Multiple-input Multiple-output Links Research Articles

Photonics-aided exceeding 200-Gb/s wireless data transmission over outdoor long-range 2 × 2 MIMO THz links at 300 GHz

Outdoor long-range terahertz (THz) communications often come at the expense of transmission rate. Moreover, the practicability of the single polarization optical/THz link, which is commonly used in the previous long-range THz demonstrations based on photonics, is extremely limited by the following two fatal defects. One is relying on active polarization control, and the other is not supporting the transparent bridging of optical polarization division multiplexed (PDM) signals for mature coherent optical communication networks. In this work, a large-capacity photonics-aided THz wireless communication system based on the outdoor long-range 2 × 2 multiple-input multiple-output (MIMO) links has been successfully demonstrated. We first build the 200-m 2 × 2 MIMO THz wireless links at the 300 GHz band. The cascaded linear and nonlinear equalizers are proposed which can significantly improve the transmission performance of 100- and 200-Gb/s PDM quadrature phase shift keying (QPSK) signals. Then an interesting 2 × 2 MIMO structure which can provide certain diversity reception gain under 200-m long-range wireless delivery using the same polarization scheme is also presented and further compared with the orthogonal polarization scheme. Since each THz receiver simultaneously receives data from both the two THz transmitters for this MIMO links, an improvement over 6 dB in receiving sensitivity and one order of magnitude in bit error ratio performance under low signal-to-noise ratio conditions can be achieved. Finally, based on the proposed cascaded equalizers and novel 2 × 2 MIMO structure, we successfully demonstrate a record-breaking 58-Gbaud (232-Gb/s) PDM-QPSK signal transmission over 200-m 2 × 2 MIMO THz wireless links. This is an important attempt for the photonics-aided THz wireless communication systems to achieve 2 × 2 MIMO transmission over a long wireless distance in the outdoors. Furthermore, attaining over 200 Gb/s at a wireless distance of 200 m also represents a key milestone for the long-range and large-capacity THz wireless communication systems.

Hard Successive Interference Cancellation for M-QAM MIMO Links in the Presence of Rayleigh Deep-Fading.

In our paper, we propose a generalized version of the Alternating Projections Digital Hard Successive Interference Cancellation (AP-HSIC) algorithm that is capable of decoding any order of constellation M in an M-Quadrature Amplitude Modulation (QAM) system. Our approach applies to Rayleigh deep-fading Multiple-Input Multiple-Output (MIMO) channels with high-level Additive White Gaussian Noise (AWGN). It can handle various destructive phenomena without restricting the number of antenna arrays in the transmitter/receiver. Importantly, it does not rely on closed-loop MIMO feedback or the need for Channel-State Information Transmission (CSIT). We have demonstrated the effectiveness of our approach and provided a Bit Error Rate (BER) analysis for 16-, 32-, and 64-QAM modulation systems. Real-time simulations showcase the differences and advantages of our proposed algorithm compared to the Multi-Group Space-Time Coding (MGSTC) decoding algorithm and the Lagrange Multipliers Hard Successive Interference Cancellation (LM-HSIC) algorithm, which we have also developed here. Additionally, our paper includes a mathematical analysis of the LM-HSIC algorithm. The AP-HSIC algorithm is not only effective and fast in decoding, including interference cancellation computational feedback, but it can also be integrated with any Linear Processing Complex Orthogonal Design (LPCOD) technique, including Complex Orthogonal Design (COD) schemes such as high-order Orthogonal Space-Time Block Code (OSTBC) with high-order QAM symbols.

Real-Time Demonstration of 100 GbE THz-Wireless and Fiber Seamless Integration Networks

We propose and experimentally demonstrate a real-time fiber-THz-fiber 2 × 2 multiple-input multiple-output (MIMO) seamless integration system at 340–510 GHz using commercial digital coherent optics (DCO) modules for baseband signals processing, which realizes a record net rate of 103.125 Gb/s dual polarization quadrature-phase-shift-keying (DP-QPSK) signals delivery over two spans of 20 km wireline single-mode fiber-link and 3 m wireless 2 × 2 MIMO link without using THz power amplifier under 15% soft-decision forward-error-correction threshold (SD-FEC) of 1.56 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> . Two kinds of THz-to-optical conversion schemes based on an integrated dual-polarization Mach-Zehnder modulator (DP-MZM) and two discrete intensity-modulators (IMs) are extensively investigated and compared. To the best of our knowledge, this is the first real-time demonstration of THz-wireless and 100 GbE fiber-optic networks seamless integration transmission, which is compatible with IEEE 802.3 and ITU-T G.798 specifications. It is a promising scheme to meet the demands of future fiber-wireless-integrations communication for low power consumption, low cost and miniaturization.

