  This letter studies the application of backscatter communications (BackCom)
assisted non-orthogonal multiple access (BAC-NOMA) to the envisioned
sixth-generation (6G) ultra-massive machine type communications (umMTC). In
particular, the proposed BAC-NOMA transmission scheme can realize simultaneous
energy and spectrum cooperation between uplink and downlink users, which is
important to support massive connectivity and stringent energy constraints in
umMTC. Furthermore, a resource allocation problem for maximizing the uplink
throughput and suppressing the interference between downlink and uplink
transmission is formulated as an optimization problem and the corresponding
optimal resource allocation policy is obtained. Computer simulations are
provided to demonstrate the superior performance of BAC-NOMA.
