Co-existence of terrestrial and non-terrestrial networks (NTN) is foreseen as an important component to fulfill the global coverage promised for sixth-generation (6G) of cellular networks. Due to ever-rising spectrum demand, using dedicated frequency bands for terrestrial network (TN) and NTN may not be feasible. As a result, certain S-band frequency bands allocated by radio regulations to NTN networks overlap with those already utilized by cellular TN, leading to significant performance degradation due to the potential co-channel interference. Early simulation-based studies on different co-existence scenarios failed to offer a comprehensive and insightful understanding of the overall performance of these networks. Besides, the complexity of a brute force performance evaluation increases exponentially with the number of nodes and their possible combinations in the network. In this paper, we utilize stochastic geometry to analytically derive the performance of TN-NTN integrated networks in terms of the probability of coverage and average achievable data rate for two co-existence scenarios. From the numerical results, it can be observed that, depending on the network parameters, TN and NTN users’ distributions, and traffic load, one co-existence case may outperform the other, resulting in the optimal performance of the integrated network. The analytical results presented herein pave the way for designing state-of-the-art methods for spectrum sharing between TN and NTN and optimizing the integrated network performance.

The sixth generation of wireless networks (6G) is expected to support the deployment of Internet of things (IoT) devices in massive scales. Finding lightweight and decentralized secret-key distribution primitives is therefore a challenge. Secret-key generation (SKG) from wireless channel coefficients is seen as a possible solution. It allows the extraction of secret keys using the channel randomness observed at the physical layer, without a centralized key distribution server. In this work an SKG approach suitable for wideband IoT devices is proposed. We investigate a filterbank-based SKG method, in which secret bits are generated through power measurements over different frequencies. To minimize dependencies and correlation among frequencies the quantile and the Karhunen-Loeve transforms are used. Finally, we perform a numerical evaluation of the achievable SKG rates, in the form of mutual-information (MI) estimates, using 3GPP channel models. Our numerical evaluation shows that the achievable SKG rate depends on the channel statistics, and, hence, to optimally harvest the information, devices need to be channel-aware.

Multi-connectivity can increase the reliability of wireless communications by orders of magnitude due to an increased diversity. However, simply copying packets and sending them on multiple links in parallel, i. e., as standardized as packet duplication, will result in a wasteful scheme. For example, there is no benefit in receiving the same information more often than once if multiple links succeed. This drawback can be resolved by channel coding. In a previous work, we proposed packet transmission schemes based on rateless codes, which could already increase resource efficiency compared to packet duplication. Here, we extend our previous studies in two different ways. First, we replace the previously used random linear fountain code by a more sophisticated raptor code, namely the R10 code. Second, we investigate a scenario, where the rateless coding scheme, which typically does not need any feedback, has acknowledgement information available. Both proposals show significant benefits of up to 15 % compared with our previous work in our mathematical analysis and empirical simulations.

The emergence of Internet of things (IoT) applications brings the challenge of finding lightweight schemes for secret key distribution. A promising solution coming from the physical layer is the so called secret key generation (SKG) from shared randomness. SKG allows two communicating parties to extract the shared randomness already present in wireless channels. This work investigates the achievable SKG rates using received signal strength (RSS) as a key generation parameter. A multi-path scenario is considered and the probability density function of the RSS in different channel conditions is evaluated. Next, through a numerical evaluation, the SKG rates are derived in the form of mutual information (MI) estimates, using a 3GPP standard channel model. The simulations are performed for different values of bandwidth and delay spread. We demonstrate that while both parameters have similar impact on how multi-path components are resolved, their effect on the MI is opposite.

In this paper, we consider different equalization techniques for chirp-based communications systems. Although chirps are more commonly used for radar systems, several studies show their potential for either low-rate communication or for high spectral efficiency using an overlapping technique. However, previous literature has dealt only with additive white Gaussian noise (AWGN) channels. In this paper, we investigate the overlapped chirps under multipath channel models. More specifically, we model the overlapped chirp-based communication system and then reformulate the receive signals to which common linear and non-linear equalizers are applicable. For the purpose of benchmarking, we demonstrate the effectiveness of those equalizers in a realistic 5G tapped delay line (TDL) channel model. Our simulation results show that, with proper equalization, overlapped chirps can also operate under multipath channels, and that non-linear equalization methods outperform linear ones.

In this work we explore the problem of physical layer security key exchange between two communicating devices in wideband wireless networks. We propose to obtain a pair of symmetric keys through a general filtering procedure, which exploits the frequency-selective characteristics of the radio channel. We focus our analysis on the keys bits mismatch with respect to the signal-to-noise-ratio and the link distance, through simulations of realistic radio channels in both indoor and outdoor scenarios. We investigated the impact of a proximity-attack by a malicious third device, founding that it is capable to eavesdrop large parts of the generated key compromising the overall security.