Demonstration of 4096QAM THz MIMO wireless delivery employing one-bit delta-sigma modulation.

We experimentally demonstrate 2-Gbaud 4096-ary quadrature amplitude modulation (4096QAM) signal transmission over 2-m wireless distance at 370 GHz in a photonics-aided terahertz (THz)-wave communication system. By using one-bit quantized delta-sigma modulation (DSM) and photonics-aided technology, we successfully achieve the transmission of high-order QAM signals over THz multiple-input multiple-output (MIMO) links. After 2 m of free space transmission, its bit error rate (BER) is below the 20% soft-decision forward error correction (20% SD-FEC) threshold of 2.4 × 10-2. To the best of our knowledge, this is the first time to apply DSM technology in a THz MIMO link transmission.

Phase Turbulence Prediction Method for Line-of-Sight Multiple-Input–Multiple-Output Links Caused by Atmospheric Environment

The multiple-input–multiple-output (MIMO) phase of the received signal is the most important parameter in the line-of-sight MIMO (LOS MIMO) system. The MIMO phase turbulence often occurs in practical applications, and the LOS MIMO system cannot operate normally in severe cases. However, there is no practical MIMO phase prediction method presently. A tropospheric radio-duct during phase turbulence was observed using a mesoscale numerical weather prediction (NWP) model. In this study, a novel MIMO phase prediction method was proposed. In the method, the tropospheric refractivity profile was predicted using a mesoscale NWP model, and then MIMO phase was predicted by the parabolic equation model. The comparison of the predicted MIMO phases with the measured results shows good agreement. MIMO phase turbulence prediction method can provide support for LOS MIMO system performance prediction.

Configurable virtual MIMO via UAV swarm: Channel modeling and spatial correlation analysis

Air-to-ground wireless channel modeling for unmanned aerial vehicle (UAV) communications has been widely studied. However, channel modeling for UAV swarm-enabled cooperative communication still needs investigation, where the impact of UAV positions on the spatial channel characteristics is of particular importance. In this paper, we consider a UAV swarm-enabled virtual multiple input multiple output (MIMO) system, where multiple single-antenna UAVs cooperatively transmit to multiple ground users (GUs). We establish a common coordinate system, as well as a UAV swarm-oriented coordinate system, to describe the relative positions of the GUs and the UAV elements, respectively. Based on the established coordinate systems, geometric ray superposition method is applied to describe the spatial channel matrix. The proposed modeling framework can be directly used to describe the line-of-sight and two-ray propagations, and can be extended for including more practical spatial features such as multipath scattering, inter-UAV blockage, and random UAV jittering, etc. Based on the proposed model, we further analyze the spatial correlation among the virtual MIMO links of GUs located at different positions. Via extensive simulations, we show that thanks to the flexible deployment of UAVs, the virtual MIMO array structure can be conveniently configured to get desired channel properties, such as the channel capacity, eigenvalue and condition number distribution, and spatial correlation distribution. This shows the possibility and importance of exploiting a new design dimension, i.e., the UAV swarm pattern, in such cooperative virtual MIMO systems.

Performance Comparison of WDM MIMO RoFSO Links for 5G Applications

The ubiquitous deployment of 5G systems is expected shortly. The 5th generation systems are envisioned to deliver a comprehensive selection of services with revolutionary data rates through a single network. Optical fiber networks are going to be the backbone of such 5G systems endowing huge data transfers. Radio over Fiber (RoF) is a promising network architecture that can provide all the provisions envisaged in the development of 5G systems. However, in certain scenarios, these fiber optic networks may encounter empirical problems in deployment. Radio over Free Space Optical (RoFSO) links can be used as an alternate in such environments. To increase the data carrying capacity of the link, Wavelength Division Multiplexing (WDM) communication equipping multiple wavelengths can be used. The free space link may suffer from atmospheric disturbances like rain, fog, and haze. In such adverse conditions, Multiple Input Multiple Output (MIMO) transceivers can be used to increase the link performance. In this work, the performance analysis of such WDM MIMO RoFSO links carrying 5G frequencies is carried out. The work also presents a comparison of single path and MIMO RoFSO links, single channel and WDM RoFSO links, and a combination of these. It is found that, transporting data over WDM channels can increase the capacity of the RoFSO system tremendously while the MIMO link setup can maintain the transmission range and link performance even when high data rates are used.&#x0D; HIGHLIGHTS&#x0D; &#x0D; A quantitative performance comparison of Radio over Free Space Optical links&#x0D; WDM increases the capacity while the MIMO links help to maintain the transmission range&#x0D; A combination of WDM and MIMO RoFSO links is identified to be the best&#x0D;

