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Backscatter communication (BC) systems are a promising technology for internet of things (IoT) applications that allow devices to transmit information by modulating ambient radio signals without the need for a dedicated power source. However, the security of BC systems is a critical concern due to the vulnerability of the wireless channel. This paper investigates the impact of side information (SI) on the secrecy performance of BC systems. SI mainly refers to the additional knowledge that is available to the communicating parties beyond transmitted data, which can be used to enhance reliability, efficiency, security, and quality of service in various communication systems. In particular, in this paper, by considering a non-causally known SI at the transmitter, we derive compact analytical expressions of average secrecy capacity (ASC) and secrecy outage probability (SOP) for the proposed system model to analyze how SI affects the secrecy performance of BC systems. Moreover, a Monte Carlo simulation validates the accuracy of our analytical results and reveals that considering such knowledge at the transmitter has constructive effects on the system performance and ensures reliable communication with higher rates than the conventional BC systems without SI, namely, lower SOP and higher ASC are achievable.

Abstract The recently introduced 5G New Radio is the first wireless standard natively designed to support critical and massive machine type communications (MTC). However, it is already becoming evident that some of the more demanding requirements for MTC cannot be fully supported by 5G networks. Alongside, emerging use cases and applications towards 2030 will give rise to new and more stringent requirements on wireless connectivity in general and MTC in particular. Next generation wireless networks, namely 6G, should therefore be an agile and efficient convergent network designed to meet the diverse and challenging requirements anticipated by 2030. This paper explores the main drivers and requirements of MTC towards 6G, and discusses a wide variety of enabling technologies. More specifically, we first explore the emerging key performance indicators for MTC in 6G. Thereafter, we present a vision for an MTC-optimized holistic end-to-end network architecture. Finally, key enablers towards (1) ultra-low power MTC, (2) massively scalable global connectivity, (3) critical and dependable MTC, and (4) security and privacy preserving schemes for MTC are detailed. Our main objective is to present a set of research directions considering different aspects for an MTC-optimized 6G network in the 2030-era.

Abstract The aim of this work is to study the impact of small receiver displacement on a signal propagation in a typical conference room environment at a millimeter wave frequency of 60 GHz. While channel measurements provide insights on the propagation phenomena, their use for the wireless system performance evaluation is challenging. Whereas, carefully executed three-dimensional ray tracing (RT) simulations represent a more flexible option. Nevertheless, a careful validation of simulation methodology is needed. The first target of this article is to highlight the benefits of an in-house built three-dimensional RT tool at 60 GHz and shows the effectiveness of simulations in predicting different characteristics of the channel. To validate the simulation results against the measurements, two different transmitter (Tx) positions and antenna types along with ten receiver (Rx) positions are considered in a typical conference room. In first system configuration, an omnidirectional antenna is placed in the middle of the table, while in the second system configuration a directed horn antenna is located in the corner of the meeting room. After validating the simulation results with the measurement data, in the second part of this work, the impact of a small change, i.e., 20 cm in the receiver position, is studied. To characterize the impact, we apply as performance indicators the received power level, root mean square delay spread (RMS-DS) and RMS angular spread (RMS-AS) in azimuth plane. The channel characteristics are considered with respect to the direct orientation (DO), i.e., the Rx antenna is directed toward the strongest incoming path. Different antenna configurations at the Tx and Rx side are applied to highlight the role of antenna properties on the considered channel characteristics. Especially, in the second system configuration the impact of different antenna half power beamwidth on different considered channel characteristics is highlighted through acquired simulation results. The validation of results shows the RMS error of only 2–3 dB between the measured and simulated received power levels for different Tx configurations in the direction of DO. Results indicate that only a small change of the Rx position may result a large difference in the received power level even in the presence of line-of-sight between the Tx and Rx. It is found that the STD of received power level across the room increases with the decrease in HPBW of the antenna. As can be expected, directed antennas offer lower value of RMS-DS and RMS-AS compared with isotropic antenna.

