Microwave Journal
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Millimeter-wave Hits The Mainstream, Along With All The Challenges of High Frequency Design & Fabrication

October 25, 2021

There are several simultaneous trends enabling the transition of legacy applications, and creation of new applications, at mmWave frequencies. One of the most discussed reasons for this transition tends to be the overall congestion of sub 1 GHz and sub-6 GHz spectrum driving consumer and enterprise wireless communications toward higher frequencies. Though these lower frequency bands are certainly becoming increasingly used, the wireless standards operating at these frequencies are also becoming more capable of managing interference and standards are also driving stricter electromagnetic compliance (EMC) requirements to combat interference as a result of this congestion. Also, “intelligent” radio systems are also being developed with knowledge of the nearby EM environment, that can “choose” the best spectrum in which to operate given a specific payload of data or communication style.

 

Another possible reason is the increasing accessibility of precision manufacturing, lower cost and more compact computing resources, and simulation/model capabilities that can now tackle the small dimensions and dense physical structures of mmWave components and devices. Moreover, there are now RF semiconductor processes and circuit substrates (PCB, ceramics, and films) that can now more affordably offer higher power and performance at tens to hundreds of gigahertz. 

 

The defense, aerospace, and satellite communications (Satcom) industries have relied on mmWave communications and sensing for decades to great effect. However, these industries tend to purchase components and devices in small volumes with extremely stringent sourcing, manufacturing, quality, testing, and documentation requirements. With more accessible mmWave design resources and RF hardware that can operate to these frequencies, developing and deploying mmWave communication and sensing technology is now seeing a much lower barrier to entry than in previous years.

 

How Do mmWave Frequencies Differ From RF or Microwaves?

The barriers to entry of developing and deploying mmWave communication and sensing technologies has largely been a result of the design and manufacturing complexity which tends to increase as a function of frequency. There are several key factors. mmWave devices and components are often necessarily proportionally smaller than lower frequency components and devices. An example of this is with ground planes for planar transmission lines, such as coplanar waveguides or striplines. It is generally recommended to use metalized via holes spaced at least 1/10 the wavelength apart to connect the multiple layers of grounds for good electrical connection and to prevent the development of undesirable transmission modes.

 

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Figure 1. A plot of the wavelengths of mmWave frequency EM radiation through dielectric media with relative permittivities varying from 1 (air) to 10.

 

It can be observed from Figure 1, that 1/10 a wavelength of mmWave signals  to 100 GHz in a dielectric media, such as a PCB substrate (~2 to 10 relative permittivity), are a fraction of a millimeter. Though this is certainly mechanically possible with some modern PCB, Low Temperature Co-fired Ceramic (LTCC), High Temperature Co-fired Ceramic (HTCC), thin-film, and thick-film  fabrication processes, it has historically been at a premium cost. With ever denser digital electronics with digital signal data rates in the hundreds of gigabits per second (Gbps), PCB technology has subsequently enhanced to make manufacturing small precision traces, planes, and via holes more cost accessible and otherwise reliable.

 

Another factor to consider is that RF losses also tend to scale with frequency, or inversely to wavelength. Phenomena including skin effect, conductor losses, and dielectric losses are also a function of frequency. In the case of dielectrics, mmWave signals also react with dielectric boundaries with greater severity than lower frequency signals and are more readily absorbed by “lossy” dielectrics. The result of this is that mmWave signals face greater losses and scattering from common dielectrics in the environment, as well as dielectrics in building structures (such as concrete and insulation). Another side of this is that certain dielectric structures can also be used in similar ways to optical lenses and waveguides in manufacturable dimensions for mmWave applications. For instance, dielectric lenses can be used to focus, concentrate, or culminate mmWave radiation in a similar way that optical lenses manipulate visible light.

 

Atmospheric attenuation also depends on the frequency, with some portions of the spectrum suffering many times the loss of other bands in the nearby spectrum due to atmospheric phenomenon. The bands with the highest atmospheric attenuation also tend to be the bands available for license-free use or are otherwise unoccupied by military, defense, or Satcom restrictions (see Figure 2). 

