Distributed antenna systems are one of the fastest-growing types of cellular infrastructure. Global annual DAS sales will top $1 billion this year, ABI Research predicts. And by 2017, more than 2.5 million DAS nodes will ship annually, Mobile Experts says. That’s double the current amount.
The DAS antenna market is booming because mobile operators and site companies understand that these solutions are key for meeting customer expectations for seamless coverage, capacity, and consistently fast data rates. DAS enables the quality of service that’s key for attracting and retaining customers, including high-value business users. DAS also is an ideal way for operators to accommodate trends such as the dramatic increase in photos and videos that consumers upload to social networking sites.
But as with any other type of wireless infrastructure, a DAS solution is only as effective as its designer’s ingenuity and attention to detail. A prime example is passive intermodulation, which plays a critical, but often under-appreciated, role in determining whether a DAS solution can deliver the kind of uplink and downlink performance that helps an operator stand out from the competition. That’s why mobile operators increasingly are requesting stringent PIM requirements to minimize interference, which undermines data, video and voice performance, and saps network capacity noticeable to customers in the form of dropped calls, slower download speeds and other KPIs (key performance indicators). In extreme cases, PIM can knock out an entire receiver channel or network, such as a stadium or building. PIM doesn’t generate revenue, and it competes with paying customers for scarce, expensive resources, including spectrum.
What causes PIM?
PIM occurs when two or more signals are present in a passive, non-linear device or element – in this case, a DAS solution. The signals mix with each other to generate other signals that are related to the original signals. PIM occurs in elements that would be expected to operate in a linear fashion. PIM is generally the result of spurious generation of non-linearities – typically occurring in connectors, switches, isolators and the like. Passive antennas can be main contributors to poor PIM performance if not designed properly as they are the main air interface for the system. Dissimilar materials, antenna design techniques, as well as oxidation and other effects are likely causes.
The PIM performance of an antenna designed to target DAS applications can be affected by antenna radiator design, material selection and connection methods internal to the antenna. Material science knowledge and EM simulations are helpful during the design process to map the near-field characteristics of the antenna to better manage field structure and intensity internal to the antenna assembly. A successful antenna design for low PIM applications will require PIM characteristics to be regularly measured during the antenna prototyping and test phase.
Poor design and manufacturing are common causes of high PIM in a DAS solution. Some examples include:
–Junctions within the system which contain dissimilar materials. The RF connector/baseplate interface in the antenna can often be a source of PIM issues if material selection and plating type is not properly considered.
–Material type and plating type selected for the radiating element of the antenna design is critical due to the high currents present.
Low-cost, low-performance cables and connectors are often contributors to poor PIM results. PIM can be generated if the coax cables don’t have tight braids, the braids’ tinning has cracks or the connectors are poorly assembled. DAS vendors can minimize these problems with quality control to identify cracked tinning and poorly assembled connectors, by putting a layer of foil under the braid or both. Some DAS antennas are required to have antenna cables or “pigtails” permanently attached or integrated into the design, making proper coaxial cable selection critical for PIM performance.
The lower the PIM specification; the better performance a DAS solution will provide. How low is low enough? Vendors typically aim for -110 dBc, but state-of-the-art solutions are as low as
-153 dBc. Ideally this spec should remain constant across all 2G, 3G and “4G” bands, from 700 MHz through 2.7 GHz, to ensure consistently great performance for all customers and all devices.
Importance of proper installation and maintenance to maximize DAS benefits
Other common sources of PIM occur during or after installation:
–Metal objects, such as rebar or suspended ceiling grids that sit in the signal path. Identifying and avoiding those situations minimizes PIM. If they can’t be avoided, using a directional antenna can mitigate the objects’ effects.
–Corrosion of cables, connectors and other components – a problem often referred to as the “rusty bolt syndrome.” Periodic inspection of a DAS installation can identify deterioration before it elevates PIM to the point that it becomes noticeable to customers. As previously mentioned, proper plating and material selection can lead to an antenna design that maintains PIM performance during the lifecycle of the DAS system.
PIM testing is an important step in the design and installation process to ensure maximum performance from the system. Some testers are designed for use during manufacturing of DAS solutions. Others are designed for use in the field to assist with installation and troubleshooting. Field testing assesses the entire DAS installation, from the feeder cables to the antennas. It’s a good idea to test each antenna before it’s installed. If subsequent testing – say, one year later – identifies failures, rusty bolt syndrome is one likely source.
Investing from the start in a low-PIM DAS solution eliminates the need for antenna upgrades due to subpar performance from a high-PIM DAS system – providing mobile operators with positive dividends in the long run. In addition to being a powerful tool for improving coverage and capacity, low-PIM DAS solutions reduce operators’ macro base station capital expenditures and operating expenditures, minimizing their need to purchase additional spectrum. Additionally, through proper, high-quality antenna design, installation and periodic testing, mobile operators can be assured that a DAS solution will provide consistently great, market-differentiating performance year in and year out.
Anthony McCray is North America Senior Sales Director (DAS and Small Cell Specialist) at Ethertronics. He has vast experience with DAS, small cell, RRH, MCPAs, TMAs, BDAs, antennas and test and measurement solutions. McCray was previously North American Director of Sales at Comba Telecom, National Sales Manager at Commscope, Senior Sales Engineer at ADC,and Business Development Manager at Northrop Grumman IS Broadband AMR/AMI, First Response, and Municipal Wireless applications. McCray’s extensive Wireless RF Engineering and RF Network management experience also includes roles at Verizon Wireless, Vodafone and Qualcomm. McCray has a rich entrepreneurial history, notably co-founding 3G Wireless Entertainment, an RF In-building and Extended Coverage consulting firm. At 3G Wireless Entertainment, he served as Director of Business Development and Engineering.