Wireless closed-loop control is of major significance for future industrial manufacturing. However, control applications pose stringent quality of service requirements for reliable operation. Contrary to traditional ultra-reliable low-latency communications design goals such as low packet loss rates and low latency, research results in the domain of networked control systems (NCS) state that depending on the sampling period, control applications inherently tolerate a few consecutive packet losses. This translates into a better-suited metric to capture control application requirements and therefore a more conclusive design goal for wireless networks: ensuring a maximum age of information (AoI). With a Markov modeling approach, we propose to exploit the tolerance through a novel dynamic multi-connectivity scheme that we term state-aware resource allocation (SARA), which temporally negatively correlates packet losses, thus avoiding long packet loss sequences. Through statistical multiplexing, SARA enables a mean time to failure (MTTF) in the order of years while keeping the per-agent average channel usage close to one, also in a multi-agent setting with competition for resources. Compared with static dual-connectivity, the MTTF can be increased 100-fold whereas the number of required channels reduces by 40%. Our approach also statistically guarantees system-wide AoI distributions, which aid to ensure control performance.

Due to the great flexibility and innate simplicity of wireless networks, many applications are now realized with wireless connectivity at their core. Countless novel applications are enabled this way and legacy applications are transferred to mobile networks to reap the same benefits. As the networks evolve, applications are becoming ever more demanding, particularly with respect to high reliability and low latency. A common way to achieve high reliability in wireless communications is adding redundant communication channels by means of multi-connectivity. This comes at the cost of additional radio resources that could have been used by other connections. However, multiple links are not always required to achieve the desired reliability. Thus, we propose to combine multi-connectivity with rateless coding as a strategy for efficient resource usage in reliable wireless communications. The proposed schemes consider an erasure channel, and can therefore be implemented on the application layer, making them suitable for use with different physical layer wireless technologies simultaneously. We start out with a single-user analysis and then extend our discussion to multi-user scenarios. Our approach is compared against standardized packet duplication with our analytical framework and by means of simulation. The results with rateless coding show significant gains with respect to the required resources and feedback transmissions.

Wireless closed-loop control is of major significance for different application areas, such as future industrial manufacturing, and ultra-reliable low-latency communications (URLLC) are designed to enable such systems. Static multi-connectivity, in which a number of independent parallel channels are allocated for each service, is a possible solution to achieve URLLC requirements, but this increases resource usage significantly, which becomes an issue particularly in multi-user systems. Building upon a control-communications codesign (CoCoCo) approach, a control-application optimized state-aware resource allocation (SARA) scheme was developed, which exploits the control cycle's inherent capability of tolerating a limited number of consecutive packet losses before ultimately failing. In essence, SARA negatively correlates packet losses through dynamic channel allocation in order to yield extraordinary availability values while keeping the average resource consumption low. This article develops a multi-user system representation of SARA with competition for limited resources using a Markov chain approach and subsequently evaluates the mean time to failure, demonstrating that SARA scales better than static multi-connectivity, fully supporting the maximum system availability at fewer channels per agent.

Control communications co-design enables robust and scalable wireless closed-loop control system design. We study the metric “control-communications availability” that allows consecutive errors until the control application is deemed dysfunctional. Multi-connectivity helps increasing the network availability, but there is a lack of dynamic and resource efficient link management. Thus, we propose the “state-aware resource allocation” scheme, whereby parallel links can be assigned adaptively to a given connection, depending on the number of previously, consecutively lost packets. We develop a Markov chain that captures the novel resource allocation approach suited for closed-loop wireless control applications. Our approach outperforms static dual connectivity by two orders of magnitude in terms of control-communications availability while reducing the amount of required resources to approximately half.

In future 5G systems, it is envisioned that the physical resources of a single network will be dynamically shared between the virtual end-to-end networks called “slices” and the network is “sliced”. The dynamic sharing of resources can bring about pooling gains, but different slices can easily influence each other. Focusing on slicing the radio access network, a slice management entity is required to steer the radio resource management (RRM) so that all of the slices are satisfied and negative inter-slice influences are minimized. The steering of RRM can be done by adjusting slice-specific control parameters in scheduler and admission controller mechanisms. We use a model-free reinforcement learning (RL) framework and train an agent as a slice manager. Simulation results show that such agents are capable of relatively quickly learning how to steer the RRM. Furthermore, a hybrid method of Jacobian-matrix approximation with RL approach has been devised and shown to be a practical and efficient solution.