Innovations in industrial automation, information and communication technology (ICT), renewable energy as well as monitoring and sensing fields have been paving the way for smart devices, which can acquire and convey information to the Internet. Since there is an ever-increasing demand for large yet affordable production volumes for such devices, printed electronics has been attracting attention of both industry and academia. In order to understand the potential and future prospects of the printed electronics, the present paper summarizes the basic principles and conventional approaches while providing the recent progresses in the fabrication and material technologies, applications and environmental impacts.

With the popularization of Internet-of-things (IoT) and wireless communication systems, a diverse set of applications in smart cities are emerging to improve the city-life. These applications usually require a large coverage area and minimal operation and maintenance cost. To this end, the recently emerging backscatter communication (BC) is gaining interest in both industry and academia as a new communication paradigm that provides high energy efficient communications that may even work in a battery-less mode and, thus, it is well suited for smart city applications. However, the coverage of BC in urban area deployments is not available, and the feasibility of its utilization for smart city applications is not known. In this article, we present a comprehensive coverage study of a practical cellular carrier-based BC system for indoor and outdoor scenarios in a downtown area of a Helsinki city. In particular, we evaluate the coverage outage performance of different low-power and wide area technologies, i.e., long range (LoRa) backscatter, arrow band-Internet of Things (NB-IoT), and Bluetooth low energy (BLE) based BC at different frequencies of operation. To do so, we carry out a comprehensive campaign of simulations while using a sophisticated three-dimensional (3D) ray tracing (RT) tool, ITU outdoor model, and 3rd generation partnership project (3GPP) indoor hotspot model. This study also covers the energy harvesting aspects of backscatter device, and it highlights the importance of future backscatter devices with high energy harvesting efficiency. The simulation results and discussion provided in this article will be helpful in understanding the coverage aspects of practical backscatter communication system in a smart city environment.

Internet of Things (IoT) is expected to change the everyday life of its users by enabling data exchanges among pervasive things through the Internet. Such a broad aim, however, puts prohibitive constraints on applications demanding time-synchronized operation for the chronological ordering of information or synchronous execution of some tasks, since in general the networks are formed by entities of widely varying resources. On one hand, the existing contemporary solutions for time synchronization, such as Network Time Protocol, do not easily tailor to resource-constrained devices, and on the other, the available solutions for constrained systems do not extend well to heterogeneous deployments. In this article, the time synchronization problems for IoT deployments for applications requiring a coherent notion of time are studied. Detailed derivations of the clock model and various clock relation models are provided. The clock synchronization methods are also presented for different models, and their expected performance are derived and illustrated. A survey of time synchronization protocols is provided to aid the IoT practitioners to select appropriate components for a deployment. The clock discipline algorithms are presented in a tutorial format, while the time synchronization methods are summarized as a survey. Therefore, this paper is a holistic overview of the available time synchronization methods for IoT deployments.

The demand for wireless services is growing on a daily basis while spectral resources to support this growth are static. Therefore, there is need for the adoption of a new spectrum sharing paradigm. Cognitive Radio (CR) is a revolutionary technology aiming to increase spectrum utilization through dynamic spectrum access, as well as mitigating interference among multiple coexisting wireless networks. In many practical scenarios, multiple CR networks may coexist in the same geographical area, and they may interfere with each other and also have to yield to the primary user (PU). In this study, we investigate how much throughput a node in a CR network can achieve in the presence of another CR network and a PU. The results of this study illustrate how the transmission probability and sensing performance affect the achievable throughput of a node in coexisting CR networks. In addition, these results may serve as guidance for the deployment of multiple CR networks.

