nal multiple-access especially when the system is heavily overloaded with a large number of multiplexed layers. Low-density spreading (LDS) is a special form of SCMA. In LDS, codewords are built by spreading of modulated symbols using LDS signatures with few non-zero elements within a large signature length. Despite the moderate complexity of detection, LDS suffers from poor performance espe- cially for large constellation sizes beyond QPSK. All CDMA schemes, and in particular LDS, can be considered as different types of repetition coding in which different variations of a QAM symbol are generated by a spreading signature. Repetition coding is not able to provide desirable spectral efficiency for a wide range of SNR. In SCMA the QAM mapper and linear operation of sparse spreading are merged to directly map incoming bits to a com- plex sparse vector called a codeword. Both LDS and SCMA are based on the idea that one- user information is spread over multiple subcarriers. However, the number of subcarriers assigned to each user is smaller than the total number of subcarriers, and this low spreading (sparse) feature ensures that the number of users utilizing the same subcarrier is not too large, such that the system complexity remains manageable [50-52]. In LDS, each user spreads its data on a small set of subcarriers. There is no exclusivity in the subcarrier allocation and more than one user can share each subcarrier. The interference pattern at the receiver will generate a low-density graph, and graph theory-based techniques can be utilized. The main features of the LDS scheme can be summarized as follows. At each subcarrier, a user will have relativity small number of interferers comparing to the total number of users. Consequently, the search space will be smaller and more complex multi- user detection techniques can be implemented. Higher SINR can be achieved at each sub- carrier, which results in reliable detection process. Each user will experience interference from different users at different subcarriers, w