In unsourced random access (URA) system, user pilot collisions are a key challenge when multiple users select identical pilot sequences, leading to poor activity detection and decoding performance. To address this issue, this paper develops a two-stage segmented pilot (TSSP) design for collision-resilient URA in massive multiple-input multiple-output (MIMO) system. The proposed scheme enables a substantial enlargement of the effective pilot space without increasing the physical codebook size and detection complexity. First, we construct a probabilistic collision model by leveraging the birthday-problem analogy, which provides theoretical characterization of collision events and closely matches the simulation results. Second, we propose a segmented pilot mapping and decoding framework in which the pilot bits are divided into two interconnected parts linked by common pilot bits. This structured segmentation allows the system to resolve partial collisions and reduce missed detection in high-density access scenarios. Third, we develop a joint-covariance linear minimum mean square error (JC-LMMSE) scheme that exploits the segmented pilot structure to suppress pilot contamination and improve estimation accuracy. Monte Carlo simulations show that the proposed TSSP framework achieves up to 3.5 dB gain in collision scenarios and approximately 1 dB gain in collision-free cases over existing methods, while maintaining comparable computational complexity.

This paper presents a novel multi-carrier differential chaos shift keying modulation with permutation index (PI-MC-DCSK) over wireless multipath fading channels, since the conventional multi-carrier DCSK (MC-DCSK) suffers from low spectral efficiency and loss of system performance. In this system, the predefined subcarriers can carry multiple data bits according to permutation indexes, while only a single subcarrier is used as the chaotic reference and shared by data subcarriers. Thus, the transmission energy is saved and the data rate can be enhanced. Moreover, the low cross-correlation property of permuted chaotic sequences is exploited in PI-MC-DCSK to effectively suppress inter-subcarrier interference. The theoretical analysis and consistent simulation results show that the proposed system can obtain significant performance gains of more than 2 dB than the conventional MC-DCSK over Rayleigh multipath channels. This performance gain can be up to 4 dB in typical ultra-wideband (UWB) channels, which proves its feasibility in practical applications. Therefore, this new design can be viewed as a low-complexity solution for chaos-based wireless communications, such as short-range Internet of Things (IoT) applications of transmitted-reference communications.

In this paper, we design a novel joint receiver that iteratively performs channel estimation (CE) and symbol detection for channel-coded multi-user multiple-input multiple-output systems with orthogonal time frequency space modulation (MIMO-OTFS). In particular, we first propose a two-stage CE method based on a single superimposed pilot (SP) with inter-user interference canceling (ICCE). The delay-Doppler domain SP is used to estimate path delays, Doppler shifts, and other channel parameters. Then the detected user symbols are utilized to update the channel parameters with mitigating inter-user and multipath interferences. Second, we construct a joint CE and signal detection scheme based on the expectation propagation (EP) algorithm (EP-ICCE). We exploit the relationship between CE and symbol detection, refining the estimated channel matrix with the detected data symbols. Finally, simulation results shows the proposed EP-ICCE can achieve performance gains up to 2 dB over the conventional methods for the different MIMO and modulations.

In this article, we investigate the performance of physical-layer network coding (PNC) with hierarchical modulation (HM) over a two-way relay channel (TWRC). While Gray mapping is the optimal for the bit-interleaved coded modulation (BICM) systems in point-to-point communications, the superimposed constellation at the relay in PNC transmission does not maintain the same characteristics as the original Gray mapping at the users. Thus, to address this issue and obtain the optimal mapping for PNC, we propose a Pseudo-Gray (Pe-Gray) constellation for the user-end hierarchical quadrature amplitude modulation (H-QAM). In particular, we develop the design criterion for the user constellation to avoid demodulation ambiguity and more importantly, to ensure that the superimposed constellation can be Gray-mapped. The achievable rate analysis and simulation results show that the proposed Pe-Gray can achieve significant performance gains of up to 2 dB over the conventional Gray, M3, and MSED constellations in power-imbalanced HM-BICM-PNC. The performance superiority of the Pe-Gray is further demonstrated over the power-imbalanced channels, which also suggests its enhanced robustness in wireless fading channels.

In this paper, we propose a polarization-aided polar coding scheme over non-orthogonal multiple access channels operated with physical-layer network coding (PN-NOMA), where M users adopt polar coding for transmission. By exploiting the characteristic of PNC and polar encoding, we found that the polar code at the users can be seen as nested codes and sub-codeword of a larger polar code. It enables us to construct a single (virtual) polar code at the receiver, which has the length M times longer than that of the original user codes. In this way, we can decode all the user messages within this single polar code. The channel polarization effect as well as the decoding performance can be improved. We further propose a Monte Carlo-based code construction for the virtual polar code with user channel coefficients, and short nested user codes can be individually derived from this code. Simulation results show that the proposed PN-NOMA can achieve significant performance gains of more than 0.5 dB as compared to conventional polar-coded NOMA (PC-NOMA) over power-imbalanced channels, and this gain is more than 2 dB over Rayleigh fading channels.

Both massive multiple-input and multiple-output (MIMO) and physical-layer network coding (PNC) techniques can effectively improve the network throughput. However, for PNC-aided massive MIMO systems, the high-order modulations superimposed at the relay particularly bring computational burden and performance degradation to the signal detection. Thus, this letter proposes an iterative MIMO-PNC detector from double expectation propagation (DEP) as a high-accuracy solution in this large-scale system. By exploiting the PNC characteristic, the summation and difference (SD) information of the user pair messages are first linearly formed and iteratively updated between the DEP detector and the channel decoder. Simulation results show that the proposed iterative method can achieve significant performance gains of 2 dB in various massive MIMO systems, as compared to the non-iterative one and traditional linear minimum mean square error (LMMSE) detector.

This letter proposes a polarization mapping (PM) for polar coded modulation with physical network coding (PM-PNC) over two-way relay channels (TWRC). The achievable information rates (AIRs) of the mapped bits within a XOR symbol from bit-wise XOR of two user symbols are significantly different, since the users adopt non-uniform PAM modulation to avoid mapping ambiguity. In the polar-coded PM-PNC, the original channels associated with the split information bit channel are mapped to the XOR bit stream with a larger AIR. In this way, the achieved rate of the split information channels can be maximized and is illustrated in distribution of split bit channel AIRs. Simulation results show that the proposed PM-PNC can achieve significant performance gains of more than 0.5 dB as compared to the bit-interleaved coded modulation (BICM) and multilevel coded (MLC) PNC systems over Gaussian and block fading channels.