Introduction: Why Does 6G Need Lens Antennas?
The telecoms industry is currently developing the next generation of mobile communication technology. Following on from 5G, 6G is expected to roll out in the early 2030s, bringing faster and more reliable cellular data transfer. One of the biggest challenges of 6G is balancing coverage against energy consumption. As frequencies increase, the ability of waves to propagate through obstacles is reduced, requiring high power levels and numerous “nano-cells” in dense urban environments. Increasing the efficiency of the network will be critical, and one approach is to use lens antennas.
A lens antenna places a material in front of the radiating element, which focuses the radiated waves into a beam. This beam concentrates the energy in the intended direction while minimizing wasted and scattered radiation. Compared to other beamforming techniques, such as antenna arrays, lenses are passive, low-cost and low-power. They do not require thermal management like active-powered solutions and they can be paired with antenna arrays to increase the array’s efficiency and field of view.
How Do GRIN Lens Antennas Work?
Gradient refractive index (GRIN) lenses take advantage of advanced manufacturing techniques such as additive manufacturing software to build complex structures with a varying refractive index. Unlike a conventional lens operating on optical principles, a GRIN lens can have an arbitrary shape, and its properties can be more precisely controlled. Animation 1 shows a plane wave incident on a conventional lens and a flat GRIN lens. The GRIN lens refracts the wave similarly to the conventional lens.
GRIN lenses have a metamaterial structure, with smaller elements repeating to build the complete device. The details of the geometry of the elements create effects at the larger scale that cannot be replicated in a natural bulk material. This unlocks the possibility to innovate new types of lens antenna for various applications.
One promising variation is the Luneburg lens for densification applications. A Luneburg lens is a spherical GRIN lens with a decreasing refractive index from the center to the edge, which focuses an incident plane wave to a single point on the opposite of the sphere. This has application potential for base stations, whereby a single Luneburg lens fed by multiple antennas can divide the network cell into radial subcells. This is an efficient solution for providing dense coverage from a single base station in venues containing large numbers of people, such as public squares and festivals.
GRIN Antenna Design Using Simulation
When designing a metamaterial-based GRIN antenna, there are two main steps in the design process – designing the individual element or unit cells, and designing the antenna as a whole. Electromagnetic simulation is important in both stages of the design process.
To illustrate the process, we use a 3D printed 6G Luneburg lens design from our partner Fortify.
GRIN Lens Unit Cell Design
A metamaterial effectively has the structure of a periodic crystal, and we can borrow techniques from the world of photonics to optimize the design efficiently.
We take a single unit cell of the structure, parameterized to allow us to vary the filling factor, which dictates how much of the unit cell is filled with dielectric. Ultimately, we want to find the relationship between the filling factor and the effective permittivity or Dk , from which we can determine the refractive index of the unit cell, which will allow us to design the complete lens.
To do so, we set up a dispersion simulation in CST Studio Suite, exciting the unit cell from different directions. SIMULIA’s CST Studio Suite includes an automatic template to set up the simulation with the appropriate properties, in order to automatically produce a dispersion diagram. For more information, see this Knowledge Base article.
As well as calculating Dk, we can also calculate the cut-off frequency for the lens. We can do this using the dispersion diagram, which plots the frequency of different modes in the structure for different phases of the incident rays. In a non-homogenous material, these modes will not be the same for all incident angles, giving rise to a situation where the same mode at the same phase has two different frequencies. This will cause reflection and give rise to a “soft cut-off frequency” (Figure 2) for the lens. Above these frequencies, the efficiency of the lens starts to decrease. By investigating the aperture efficiency across the full range of filling factors, engineers can ensure that their lens maintains high efficiency in the 6G frequency bands.
Full GRIN Lens Design
Once the unit cell has been designed, the full lens has to be analyzed to ensure that it operates as expected when constructed and installed in an antenna system. There are several methods for modeling the full lens. The most direct is to construct a full 3D model of the lens, including all the details of the metamaterial. However, this will be very computationally expensive, as the detailed geometry will require a very fine mesh.
Another option is to build a dielectric mosaic or dielectric shell model. The lens is broken down into blocks, and each block is assigned the average dielectric properties of the metamaterial. This is more efficient to compute, although it requires more up-front modeling work by the user.
The third option is to use a spacemap. This creates a bulk material with the same spatially-varying dielectric properties as the metamaterial, which can be easily modelled and efficiently meshed. CST Studio Suite includes a Python interface that can be used to automate spacemap generation and link spacemaps to automatic optimization routines.
The approaches are compared in Figure 5.
The final step of design is to “focus” the lens, using optimization to fine-tune its properties as fed by a real antenna system. The focal point and the phase center of the lens can be aligned using automatic optimization, and for this purpose, the simulation speed and flexibility of the spacemap method have a distinct advantage.
The final results of the optimization are shown in Figure 6. The lens works as specified to provide acceptable coverage across a 120° field of view, with all 7 sectors of the antenna forming strong beams that are well isolated from each other.
Conclusion
GRIN lenses are a promising technology to enable high-performance 6G base stations with better coverage and reliability, more simultaneous users and reduced power consumption. Electromagnetic simulation can be used through the GRIN lens design process in order to optimize the lens design and meet requirements. Simulation helps to develop both the individual metamaterial element and the whole antenna system. Using simulation accelerates GRIN lens and reduces the risk involved in bringing this innovative new technology to market.
Find the webinar on GRIN Lens, here.
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Stephen is an Advocacy Offer Marketing Specialist in Global Marketing.