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Enhanced Generalized OFDM with Index Modulation

In recent years, many attempts have been made to enhance Orthogonal Frequency Multiplexing with Index Modulation (OFDM-IM) in terms of spectral efficiency and error performance. Two challenges typically erupt when using OFDM-IM. First, the degradation in spectral efficiency due to the subcarrier's deactivation, especially when using higher order modulation (M-ary) where every inactive subcarrier will cost $Log_2(M)$ bits loss. Second, using a fixed number of active subcarriers within a sub-block forces the error to be localized within the sub-block. Yet, it loses the advantage of exploiting all possible pattern combinations degrading the overall spectral efficiency. In this paper, we introduce a solution to tackle those problems. The Enhanced Generalized Index Modulation (EGIM) is a simple systematic way to generate and detect the OFDM-IM frame. Unlike the classical OFDM-IM generation by splitting the frame into sub-frames which increases the complexity of the OFDM-IM transmitter and reflects on the receiver Maximum likelihood detector, EGIM Makes full use of all possible combinations of active subcarriers within the frame by using variable active subcarriers (k) depending on the incoming data. The EGIM is still susceptible to error propagation if the OFF symbol is wrongly mapped to one of the ON symbols or vice versa. For that reason, we offer an OFDM-IM autoencoder to overcome this problem. The encoder generates the (ON/OFF) symbols systematically to achieve the advantage of sending all possible frame indices patterns depending on the input bit stream offering an average of 3dB gain in terms of power efficiency. The proposed encoder performance was compared to the standard encoder with the same effective coding rate using soft and hard decision Viterbi decoding utilizing the power gain achieved.

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