Virtual Multiple-input Multiple-output System Research Articles

Path Loss and Auxiliary Communication Analysis of VANET in Tunnel Environments

Vehicular ad hoc network (VANET) communications face severe fading problems due to the signal reflections and diffractions within tunnels. Unlike the open road, the space of a tunnel is very limited, so VANET communication performance in a tunnel is seriously affected. In the process of signal transmission, the reflected signal is symmetrical with the incident signal after it is reflected by the road and the wall. In this paper, we establish a mathematical model of path loss for V2V (Vehicle-to-Vehicle) communication based on the principle of signal reflection symmetry in tunnels and considering several factors, such as the tunnel surface and the color of the tunnel wall. In addition, we use cooperative communication to form a virtual multiple-input multiple-output (V-MIMO) system, to improve the communication quality in tunnels. In the proposed system, the OBU (On-Board-unit) and RSU (Road-Side-Unit) share each other’s antennas, so that wireless cooperative communication can be employed, without increasing the number of antennas in a one-way tunnel. Therefore, this multipath fading internal electromagnetic wave propagation model can be used to improve performance. A deep reinforcement learning algorithm was used to solve the pairing problem to obtain a more accurate OBU and RSU pair, to form a V-MIMO system. Here, the RSU is regarded as an agent and interacts with the OBU in the tunnel. The optimal strategy was learned in a real-time changing simulation environment, and the experiment verified the convergence of the algorithm. The simulation results showed that, compared with the Q-learning based scheme, the optimal matching algorithm based on V-MIMO and a DQN (Deep Q-network) could effectively reduce the probability of transmission outages and improve the communication efficiency in tunnels.

Adaptive LDPC coded high data rate based IR-UWB combined with VMIMO system for vehicular communication links

The automotive industry is developing significantly with the evolution of vehicular networks, intelligent transportation systems, and autonomous cars. These technologies are made achievable by recent advances in software, hardware, and communication systems, along with various applications and standardization efforts. On the other side, many impairments have existed due to these evolving, limiting the process of vehicular networks. So, this paper proposes an implementation of adaptive LDPC-coded, high data rate-based Impulse Radio-Ultra Wide Band (IR-UWB) combined with Virtual Multiple Input Multiple Output (VMIMO) systems for Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure (V2I) wireless communications. Furthermore, they are merged with orthogonal pulse shaping using a modified Hermite pulse. Besides, the paper compares Zero-Forcing (ZF) and Minimum Mean Square Error (MMSE) detection techniques. To enhance and mitigate the existing impairments facing the vehicular networks. The proposed system’s performance is evaluated using both techniques, utilizing Bit Error Rate (BER), processing time, and resultant throughput analysis for both channel models. The simulation results include Non-Line of Sight (NLoS) and Line of Sight (LoS) scenarios for both channel models. Moreover, the simulation considers the effect of path loss and noise on the transmitted signal. In addition, the recent LDPC decoding algorithm termed “Modified Implementation Efficient Reliability Ratio Weighted Bit Flipping” (MIERRWBF) is evaluated through all latter channel models. At last, a novel adaptive LDPC decoder is proposed to further enhance the performance of all vehicular communication links under study. Consequently, the paper proposed an integrated adaptive LDPC-coded high data rate communication system to enhance vehicular communications using VMIMO mixed with modified Hermite orthogonal pulse shaping and deploying the MMSE recommended detection technique.

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.

MIMO-OFDM in a Cooperative Frequency Selective Fading Channel: An Analysis

Diversity is an integral technique in the advancement of wireless communications. Multiple-input multiple-output (MIMO) technology continues to occupy a pivotal position in the fourth-generation (4G) standards and beyond. With the advent of full-dimension (FD) MIMO and massive MIMO, new paradigms are being targeted in the disruptive technology. Third-generation partnership project (3GPP) has also emphasized the advanced MIMO technology for achieving its vision of International Mobile Telephony (IMT) 2020. However, implementing multiple antennas can be cumbersome due to limiting factors such as size, cost, and hardware limitations. In such a scenario, where implementing many antennas is not feasible, the benefits of diversity can be reaped by employing cooperative communication. Single antenna mobiles can share their mobile stations giving rise to a virtual MIMO system. In this paper, the authors have analyzed the performance of cooperative mobile stations in a frequency selective channel. Orthogonal frequency division multiplexing (OFDM), known for its robustness against multipath fading, is the modulation technique employed to circumvent the hazardous effects of intersymbol interference (ISI). MIMO is the key technology for achieving capacity advantage and improving link reliability. The blend of these two cutting edge technologies in a cooperative relay system presents encouraging results and inferences. Maximal Ratio Combining (MRC) is employed at the relay and the receiver to maximize the signal-to-noise ratio (SNR). Decode and forward (DF) protocol is used at the relay node owing to its regenerative nature and adaptability. The closed-form expressions for bit error rate (BER) and outage probability (Pout) are derived and compared with the experimental results. The effect of power decay factor is analyzed and simulation is done for 2 × 2, 2 × 3, and 2 × 4 transmit and receive antennas. The number of antennas at the relay is kept as 2. The analytical results agree with the simulation results.

