SS-OFDMA: Spatial-Spread Orthogonal Frequency Division Multiple Access for Terahertz Networks

How to achieve efficient multiple access for Terahertz (THz) networks is still an open problem. The key obstacle is the extremely narrow angular coverage by a highly directional THz beam. Thus, a hybrid THzPrism beamforming (HTB) architecture is designed in this paper to greatly enlarge angular coverage by spreading frequency subcarriers to different directions. Based on the HTB architecture, a spatial-spread orthogonal frequency division multiple access (SS-OFDMA) scheme is developed. To serve users dispersed in a large angular range, a user grouping mechanism is first designed for SS-OFDMA to utilize SDMA and suppress the inter-group interference. To improve the spectrum and energy efficiency for sporadic users within a group, a non-uniform beam spreading mechanism is then developed for SS-OFDMA by joint design of digital and analog beamforming. Finally, a resource allocation algorithm is designed to minimize the transmit power of the base station by optimizing the subarray allocation among groups as well as the subcarrier and power allocation for each user within a group. Compared with the existing schemes, SS-OFDMA increases the achievable data rate by up to 124%, reduces the power consumption by up to 71%, and increases the average number of concurrently served users by up to 147%.