May 23, 2017
Mobile operators continue to densify their networks to meet the ever-growing consumer demand for high-bandwidth services. They have deployed DAS, small cells and similar solutions to maximize the RF spectrum and maintain customer satisfaction as mobile usage continues to rise.
The integration of these solutions helps to alleviate bandwidth pressure points on mobile networks, helping them achieve stated KPIs set forth by operators. For field technicians and engineers responsible for the performance of these networks, test equipment remains a priority. To meet the verification processes associated with DAS, macro cells, and small cells, test instruments need to evolve and offer new capabilities.
In today’s post, we will answer questions about network densification and how test solutions are meeting the challenges they have created.
Q: As networks densify with DAS, small cells and other solutions, how is testing evolving?
A: Densification makes capacity increasingly important, especially with LTE. Because of this, the focus is on received signal quality and spectrum assurance. There are a variety of ways to ensure a clean uplink to significantly improve capacity. This focus on capacity also means, in some cases, new measurement techniques are required. DAS systems, such as the one shown in figure 1, are a prime example of shifting measurement techniques.
Q: What makes DAS testing different from other network elements?
A: The number of tests drastically multiplies in DAS. Typical tower installation tests consist of antennas, splitters and other passive RF components. These tests can have up to 150 sweeps to determine return loss, distance-to-fault, cable loss, and PIM. Some additional fiber tests may also be necessary. These tests can be done with traditional processes.
There are considerably more tests associated with DAS, as procedures may require testing three or four frequency bands on every cable. Even a medium-sized DAS install could require several thousand tests. For example, a football stadium can have as many as 15,000 tests, which means that the testing, troubleshooting, and reporting process can take months.
Q: How will testing transform to address the increased complexity of DAS?
A: Because of the large number of tests needed, existing processes based on tower techniques don’t scale, so new procedures need to be developed. Automation of field tests, as well as reporting, is currently possible, and is one big step towards making DAS testing more effective.
Q: How do you deal with the increased amount of interference opportunities brought by densification?
A: The first step is to understand that interference is a receive issue. Signals that reach a radio’s receiver will affect the radio’s front end, even if these signals aren’t wanted. These unwanted signals create a reduction in the radio’s sensitivity, causing increases in bit error rate (BER), frame rate, dropped calls and other negative effects. This occurrence is called receiver desense or desensitization. If severe enough, it’s known as receiver blocking.
Interference does not have to be on a receive channel. It just must arrive at the input of a receiver, and it can be on a different frequency than the receive channel. In other words, it can be in band, but it doesn’t have to be on channel. This makes the receive pre-filter very important, as it can be selected to block out-of-channel, in-band, signals. If an unwanted signal does reach the receiver’s input, however it can cause desense.
Q: How do interference sources change from macro cells to small cells?
A: The biggest difference between macro cells and small cells is that small cells are likely closer to the source of interference. Since RF signals exhibit path loss, the closer a receiver is to the interference, the stronger the interfering signal will be. Small cells, because of their multiple locations, will be closer to some RF sources. They will be affected by interference that would not affect macro cells.
Anritsu offers various solutions for conducting more efficient interference hunts. Starting with the simple methods, there are traditional direction-finding tools. Using a spectrum analyzer and a Yagi antenna (figure 2), the user can look for the strongest signal and triangulate. There are also tools that can actually put a map on a spectrum analyzer. A higher level is car-based signal location, where a user can drive around and seek the strongest signal. This method tends to lessen the effect of multi-path, an issue in dense urban areas where small cells are likely to be deployed.
Another solution is remote monitoring, because interfering signals aren’t always present. A signal’s occurrence times can be characterized through short-term monitoring. All Anritsu spectrum analyzers are web-enabled, meaning they can be controlled from a distance via web browser. So, a user can configure one at a site and leave it for weeks. They can check the site remotely to save time and money, and maintain network performance.
There is also long-term monitoring. A set of headless spectrum analyzers can be either temporarily or permanently put in place. Software for these devices allows users to monitor and maintain signal quality through their networks as aspects change. The devices can be comfortably monitored remotely.
Q: What impact does densification have on spectrum assurance?
A: Spectrum assurance is an umbrella term for a family of spectrum monitoring, interference hunting and signal mapping tools that can be used in-building and outdoors. Anritsu is developing these in response to our customers’ requirements for the capacity associated with LTE and densification.
Q: Where do you see testing in the future?
A: We’re going to be moving closer to real time. Right now, cloud-based solutions are tremendously efficient. The test automation for DAS is cloud-based and that will continue. There will be more control, remote expertise and remote dispatch in the near future, as well.
Q: Monitoring allows you to identify and fix a problem. Can it also allow operators to optimize the use of the network resources?
A: Yes. One example is a service-assurance solution such as Master Claw, a big-iron software application. Another is a software application that works with long-term RF monitoring probes to help characterize when interfering signals occur and how they appear. Through triangulation, this application helps determine the interfering signal’s location and provides all the necessary information to find it.
There is much to learn about DAS, interference and the test tools necessary to address network densification. Anritsu has published an application note that explains DAS testing procedures, as well as an interference hunting application note. Anritsu has also designed a poster that provides examples of interference sources. All are helpful resources for field engineers and technicians.