February 2, 2017
Base stations have undergone quite the evolution as wireless networks advanced from 3G to 4G. To accommodate the fast speeds and high bandwidth associated with LTE networks, Common Public Radio Interface (CPRI) technology is now commonplace, used to connect a Baseband Unit (BBU) and one or more Remote Radio Heads (RRH) sitting atop the tower. As part of this progression, fiber optic cable, which has extremely low transmitted signal loss to accommodate LTE networks, is prevalent in base station design.
Technicians spent decades handling copper. Now they are installing and maintaining fiber, which has many different characteristics than coax. There are several causes of failure in fiber connections, but here are the most common – and ways to prevent them.
1. Dirty Fiber – Simply touching a connector end with a bare finger can leave oil on the fiber, which will affect performance. That is one reason why 60%-75% of the alarms, failures and poor throughput issues found in modern cellular systems are due to dirty fiber. As you can see in figures 1 and 2, a dirty fiber connector can result in failed International Electrotechnical Commission (IEC) tests. These can include the IEC 61300-3-35 pass/fail test profile, which is an industry-accepted procedure that ensures that a fiber is clean and damage free.
Figure 1. A dirty fiber connector end fails IEC 61300-3-35 standard based test.
Figure 2. After a cleaning, the same fiber connector end easily passes IEC 61300-3-35 standard test.
2. Mismatched Small Form-Factor Pluggable (SFP) Modules – Using improper line rate, optical wavelength or fiber type will lower network performance. Single mode (SM) fiber’s most frequent wavelengths are 1550 nm and 1310 nm, while the most common wavelengths used with multimode (MM) fiber are 850 nm and 1300 nm. For best results, matched pairs of modules between BBU and RRH should be used.
3. Incorrect Fiber Type – It is crucial to use the correct fiber type that is matched to the SFP or SFP+ type in a particular system. If MM fiber is used in a SM system, or vice-versa, the system may work over very short distances, but there will be propagation issues if used for long distances. The center conductor of the MM and SM fibers have different diameters. MM typically uses 850 nm lasers; the propagation of its signal would be affected if passed through a SM fiber and vice versa.
4. Distance Between BBU and RRH – Problems may occur when there is too much distance between the BBU and RRH for the required line rate associated with the SFP and fiber. MM fiber is typically not capable of high-speed data transmission of greater than 1 Gb/s over long distances. SM fiber should always be used in this case. Costs of SM fiber and modules have decreased significantly, and with data rates constantly increasing, SM equipment is being used in the majority of systems being deployed.
5. Damaged Fiber – As any technician knows, fiber is more sensitive than coax. Surface damage, such as scratches or chips, as well as pinching from excessive bending can cause a significant loss in optical transmit power. Something as simple as over tightening a cable tie can cause sharp bends that damage the fiber. Prevent these issues by properly protecting, housing and installing optical fiber.
6. Incorrect Tx/Rx Connections – Many of today’s systems use hybrid cable assemblies that bundle power, fiber and sometimes CAT 5e cables. The fiber ends may be terminated with single LC connectors, therefore it is important to make sure that the Tx and RX are connected to the correct ports on the SFP. Other examples of incorrectly connected Tx or Rx can be found in systems with redundancy protection.
7. Connectors Not Fully Seated in SFP Modules – Both duplex and simplex LC connectors are quite small and rather delicate. Small plastic tabs lock the LC connectors in place once they are inserted far enough into the SFP modules. Similar to Cat53 or Cat 6, the tabs must be pressed downward to unlock before being carefully pulled out. It is incredibly vital for the connectors to be fully locked in order for the system to function correctly. Even the smallest gap between the fiber end and the receiver or transmitter will result in significant signal loss.
8. Wrong Sectors Connected – It is possible to improperly connect the fiber pair to the wrong sector RRH in a typical multi-sector RRH system being fed from a single BBU. As systems become more complex there will be many fiber pairs, and if they are not all properly identified and connected to the right locations, the system will not function at its optimal performance.
9. Improper SFP Connections - Many RRHs have additional SFP ports to support daisy chain topologies. Plugging the SFP module into the wrong SFP port on the RRH will make the system inoperable. The BBU generates the master clock, and the RRH has a master clock output on its second SFP port. If the connection between the BBU and RRH is using these two connectors, they each try to set the master clock, and the CPRI link will not configure.
Test Tools
Technicians can prevent these fiber issues through specific pass/fail tests and inspections. Fortunately, there are several handheld products, such as the Anritsu Site Master™ S331L and the Microwave Site Master™ S820E, that offer Visual Inspection Probe (VIP) capability. VIP mode captures connector images digitally and shows them on the analyzer’s large display so technicians can perform visual inspections on fiber optic connectors and automatically conduct pass/fail testing per the IEC 61300-3-35 standard. These instruments can help facilitate these tests and be part of an inspection process that is absolutely crucial to ensure functioning systems.
A new technology page focusing in the today’s base stations has just been published by Anritsu. There, you can access a new application note that details the importance of inspecting and cleaning fiber in an optical BBU/RRH link.