Millimeter-wave has become an integral part of 5G networks to meet the ever-increasing demand for user data throughput. Employing higher carrier frequencies introduces new challenges for the propagation channel such as higher path loss and rapid signal degradations. On the other hand, higher frequencies allow deployment of small-sized antenna elements that enable beamforming. To investigate user mobility under these new propagation conditions, a proper model is needed that captures spatial and temporal characteristics of the channel in beamformed networks. Current channel models that have been developed for 5G networks are computationally inefficient and lead to infeasible simulation time for most user mobility simulations. In this paper, we present a simplified channel model that captures the spatial and temporal characteristics of the 5G propagation channel and runs in feasible simulation time. To this end, coherence time and path diversity originating from fully fledged Geometry based Stochastic Channel Model (GSCM) are analyzed and adopted in Jake's channel model. Furthermore, the deviation of multipath beamforming gain from single ray beamforming gain is analyzed and a regression curve is obtained to be used in the system-level simulations. We show through simulations that the proposed simplified channel model leads to mobility results comparable to Jake's model for high path diversity. Moreover, the multi-path beamforming gain increases the interference in the system and in turn number of mobility failures.

Increased usage of wireless technologies in unlicensed frequency bands inevitably increases the co-channel interference. Hence, for applications such as ultra-reliable-low-latency-communications (URLLC) in factory automation, the interference should be avoided. An intelligent coexistence management entity, which dynamically distributes the time and frequency resources, has been shown to be greatly beneficial in boosting efficiency and avoiding crippling interruptions of the wireless medium. This entity also supports multi-connectivity schemes, which are crucial for industry-level reliability requirements. The proposed governing technique of the coexistence management is a deep reinforcement learning (DRL) method, which is a model-free framework and channel allocation decisions are learned merely by interactions with the environment. The simulation results have shown that the employed method can greatly increase the reliability of the wireless network, when compared with legacy methods.

Achieving extremely high reliability is one of the key targets in the development of fifth generation mobile networks. To meet this ambitious aim, usually redundancy is introduced by simultaneously utilizing multiple links that are separated in frequency and/or space. However, current standards simply duplicate packets on these links, resulting in an inefficient high usage of resources that could have been used for other applications. To address this problem, rateless coding using multiple links is proposed for ultra-reliable communications, mathematically modeled, and evaluated in this paper. The benefits comprise efficient resource usage and a simplified feedback mechanism. Finally, they can be implemented in a technology-agnostic manner on application layer to easily exploit interface diversity.

In the context of 5G systems, the emergence of various use cases with diverse requirements has attracted great attention to network slicing. In a single physical network, several instances of logical end-to-end networks, i.e. slices, will be instantiated to fulfill these requirements. To this end, slices should share the resources of the physical network, which consist of Core Network (CN) and Radio Access Network (RAN) resources. Herein, we focus on the Radio Resource Management (RRM) in the context of network slicing. To maximize the pooling gains, dynamic resource sharing is preferred over static sharing. However, dynamic resource sharing can lead to undesirable inter-slice influences, in particular, the contention on radio resources. In this article, we show that, although the radio resources are dynamically shared among the users of different slices, proper slice management can realize slice protection. This is achieved by adjusting the fraction of radio resources allocated to the different slices by the Packet Scheduler (PS) and by limiting the number of users admitted to the network via Admission Control (AC). We propose an iterative algorithm to optimize the parameters of PS and AC in order to ensure that the service level agreements are satisfied. Extensive system-level simulations have shown that a central entity that tunes these control parameters can greatly increase the network's performance.

To study critical 5G mobility features like zero-millisecond interruption time and multi-connectivity, an average downlink Signal-to-Interference and Noise Ratio (SINR) is needed for radio link failure detection and throughput calculation. This paper presents an accurate approximation of the average downlink SINR with low computational complexity in 5G networks where the base station forms multiple beams simultaneously. To this end, geometry-based link budget is formulated first for both desired and interfering downlink signals. Then, a closed form expression of the average downlink SINR is derived for multi-beam scheduling system and approximated by Monte-Carlo experiment using beam scheduling probabilities. In addition, the SINR model is derived for both strict and opportunistic resource fair scheduler where the latter targets a higher utilization of radio resources. Results have revealed that with increasing number of scheduled beams, the average downlink SINR generally degrades while the network throughput improves. Moreover, it has been also shown that the opportunistic resource fair scheduler performs better than the strict in terms of utilizing available network resources.