The key bottleneck for secondary spectrum usage is the aggregate interference to the primary system receivers due to simultaneous secondary transmissions. Existing power allocation algorithms for multiple secondary transmitters in the TV white space either fail to protect the TV service in all cases or they allocate extremely low power levels to some of the transmitters. In this paper, we propose a power allocation algorithm that favors equally the secondary transmitters and it is able to protect the TV service in all cases. When the number of secondary transmitters is high, the computational complexity of the proposed algorithm becomes high too. We show how the algorithm could be modified to reduce its computational complexity at the cost of negligible performance loss. The modified algorithm could permit a spectrum allocation database to allocate near optimal transmit power levels to tens of thousands of secondary transmitters in real time. In addition, we describe how the modified algorithm could be applied to allow decentralized power allocation for mobile secondary transmitters. In that case, the proposed algorithm outperforms the existing algorithms because it allows reducing the communication signalling overhead between mobile secondary transmitters and the spectrum allocation database.

Exploitation of wireless communications in various automation applications brings indisputable benefits, which has proliferated research efforts in the field during the recent years. However, underlying wireless communication systems introduce problems to the wireless control systems, such as variable delays, information loss, and limited throughput. By using multiple frequency bands simultaneously, the performance of a wireless network can be enhanced. Since many control applications have strict requirements on delay and delay variations, it is important to understand the impact of communication systems on the performance of wireless automation systems with respect to delay. Hence, in this paper, we study how different communication parameters, such as packet arrival rate, number of channels, and packet size, affect the performance of control systems. We also derive bound on control system performance given the network properties. Our results show the tradeoff between the performance of the wireless multichannel communication system and the automation system. These results can, for example, be further exploited in system design to find out the maximum packet arrival rate with given control parameters.

With the introduction of IEEE 802.11 power save mode (PSM), a lot of work has been done to enhance the energy saving ability of the wireless nodes. The ultimate goal of the research is to make the networking equipment carbon neutral and prolong the lifetime of the energy limited device for various applications; in some cases it is a trade-off between energy efficiency and delay. However, few studies have been made until now in the area of IEEE 802.11s based link specific power mode. The essence of this method is the ability of a node to maintain different power modes with its different peer nodes at the same time. A new peer service period (PSP) mechanism is also proposed in IEEE 802.11s amendment for transmitting to a receiver operating in PSM. In this paper the performance of the link specific power mode is studied for a single- and a multilink network in terms of energy, delay throughput, and sleep duration. It is found that at small load the energy saving could be as high as eighty percent when compared with the active mode operation. A stochastic model, based on discrete time discrete state Markov chain, is developed for one peer link operation to study the system behavior closely during PSM operation.

The Electronic Communication Committee (ECC) in Europe proposed a location-based transmission power allocation rule for secondary devices operating in the TV white space (TVWS). The further the secondary device is located from the TV cell border the higher transmission power level it can utilize. The Federal Communication Committee (FCC) in the US proposed a fixed transmission power allocation rule for all secondary transmitters. Both rules do not consider the secondary system’s self-interference while setting the transmission power levels. In this paper, we propose a power allocation scheme for a cellular secondary system. Unlike the ECC and the FCC proposals we do the power allocation by considering the self-interference. We define the power allocation scheme as an optimization problem. The sum cell border data rate of the secondary network is selected to be the optimization objective. We observe that the optimal transmission power levels become approximately constant over the secondary deployment area. The FCC rule captures the general trend for cellular deployment in the TVWS, since it suggests the use of constant power. However, the transmission power should not be set equal to 4 W but according to the allowable generated interference at the borders of the TV and secondary cells.

We present an analysis of a femtocellular communications network and the impact of cochannel interference on link performance. Furthermore, we propose a method whereby user terminals can maintain a controlonly connection to an adjacent femtocell for interference mitigation purposes. Specifically, we provide an emphasis on suboptimal but practical methods that rely on transmit beamforming. Our numerical results demonstrate that even simple multiantenna methods can be effectively used to suppress co-channel interference provided that control channel connection between interfering femto-base station and user terminal is allowed.

