Sphere Decoding for Spatial Modulation Systems with Arbitrary Nt
  Recently, three Sphere Decoding (SD) algorithms were proposed for Spatial Modulation (SM) scheme which focus on reducing the transmit-, receive-, and both transmit and receive-search spaces at the receiver and were termed as Receiver-centric SD (Rx-SD), Transmitter-centric SD (Tx-SD), and Combined SD (C-SD) detectors, respectively. The Tx-SD detector was proposed for systems with Nt \leq Nr, where Nt and Nr are the number of transmit and receive antennas of the system. In this paper, we show that the existing Tx-SD detector is not limited to systems with Nt \leq Nr but can be used with systems Nr < Nt \leq 2Nr - 1 as well. We refer to this detector as the Extended Tx-SD (E-Tx-SD) detector. Further, we propose an E- Tx-SD based detection scheme for SM systems with arbitrary Nt by exploiting the Inter-Channel Interference (ICI) free property of the SM systems. We show with our simulation results that the proposed detectors are ML-optimal and offer significantly reduced complexity. 
Rakshith Rajashekar and K.V.S. Hari
cs.IT math.IT