In view of the need to find novel means to utilize the unlicensed spectrum to meet the rising latency and reliability requirements of new applications, we propose a novel mechanism that allows devices to transmit anytime that a packet has to be delivered. The proposed mechanism, Contention-free with Power Adaptation (ConPA), aims to bypass the contention periods of current Listen-Before-Talk (LBT) approaches, which are the main source of unreliability in unlicensed technologies like Wi-Fi. To assess the feasibility of ConPA, we provide an analytical method based on Markov chains, which allows deriving relevant performance metrics, including throughput, airtime, and quality of transmissions. Using such a model, we study the performance of ConPA in various scenarios, and compare it to baseline channel access approaches like the Distributed Coordination Function (DCF) and the IEEE 802.11ax Overlapping Basic Service Set (OBSS) Packet Detect (PD)-based Spatial Reuse (SR). Our results prove the effectiveness of ConPA in reusing the space to offer substantial throughput gains with respect to the baselines (up to 76% improvement).

The concept of sub-networks has been recently identified as an important component of 6G to enable high-demanding critical services with capabilities of hyper reliable low-latency communications (HRLLC) at the local area of the sub-network. In this paper, we formulate the power control problem for sub-networks as finding the transmit power vector, corresponding to the sub-networks, to minimize the sum interference-to-signal power ratios across all the sub-networks. We propose then a heuristic sequential iterative power allocation (SIPA) and an optimal power allocation based on gradient descent-based algorithm (GDPA). Extensive system simulations validated the proposed methods. In particular, the SIPA achieves significant performance gains over the fixed transmit power setting, with limited performance loss over the GDPA. In most cases, the proposed methods outperform the max-min power allocation with much lower feedback signaling overhead and complexity. Furthermore, to keep 99.9% of all the sub-network link instances to achieve reliability of 6 nines, the SIPA can save radio resources by about 27% compared to the fixed transmit power setting.

In this paper, we firstly analyze potential use cases and new requirements related to ultra-reliable and low-latency communications (URLLC) in 6G era. With the objective of showing feasibility of round-trip latency less than 100 µs, we propose a system design that introduces novel features in comparison to 5G design such as larger SCS, new frame structure and paired scheduling, and create a proof-of-concept (PoC). A verification test in a real indoor scenario shows that our system can achieve 78 μs round-trip latency and higher reliability than 5G under static environment. Furthermore, we describe challenges, potential solutions and research directions for 6G hyper reliable and low-latency communication (HRLLC).

The concept of in-X subnetworks has been recently proposed to meet extreme communication requirements such as sub-millisecond latency and up to 9 nines reliability in 6thgeneration (6G) networks. On the other hand, many open challenges have already been recognized for this new concept, from air interface design to interference management in dense and dynamic scenarios. In this paper, we focus on subnetworks for industrial wireless control applications and propose an advanced frequency resource allocation scheme, denoted as sequential iterative subband allocation (SISA), which is designed to minimize the sum interference-to-signal ratio over all subnetwork links. Through extensive system level simulations, we evaluate the benefits of the proposed SISA scheme and compare it with the state-of-the-art. Numerical results show that SISA with interference weighting strongly outperforms a greedy distributed scheme, by reducing by half the frequency resources needed to enable 99.9% of all the subnetwork link instances to achieve reliability of 6 nines.

This paper studies communications service availability for industrial wireless control systems. We consider a motion controller with a continuous closed-loop control link to a group of actuator devices on a factory floor. The goal is to satisfy end-to-end latency for each packet and to guarantee that the communication service will not be un-available for longer than a survival time. We propose to decouple the scheduling operation between the normal and survival modes of operation, enabling a dual-mode ultra-reliable and low-latency communications (URLLC) scheduler. Scheduler strategies for the survival mode are presented, targeting link adaptation and signal to interference and noise ratio (SINR) estimation in presence of temporal and spatial channel correlation. Through numerical examples, we investigate the impact of channel correlation on the schedulers ability to target the required reliability for each mode. We further present our findings on system-level performance evaluation of such scheduling strategies by adopting a realistic system setup and channel model to obtain insights with high level of realism. Extensive simulation results are presented which demonstrate significant reduction in resource utilization with the proposed dual-mode scheduler when compared to single-mode URLLC scheduling. Specifically, our results demonstrate that the scheduler should target moderate packet error rate (PER) for normal mode of operation and very low PER for the survival mode; the latter guarantees service availability while the former saves radio resources.

