Capacity Of Wireless Systems Research Articles

Joint Base Station Selection and Power Allocation Design for Reconfigurable Intelligent Surface-Aided Cell-Free Networks

Cell-free (CF) networks can reduce cell boundaries by densely deploying base stations (BSs) with additional hardware costs and power sources. Integrating a reconfigurable intelligent surface (RIS) into CF networks can cost-effectively increase the capacity and coverage of future wireless systems. This paper considers an RIS-aided CF system where each user is supported by a devoted RIS and can establish connections with multiple BSs for coherent transmission. Specifically, each RIS can enhance signal transmission between users and their selected BSs through passive beam-forming, but also randomly scattered signals from other non-selected BSs to users, causing additional signals and interference in the network. To gain insights into the system performance, we first derive the average signal-to-interference-plus-noise ratio (SINR) received by each user in a closed-form expression. Subsequently, we formulate an optimization problem aimed at maximizing the weighted sum-SINR of all users in the RIS-CF network. This optimization considers both BS transmit power allocation and BS selections as variables to be jointly optimized. To tackle the complexity of this nonconvex optimization problem, we develop an innovative two-layer iterative approach that offers both efficiency and efficacy. This algorithm iteratively updates the transmit power allocation and BS selections to converge to a locally optimal solution. Numerical results demonstrate significant performance improvement for the RIS-CF network using our proposed scheme. These results also highlight the effectiveness of jointly optimizing BS transmit power allocation and BS selections in the RIS-CF network.

Metamaterial inspired electromagnetic bandgap filter for ultra-wide stopband screening devices of electromagnetic interference

Presented here is a reactively loaded microstrip transmission line that exhibit an ultra-wide bandgap. The reactive loading is periodically distributed along the transmission line, which is electromagnetically coupled. The reactive load consists of a circular shaped patch which is converted to a metamaterial structure by embedded on it two concentric slit-rings. The patch is connected to the ground plane with a via-hole. The resulting structure exhibits electromagnetic bandgap (EBG) properties. The size and gap between the slit-rings dictate the magnitude of the reactive loading. The structure was first theoretically modelled to gain insight of the characterizing parameters. The equivalent circuit was verified using a full-wave 3D electromagnetic (EM) solver. The measured results show the proposed EBG structure has a highly sharp 3-dB skirt and a very wide bandgap, which is substantially larger than any EBG structure reported to date. The bandgap rejection of the single EBG unit-cell is better than − 30 dB, and the five element EBG unit-cell is better than − 90 dB. The innovation can be used in various applications such as biomedical applications that are requiring sharp roll-off rates and high stopband rejection thus enabling efficient use of the EM spectrum. This can reduce guard band and thereby increase the channel capacity of wireless systems.

Noise‐Robust Optimization of Impedance Loading for Single‐RF MIMO Systems

Multiple‐Input‐Multiple‐Output (MIMO) systems are widely used to enhance the system capacity in wireless communications systems. The complexity and the cost for implementing the multiple radio frequency‐chain (RF‐chain) increases proportional to the number of signal streams. Therefore, so‐called load modulating single‐RF MIMO system is one promising technology in the practical aspects. Load modulating single‐RF MIMO can utilize two‐port network for controlling the load and the desired signal can be generate by loading proper set of impedance. In this paper, the optimization method for choosing the two‐port network impedance solution that is robust to the impedance noise is proposed and the simulations are conducted to verify the robustness. © 2023 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Over-the-Air Phase Calibration of Reconfigurable Intelligent Surfaces

The reconfigurable intelligent surface (RIS) has emerged as a promising technology to enhance the capacity of wireless systems, as it can configure the wireless channel using low-cost passive devices, i.e., printed dipoles and phase shifters. But the real phases of the RIS elements usually deviate from the nominal values in practice, and the phase deviations need to be precisely estimated and calibrated for some applications. This letter proposes a novel over-the-air (OTA) approach that can estimate the real phases of the RIS elements at each gear. We also derive the Cramér Rao Bounds (CRBs) for the RIS calibration as a performance benchmark. The simulation results verify the effectiveness of the proposed algorithm as the root mean square errors (RMSEs) of the phase estimates are close to the CRBs.

