  In this paper, a novel variation of codeword position index based sparse code
multiple access (CPI-SCMA) system, which is termed as hybrid codeword position
index modulated sparse code multiple access (HCPI-SCMA), is proposed to further
improve the transmission efficiency (TE). In this scheme, unlike the
conventional CPI-SCMA that uses only one kind of bits-toindices (BTI) mapper,
the codeword positions which are padded with zeros in CPI-SCMA are also
utilized to transmit additional information. Since multiple index selectors are
used in a HCPISCMA codeword, the original message passing algorithm (MPA) no
longer works in HCPI-SCMA; hence, a modified MPA is proposed to detect the
received signals. It is shown in the simulations and analysis that the proposed
scheme can achieve both higher TE and better error rate performance in the
region of high signal-to-noise ratio (SNR) compare to the conventional SCMA
(C-SCMA). Moreover, compared with CPI-SCMA, HCPISCMA can achieve higher TE with
approximately the same error rate performance compared to CPI-SCMA at high
SNRs.