In the framework of 5G-and-beyond Industry 4.0, jamming attacks for denial of service are a rising threat which can severely compromise the system performance. Therefore, in this paper we deal with the problem of jamming detection and mitigation in indoor factory deployments. We design two jamming detectors based on pseudo-random blanking of subcarriers with orthogonal frequency division multiplexing and consider jamming mitigation with frequency hopping and random scheduling of the user equipments. We then evaluate the performance of the system in terms of achievable block error rate (BLER) with ultra-reliable low-latency communications traffic and jamming missed detection probability. Simulations are performed considering a 3rd Generation Partnership Project spatial channel model for the factory floor with a jammer stationed outside the plant trying to disrupt the communication inside the factory. Numerical results show that jamming resiliency increases when using a distributed access point deployment and exploiting channel correlation among antennas for jamming detection, while frequency hopping is helpful in jamming mitigation only for strict BLER requirements.

Traditional link adaptation (LA) schemes in cellular network must be revised for networks beyond the fifth generation (b5G), to guarantee the strict latency and reliability requirements advocated by ultra reliable low latency communications (URLLC). In particular, a poor error rate prediction potentially increases retransmissions, which in turn increase latency and reduce reliability. In this paper, we present an interference prediction method to enhance LA for URLLC. To develop our prediction method, we propose a kernel based probability density estimation algorithm, and provide an in depth analysis of its statistical performance. We also provide a low complexity version, suitable for practical scenarios. The proposed scheme is compared with state-of-the-art LA solutions over fully compliant 3rd generation partnership project (3GPP) calibrated channels, showing the validity of our proposal.

Security attacks at the physical layer, in the form of radio jamming for denial of service, are an increasing threat in the Industry 4.0 scenarios. In this paper, we consider the problem of jamming detection in 5G-and-beyond communication systems and propose a defense mechanism based on pseudo-random blanking of subcarriers with orthogonal frequency division multiplexing (OFDM). We then design a detector by applying the generalized likelihood ratio test (GLRT) on those subcarriers. We finally evaluate the performance of the proposed technique against a smart jammer, which is pursuing one of the following objectives: maximize stealthiness, minimize spectral efficiency (SE) with mobile broadband (MBB) type of traffic, and maximize block error rate (BLER) with ultra-reliable low-latency communications (URLLC). Numerical results show that a smart jammer a) needs to compromise between missed detection (MD) probability and SE reduction with MBB and b) can achieve low detectability and high system performance degradation with URLLC only if it has sufficiently high power.

The strict latency and reliability requirements of ultra-reliable low-latency communications (URLLC) use cases are among the main drivers in fifth generation (5G) network design. Link adaptation (LA) is considered to be one of the bottlenecks to realize URLLC. In this paper, we focus on predicting the signal to interference plus noise ratio at the user to enhance the LA. Motivated by the fact that most of the URLLC use cases with most extreme latency and reliability requirements are characterized by semi-deterministic traffic, we propose to exploit the time correlation of the interference to compute useful statistics needed to predict the interference power in the next transmission. This prediction is exploited in the LA context to maximize the spectral efficiency while guaranteeing reliability at an arbitrary level. Numerical results are compared with state of the art interference prediction techniques for LA. We show that exploiting time correlation of the interference is an important enabler of URLLC.

Factory automation is one of the most challenging use cases for 5G-and-beyond mobile networks due to strict latency, availability and reliability constraints. In this work, an indoor factory scenario is considered, and distributed multiple- input multiple-output (MIMO) schemes are investigated in order to enable reliable communication to the actuators (ACs) active in the factory. Different levels of coordination among the access points serving the ACs and several beamforming schemes are considered and analyzed. To enforce system reliability, a max-min power allocation (MPA) algorithm is proposed, aimed at improving the signal to interference plus noise ratio (SINR) of the ACs with the worst channel conditions. Extensive system simulations are performed in a realistic scenario, which includes a new path-loss model based on recent measurements in factory scenarios, and, also, the presence of non-Gaussian impulsive noise. Numerical results show that distributed MIMO schemes with zero-forcing (ZF) beamforming and MPA have the potential of providing SINR gains in the order of tens of dB with respect to a centralized MIMO deployment, as well as that the impulsive noise can strongly degrade the system performance and thus requires specific detection and mitigation techniques.

