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In recent years, the standardization for the fifth generation (5G) mobile communication systems has been actively discussed, and it is expected that a large number of wireless communication terminals, and beyond, in the current systems are accommodated in 5G systems. One of the steps in establishing this system is an advanced multiple access technology. In this paper, we propose a chaos non-orthogonal multiple access (C-NOMA) scheme for downlink transmission that offers high capacity allocation and secure wireless multiple access with physical layer security. In 5G systems where many terminals concurrently communicate, it is also important to ensure communication integrity for each user while maintaining large capacity communication. As a secure wireless channel-coded communication scheme, we have proposed a chaos multiple-input multiple-output (C-MIMO) scheme using the principle of chaos communication. By applying C-MIMO into a power-domain non-orthogonal multiple access transmission, we were able to demonstrate the operation and suitability of the C-NOMA configuration for handling both large capacity and physical layer security against eavesdroppers. We also demonstrated its improved performances through numerical simulations, and provided comparisons with those of conventional NOMA and chaos orthogonal frequency division multiple access schemes. In addition, we evaluated the security capability of the proposed technique based on the channel capacity of eavesdroppers and showed that secure transmission can be achieved using floating-point calculations.

In the fifth-generation mobile communications system (5G), wireless connections among numerous Internet of things (IoT) devices will be accommodated in the massive machine-type communications (mMTC) scenario. To realize such massive connectivity, non-orthogonal multiple access (NOMA) technologies have been proposed for 5G. As one of the NOMA schemes, we have proposed a grant-free sparse chaos code multiple access (GF-SCCMA) scheme exploiting the principle of chaos communications for mMTC. It realizes both physical layer security and large-capacity transmission. However, in conventional studies, a message-passing algorithm (MPA) was used in the receiver, which required relatively high calculation complexity, and the maximum number of users in GF-SCCMA was limited to approximately 10. Therefore, we propose an application of the Gaussian belief propagation (GaBP) algorithm for user signal detection to realize a large-scale transmission of more than 50 users with low calculation complexity. The improved transmission performances are shown through numerical simulations, by comparison with a conventional grant-free low-density signature scheme.

Massive machine-type communications (mMTC) scenarios that can accommodate Internet of things (IoT) device communications are currently being considered for implementation in fifth generation mobile communication systems. To support the dynamic traffic present in mMTC systems, grant-free non-orthogonal multiple access schemes have been proposed. In these schemes, both system capacity enhancement and transmit protocol simplification are achieved, and an overloaded transmission of more than one hundred percent of the capacity of the number of transmit samples is conducted. However, demand still exists for more capacity to accommodate massive devices. On the other hand, a simple method for ensuring the security of mMTC systems is required to suppress the energy consumption of IoT devices and reduce the amount of signal processing required at the central receive stations. In this paper, to satisfy these requirements, we propose a novel grant-free sparse chaos code multiple access (GF-SCCMA) scheme for mMTC systems in which sparse code multiple access is supplied based on chaos signals and physical layer security is ensured. GF-SCCMA is a non-orthogonal multiple access scheme in which only pairs sharing common keys can decode data by utilizing a chaotic codebook of transmission sequences. Furthermore, the distribution of the output log likelihood ratio is improved by using chaos-based quasi-Gaussian modulation, and enhancement of the capacity is achieved for conventional schemes when the outer channel code is concatenated. The improved performances in terms of capacity and security are shown through numerical simulations.

The multi-user multiple-input multiple-output (MU-MIMO) scheme is one of the main methods for improving the frequency-efficiency in wireless communication systems. In MU-MIMO systems, a base station equipped with multiple antennas and a large number of receivers with multiple antennas communicate simultaneously. Conventionally, we adopted the principle of chaos communications to MU-MIMO and proposed a multi-user chaos MIMO (MU-C-MIMO) scheme that has physical layer security between each receiver. In the previous study, we utilized block diagonalization (BD) — a liner precoding method — to remove interference components in MU-C-MIMO. On the one hand, channel gain for each user is not maximized in BD. On the other hand, the gain maximization can be realized using nonlinear precoding, and further improvement of transmission quality is expected in MU-C-MIMO. Therefore, in this paper, we propose a MU-C-MIMO scheme using dirty paper coding (DPC), a nonlinear precoding method, in order to realize the most ideal precoding characteristics. The effectiveness of this scheme is evaluated through numerical simulations with comparisons to a conventional MU-C-MIMO system with BD.

A chaos multiple-input multiple-output (C-MIMO) transmission scheme is developed, which has both physical layer security and channel coding gain with a coding rate of one, by adopting a principle of chaos communications to a MIMO block transmission system. In our previous study, further improvement of transmission quality is achieved by introducing a log-likelihood ratio (LLR) with turbo-coded transmission. Moreover, the increased coding gain is obtained by convolutional code concatenation with symbol maximum a posteriori (MAP) detection that calculates the true-valued LLR, or with a sequential LLR that is an approximate LLR. However, symbol MAP detection has not been considered in LDPC code concatenation. Therefore, in this paper, we propose a C-MIMO transmission system with LDPC code concatenation using symbol MAP detection, and the effectiveness of this scheme is evaluated through numerical simulation. Furthermore, we propose an application of space-time block coding (STBC) to the C-MIMO scheme, and show the improvement of bit error rate in lower SNR regions when the LDPC coding rate is high.