Optimizing RF Interconnect Technology in 5G Space-to-Ground Networks

Designing a crucial interconnect system that will perform well and withstand the extraordinary environmental and technical conditions of space, reliably and consistently over long periods of time, is not like designing just any interconnect. Cable assemblies used in spacecraft need to brave high shock and vibration, extreme temperatures, and intense radiation. In addition, satellites have limited space for equipment so minimizing size and weight are also key goals. Cable assemblies must be designed to perform reliably while taking up the smallest footprint possible.

The cables, connectors and RF solutions deployed in space are integral components enabling industry to successfully move satellites, enable earthbound communications and to transmit information contributing to climate science, global security, communications relays between continents, high-speed Internet, and more. The enormity of global connection and data sharing needs is growing by the day—and the communication infrastructure’s performance is vital.

Key considerations for optimizing RF interconnect systems, selecting the right technologies, and accommodating the unique requirements of deployment in space are discussed below.

High Density

In urban and suburban areas, direct line-of-sight MIMO antennas are capable of meeting the speed, coverage and latency requirements of 5G – but operating at mmwave frequencies, their range is short, requiring a dense network of antennas, often located not much more than a hundred yards apart. Much emphasis has been placed on the aesthetics of these antennas in recent years as well so that in some cities, they are being integrated into lamp poles or other urban furniture.

Such a dense network of massive MIMO antennas is often impractical, especially in rural areas, ships and airplanes. Commercial satellites will play a critical role in extending 5G performance anywhere outside of population centers.

In terms of RF feeders/interconnects used with ground-based satellite dishes, there are a multitude of different requirements that may apply such as high frequency performance, needs for low attenuation, phase stability, low PIM performance. Times Microwave’s TCOM, MaxGain and LMR products are designed to address these needs.

In urban and suburban areas, direct line-of-sight MIMO antennas are capable of meeting the speed, coverage and latency requirements of 5G – but operating at mmwave frequencies, their range is short, requiring a dense network of antennas, often located not much more than a hundred yards apart. Much emphasis has been placed on the aesthetics of these antennas in recent years as well so that in some cities, they are being integrated into lamp poles or other urban furniture.

Such a dense network of massive MIMO antennas is often impractical, especially in rural areas, ships and airplanes. Commercial satellites will play a critical role in extending 5G performance anywhere outside of population centers.

In terms of RF feeders/interconnects used with ground-based satellite dishes, there are a multitude of different requirements that may apply such as high frequency performance, needs for low attenuation, phase stability, low PIM performance. Times Microwave’s TCOM^®, MaxGain and LMR® products are designed to address these needs.

At the same time, technology providers are working on advanced designs that accommodate extremely restricted space constraints, as well as to produce spaceflight connectors that successfully operate up to 70 GHz. Times Microwave’s new InstaBend® high-performance microwave assemblies provide a flexible preassembled design for interconnects between RF circuit cards, modules and enclosure panels, enabling space-efficient implementation for higher frequencies.

The high-performance microwave assemblies are ideal for in-the-box applications such as space flight as the cable can be bent very closely behind the connector, minimizing footprint, saving space and simplifying cable routing. This also eliminates the need to protect the back of the connector.

Materials

In addition to the issues of smaller cables tightly packed and connected, space applications require materials and constructions that withstand radiation, sand blast storms, extreme temperatures, and pressure variations. These factors require the latest materials technology and manufacturing processes.

In the past, semi-rigid cables have been the standard cabling solution in space applications as they have a solid copper outer conductor which protects the dielectric material inside. Today, special semi-rigid cable solutions based on silicon dioxide dielectrics are available. For example, Times’ SiO2 cable assemblies are highly temperature and radiation resistant. But, as size requirements shrink, semi-rigid cabling becomes delicate and difficult to install. Active research is ongoing to develop alternate materials for manufacturing flexible cables that can withstand severe temperature swings and high radiation as an alternative to traditional semi-rigid configurations. Such cables will greatly ease the 5G system packaging challenges.

