Combating PIM in cellular networks

In 2013, a strategic alliance was formed between the Biomedical and Communications research group at London South Bank University (LSBU) and Hughes Electronics, London. Its aim: to develop innovative and disruptive technologies to combat passive intermodulation (PIM) in cellular networks.

PIM is a series of resulting signal harmonics and distortions that emerge between two or more interacting signals when transmitted through passive components such as cables and connectors, causing unwanted interference and degradation of wireless communications. 4G and future-generation networks require stricter signal-to-noise ratios (SNRs) and the issue of PIM must be addressed if cellular capacity is to be maintained.

Investigations between Hughes Electronics and LSBU demonstrated that metal connectors are particularly vulnerable to PIM, with studies concluding that the universally accepted multiple metal-segment design results in return signal paths and the creation of multipath signal transmissions leading to ‘ghost signals’ and harmonics. Moreover, each conductive segment creates micro-separations in the transmission path forming intra-media signal transition points. This results in reflections, energy loss, distortion, arcing and capacitive junctions, all of which negatively interfere with legitimate signal transmissions.

The research project also unearthed a number of other sources of hitherto unrecognised sources of PIM, including humidity, temperature and material phase change. Temperature changes can add up to 10 dBc of PIM in the third harmonic, with metal connectors being particularly vulnerable, while phase change can add to PIM by 11 dB in the worst case (complex multiconnection) systems. Metal connectors are also susceptible to the electrical fields generated in and around cellular base station environments and to vibrations and microphonics from cables — all of which increase PIM.

Seeking to mitigate sources of PIM, Hughes Electronics developed the WaveWay connector system. By implementing a single-piece forward and return path mechanism, separations are removed and signals constrained, severely restricting multiple-signal path opportunities and the consequent PIM effects. The system is also constructed from non-metal composite material, creating an isolation layer from energy fields while at the same time absorbing microphonic vibrations, resulting in reduced PIM readings.

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