Virtual MIMO Wireless Sensor Networks: Propagation Measurements and Fusion Performance

In this paper, we investigate the practical implications of employing virtual multiple-input multiple-output (MIMO) systems for prototyping future-generation wireless sensor networks (WSNs), especially in the light of recently proposed distributed detection based decision fusion (DF) rules. In order to do that, an indoor-to-outdoor measurement campaign has been conducted recently for investigating the propagation characteristics of an $8 \times 8$ virtual MIMO system. The campaign is conducted with transmit antennas representing the sensors deployed in different indoor environments and receiver antennas mounted on an outside tower representing the DF center. Channel measurements are reported when a 20 MHz wide signal is transmitted at 2.53 GHz. Measurements are collected for different spatial combinations of the transmit antennas. After analyzing the collected data, the performance of different DF rules is compared and tested over the measured channel. The results show that the fusion rules perform differently over different sets of measured channels. The results obtained here are important for maximizing performance and enabling the air-interface design of next-generation WSNs.

A windowing-based joint user pairing and resource allocation algorithm for V-MIMO systems

In this paper, we investigate the user pairing and resource allocation (UP-RA) problem which is a critical step in uplink virtual multiple input multiple output (V-MIMO) systems. It becomes a joint problem under the framework of Long Term Evolution (LTE) where single carrier frequency division multiple access (SC-FDMA) is adopted in uplink due to the intrinsic consecutive resource allocation constraint imposed therein. Traditional two-step algorithms tackle this joint problem by separating the UP step and the RA step and turning them into two independent and successive procedures. Different from them, in this paper a windowing-based UP-RA algorithm is developed in which the user pairs are selected by maximizing their performance on a series of contiguous RBs rather than a single one as discussed in those traditional algorithms. With the windowing technique introduced in the new algorithm heuristically, improved performance in uplink V-MIMO systems can be achieved. In addition, we provide the relationship between the performance and the window size, and present that the window size can be selected adaptively to further exploit the diversity gain. Furthermore, to corroborate the universal validity of the proposed windowing technique, another algorithm named pairing and extension (P&E) is also modified accordingly by it. Corresponding simulation results manifest the great potential of the windowing technique to be applied to other general two-step algorithms in V-MIMO systems and improve their performances.

Preference‐based unified criterion in virtual MIMO systems

Virtual multiple-input multiple-output (V-MIMO) is an advanced technology in uplink systems, and a well-designed user pairing and resource allocation scheme in V-MIMO systems can effectively exploit multi-user diversity to yield a significant performance gain. Recently, main attention has been devoted to the crucial user pairing issue with consideration of three key metrics, i.e. spectrum efficiency, energy efficiency and fairness. In this study, the authors investigate the unified tradeoff among all the three metrics for V-MIMO systems with user pairing and resource allocation. They first formulate the three key metrics in a united form and derive a straightforward unified criterion based on the user-specific preference to depict system performances. Through discussing several preference-based cases, they observe that a portion of spectrum efficiency is considered redundantly. Thus, a modified unified criterion is proposed to remove the partial redundancy provided by spectrum efficiency contribution to the objective function. Based on the two unified criteria, they propose a user pairing scheme in flat fading channels and a joint user pairing and resource allocation scheme in frequency-selective fading channels to balance the tradeoff flexibly. Simulation results show that, compared with existing schemes, the proposed scheme achieves a dynamical tradeoff among spectrum efficiency, energy efficiency and fairness.