All-Optical Distributed MIMO for LiFi: Spatial Diversity Versus Spatial Multiplexing

LiFi or networked optical wireless communication is likely to play an important role in offloading mobile data traffic from radio into the optical spectrum. As the number of Internet of Things (IoT) devices is growing, the RF spectrum becomes a rare resource. Imaging IoT sensors like cameras, ultrasonic devices, and Lidars have real-time requirements, need a high-capacity uplink, and operate in environments that cause or are sensitive against electromagnetic interference. In this paper, for the first time, we present realtime communication over an all-optical fixed-wireless LiFi link based on the distributed multiple-input multiple-output concept. For distributing the wireless signals, plastic optical fibers are used as an analog front-haul. We study the operation of the distributed multiple-input multiple-output link in two modes, i.e., spatial diversity and spatial multiplexing. For the diversity mode, a new combiner is presented, which can support equal gain as well as selection combining. We demonstrate that selection combining is highly effective and enables a similar LiFi performance in up- and downlink, as it is desirable for industrial applications. For the spatial multiplexing mode, we observe that the channel rank and the achievable throughput depend strongly on the user location. As effective solutions, we study the benefits of angular diversity and multiple-input multiple-output mode switching together with multi-user multiplexing and conclude that a dynamic switching between spatial diversity and spatial multiplexing is a practical approach.

Conceiving Inferential Prototypes of MIMO Channel Models via Buckingham’s Similitude Principle for 30+ GHz through THz Spectrum

Facilitating newer bands of ‘unused’ segments (windows) of RF spectrum falling in the mm-wave range (above 30+ GHz) and seeking usable stretches across unallocated THz spectrum, could viably be considered for Multiple Input Multiple Output (MIMO) communications. This could accommodate the growing needs of multigigabit 3G/4G applications in outdoor-based backhauls in picocellular networks and in indoor-specific multimedia networking. However, in contrast with cellular and Wi-Fi, wireless systems supporting sub-mm wavelength transreceive communications in the outdoor electromagnetic (EM) ambient could face “drastically different propagation geometry”; also, in indoor contexts, envisaging pertinent spatial-multiplexing with directional, MIMO links could pose grossly diverse propagation geometry across a number of multipaths; as such, channel-models based on stochastic features of diverse MIMO-specific links in the desired test spectrum of mm-wave/THz band are sparsely known and almost non-existent. To alleviate this niche, a method is proposed here to infer sub-mm band MIMO channel-models (termed as “prototypes”) by judiciously sharing “similarity” of details available already pertinent to traditional “models” of lower-side EM spectrum, (namely, VLF through micro-/mm-wave). Relevant method proposed here relies on the “principle of similitude” due to Edgar Buckingham. Exemplar set of “model-to-(inferential)-prototype” transformations are derived and prescribed for an exhaustive set of fading channel models as well as, towards estimating path-loss of various channel statistics in the high-end test spectrum.

Analog 28 GHz LoS MIMO Relay System Using a 90° Hybrid Coupler

To enlarge the range of wireless fronthaul and backhaul communication links operating close to the millimeter-wave region, this letter presents a novel line-of-sight (LoS) multiple-input–multiple-output (MIMO) relay system. The analog relay system consists of a simple 90° hybrid coupler to separate two uncorrelated transmit data streams realizing a low complex and cheap system. The functional capability is proven within measurements of two independent 2 × 2 LoS MIMO links at 28 GHz connected by the relay system. A high decoupling of the two independent transmitted data streams of more than 27 dB after the 90° hybrid coupler and a high modulation error rate at the last receiver of around 22 dB could be reached for quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (QAM) modulated signals.

Optimal Best Path Selection Algorithm for Cluster-Based Multi-Hop MIMO Cooperative Transmission for Vehicular Communications

Multiple-input multiple-output (MIMO) cooperation in cluster-to-cluster communication has shown improved network quality of services by utilizing less transmission power over the fading environment. This is an enhancement from the traditional cluster-based single-input single-output multi-hop relaying used in vehicular communications. In this paper, we deploy multi-hop MIMO clustering relay into vehicular communication for a two-lane road transportation scenario. We deploy precoder and receiver filter in each MIMO link that is characterized by a narrowband classical three-dimensional time-varying channel. With the aid of the precoder and minimum mean square error receiver filtering algorithm in MIMO communication, we derive the end-to-end network signal-to-noise ratio to find the optimal best path using max-min theory with equal power allocation in precoder matrices that minimizes the network mean square error. A low-cost close-optimal best path selection method is also proposed through mathematical modeling. Computer simulation results demonstrate the validity of the theoretical investigations in this work.