Latency is the one of the critical performance metrics for 5G and beyond mobile networks, particularly for ultra-reliable and low-latency communications (URLLC). In URLLC applications, it is required that the transmitted packets reach the destination within a certain time and the packets that are unable to meet this strict latency requirement will be discarded. In this paper, we compute the waiting time in the packetized fronthaul at the Ethernet switch and compute packet loss rate (PLR) incurred due to the inability of the transmitted packets to meet the FH latency threshold. In addition, we derive the tractable closed-form solution for the waiting time distribution and verify it with simulation results. Our results show that PLR is affected mainly by packet size, spectral efficiency, switch speed and arrival rate.

High-Altitude Platform Systems (HAPS) are being studied as a viable, easy to deploy and to maintain technology to provide isolated areas with connectivity. They can be integrated to existing cellular networks and have a wide coverage area, presenting a lower cost than satellite systems. The deployment of HAPS, however, demands that country administrations allocate and release spectrum for the system. That is why ITU is studying the possibility of identifying the 24.25- 27.5 GHz and 38-39.5GHz bands for HAPS. Even though these bands are already allocated to the Fixed-Satellite Service (FSS), the bands could be shared, as long as the HAPS stations do not cause harmful interference to the existing and future FSS stations. This paper presents a sharing study between these two systems, to assess the amount of interference the deployment of the HAPS in these bands would cause to the FSS. We conclude that, with some basic coordination, the two systems can share the same frequency band without damage to the FSS.

With the emergence of cloud radio access network (C-RAN) architecture, latency in fronthaul (FH) network is a critical performance metric especially for ultra-reliable and low-latency communication applications. The stringent FH capacity and latency requirements of C-RAN can be relaxed by offloading some baseband functionalities to remote radio unit (RRU), referred to as functional splitting. This allows packetized FH network solutions such as ubiquitous Ethernet. In this paper, we calculate the FH latency in the uplink of a C-RAN system with massive MIMO-based RRUs and 3GPP functional Split 7, wherein MIMO equalization is done at the RRU. We derive tractable, closed-form expressions for the steady-state probabilities of queue length and sojourn time distribution at the output port of an Ethernet switch in the FH network. We first present these results for Poisson file arrivals from users in the network and exponential file size distribution. We then extend the results to general file size distribution. The numerical results show that the file size and spectral efficiency of the users are critical in determining the FH latency. Further, results show that switch speed can be decreased without incurring significant increase in FH latency revealing the possibility for statistical multiplexing gains.

This work presents an investigation on the use of the IEEE 1902.1 (RuBee) protocol in order to communicate with buried assets. This is an important requirement in some applications, particularly in the mining industry. RuBee is a low-rate protocol, but its ability to transmit through solid material makes it the first choice for this kind of application. By means of an event-driven simulation, we evaluate the performance of RuBee in different scenarios, in which several buried tags attempt to communicate with a surface reader.

This article describes image transmission in Through-The-Earth Communications (TTE), which are characterized by non-Gaussian impulsive atmospheric noise. We analyze the transmission of compressed and uncompressed image files. We verify the possibility of transmitting uncompressed analog images, which can be restored even with signal noise ratio (SNR) equal to −10 dB, and, based on the Structural Similarity (SSIM) metric, we verify the existence of an SNR threshold, at which we should switch between compressed and uncompressed images.

PAM-DMT is a transmission technique for IM/DD channels that can be effectively employed without a DC bias, by using only the quadrature component of QAM constellations. In this contribution, and based on an algorithm previously employed for AC-DMT [8], we show that the non-linear distortion that falls onto the in-phase component of the received signal also contains useful information, and can be adequately combined with the quadrature component to improve the detection performance.

Iterative correction can be employed to mitigate the distortion caused by nonlinear power amplifiers in OFDM systems. However, proposals found in the literature suppose that the receiver has perfect knowledge of the nonlinear conditions encountered in transmission. In this paper, we propose the use of the Extended Kalman Filter and the Unscented Kalman Filter to estimate the nonlinear characteristics of the amplifier before performing the iterative correction, and see that by using these filters, the amplifier clipping level can be correctly estimated even without a pilot signal.

A receiver for a MIMO OFDM communication system using direct conversion transceivers is proposed. The novel receiver takes advantage of the I/Q imbalance inherent to the direct conversion operation. It can be shown that the I/Q imbalance in multicarrier communication systems creates an additional frequency diversity. By adequately exploiting such diversity at the receiver, the overall performance can be considerably improved. Simulation results considering a MIMO OFDM system using V-BLAST show that, when exploiting the I/Q imbalance, gains of up to 5 dB at a BER of 10-4can be obtained with respect to the ideal system, without I/Q imbalance.

