# Enhanced LDM for Next-Generation Digital Broadcasting Transmission
## 3 Proposed En-LDM Scheme
### System Model
#### 3.1.2 En-LDM Receiver  
The block diagram of the receiver in the proposed En-LDM scheme is shown in Figure 2b. After the signal is synchronized in time and frequency, CP removal, fast Fourier transform (FFT) de-mapping, channel estimation, and equalization, etc., the CL signal of LDM is first demodulated. At the receiver, the received signal is expressed as  
\[Y_{LDM}(k)=\sqrt{\frac{1}{1+\alpha}}X_{\mathrm{C}}(k)\cdot H(k)+\sqrt{\frac{ \alpha}{1+\alpha}}X_{\mathrm{E1}}(k)\cdot H(k)+N(k) \tag{4}\]  
where \(H(k)\) represents the transfer function, and \(N(k)\) stands for the Gaussian noise and other additive interference signals. After the receiver performs channel estimation on the signal, the CL symbol can be demodulated by the following equalization method, i.e.,  
\[\hat{X}_{\mathrm{C}}(k)=\frac{Y_{LDM}(k)\sqrt{1+\alpha}}{\hat{H}(k)}=X_{ \mathrm{C}}(k)\frac{H(k)}{\hat{H}(k)}+\frac{\sqrt{\alpha}X_{\mathrm{E1}}(k) \cdot H(k)+\sqrt{1+\alpha}N(k)}{\hat{H}(k)} \tag{5}\]  
where \(\hat{H}(k)\) indicates the estimated value of the channel frequency response of the \(k\)-th subcarrier. Since CL includes mobile service and fixed service, after de-multiplexing the CL demodulation symbol \(\hat{X}_{C}(k)\) in the frequency domain, the demodulated mobile service symbol \(\hat{X}_{M}(k)\) and the fixed service symbol \(\hat{X}_{E2}(k)\) can be obtained.  
The mobile service signal is demodulated by the BICM decoder to obtain mobile service information bits. The fixed service signal uses hard decision demodulation to obtain bit information and remaps to obtain the symbol \(\hat{X}_{E2}(k)\). The decoded mobile service information bits are subjected to BICM encoding to reconstruct the mobile service symbol \(\widetilde{X}_{M}(k)\), which is multiplexed with \(\widehat{X}_{E2}(k)\) and reconstructed to obtain the replica symbol \(\widetilde{X}_{\mathrm{C}}(k)\) of CL. The symbol \(\widetilde{X}_{\mathrm{C}}(k)\) regenerated by LDM-CL can be expressed as  
\[\widetilde{X}_{\mathrm{C}}(k)=\left\{\begin{array}{l}\widetilde{X}_{M}(k), k\leq L-1\\ \widetilde{X}_{E2}(k-L),k>L-1\end{array}\right. \tag{6}\]  
where \(L\) represents the number of subcarriers occupied by the mobile service symbol \(\hat{X}_{M}(k)\) in the LDM-CL.  
Before generating the copy symbols of the core layer, the LDM symbols are buffered in \"Buffering\". The fixed service symbol can be obtained by subtracting the generatedduplicate symbol from the buffered LDM symbol. Then, the demodulation of the LDM-EL fixed service symbols can be expressed as follows:  
\[\hat{X}_{E1}(k) =\frac{\gamma_{LDM}(k)\sqrt{1+a}}{H(k)\sqrt{a}}-\frac{1}{\sqrt{a}} \widetilde{X}_{C}(k) \tag{7}\] \[=X_{E1}(k)\cdot\frac{H(k)}{H(k)}+\frac{\Delta_{C}(k)+\frac{\sqrt{1 +a}}{H(k)}N(k)}{\sqrt{a}}\]  
\[\Delta_{C}(k)=X_{C}(k)\frac{H(k)}{H(k)}-\widetilde{X}_{C}(k) \tag{8}\]  
where \(\Delta_{C}(k)\) represents the CL signal left after interference cancellation.  
The fixed service symbols \(\hat{X}_{E1}(k)\) and \(\hat{X}_{E2}(k)\) are respectively de-mapped and de-interleaved, and then, the fixed service bit information before puncturing is recovered and decoded to obtain the fixed service bit data according to the transmitter bit puncturing rule.