Transmit Precoding and Receive Power Splitting for Harvested Power Maximization in MIMO SWIPT Systems

We consider the problem of maximizing the harvested power in Multiple Input Multiple Output (MIMO) Simultaneous Wireless Information and Power Transfer systems with power splitting reception. Different from recently proposed designs, with our optimization problem formulation we target for the jointly optimal transmit precoding and receive uniform power splitting ratio maximizing the harvested power, while ensuring that the quality-of-service requirement of the MIMO link is satisfied. We assume practical radio-frequency Energy Harvesting (EH) receive operation that results in a non-convex optimization problem for the design parameters, which we first formulate in an equivalent generalized convex problem that we then solve optimally. We also derive the globally optimal transmit precoding design for ideal reception. Furthermore, we present analytical bounds for the key variables of both considered problems along with tight high signal-to-noise ratio approximations for their optimal solutions. Two algorithms for the efficient computation of the globally optimal designs are outlined. The first requires solving a small number of non-linear equations, while the second is based on a two-dimensional (2-D) search having linear complexity. Computer simulation results are presented validating the proposed analysis, providing key insights on various system parameters, and investigating the achievable EH gains over benchmark schemes.

120 Gb/s Wireless Terahertz-Wave Signal Delivery by 375 GHz-500 GHz Multi-Carrier in a 2 × 2 MIMO System

We propose and experimentally demonstrate a photonics-aided 2 × 2 multiple-input multiple-output (MIMO) wireless Terahertz-wave (THz-wave) signal transmission system, which realizes 6 × 20-Gb/s six-channel polarization division multiplexing quadrature-phase-shift-keying THz-wave signal delivery over 10-km wireline single-mode fiber- link and 142-cm wireless 2 × 2 MIMO link with a bit-error ratio under the hard-decision forward-error-correction threshold of 3.8 × 10−3. Our employed multi-carrier frequencies are located within the range of 375 GHz to 500 GHz. To the best of our knowledge, this is the first experimental demonstration of 2 × 2 MIMO wireless transmission of multi-channel THz-wave signal. Here, it is worth noting that our wireless 2 × 2 MIMO link, which offers point-to-point straight transmission and brings neither interference nor gain, is different from the traditional MIMO link defined in the field of wireless communications.

Product of Two Envelopes Taken From $\alpha $ - $\mu $ , $\kappa $ - $\mu $ , and $\eta $ - $\mu $ Distributions

This paper presents exact, novel formulations for the probability density function and cumulative distribution function for the product of two independent and non-identically distributed $\alpha $ - $\mu $ , $\kappa $ - $\mu $ , and $\eta $ - $\mu $ variates. The expressions are given in terms of both 1) generalized Fox H-function and 2) easily computable series expansions. The formulations derived can be directly used to explore the performance of a number of wireless communication processes, including multihop systems, cascaded channels, radar communications, multiple-input multiple-output links, and others. Due to the high flexibility of the above-mentioned distributions, the results presented here comprise a substantial number of useful product distributions. As application examples, performance metrics for the cascaded fading channel are derived. The validity of the expressions is confirmed via Monte Carlo simulation. It is noteworthy that, because any composite multipath-shadowing fading model is obtained as a particular case of the product of two fading variables, the results given here provide a plethora of composite multipath-shadowing fading scenarios.

Jointly Optimal Spatial Channel Assignment and Power Allocation for MIMO SWIPT Systems

The joint design of spatial channel assignment and power allocation in multiple input multiple output (MIMO) systems capable of simultaneous wireless information and power transfer is studied. Assuming availability of channel state information at both communications ends, we maximize the harvested energy at the multi-antenna receiver, while satisfying a minimum information rate requirement for the MIMO link. We first derive the globally optimal eigenchannel assignment and power allocation design, and then present a practically motivated tight closed-form approximation for the optimal design parameters. Selected numerical results verify the validity of the optimal solution and provide useful insights on the proposed designs as well as the Pareto-optimal rate-energy tradeoff.

