The challenge of how to get power to devices mounted on lamp posts, tops and sides of buildings, security fences, and many other unusual places can often be the very last thing on an engineer’s mind.
Take the case of small cells. What’s the first thing any good 4G-network planning engineer should ask? Probably, “What does my 4G coverage map need to look like?”
That makes perfect sense. So we take a nice map, position every small-cell device for maximal 4G coverage – ensuring coverage of all those hard-to-reach places in between buildings – and everything looks great. But then a site survey is conducted and everyone realizes there’s no electrical power in the middle of that public park as well as several other places.
Of course, electrical power may be nicely available from the local utility company for the small cells to be positioned on lamp posts, except the utility wants a power meter placed on every device. What if negotiations to tap power from a strategic apartment complex suddenly go south when the building owner refuses to allow access to the building because of tenant complaints?
These are real life examples drawn from network access deployments over past years.
One way to solve the challenge of delivering power to odd locations is to use the same cable that connects the optical fiber, eliminating the reliance on local power availability near the device. So-called hybrid optical/copper cables are not new – several designs have been on the market for 20 years or longer.
If that’s the case, then why haven’t such hybrid cables been sold in large volumes until now?
It seems logical that combining power and optical communications delivery into one cable should provide time and cost savings to network providers. Isn’t that, after all, what Power-over-Ethernet (PoE) technology is really all about – combining power and communications under one sheath? There are two reasons that until recently these cables haven’t taken the market by storm.
First, the industry’s hybrid cables up to now have proven very expensive, large in size, and time-consuming to terminate due to very complex cable designs. Second, getting power through these cables and accessing it from the other end of the device required significant engineering expertise. These systems have been difficult to deploy, and there are no “off the shelf” options generally available.
Let’s take a look at the cable designs for hybrid optical/copper cables. Until recently, common hybrid cables typically used some type of loose tube optical cable design in which one or more buffer tubes were replaced with conductors (see Figure 1).
A less common design referred to as a composite design, involved taking individual optical and copper cables and bonding them together in a “figure eight” arrangement (see Figure 2) – more specifically, two side-by-side cables possibly bonded together by a third plastic sheath.
These cable designs are complicated in how the cables are manufactured and how they’re accessed for fiber splicing and copper termination. Also, making these kinds of cables requires three to six separate manufacturing steps, depending on the cable design, increasing the cost of the cable.
Accessing a typical loose tube style of hybrid cable requires several steps as well as specific optical and copper cable access tools.
A survey of several customers with experience in deploying such cables reveals that it typically takes a minimum of two or three minutes to terminate them. In some cases, the amount of time could be as long as five minutes or more to prepare one cable end.
That amount of time may have been acceptable in the past for unique projects where only a few cables were deployed. But considering the current massive deployments of small cells, Wi-Fi access points, HD security cameras, and other access devices, the high cost and time constraints of the past are totally unacceptable.
For hybrid cables to become viable options for these markets, the cable must have characteristics resembling fiber to the home (FTTH) optical drop cable. The cable must be cost-effective and fast to deploy. It’s most desirable for such a cable design to be simple, ideally produced in a single manufacturing step, and able to be terminated in well under a minute.
But having a great hybrid cable is not necessarily enough. The majority of providers seeking to deploy hybrid cable don’t understand the challenges involved. For instance, there are voltage drop penalties over distance; electrical protection may be needed, such as surge protection or protection against lightning strikes or cable cuts.
Providers must also know how to convert an optical signal to an electrical or PoE input at distances greater than a kilometer. Typically, most access devices such as small cells, Wi-Fi access points, and security cameras are not designed for direct fiber input. Therefore, media conversion is often necessary. Additionally, what type of power supply should be used?
There are lots of considerations beyond the cable itself. In short, every hybrid optical/electrical deployment of the past has required a unique electrical engineering design because no off the shelf options were available.
Another consideration is battery backup or Uninterruptible Power Supply (UPS). It’s highly desirable to have emergency battery backup for small cells to handle emergency phone service. But backup power for security cameras, critical Wi-Fi applications, and other access devices may be equally important.
But if we truly envision millions of these access devices deployed globally in the coming years, then deploying and supporting millions of individual batteries probably isn’t the most effective strategy. Cost, environmental concerns, maintenance, and other such issues would seem to discount that approach as a viable alternative.
A centralized battery backup system is a much better alternative. Delivering power via hybrid cabling to many devices from a central location (perhaps in a star network arrangement) as a UPS service could support this approach.
Such a central location could be an outdoor distribution cabinet, a telecom closet, a macro base station location, etc. Any location that has access to power and the optical network could be a potentially viable alternative for such a network architecture (see Figure).