 

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Figure 2. Total, dry air, and water-vapor zenith attenuation at sea level *Source ITU-R P.676-9 02/2012 (https://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.676-9-201202-S!!PDF-E.pdf).

 

Therefore, mmWave systems often require higher gain amplification and lower loss materials as part of their manufacture to overcome these losses. Fortunately, RF Front-end (RFFE) and antennas that operate in the mmWave have become increasingly integrated and more affordable, allowing for higher gain and lower loss mmWave components in smaller form factors and cost structures than in previous decades. 

 

A caveat of this is that with higher gain, mmWave antennas are also necessarily more directive and therefore have a narrowing beampattern for transmission and reception. This is why AAS (Active Antenna Systems) are seen as an enabling technology for 5G, and future 6G, mmW technology. AAS using phased arrays or other beamsteering technology can provide higher gain and an antenna pattern that can be pointed at a target within a limited scanning angle of the antennas broadside face. The trade-off is that AAS requires more RF components in an often more compact space.

 

Fabrication, Design, & Integration Challenges For Millimeter-wave Devices
Though there are more accessible mmWave fabrication, design, and testing technologies that are now relatively affordable for higher volume applications, there are still many material and design challenges that need to be tackled to provide mainstream use of mmWave systems. Part of these challenges are overcoming the innate losses and fine tolerances of mmWave components and devices, but there are also still materials and fabrication challenges, specifically with dielectrics. Integration challenges of fitting complex combinations of mmWave components and devices is also a limiting factor in deploying mmWave technology. 

 

Fabrication Challenges

High RF Losses & Capacitive/Inductive Parasitics

The general guidance is that the effects of conductors, dielectrics, and semiconductor geometries on electrical performance are a function of frequency. Depending on the dimensions, there is a maximum frequency to which many of the EM effects (skin effect, dielectric loss, conductor loss, surface roughness, etc.) are negligible for a given structure and application. Due to the extremely small wavelength of mmWave signals, these frequency dependent effects generally must be considered. 

 

For instance, the insertion loss impact of the skin effect, conductive losses, dielectric losses, and other possible effects like dispersion, increase with frequency. A transmission line of the same length tends to exhibit greater loss for higher frequency signals than lower frequency signals (see Figure 3). 

 

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Figure 3. A 2-Port Network Diagram showing transmission (S12/S21), reflection (S11/S22), and Scattering parameter assignments with insertion loss of a transmission line depicted as an inline resistance.

 

Moreover, for many transmission line types, there is a maximum frequency for which the transverse-electromagnetic (TEM) or quasi-TEM mode are dominant, and higher frequency signals will experience additional transmission modes that could further reduce the performance of the transmission line.

 

The phase and velocity of propagation (phase velocity) of the signals passing through a transmission line are also a function of a variety of frequency dependent effects, including the surface roughness of the conductors used in fabricating the transmission line [2.1.2,2.1.3,2.1.4]. A result of this is that it is often a challenge to maintain the group delay (phase delay across a frequency band) for wide bandwidth signals, which is increasingly crucial for highly complex, modern modulation schemes.

 

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Where Xc (capacitive reactance), XL (inductive reactance), C (capacitance), L (inductance), ⍵ (frequency).

 

The capacitive (Xc) and inductive (Xl) reactance are also a function of frequency. This means that the effect of capacitive and inductive parasitics are more significant at mmWave frequencies, even though the inductance and capacitance values derived from electrostatic effects remain the same. This has a direct impact on the performance of mmWave field-effect transistors (FET), which suffer from frequency operation limitations due to the gain-to-drain parasitic capacitance being amplified by the Miller effect in some designs. This is also a challenge as reaching higher frequency operation also necessitates a shrinking of geometries, for which parasitic capacitances often increase as conductors and semiconductors become more closely spaced. In order to keep parasitic capacitances small at mmWave frequencies, dielectrics with low dielectric constants are used, such as in circuit substrates and coatings.