Some antennas are also designed to fold into the satellite when not in use and unfold upon arrival at the satellite’s destination. There, the antennas will point to other satellites, get a position and lock mechanically. Flexible cables are needed to work around the elbow that enables the antenna to fold and unfold.

Phase stability

Tight phase control is a key parameter for optimizing system performance in 5G smart antennas. Electronically steered antennas use multiple antenna arrays and vary the phase relationships of these elements to control the radiation pattern, so they can switch from search to tracking radiation patterns, or shift direction very quickly.

The arrays are fed by transmission lines; beam accuracy depends upon the phase relationships between those cables. Phase is also responsible for precision in some of the more time-sensitive satellite applications like GPS, radar and more. Phase must be accurately controlled in the components within those systems, and phase-stable cable assemblies (such as Times Microwave Systems’ PhaseTrack® line) are important in today’s increasingly sophisticated electronics.

The concept of phase starts with the fact that microwave signal propagates in the form of a sine wave. Every cycle of a sine wave will accumulate 360 degrees of electrical length. In the higher frequency range of 5G applications, millions or billions of cycles per second will accumulate, and the wavelengths are very short so maintaining phase accuracy across a length of cable, phase length becomes exponentially more challenging as frequency increases.

Frequency, time delay and physical properties like length, dielectric constant and propagation velocity all affect electrical length. Coax cables contain a consistent dielectric material throughout the length of the cable and hence have a constant velocity factor. Even though the material is consistent, environmental factors can alter the electrical properties of the cables—like temperature fluctuations, flexure, handling, twisting, pulling, and crushing that can happen to a cable during installation and maintenance.

A cable assembly gets electrically longer as it gets colder, and shorter as it gets warmer—contrary to what one might expect. Electrical length is proportional to physical length. Metals expand as they get warmer and contract as they get colder but as that happens, the dielectric constant expands and contracts as well and its density changes, altering the velocity. The dielectric effects of the plastic offset and dominate the metal effects.

Phase-matched cables are in pairs or groupings. As temperature changes to cold or warm extremes, they do not exactly track together; the phase match degrades just slightly. That small amount of degradation, known as its phase tracking characteristic, adversely affects antenna performance.

RF coax cables are often designed using PTFE (also known as Teflon™) dielectrics, because PTFE can operate across a broad 200-degree Celsius temperature range (-50ºC to 150ºC) with low dielectric loss. The challenge with PTFE, however, is that the material goes through a phase transition around room temperature which causes the phase length to vary non-linearly with temperature and introduces significant hysteresis as the temperatures vary up and down. Controlling phased array antennas with PTFE-based cables is challenging in temperature varying environments.

Cable assemblies must be designed to minimize temperature effects on phase using special materials, such as cables made from silicon dioxide (e.g., Times’ SiO2™). The challenge is to find flexible dielectric materials that not only meet the physical requirements but can also be phase stable across a broad temperature range. Novel options, such as Times’ proprietary dielectric, TF4®, are being developed with the goal of creating flexible cables that can match the temperature range and phase stability of a silicon dioxide-based cable. This developing science will help system designers optimize components for onboard spatial density and ideal positioning within the host structure.

Parting Words

Searching and qualifying an RF interconnect supplier for space applications can be a lengthy and costly process. Some suppliers offer standard qualifications and documentation for basic products in hopes of simplifying this process. But nothing is standard, or easy, in space.

Custom designs, special testing and qualifications, and new product development for space applications require experience and commitment. “Standard” RF systems are not good enough in space. It is important to thoroughly evaluate the capabilities of an RF supplier to ensure a positive outcome.

In the design phase, keep in mind the key considerations to be factored into any RF system that will operate in space: high-density equipment installation for minimal footprint, materials selection to withstand environmental challenges and optimize fit, weight and durability, and phase stability for optimal signal transmission. Work with a reputable RF solutions provider that understands the unique requirements of space and can customize components to meet your application needs. A qualified provider will have experience with designing custom solutions for defense, military, avionics and/or aerospace installations.