Spatial Multiplexing Research Articles

Fair Scheduling of Wireless Power Transfer to Nonlinear Energy Harvesters

The performances of two wireless power transfer (WPT) scheduling schemes, time sharing (TS) and spatial multiplexing (SM), in terms of provisioning fairness among energy receivers (ERs), are studied and compared while taking into account the nonlinearity of the harvesting circuits. In the network, the multiple-antenna energy transmitter attempts to maximize the harvested energy by the ER which has accumulated the minimum amount of energy among all single-antenna ERs during the WPT block—hence the max-min fairness criterion. Two network scenarios are studied: homogeneous, where similar channel coefficients are assumed for all the well-apart ERs, and heterogeneous, where the said coefficients can take arbitrary values. For WPT in single-band or multi-band, we analytically prove that the optimal scheduling policy in the homogeneous scenario is to allocate each ER the full transmit power with uniform distribution of charging times among ERs, rather than to allocate the full power transfer block with uniform distribution of the power among ERs. Generalization of the network to the heterogeneous scenario aims to find the optimal beamforming vectors for the SM scheme and the optimal time sharing vector for the TS scheme. We form the max-min fairness optimization problems for both scheduling techniques, in single-and multi-band WPT scenarios. The problems, which are nonlinear and non-convex, are solved through exhaustive search algorithms. It is proven that the TS scheduling outperforms the SM one in terms of max-min fairness as a result of taking into account the inherent non-linearity of the harvesting devices.

Linearized Model for MIMO-MFSK Systems With Energy Detection

For user terminals, energy-detection based multiple-input-multiple-output (MIMO) has the advantages of low complexity and anti-Doppler capability. However, the energy detector makes it difficult to linearly separate the signals in spatial multiplexing. In this letter, the linearized model is introduced in <inline-formula> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> M-ary frequency-shifted keying (MFSK) V-BLAST system with energy detection. To avoid rank deficiency of the model, we propose nonlinear virtual antennas (NVA) to increases both the dimension and receive diversity gain of MIMO system. Then the full-rank condition is discussed. In multiple-antenna systems, NVA is further generalized by spatial index modulation with dual antennas selection. By simulations, linear detectors are verified to be feasible for non-coherent spatial-multiplexing MIMO, and its diversity gain is obviously increased by the proposed NVA.

Design and Analysis of MIMO Systems Using Energy Detectors for Sub-THz Applications

The significant amount of unused spectrum in sub-TeraHertz frequencies is contemplated to realize high rate wireless communications for beyond 5G networks. Yet, the performance of radio-frequency sub-TeraHertz systems is severely degraded by strong oscillator phase noise. Therefore, we investigate in this paper the use of multiple-input multiple-output (MIMO) systems with energy detection receivers to achieve high rate communications robust to phase noise. First, the design of the receiver detection algorithm is addressed. Two detectors are proposed for the studied nonlinear MIMO channel, either derived from the maximum likelihood decision rule by using a Gaussian approximation, or based on the use of neural networks. Second, the communication performance is assessed through numerical simulations for uncoded and coded systems. We consider a realistic scenario modeling an indoor wireless link in D-band with directive antennas and strongly correlated line-of-sight channels. Our results demonstrate that spatial multiplexing with non-coherent sub-TeraHertz transceivers can be realized on strongly correlated line-of-sight channels using the proposed detection schemes. Thereby, we highlight that high-rate radio-frequency sub-TeraHertz systems can be implemented with low-complexity and low-power architectures using MIMO systems with energy detection receivers.

Spatial Multiplexing Superimposed Pilots for Multicell Multiuser MIMO Uplink Systems

For a multiple-input multiple-output (MIMO) system, or a multiuser MIMO system, it is important to acquire the accurate channel state information. In this article, a superimposed-pilot-based channel estimation scheme is proposed for multicell multiuser MIMO uplink systems. In this scheme, the pilot sequences and data sequences are superimposed, but the space generated by the pilot sequences and the space spanned by data sequences are separated far enough in order to reduce interference. Based on this scheme, the minimum mean-square error channel estimation is deduced, and the optimal power allocation between the pilots and the data signals is derived to maximize throughput. These results show that our proposed scheme can overcome the pilot contamination and achieve better performance than the traditional time-multiplexing scheme with mutually orthogonal pilot sequences for different users when the coherent time slot is limited.

