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Antennas Face Up to the Challenge of Crowded Spectrum

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by Justin Pollock, KP Performance Antennas

After nearly a century of wireless communications, it’s reasonable to assume that every type of antenna has already been designed, deployed, and in some cases replaced by more modern variants. However, nothing could be further from the truth, as the wireless industry is moving in more directions than ever and system operators increasingly demand antennas that are smaller while still delivering high performance, as well as antennas designed to optimize signal propagation, to name just two. So there’s no shortage of good antenna problems to solve.

One challenge increasingly faced by the wireless industry is reducing the potential for interference with nearby services. This is becoming extremely difficult at frequencies below 6 GHz that are already densely packed with signals and will contain even more in the future. The potential for interference becomes even more likely the further the transmitting antenna is from the receiver, as strength is significantly reduced.

The force driving the initiative to “create” more spectrum is the need by 5G for greater bandwidth to accommodate its very wide channel widths required by bandwidth-intensive applications such as 4K video streaming. In addition, Wi-Fi is poised to achieve huge increases in speed and thus also require more spectrum in the coming years, as it moves into the 6 GHz region. The so-called “mid-band” frequencies at 6 GHz and below have far better propagation characteristics than at higher frequencies, so the Federal Communications Commission is concentrating on making the use of these bands more efficient.

The most recent versions of LTE, including LTE-Advanced as well as Licensed-Assisted Access (LAA) and LTE Unlicensed (LTE-U) have several mechanisms designed to mitigate interference issues. In practice, however, there are so many signals present in every band that eliminating interference is basically impossible, but prudent use of known antenna techniques can play a role in reducing it.

Figure 1: This four-port 33-deg. sector antenna for 3.5 GHz to 4.2 GHz operation provides significant interference mitigation through suppression of azimuth and elevation side lobes. It has gain of 18.8 dBi, and uses +/-45 slant polarization with 4-deg. fixed electrical downtilt.

One of these is polarization diversity, which has many benefits and has been used for years in most wireless applications. Polarization diversity is basically the use of antennas that radiate signals in more than one polarization, such as horizontal and vertical. “Dual” horizontal and vertical polarization was the approach used for many years but has often been replaced by “slant” polarization, in which two linearly-polarized antennas radiate at 45-deg. angles (+45 deg. and -45 deg.) from horizontal and vertical, midway between the two. For example, the four-port KP-5SX4-33 is a 33-deg. sector antenna from KP Performance Antennas (Figure 1) that operates in the 3.5 GHz to 4.2 GHz range with gain of 18.8 dBi, +/-45 slant polarization and 4-deg. fixed electrical downtilt. The antenna provides significant interference mitigation through suppression of azimuth and elevation side lobes.

Polarization “slants” don’t have to be 45 deg. but as the wireless industry began using it, this angle rapidly replaced its “H and V” predecessors, although the latter configuration remains in use. An antenna with +/-45 deg. slant polarization can provide benefits that “H and V” configurations do not because the signal propagates much more effectively in NLOS applications.

In addition to its primary benefits, slant polarization can potentially reduce interference in situations where there are simultaneous nearby (i.e., strong) emitters, many of which are likely to be stronger than the desired signal. As received signals are typically more vertically than horizontally polarized, an unequal relationship is created as vertical polarization often delivers a stronger received signal than its horizontal counterpart. Slant polarization can minimize this mismatch by equalizing the signal levels from both orientations, effectively producing the effect of increasing signal strength at the receive location. 

Smaller, Please

In a recent online forum someone asked: “What are today’s design challenges?” The answers he received ranged from ultrawideband antennas for biomedical sensors, small form factor massive MIMO, adaptive antennas for cognitive radios, electrically small antennas with high gain and wide bandwidths, suppressing nearfield interaction between antenna elements in smartphones employing MIMO. In each case, whether directly or indirectly, there was a need for electrically-small antennas to accommodate the confines of small devices. Unfortunately, electrically-small antennas are inherently less efficient and narrowband than larger ones at a given frequency, so creating a small antenna that covers reasonably-wide bandwidth while also radiating effectively is a major challenge. 

As wireless applications move to higher frequencies, this becomes far less an issue as a full wavelength at, say, 28 GHz, is only 10.7 mm. This not only makes it possible to create a tiny antenna that is not electrically small, but also makes it possible to create array-type antennas with many elements in a small package. At least one manufacturer has demonstrated a SoC that integrates not only a millimeter-wave transceiver but a phased-array antenna as well, in a package measuring 18 x 5 mm.

However, a very promising approach using metamaterials is beginning to challenge the traditional antenna design and fabrication paradigm. Metamaterial-based antennas are just now entering the market after many years of research by industry, the Defense Department, and academia.

Metamaterials are artificially-created structures that can be engineered to have material properties that do not occur in natural materials. Metamaterials are composed of an array of electrically small electric and magnetic scatters (unit cells),  such as thin wires, split ring resonators, or patterned frequency-selective surfaces (FSS). At wavelengths larger than the unit cell size, the metamaterial appears as a homogeneous medium with exotic effective medium properties. The geometry, size, and orientation of the cells in the array allows them to block, absorb, enhance, or bend electromagnetic waves. This enables control over material parameters such as electric permittivity and magnetic permeability that together determine the propagation of electromagnetic waves, whether in the region of radio or optical wavelengths. 

The result is a structure that can achieve remarkable capabilities beyond the means of conventional materials. For instance, a planar array of metamaterial scatters that form a planar antenna can produce similar or better performance characteristics as conventional three-dimensional antennas. This includes enhancing its bandwidth, efficiency, and scanning range, and dramatically reducing the antenna size, allowing for reduced fabrication cost. In cases where the antenna is destined for satellite communications, it also reduces payload cost.

One of the first industries to benefit from this type of antenna is satellite communications, and, in 2017, Kymeta introduced the first commercial flat-panel, electronically scanned antenna (ESA) for spacecraft, and has since moved into other markets as well. The company, as well as competitors such as Phasor and Electrodyne, use a variety of proprietary techniques to achieve their results. Metamaterials have huge potential for almost any high-frequency application, a fact that is just now revealing itself in commercial products. However, there are still significant challenges to be overcome in the metamaterial narrowband, highly resonant, and lossy characteristics.

Summary

The techniques and technologies described in this article are just a few of the tools being used by antenna designers to solve challenges that vary with frequency and application. Small-cell base stations and small IoT transceivers, for example,  can benefit from any improvement, owing to their dense deployment in many situations. For designers, it’s a wide open field.

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