# LDM-Ex-FDM: A Novel Multi-Service Transmission Scheme for the ATSC 3.0 System
## 2 Advanced Television System Committee (ATSC) 3.0 System Overview
### Direct-Interference-Cancellation Scheme  
For fixed receivers, usually only the \(EL\) signal in the two-layer \(LDM\) is demodulated. To solve this problem, this paper proposes a novel DIC scheme. Note that in the DIC scheme, QPSK modulation is adopted for the \(CL\) of the \(LDM\) signal. Actually, to ensure the robustness of \(CL\), the \(CL\) of \(LDM\) is usually equipped with low code rate FEC coding and a low-order modulation scheme. By adopting QPSK modulation and appropriate code rate, the 720p mobile service of \(CL\) transmission can be sufficiently fulfilled. If the \(CL\) adopts non-uniform 16 QAM or other higher-order modulation, further optimization is needed for this scheme.  
The core idea of DIC is to treat the \(LDM\) symbols as the \(EL\) symbols with constellation offset. The procedure of the proposed DIC scheme can be divided into the following steps: (1) Determine the distance of the constellation offset of the \(LDM\) symbol after channel equalization. If the equalized symbol constellation is within the first quadrant, the constellation offset distance will be determined as \(\sqrt{2}+\mathrm{j}\sqrt{2}\). Similarly, if the equalized symbol constellation is within the second, third, and fourth quadrants, the constellation offset distance will be determined as \(-\sqrt{2}+\mathrm{j}\sqrt{2},-\sqrt{2}-\mathrm{j}\sqrt{2}\), and \(\sqrt{2}-\mathrm{j}\sqrt{2}\), respectively. (2) The second step is to subtract the corresponding constellation offset from the equalized  
Figure 7: Comparison of the constellation of the \(LDM\) symbols with and without NEISP in the LDM-Ex-FDM scheme, where the injection level was set to \(\Delta=0\) dB, the \(CL\) constellation was set to quadrature phase-shift keying (QPSK), and the \(EL\) constellation was set to NU\(-\)64 QAM (non-uniform quadrature amplitude modulation): (**a**) \(LDM\) symbol constellation before puncturing; (**b**) \(LDM\) symbol constellation after puncturing.  
_LDM_ symbol to obtain the repaired _EL_ symbol. (3) The third step is to pass the repaired _EL_ symbol through the BICM decoder to decode the _EL_ bit data out.  
Compared to the traditional SIC scheme, the demodulation of _CL_ symbols and the re-construction of \(X_{CL}(k)\) can be omitted in the DIC scheme during the process of demodulating _EL_ symbols, thus the computational complexity and the processing delay can be significantly reduced at the receiver. In fact, the constellation offset \(X_{CL}{{}^{\prime}}(k)\) here can be regarded as the reconstructed signal \(\widetilde{X}_{CL}(k)\) in the above SIC scheme. Furthermore, the process of repairing the constellation offset of the _EL_ signal in DIC is equivalent to eliminating the interference of _CL_ to the _EL_ signal in SIC.  
According to (5), at the receiver, the demodulated _EL_ symbol can be rewritten as:  
\[\begin{array}{ll}X_{EL}{{}^{\prime}}(k)&=\frac{Y_{\text{LDM}}(k)\sqrt{1+a}}{ \widehat{H}(k)\sqrt{\bar{a}}}-\frac{1}{\sqrt{a}}X_{CL}{{}^{\prime}}(k)\\ &=X_{EL}(k).\frac{H(k)}{\widehat{H}(k)}+\frac{\Delta_{CL}{{}^{\prime}}(k)+ \frac{\sqrt{1+a}}{\widehat{H}(k)}N(k)}{\sqrt{\bar{a}}}\\ &\Delta_{CL}{{}^{\prime}}(k)=X_{CL}(k)\frac{H(k)}{\widehat{H}(k)}-X_{CL}{{}^{ \prime}}(k)\end{array} \tag{7}\]  
where \(\Delta_{CL}{{}^{\prime}}(k)\) indicates the error of repairing the constellation offset of the _EL_ signal. The bit data transmitted by _EL_ can be obtained through BICM decoding of the demodulated symbols of _EL_.  
The receiver structure of the DIC scheme for demodulating _EL_ signal is shown in Figure 8, while the structure of the receiver for demodulating the _CL_ signal can be referred to Figure 1. The processing procedure of the receiver is as follows: after the signal is synchronized with time and frequency, CP removal, FFT and channel equalization, the demodulated _EL_ signal with constellation offset will be repaired using the proposed DIC scheme. Then, the repaired _EL_ symbol will be decoded by a BICM decoder to obtain the _EL_ bit data.