Making waves: A new method for mmWave antennas and components

Today, millimeter-wave (mm-wave) radio links are the dominant backhaul solution in telecommunication infrastructure, particularly for cellular wireless access in dense urban areas where high capacity and compact size are critical. An example of the work done by Ericsson Research in this area is the demonstration of 139 Gbps over a 1.5km link using a multiple-input-multiple-output (MIMO) setup at the E-band (70/80GHz). Our experience in the implementation of antenna systems operating at the mm-wave frequencies (30-300GHz) has shown that there are certain challenges from a hardware perspective.

First, mm-wave front-end modules cost more than the ones operating at microwave bands. The reason lies on both employed active devices as well as passive elements.  For instance, critical passive components, such as filters and antennas, shrink in size and require high precision manufacturing and assembly, which is not only expensive but also slows down the development cycle of new products.

Secondly, hardware integration of mm-wave front-end subsystems requires low-loss and cost-efficient interconnect and packaging solutions, in order to minimize the loss of the precious signal power which is hard to generate at mm-wave range.

A novel integration technique for mmwave technology

By using metasurfaces, an innovative hardware integration technology, so-called Multi-layer waveguide (MLW), can provide the desired features for an optimum hardware technology: high performance, simple integrability, cost effectiveness and mass production capability. MLW technology has been developed by Metasum AB, and Ericsson Research has been involved in several project collaborations related to further development and industrialization capabilities of MLW passive components.

As initial validation, E-band bandpass filters (BPF) based on MLW have been designed and implemented in a large number of samples to assess the filter performance, massive producibility and the stability to temperature variation. Nevertheless, the MLW technology should not be viewed as a technique for specific passive components only, but a HW integration solution for mm-wave systems. MLW provides a unique compact modular concept that can include all critical building blocks of a mm-wave system. An additional advantage is that upon modifications of the system specifications, different MLW modules can be developed and assembled by using the same fabrication method, therefore achieving a customized system with the benefit of a considerable cost and design time saving for high-volume productions.

Metasurfaces are artificial materials, and can provide special electromagnetic properties that are not expected from standard materials found in nature. They are usually arranged in periodic patterns at a scale smaller than the wavelength of the electromagnetic waves they influence. MLW technology is a novel, cost-effective air-filled waveguide for mm-wave applications, which is made by stacking thin but unconnected metal layers. Since there is an air gap between the layers, electromagnetic field may leak and cause unwanted losses.  By applying an Electromagnetic Bandgap (EBG) structure (a type of metasurface) created by means of periodic all-through holes allocated in a glide-symmetric configuration, the expected field leakage among the unconnected layers is suppressed. The next figure illustrates this concept:

Rectangular waveguide consisting of 5 unconnected layers without metasurface (left) and with metasurface (right).

Since the field propagates in air, the loss is low. A clear advantage, as compared to standard metallic waveguides, is the manufacturing simplicity of the MLW by using technique like metal chemical etching, avoiding expansive and time-consuming high-precision metal milling. Moreover, the assembly of the layers is done simply by gluing, and there is no need to use screws. The first demonstrator operating at D-band (110-170 GHz) is shown in the next picture.

First demonstrator operating at D-band

The details about this proof of concept demonstration is described in the paper “Novel air-filled waveguide transmission line based on multilayer thin metal”, published in IEEE Transactions on Terahertz Science and Technology, vol. 9, no. 3, 2019.

Putting MLW into practice

Our vision is that MLW can be applied for a wide range of applications, such as interconnect and packaging of active circuits as well as passive components like antennas and filters. Ultimately, the MLW technology is envisioned as a System-in-Package (SiP) platform for mm-wave modules, offering an integration path from component level to system level including mm-wave/THz antennas, as it is illustrated below:

In the next graph, we compare MLW with alternative emerging waveguide (WG) technologies: Gap waveguide, 3D printed waveguide, micromachined waveguide and substrate integrated waveguide (SIW). The comparison is qualitative in terms of compactness, manufacturing cost and RF performance. We can observe that the MLW technology provides a balanced tradeoff among the three key parameters.

As an example of design flow for a passive component using MLW technology, here we present the results of our study where an E-band bandpass filter is implemented. As mentioned, one of the main goals was to investigate the potential for industrialization of the technology. The design flow is specified as follows, and can be generalized to any MLW passive component:

First, a basic bandpass filter is designed by following a method based in low-pass and high-pass filtering structures. The method is described in the paper “High-performance compact diplexers for Ku/K-band satellite applications”, published in IEEE Transactions on Microwave Theory and Techniques, 2017. Afterwards, an EBG unit-cell design should be realized in order to be embedded in the basic filter with MLW. The number of layers is chosen in step 1 to fulfill the filter requirements. We would like to remark that the total thickness of MLW filter is less than 6 mm, which is extremely compact.

For experimental validation of the MLW filter components, a high-volume fabrication method and simple assembly process have been adopted, which consists of three steps, as summarized below:

Each metal layer in the MLW filter is made of silver-plated brass panels. The desired EBG pattern on each layer is formed by using metal chemical etching which is a highly accurate manufacturing process capable of producing metal parts with any desired shape. A set of 19 panels were fabricated, and each of them contains 12 rows and 18 columns of the corresponding pattern for each MLW filter layer, resulting in a total of 216 filter samples. 

Each panel was placed in a fixture, and a dispenser machine was programmed to apply adhesive epoxy at the desired position. After securing and aligning all layers on the fixture, the panels were heated in an oven to cure the epoxy and achieve a proper attachment. The last step is to separate the 216 samples – waterjet cutting is used in this case. The whole assembly process including panel attachment and sample separation took around 5 hours.  The final bandpass filter samples are well defined and have very smooth side walls, as can be observed in the next photo:

The experimental validation consisted of measuring 20 filter samples obtained from this production batch, and the results were compared with simulations to analyze the repeatability and robustness of the design. The S-parameters are depicted below and excellent agreement between simulation and measurement is obtained. The frequency shift is almost negligible, showing that the MLW filter is robust, low loss and high-volume producible.

To complete the experimental validation, a thermal response evaluation of the filters was carried out at  Ericsson by using a thermal chamber. The measurement setup is shown in the next picture, as well as the measured results when the chamber temperature was varied from -30°C to +70°C:

Thermal measurement set-up

The MLW-type filters were found to be less temperature sensitive than any other waveguide-based E-band filters found in the literature or in commercial market, as the relative frequency drift is less than 0.12 percent over the entire tested temperature range (-30°C to +70°C).

The results described above confirm the capability for realizing low loss and ultra-compact MLW passive components in large volumes.

These excellent results were submitted to EuCAP2020, one of the largest and most well-known antenna conferences in the world. We wrote out paper together with the project partners Abbas Vosoogh (Metasum AB) and Zhongxia Simon He (Chalmers). Proof of the significant interest of the MLW concept from the reviewers of the conference came in the form of the Best Antenna Paper Award which we gladly received and was selected among 1200 accepted contributions.

Learn more

Read our Ericsson Microwave Outlook 2019 report