Understanding Passive Intermodulation in Wireless Communication Systems

Key Takeaways

• The interference in wireless communication networks caused by non-linear passive components such as cables, connectors, antennas, bolts, and joints results in intermodulation distortions called passive intermodulation or PIM.

• When two signals of frequencies f₁ and f₂ mix together due to non-linearities in passive components, they form passive intermodulation products of frequencies  ± M ⋅ f₁   ± N ⋅f₂, where M and N are harmonic numbers.

• The order of intermodulation products is given by M+N. The intermodulation products can be additive or subtractive. The terms  M ⋅ f₁   + N ⋅f₂ fall under additive products and M ⋅ f₁  - N ⋅f₂ forms the subtractive products.  

Passive intermodulation is one of the causes of poor wireless communication performance

The poor performance of communication lines is always a concern in wireless communication systems. When it comes to poor performance, we often think of the faults in active component-based circuits such as amplifiers, attenuators, shifters, and detectors.  But the ferrous metal parts, adhesion of two unmatched metals, and poor mechanical contacts can also adversely affect the performance of wireless communication systems. Passive components in the network exhibiting non-linear characteristics to signals generate interference in the system and are called passive intermodulation. 

Passive Intermodulation 

The interference in wireless networks caused by non-linear passive components such as cables, connectors, antennas, bolts, and joints results in intermodulation distortions called passive intermodulation or PIM. The non-linearities in passive components are mainly due to defective mechanical parts, wrong installation, poor contacts, metal flakes in parts, corrosion, or poor termination. Some linear components are also sources of PIM. 

The common causes of PIM are  summarized below:

1. Unmatched metal joints-The non-linearity similar to that of diodes is developed at joints of unmatched metals.

2. Oxidation-The oxidation of dissimilar metal joints and co-axial connectors can generate PIM. 

3. Poor assembly-The poor assembling of parts in antennas, cables, and connectors can produce higher-order PIM. 

4. Feeders-The interfacing of the outer metal layer of the co-axial feeders with other mechanical parts in the system can induce PIM. 

5. Contacts and connections-The tight connections and proper contacts of the passive components in wireless systems reduce PIM amplitudes. Improper contacts or loose and dirty connections are considered sources of PIM.

6. Presence of ferrous metals-Ferrous metals such as iron, steel, and nickel have hysteresis and eddy current effects which are non-linear characteristics. The non-linearity of the metal can cause PIM in the networks. 

7. Spark discharges-The poor connection or disconnection of connectors in wireless system circuits causes sparks. These sparks create holes or dents in the connectors and this will give rise to non-linearity.

The amplitude of the PIM increases with the increase in passive component non-linearities. High power rated wireless systems are seen as more susceptible to PIM problems. High power and high data transfer rates in communication systems can exert constant strain on mechanical components and the strain-induced non-linearities in passive components results in PIM. 

PIM problems are usually reported in base transceiver station antennas and communication lines. The PIM amplitudes are capable of covering up the wanted signals. It is always necessary to remove the passive components that are PIM sources. 

Older systems are generally crucial in generating PIM. Even the newly installed mechanical components undergo accelerated aging due to PIM interference. Temperature, a brine filled atmosphere, pollution, and mechanical vibrations are some of the physical factors that aggravate the passive component non-linearities and PIM problems. 

The non-linear nature of passive components makes them behave as mixers causing interference. In the presence of non-linear mechanical components, signals mix together to produce sum and difference intermodulation product signals that cause signal interferences. The passive intermodulation products remain within the operational frequency band and follow the same principles as that of the wanted modulation products in a mixer. 

Passive Intermodulation Products

When two signals of frequencies f₁ and f₂ mix together due to non-linearities in passive components, they form passive intermodulation products of frequencies  ± M ⋅ f₁   ± N ⋅f₂. where M and N are harmonic numbers.  

The order of the intermodulation products is very important when describing PIM. The order of intermodulation products is given by M+N. PIM products become problematic only when they start interfering with other signals and other components in the system. The order of the PIM products is crucial when it comes to wanted signal distortion. The signals that mix with second-order and higher-order harmonics are the main problem makers in PIM products. As the order of the PIM products gets higher, there is more spread of interference, affecting the system performance. 

In cellular communication, using broadband methods such as LTE, CDMA, and PIM products reduces the sensitivity of the receiver. Some of the outcomes of poor receiver sensitivity are dropped calls, low data transfer rates, and poor system capacity. The PIM products also form the background noise and affect the quality of the desired signals in cell phone communications. 

The transmit/receive modules in wireless communication systems are the other units that suffer from PIM products. In certain systems, the transmit chain and the receive chain use the same antenna and a common feeder. In such a duplex system, the transmitter and receiver may be handling two different frequencies. PIM products pose a serious issue in duplex systems. 

PIM Mitigation 

Methods used to mitigate PIM in wireless communication systems include:  

1. Checking for loose connection and proper contacts. Whenever connecting passive components such as bolts, antennas, or connectors in a wireless communication system, make sure they are properly connected and the contacts are even and tight. 

2. Older passive components are very much susceptible to PIM. Never use worn-out passive components. Procure components of low PIM values that are less than the thermal noise level in the system. 

3. Cleansing the connectors—removal of dirt and loose particles in the connectors is important for reducing PIM. Cleaning with cotton dipped in isopropyl alcohol and blowing low-pressure air are some of the methods to remove pollutants and dirt present in the connectors. 

PIM is one of the performance limiters in wireless communication systems. Small imperfections such as a rusty bolt, a loose connection in connectors, or a metal dent on the antenna are harmful enough to produce higher-order PIM products. You can minimize the PIM in the communication circuit through proper inspection and the right selection of passive components with low PIM values.