  This paper proposes a novel spread spectrum and sub-carrier index modulation
(SS-SIM) scheme, which is integrated to orthogonal frequency division
multiplexing (OFDM) framework to enhance the diversity over the conventional IM
schemes. Particularly, the resulting scheme, called SS-SIMOFDM, jointly employs
both spread spectrum and sub-carrier index modulations to form a precoding
vector which is then used to spread an M-ary complex symbol across all active
sub-carriers. As a result, the proposed scheme enables a novel transmission of
three signal domains: SS and sub-carrier indices, and a single M-ary symbol.
For practical implementations, two reduced-complexity near-optimal detectors
are proposed, which have complexities less depending on the M-ary modulation
size. Then, the bit error probability and its upper bound are analyzed to gain
an insight into the diversity gain, which is shown to be strongly affected by
the order of sub-carrier indices. Based on this observation, we propose two
novel sub-carrier index mapping methods, which significantly increase the
diversity gain of SSSIM-OFDM. Finally, simulation results show that our scheme
achieves better error performance than the benchmarks at the cost of lower
spectral efficiency compared to classical OFDM and OFDM-IM, which can carry
multiple M-ary symbols.