Joint User Pairing and Power Allocation in Virtual MIMO Systems

Virtual multiple-input multiple-output (V-MIMO) is an effective way to achieve a diversity gain in uplink channels. A well-designed user pairing scheme in V-MIMO systems can exploit multiuser diversity to yield a significant performance gain. This paper investigates joint user pairing and power allocation problems in V-MIMO systems to maximize energy efficiency (EE) while guaranteeing spectrum efficiency (SE) under a total power consumption constraint. We formulate such a joint problem as a complex nonlinear optimization problem, which can be solved in two steps in $M\times K$ V-MIMO systems, i.e., power allocation with known user group and joint user pairing and power allocation. A multi-level water-filling method is used, and an iterative algorithm based on the outputs from the first step is employed in the second step. Finally, we propose a joint user pairing and power allocation scheme to find a user group that can achieve the maximum EE with power allocation. Simulation results and performance comparisons illustrate improved performance of the proposed scheme. Compared to the existing user pairing approaches, the proposed scheme achieves a significantly high EE gain while guaranteeing SE of a V-MIMO system.

Protocol Design and Performance Analysis of Multiuser Mixed RF and Hybrid FSO/RF Relaying With Buffers

In this paper, a multiuser mixed radio frequency (RF) and hybrid free-space optical (FSO)/RF system is considered, where multiple mobile users transmit their data to an intermediate decode-and-forward relay node through RF links using a virtual multiple-input multiple-output (MIMO) system, and the relay node forwards the multiplexed data of all users through a FSO link that is supported by a RF MIMO backup system to the destination. The relay node is equipped with a buffer in the physical layer for temporal storage of the users’ data until the best channel conditions at the relay-destination link are met. For this communication setup, we first propose a transmission protocol that achieves a multiplexing gain through a virtual MIMO system. After that, we derive closed-form expressions for the end-to-end outage probability, asymptotic outage probability, average symbol error rate, and the ergodic capacity when considering the delay-tolerant (finite buffer size) scenario. The results show that buffering in the physical layer provides a significant enhancement to the system performance (outage, error rate, and ergodic capacity). It is also found that pointing error and severe weather turbulence conditions become more tolerable with the existence of the relay’s buffer and RF backup link (in the second hop). In addition, the proposed virtual MIMO scheme shows a significant performance enhancement at a high number of receiving antennas, which introduces potential low-complexity diversity gain-based massive MIMO schemes.

Effective Interference Nulling Virtual MIMO Broadcasting Transceiver for Multiple Relaying

In multi-hop relay networks, the improving end-to-end sum rate is a challenging issue. Although a new cooperative beamforming called virtual multiple-input multiple-output (MIMO) has been introduced as a possible solution, conventional virtual MIMO systems have severe signaling overhead among master and source nodes when the number of hops increases. To resolve this problem, a broadcast virtual MIMO system and a virtual MIMO broadcasting transceiver (VMBT) have been proposed to reduce the computational complexity and improve the end-to-end sum rate. However, the application of such methods is limited to environments wherein interference from other links and networks is not considered. Thus, it is difficult to apply them to practical multi-hop relay networks. In this paper, we propose a generalized version of the VMBT in which intra- and inter-network interferences are simultaneously caused by multiple transmission of clusters in the same network and each network’s usage of the same frequency band, respectively. To achieve this goal, we propose an effective interference nulling VMBT scheme. Simulation results show that the proposed scheme has efficient performance in terms of average end-to-end sum rate compared with conventional schemes.

Genetic optimization channel estimation for multi-hop cooperation V-MIMO

Considering the channel estimation of a multi-hop cooperation virtual multiple input multiple output (V-MIMO) system, we research and analyze the destructive effect of a multi-hop concatenate channel and energy leakage on channel sparseness. The decode-and-forward (DF) cooperation method can avoid estimation error accumulation of concatenate channel estimation. Moreover, the optimized orthogonal matrix with a genetic algorithm (GA) can enhance channel sparsity. This led us to propose a channel-estimation algorithm utilizing GA to optimize the orthogonal matrix of the multi-hop cooperation V-MIMO system based on a DF protocol. The simulation results show that, compared with the amplify-and-forward (AF) method, the proposed algorithm can improve the channel estimation accuracy and reduce the bit error rate of the multi-hop cooperation V-MIMO system.