In order to accommodate the growing traffic demand in GSM cellular networks, several features are continuously being developed to improve the voice capacity of GSM/EDGE radio access network (GERAN). In this paper we investigate the capacity of a GSM network considering the use of some of these features, namely, interference rejection combining (IRC), single-antenna interference cancellation (SAIC), source-adaptive AMR (SA-AMR), dynamic frequency and channel allocation (DFCA), and 8-PSK voice channels. These techniques have been previously investigated individually in the literature, but here we focus on the system-level impact of the interactions among them. We show that the proper combination of these features increases the spectral efficiency of speech channels in GERAN.

Adaptive multi rate (AMR) is a variable-rate speech codec, which was introduced in GSM specifications to improve speech quality and transmission robustness. Based on existing AMR codecs, source-adaptive AMR is a new proposal to improve system capacity in GSM and WCDMA networks. In GSM, by decreasing the codec bit rate we can reach a more robust transmission, due to the increased redundancy available. Because some speech periods have low entropy, they can be encoded with lower bit rate with no noticeable loss in speech quality. This implies a robustness gain, which can be used to increase system capacity. In this paper we investigate the potential capacity increase that can be obtained in GSM networks using this technique

We present a simple but powerful dynamic link adaptation mechanism for wireless LANs, which are based on the IEEE 802.11a standard. Using this method, a transmitter is able to detect whether the quality of the link is improving or deteriorating, and based on this information to switch to a higher or lower transmission rate, respectively. For determining the link quality, the transmitter employs only information that is available locally. Therefore, the proposed method can be implemented without changes or enhancements to the current IEEE 802.11 standard. The efficiency of our method is investigated and evaluated by means of simulations.

The downlink capacity tends to be the limiting factor in future communications systems. We propose two schemes for the downlink of code-division multiple-access systems that help reduce the multiuser interference by jointly precoding the transmitted signal based on knowledge of the downlink channel. Methods that operate on blocks of bits are first developed and a bitwise simplification of these for systems with large processing gains are derived. The use of multiple transmit antennas are also considered. These methods provide a substantial capacity increase without requiring complex multiuser detectors at the mobile terminals, and can be applied for instance to time-division duplex systems, which have reciprocal channels.

We investigate the use of adaptive bit loading in existing OFDM-based W-LAN standards. The performance gain is assessed through simulation and the use of higher-rate codes is considered to improve the performance. Some practical aspects, such as peak-to-average power ratio, channel estimation, signalling and spectral flatness are also addressed.

We investigate several transmit-diversity techniques for wireless OFDM systems employing multiple antennas. We concentrate our investigation on methods that can be applied with few modifications to existing standards. Existing methods such as delay diversity, clustering and antenna interleaving are investigated and compared. A new approach that yields better results, consisting in applying different phase shifts on each subcarrier and on each antenna is also presented. Some practical implementation issues are also addressed.

The high peak-to-average power ratio of multicarrier signals is one of the greatest difficulties in the implementation of OFDM for wireless LANs. We investigate how we can deal with this problem without major modifications in existing standards. We analyse several methods previously found in the literature, but also propose a new approach consisting of applying different phase shifts to the subcarriers, based on a packetwise minimisation of the clipping distortion.

In this paper we propose two precoding schemes for the downlink of CDMA systems which help reduce the multiuser interference by jointly precoding the transmitted signal based on knowledge of the downlink channel. These methods provide a substantial capacity increase without requiring complex multiuser detectors at the mobile terminals and can be applied for instance to TDD systems, which have symmetrical channels.

Mobile communications is characterized by a fast growing number of users and an increasingly heterogeneous data traffic due to emerging multimedia packet services. Likewise, users desire more mobility support, thus demanding permanent availability at any place and any speed. The conventional answer to this problem is building new wireless standards. But these standards require totally new hardware, users cannot use their old terminals, and no wireless communication among networks is possible which considerably constrains the availability demand. Instead, we propose a unified way for basic network access and connectivity maintenance. It is embodied by a perpetually available signalling channel-the network access and connectivity channel (NACCH) and a common functionality trunk, shared by several networks. With this concept, networks can communicate and, hence, form a mega-network where users have universal access and roaming support.

We present an interference analysis which reveals capacity reserves for a hierarchical GSM radio network. In the investigated concept a low-power cordless telephony system reuses the frequencies of a cellular GSM system based on total frequency hopping (T-FH) multiple access. Based on a novel simulation method, which considers the dynamics of a T-FH cellular network, we present results for different cordless user densities and numbers of T-FH frequencies which exhibit the benefits of our CTS concept. Moreover, we show two possible ways to further improve the CTS downlink performance in case of difficult scenarios.