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A Perspective of Antennas for 5G and 6G

The roll-out of the fifth generation (5G) of wireless and mobile communications systems has commenced, and the technology race on the sixth-generation (6G) mobile and wireless communications systems has started in earnest [1, 2]. 5G promises significantly increased capacity, massive connections, low latency, and compelling new applications. For example, device-to-device (D2D) and vehicle-to-vehicle (V2V) communication systems will help facilitate the realization of autonomous transport. The rapid access to and exchange of "Big Data" will increasingly impact real-time economic and political decisions. Similarly, highly integrated, accessible "infotainment" systems will continue to alter our social relationships and communities. Wireless power transfer will replace cumbersome, weighty, short-life batteries enabling widespread health, agriculture, and building monitoring sensor networks with much less waste impact on the environment. 6G networks aim to achieve a number of new features such as full global coverage, much greater data rates and mobility, and higher energy and cost efficiency. These will usher in new services based on virtual reality/augmented reality and artificial intelligence [3].

At the core of wireless devices, systems, networks, and ecosystems are their antennas and antenna arrays. Antennas enable the transmission and reception of electromagnetic energy. Antenna arrays enhance our abilities to direct and localize the desired energy and information transfer. To achieve the many stunning and amazing 5G and 6G promises, significant advances in antenna and antenna array technologies must be accomplished.

1.1 5G Requirements of Antenna Arrays

One of the most important features of 5G is the employment of massive antenna arrays, with the size of the array currently varying from 64 to 128 and 256 elements. Such a large number of antenna elements in an array provide an unprecedented variety of possibilities. These include a means to increase the network capacity; the distance and data rates of individual links between the base station and mobile users; and the reduction of interference between different users and cells.

1.1.1 Array Characteristics

Generally speaking, there are three ways to exploit the benefits of antenna arrays in 5G wireless communication systems [4, 5], namely diversity, spatial multiplexing, and beamforming. These concepts are explained as follows.

• a) Diversity and Diversity Combining

It is a fact that mobile wireless communication channels typically suffer from both temporal fading and frequency fading. As a consequence, the quality of the channel varies with time and across different frequencies. Thus, the specific characteristics of the two propagation channels observed between any two pairs of transmitting and receiving antennas are usually different due to the variation in the scattering along the corresponding propagation paths. The peaks and troughs of the strength of the received signal at one antenna would be different from those at another antenna in a rich scattering environment. If the correlation between those two signals is low, one can combine them through so-called diversity combining to obtain a greater signal-to-interference-and-noise ratio (SINR). The latter is also known as diversity gain. A simple viewpoint is that diversity combining techniques aim to improve the quality of the individual links between the base stations and the user terminals by increasing the SINR.

From an antenna point of view, diversity can be obtained by exploiting either the distance between adjacent antennas, i.e., their positions, or different polarizations at the receiver and the transmitter. However, a fundamental requirement is that the mutual coupling between these diversity antennas must be low. Most modern base station antennas employ polarization diversity, i.e., each antenna element is dual-polarized typically with two pairs of slanted dipole "arms" in the ±45° directions. In 5G millimeter-wave (mm-wave) systems, for example, a popular antenna configuration is to have beamforming antenna arrays with ±45° polarizations, respectively.

• b) Spatial Multiplexing

Multiplexing is the process of combining multiple digital or analog signals into a data stream for their transmission over a common medium, thus sharing a scarce resource. Spatial multiplexing aims to establish separate data streams in parallel using the same time/frequency resources. Thus, the space dimension is reused, i.e., multiplexed.

The simplest spatial multiplexing scheme is to employ sectorized antennas, a conventional technique for frequency reuse. More advanced spatial multiplexing schemes employ spatial-temporal (or frequency) coding by virtue of multiple input and multiple output (MIMO) antennas. A MIMO system requires the use of multiple antennas at least at the base stations. MIMO is implemented with two basic schemes as described below.

The first spatial multiplexing scheme is known as single user MIMO (SU-MIMO). By virtue of multiple antennas at both the base station and the user terminals, SU-MIMO first splits the data stream transmitted toward a specific user into multiple data streams. It then recombines them together at the user terminal to improve the information throughput and system capacity. One major challenge to SU-MIMO is the need for the tightly packed multiple antennas in the terminals to be decoupled.

The second spatial multiplexing scheme is known as multiuser MIMO (MU-MIMO). MU-MIMO aims to maximize the overall data throughput between all of the users and their associated base station. While it employs an antenna array at the base station, only one or a few antenna elements are present at each user terminal. Since user terminals are typically well dispersed within a radio cell and their individual channels are likely to be uncorrelated, the benefits of MU-MIMO are easier to achieve.

Both SU-MIMO and MU-MIMO protocols are intended for implementation in most 5G systems.

• c) Beamforming

Spatial filtering can be regarded as a simple version of MU-MIMO. Beamforming achieves this spatial filtering by coherently combining the fields radiated by the array elements to direct their radiated energy into particular directions. These multiple beams are created at the base station to communicate with different users simultaneously.

Beamforming offers two benefits to a communication system. The first is capacity. If there is no overlap of the beams, simultaneous communications can take place in the same frequency band and at the same time without causing much interference. The second is the gain of the antenna array. Higher gain translates into information exchange over greater distances or higher data rates due to increased SINR values. Unlike 3G and 4G antenna arrays that provide coverage with fixed beam patterns and directivity, 5G arrays must support on-demand beam coverage according to real-time application scenarios and user distributions. Moreover, they must be able to support beam management in order to deliver precise coverage in target areas while significantly suppressing interference in other areas.

For beamforming to be effective, large antenna arrays are necessary to generate narrow beams and produce scattering from mobile users with small angular spreads. The latter is to ensure that the majority of the signals transmitted and received from a mobile platform is covered by a narrow base station antenna beam. These requirements, in conjunction with wide bandwidths, support the use of millimeter wave (mm-wave) communications for 5G. In particular, mm-waves propagate in a pseudo-light fashion so the scattering of the signals to and from a mobile platform is highly localized. Furthermore, since their wavelengths are small, an electrically large mm-wave array can be fit easily into a physically small space.

1.1.2 Frequency Bands

Another major challenge associated with 5G antenna arrays is the simultaneous support of all allotted frequency bands [6]. As the number of bands being considered to meet current and future 5G needs increases, significant antenna array innovations are required to support all of them. Moreover, existing 4G bands must be supported as well [7].

Owing to the stringent requirements placed on the radiation patterns produced by cellular systems and on the levels of their impedance matching to sources to maximize their realized gains, the mobile communication industry has so far adopted an approach of using different antennas to support different frequency bands. However, because of the limited space at base station antenna sites and in mobile platforms, the coexistence of these different antennas has posed serious challenges already. It is extremely difficult to maintain low coupling levels between antennas operating over the same band and even harder to suppress the scattering interactions between antennas that operate over different bands. The latter can cause significant distortions to the radiation patterns. It is with this background that the decoupling and de-scattering issues will be addressed in Chapters 2 and 3, respectively.

1.1.3 Component Integration and Antennas-in-Package (AiP)

Clearly, the number of...