Clearing up the signals

With wireless communications fast becoming part and parcel of everyday life, wireless network providers face the challenge of having to maintain an efficient network that operates at optimal capacity.

Many operators make investments to better their networks and meet the demand of the masses by overlaying on existing networks. This means utilizing existing tower equipment such as feeder coaxial cable lines and base station antennas. One of the obstacles faced by operators is the phenomenon of passive intermodulation (PIM).

PIM is the result of two or more wireless signals mixing together to create additional, undesired frequencies that cause interference or degrade transmission of desired signals in wireless networks. Antennas and radios are now hyper-sensitive and susceptible to smaller and smaller levels of distortion. A 1dB drop in uplink sensitivity due to PIM can reduce coverage by as much as 11 per cent.

PIM Costs

Wireless operators have invested a lot of money into 3G networks and will continue to do so for the next generation of technologies, whether it is HSPA+, LTE, or others. These investments are often overlaid on the existing network, utilizing existing tower equipment such as coaxial cable feeder lines and base station antennas.

PIM levels that approach those of thermal noise or other interference can desensitize the receiver, causing performance to deteriorate and usually results in signal blockage or loss of reception. PIM may occur in passive non-linear radio frequency (RF) circuits with two or more common components.

The two fundamental causes are current rectification at the conductor joints, and/or varying magnetic permeability due to the ferromagnetic material in or near the RF path. This translates to reduced network efficiency, channel capacity and bottom line profit, making PIM a critical threat and growing problem for network operators.

As such, operators are increasingly taking steps to avoid potential signal degradation that can result from overlays, especially when adding new frequency bands. In fact, many operators are addressing by deploying PIM testing equipment to the field. But using PIM equipment correctly requires greater care than standard voltage standing wave ratio (VSWR) site sweeps or distance-to-fault transmission line testing.

PIM Approach

Operators need not fret as long as they adopt a holistic and proactive approach towards dealing with PIM. Firstly, network operators need to realise that PIM is not just confined to those involved with network testing. It affects everyone, from installers to those working on system design. The more knowledgeable each person is regarding the root causes and effects of PIM, the better the chances of minimizing its effects.

One of the best ways to minimize the cost and problems created by PIM is to try and prevent it during field installation. Operators should invest in getting trained technicians to deal with connectorization and installation — two of the most prevalent causes of PIM.

PIM is not a phenomenon that can be avoided entirely. Hence operators should ensure that regular testing is conducted to see if unacceptable levels of PIM are detected in the RF path. Ideally PIM testing should be done yearly as the RF path must be off line during PIM testing.

However, the problem with taking PIM measurements is that readings can be skewed by test equipment and surroundings which could result in false failures. Sources of irregularities or offending components should be identified and located as quickly as possible. In the event that a component is isolated and identified as lacking PIM compliance or exceeding system specified PIM performance level, it should be replaced as soon as possible.

Operators can undertake some preventive maintenance by checking PIM levels when upgrades or changes are made to the RF path. Additionally, technicians should be sent for regular training on installation and connectorization.

(The author is director, Wireless Sales, India, CommScope. The views expressed in this article are that of the author and do not necessarily reflect the views or policies of CIOL).