Abstract
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-SIM-OFDM, 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 SS-SIM-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.
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Notes
Note that since we aim to spread the M-ary symbol over K active sub-carriers, the length of the spreading codes is equal to K.
In our future work, we will attempt to increase the SE of the proposed scheme through conveying multiple M-ary symbols similar to OFDM-IM, while still ensuring the benefit of three signal domains.
Note that our analysis here can be applied to any channel model, and hence is not limited to Rayleigh fading. In particular, for a new channel model such as Rician, we just need to apply the MGF of that channel to (14) for achieving the corresponding unconditional PEP. We leave such performance evaluation of our scheme over generalized fading channels as our future work.
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Ngo, VD., Van Luong, T., Luong, N.C. et al. Enhancing diversity of OFDM with joint spread spectrum and subcarrier index modulations. Wireless Netw 28, 3739–3751 (2022). https://doi.org/10.1007/s11276-022-03092-9
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DOI: https://doi.org/10.1007/s11276-022-03092-9