Most all of us know that all wireless systems are interference limited. In Macro cellular design, great efforts are made to minimize interference, both internally generated and externally generated from various sources in order to maximize capacity. However, modern wireless systems including LTE and LTE-A (Advanced) are increasingly susceptible to interference due in part to the use of advanced modulation schemes. In the Macro environment most of the interference can be mitigated at the design stage but some is just a function of environmental and deployment factors. Careful cell site selection, location and design are increasingly important. Adding new frequency bands and increasing capacity utilization of existing bands all serve to expand the likelihood and effect of interference. The latest 600MHz band offered in the most recent auction is known to be particularly susceptible to Intermodulation interference from other cellular systems (intermod is caused by a mixing of frequencies, somewhat like notes from two different instruments creating a beat note which falls on top of another desired note).

Complex as this may be, it is far worse in a building where not only is the RF environment challenging but continually changing as the building itself is repurposed.

Not only are internal factors a consideration, but external interference from local radiating sources must be considered and designed for (or at least contemplated and quantified). Finding the sources of interference can be tricky and buildings components (such ceiling tiles with RF reflective backings or other materials with adverse characteristics) can also play a contributory role in RF optimization and design.

In addition, Passive Intermodulation (PIM) is a growing concern in all Wireless systems.

What is PIM

Essentially, PIM is self interference caused by high power signals mixing together and causing interference, often at connectors or bent cables. This can be a problem if the interference lands in the band of the desired signal. It’s a bit like strumming two guitar strings at the same time. If the frequencies are ‘correct’ you will hear a beat note or harmonic. What you are hearing is the sum and difference products of the two notes mixing to create another harmonic tone. If you looked on an oscilloscope you would see the original tones and a series of mixing products spaced at equal intervals. That’s essentially what happened with PIM.

PIM occurs in antenna elements, coax connectors, coax cable, and grounds and can be caused by rust, corrosion, loose connections, dirt, oxidation, and any contamination in the signal path. PIM can even be caused by reflections from nearby metal objects. This is the so called ‘rusty bolt’ effect where two dissimilar metals create a diode that serves to mix the signals. The greater the non linearity and the input signal, the greater the power of the Passive Intermodulation (PIM) energy produced.

Effects of PIM

No system is perfectly linear so PIM is present to some degree in almost any wireless system but is usually small in relation to the desired signal. However, it is emerging as a significant problem in cellular basestation antennas and related components including jumpers, filters and tower mounted amplifiers; where power levels are high and in close proximity to the receiver.

The problem scenario is typically where two strong RF signals are close in frequency, such as in a base station, mix to create PIM effects. If the PIM generated signal is the same frequency as the desired received signal it will sit under the desired signal raising the noise floor, blocking signals and reducing receiver sensitivity. This can result in dropped calls, decreased system capacity, slower data rates and even shrinking of the affected cell radius, resulting in phones and devices at the planned cell edge experiencing poor or no service at all.

PIM tends to get worse as components age and in harsh environments where there are wide variations in temperature or vibration causing flexing and subsequent ingress of moisture, particularly sea air. PIM can be created by bent cables (which cause undesirable reflections and induce non linearities.) Additionally, as more frequencies are deployed, the probability of adverse combinations increases, which in turn increases the likelihood that a PIM signal will be created. The addition of new frequency bands and the increasing use of carrier aggregation have all served to increase the probability and magnitude of PIM issues.

Detecting PIM

The general technique for measuring PIM is to inject two high-power RF signals similar to the ones used in the actual system, so on the order of 20 W or 43 dBm for a base station, and apply them to the system under test . The the resultant signals are observed on a spectrum analyzer or by using specialized PIM testing equipment. Usually, the biggest issue is the third order component or IM3, but other combinations and harmonics can significantly impact service in many circumstances.

PIM in DAS systems

When Distributed Antenna Systems (DAS) were first deployed PIM was a seldom a problem. However, due to the increasing complexity and number of frequency bands, connections, feeds and antennas in a typical DAS system, PIM has grown to become a significant and critical factor in deploying a successful,high performance DAS system.

DAS remote unit output ports generate multiband and multicarrier signals which can mix together at every passive stage, generating a wide variety of interference which can fall in multiple uplink bands. As such, PIM specifications for passive components are critical.

PIM can affect both Active and Passive DAS, as there will always be areas where PIM can occur. In an Active DAS, PIM can be generated in the passive RF sections between the Carrier’s radios and the DAS head end, as well as in any RF sections beyond the Remote Radio Heads. In a Passive DAS, everything beyond the Carrier’s RF Source can generate harmful PIM signals.

The DAS OEMs understand this well and many now offer extremely high linearity connectors, splitters,couplers and employ rigorous testing and strict cable deployment guidelines to minimize this issue.

Mitigating PIM Interference

Wireless is complex and its deployment needs are very particular. Mitigating PIM begins with good design practice and a deep understanding of the causes and potential resolution of PIM induced performance problems. Progressive businesses understand these requirements; they will tackle the issues and will design for the future.

MobileNet’s engineers work on all steps involved in pre-launch optimization, including building neighbor lists & work orders, driving route packages, site testing, post-processing of data, parameter and neighbor list audits, alarm checks, and detailed reporting that includes coverage & event analysis. We gather and analyze detailed field RF measurement data to detect and resolve issues caused by radio problems, improper parameter settings, or network faults and ensure that the system passes any defined evaluation criteria, or Key Parameter Indicators (KPI).

MobileNet is adept at identifying the presence of PIM and in testing and quantifying its effects in both Macro and DAS systems. Our years of experience place us at the forefront of wireless performance analysis and improvement, making us an ideal partner in the ongoing battle between signal and noise.

MobileNet provides a full range of turnkey engineering services for every stage of network deployment, integration, optimization, performance validation and compliance testing to major telecom operators and OEMs across the nation. We have led deployment of 2G, 3G, and 4G networks for over a decade and have launched over 25,000 UMTS sites and over 5,500 LTE turnkey sites across the U.S, with deep domain expertise that spans GSM through CDMA, to LTE, WiFi, VoLTE, and beyond.

We are a leading provider of DAS System Verification, Optimization and Design services with extensive experience testing over 2500 high profile venues including airports, stadiums, hotels, amusement parks, universities, and business parks. Our long lasting customer relationships attest to our reputation for Quality, Expertise, and Reliability and reflect the integrity we bring to delivering superior services, on time and on budget.