 

mmWave Minimum Physical Constraints

The size, tolerances, and repeatability of electrically significant structures at mmWave frequencies are key considerations when designing and fabricating mmWave components in devices. Certain fabrication processes that can only achieve trace and feature resolution to a few millimeters will likely be unsuitable for mmWave applications, which generally require trace width and feature sizes in the hundreds of micrometers (microns). With base feature sizes so small, the tolerances of the features also become more important. RF and microwave frequency fabrication processes may have manufacturing tolerances as high as the feature sizes for mmWave components and devices.

 

The EM field lines are also concentrated closer to the conductors and dielectrics of the physical structures, which further exacerbates the impact of manufacturing tolerances, surface roughness of conductors, and material property conformality at mmWave frequencies. Using planar transmission lines and waveguides as examples, the efficient transmission of signals through these signal carrying lines depends on the exact dimensions ratios of the geometries.

 

If a 60 GHz stripline with a strip width in the hundreds of microns and substrate height also in the hundreds of microns, suffers from stripe width or substrate thickness tolerances in the tens of microns, then the characteristic impedance of the stripline can be thrown off by over 10% at any given cross section along the stripline. This would amount to creating additional reflections and the development of standing waves within the stripline that could dramatically reduce the transmission lines performance. Maintaining an acceptable impedance tolerance with such a stripline may require geometric tolerances on the micron level and material conformality to less than 1% deviation. This is generally beyond what can be done with PCB manufacturing, which typically has minimal trace widths and spacing around 76.2 micrometers (0.003”) with tolerances in the 10s of micrometers. Other non-semiconductor fabrication technologies for RF structures (LTCC, thin-film, thick-film, etc) also have tolerances limits from several micrometers and upward. This is why manufacturing circuits for high-speed digital and RF devices often requires special attention and sacrificing of absolute dimensional accuracy for relational accuracy between specific critical dimensions, such as the trace width and spacing to ground traces for coplanar waveguides.

 

Summarily, to fabricate mmWave devices and components, the manufacturing tolerances, surface roughness, and material property conformality requirements must become proportionally tighter as frequencies increase. This also directly applies to repeatability where the tolerances stackups and material property deviations could also lead to components and devices within a batch to vary considerably.

 

mmWave Material Constraints

The level of manufacturing sophistication needed to reliably produce some mmWave components and devices is available in semiconductor manufacturing, where dimensions and features can be on the sub-nanometer scale and high volume production is possible with an emphasis on repeatability. However, the processes of designing a semiconductor component/device or otherwise producing a mmWave part generally requires access to design and production experts familiar with such part manufacture and access to specialized manufacturing equipment/processes. In some cases the equipment or technology needed for mmWave component or device development may even be under export control and access may be limited to the necessary technology in some regions of the world.

 

Furthermore, there are fewer material choices for conductors and dielectrics that can be fabricated to the necessary sizes, tolerances, and conformality requirements for mmWave. There is also often a vertical market of services that are needed to produce mmWave technology. For instance, producing ceramics that can be used for LTCC, HTCC, thin-film or thick-film mmWave circuits often requires first sourcing the ceramic substrate, lapping/polishing, machining the substrate to appropriate size sections for the application, adding metallization, plating the metallization, and other steps for which there may only be one or a few available regional suppliers/vendors. Consequently, developing a mmWave technology often involves overcoming design and production challenges at each stage of the vertical, that further increase overall system costs and lead to longer design and production cycles (i.e. time-to-market). This is a major area of innovation and academic research with teams and organizations round the world developing new solutions and demonstrating prototypes, such as nScrypt, Nano-Fabrica, Nuvotronics, and many others. Though capable, these new technologies and approaches tend to fall short in the task of developing relatively large volume dielectrics at high resolutions with extreme levels of material property control.



 

Dielectric Specific Fabrication Challenges

Where conductors are formed of metals, which can generally be reliably deposited, sputtered, formed, and plated to reasonably high tolerances and readily measured, dielectrics are most often ceramics, glasses, or polymers, which often present their own nuanced production and manufacturing challenges. The most significant electrical characteristics of conductors at mmWave is generally the surface of a conductor, depending on how thick the conductor is and the relative depth of the carrier concentration, which can be plated or otherwise coated with gold or other noble metals to provide for higher conductivity and more conformal outer surface. 

