23 Feb 2014

Musings from a Wireless Engineer

Erik Lilieholm Applications Engineering Manager, CommScope provides his views on RF topics including terminology, reliability, power and PIM

Honour or Honor? Organization or Organisation? The debate of the ‘correct’ spelling of certain words has long been the topic of many trans-Atlantic debates between us Americans and our British cousins. Much like the variations in spelling within British and American English, the world of RF engineering has many differences throughout the world.

This diversity in terminology became conspicuous as I grappled with a recent inquiry that landed on my desk. A customer was asking for “something that will combine two 850 MHz signals.” Simple as it sounds, I can easily think of half a dozen solutions that meet that description in one way or another.

I had to find out more about specific frequencies and power levels before I could make a recommendation. This need prompted a call to the requestor, yielding not only the required information but also the usual pleasant conversation with a fellow RF professional. That’s one of the many things I enjoy about application engineering, but that’s beside the point.

What the customer needed is what we call a Same-Band Combiner (SBC). As a product category, SBCs encompass a broad variety of devices incorporating filters, couplers, amplifiers, and other RF building blocks. They combine signals within one band, such as the CELL-850 or the DCS-1800 band. Aside from that common trait, they come in many shapes and forms, each optimized for a specific purpose and application. “Surely there must be an established categorisation!” you say. Not really.

Same Band Combiner

Same Band Combiner, SBC

In this endeavour, I began to realise how lacking we are for unambiguous terminology in this area. There are numerous solutions and numerous names for those solutions. But who am I to decide what the correct name for a particular device is? In so doing would I be suggesting that those who disagree are wrong?

That certainly is not my intent, and I know everyone’s familiar terms will not be changed overnight. But I think greater uniformity in terminology will make creating, discussing and analysing RF solutions easier for all parties involved.

I propose that we categorise these cell site products according to their application, that is, their function in the RF path. SBCs, for example, will be categorised as Duplex/Duplex, Duplex/RX, for example. Construction and performance can differ between products within a category, but it will be easier to find and compare solutions to a particular co-siting problem. As we welcome new talent to our guild, we can give them a better structured introduction to the available methods, products and solutions. Meanwhile, existing products can retain the names by which they are already known. But who knows—in time we may all speak the same language.

Reliability

One area of RF engineering that I think does well with defining its terms is reliability engineering. Reliability is often defined as the probability that a product or service will work as needed for a certain time and under certain operating conditions. Like other performance features, reliability is “designed-in” to products to meet needs. But unlike other features, reliability describes future performance and how it changes over time and varies with use conditions. What is often not understood is that only past reliability can be measured. Future reliability must be predicted by considering issues like:

  • Does a product or service work where and how it will be used?

  • How often will it fail and, when it does, will it be repaired or replaced?

  • How does it fail and what happens when it does?

  • How long can it last until it has to be replaced?

The first of these questions can be answered with validation tests to show that specifications are met and 100 percent reliability is possible—at least at the start of operating life. The other issues deal with predicting reliability after that. Two approaches are used for this, often in combination. One considers the statistics of failure and the other the physics of failure.

The statistics approach looks at past failure trends in large populations to predict future rate and mean time between failures, MTBF. The physics approach considers why specific failures happened and how to improve designs to reduce failure rates or extend useful life. Predicting the annual failure rate of a new product like a remote radio head shows how these approaches are used together.

Bath-tube reliability curve

Bath-tube reliability curve

The past reliability of electronics components is known so well that failure rates for each part in a system can be combined to predict a basic rate for the whole system. While doing this, each part’s failure rate is adjusted according to how it is used and physically stressed by temperature, voltage or other factors. In this way, the future failure rate for a product can be predicted, even before it exists.

Power

Reliability is an important concept to understand because it impacts total cost of ownership and quality of service, among other network concerns. Something else that impacts total cost of ownership, especially in the area of operating costs, is power. There are many things wireless engineers must consider in the design for powering cell sites, one of them being the backup system to keep power—and communication—flowing in the event of a power outage.

Integrated into most standard power systems are backup batteries. Lead-acid batteries are one of the most commonly used power alternatives at cell sites. They are compact and are similar to the one under the hood of your car. While these batteries are charged by the cell site power system, it will not discharge until grid power is interrupted. When this occurs, the batteries seamlessly replace grid power. These batteries can last from two to eight hours, depending on their backup configuration design. CommScope provides several outdoor cabinets, which can accommodate power systems, batteries and combinations thereof.

