Multiple-input multiple-output (MIMO) and sparse code multiple access (SCMA) are two effective techniques supporting high spectral efficiency and massive connectivity, respectively. Although the combination of these two techniques, namely MIMO-SCMA, can further improve the spectral efficiency, it deteriorates the error performance and greatly increases the computational costs. In this letter, in order to enhance the error performance and reduce the receiver complexity of the uplink MIMO-SCMA system, we propose a new scheme named spatial modulation sparse code multiple access (SM-SCMA). At the receiver, a low complexity joint message passing algorithm based on the principle of the maximum a posterior probability and the simplified factor graph is proposed. Simulation results show that the SM-SCMA can achieve a better bit-error-rate performance while significantly reducing the receiver complexity compared with the conventional MIMO-SCMA.

SCMA and polar coding are possible candidates for 5G systems. In this paper, we firstly propose the joint iterative detection and decoding (JIDD) receiver for the uplink polar coded sparse code multiple access (PC-SCMA) system. Then, the EXIT chart is used to investigate the performance of the JIDD receiver. Additionally, we optimize the system design and polar code construction based on the EXIT chart analysis. The proposed receiver integrates the factor graph of SCMA detector and polar soft-output decoder into a joint factor graph, which enables the exchange of messages between SCMA detector and polar decoder iteratively. Simulation results demonstrate that the JIDD receiver has better BER performance and lower complexity than the separate scheme. Specifically, when polar code length $N=256$ and code rate $R = 1/2$ , JIDD outperforms the separate scheme 4.8 and 6 dB over AWGN channel and Rayleigh fading channel, respectively. It also shows that, under 150% system loading, the JIDD receiver only has 0.3 dB performance loss compared to the single user uplink PC-SCMA over AWGN channel and 0.6 dB performance loss over Rayleigh fading channel.

Researches on 5th generation mobile communication networks (5G) are emerging to meet the demands of rapidly developing communications applications. The reliability and security of data have become key factors of its development, and thus 5G has put forward higher requirements for new coding and encryption technologies. In this paper, a physical layer encryption algorithm based on polarization codes and chaotic sequences is proposed to improve the reliability and confidentiality of transmission. In our scheme, chaotic sequences are allocated in the frozen bits of polar codes. Therefore, error correction and encryption can be performed simultaneously. Since the frozen bits are unknown to the eavesdroppers, it is difficult for them to decode the transmitted bits without the knowledge of the chaotic sequences. To further improve the security, we propose to generate the chaotic sequences by using the channel state information. Besides, the chain effects of delayed feedback are applied to increase the decrypted complexity of the eavesdroppers in this paper. Theoretic analysis and simulation results both show that the proposed algorithms can achieve high security without any error rate performance loss.

Multiple-input multiple-output (MIMO) and sparse code multiple access (SCMA) are two key techniques supporting high spectral efficiency and massive connectivity. In this paper, we consider the downlink MIMO communication scenario where SCMA is used for multiuser access. We aim to improve the performance of the traditional single input single output SCMA (SISO-SCMA) by exploiting the transmit diversity of multiple antennas at the transmitter. Specifically, we propose the multi-dimensional space time block code (MSTBC) aided downlink MIMO-SCMA and derive the design criteria of MSTBC that can achieve full diversity while keep the symbol rate same as the traditional SISO-SCMA system. In particular, the MSTBC-2 for two transmit antennas and the MSTBC-4 for four transmit antennas are designed by extending the Alamouti code and quasi-orthogonal STBC into multi-dimension. Due to the orthogonality/quasi-orthogonality of the two MSTBCs, a linear decoder is available at the receiver, which keeps the complexity of the proposed MSTBC aided MIMO-SCMA almost the same as the conventional SISO-SCMA. Furthermore, the performance of the proposed schemes is evaluated based on both Monte-Carlo simulations and analytical results. Simulation results show that the proposed MSTBC aided MIMO-SCMA can significantly outperform the conventional SISO-SCMA, STBC-MIMO-OFDM, and multiplexing MIMO-SCMA schemes.

Channel coding and security are important in a communication system. The 5th generation (5G) mobile communication networks call for higher requirements of new coding technologies and encryption technologies. As an efficient coding method, polar codes have attracted more attention in recent years. Besides, the peak-to-average-power ratio (PAPR) is a major problem of the orthogonal frequency-division multiplexing (OFDM) system, which will significantly affect the performance of the OFDM system. In this paper, joint physical-layer encryption and PAPR reduction scheme is proposed, aiming to solve the PAPR problem and achieve high security of the transmission system. In this scheme, we utilize the key generation technology of wireless channels to get the initial value of chaotic sequences. Then, the chaotic sequences can be used to encrypt the information and reduce PAPR simultaneously. Moreover, we use the best information bits of polar codes to store the serial number index of chaotic sequences. The theoretic analysis and simulation results show that the proposed scheme can not only achieve high security in the physical layer but also reduce PAPR in the OFDM system without increasing system complexity and latency.