The concept of Reconfigurable Intelligent Surfaces (RIS) has emerged as a promising method for communications and the localization of aeronautical vehicles. In this paper, we explore the impact of hardware impairments on three-dimensional (3D) drone localization within a single-input single-output (SISO) system assisted by RIS. Our methodology begins by modeling the channel from the base station (BS), equipped with a single-antenna transmitter, to each RIS at known positions. This model accounts for hardware impairments at the BS, particularly beam downtilt, which influences the accuracy of drone location estimation. Moreover, we model the channel from the RIS to the drone, employing exhaustive beam sweeping in both azimuth and elevation angles to estimate the Angles of Departure (AODs) from the RIS to the drone. We adopt a unique phase noise (PN) model for each element within the RIS and assess the impact of these impairments on angle and location estimation accuracy through extensive simulations. Additionally, we examine the effects of RIS configuration and the Inter-Site Distance (ISD) between two RIS units on localization performance. An Unscented Kalman Filter (UKF) algorithm is integrated for tracking of the drone trajectory. Our simulation results demonstrate that the RIS-assisted 3D drone localization approach achieves significant accuracy despite various impairments. The findings of this paper underscore the potential of RIS-enabled 3D drone localization to maintain high accuracy under hardware impairments, paving the way for future research in RIS-enabled drone localization systems.

In this paper, we investigate frequency shift keying (FSK) over ambient orthogonal frequency division multiplexed (OFDM) signals. By cycling through a sequence of antenna loads providing different phase shifts at the tag, we are able to unidirectionally shift the ambient OFDM spectrum either up or down in frequency to disjoint subsets of the subcarriers allowing the implementation of FSK. We exploit the guard bands and the orthogonality of the OFDM subcarriers to avoid both direct-link and adjacent channel interference. Different from energy detection based techniques that suffer from asymmetric error probabilities and rely on signal-to-noise ratio (SNR) dependent detection thresholds, the proposed scheme has symmetric error probabilities and allows simple detection without the need for a threshold. We present both binary and four-ary schemes, and analyze the error performance of the optimal noncoherent detectors. For the binary scheme, we obtain exact expressions for the average probability of error, while for the four-ary scheme, a union bound is used to characterize the error performance. Single and multi-antenna receivers are considered, and their performance is analyzed. Finally, we present simulation results to corroborate our analysis and investigate the effects of multiple system parameters. The results show that the proposed scheme outperforms the baseline energy detection based schemes available in the literature in various scenarios by up to 5 dB.

Microwave quantum information networks require reliable transmission of single-photon propagating modes over lossy channels. In this article, we propose a microwave noiseless linear amplifier (NLA) suitable to circumvent the losses incurred by a flying photon undergoing an amplitude damping channel (ADC). The proposed model is constructed by engineering a simple 1-D four-node cluster state. Contrary to conventional NLAs based on quantum scissors (QS), single-photon amplification is realized without the need for photon number resolving detectors. Entanglement between nodes comprising the device's cluster is achieved by means of a controlled phase gate. Furthermore, photon measurements are implemented by quantum nondemolition detectors, which are currently available as a part of the circuit quantum electrodynamics toolbox. We analyze the performance of our device practically by considering detection inefficiency and dark count probability. We further examine the potential usage of our device in low-power quantum sensing applications and remote secret key generation (SKG). Specifically, we demonstrate the device's ability to prepare loss-free resources offline, and its capacity to overcome the repeaterless bound of SKG. We compare the performance of our device against a QS-NLA for the aforementioned applications, and highlight explicitly the operating conditions under which our device can outperform a QS-NLA. The proposed device is also suitable for applications in the optical domain.

