June 30, 2017
Today’s base stations integrate multiple technologies. From traditional RF cables and fiber optic lines to small cells and macro cells, field engineers and technicians must test and verify more elements than ever before.
This integration is part of a decade-long evolution that has also seen radios travel from the base station on the ground to atop the tower, with a baseband unit (BBU) installed in the equipment room. While this configuration has helped reduce operating expenses (OpEX) by shrinking the size of the building and lowering the equipment room air conditioning cost, it has created new testing challenges.
To meet these challenges, test solutions have evolved, as well. Fiber tests have become a necessity, which may appear foreign to field technicians who have spent their careers in the RF space. Solutions that integrate RF and fiber testing with built-in measurements are now available to address this aspect. Another issue – a costly one at that – is that by moving radios to the top of the tower the installation process can be more time consuming and expensive. New techniques are available on this front, as well, to help reduce costs while ensuring operation.
CPRI was integrated into base stations to connect the Remote Radio Heads (RRHs) atop the tower with the (BBU) in the base station. Some of the new test considerations faced by the incorporation of CPRI are listed in figure 1. A few things to note with this list is that absolute measurements can be made in dBm on RF signals, while CPRI has relative measurements in dB. Another big difference is that a center frequency can be specified when conducting spectrum-based measurements in the RF domain. Because everything is baseband with CPRI, tests are frequency agnostic. Also, a full span can be covered with an RF test whereas with CPRI only so much IQ data is transmitted, so there is a limit to the maximum span seen on the analyzer.
Logically, CPRI integration in base stations has expanded the tests that are necessary to ensure network operation. Common CPRI-based tests are shown in figure 2.
Faster Turn Up Time; Lower OpEx
The integration of RRHs on top of the tower has led to a change in the deployment of base stations. During the initial stage, the RRH is installed with the fiber optic cable, antennas and other elements. Once that is completed, the BBUs are commissioned. This multi-step process adds time and costs to the project. A new test technique – BBU emulation – has been developed to solve this problem.
There are two main advantages of BBU emulation:
- Faster turn up time, resulting in carriers collecting revenue earlier from a new site and greater profits for contractors
- Lower OpEx
The latter benefit can be realized in both new construction and existing sites. We have talked to people in the field and 3-6 truck rolls are typically necessary to fully commission a new site. One crew is deployed to install the RRH and associated elements. A second crew is then sent to commission the BBU. Upon completion of the BBU install, testing is commenced. If there is an issue with the RRH, fiber, antennas, etc., it will be revealed at this time, requiring the original crew to return to fix any issues. The BBU crew will then return a second time to verify the base station.
BBU emulation streamlines the entire process by allowing technicians to test the RRH infrastructure without a commissioned BBU. The result is that tests are made with the construction equipment still at the site, so if any repairs need to be made the crew is already at the location.
BBU emulation can also be used to monitor and test existing base stations. Currently, the general practice is to replace the BBU when a problem is detected. If the issue persists, the RRH is swapped out. This approach can create “No Trouble Found” (NTF) costs from the manufacturer, all of which are eliminated when using BBU emulations.
All necessary installation tests can be made with BBU emulation, including line sweep of jumper cables and antennas, and cleaning and viewing of all fiber connectors. Verifying SFP port location and suitability can also be conducted with BBU emulation. By SFP port suitability, we mean to ensure that the port is compatible with the CPRI line rates used at the base station. (One note: the most common are Line Rate 3, Line Rate 5 and Line Rate 7.)
Conducting BBU Emulation Tests
The benefits of BBU emulation can be realized using an appropriately configured handheld analyzer, such as the BTS Master MT8220T (figure 3). When conducting tests, the user must ensure there is power to the RRH and the SFP supports the required line rate. Both ends of the fiber must be cleaned with the proper tools. The duplex fiber pair between the RRH SFP and tester SFP must then be connected. Once that is completed, the analyzer should be configured in BBU emulation mode, allowing the instrument to provide the system clock and serve as the master.
When it is in BBU emulation mode, the analyzer immediately starts an auto negotiation of line rate. Communications with the RRH are established over CPRI Link via Ethernet link, HDLC (High level Data Link Control), or a proprietary method developed by the vendor. The field technician can select the RRH to be tested from the list (IP address), and OEM Command and Management messages can be exchanged over the Ethernet connection between the BTS Master and RRH.
After the BBUs have been commissioned, overall system performance needs to be validated by checking uplink spectrum for interferers. One method is to install an optical TAP in the system, which facilitates performance testing from ground level in the equipment room without taking the tower down. When conducting this test, field technicians must remember that the analyzer needs to be in normal CPRI mode to monitor the CPRI link and that it is no longer supplying the system clock.
Summary Carriers and contractors can save valuable time and money by using BBU emulation to test new and existing base stations. It can verify that RRH system components are installed correctly, as well as RRH system functionality, while also testing for interference issues. It allows for quicker commercialization of new construction sites and is useful for troubleshooting existing installations.
To learn more about BBU emulation, watch this informative webinar.