Fast, Efficient DAS Optimization with the SIQMA CPRI Analyzer
Challenge: Identifying and optimizing distributed antenna system (DAS) performance for one of the nation’s largest football stadiums
Solution: Utilizing the SignalCraft SIQMA CPRI analyzer to accurately and simultaneously capture multiple real-time RF signals for increasing troubleshooting efficiency
Imagine one of the nation’s largest football stadiums packed with over a hundred thousand people, and each attendee is equipped with his or her own cell phone. The difficulty is making sure the facility can service a high density of simultaneous mobile connections while maintaining a high level of signal quality. Wireless service providers typically rely on the stadium’s distributed antenna system (DAS) to get their signals into the building, and after each major event, the carriers receive reports on which sectors experienced performance degradation and need adjustment before the next sports game.
Once the carrier’s RAN engineers or cell site technicians troubleshoot and identify the root problem, they can present their findings to the third-party DAS operator and request the appropriate adjustments. Since each carrier is asking for DAS changes that may inadvertently affect another carrier’s signal, the wireless service providers must iteratively improve their performance levels to avoid getting a bad reputation for poor connectivity.
Choosing the Right Tool for DAS Optimization
Our customer is a senior RAN engineer at one of the largest wireless telecommunications providers in the United States, and his job responsibilities include the investigation of signal integrity issues at a major college football stadium. It would be very time-consuming to plug in an RF spectrum analyzer into each radio head to study the signals. Instead, he needed a CPRI analyzer that could tap into the fiber optic line between the radio heads and baseband unit to easily pull out the IQ samples and convert them into spectral data in the frequency domain for analysis.
The problem was that other CPRI analyzers on the market were unable to display multiple signals simultaneously, making it harder to detect imbalances. Plus, they took a long time to boot up, and the available sweep rates were slower than expected. When we introduced him to the SignalCraft SIQMA CPRI analyzer, the performance improvement was significant; all the functions ran faster with very little lag time. Plus, SIQMA is able to capture every CPRI frame for 100% interference detection and provide simultaneous multiple antenna support, offering the highest quality CPRI analysis for the lowest cost.
Dealing with Common DAS Troubleshooting Issues
When optimizing DAS performance, CPRI analysis helps with interference detection and hunting, passive intermodulation (PIM) detection, and many other applications. Here are some common issues the senior RAN engineer uncovered at the stadium using SIQMA:
Issue #1: Diversity Imbalance
During the commission and optimization of the DAS, overlapping coverage between adjacent antennas makes power balance critical to the proper operation of the overall system. This screenshot from SIQMA displays the channel noise for two antennas from the same radio head. The sudden blip on the purple signal represents the actual LTE signal or someone’s mobile data coming through. Unfortunately, the noise floor of the blue signal was so high that all the mobile traffic was masked and could not be detected. SIQMA’s ability to view signals from multiple antennas at the same time allowed the RAN engineer to quickly detect the diversity imbalance that existed.
For optimal performance, the signals from both antennas must have similar noise floors and traffic levels. When looking at amplitude alone with no spectral component, the RAN engineer cannot verify whether the issue is caused by gain, external interference, hardware, or signal-to-noise ratio (SNR). All he or she can tell is the amplitude of one antenna is much higher than the other one. Therefore, the natural reaction is to lower the uplink gain on the antenna path with higher amplitude level. If the diversity imbalance is caused by something other than uplink gain, this will simply reduce receiver sensitivity and offer no real benefit. Instead, SIQMA allowed the client to simultaneously view both antennas’ spectrums, clearly seeing the relationship between the traffic and noise floor. Intelligent adjustments could then be made, and the results were monitored in real-time to improve overall SNR.
Issue #2: Passive Intermodulation
Intermodulation products are a type of distortion created when two or more signals pass through non-linear systems, often caused by saturated RRU amplifiers or corroded RF connectors. When these distortion products fall within the radio head’s uplink channel or Rx band, system performance is degraded. A classic sign of PIM is that undesirable slope in the noise floor of the channel.
For troubleshooting purposes, it can be helpful to disconnect the transmit line for the DAS or reduce the site Tx power. If disconnecting or reducing the transmit power causes the slope to go away, there is clearly a PIM problem at hand. Another technique that some RAN engineers use is called artificial loading – also known as Orthogonal Channel Noise Source (OCNS) or Air Interface Load Generator (AILG) – to ensure that the DAS is transmitting all available resource blocks at maximum power.
Ultimately, our client did not find equipment with automated testing features to be useful for PIM troubleshooting, since the tools couldn’t account for the complexity of DAS applications (such as which zones were combined).
Issue #3: External Interference
As spectrum occupancy increases, another important aspect of DAS optimization is the detection and identification of in-band interference that affects network capacity and customer quality of service. When viewing the following traces shown by SIQMA, it’s clear that we’re not dealing with a mixing problem. The question becomes whether the spikes are caused by external interference or perhaps faulty hardware in the DAS chain (such as an amplifier issue).
Oftentimes the base station or sector is connected to multiple DAS remotes, so in order to easily isolate the issue, the client simply turned off the remotes one at a time to see if the issue disappeared. This troubleshooting could be accomplished with remote tools, but this task quickly becomes tedious if the CPRI analyzer only offers slow sweep rates. Even if the scans are run manually by the engineer, other RF tools often just report the amplitude. With SIQMA’s fast sweep rates that can detect and trigger on intermittent interferers, the client was quickly able to detect the interference and narrow down the list of possible causes based on seeing the spectrum.
Increasing Troubleshooting Efficiency with SIQMA
Because of SIQMA’s easy-to-use interface and fast response times, our customer was able to quickly resolve several DAS issues within the span of a couple of hours. Our CPRI analyzer provided much greater accuracy with troubleshooting and allowed him to record video of waveforms to further analyze and show the DAS operator. Also, the client appreciated the portability of SIQMA with its ability to run all day on battery power or plug into AC power.
Recently, he ran into a CPRI signal that contained eight antennas on one line, something he had never seen before. At the time, that version of SIQMA could only display four antennas, so he captured the CPRI data to send to our engineering team. We were able to quickly turn around and provide an update that allowed him to view any four signals out of the eight.
All-in-all, the superior performance and excellent customer support for the SIQMA CPRI analyzer exceeded the customer’s expectations and allowed him to complete his DAS optimization responsibilities with greater efficiency and productivity.
Visit the SIQMA product page to learn more about our latest and greatest CPRI analyzers.