The Uplink/Downlink transmission mode or Transmission Order (TO) optimization has recently appeared as a new optimization domain in radio resource management. Such optimization is a combinatorics problem and requires good heuristic algorithm to be approximately solved within short time for the dynamic radio environment. This paper shows how the TO optimization problem in Time Division Duplex (TDD) indoor femtocells can be formulated and solved by the Hopfield Neural Network (HNN) based TO schedulers. Both centralized and distributed versions are analyzed in the context of indoor femtocells. We also examine proposed TO schedulers’ system performance in TDD indoor femtocells environment by extensive simulation campaigns. Our simulation results for a large 3-story building including 120 femtocells show that (i) the indoor femtocell system performance is improved up to 13 to 20 percent by the proposed HNN schedulers depending on the number of femtocells, (ii) the proposed TO schedulers converge within the first few epochs. (iii) The performance of the proposed schedulers are justified by a time-consuming but a thorough Genetic Algorithm Scheduler.

In quantum illumination (QI) the non-classical correlations between continuous variable (CV) entangled modes of radiation are exploited to detect the presence of a target embedded in thermal noise. The extreme environment where QI outperforms its optimal classical counterpart suggests that applications in the microwave domain would benefit the most from this new sensing paradigm. However all the proposed QI receivers rely on ideal photon counters or detectors, which are not currently feasible in the microwave domain. Here we propose a new QI receiver that utilises a CV controlled not gate (CNOT) in order to perform a joint measurement on a target return and its retained twin. Unlike other QI receivers, the entire detection process is carried out by homodyne measurements and square-law detectors. The receiver exploits two squeezed ancillary modes as a part of the gate’s operation. These extra resources are prepared offline and their overall gain is controlled passively by a single beamsplitter parameter. We compare our model to other QI receivers and highlight the conditions in which it outperforms others and achieves optimal performance. Although the main focus of this study is microwave quantum sensing applications, our proposed device can be built as well in the optical domain, thus rendering it as a new addition to the quantum sensing toolbox in a wider sense.

In this paper, we address the challenge of establishing secure communication within a symbiotic radio (SR) network. This network comprises a primary transmitter (PT), a primary receiver (PR), a passive backscatter device (BD), and an eavesdropper (ED) attempting to intercept the BD’s transmitted information signal. The PT simultaneously transmits an information-bearing signal to the PR and artificial noise (AN) to confound the ED. The objective is that the BD conveys confidential information to the PR by leveraging the PT’s signal. The PR performs joint decoding of both the symbols transmitted by the PT and the BD. In this system configuration, we derive a closed-form expression for the ergodic secrecy rate of the BD, providing an analytical framework for evaluating its security performance. Furthermore, we derive an expression for the secrecy rate in the asymptotic regime characterized by a large number of transmit antennas. These derived expressions allow us to optimize both the reflection coefficient and power allocation factor, enabling the maximization of the BD’s ergodic secrecy rate while considering the quality of service (QoS) requirements of the primary system. The derived analytical results provide valuable insights into the influence of key system parameters on the secrecy performance. Notably, the derived analytical results quantify the effect of key system parameters on the secrecy performance. In particular, we show that the using AN can always improve the secrecy rate given the QoS constraints of the primary user. Moreover, we show that the secrecy rate can be improved by increasing the reflection coefficient at the BD even with better channel condition for the ED than the legitimate link.

When a vehicle moves from one base station (BS) to another, a large number of on-board user equipments (UE) may simultaneously and individually perform a handover (HO) procedure, resulting in increased HO overheads. A mobile relay node (MRN), connected via a wireless backhaul to a donor base station (DBS), is deployed on the rooftop of a bus to improve the link quality and reduce the associated HO overhead via group mobility. However, at moderate to high speeds, the on-board UEs can still suffer from frequent HOs due to the MRN failing to HO to a new DBS using the legacy downlink measurement-based HO (DL-HO) method. As a consequence, the connection towards all associated mobile users will be lost which poses tight reliability requirements on the backhaul link to avoid becoming a single point of failure (SPoF). In order to improve the reliability during group handover, in this work, we propose an uplink reference signal (UL RS) based HO procedure (coined as UL-HO) for the MRN which relies on the existing sounding reference signal in long term evolution (LTE) /new radio (NR). In the proposed scheme, and unlike the legacy DL-HO procedure in LTE/NR, the measurement report (MeasReport) transmission is not required between MRN and the DBS, therefore the HO delay can be reduced, decreasing the SPoF chances and thus, uninterrupted services can be provided to on-board UEs. We investigate the gain in terms of HO rate, HO failure rate, ping-pong rate and power consumption (both at the UE and the BS). Performance evaluations demonstrate that the proposed UL-HO scheme outperforms the legacy DL-HO scheme in current cellular networks.