Increasing Spatial Multiplexing Gain in Future Multi-AP WiFi Systems via Joint Transmission

There has recently been a growing interest in multi-AP coordination technologies for the next generation of WiFi systems. Of the several flavors under consideration, the most sophisticated is joint transmission (JT), which can potentially achieve multiplicative gains in network throughput proportional to the number of participating APs via concurrent distributed MU-MIMO. In this article, we provide a survey of the practical challenges envisioned in realizing gains for JT and the methods proposed to address them in the context of 802.11. We also briefly review other multi-AP schemes that involve coordination in the spatial domain, and contrast their requirements and achievable gains vs. JT.

Divergence-degenerate spatial multiplexing towards future ultrahigh capacity, low error-rate optical communications

Spatial mode (de)multiplexing of orbital angular momentum (OAM) beams is a promising solution to address future bandwidth issues, but the rapidly increasing divergence with the mode order severely limits the practically addressable number of OAM modes. Here we present a set of multi-vortex geometric beams (MVGBs) as high-dimensional information carriers for free-space optical communication, by virtue of three independent degrees of freedom (DoFs) including central OAM, sub-beam OAM, and coherent-state phase. The novel modal basis set has high divergence degeneracy, and highly consistent propagation behaviors among all spatial modes, capable of increasing the addressable spatial channels by two orders of magnitude than OAM basis as predicted. We experimentally realize the tri-DoF MVGB mode (de)multiplexing and data transmission by the conjugated modulation method, demonstrating lower error rates caused by center offset and coherent background noise, compared with OAM basis. Our work provides a potentially useful basis for the next generation of large-scale dense data communication.

Fiber-Optic Distributed Sensing Network for Thermal Mapping of Gold Nanoparticles-Mediated Radiofrequency Ablation.

In this work, we report the design of an optical fiber distributed sensing network for the 2-dimensional (2D) in situ thermal mapping of advanced methods for radiofrequency thermal ablation. The sensing system is based on six high-scattering MgO-doped optical fibers, interleaved by a scattering-level spatial multiplexing approach that allows simultaneous detection of each fiber location, in a 40 × 20 mm grid (7.8 mm2 pixel size). Radiofrequency ablation (RFA) was performed on bovine phantom, using a pristine approach and methods mediated by agarose and gold nanoparticles in order to enhance the ablation properties. The 2D sensors allow the detection of spatiotemporal patterns, evaluating the heating properties and investigating the repeatability. We observe that agarose-based ablation yields the widest ablated area in the best-case scenario, while gold nanoparticles-mediated ablation provides the best trade-off between the ablated area (53.0–65.1 mm2, 61.5 mm2 mean value) and repeatability.

Spiraling light: Generating optical tornados

We experimentally generate optical tornado waves using spatial multiplexing on a single-phase modulation device. In their focal region, the intensity pattern outlines a spiral of decreasing radius and pitch. We examine the propagation dynamics of such novel waves and reveal the key factors that lead to angular acceleration. Moreover, we propose a two-color scheme that makes it possible to generate dynamically twisting light, an optical analog of a drill, that can rotate at THz frequencies.

A Novel Meander Line Metamaterial Absorber Operating at 24 GHz and 28 GHz for the 5G Applications

Fifth generation (5G) communication systems deploy a massive MIMO technique to enhance gain and spatial multiplexing in arrays of 16 to 128 antennas. In these arrays, it is critical to isolate the adjacent antennas to prevent unwanted interaction between them. Fifth generation absorbers, in this regard, are the recent interest of many researchers nowadays. The authors present a dual-band novel metamaterial-based 5G absorber. The absorber operates at 24 GHz and 28 GHz and is composed of symmetric meander lines connected through a transmission line. An analytical model used to calculate the total number of required meander lines to design the absorber is delineated. The analytical model is based on the total inductance offered by the meander line structure in an impedance-matched electronic circuit. The proposed absorber works on the principal of resonance and absorbs two 5G bands (24 GHz and 28 GHz). A complete angular stability analysis was carried out prior to experiments for both transverse electric (TE) and transverse magnetic (TM) polarizations. Further, the resonance conditions are altered by changing the substrate thickness and incidence angle of the incident fields to demonstrate the functionality of the absorber. The comparison between simulated and measured results shows that such an absorber would be a strong candidate for the absorption in millimetre-wave array antennas, where elements are placed in proximity within compact 5G devices.

