Mobile communications have been undergoing a generational change every ten years or so. However, the time difference between the so-called “G’s” is also decreasing. While fifth-generation (5G) systems are becoming a commercial reality, there is already significant interest in systems beyond 5G, which we refer to as the sixth generation (6G) of wireless systems. In contrast to the already published papers on the topic, we take a top-down approach to 6G. More precisely, we present a holistic discussion of 6G systems beginning with lifestyle and societal changes driving the need for next-generation networks. This is followed by a discussion into the technical requirements needed to enable 6G applications, based on which we dissect key challenges and possibilities for practically realizable system solutions across all layers of the Open Systems Interconnection stack (i.e., from applications to the physical layer). Since many of the 6G applications will need access to an order-of-magnitude more spectrum, utilization of frequencies between 100 GHz and 1 THz becomes of paramount importance. As such, the 6G ecosystem will feature a diverse range of frequency bands, ranging from below 6 GHz up to 1 THz. We comprehensively characterize the limitations that must be overcome to realize working systems in these bands and provide a unique perspective on the physical and higher layer challenges relating to the design of next-generation core networks, new modulation and coding methods, novel multiple-access techniques, antenna arrays, wave propagation, radio frequency transceiver design, and real-time signal processing. We rigorously discuss the fundamental changes required in the core networks of the future, such as the redesign or significant reduction of the transport architecture that serves as a major source of latency for time-sensitive applications. This is in sharp contrast to the present hierarchical network architectures that are not suitable to realize many of the anticipated 6G services. While evaluating the strengths and weaknesses of key candidate 6G technologies, we differentiate what may be practically achievable over the next decade, relative to what is possible in theory. Keeping this in mind, we present concrete research challenges for each of the discussed system aspects, providing inspiration for what follows.

6G Wireless Systems: Vision, Requirements, Challenges, Insights, and Opportunities

Multiple antenna technologies have attracted much research interest for several decades and have gradually made their way into mainstream communication systems. Two main benefits are adaptive beamforming gains and spatial multiplexing, leading to high data rates per user and per cell, especially when large antenna arrays are adopted. Since multiple antenna technology has become a key component of the fifth-generation (5G) networks, it is time for the research community to look for new multiple antenna technologies to meet the immensely higher data rate, reliability, and traffic demands in the beyond 5G era. Radically new approaches are required to achieve orders-of-magnitude improvements in these metrics. There will be large technical challenges, many of which are yet to be identified. In this paper, we survey three new multiple antenna technologies that can play key roles in beyond 5G networks: cell-free massive MIMO, beamspace massive MIMO, and intelligent reflecting surfaces. For each of these technologies, we present the fundamental motivation, key characteristics, recent technical progresses, and provide our perspectives for future research directions. The paper is not meant to be a survey/tutorial of a mature subject, but rather serve as a catalyst to encourage more research and experiments in these multiple antenna technologies.

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Prospective Multiple Antenna Technologies for Beyond 5G

Multiple antenna technologies have attracted large research interest for several decades and have gradually made their way into mainstream communication systems. Two main benefits are adaptive beamforming gains and spatial multiplexing, leading to high data rates per user and per cell, especially when large antenna arrays are used. Now that multiple antenna technology has become a key component of the fifth-generation (5G) networks, it is time for the research community to look for new multiple antenna applications to meet the immensely higher data rate, reliability, and traffic demands in the beyond 5G era. We need radically new approaches to achieve orders-of-magnitude improvements in these metrics and this will be connected to large technical challenges, many of which are yet to be identified. In this survey paper, we present a survey of three new multiple antenna related research directions that might play a key role in beyond 5G networks: Cell-free massive multiple-input multiple-output (MIMO), beamspace massive MIMO, and intelligent reflecting surfaces. More specifically, the fundamental motivation and key characteristics of these new technologies are introduced. Recent technical progress is also presented. Finally, we provide a list of other prospective future research directions.

Multi-beam antennas are critical components in future terrestrial and non-terrestrial wireless communications networks. The multiple beams produced by these antennas will enable dynamic interconnection of various terrestrial, airborne and space-borne network nodes. As the operating frequency increases to the high millimeter wave (mmWave) and terahertz (THz) bands for beyond 5G (B5G) and sixth-generation (6G) systems, quasi-optical techniques are expected to become dominant in the design of high gain multi-beam antennas. This paper presents a timely overview of the mainstream quasi-optical techniques employed in current and future multi-beam antennas. Their operating principles and design techniques along with those of various quasi-optical beamformers are presented. These include both conventional and advanced lens and reflector based configurations to realize high gain multiple beams at low cost and in small form factors. New research challenges and industry trends in the field, such as planar lenses based on transformation optics and metasurface-based transmitarrays, are discussed to foster further innovations in the microwave and antenna research community.

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Quasi-Optical Multi-Beam Antenna Technologies for B5G and 6G mmWave and THz Networks: A Review

Multiple antenna technologies have attracted large research interest for
several decades and have gradually made their way into mainstream communication
systems. Two main benefits are adaptive beamforming gains and spatial
multiplexing, leading to high data rates per user and per cell, especially when
large antenna arrays are used. Now that multiple antenna technology has become
a key component of the fifth-generation (5G) networks, it is time for the
research community to look for new multiple antenna applications to meet the
immensely higher data rate, reliability, and traffic demands in the beyond 5G
era. We need radically new approaches to achieve orders-of-magnitude
improvements in these metrics and this will be connected to large technical
challenges, many of which are yet to be identified. In this survey paper, we
present a survey of three new multiple antenna related research directions that
might play a key role in beyond 5G networks: Cell-free massive multiple-input
multiple-output (MIMO), beamspace massive MIMO, and intelligent reflecting
surfaces. More specifically, the fundamental motivation and key characteristics
of these new technologies are introduced. Recent technical progress is also
presented. Finally, we provide a list of other prospective future research
directions.