Non-orthogonal multiple access (NOMA) is considered as one of the most promising technologies to handle the issue of spectrum scarcity in beyond fifth-generation (5G) networks. Integrating NOMA in cognitive radio (CR) network is expected to usher a new era of reliable, seamless, and massive connectivity. In this regard, this work explores the relay selection problem in CR networks when operating in the spectrum sharing model. Specifically, the secondary users (SUs) in the CR-NOMA network opportunistically access the licensed spectrum resources to boost the number of accessible SUs sharing the limited and dynamic spectrum resources. Moreover, to improve the performance of far users, partial relay selection architecture is exploited at full-duplex (FD) and half duplex (HD) relays for both uplink and downlink communications. For in-depth performance evaluation, we provide closed-form expressions of the outage probabilities of the users in FD and HD relay-aided CR-NOMA networks. In addition to this, the analytical expressions of asymptotic outage probabilities and ergodic capacity are also provided which unveils the critical factors affecting the performance of CR-NOMA networks. To validate the derived expressions, extensive simulations are performed that demonstrate the accuracy of analytical expressions for FD and HD relay-aided CR-NOMA networks.

Vehicular communication is gradually evolving from the rudimentary data exchange techniques to innovative, efficient, and large-scale communication platform. The future vehicular networks aim to offer new services and achieve massive connectivity with the aid of advanced wireless communication techniques. In this regard, one of the emerging paradigms is non-orthogonal multiple access (NOMA) which holds the promise to meet the resource-intensive demands of future networks. Due to this reason, the applications of NOMA are being investigated by the researcher of academia and industry alike. This article aims to build on the state-of-the-art by providing a novel multiobjective optimization framework for vehicle-to-infrastructure (V2I) communications. Specifically, this work aims to improve upon the sum-rate and energy efficiency of uplink NOMA-enabled V2I communications. The formulated problem is based on the tradeoff between sum-rate and total transmit power while maintaining the quality of service (QoS) requirements of the network. To provide a reliable solution, a multiobjective optimization framework has been proposed by applying the weighted-sum method and handle joint metrics of sum-rate maximization and total transmit power minimization. Extensive simulations have been performed to validate the superiority of the proposed framework against conventional benchmark techniques. The results clearly indicate that the proposed framework is feasible for NOMA-enabled V2I communication and can be scaled up for practical implementation.

In Ambient Backscatter Communications (AmBC), a backscatter device communicates by modulating the ambient radio frequency (RF) signal impinging at its antenna. In many cases, the system setup is bi-static such that the receiver and the ambient signal source are separated in space. This configuration suffers from the direct path interference problem. The direct signal component can be several orders of magnitude stronger than the scattered one. This imposes a challenge for the receiver that needs to have high dynamic range in order not to lose the scattered signal component to the quantization noise. In this paper, we propose a novel AmBC system concept, in which a polarization conversion between the direct and scattered path is introduced at the backscatter device and exploited at the dual polarization based receiver antenna to mitigate the direct path interference. The proposed system is agnostic to the ambient signal source characteristics as long as it uses linearly polarized antennas. The backscatter device changes the polarization from linear to circular. The receiver antenna is a circularly polarized patch antenna with a 180°-hybrid to obtain the difference between the left- and right-hand polarized fields. Ideally, this receiver antenna and 180°-hybrid combination would completely remove linearly polarized direct path and reflected components. In this paper, we propose a robust design that can mitigate the direct path signal power more than 25 dB despite nonidealities in the antenna manufacturing.