 

For dielectrics, which are often used to support, separate, or load conductive structures, the surface and bulk of the dielectric are critical and the electric field typically penetrates into, or through, the entire dielectric. As the electric field passes through a dielectric, some of the overall energy is absorbed (dielectric loss tangent, or its reciprocal, quality factor) and converted to heat. For many dielectrics, a relatively small change in temperature can result in a change of the overall dielectric performance and dimensional stability of the dielectric structure. Consequently, dielectrics with extremely low loss tangents, less than 10^-4 at 1 kHz and 10^-3 in the mmWave are desirable [2.1.5].

 

Conformality of the dielectric properties and dimensions of dielectrics are subsequently critical, as even small variations or gradients of the dielectric performance of the material can change the behavior of the electric field passing through the dielectric. Spatially layering or mixing dielectric materials result in generally difficult to determine dielectric effects which may result in the effective permittivities of a given dielectric structure to be much different than the dielectric performance of the individual dielectrics. For instance, in a parallel plate capacitor, the direction of the dielectric layers in respect to the capacitive plates determines the effective permittivity of the overall dielectric. For perpendicularly stacked dielectric layers the effective dielectric is essentially cumulative, where parallel stacked dielectric layers (perpendicular to the electric field with equal charge) results in an effective permittivity which is the cumulative of the reciprocals of the layer’s dielectric constant (see Figure 4). For more complex 3d structures, this computation becomes increasingly complicated, and thus hard to analytically determine (i.e. using effective medium theory). 

 

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Figure 4. Parallel plate capacitor with series and parallel dielectrics.

 

Thus dielectrics for mmWave technology need to be machined, deposited, developed, or layered with exacting precision, and such precision effectively acts as a limiting factor to what materials and fabrication processes may be viable for a given mmWave frequency. An important note is that this behavior of dielectrics can be intentionally exploited to develop dielectric metamaterials and metasurfaces. A new class of electromagnetic metastructures (metamaterials, metasurfaces, meta-atoms, metalfilms, metascreens, etc) can be made to behave in ways that traditional dielectrics in nature, or as conformal bulk structures, cannot.

 

Essentially, within a dielectric medium, the dipole moments are induced by the electric polarizabilities of the scatterers embedded within a medium when exposed to an electric field [2.1.6]. By volume averaging the dipole moments into a polarization density (P) and the electric field (E), the result is an electric displacement vector (D) and permittivity (epsilon):

 

 
The “bulk” electric displacement vector (D), as a function of the permittivity of free space (epsilon_0), the electric field (E), and the polarization density (P).

 

Though natural dielectrics and materials only have positive values for permittivity, man made materials with special properties or structures have since been made/arranged in such a way as to yield a negative refractive index, or negative permittivity. This is also the case for other classes of magnetic metastructures impacting the magnetic permeability, but that is beyond the discussion of this paper. An interesting note about these metastructures is that the nontraditional permittivity behavior is frequency dependent, where frequencies outside of the range of the structures interact with the structure in more traditional ways. In this way these materials behave as a continuous effective medium with bulk effective material properties. Within that frequency range of operation, composite/engineered materials can result in effects that dramatically change the electric performance of the structure.

For instance, creating a grade of refractive indices within a dielectric volume can result in behavior that physically resembles that of optical lenses, but on electromagnetic radiation well below that of visible light. These gradient refractive index (GRIN) structures can be specifically designed to act as complex optical structures that yield a desired radiation pattern from that of traditional lenses, and also more complex and nuanced behavior. 

Other mmW metastructures can be fabricated that allow for amplitude and phase manipulation, near-field interactions, and even nonlinear behavior depending on the design and materials involved. A main advantage of mmWave metastructures is the enhanced degrees of freedom and more compact dimensions that can result from periodic, unit cell, or fractal metastructure design methods.

 

The precision and tolerance of the manufacturing process are intrinsically significant with 2D and 3D mmWave metastructures, as the structures require structure sizes on the order of fractions of a wavelength and tolerances a few percent of that minimum structure size. Otherwise, such a mmWave metastructure would operate less efficiently, or even fail to function as intended.

