# Enhanced LDM for Next-Generation Digital Broadcasting Transmission
## 1 Introduction  
In recent years, as the types and quantities of handheld portable devices and fixed terrestrial broadcast receiving devices have developed in full swing, people's needs for multimedia content are becoming increasingly diverse. In order to meet the growing demand for multi-service transmission and to make more efficient use of spectrum resources in future broadcasting networks, several next-generation digital TV broadcasting standards have been announced successively around the world. For example, in June 2008, the European second-generation digital terrestrial TV broadcast transmission standard DVB-T2 was announced [1]. In July 2015, China's next-generation digital TV broadcasting system digital terrestrial television multimedia broadcasting-advanced (DTMB-A) was adopted by the International Telecommunication Union (ITU) [2]. In June 2017, the physical layer protocol (A/322) of the Advanced Television Systems Committee 3.0 (ATSC 3.0) was approved as the final standard [3, 4, 5, 6]. Compared to existing standards, ATSC 3.0 standard can provide better flexibility, more robustness, and higher spectrum efficiency [7, 8, 9, 10, 11, 12, 13]. A key physical layer technique in ATSC 3.0 is the so-called layer division multiplexing (LDM). LDM is a non-orthogonal multiple access (NOMA) technology. Both NOMA and point-to-multipoint (P2 MP) have been identified as essential technologies for 5G to make more efficient use of the spectrum [14, 15, 16]. Ref. [17] discussed the application of LDM as an enabling technology for 5G and proved that the P2 MP subsystem with LDM can deliver high-quality broadcast services using thebroadband infrastructure. The basic principle of LDM is that the second layer of the data stream is superimposed to the original data layer to form a composite signal. Usually, the second layer of the data stream is called the enhanced layer (EL, also called the upper layer) signal, and the original data layer is the core layer (CL, also called the lower layer) signal. All the two signals share the same bandwidth except that the power of the CL signal is higher than that of the EL signal. The power difference between CL and EL signals is characterized by the power injection level. Compared with traditional time division multiplexing (TDM) and frequency division multiplexing (FDM) technologies, the LDM technology can provide a lower received signal-to-noise ratio (SNR) threshold and higher spectral efficiency [18; 19; 20]. Consider a typical application scenario of LDM where there are two differentiated services, namely mobile services and fixed services. The former is used for outdoor and portable reception of high-definition signals, which can usually tolerate lower data rates and is carried by the more robust CL; the latter is exploited to transmit ultra-high-definition television signals, which usually require higher data rates and are carried by the lower power EL [21; 22; 23; 24; 25]. In terms of coverage and networking, note that there is a phenomenon that receiving methods in different scenarios have different gain differences. In order to achieve the same coverage for different services in the LDM system, the requirements for the receiver antenna height correction and gain correction value will be more stringent. Therefore, in actual reception, the receiver antenna form and installation will directly affect the service effective coverage. For example, for the high bit rate services transmitted by EL, when the antenna gain and installation height cannot reach the value of the service plan, a large range of EL holes may appear within the effective coverage of CL, which is more obvious in urban environments [23; 26]. Although the coverage of the EL signal can be enhanced by simply adjusting the injection level, it will significantly affect the reception performance of CL mobile services. Therefore, it is necessary to improve the performance of EL service reception without affecting the reception threshold of CL service.