Research on spatial multiplexing using BGSM beam in FSO communication

We propose a spatial division multiplexing scheme based on the Bessel–Gaussian​ Schell-mode beam. Sucha communication scheme can enhance the capacity of free-space optical communication (FSO) in turbulence. This multiplexing utilizes the relationship between the characteristics of coherence degree distribution and the distant facula, realizing the separation of several BGSM beams with different Bessel parameter from each other, which makes the multiplexing technique deploying coherence modulation possible. The advantage of this scheme is its resistance to the atmospheric influence which exists widely in the optical wireless communication’s application environment. According to the simulation, for the transmission length of about 1300 m, under the weak turbulence environment (Cn2=10−16 m2/3), the BGSM-SDM scheme can support about 5 times more capacity compared with the traditional optical wireless communication scheme with Gaussian beam, and while the turbulence becomes strong (Cn2=10−13 m2/3), the SDM scheme degenerates to single channel because of the beam expansion, but it can still support more than 2 times more capacity. The scheme is proved work stable for each channel, and applicable in strong turbulent environment by measuring the signal and noise in the experiment. I believe it provides a promising choice for the future 6G ultrahigh-speed wireless communication.

Performance evaluation of MIMO DFT-Spread WR-OFDM system for spectrum efficiency and power efficiency

ABSTRACT It is important to design a spectrum-efficient and power-efficient wireless communication system, which is the main design target in cellular and wireless communication engineering. Basically, to make it, the desired system must have a low peak-to-average power ratio (PAPR), low-level of out-of-band (OOB) power emission and can provide a high throughput. In this paper, we propose a high-throughput cellular communication system, namely Discrete Fourier Transform-Spread Windowing and Restructuring Orthogonal Frequency Division Multiplexing (DFT-Spread WR-OFDM), which has considerably low PAPR and low OOB power emission for upcoming next-generation beyond 5G (B5G) and 6G cellular communication systems. High PAPR is still one of the challenging issues for the MIMO-OFDM system. The proposed system is an effective arrangement of DFT-Spreading and windowing techniques to reduce PAPR and OOB power emission, respectively. Multiuser diversity is obtained using localized subcarrier mapping. We take advantages of spatial multiplexing to enhance the data throughput significantly. Additionally, frequency-domain equalization (FDE)-minimum mean squared error (MMSE) is used to combat the inter-symbol interference (ISI) effect. Simulation results verify that the suggested system considerably lowers PAPR and OOB power emission compared to the conventional systems. Furthermore, the proposed scheme shows better bit error rate (BER) performance over the MIMO Rayleigh fading channel environment. Simulation results also confirmed that the channel capacity of our suggested system is better than the conventional multi-carrier systems.

Reconfigurable Intelligent Surface-Assisted Massive MIMO: Favorable propagation, channel hardening, and rank deficiency [Lecture Notes

Massive multiple-input multiple-output (MIMO) and reconfigurable intelligent surface (RIS) are two promising technologies for 5G-and-beyond wireless networks, capable of providing large array gain and multiuser spatial multiplexing. Without requiring additional frequency bands, those technologies offer significant improvements in both spectral and energy efficiency by simultaneously serving many users. The performance analysis of an RIS-assisted massive MIMO system as a function of channel statistics relies heavily on fundamental properties, including favorable propagation, channel hardening, and rank deficiency. The coexistence of both direct and indirect links results in aggregated channels, whose properties are the main concerns of this “Lecture Notes” article. For practical systems with a finite number of antennas and engineered scattering elements of the RIS, we evaluate the corresponding deterministic metrics, with Rayleigh fading channels as a typical example.