On the Use of High-Rise Topographic Features for Optimal Aerial Base Station Placement

The use of unmanned aerial vehicles as aerial base stations (ABSs) can significantly enhance the capacity and coverage of wireless systems. In this paper, the problem of optimal ABS placement is studied while exploiting high-rise topographic features to maximize wireless coverage. In contrast to prior art that relies on simplified full line-of-sight (LoS) channel models or impractical probabilistic LoS channel models, this paper presents a novel feature-aware channel model that decisively discerns whether an air-to-ground (A2G) link is in LoS or non-LoS (NLoS) based on the topographical environment data for the target area. To resolve the challenges created by the dependence between the channel gain and the topographical environment in this feature-aware channel model, the LoS and NLoS zones in the target area are analyzed from a geometrical point of view. Then, based on the analysis results, the coverage area is derived in a tractable form and then used to develop a feature-aware ABS placement algorithm, called ABS-FA, based on particle swarm optimization (PSO). The effectiveness of the proposed approach is compared with two other baseline algorithms based on the full LoS and probabilistic LoS channel models, called ABS-LoS and ABS-Prob, respectively. Simulation results show that, depending on the topographical environment, ABS-LoS may outperform ABS-Prob, or vice versa, and even both may be very limited in some cases, because the full LoS and probabilistic LoS channel models cannot properly capture whether an A2G link is in LoS or NLoS. The results also show that the proposed ABS-FA scheme always outperforms these baseline algorithms and that, for instance, it can provide approximately 25% and 50% higher coverage performance compared to ABS-Prob and ABS-LoS, respectively. These results verify that considering a feature-aware channel model can be a very effective approach for determining the ABS location.

Data Aided Channel Estimation for MIMO-OFDM Wireless Systems Using Reliable Carriers

Wireless channel estimation is one of the challenging problems in Multiple Input Multiple Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) wireless systems. The MIMO-OFDM exploits the spatial resources and increases the reliability and capacity of wireless systems. However, the performance of these systems depends on accurate channel estimation since the receivers require perfect Channel State Information (CSI) for coherent signal detection. Thus, wireless channel estimation is a necessary component of OFDM systems. In this article, we have proposed an algorithm for MIMO-OFDM systems that combines pilot symbols with reliable data symbols for channel estimation. The reliable data symbols serve as virtual pilots and enhance the spectral efficiency. The proposed Data Aided Channel Estimation (DACE) algorithm eliminates the requirement of any additional resources such as excessive number of training symbols to attain the desired performance. Also, it outperforms the traditional Least Square (LS) and Linear Minimum Mean Square Error (LMMSE) methods for channel estimation in terms of Mean Square Error (MSE) and Bit Error Rate (BER) performance of the system.

Unmanned Aerial Vehicle Communications for Civil Applications: A Review

The use of drones, formally known as unmanned aerial vehicles (UAVs), has significantly increased across a variety of applications over the past few years. This is due to the rapid advancement towards the design and production of inexpensive and dependable UAVs and the growing request for the utilization of such platforms particularly in civil applications. With their intrinsic attributes such as high mobility, rapid deployment and flexible altitude, UAVs have the potential to be utilized in many wireless system applications. On the one hand, UAVs are able to operate as flying mobile terminals within wireless/cellular networks to support a variety of missions such as goods delivery, search and rescue, precision agriculture monitoring, and remote sensing. On the other hand, UAVs can be utilized as aerial base stations to increase wireless communication coverage, reliability, and the capacity of wireless systems without additional investment in wireless systems infrastructure. The aim of this article is to review the current applications of UAVs for civil and commercial purposes. The focus of this paper is on the challenges and communication requirements associated with UAV-based communication systems. This article initially classifies UAVs in terms of various parameters, some of which can impact UAVs’ communication performance. It then provides an overview of aerial networking and investigates UAVs routing protocols specifically, which are considered as one of the challenges in UAV communication. This article later investigates the use of UAV networks in a variety of civil applications and considers many challenges and communication demands of these applications. Subsequently, different types of simulation platforms are investigated from a communication and networking viewpoint. Finally, it identifies areas of future research.

