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A quadrature spatial modulation with the fourth order transmit diversity (FO-QSM) scheme is presented in this paper. In this scheme, two dispersion-matrix (DM) sets are assigned at the transmitter, and $P$ out of $Q$ DMs are activated in each set to extend the real and imaginary parts of $P$ QSM signals respectively, to large number of transmit antennas. The two DM sets are constructed by performing Kronecker product between two simple extension matrices and the DMs of Sezginer-Sari-Biglieri (SSB) code. The framework designs of FO-QSM and the two DM sets make FO-QSM possess the properties: 1) it guarantees the fourth order transmit diversity without requiring any parameter optimization; 2) it has higher spectral efficiency than the newly proposed generalized space-time block coded spatial modulation (GSTBC-SM) scheme. Furthermore, a repeated computation reduced sphere decoding (RCR-SD) is proposed. RCR-SD is very suitable for the schemes constructed by a large number of DMs, including FO-QSM. By sorting the index vectors, the RCR-SD detector can efficiently reduce the repeated computations in the SD searching, which significantly reduces the decoding complexity without sacrificing optimal decoding performance. Simulation results show that FO-QSM has obvious better bit error rate (BER) performance than GSTBC-SM. [ABSTRACT FROM AUTHOR]

In this letter, a new diagonal and Alamouti space-frequency coded quadrature index modulation (DASFC-QIM) scheme for MIMO-OFDM systems is proposed to achieve higher transmit diversity. In the proposed DASFC-QIM scheme, firstly two modulated symbols are associated through a Hadamard matrix and a rotation matrix to construct a diagonal code. Next, by independently selecting the indices of the space-frequency units in each resource block (RB) twice, the in-phase and the quadrature components of the diagonal code are put into each RB. Finally, the signal in each RB is divided into two parts which are used to construct an Alamouti space-frequency code fitting for four transmit antennas. It can be proved that the DASFC-QIM scheme achieves the fourth order transmit diversity so that it achieves better bit error rate (BER) performance than some existing typical IM schemes. Simulation results show that DASFC-QIM exhibits superior BER performance over both the independent and correlated Rayleigh fading channels. [ABSTRACT FROM AUTHOR]