In addition to backup batteries, generators are usually another line of defence against service interruption. Unlike batteries, generators require fuel and can supply the needed power for a longer period of time. There are different types and configurations for generators so factors like space, cost and service expectations must be considered. Diesel generators contribute to air and noise pollution, even when there is no actual power outage, due to periodic maintenance runs. Operators may need to find out what kind of noise restrictions or codes apply to the area where the cell site is located. Some operators can receive fines if the generators are too loud, or stay on for a certain period of time, contributing to the air and noise pollution in the area.

The latest backup power solution gaining favour with wireless operators is hydrogen fuel cell technology. Hydrogen fuel cells use proton electrolyte membrane technology to generate power. There are several key advantages to this technology: low cost, higher efficiency than typical diesel generators, the ability to operate in extreme temperatures, high density, silent operation and zero emissions. CommScope has a unique fuel cell solution, which unlike competing solutions doesn’t require monthly operation for hydration of the fuel cell membranes. This helps conserve the hydrogen fuel, which is used only when there is an actual outage.

Cellular basestation using a fuel cell

Cellular basestation using a fuel cell

These are some of the factors we want customers and wireless engineers to be aware of regarding selection of a cell site backup system. In addition to these factors, cell site power systems can have fairly sophisticated monitoring reporting software that communicate with the operator on the status of the site battery, site alarms, temperature and condition. It is important to know what can be monitored and reported in regards to the power and backup system.

PIM

What kind of wireless engineer would I be if I didn’t start thinking about passive intermodulation (PIM) at some point here? As we all know, PIM has been a big deal in the wireless industry for a number of years. PIM can occur at any point in the transmission path as the result of two or more wireless signals mixing together, creating additional, undesired frequencies that cause interference or degraded transmission of desired signals. PIM has long been recognised as an obstacle to network efficiency and engineers have acknowledged its existence and disruption potential.

Intermodulation distortion
Frequencies generated by intermodulation distortion

Isolation between the transmit and receive paths is important to keep unwanted intermods created in the transmit path from ending up in the receive path. A typical duplex communications system will use the same antenna to transmit and receive. It will also use this same antenna to transmit many frequencies and receive their partnering frequencies. This involves using a duplexer to put the transmit frequencies onto the same port that the receive signals are coming in on. The system benefits from any PIM components that are produced in the transmit path before the duplexer having a high isolation to the receive path. The typical rejection of receive frequencies on the transmit port of a duplexer is around 70–80 dB.

Passive intermodulation distortion
How passive intermodulation affects transmitter receiver systems

However, this highlights the importance of using products that have superb PIM performance after the duplexer. PIM elements that are produced after the duplexer will be allowed to travel directly down the transmission path to the receiver. This will raise the noise floor of the receiver, effectively reducing the uplink footprint of the base station, or if it is severe enough, blocking all traffic on that sector.

I recently used some old power amplifiers that we had acquired to do some PIM testing within our lab. By using one of our combiners that had excellent isolation, any intermods produced by the old amplifiers were filtered out. Remembering, of course, that “filtered out” means “reduced to a point of inconsequence.” PIM has always been there and will always be there; it’s how we manage it that makes it inconsequential.

It is unlikely that there will ever be a universal version of English but the possibility of a global RF ‘dialogue’ is much higher. Through ongoing collaboration and education we can be hopeful that when it comes to product terminology, reliability, backup power and PIM we’ll all be singing from the same hymn sheet, regardless of whether we’re working in Liverpool or Kansas City.

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About the author

Erik Lilieholm is applications engineering manager, Active Wireless Products. Erik’s diverse background in North America’s wireless communications industry dates back to the launch of the first cellular communications networks. He has built his expertise with Allen Telecom, LGP Telecom and Ericsson. With more than 25 years in RF design, product management and technical marketing, Erik provides critical leadership to CommScope’s families of wireless solutions, helping each product fulfill its specific role and customer need. He holds several patents in the field of RF filter technology. Erik earned a master of science degree in electrical engineering from the Royal Institute of Technology in Stockholm, Sweden, and an MBA from the University of Nevada, Reno.

CommScope has played a role in virtually all the world’s best communication networks. We create the infrastructure that connects people and technologies through every evolution. Our portfolio of end-to-end solutions includes critical infrastructure our customers need to build high-performing wired and wireless networks. As much as technology changes, our goal remains the same: to help our customers create, innovate, design, and build faster and better. We’ll never stop connecting and evolving networks for the business of life at home, at work, and on the go.

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