Energy Efficiency Maximization in RIS-Aided Cell-Free Network With Limited Backhaul

Integrating the reconfigurable intelligent surface in a cell-free (RIS-CF) network is an effective solution to improve the capacity and coverage of future wireless systems with low cost and power consumption. The reflecting coefficients of RISs can be programmed to enhance signals received at users. This letter addresses a joint design of transmit beamformers at access points and reflecting coefficients at RISs to maximize the energy efficiency (EE) of RIS-CF networks, taking into account the limited backhaul capacity constraints. Due to a very computationally challenging nonconvex problem, we develop a simple yet efficient alternating algorithm for its solution. Numerical results verify that the EE of RIS-CF networks is greatly improved, showing the benefit of using RISs.

A Broadband Multistage Self-Interference Canceller for Full-Duplex MIMO Radios

Full-duplex (FD) antenna arrays are desirable for expanding the channel capacity of wireless systems up to 2, which is particularly important in the crowded sub-6-GHz spectral bands. Conventional FD solutions based on circulators do not scale to antenna arrays due to the presence of significant mutual coupling between antenna elements. This article describes a two-stage self-interference cancellation (SIC) method that is suitable for FD arrays. The first-stage SIC uses a replica-antenna (RA)-based network that is passive and linear and can thus be placed before the low-noise amplifier (LNA) without significantly degrading receiver noise figure (NF) and linearity. The RA-based SIC also simplifies the hardware requirements for the second-stage SIC, which uses an analog tapped delay line (ATDL) network. The proposed two-stage SIC was realized using off-the-shelf components and tested in the 1–3-GHz range using strongly coupled pairs of ultrawideband antennas. Network analyzer measurements show 20–30 dB of SIC using the RA network over the entire frequency range and >20 dB of SIC using a single-stage ATDL SIC over instantaneous bandwidths of 0.53–0.65 GHz. The combined transmit (Tx)–receive isolation is 66.5, 73.5, and 85.8 dB around the center frequencies of 1, 2, and 3 GHz, respectively. The amount of isolation is shown to be sufficient for realizing practical FD wireless links at 2 GHz with reasonable Tx power levels (0 dBm).

DESIGNING OF HEXAGON SLOTTED MIMO ANTENNA FOR WIRELESS APPLICATION

The wireless system capacity can be increased by the use of multiple antennas at transmitting and receiving side. The technology which uses multiple antennas called multiple inputs multiple outputs used to fulfils the demand of higher bit rate. In this paper two element antennas has designed using MIMO technique for dual band application and produced better isolation characteristics. In the proposed structure the high isolation was achieved using triangular and hexagonal cut structure. The presented simulated result produced low value of ECC and optimum value of VSWR in the given frequency band. The value of S12 is found less than -18 dB. The value of ECC is obtained 0.015 at 5.3 GHz. The proposed antenna operates in two frequency applications which are 3.59 GHz and 5.3 GHz.

Millimeter wave based 3D clustered MIMO channel modeling and system simulation for vehicle-to-vehicle communication

With the era of the fifth generation (5G) drawing near, there is an insistent demand for high data rates in wireless communication. But the spectrum in current cellular bands (below 6 GHz) is faced with an increasing shortage. Millimeter wave (mmWave) with a wide range of frequencies between 30 and 300 GHz is one of the key technologies to broaden the spectrum and improve the capacity of wireless systems. However, deploying mmWave is challenging due to its large path attenuation in the air and weak diffraction around obstacles. Moreover, the physical propagation in mmWave bands is characterized by a “clustered” channel model. To this end, multiple antenna arrays at the transmitter and the receiver can be a complement to enhance the signal gain. In this paper, we build upon the previous channel models containing various features and provide a clustered Multiple Input Multiple Output (MIMO) system framework in the case of time-variant non-stationary Vehicle-to-Vehicle (V2V) communication. Simulation results validate that the numbers of antennas, velocity of the users, link length, and the number of symbols will impact the transmission performance of the system. Mobility plays a more conspicuous role compared to antennas. The performance of the communication system is almost stable and controllable within a certain range of height. Our system outperforms the similar work in terms of spectral efficiency under the same-level deployments.