 

This is a particular challenge with dielectrics, as there are limited manufacturing processes that can produce ceramic and glass structures with such small dimensions, conformally, and with tight tolerances [2.1.5-2.1.10]. Furthermore, to achieve other performance goals, these metastructures also need to be constructed of materials with extremely low dielectric losses (loss tangent), which rules out many polymers and other materials. Composite materials can be produced with the desired behavior and assembled through additive manufacturing approaches. However, there are few additive manufacturing processes that are compatible with complex dielectric composite materials that can produce relatively large dielectric structures (centimeters) with high resolution features (tens of microns). There are promising new composite materials and fabrication approaches that use innovative methods to ensure conformal mixing, hence material properties, and can produce structures on such scales as to be useful well into the mmWave frequencies such as what Fortify is doing with Rogers materials.

 

Integration Challenges

With increased accessibility to mmWave technology has come the desire to embed and integrate mmWave devices and systems in a variety of form factors and in applications that don’t have the power or space available for traditional mmWave systems composed of discrete mmWave components. As a result, there is increasing incentives for manufacturers to enhance the level of integration of mmWave components, devices, and systems. To overcome the high atmospheric attenuation and other intrinsic RF losses within a mmWave system, much of the effort in realizing viable mmWave systems has largely been on integrating and developing enhancements to AAS.

 

mmWave Integration Design Considerations

  • EMC/EMI close proximity & high component densities
  • Complex signal routing requiring high performance transmission line or waveguide interconnect
  • Analog, digital, power, and RF coexistence
  • Thermal management
  • Power Routing
  • Enclosure/housing that doesn’t lead to unintended effects
  • Integrating microwave and mmWave antennas/structures without sacrificing gain
  • mmWave AAS Specific Challenges

 

For instance, previous generations of mmWave communication (mainly backhaul, radar, and Satcom), were generally built with digital signal processing and networking electronics that were highly integrated, and mmWave electronics that were assemblies of discrete mmWave components and devices. Such assemblies are often composed of discrete power amplifier pallets, a monolithic microwave integrated circuit (MMIC) low noise amplifiers, mixers, local oscillators (LOs), switches, circulators/isolators, power divider/combiners, and transmission line/waveguide based interconnect. Depending on the application, the filters in the system may be discrete, connectorized, or contained within their own integrated microwave assembly (IMA) as a filter bank. In some cases, these integrated modules have performance requirements that necessitate specialized interconnect, such as wire bonding or using coaxial spring probe connector assemblies between integrated modules.

 

With the advent of 5G beamsteering and multi-input multi-output (MIMO) systems, actively electronically steered antenna arrays (AESAs), and phased array-based Satcom systems, the number of RF Front-ends (RFFEs) and interconnects have been increased. Hence, a new level of mmWave integration is needed. In many cases, design teams have turned toward semiconductor, thin-film/thick-film, LTCC, and other highly compact integration processes to even further compress the overall size of mmW components and devices in a system.

 

mmWave Integration Technologies

  • MMIC built with silicon or III-V type semiconductors
  • LTCC or mmWave PCB with System-in-Package (SiP) RFFE hardware
  • Integrated Microwave Assemblies (IMAs) that use suspended substrate (SS), LTCC, thin-film/thick-film, or other compact components/devices
  • Heterogeneous or 2.5D/3Dintegration of class III-V semiconductor devices and silicon (Si) complementary metal-oxide semiconductor (CMOS) devices, similar to silicon-germanium (SiGe) BiCMOS processes, for the development of digital/RF systems-on-chip (SoCs).
  • Micro-electromechanical systems (MEMS) switches, filters, phase shifters, attenuators, etc.
  • Digitization of modulation, demodulation, frequency conversion, filtering, and signal enhancing features

 

