High-resolution delay-Doppler estimation is an important requirement for automotive radar systems, especially in multi-target scenarios that require better target separation performance. Orthogonal frequency division multiplexing (OFDM) is a promising candidate waveform for future intelligent transport networks, since it enables the integration of both radar and communication functionalities. Exploring the dual functionality enabled by OFDM, this paper presents a new cooperative method for high-resolution delay-Doppler estimation. The proposed subspace-based method exploits the combination of both the radar and received communication signals to estimate target parameters. The procedure achieves high-resolution delay-Doppler estimation for both uncorrelated, partially correlated and coherent signals, and enables a significant reduction in the required bandwidth when compared to previous approaches which did not exploit the knowledge of the communication signals. Laboratory measurements at 24 GHz and simulation results demonstrate the efficacy of the proposed method for the estimation of multiple targets.

Cell-free massive multiple-input multiple-output (CF mMIMO) can be considered as a potential physical layer technology for future wireless networks since it can benefit from all the advantages of distributed antenna systems (DASs) and network MIMOs, such as macro-diversity gain, high channel capacity, and link reliability. CF mMIMO systems offer remarkable spatial degrees of freedom and array gains to mitigate the inherent inter-cell interference (ICI) of cellular networks. In such networks, several distributed access points (APs) together with precoding/detection processing can serve many users while sharing the same time-frequency resources. Each AP can be equipped with single or multiple antennas, and hence, can provide a consistently adequate service to all users regardless of their locations in the network. This paper presents a detailed overview of the current state-of-the-art on CF systems. First, it performs a literature review of the conventional CF and scalable user-centric (UC) CF mMIMO systems in terms of the limited capacity of the fronthaul links and the connection between APs and user equipments (UEs). As beyond networks will rely on higher frequency bands, it is of paramount importance to discuss the impact of beamforming techniques that are being investigated. Finally, some of the CF promising enabling technologies are presented to emphasize the main applications in these networks.

The distributed antenna system concept promises to enhance the capacity and diversity of next-generation wireless communication networks, due to the inherently added micro and macro diversity. In this article we first give an overview of the main benefits of a DAS in relation to a collocated antenna system. Next we study the sum-capacity scaling of a multi-user DAS with the number of jointly processed transmit antennas in the downlink. In a practical system this scaling will have implications on the number of antennas worth jointly processing, since the costs of processing an additional antenna can be higher than the additional benefits obtained. Results show that the most important system property to attain the highest capacity gains is symmetry, and the users that attain the maximum gain are those at cell borders. They also confirm that the main DAS feature that makes possible its gains over the CAS architecture are the additional degrees of freedom/diversity provided by such an architecture, which increase the probability of finding a system state with high symmetry and of each user being near one of the transmit antennas.

Network densification through the deployment of small cells along the coverage area of a macro-cell, employing massive multiple input multiple output (MIMO) and millimeter-wave (mmWave) technologies, is a key approach to enhancing the network capacity and coverage of future systems. For ultra-dense mmWave heterogeneous scenarios with a massive number of users, one should address both inter- and intra-tier interferences contrary to the low-density scenarios, where the network is mainly noise limited. Therefore, this paper proposes low complex hybrid analog–digital receive and transmit beamforming techniques for ultra-dense uplink massive MIMO mmWave heterogeneous systems to efficiently mitigate these interferences. At the small cells, the hybrid analog–digital receive beamforming/equalizer is computed in a distributed fashion, where the analog processing is performed at the small cell base stations or access points and the digital part at a central unit for joint processing. To optimize the analog part of the hybrid equalizer and the precoders used at the user terminals, we consider as a metric the distance relative to the fully digital counterpart induced by the Frobenius norm. In the optimization problem, apart from the analog constraints usually considered in the homogeneous systems, we further impose the constraints inherent to the distributed nature of the access points. To cancel the inter-tier interference, the digital parts of the precoders employed at the small cell user terminals are designed so that this interference resides in a low-dimensional subspace at the macro base station. The results show that the performance of the proposed hybrid analog–digital precoder/equalizer scheme is close to the fully digital counterpart and is able to efficiently remove both inter- and intra-tier interferences.

The 5G and beyond future wireless networks aim to support a large variety of services with increasing demand in terms of data rate and throughput while providing a higher degree of reliability, keeping the overall system complexity affordable. One of the key aspects regarding the physical layer architecture of such systems is the definition of the waveform to be used in the air interface. Such waveforms must be studied and compared in order to choose the most suitable and capable of providing the 5G and beyond services requirements, with flexible resource allocation in time and frequency domains, while providing high spectral and power efficiencies. In this paper, several beyond 5G waveforms candidates are presented, along with their transceiver architectures. Additionally, the associated advantages and disadvantages regarding the use of these transmission techniques are discussed. They are compared in a similar downlink transmission scenario where three main key performance indicators (KPIs) are evaluated. They are the peak-to-average power ratio, the overall system spectral efficiency (wherein the out of band emissions are measured, along with the spectral confinement of the power spectral density of the transmitted signals) and the bit error rate performance. Additionally, other KPIs are discussed.