequency band utilization in satellites are code division multiple access and improved modulation methods. In the mobile communication system, the extreme shortage of spectrum resources can be alleviated by the TDD mode. By transmitting and receiving at different times, it can utilize all available bandwidth efficiently. TDD technology plays an important part in the long-term evolution (LTE) standard, saves spectrum resources, and addresses users' needs [15; 16]. For satellite and aircraft systems, compared with those in FDD, the frequency resources in TDD are easier to obtain, and the costs are lower. However, TDD technology is not suitable for high-precision ranging and cannot measure velocity. Because the transponder cannot receive the signal during the transmission period, the tracking loop is interrupted periodically. With these observations in mind, in this paper, we increase the utilization of the frequency band by using an integrated signal and the TDD mode. The BOC and amplitude-phase shift keying (APSK) integrated signal is proposed. The TT&C signal is modulated by the BOC, and the data transmission is modulated by APSK. The BOC signal is expected to be used in future navigation systems because of its advantages. The BOC signal offers better performance for code tracking and multipath rejection performance. In addition, compared with the traditional spread spectrum modulation, the existence of subcarriers provides designers with more freedom to design modulation mode and reduce the interference of other signals at the same frequency [17; 18]. It can be regarded as a direct spread spectrum sequence (DSSS) that multiplies the square wave subcarrier sequence [19]. The modulation subcarrier causes the frequency spectrum to shift to both sides of the carrier frequency.