The move toward beamsteering antenna and other AAS technologies has also resulted in the addition of mmWave signal chain components that require integration, mainly variable attenuators/amplifiers and phase shifters. Though more capable digital signal processing (DSP), FPGAs, application specific integrated circuit (ASIC), central processing units (CPUs), analog-to-digital converters (ADCs), and digital-to-analog converters (DACs) are now capable of sophisticated signal synthesis, manipulation, and conversion, these digital electronics are not able to fully address the RF performance requirements from the RFFE onwards in the signal chain. This is why there is still substantial investment in developing more compact and capable mmWave components and devices and to enhance the interoperability of these technologies for wider applicability [2.2.4-2.2.7]. For many mmWave applications, direct-digital-conversion (DDC) and direct-digital-synthesis (DDS) electronics are not capable of synthesizing or converting to the desired mmWave operating frequencies, and there is often still a need for frequency conversion devices beyond 6 GHz (X-band and beyond). Consequently, the RFFE and antenna systems for AAS still consume a significant amount of the power, space, cost, and design complexity budget of a mmWave system and are often a limiting factor in the performance and capability of modern system (see Figure 5).

 

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Figure 5. An example of a highly integrated antenna array stack built as a layered module. 

 

To achieve the desired operating range, throughput, or resolution in a given size and power budget with an AAS, there is substantial emphasis on power amplifier efficiency and receiver dynamic range [2.2.4-2.2.7]. For example, the F-22 Intra-Flight Data Link (IFDL) upgrade, the F-35 Multi-function Advanced Data Link (MADL) is designed using an active analog beamformer with the assumption that the extremely narrow antenna beams and high atmospheric attenuation of mmWave system provides enhanced physical security and shorter multi-beam neighbor discover times. 

 

In much the same way, 5G AAS are relying on thin mmWave links to establish extremely high data rates, and face many of the same challenges of getting the most from highly integrated RFFE signal chain components. Therefore, methods of improving the antenna gain, efficiency, or directivity of the AAS without sacrificing, and even enhancing, the maximum scanning angle can lead to improved overall mmWave AAS performance or alleviate some of the performance requirements from the RFFE hardware. Accordingly, dielectric metastructures that can reduce side-lobe levels, enhance the maximum scanning angle, and/or improve antenna efficiency may provide benefit in the design of mmWave AAS to meet these latest challenges, be it for aerospace/defense, Satcom, or telecommunications [2.1.11].

 

Complex Dielectric Specific Integration Challenges

In order to benefit from dielectric metastructures, they must be carefully designed as a key feature of an AAS, or as an enhancement addon. Either way, the design and fabrication of the dielectric metastructure, in the majority of cases, will need to be customized to a given design. This is because virtually every physical dimension of a dielectric metastructure needs to be designed in respect to the desired function of the structure. Unlike a radome, which is mainly a necessary protection of a radar/communication external hardware but doesn’t necessarily benefit the function of the system, dielectric metastructures in such systems are key components that critically change the behavior of the electric fields within a mmWave system or radiated/received by the system [2.2.8,2.2.9].

 

Integration of dielectric metastructures, in most cases, will require full EM simulation and optimization of the mmWave system as it will be integrated to fully determine the end-design. This is due to the impact of the design integration and fabrication of the integrated assembly on the radiation and the electric field behavior within a device and from it's antenna structure. This type of simulation is computationally expensive for mmWave structures of any size or complexity, as the mesh refinement and resolution required to achieve accurate simulation results is quite sophisticated. Optimization of a dielectric metastructure could require a substantial amount of time, as iterative simulation refinement may be needed. It is also likely necessary in order to characterize the behavior of the lens feed system, for a dielectric lens antenna, or the placement and fixture tolerance within the assembly housing [2.2.10,2.2.11].

 

Manufacturing dielectric metastructures for integration with a mmWave system applications using traditional layered, cast/molded, or CNC subtractive manufacturing approaches may also incur significant expense and development of tooling, dies, molds, etc. Consequently, there is an active tradeoff between the time and resources spent on design optimization and manufacturing optimization, either of which have their own expense considerations, risk, and unknowns. Therefore, it is likely desirable to have access to flexible methods of fabricating dielectric metastructures with a high degree of design flexibility and tight manufacturing tolerances. These are both achievable with recently developed dielectric materials and digital light processing (DLP) 3D printing technology [2.1.11].

