# 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
### System Model  
In the typical scenario of using _LDM_ in the ATSC 3.0 system, simply adjusting the injection level can improve the reception performance of the high throughput fixed service carried in the _EL_ of the _LDM_. However, it will significantly affect the reception of the mobile service of the _CL_. Therefore, how to improve the reception performance of the _EL_ without affecting the system performance of the _CL_ is meaningful. This paper attempts to tackle this problem by resorting to the combination of _LDM_ with FDM. Specifically, the data symbols for the mobile service stream are transmitted through the _CL_ of _LDM_ while the data symbols for fixed service stream are divided into two parts: _EL_1 and _EL_2. _EL_1 is assigned to the lower layer of _LDM_. The _LDM_ signal formed by the multiplexing of _EL_1 and _CL_ is frequency division multiplexed with _EL_2, which then forms the final composite frame structure. In this case, the SNR of _EL_1 is much lower than that of _EL_2. To perform the error correction of the BICM more effectively, puncturing at equal intervals is adopted when dividing the data symbols of the fixed service stream. The remaining symbols after puncturing are _EL_1 and the symbols removed by puncturing are _EL_2. The amount of information carried by _EL_1 is larger than that by _EL_2. According to different service requirements, the information to be transmitted is formed into modulated symbols of different layers through different forward error-correction (FEC) coding and the BICM process, and then the modulated symbols are mapped to OFDM subcarriers. The transmitted symbols of the _LDM_ signal can be expressed as:  
\[X_{LDM}(k)=\sqrt{\frac{1}{1+\alpha}}X_{\text{CL}}(k)+\sqrt{\frac{\alpha}{1+ \alpha}}X_{\text{EL}1}(k) \tag{1}\]  
Figure 1: The two-layer layered division multiplexing (_LDM_) transmitter and receiver of the traditional Advanced Television System Committee (ATSC) 3.0 system: (**a**) two-layer _LDM_ transmitter; (**b**) two-layer _LDM_ receiver. \[\Delta=10\text{-}\text{lg}\frac{1}{\alpha} \tag{2}\]  
where \(k\) represents the subcarrier index, \(\alpha\) denotes the average power ratio of the \(LDM\), \(\Delta\) indicates the injection level. \(X_{CL}(k)\) and \(X_{EL1}(k)\) stand for the transmitted symbols for \(CL\) reception and \(EL\) reception on the \(k\)-th sub-carrier, respectively. Note that the average power of \(X_{CL}(k)+X_{EL1}(k)\) is normalized. Through a time-invariant channel, at the receiver, the received symbols can be denoted as:  
\[ \begin{array}{ll}Y_{LDM}(k)&=\sqrt{\frac{1}{1+\alpha}}X_{CL}(k)\cdot H(k)\\ &+\sqrt{\frac{\alpha}{1+\alpha}}X_{EL1}(k)\cdot H(k)+N(k)\end{array}  \tag{3}\]  
where \(H(k)\) represents the transfer function, \(N(k)\) stands for the Gaussian noise and other additive interference. After channel estimation, the \(CL\) symbols can be demodulated through the following single-tap frequency-domain (FD) equalization, i.e.,  
\[ \begin{array}{ll}\hat{X}_{CL}(k)&=\frac{Y_{LDM}(k)\sqrt{1+\alpha}}{\hat{H}( k)}\\ &=X_{CL}(k)\frac{H(k)}{H(k)}\\ &+\frac{\sqrt{\alpha}X_{EL1}(k)\cdot H(k)+\sqrt{1+\alpha}N(k)}{H(k)}\end{array}  \tag{4}\]  
where \(\hat{H}(k)\) expresses the estimated channel frequency response of the \(k\)-th sub-carrier. The bit data of the mobile reception can be obtained by decoding the \(CL\) symbols, which then is reconstructed by BICM to generate \(\widetilde{X}_{CL}(k)\). The demodulation of the \(EL1\) symbols for fixed service can be expressed as follows:  
\[ \begin{array}{ll}\hat{X}_{EL1}(k)&=\frac{Y_{LDM}(k)\sqrt{1+\alpha}}{\hat{H}( k)\sqrt{\alpha}}-\frac{1}{\sqrt{\alpha}}\widetilde{X}_{CL}(k)\\ &=X_{EL1}(k)\cdot\frac{H(k)}{H(k)}+\frac{\Delta_{CL}(k)+\frac{\sqrt{1+\alpha} }{\hat{H}(k)}N(k)}{\sqrt{\alpha}}\end{array}  \tag{5}\]  
\[\Delta_{CL}(k)=X_{CL}(k)\frac{H(k)}{\hat{H}(k)}-\widetilde{X}_{CL}(k) \tag{6}\]  
where \(\Delta_{CL}(k)\) denotes the remaining \(CL\) signals after interference cancellation.