Energy-Spectral Efficiency Trade-Off in Virtual MIMO Cellular Systems

Virtual multiple-input multiple-output (V-MIMO) technology promises significant performance enhancements to cellular systems in terms of spectral efficiency (SE) and energy efficiency (EE). How these two conflicting metrics scale up in large cellular V-MIMO networks is unclear. This paper studies the EE-SE trade-off of the uplink of a multi-user cellular V-MIMO system with decode-and-forward type protocols. We first express the trade-off in an implicit function and further derive closed-form formulas of the trade-off in low and high SE regimes. Unlike conventional MIMO systems, the EE-SE trade-off of the V-MIMO system is shown to be susceptible to many factors including protocol design (e.g., resource allocation) and scenario characteristics (e.g., user density). Focusing on the medium and high SE regimes, we propose a heuristic resource allocation algorithm to optimize the EE-SE trade-off. The fundamental performance limits of the optimized V-MIMO system are subsequently investigated and compared with conventional MIMO systems in different scenarios. Numerical results reveal a surprisingly chaotic behavior of V-MIMO systems when the user density scales up. Our analysis indicates that low frequency reuse factor, adaptive resource allocation, and user density control are critical to harness the full benefits of cellular V-MIMO systems.

Mode Selection and Power Optimization for Energy Efficiency in Uplink Virtual MIMO Systems

Driven by green communications, energy-efficient transmission is becoming an important design criterion for wireless systems, aiming to extend the life cycle of batteries in mobile devices. In this paper, we tackle the energy efficiency (EE) issue in uplink virtual multiple-input multiple-output (MIMO) systems, which requires the optimization of two interlaced parameters: the number of constituent mobile users in the virtual MIMO and their corresponding power allocation. The former parameter is a structural parameter defining the size of the virtual MIMO (usually known as the transmission mode) and its optimization relies on the method of enumeration. The difficulty is further aggravated by the fact that the EE is a non-convex function of power, even for a given transmission mode. By exploiting the fact that increasing the number of active users can increase the number of contributors to the total EE on one hand but reducing the diversity order for each single user on the other, we can show the existence of an optimal transmission mode and find a simple way for its search. Through in-depth analysis, we show the existence of a unique globally optimal power allocator for the case without power constraints under the assumption of zero-forcing receivers, and further reveal the impact of power constraints upon power allocation, as compared to its global counterpart, aiming to provide a powerful means for power-constrained EE optimization. Finally, we establish theories, for isometric networks, to narrow down the search range for possible transmission modes, leading to a significant reduction of computational complexity in optimization. Simulation results are presented to substantiate the proposed schemes and the corresponding theories.

Stability Properties of Network Diversity Multiple Access with Multiple-Antenna Reception and Imperfect Collision Multiplicity Estimation

In NDMA (network diversity multiple access), protocol-controlled retransmissions are used to create a virtual MIMO (multiple-input multiple-output) system, where collisions can be resolved via source separation. By using this retransmission diversity approach for collision resolution, NDMA is the family of random access protocols with the highest potential throughput. However, several issues remain open today in the modeling and design of this type of protocol, particularly in terms of dynamic stable performance and backlog delay. This paper attempts to partially fill this gap by proposing a Markov model for the study of the dynamic-stable performance of a symmetrical and non-blind NDMA protocol assisted by a multiple-antenna receiver. The model is useful in the study of stability aspects in terms of the backlog-user distribution and average backlog delay. It also allows for the investigation of the different states of the system and the transition probabilities between them. Unlike previous works, the proposed approach considers the imperfect estimation of the collision multiplicity, which is a crucial process to the performance of NDMA. The results suggest that NDMA improves not only the throughput performance over previous solutions, but also the average number of backlogged users, the average backlog delay and, in general, the stability of random access protocols. It is also shown that when multiuser detection conditions degrade, ALOHA-type backlog retransmission becomes relevant to the stable operation of NDMA.

M2-m2 Beamforming for Virtual MIMO Broadcasting in Multi-Hop Relay Networks

The emerging demand for multi-hop relay networks requires significant improvement of the end-to-end throughput through the development of a more advanced transmission technology. Research on multiple-input multiple-output (MIMO) has been actively pursued for achieving channel throughput improvements for cellular networks. In a multi-hop ad-hoc network, it is difficult for each sensor node to have multiple transmit antennas, therefore, virtual MIMO has been introduced as an extended version of cellular MIMO. The conventional virtual MIMO scheme requires additional complexity in order to implement the same functionality used for cellular systems. Therefore, we propose a new framework for the broadcast virtual MIMO system (BVMS) by developing an innovative max-min/min-max (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) beamforming technology optimized for the multi-hop relay network. Compared to the conventional singular value decomposition (SVD)-based or random beamforming technologies, M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> beamforming significantly improves the end-to-end channel throughput up to the optimal bound for the BVMS over a multi-hop relay network.