 

Conclusion

There are a slew of market forces and technology trends that are driving development of wireless communication and sensing systems designed to operate in frequency bands previously only used in satellite communication, backhaul, and military radar. An example of this is the recently released and licensed 5G and Wi-Fi frequency bands beyond 6 GHz, as well as the extended V-band allowing unlicensed use to 71 GHz. With the growing spectrum congestion below 6 GHz, many applications are being redesigned to operate at higher spectrum regimes, which in turn, is causing yet other applications to seek even further heights of spectrum to avoid interference. 

 

There are also other attractive features of operation at mmWave frequencies, namely that the size of many RF electronics scales in proportion to wavelength. mmWave components and devices can often be fabricated in much smaller form factors than lower frequency components and devices (i.e. smaller antenna and other RF hardware). Examples of this include mmWave 5G gnodeB and antenna array, as well as mmW sensing devices, such as mmW imaging systems and radar. 

 

This is a double-edged sword of sorts, as the relative sizes of mmWave hardware shrinks, the manufacturing tolerances and precision material requirements become stricter proportionally due to a variety of phenomenon (skin effect, high RF losses). Though the promise of broad swaths of available spectrum in the mmWave frequencies seems attractive, there are many phenomenon and fabrication considerations that come into play at these frequencies that are often either negligible or ignored at lower frequencies. There are also some technologies, now emerging, whose benefits could not fully be realized below mmWave frequencies, such as dense active/advanced antenna arrays (AAS). Now is the dawn of mmWave emerging from the shadows into mainstream use, as well as the emergence of mmWave design and fabrication challenges that may require designers to step outside of their familiar solutions to solve.

 

References

  1. Introduction
    1. The Trend Toward Millimeter-waves
      1. FCC: Millimeter Wave 70/80/90 GHz Service
      2. 5G mmWAVE Technology Design Challenges and Development Trends
      3. Issues, challenges, and research trends in spectrum management: A comprehensive overview and new vision for designing 6g networks view and new vision for designing 6g networks
    2. How Do Millimeter-wave Frequencies Differ From RF or Microwaves?
      1. Understanding mmW Spectrum For 5G Networks
      2. Millimeter Waves Technologies and Challenges for EMC & Wireless
      3. 5G Millimeter-wave Antenna Array: Design And Challenges
      4. 6G WIRELESS CHANNEL MEASUREMENTS AND MODELS Trends and Challenges
  2. Fabrication, Design, & Integration Challenges For Millimeter-wave Devices
    1. Fabrication Challenges
      1. Semiconductor technologies for 5G implementation at millimeter wave frequencies – Design challenges and current state of work
      2. PCB Design and Fabrication Concerns for MILLIMETER-WAVE CIRCUITS
      3. Effect of conductor profile on the impedance and capacitance of transmission lines
      4. Transmission Line Loss Properties of Dielectric Loss Tangent and Conductive Surface Roughness in 5G Millimeter Wave Band
      5. Silicate dielectric ceramics for millimetre wave applications,
      6. Dielectric Materials for Wireless Communication
      7. Dielectric Metamaterials
      8. Dielectric Metamaterials and Metasurfaces in Transformation Optics and Photonics 1st Edition
      9. Integrating microsystems with metamaterials towards metadevices
      10. All-Dielectric Metamaterial Fabrication Techniques
      11. 3D Printed Dielectric Lenses Increase Antenna Gain and Widen Beam Scanning Angle
    2. Integration Challenges
      1. Packaging Trends for Millimeter-Wave Radar and Communication Systems
      2. Stacking GaN And Silicon Transistors On 300 Mm Silicon
      3. Packaging and Antenna Integration for Silicon-Based Millimeter-Wave Phased Arrays: 5G and Beyond
      4. The Microwave and Millimeter Wave Integrated Circuit (MIMIC) DARPA
      5. Millimeter Wave GaN Maturation (MGM) DARPA
      6. Millimeter Wave Digital Arrays (MIDAS) DARPA
      7. The DARPA Millimeter Wave Digital Arrays (MIDAS) Program
      8. Effect of dielectric materials on integrated lens antenna for millimeter wave applications
      9. Modeling and validation of a mm-wave shaped dielectric lens antenna
      10. Review of 20 Years of Research on Microwave and Millimeter-wave Lenses at “Instituto de Telecomunicações”
      11. Multi Beam Dielectric Lens Antenna for 5G Base Station