Multiple Antenna Technologies for Beyond 5G.

This paper presents a compact low profile microstrip patch antenna suitable for Global System for Mobile Communications (GSM) and Worldwide Interoperability for Microwave Access (WiMAX) applications. FR4 epoxy is used as substrate with relative permittivity 4.4 and tangent loss 0.02. The prototype contains slots, shorted pin, partial ground, tuning stub and dual feeding techniques. Parametric analysis has been carried out to observe the effects of techniques. Simulation results show that antenna has enough impedance bandwidth to cover the desired frequency bands. The prototype is fed using transmission line. Antenna exhibits omni-directional radiation patterns with reasonable gain on all operating frequency bands. Characteristic analysis and simulation of prototype was done using Ansoft HFSS (High Frequency Structure Simulator).

A Compact Multiband Antenna for GSM and WiMAX Applications

This work demonstrates the sensitivity of lens antenna arrays operating at millimeter-wave (mmWave) frequencies. Considering a Rotman lens array in receive mode, our investigation focuses on its most imperative defect: aberration of electromagnetic (EM) energy. Aberration leads to spillover of electric fields to neighboring ports, reducing the lens’ ability to focus the EM energy to a desired port. With full EM simulations, we design a 28 GHz, 13 beam and 13 array port Rotman lens array to characterize its performance with the aforementioned impairment. Our findings show that the impact of aberration is more pronounced when the beam angles are close to the array end-fire. More critically, the corresponding impact of aberration on the desired signal and interference powers is also investigated for an uplink multiuser cellular system operating at 28 GHz. The presented results can be used as a reference to re-calibrate our expectations for Rotman lens arrays at mmWave frequencies.

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On the Impact of Spillover Losses in 28 GHz Rotman Lens Arrays for 5G Applications

Phase shifter–based hybrid beamforming has received a lot of attention at millimeter–wave frequencies for cellular communications. Nevertheless, the implementation complexity of such beamformers is rather high due to the complexities involved in designing and fabricating the required radio– frequency (RF) circuits. In contrast, lens–based RF beamformers significantly reduce the implementation complexity, as all active circuits can be replaced by a passive device. In this paper, we present the sum spectral efficiency performance of an uplink multiuser multiple–input multiple–output (MU–MIMO) system with a 28 GHz Rotman lens. An asymmetric two–stage stacked design is fabricated with a 15 element (3×5) uniform rectangular array feeding 9 RF down–conversion chains towards baseband. Zero–forcing processing is employed at baseband for interference nulling and multistream recovery. Our results show that the MU– MIMO gains are substantially more pronounced for the two– stage architecture relative to a single–stage design due to the inclusion of the elevation multipath components. Moreover, we show that the asymmetric design can help to further reduce the implementation complexity, since the conventional beam selection network can be omitted from the RF front–end.

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Performance of a 28 GHz Two–Stage Rotman Lens Beamformer for Millimeter Wave Cellular Systems

A two-stage Rotman lens based millimeter wave (mmWave) hybrid beamforming architecture for multiuser multiple-input multiple-output (MU-MIMO) operation is investigated. We identify a set of non-trivial losses within the complex analog phase-shifter network with the aid of a subscaled beamformer prototype, consisting of 15 element uniform rectangular array (URA) and two stage phase-shifter with 6 stacked Rotman lenses. The prototype is designed to operate at 28 GHz and is capable of 9 beam projections in azimuth and elevation zones. We evaluate the ergodic sum spectral efficiency of a 9-user uplink MU-MIMO system when the user equipments’ (UE) dominant paths are assumed to be aligned with the predefined beam directions. We point out the primary sources of some of the most prominent EM losses and discuss design guidelines for their possible mitigation at both the RF front-end and the signal processing (SP) blocks. Our investigation shows the practical limitations of the system and predicts the achievable performance. Interestingly, our conclusions hold valid for a scaled version of the same hybrid beamformer architecture supporting a larger number of antenna elements.

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Millimeter Wave Hybrid Beamforming with Rotman Lens: Performance with Hardware Imperfections

In this paper, a modified ground structure capable of reducing mutual coupling to provide isolation between adjacent antenna elements is presented. The proposed modified ground structure is a combination of a strategically located ground slot, asymmetric partial ground and a substrate-integrated pin wall. The use of the modified ground structure causes a more than 28 dB (measured value) mutual coupling reduction. The modified ground structure has been optimized and validated with a finite spaced planar 2 × 1 antenna array operating at 4.16 GHz, intended for unmanned aerial vehicle radar altimeter applications. The patch antennas built with the MGS exhibit high gain (greater than 6 dBi throughout the operational band), along with inter-element coupling as low as −65 dB. Mutual coupling reduction at lower contours is beneficial for altimeter applications.

M. Ali Babar Abbasi

Papers

A Compact Multiband Antenna for GSM and WiMAX Applications

On the Impact of Spillover Losses in 28 GHz Rotman Lens Arrays for 5G Applications

Performance of a 28 GHz Two–Stage Rotman Lens Beamformer for Millimeter Wave Cellular Systems

Millimeter Wave Hybrid Beamforming with Rotman Lens: Performance with Hardware Imperfections

Mutual Coupling Reduction between Finite Spaced Planar Antenna Elements Using Modified Ground Structure