The ever-growing data rate demand from cellular users increases the associated power consumption that directly contributes to the global warming of the environment. Also, achieving high system capacity by increasing the density of the base stations (BSs) increases the number of handovers (HOs) which for moderate-to-high-speed users implies significant signaling traffic overhead. One of the key research objectives of this paper is to identify the different signaling overheads occurring during the HO procedure in current 3GPP cellular networks (e.g. Long Term Evolution (LTE)/ New Radio (NR)) and, among those, which are the main contributors to power consumption. Specifically, we analyze the impact of signaling messages transmitted and received during HO on the power consumption for both the BS and the User Equipment (UE). System-level simulations are performed for a detailed quantitative analysis. Our analysis shows that the transmission of the measurement reports is the largest contributor to air-interface signaling and that its contributed total power consumption is higher than the random access channel (RACH) signaling and the signaling confirming the HO. To eliminate measurement reports and effectively reduce the power consumption associated with the HO in future networks, we propose a HO procedure that exploits uplink (UL) reference signals (RSs), namely the sounding reference signal (SRS), transmitted by UEs. The numerical results show that the proposed SRS-based method reduces the total power consumption during the HO procedure by 30% in comparison to the legacy downlink RS based measurement method in current cellular networks. Also, this method improves the UE battery lifetime by reducing the RS transmissions and measurements significantly, UE transmitted power consumption by 48% and received power consumption by 27%.

Cellular systems are undergoing a transformation toward the fifth generation (5G). Envisioned applications in 5G include intelligent transport system (ITS), autonomous vehicles, and robots as a part of future roads, factories, and society. These applications rely to a great extent on accurate and timely location information of connected devices. This paper proposes a practical scheme for acquiring precise and timely position information by means of a user-centric ultra-dense network (UDN) architecture based on an edge cloud. The considered solution consists of estimating and tracking the azimuth angle-of-arrival (AoA) of the line-of-sight (LoS)-path between a device and multiple transmission-reception points (TRPs), each having a uniform linear antenna array (ULA). AoA estimates from multiple TRPs are fused into position estimates at the edge cloud to obtain timely position information. The extensive measurements have been carried out using a proof-of-concept software-defined-radio (SDR) testbed in order to experimentally assess the achievable positioning accuracy of the proposed architecture. A realistic UDN deployment scenario has been considered in which TRPs consist of antenna arrays mounted on lamp posts. Our results show that practical UDNs can provide sub-meter positioning accuracy of mobile users by employing ULAs with at least four antennas per TRP and by taking into account the non-idealities of the ULAs' phase and magnitude response.

To satisfy the high data demands in future cellular networks, an ultra-densification approach is introduced to shrink the coverage of base station (BS) and improve the frequency reuse. The gain in capacity is expected but at the expense of increased interference, frequent handovers (HOs), increased HO failure (HOF) rates, increased HO delays, increase in ping pong rate, high energy consumption, increased overheads due to frequent HO, high packet losses and bad user experience mostly in high-speed user equipment (UE) scenarios. This paper presents the general concepts of radio access mobility in cellular networks with possible challenges and current research focus. In this article, we provide an overview of HO management in long-term evolution (LTE) and 5G new radio (NR) to highlight the main differences in basic HO scenarios. A detailed literature survey on radio access mobility in LTE, heterogeneous networks (HetNets) and NR is provided. In addition, this paper suggests HO management challenges and enhancing techniques with a discussion on the key points that need to be considered in formulating an efficient HO scheme.