Analyzing 5G RF System Performance and Relation to Link Budget for Directive MIMO

Wideband fifth generation systems utilizing high carrier frequency and multiple-input multiple-output (MIMO) raise major challenges for the system design. Wave propagation and practical hardware tradeoffs at higher frequency ranges provide new boundary conditions for the implementation. This paper addresses system performance boundaries and the analysis method toward multibeam communications at millimeter wave. We combine analysis from antennas and propagation to the radio frequency (RF) transceiver specifications and beamforming requirements. Realistic propagation model and antenna implementation are used to generate beam-specific path gains and provide a wide variety of user scenarios. Using this approach, system level interdependencies and RF performance boundaries can be verified with different antenna configurations in various propagation environments. As an example, we present MIMO link budget analysis targeting 10 Gbits/s for multiple devices in the office scenario at 27 GHz.

Delivery of 54-Gb/s 8QAM W-Band Signal and 32-Gb/s 16QAM K -Band Signal Over 20-km SMF-28 and 2500-m Wireless Distance

We propose and experimentally demonstrate a large-capacity long-distance high-spectral-efficiency fiber-wireless-integration transmission system, which can realize photonics-aided multiband delivery of 54-Gb/s W -band polarization division multiplexing 8-ary quadrature-amplitude-modulation downlink signal and 32-Gb/s K -band PDM-16QAM uplink signal over 20-km single-mode fiber-28 and 2500-m wireless 2 × 2 multiple-input multiple-output link, with a bit-error ratio under $\text{3.8}\times \text{10}^{{-3}}$ . Transmitter-based predistortion and receiver-based decision-directed least-mean-square equalization are employed to improve the system performance. To the best of our knowledge, this is the first demonstration of >2-km K -band wireless signal delivery with 16QAM, and the first demonstration of >2-km W -band wireless signal delivery with 8QAM.

Compact Full Duplex MIMO Radios in D2D Underlaid Cellular Networks: From System Design to Prototype Results

This paper considers the implementation and application possibilities of a compact full duplex multiple-input multiple-output (MIMO) architecture, where direct communication exists between users, e.g., device-to-device (D2D) and cellular link coexisting on the same spectrum. For the architecture of the compact full duplex radio, we combine an analog self-interference canceler-based dual polarization with high cross-polarization discrimination and long-term evolution (LTE)-based per-subcarrier digital self-interference canceler. While we consider the compactness and power efficiency of an analog solution, we focus on the digital canceler design with robustness to a frequency-selective channel and high compatibility with a conventional LTE system. For an over-the-air wireless experiment of full duplex testbed with a two-user-pair, we implement a full duplex MIMO physical layer, supporting 20-MHz bandwidth, on an Field-Programmable Gate Array-based software-defined radio platform. Furthermore, we propose a novel timing synchronization method to construct a more viable full duplex MIMO link. By having the full duplex link prototype fully operating in real time, we present the first characterization of the proposed compact full duplex MIMO performance depending on the transmit power of the full duplex node. We also show the link quality between nodes. One of the crucial insights of this paper is that the full duplex operation of a user is capable of acquiring the throughput gain if the user has self-interference capability with guaranteed performance.

Experimental Throughput Analysis of Low-THz MIMO Communication Channel in 5G Wireless Networks

This letter presents first results of a Terahertz (THz) band line-of-sight 2 × 2 multiple-input multiple-output (MIMO) channel. The system architecture is based on a subharmonic mixer that translates the measuring frequency of vector network analyzer in the range of 298-313 GHz. The system uses a virtual antenna array technique to characterize a MIMO channel. The antenna element spacing is derived from the principles of diffraction limited optics to establish parallel channels for higher system throughput and reliability. The MIMO channel measurements are then used in simulations to evaluate the performance of the communication system operating in THz band MIMO communication channel. Finally, we have shown the MIMO link in THz band operating at 7 Gb/s, with higher reliability, in comparison with the single channel operating at 5.55 Gb/s.

A static/opportunistic hybrid-scheduling scheme for MIMO wireless networks

Multiple Input Multiple Output (MIMO) based Spatial Time Division Multiple Access (STDMA) Wireless Mesh Networks (WMNs) have attracted extensive research attention. However, there are two problems in existing studies: (1) the employed MIMO link rate models are not suitable for a MIMO link of a practical STDMA WMN, and (2) the designed scheduling algorithms usually cannot take advantage of the multi-user diversity in a WMN. In this paper, we develop an analytical model for determining the MIMO link rate of an STDMA WMN. Based on a node-based slot assignment and scheduling algorithm (Chen and Lea in IEEE Trans Veh Technol 62(1):272–283, 2013), we propose a static/opportunistic hybrid scheduling framework that can exploit multi-user diversity and channel fading. The performance evaluation shows that the proposed framework has 33–46 % throughput gain over the prior joint routing and time slot assignment schemes for MIMO WMNs.