Fluid Antenna Grouping-Based Index Modulation: Transceiver Design for MIMO Systems

The fluid antenna (FA)-enabled multiple-input multiple-output (MIMO) system based on index modulation (IM), referred to as FA-IM, significantly enhances spectral efficiency (SE) compared to the conventional FA-assisted MIMO system. This paper proposes an innovative FA grouping-based IM (FAG-IM) system to improve performance in mitigating the high spatial correlation between multiple activated ports. A block grouping scheme is employed based on the spatial correlation model and the distribution structure of the ports. Then, a closed-form expression for the average bit error probability (ABEP) upper bound of the FAG-IM system is derived. In order to reduce the receiver complexity of the proposed system, the message passing mechanism is first incorporated into the FAG-IM system. Subsequently, within the approximate message passing (AMP) framework, an efficient structured AMP (S-AMP) detector is devised by leveraging the structural characteristics of the transmission signal vector. Simulation results confirm that the proposed FAG-IM system significantly outperforms the existing FA-IM system in the presence of spatial correlation. The derived ABEP curve aligns well with the numerical results, providing an efficient theoretical tool for evaluating the system performance. Additionally, simulation results demonstrate that the proposed low-complexity S-AMP detector not only reduces the time complexity to a linear scale but also substantially improves bit error rate (BER) performance compared to the minimum mean square error (MMSE) detector, thus facilitating the practical implementation of the FAG-IM system.

Fluid Antenna Grouping Index Modulation Design for MIMO Systems

Index modulation (IM) significantly enhances the spectral efficiency of fluid antennas (FAs) enabled multiple-input multiple-output (MIMO) systems, which is named FA-IM. However, due to the dense distribution of ports on fluid antennas, the wireless channel exhibits a high spatial correlation, resulting in severe performance degradation in the existing FA-IM scheme. This paper proposes a novel fluid antenna grouping index modulation (FA-GIM) scheme to mitigate the spatial correlation of the FA-IM channel, further enhancing system performance. Based on the spatial correlation model of two-dimensional (2D) fluid antenna surfaces, this paper specifically adopts a block grouping method where adjacent ports are allocated to the same group. The numerical results demonstrate that the proposed scheme exhibits superior bit error rate (BER) performance compared to the state-of-the-art scheme, enhancing the robustness of FA-assisted MIMO systems.

PAPR Reduction with Pre-chirp Selection for Affine Frequency Division Multiple

Affine frequency division multiplexing (AFDM) is a promising new multicarrier technique based on discrete affine Fourier transform (DAFT). By properly tuning pre-chirp parameter and post-chirp parameter in the DAFT, the effective channel in the DAFT domain can completely avoid overlap of different paths, thus constitutes a full representation of delay-Doppler profile, which significantly improves the system performance in high mobility scenarios. However, AFDM has the crucial problem of high peak-to-average power ratio (PAPR) caused by phase randomness of modulated symbols. In this letter, an algorithm named grouped pre-chirp selection (GPS) is proposed to reduce the PAPR by changing the value of pre-chirp parameter on sub-carriers group by group. Specifically, it is demonstrated first that the important properties of AFDM system are maintained when implementing GPS. Secondly, we elaborate the operation steps of GPS algorithm, illustrating its effect on PAPR reduction and its advantage in terms of computational complexity compared with the ungrouped approach. Finally, simulation results of PAPR reduction in the form of complementary cumulative distribution function (CCDF) show the effectiveness of the proposed GPS algorithm.

Capacity-based Spatial Modulation Constellation and Pre-scaling Design

Spatial Modulation (SM) can utilize the index of the transmit antenna (TA) to transmit additional information. In this paper, to improve the performance of SM, a non-uniform constellation (NUC) and pre-scaling coefficients optimization design scheme is proposed. The bit-interleaved coded modulation (BICM) capacity calculation formula of SM system is firstly derived. The constellation and pre-scaling coefficients are optimized by maximizing the BICM capacity without channel state information (CSI) feedback. Optimization results are given for the multiple-input-single-output (MISO) system with Rayleigh channel. Simulation result shows the proposed scheme provides a meaningful performance gain compared to conventional SM system without CSI feedback. The proposed optimization design scheme can be a promising technology for future 6G to achieve high-efficiency.

Reconfigurable Intelligent Surface-Based Receive Generalized Spatial Modulation Design

In this paper, the receive generalized spatial modulation (RGSM) scheme with reconfigurable intelligent surfaces (RIS) assistance is proposed. The RIS group controllers change the reflected phases of the RIS elements to achieve the selection of receive antennas and phase shift keying (PSK) modulation, and the amplitudes of the received symbols are adjusted by changing the activation states of the elements to achieve amplitude phase shift keying (APSK) modulation. Compared with the existing RIS-aided receive generalized space shift keying (RIS-RGSSK) scheme, the proposed scheme realizes that the selected antennas respectively receive different modulation symbols, and only adds the process to control the modulated phases and the activation states of elements. The proposed scheme has better bit error rate (BER) performance than the RIS-RGSSK scheme at the same rate. In addition, the results show that for low modulation orders, the proposed scheme will perform better with PSK, while for high modulation order, APSK is better. The proposed scheme is a promising scheme for future wireless communication to achieve high-efficiency.