Cell-free massive multiple-input multiple-output (MIMO) is a promising cellular technology in which a large number of distributed access points (APs) jointly serve a small number of user equipments (UEs). In this work, we investigate the impact of the spatial distribution of APs on the performance of a cell-free massive MIMO system, considering different downlink power control policies. Further, we analyze the performance for the case where only subsets of all APs serve the individual UEs: this scenario has lower backhaul requirements and associated CAPEX/OPEX costs. In this framework, we first propose a novel, tractable approximation for the average spectral efficiency (SE) of the transmission to a UE conditioned on the estimated channel gains. This approximation is then used to develop different downlink power control policies. Further, we extend the policies to the scenario when power control is coordinated only among subsets of APs and not across the subsets. Through extensive system-level simulations, we evaluate the improvement in SE by spreading out the APs and the SE loss when a subset of all APs serve a UE and coordination across subsets is absent.

A key aspect of the fifth-generation wireless communication network will be the integration of different services and technologies to provide seamless connectivity. In this paper, we consider using massive multiple-input multiple-output (mMIMO) to provide backhaul links to a dense deployment of self-backhauling (s-BH) small cells (SCs) that provide cellular access within the same spectrum resources of the backhaul. Through a comprehensive system-level simulation study, we evaluate the interplay between access and backhaul and the resulting end-to-end user rates. Moreover, we analyze the impact of different SCs deployment strategies, while varying the time resource allocation between radio access and backhaul links. We finally compare the above mMIMO-based s-BH approach to a mMIMO direct access (DA) architecture accounting for the effects of pilot reuse schemes, together with their associated overhead and contamination mitigation effects. The results show that dense SCs deployments supported by mMIMO s-BH provide significant rate improvements for cell-edge users (UEs) in ultra-dense deployments with respect to mMIMO DA, while the latter outperforms mMIMO s-BH from the median UEs' standpoint.

Providing consistently high wireless capacity is becoming increasingly important to support the applications required by future digital enterprises. In this paper, we propose Eigen-direction-aware ZF (EDA-ZF) with partial coordination among base stations (BSs) and distributed interference suppression as a practical approach to achieve this objective. We compare our solution with Zero Forcing (ZF), entailing neither BS coordination or inter-cell interference mitigation, and Network MIMO (NeMIMO), where full BS coordination enables centralized inter-cell interference management. We also evaluate the performance of said schemes for three sub-6 GHz deployments with varying BS densities — sparse, intermediate, and dense — all with fixed total number of antennas and radiated power. Extensive simulations show that: (i) indoor massive MIMO implementing the proposed EDA-ZF provides uniformly good rates for all users; (ii) indoor network densification is detrimental unless full coordination is implemented; (iii) deploying NeMIMO pays off under strong outdoor interference, especially for cell-edge users.

To guarantee the success of massive multiple-input multiple-output (MIMO), one of the main challenges to solve is the efficient management of pilot contamination. Allocation of fully orthogonal pilot sequences across the network would provide a solution to the problem, but the associated overhead would make this approach infeasible in practical systems. Ongoing fifth-generation (5G) standardisation activities are debating the amount of resources to be dedicated to the transmission of pilot sequences, focussing on uplink sounding reference signals (UL SRSs) design. In this paper, we evaluate the performance of various UL SRS allocation strategies in practical deployments, shedding light on their strengths and weaknesses. Furthermore, we introduce a novel UL SRS fractional reuse (FR) scheme, denoted neighbour-aware (FR-NA). The proposed FR-NA generalizes the fixed reuse paradigm, and entails a trade-off between i) aggressively sharing some UL SRS resources, and ii) protecting other UL SRS resources with the aim of relieving neighbouring BSs from pilot contamination. Said features result in a cell throughput improvement over both fixed reuse and state-of-the-art FR based on a cell-centric perspective.