Precise spatial multiplexing of immune cell diversity in clinical FFPE tumor samples with ChipCytometry™

Abstract Highly multiplexed spatial biomarker analysis has demonstrated the potential to advance our current understanding of the immune system and its role in cancer – from tumor initiation to metastatic progression. Previously, a trade-off between plex and spatial context meant that our understanding of immune cell involvement in cancer was limited by either single-plex technologies with spatial context (i.e., immunohistochemistry) or highly multiplex technologies without spatial context (i.e., single-cell RNA sequencing). ChipCytometryTM is a novel, highly multiplexed technology that preserves both plex and spatial context to deeply profile immune cell diversity at single-cell resolution. ChipCytometry uses commercially available antibodies and combines iterative immuno-fluorescent staining with high-dynamic range imaging to profile dozens of protein biomarkers in a single tissue specimen. Cellular phenotypes are identified with via flow cytometry-like hierarchical gating from standard multichannel OME-TIFF images, compatible with a variety of computational tools being developed for multiplexed analysis and visualization. Here, we use ChipCytometry to identify and quantify key immune cell subtypes in both normal (i.e., appendix) and tumor tissues (i.e., melanoma and non-small cell lung cancer) FFPE samples. The results show precise expression levels for each biomarker in the assay in each individual cell in the sample, while maintaining spatial positioning of each cell. Spatial analysis reveals quantifiable heterogeneity of immune cell infiltration within the tumor samples, demonstrating the utility of the ChipCytometry platform for the in-depth immune profiling in clinical FFPE samples.

Efficient MIMO Signal Predistortion for Secrecy-Enhancing

Massive machine type communications or internet of things (IoT) over wireless systems have invoked attention in terms of security problems, especially for multi-input multi-output (MIMO) schemes. In this paper, we propose efficient physical layer security (PLS) enhancing methods for space-time block coding (STBC), as well as spatial multiplexing (SM) schemes. The proposed methods pre-distort the transmit signals by using the phase information of the legitimate MIMO channel, and, as a result, the illegal eavesdropper cannot extract any information. The proposed predistortion for the STBC schemes is done so that the channel matrix at the receiver is a real-valued one, which results in full-rate and full-diversity gain for more than two transmit antennae. Therefore, compared to the conventional schemes, the proposed scheme eventually leads to the higher performance gain and lower detection complexity in addition to the high security protection. By extending the principle of the proposed method for STBC, a predistortion scheme is also proposed for SM by using the phase information of the column space of the channel matrix. The simulation results investigated in this paper reveal that the proposed methods can achieve enhanced error rate performances, as well as high security protection.

Pixelated volume holographic optical element for augmented reality 3D display.

Augmented reality (AR) three-dimensional (3D) display is the hardware entrance of metaverse and attracts great interest. The fusion of physical world with 3D virtual images is non-trivial. In this paper, we proposed an AR 3D display based on a pixelated volume holographic optical element (P-VHOE). The see-through combiner is prepared by spatial multiplexing. A prototype of AR 3D display with high diffraction efficiency (78.59%), high transmission (>80%) and non-repeating views is realized. Virtual 3D objects with high fidelity in depth is reconstructed by P-VHOE, with a complex wavelet structural similarity (CW-SSIM) value of 0.9882. The proposed prototype provides an efficient solution for a compact glasses-free AR 3D display. Potential applications include window display, exhibition, education, teleconference.

19-ring-air-core fiber supporting thousands of OAM modes for spatial division multiplexing.

We propose and design a 19-ring-air-core fiber that can support about 3000 orbital angular momentum (OAM) modes (156 modes in each ring) with <-80 dB inter-ring cross talk across the entire C and L bands after 100-km fiber propagation. Moreover, the eigenmodes are all separated from their adjacent modes by effective index differences >2.67 × 10-4 and mode groups by > 1.90 × 10-2, which can guarantee the stable transmission of OAM modes. This designed fiber is a potential candidate for applications in spatial division multiplexing (SDM) of optical channels to improve the capacity of next-generation high-speed optical communication systems, especially in short-distance applications. In this Letter, we also show the relationship between supported OAM mode numbers, total cross talk, and effective refractive index of intra-ring modes during the optimization of fiber through numerical simulations. It can provide a related reference for the future design of multi-ring-core fibers.

