Overlaying a bane of networks

Dipesh Rattan/CommScope
With wireless communications fast becoming part and parcel of everyday life, wireless network providers need to maintain an efficient network that operates at optimal capacity.
 
Many operators try to do this by overlaying on existing networks. This means using existing tower equipment such as feeder coaxial cable lines and base station antennas.
 
In overlaying, operators encounter 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.
 
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.
 
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. This translates to reduced network efficiency, channel capacity and bottom line profit, making PIM a critical threat and growing problem for network operators.
 
Also, 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%.
 
As such, operators are increasingly taking steps to avoid potential signal degradation that can result from overlays, especially when adding new frequency bands. Many operators are addressing the problem by deploying PIM testing equipment to the field.
 

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