Full-duplex medium access control protocols in wireless networks: a survey

Medium Access Control (MAC) protocol plays an important role in full-duplex wireless networks. Theoretically, full-duplex communications have this ability to increase the capacity of traditional half-duplex wireless systems by up to twice. However, designing and implementing an efficient MAC protocol is a central issue in this way. Increasing throughput, collision avoidance, and fairness are common challenges in designing an efficient full-duplex MAC protocol. After giving a short introduction to the general classification of traditional MAC protocols, IEEE 802.11 MAC protocol, and the identified challenges of full-duplex communications in wireless networks, this review paper provides a comprehensive survey of current major MAC protocols for full-duplex wireless communications and classifies them in to three categories i.e. distributed wireless local area networks, centralized wireless local area networks, and other types of networks. This paper also explores and gives a comparative analysis of current full-duplex MAC protocols with stating their advantages and disadvantages as well as comparing them one with another.

Outage-Capacity-Based Cross Layer Resource Management for Downlink NOMA-OFDMA Video Communications: Non-Deep Learning and Deep Learning Approaches

Prior works either considered outage capacity of wireless video transmission systems but did not consider NOMA which is a key technology for 5G ultra-reliable low-latency (URLLC), or concerned the ergodic capacity of NOMA-OFDMA systems but did not consider the outage capacity emphasized in 5G URLLC scenario. In this paper, outage capacity (as well as ergodic capacity) maximization in a 5G URLLC scenario, are considered, using two proposed resource management schemes (i.e. B, C) and finally, proposed deep-learning-based versions of Schemes B and C (i.e. Schemes B' and C') to reduce the complexity and the latency for 5G URLLC. The proposed schemes re-allocate subcarrier according to outage capacity- instead of ergodic capacity only-maximization objective and choose the candidate user to gain subcarrier in a new way to improve the outage capacity. The numerical results show the proposed Scheme B and C increase the outage capacity (the percentage of satisfied users) from 79.2% for a prior work (Scheme A) to 85.2% and 92.6%, respectively. Scheme C also increases the ergodic capacity (average PSNR) from 34.7dB for Scheme A to 35.5dB. The deep learning based Schemes B' and C' perform slightly poorer than the corresponding non deep learning based Schemes B and C but the execution time/latency of Scheme B'/C' is less than Schemes B/C.

Full-Duplex Non-Orthogonal Multiple Access for Next Generation Wireless Systems

In this article, we study the combination of NOMA and full-duplex operation as a promising solution to improve the capacity of next-generation wireless systems. We study the application of full-duplex NOMA transmission in wireless cellular, relay and cognitive radio networks, and demonstrate achievable performance gains. It is shown that the effects of self-interference and inter-user interference due to full-duplex operation can be effectively mitigated by optimizing/enhancing the beamforming, power control, and link scheduling techniques. We also discuss research challenges and future directions so that full-duplex NOMA can be made practical in the near future.

CPPHA: Capability-based Privacy-Protection Handover Authentication Mechanism for SDN-based 5G HetNets

Ultra-dense Heterogeneous network (HetNet) technique can significantly improve wireless link quality, spectrum efficiency and system capacity, and satisfy different requirements for coverage in hotspots, which has been viewed as one of the key technologies in fifth Generation (5G) network. Due to the existence of many different types of base stations (BSs) and the complexity of the network topology in the 5G HetNets, there are a lot of new challenges in security and mobility management aspects for this multi-tier 5G architecture including insecure access points and potential frequent handovers among several different types of base stations. In this paper, we integrate user capability and Software Defined Network (SDN) technique, and propose a capability-based privacy protection handover authentication mechanism in SDN-based 5G HetNets. Our proposed scheme can achieve the mutual authentication and key agreement between User Equipments (UEs) and BSs in 5G HetNets at the same time largely reduce the authentication handover cost. We demonstrate that our proposed scheme indeed can provide robust security protection by employing several security analysis methods including the BAN logic and the formal verification tool Scyther. In addition, the performance evaluation results show that our scheme outperforms other existing schemes.