We investigate the asymptotic behavior for an overlooked aspect of spectrum-sharing systems when the number of transmit antennas nt at the secondary transmitter (ST) grows to infinity. Considering imperfect channel state information (CSI), we apply the transmit antenna selection and the maximal-ratio combining techniques at the ST and the secondary receiver (SR), respectively. First, we obtain the signalto-noise ratio (SNR) distributions received by the SR under perfect and imperfect CSI conditions. Then we show that the SNR distributions are tail-equivalent in the sense that the right tails of the two distributions decay in the same rate as the number of transmit antennas nt grows to infinity. Based on the extreme value theory, when the transmit power of the ST is solely limited by the interference constraint, we show that the limiting SNR at the SR is Fréchet-distributed and the limiting rate scales as log(nt). When the transmit power of ST is determined by both the maximal transmit power and the interference power constraints, the limiting SNR is Gumbel-distributed and the limiting rate scales as log(log(nt)). We further show that the average rate can be estimated by the corresponding easier-to-obtain outage rate. Numerical results indicate that the derived asymptotic rate expressions represent accurate approximations even when nt is “not-solarge”. Finally, we study the robustness of the secondary transmissions by analyzing the corresponding average symbol error rates (SER) under general modulation and coding schemes. The findings indicate that the SER is Weibull distributed, when the maximal transmit power and interference power constraints are comparable.

One of the main challenges for wide-scale deployment and timely adoption of ultra-dense networks (UDNs) in future 5G is the backhaul. Typically, mmW technologies for backhaul require line-of-sight conditions while high-capacity wired-based solutions need a significant investment in infrastructure. Such limitations pose practical constrains on the scalability of UDNs and increase the deployment cost of dense networks. In this paper, we consider in-band backhaul for UDNs based on massive MIMO systems in sub-6 GHz. In particular, we propose a scheme for allowing simultaneous downlink transmissions in backhaul and access network on a single frequency band that exploits a novel combination of the state-of-the-art practical transmit and receive beamforming techniques. A novel frame structure for allowing a co-existence between massive MIMO-based backhaul and UDNs is also proposed. Moreover, a solution for in-band uplink transmissions that exploits time-division-duplex (TDD) and spatial multiple-access is also provided. Extensive numerical results using a realistic system-level simulator are given. Results show that the performance of a UDN with the proposed in-band backhaul scheme reaches ~58% of the throughput of a similar access network with ideal (e.g., wired) backhaul. Our results also show that the proposed scheme provides an increase in the throughput of ~30% compared with a TDD scheme for in-band backhaul. Further advantages of the proposed massive MIMO-based in-band backhaul scheme for UDNs include reusing both the (scarce) spectrum in sub-6 GHz and acquired macro-sites, thus providing a seamless transition from LTE to 5G networks.

A cloud radio access network (Cloud-RAN) is a new cellular technology that brings baseband processing units for a set of base stations into a central server retaining only the radio front-ends at the cell sites. This new architecture opens up opportunities for algorithms that require centralized processing. However, efficient implementation of the algorithms presents a number of challenges the most critical being latency, fronthaul capacity, and resource control. In this paper, we propose a software-defined radio-based architecture that addresses these problems and can be implemented on a cloud of general purpose computing platforms. We also present the practical implementation of Cloud-RAN running on an off-the-shelf server to validate the flexibility of the architecture. The implementation is able to realize various cellular networks, including heterogeneous networks, distribute-antenna systems, and transmission schemes, such as transmit antenna selection and open-loop transmit diversity.

Many operations, be they military, police, rescue, or other field operations, require localization services and online situation awareness to make them effective. Questions such as how many people are inside a building and their locations are essential. In this paper, an online localization and situation awareness system is presented, called Mobile Urban Situation Awareness System (MUSAS), for gathering and maintaining localization information, to form a common operational picture. The MUSAS provides multiple localization services, as well as visualization of other sensor data, in a common frame of reference. The information and common operational picture of the system is conveyed to all parties involved in the operation, the field team, and people in the command post. In this paper, a general system architecture for enabling localization based situation awareness is designed and the MUSAS system solution is presented. The developed subsystem components and forming of the common operational picture are summarized, and the future potential of the system for various scenarios is discussed. In the demonstration, the MUSAS is deployed to an unknown building, in an ad hoc fashion, to provide situation awareness in an urban indoor military operation.