# LDM-Ex-FDM: A Novel Multi-Service Transmission Scheme for the ATSC 3.0 System
## 1 Introduction
Reference [17] proposes a low-rate low-density parity-check (LDPC) code applied to _CL_ of _LDM_, aiming to increase the power of _EL_ by reducing the injection level of _LDM_, and finally, the improved reception performance of _EL_ is obtained. However, this approach is only suitable for very robust services delivered using _CL_. Although reference [17; 18] mentioned the combination between _LDM_ and FDM, unfortunately, the detailed analysis is absent. In this paper, we will investigate the transceiver structure and evaluate the performance of the combination between _LDM_ and FDM in a comprehensive manner. The first objective of this work is to develop an efficient transceiver structure with the aid of symbol puncturing techniques, thus improving the performance of the _EL_ services without affecting the reception threshold of the _CL_ services. The key idea of the proposed scheme is that the symbols punctured from the _EL_ of fixed services will be independently transmitted on a certain number of subcarriers orthogonal to the _LDM_ signal of the _CL_ and most of the _EL_. Then, the punctured symbols will be demodulated with a higher SNR on the receiver side and can be recovered well, thus improving the reception performance of the _EL_ fixed services. For the demodulation of _LDM_ signals, the existing detection methods usually use a successive interference cancellation (SIC) detector to perform multi-level signal detection at the receiver. However, for a receiving terminal that only demodulates the _EL_ signal of the _LDM_, if it is forced to demodulate the _CL_ signal of the _LDM_ first, and then performs the bit-interleaved coded modulation (BICM) signal reconstruction on it, additional complexity and signal processing delay will inevitably increase. In view of the increase in complexity at the transmitter side as a result of the introduction of symbol puncturing, the second objective of this work is to further decrease the computational complexity at the receiver side. Fortunately, note that _LDM_ can be regarded as the extension of low-order modulation to high-order modulation during _EL_ subcarrier modulation. Compared with the single-layer signal structure, the constellation after the _EL_ symbol mapping has a constellation offset. Therefore, correcting the constellation offset of the _EL_ symbol is equivalent to eliminating the interference of the _CL_ to the _EL_, so that the _EL_ signal can be demodulated. Motivated by that, a direct interference cancellation (DIC) detector is also developed in this paper, where the fixed receiver can decode the _EL_ symbol just by correcting the constellation offset of the _EL_ symbol on the received _LDM_ signal. In summary, the novel contributions of this paper are as follows:  
* We propose a novel combined FDM/_LDM_ multi-service transmission scheme, namely LDM-Ex-FDM, which is used to improve the reception performance of _EL_ services without degrading the SNR threshold of _CL_ services, thus enhancing the system coverage performance. * Two BICM symbol puncturing strategies of the LDM-Ex-FDM scheme are proposed, i.e., equal interval symbol puncturing (EISP) and non-equal interval symbol puncturing (NEISP). The LDM-Ex-FDM scheme with EISP can be used for multi-service transmission under typical scenarios while the LDM-Ex-FDM scheme with NEISP performs better in multi-service transmission scenarios with low power injection levels, which will bring about the serious overlap of the constellation symbols of the _LDM_ composite signal. * At the receiving end, a DIC detector is also proposed to reduce the computational complexity and the signal processing delay. The effectiveness of the proposed detector is verified by simulation. The remainder of this paper is briefly described as follows. Section 2 gives an overview of the ATSC 3.0 system. Section 3 introduces the transceiver structure of the LDM-Ex-FDM multi-service transmission scheme for the ATSC 3.0 system. The BICM capacity of the proposed scheme will also be analyzed in Section 3.