Robust user pairing algorithm under channel estimation errors for uplink virtual multiple-input multiple-output systems

Employing user pairing schemes in uplink virtual multiple-input multiple-output (V-MIMO) systems can significantly improve system capacity owing to multi-user diversity. Most of the existing user pairing algorithms were developed under the assumption that a scheduler always possesses the perfect channel state information (CSI). In this paper, taking into account the effects of channel estimation error (CEE), the authors propose a robust user pairing algorithm based on a closed-form average sum rate under CEE. In addition, the authors further propose an adaptive user selection scheme as an effort to maximise the sum rates owing to the rate constraint under CEE. The gain of the proposed method is shown in simulation results.

Performance assessment of virtual multiple-input multiple-output systems with compress-and-forward cooperation

A cooperative virtual multiple-input multiple-output (MIMO) system using two transmit antennas that implements bit-interleaved coded modulation (BICM) transmission and compress-and-forward (CF) relay cooperation among two receiving nodes is presented here. To perform CF cooperation, we propose to use standard source-coding techniques for virtual MIMO detection, based on the analysis of its expected rate bound and the tightness of the bound. Since the relay and the destination are closely spaced, the authors first assume an error-free conference link between them, to focus on investigating the achievable gain from the CF cooperation. Then the system throughput expression and upper bounds on the system error probabilities over block fading channels are derived. The results show that the relay enables the proposed cooperative virtual-MIMO system to achieve almost ideal MIMO performance with low source-coding rates. Furthermore, when we consider a non-ideal cooperation link for practical considerations, a channel-aware adaptive CF scheme is proposed, so that the relay could always adapt its source-coding rate to meet the data rate on the non-ideal link. Owing to the short-range communication and the proposed scheme, the impact of the non-ideal link is too slight to impair the system performance significantly.

Propagation Characteristics, Metrics, and Statistics for Virtual MIMO Performance in a Measured Outdoor Cell

Abstract-Distributing the construction of a multiple-input multiple-output (MIMO) system among more than one mobile user reduces hardware and processing complexity at the terminal end, and may result in improved channel propagation conditions, particularly those related to spatial correlation. In this paper, measured outdoor propagation data is used to study a 4 × 4 virtual MIMO system formed from a two-user 4×4 classical system using only two of the antennas at each constituent user. The capacity, if-factor, and spatial correlation are evaluated for 226 possible pairs of users whose channels were measured while standing and walking. The proportions of pairings that result in a capacity increase and if-factor and correlation reductions over both, only one and neither of the constituents are determined. It is found that correlation reduction appears to be a stronger sign of capacity improvement than is if-factor reduction. When choosing a partner for a given single user, results show that it is easier to improve these three parameters as the user's value of them becomes poorer, so thresholds applicable to this data set are found which specify values of capacity, if, and correlation beyond which at least 50% of possible pairings improve matters.

Blind Separation of Two Users Based on User Delays and Optimal Pulse-Shape Design

A wireless network is considered, in which two spatially distributed users transmit narrow-band signals simultaneously over the same channel using the same power. User separation is achieved by oversampling the received signal and formulating a virtual multiple-input multiple-output (MIMO) system based on the resulting polyphase components. Because of oversampling, high correlations can occur between the columns of the virtual MIMO system matrix which can be detrimental to user separation. A novel pulse-shape waveform design is proposed that results in low correlation between the columns of the system matrix, while it exploits all available bandwidth as dictated by a spectral mask. It is also shown that the use of successive interference cancelation in combination with blind source separation further improves the separation performance.

Multiuser Pairing-up Schemes under Power Constraints for Uplink Virtual MIMO Networks

Orthogonal user pairing (OUP) was proposed for virtual multiple-input multiple-output (V-MIMO) in uplink 3G LTE system. However, its complexity is too high for practical implementation, especially when user population is large. To improve on this, semi-orthogonal user pairing (SUP) was suggested as an alternative. In this paper, two novel SUP algorithms, namely correlative angle-based SUP (CA-SUP) and Frobenius norm-based SUP (FN-SUP) algorithms are proposed. In view of the fact that ideal power control is difficult to implement, fixed power allocation (FPA) and adaptive power allocation (APA) constraints for the user pairing algorithms are introduced. Ergodic capacities for CA-SUP and FN-SUP algorithms with APA and FPA constraints, in terms of the numbers of users and antennas at Node-B, are evaluated and their capacity upper bounds are derived. Analysis and simulation results show that FN-SUP and CA-SUP algorithms requiring only limited channel state information (CSI) perform well and suit in particular for virtual V-MIMO systems.