Multilevel Spatial Hierarchical Modulation: An Efficient Scheme for Unequal Error Protection Under Rician Fading

Under strong line-of-sight (LOS) conditions, in a conventional spatial modulation scheme, the signal domain exhibits better error performance than the spatial domain, thus rendering the accomplishment of unequal error protection (UEP) a difficult task. To this end, multilevel spatial hierarchical modulation (MSHM) is proposed to provide UEP in Rician fading channels when both the high priority (HP) and the low-priority information streams are jointly encoded in the spatial and signal domains. A phase-aided multilevel space-shift keying scheme is employed in the spatial domain to enhance HP performance, whereas in the signal domain, hierarchical modulation is employed. The joint space-signal constellation related to the LOS component of the received signal is appropriately formed to provide higher protection to the HP stream while, at the same time, being capable of providing different levels of protection by adjusting the transmission parameters of the spatial and signal schemes when they are jointly employed. By incorporating the Rician factor to the parameters of the proposed MSHM scheme, the latter is made adaptive to the fading conditions. Simulations have been performed to assess the performance and demonstrate the design options offered by the proposed MSHM scheme.