August 25, 2022
Base stations evolve as each wireless generation is rolled out. It’s no different with the deployment of 5G, as tower design has changed to ensure networks can perform as necessary to achieve the bandwidth, latency, and speed specifications associated with 5G use cases. One key element is the transition from coax to fiber optic connections, which adds complexity for technicians who have traditionally focused on RF-only signals and associated tests.
Fiber has replaced coax for fronthaul and backhaul 5G connections for a number of reasons:
- Increased speed
- Lower attenuation
- Immunity to electromagnetic interference
- Small size
One new testing consideration for field technicians to verify mobile fronthaul and backhaul for C-band base stations is Fiber to the Antenna (FTTA). Implementing a sound test process and utilizing the necessary test solutions are integral for C-band networks to achieve specified key performance indicators (KPIs).
FTTA is a wireless site architecture developed to satisfy 5G requirements more effectively than traditional coax. In a FTTA, fiber optic lines connect a baseband unit (BBU) located at the bottom of the tower to a remote radio head (RRH) that sits at the top of the tower near the antennas. Digital signals that have traveled through the optical cable are converted to analog by the RRH for transmission over the network.
"The integration of fiber into C-band base stations has changed the test toolbox for field technicians and engineers."
Fiber Test Tools for C-band Base Stations
The integration of fiber into C-band base stations has changed the test toolbox for field technicians and engineers. Here is a brief summary of the equipment to conduct fiber optic testing and their respective roles:
- Inspection equipment (microscopes) – A safe and effective method to inspect the cleanliness and condition of optical connections.
- Visual fault locator – An instrument that sends a red light similar to a laser pointer down the fiber. If there is loss in the fiber, a red glow will be produced.
- Power meter and light source – A tandem solution that simulates the transmission (Tx) and receive (Rx) of an optical signal to provide a direct loss measurement. It is also known as a loss test set.
- Optical Time Domain Reflectometer (OTDR) – The workhorse instrument because it provides a detailed graph of all fiber characteristics, including loss, length, optical return loss, reflectometry, and bending/stress.
Importance of Clean Fiber
If you ask field technicians who conduct optical tests at a base station, 80% of them will tell you they have experienced a performance issue because of dirty connectors. This statistic puts a spotlight on the importance of verifying the fiber end face to make sure it’s clean before inserting it into the BBU.
To ensure dirt-free end faces, the connector ferrules should be inspected for scratches, chips and contamination using a dedicated fiber-optic microscope. All these issues can reduce the quality of signal transmission and create easy-to-prevent errors. By taking this simple and necessary first step, costly added installation and maintenance (I&M) work can be eliminated. Given the price tag of a tower crew, this can translate to tens of thousands of dollars, not to mention potential customer churn.
One example of an advanced microscope is the Video Inspection Probe (VIP) application from Anritsu. A connector inspection microscope kit, the VIP captures images digitally and displays them on an instrument display, such as the Network Master™ Pro MT1000A All-in One field tester. When an external optical fiberscope is connected, scratches and dirt on the optical connector end face can be confirmed visually. Figure 1 displays a clean end face. The connector image and detailed PASS/FAIL status is displayed as defined by IEC 61300-3-35.
Technicians can conduct the test in four easy steps:
- Take a microscope, such as the VIP probe, and insert it directly into the fiber end face
- The VIP probe will conduct automated pass/fail measurement once connection is made
- If the light is green, insert fiber end face into SFP interface of BBU
- A red light indicates a dirty end face that technician must clean following procedures and equipment according to manufacturer’s guidelines
Standardizing the Testing Process
In an attempt to create uniformity throughout the industry, all FTTA tests are conducted based on IEC 61300-3-35 developed by the International Electrotechnical Commission. This ensures that there is consistency from base station to base station and from technician to technician.
A standard method to quantitively measure the end face of a fiber optic connector or transceiver using a designated interface is detailed in IEC 61300-3-35:2015. Using an independent third-party standard establishes a criteria for all tests. It creates consistency while also making it straightforward for the instrument to conduct automated pass/fail measurements with only a few keystrokes. The result is a simple process that ensures technicians of any optical experience level can perform the measurements.
Other Testing Requirements
Ensuring the fiber end face is clean cures a lot of poor performance ills – but not all of them. A visual fault locator is key during installation, as well. Often times, technicians will wrap fiber lines together using zip ties. There are instances in which the fiber cables are tied too tightly. Technicians can use the visual fault locator to determine if the fiber has been bent or crimped during the installation step.
Ensuring fiber-to-the-antenna performance in maintenance environments is where the OTDR shines. The reason is simple – it is a single instrument that can answer all the fiber testing questions. An OTDR sends a pulsed light into the fiber and the backscatter is plotted to show loss vs. distance. A complete summary or graph of all fiber characteristics is shown to easily determine pass/fail (figure 2). Every loss will be identified, as well as its cause – connector, splice, break, bend, or anything else. Not only that, the OTDR will reveal where the loss occurs and how much loss is experienced at each event.
The importance of fiber testing, as well as RF measurements and other testing considerations are further explained in the Anritsu 5G C-band virtual showcase. Visit it today.