Among the many proposals to meet the fifth generation (5G) data rate demands, current spectrum scarcity has motivated the use of millimeter-Wave (mmW) bands for cellular communication. It is well known that mmW communication involves many propagation challenges, which can be compensated through massive multi-antenna techniques. However, implementing fully digital precoding schemes with massive arrays entails huge complexity and costs, which have recently boosted the interest for hybrid beamforming solutions. Although hybrid schemes have shown good performance in simplified setups, their suitability for realistic cellular systems with many interfering base stations and users is still unclear. In this sense, this paper assesses the performance of hybrid beamforming in a dense urban cellular system with a realistic mmW channel model and shows that it can reach the performance of fully digital maximum ratio transmission under line of sight conditions and with a sufficient number of parallel radio-frequency chains. Another important result is that multi-user hybrid beamforming provides a substantial capacity increase with respect to its single-user (SU) counterpart. This finding complements the views on mmW communication usage, which has been so far mainly intended for SU communication to provide high beamforming gains.

The traffic profile of future mobile networks is foreseen as becoming more variable both in time and among base stations (BSs) due to the widespread heterogeneity in applications and services. Dynamic time division duplex (TDD) has been recognized as an important enabler to cope with this traffic variability, especially in dense networks with many BSs and a small number of user equipments (UEs) served by each BS. In this context, one of the main issues is to develop an incisive scheme to manage the BS-to-BS and UE-to- UE interference that arises. In this work, we propose a novel long-term BS clustering scheme that groups BSs that have a similar traffic profile and would be characterized, without clustering, by strong BS-to-BS interference. Due to the complexity of the optimal solution, we propose a heuristic algorithm which solves the optimization problem efficiently. Numerical evaluations in a dense homogeneous pico BS network show that the proposed scheme strongly outperforms even the baseline dynamic TDD without clustering by ensuring a reduction in packet delay of up to 55% and a gain in the UE packet throughput of about 75%.

The air interface for millimeter wave (mmWave) communications must be designed by properly taking into account the specific characteristics of the wireless channel at higher frequencies. In this work, we start by considering a channel model recently proposed in the literature for mmWave communications in outdoor urban scenarios. First, on top of this channel model we implement a sectorized beamforming model necessary to compensate the large path-loss at mmWave range and study how channel statistics, namely, delay spread and angle spread, are influenced by employing different beamwidths. Subsequently, adopting this beamforming model in the mmWave channel, orthogonal frequency division multiplexing (OFDM) and single carrier frequency domain equalization (SC-FDE) systems are compared. Extensive link level simulations are performed by considering different beamwidths, line-of-sight (LOS) coverage and channel coding. Numerical results show that SC-FDE using minimum mean square error (MMSE) equalization performs close to OFDM in coded systems. However, SC-FDE might be beneficial in practice due to much lower peak to average power ratio (PAPR) than OFDM.

The usage of the millimeter wave (MMW) band in the 5th generation (5G) networks relies on beamforming to compensate the strong path-loss suffered at higher frequencies. To exploit the beamforming implemented by multiple antenna devices, proper algorithms to estimate the channel need to be designed. In this work we propose a novel channel estimation method for MMW systems where both transmitter and receiver are equipped with fewer radio frequency chains than antennas and implement hybrid analog-digital beamforming. First, we define a training sequence which includes a set of analog and digital beamformers to probe the channel. Then, we develop an algorithm which estimates the channel parameters directly rather than the multiantenna channel matrix and it is based on the two dimensional (2D) discrete Fourier transform (DFT) of the received training samples. Numerical results show the effectiveness of the proposed channel estimation method even in low signal to noise ratio (SNR) conditions.

One of the key enablers for 5th Generation (5G) networks is the on-demand overlay of co-channel pico cells on homogeneous macro layer. However, the problem of inter-cell interference (ICI) is very challenging in co-channel heterogeneous deployments. Therefore, we propose a combination of enhanced inter cell interference coordination (eICIC) and coordinated beamforming (CB) as an evolutionary step to approach the 5G performance requirements. We refer to this step as enhanced coordinated beamforming (eCB). With the help of system level performance assessment for large cellular networks, we show that eCB achieves higher spectral efficiency than eICIC. We propose different clustering approaches which can be applied to achieve a given target metric. Moreover, we evaluate the impact of the pico-offset value, the macro resource coordination factor and the number of pico cells on the performance of a heterogeneous deployment.