Robust Vehicular Communications Using the Fast-Frequency-Hopping-OFDM Technology and the MIMO Spatial Multiplexing

Vehicle-to-Vehicle communication is one of the more emerging technologies in the 21st century from either the comfortable transportation or safer transportation point of view. Vehicle-to-Vehicle communication has one crucial factor, which is the huge information to be shared among vehicles, such as the position, the road data. In such situation, the accurate information sharing process is the most important factor in order to make the vehicles operating in the most feasible way. This work proposes a more robust vehicle communication system to make the existing vehicle transportation system more efficient. In this paper, we propose a fast frequency hopping orthogonal frequency division multiplexing to mitigate the Doppler spread effect on our previously published clustering benchmark. This benchmark contains both of a clustering weighting factor based stage and a multiparallel processing stage. This is in addition to modify the PHY layer of the existing IEEE 802.11p standard in order to impose Multiple Input Multiple Output for higher throughput purposes.The results show a noticeable stability compared to our previously published work. Furthermore, the results are almost exceeds the achieved results from the Lower-ID Distributed Clustering Algorithm (DCA) from both of the speed and communication range.

Distributed Q-Learning-Enabled Multi-Dimensional Spectrum Sharing Security Scheme for 6G Wireless Communication

In 6G wireless communication scenarios, the ultra-dense network (UDN) scenario has received extensive attention, as it is an inevitable case of 6G networks. How to keep users safe while providing universal service to a number of users is becoming a crucial topic of UDN. To battle this issue, we propose a distributed Q-learning-enabled multi-dimensional spectrum sharing security scheme for the millimeter-wave frequency band. With this scheme, a fine-grained spectrum sharing security framework based on spatial multiplexing is established, which balances UDN performance and security. Aiming at the multi-dimensional spectrum sharing security and optimal allocation problem, non-cooperative game theory is used for modeling and analysis. To prove the validity of this scheme, we used numerical simulations to verify the performance of the proposed distributed Q-learning algorithm for solving the spectrum sharing optimization problem in the UDN scenario. The results show that the proposed multi-dimensional spectrum sharing security scheme can significantly reduce the access delay of users, increase the number of unauthorized users that can be accommodated, and improve throughput, while achieving good security performance of the network.

Throughput modeling of cellular network systems with spatial precoding

The paper discusses wireless cellular radio communication systems along highways using Multiple-input Multiple-output (MIMO, multiple transmit / receive antennas) technologies, in particular spatial multiplexing and diversity reception, and also proposes a model for assessing the potential gains in a multi-antenna system from MIMO, which takes into account the multipath of the channel and the relative orientation of the antennas. The work was carried out by transferring the results of the calculated correlation matrix from stochastic channel model into the physical layer simulator of the cellular system protocol. A methodology for evaluating the performance of multi-antenna systems using spatial multiplexing for roadside cellular networks has been developed. The correlation properties of the channel between the antennas of the two roadside units (RSU), each of which has two perpendicular linearly polarized antennas and a user terminal with the same two orthogonally polarized antennas, have been investigated. A prediction scheme for the type of correlation matrix has been developed, which makes it possible to more accurately set the correlation matrix in simulators of the physical layer. The obtained results showed that for properly designed systems the throughput will be close to the throughput of low spatial correlation, and the case of high correlation proposed by the standard does not need to be modeled. It is also shown that the channels between Tx / Rx pairs that undergo similar polarization changes (the same relative spatial rotation of the antennas) will be strongly correlated, which must be taken into account when developing MIMO systems.

Binary volume acoustic holograms

Acoustic holograms or phase conjugate acoustic lenses are a simple inexpensive method for forming complex acoustic fields. They are 3D printable phase plates that can map an incident field onto a pre-defined phase distribution such that it diffracts to form a desired field. These phase plates areanalogues of thin optical holograms with a thickness d ≅ λ the design wavelength. Another class of optical holograms are “thick” or “volume” holograms, composed of weak periodic variations in the refractive index, for which d ⪢ λ. These volume holograms are significantly more selective to the wavelength and direction of the incident field allowing for greater ability to multiplex patterns within a single hologram. In this work, we explore the generation of volume acoustic holograms using multi-material 3-D printing. First, we numerically assess the impact of sound speed contrast, absorption, and quantisation on efficiency. A greedy optimization approach is then introduced for the design of a volumetric hologram from a desired set of input/output fields. We then validate the approach experimentally using test samples printed on an Objet Connex 350. Several holograms are fabricated demonstrating both spatial and frequency multiplexing of different patterns in a single volume.