Capacity of Optical Wireless System over Log-Normal Channels with Spatial Diversity in Presence of Atmospheric Losses

Abstract In this paper, average channel capacity of optical wireless communication system is evaluated under the combined effect of geometrical loss, attenuation due to weather conditions and weak atmospheric turbulence using a simple closed form expression. Fading induced due to atmospheric turbulence is modeled by log-normal distribution. Considering the fact that the sum of log-normal random variables can be well approximated by another log-normal random variable, the proposed expression has been utilized to compute the channel capacity for spatial diversity reception employing maximum ratio combining and equal gain combining over uncorrelated turbulence-induced fading conditions. It is shown that spatial diversity is an effective technique to mitigate the impairments caused by various atmospheric conditions such as haze, rain and fog. The quantitative improvement in channel capacity achieved by using diversity techniques is investigated and compared. Accuracy of the results is validated with exact results computed using Monte Carlo simulation.

Investigation of Secret Key Capacity in MIMO-OFDM Wireless Systems

MIMO and MIMO-OFDM systems are the enabling technologies for the next generation wireless systems. However, the radio signals transmitted are available for reception by a illegitimate user leading to eavesdropping. This raises severe concerns on the security of the transmission. Physical Layer security aims to exploit the wireless channel characteristics and in turn provide secure transmission robust against eavesdropping. In this paper, we investigate the Secret Key Capacity (SKC) in MIMO-OFDM systems. We derive expressions for SKC in MIMO systems and extend further to OFDM waveforms. Numerical simulations are performed for various transmit and receive antenna configurations without any estimation errors.

Achieving 3D-MIMO With Massive Antennas From Theory to Practice With Evaluation and Field Trial Results

Exploiting multiple-input–multiple-output (MIMO) techniques by using massive antennas can significantly improve the capacity and reliability of wireless systems and has been considered as one key technique in future fifth-generation mobile communications system. Considering the limitation of antenna panel size, placing the massive antennas in 2-D grid is an effective way for its commercialization, which is termed as “3D-MIMO.” In this paper, we study the performance of 3D-MIMO with massive antennas by system-level simulation using practical assumption and 3-D channel model and test the 3D-MIMO in field trial with commercial terminal and networks. In addition, we compare the system-level simulation results and the field trial test measurements. Our system-level simulation results show that the 3D-MIMO with 64 antennas can significantly improve the performance with 56% and 147% gain on cell-average and cell-edge throughputs compared with 2D-MIMO. We perform field trial to test the 3D-MIMO, and the results verify its performance gain obtained by simulation. Furthermore, the measurement in typical high-rise scenario shows that the 3D-MIMO can significantly enhance the data rate of users located at higher floors due to the capability of flexible elevation beamforming.

Millimeter-Wave Wireless Channel Control Using Spatially Adaptive Antenna Arrays

A wireless channel control concept based on spatially, i.e., position adaptive antenna arrays is introduced. This technique simultaneously utilizes beam-steering and spatial adaptation to enhance the wireless channel gain and system capacity. The concept is inspired by the microfluidically reconfigurable RF devices as they can enable compact systems with spatial adaptation capability. Specifically, a five element linear 28 GHz mm-wave antenna array design that can achieve beam-steering via phase shifters and spatial adaptation via microfluidics is detailed. Simulated realized gain patterns at various array positions and phase shifter states are subsequently utilized in link and system level simulations to demonstrate the advantages of the proposed concept. It is shown that a wireless communications system can achieve 51% improvement in the mean signal-to-interference ratio due to the spatial adaptation capability.

Enhanced FFR Scenario for Pilot Contamination Mitigation in 5G Systems with Massive Mimo

Massive multiple-input multiple-output (MIMO) is the technology of the future, because it has the capability to increase a wireless system capacity without costing the system to purchase an additional bandwidth. It also has the capability to decrease the energy consumption of the system. All these capabilities have promoted the massive MIMO to be a promising technology for the fifth generation (5G) wireless communication systems. However, it has an important issue that limits the benefits which can be acquired by deploying it in 5G. This issue is the pilot contamination, which means the interference between pilot sequences. This occurs when the same pilot sequence is used by more than one terminal in the channel estimation phase at the same time and this causes interference between data symbols later. In this paper, we propose an enhanced fractional frequency reuse (FFR) scenario to mitigate the effects of pilot contamination by following an algorithm in assigning the pilot sequences to the system terminals instead of the random assignment. Simulation results show that our proposed scenario outperforms the traditional FFR in terms of signal to interference and noise ratio (SINR) and capacity