LTE forces Wireless Network Designers to Re-Think

wolfgang damm, product marketing director, WTG

 Smart phones  and tablet PCs claim huge data band-widths and wireless network operators must ensure subscribers’ data demand is sufficiently satisfied. Research conducted by CISCO® in 2011 found that compared to a regular mobile phone, smart phones require 24 times the data bandwidth. Tablets have an even higher demand; they require 122 times the data bandwidth of standard mobile phones. Wireless network operators are upgrading their networks to LTE in order to satisfy the massive data hunger.  Succees requires special network design and installation executed with extra care.

“Designing low PIM into modern wireless high speed data networks is paramount for bandwidth optimization and maximum Quality of Service (QoS)”.

Passive Intermodulation (PIM) has existed since two or more frequencies were combined in one RF path. With the exception of satellite links, early technologies were less susceptible to PIM and adequate bandwidth was available. Network operators were mainly concerned about increasing their subscriber base, but now huge investments are required to expand network capacity.

What is PIM?

PIM is an unwanted effect caused by passive components. Since they are passive one would expect a perfectly linear behavior. Reality is, every single passive component comes with a certain degree of non-linearity. Indeed, any component generates PIM. This is of little concern if only one frequency at a time is transmitted; as soon as two or more frequencies share the same RF path, PIM avoidance becomes very critical. What distinguishes “The Good”  components from “The Bad” is the degree to which it causes PIM.

Frequency spectrum with PIM interference. In this example the IM products are caused by simple CW tones. Broadband carriers generate also broad intermodulation products, which can have a significant influence on the operational quality of cell sites.

Components that have insufficient PIM characteristics cause RF power diversion into other frequency ranges. These intermodulation products interfere with other RF signals.  Obviously this is not desired because for one, transmission in other than the assigned bands is  in most countries plainly illegal.  Furthermore, the most powerful PIM signal is usually the third order intermodulation product, IM3. This signal corresponds very often with the receiving bands of  telecommunication transmitters. Whenever they transmit a multi frequency signal, PIM is generated, disturbing receiver frequencies. This can lead to reduced cell site bandwidth, and reduced QoS. Sometimes PIM signals are so disturbing that they can take our complete Rx channels. This is the last thing operators can afford in a system with already strained capacity.

Prevention

PIM can be prevented with proper planning, high quality components and well trained field personnel. Some materials cause significant PIM when mated with each other. Quality components are specifically designed and manufactured to ensure the lowest PIM values, even after years of operation under challenging environmental conditions.  

Further reading

The following links provide  further information on PIM and how to prevent it:

Importance of Low PIM components (recorded webinar)

Passive Intermodulation

Proper Torque prevents PIM

PIM 21 Manual

Conclusion:

LTE systems are particularly receptive to interference caused by unwanted passive intermodulation effects. Diligent network design, highly trained field personal and high quality components with guaranteed low PIM specifications are paramount to prevent bandwidth-reducing PIM issues. While specific PIM testers can determine if sites present high PIM values, the best and least costly way to deal with PIM is avoiding it in the first place.

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