For a cellular system where mobile terminals transmit in the uplink to base stations (BSs) using single carrier-frequency division multiple access (SC-FDMA), we consider multicell processing among BSs. Received signals are first quantized on a per-subcarrier basis and then forwarded to the serving BS on a backhaul with limited rate. With the aim of maximizing the network throughput we a) design an efficient composite signal representation and b) propose a rate allocation algorithm for the backhaul. Using a closed-form expression of the achievable throughput in the presence of quantization noise, an iterative greedy algorithm for the backhaul rate allocation is developed, where at each iteration we select the signal to be exchanged as the one providing the maximum network throughput increase per backhaul bit. In order to determine how many quantization bits are used for each received signal, we consider either a static bit allocation with a fixed number of bits, or a dynamic bit allocation (which ensures a predetermined network percentage throughput loss with respect to the unquantized case). In an LTE scenario, it is seen that the proposed bit allocation methods flexibly adapt to channel and backhaul conditions and yield similar performance, hence the static approach is preferred due to its lower complexity.

In cellular networks coordination among base stations (BSs) has been recognized as an important solution to handle inter-cell interference and increase spectral efficiency. In frequency division duplex systems one of the main issue that sensibly degrades the performance of coordinated multipoint (CoMP) transmission techniques is the imperfect channel state information (CSI) due to the limited bandwidth available for the feedback transmission. In this paper we focus on a CoMP scenario with data and CSI sharing among the BSs and we consider a feedback transmission scheme where each user equipment (UE) quantizes the different channels by using codebooks designed for a single-cell scenario. Due to the different propagation characteristics of the channels between a UE and each BS and by considering a constraint on the number of available feedback bits, we propose two practical algorithms depending on the large-scale fading to a) select the subset of BSs from whom the UE prefers to be served and b) optimize the number of feedback bits allocated to each channel. The developed techniques allow a UE to send more feedback bits to the BSs with a stronger signal and numerical results show the merits of the proposed approach.

In this paper we consider a downlink multi-cell scenario where a central processor is connected by a finite-throughput backhaul to base stations (BSs) employing beamforming and QAM constellations. Both the serving BS and auxiliary BSs transmit a signal that combines at the mobile terminal (MT) to provide a QAM symbol while achieving a diversity gain. In order to reduce backhaul occupation, auxiliary BSs receive from the central processor only a quantized version of the QAM symbols to be forwarded to MTs. We formalize the problem of maximizing the network spectral efficiency on air for all the MTs within an area illuminated by the cooperative BSs optimizing a) the QAM constellation size, b) the quantization rate dictated by the finite-throughput backhaul and c) the power allocated by each BS. Since the resulting optimization is a mixed integer programming problem, we investigate a suboptimal solution where the same power is allocated to each MT and obtain a simple iterative algorithm for the rate optimization. Numerical results show that for typical cellular scenarios the suboptimal approach yields a network spectral efficiency close to the theoretical limit of Slepian-Wolf encoding.

In a wide band and multipath rich environment, precise channel estimation allows authenticating the source and protecting the integrity of a message at the physical layer without the need of a pre-shared secret key. This allows also a reduction of the burden on the authentication protocols at higher layers. In this paper we develop an authentication scheme in the framework of hypothesis testing that suits a multiple wiretap channels environment with correlated fading, as is the case of multiple input multiple output (MIMO) systems and/or orthogonal frequency division multiplexing (OFDM) modulation. By allowing some degree of correlation among the channels, we formulate the optimal attack strategy for the cases of both single attempt and multiple repeated trials. For the latter scenario, due to the complexity of the optimal solution, we also develop a simpler suboptimal attack strategy. The performance of the proposed methods is evaluated in a MIMO/OFDM scenario and numerical results show the merits of the proposed approaches that can be adopted as a layer one authentication mechanism.

In orthogonal frequency division multiplexing (OFDM) communication systems mobility results in time-variations of the channel, which yield intercarrier interference (ICI), especially when large OFDM blocks are employed in order to achieve a high spectral efficiency. In this letter we focus on systems with very long OFDM blocks, where many of the existing ICI mitigation techniques can not be applied due to complexity constraints. To mitigate ICI we propose a pre-equalizer, operating on sub-blocks of the received OFDM block, whose aim is to force all sub-blocks to have almost the same equivalent channel. In other words, the pre-equalizer combats only time variations of the channel. Next, after OFDM demodulation, the classical equalizer compensates frequency selectivity of the target channel. Performance of the proposed scheme, together with a suitable channel estimate implemented on a per sub-block basis, is evaluated for a digital video broadcasting scenario, according to the DVB-T2 standard, where the OFDM block size may be 32k, and its possible extension to hand-held devices in a next-generation DVB-H.