September 25, 2015
In our last post, we mentioned how the mobile communications evolution from analog to digital has changed network infrastructure nearly as much as it has altered mobile devices. Today, we will continue to explain how changes to base station design have forced field engineers and technicians to incorporate new test tools and procedures to ensure optimal network operation.
A decade ago, RAN was suitable for the analog transmissions associated with commercial wireless. Current networks, which must accommodate high bandwidth 4G signals, are designed with fiber-based macro cells and C-RANs. The integration of small cells and C-RANs has forced test companies and those responsible for wireless networks to take a different approach to verifying base station performance.
C-RAN designs feature centralized Base Band Units (BBUs) that control multiple Remote Radio Heads (RRHs) at the antenna sites. Typically, a Common Public Radio Interface (CPRI) is used to connect the BBU to the RRH. While many people focus on how CPRI links have brought about the need for fiber testing, those dealing with network operations on a daily basis understand another challenge – testing the RRH. CPRI links are a key to unlocking a cost-effective and time-saving method while having both feet on the ground.
What is CPRI?
CPRI is an open standard that has been evolving for more than a decade. In the current CPRI standard, bit rates are referred to as Option 1-9 (figure 1). Most U.S. installs use Option 3, which has a bit rate of 2.4576.
Figure 1
Two main CPRI layers run over C-RAN. Layer 1 provides the physical transport while Layer 2 is split into several areas. Those responsible for installing and maintaining base stations need to understand the L1 in-band protocol area in Layer 2 to help troubleshoot errors and alarms. Much of that topic was outlined in our previous blog. Today, we are going to answer the question, “How can CPRI links help conduct cost-effective RF measurements on RRHs?”
A Problem Upon High
RF performance engineers and NEM performance engineer groups in charge of optimizing a wireless network must take a new approach to testing RRHs. They need a way to achieve easy access to RRHs installed atop monopoles or other sites with restrictive access. If operators can’t find a solution, they have to resort to calling in a tower crew, which is an unwanted expense, and a large one at that. Further hurting the bottom line is that the site/sector may go down due to interference while the tower crew is dispatched, arrives, and finally repairs the site.
A key need is the ability to analyze the uplink RF spectrum from ground level via the CPRI optical fiber link, as it can reduce operational cost and increase response time to resolve interference issues. This eliminates the need to call a tower crew to climb to the top of the antenna, and connect a spectrum analyzer to the RF sniffer port to look for interference.
Certain handheld analyzers (figure 1) permit technicians to make measurements using CPRI links. The handheld analyzer converts the optical CPRI link information to IQ data so that RF spectrum measurements can be made to locate and subsequently correct interference that may occur in the uplink. It allows field technicians to use an instrument in which they are familiar, as well as conduct measurements in a manner they have experience. Tests that can be made are:
Interference analysis – Uplink RF data can be measured to determine if there is an interfering signal impacting the RRH. This can be accomplished by using a spectrum analyzer view, as well as a Spectrogram display.
PIM detection – Technicians can also measure uplink IQ RF data to determine if there is an obvious PIM-based interference by viewing a rising noise floor on the spectrum analyzer measurement display.
Signal strength – Downlink RF measurements of a BBU stream can also be conducted.
The new era in wireless access – one in which consumers currently enjoy streaming video and will soon experience broadcast applications – has changed base station design and made it more difficult to properly install and maintain networks. Using new techniques that take advantage of CPRI links and industry accepted analyzers can make testing RRHs from the ground possible, reducing the cost and saving time.
While this post focused on testing RRHs to eliminate interference, outside sources can also negatively affect network performance. In these instances, mobile interference hunting systems are beneficial. To learn more about this solution, visit a webpage with white papers, application notes, and videos.
