EuMW 2016: Passive intermodulation technique unveiled

6th October 2016
Mick Elliott

AceAxis has developed a ground-breaking technique for the detection, in-service monitoring, and mitigation of passive intermodulation (PIM), along with a demonstration platform to assist in its integration into mobile base stations. The new solution can help network service providers prevent the serious performance degradation that can result from PIM in the digital uplink path of mobile base stations, thus maintaining user experience and helping to prevent churn.

PIM is an increasingly serious problem in mobile base station receivers, particularly as the number of carriers is increasing, bandwidths are becoming wider, and more frequency bands are being co-sited.

PIM has traditionally been defined as a type of distortion generated by the nonlinear characteristics of passive components such as antennas, connectors, and filters in the RF chain at the cell site. #

However, the rapid growth in the deployment of LTE carrier aggregation, and the consequent multiple frequency bands that are now prevalent in networks, mean that this ‘self-PIM‘ is no longer the dominant effect.

Far more significant and potentially damaging is ‘multi-band PIM’, which occurs when the intermodulation products from one frequency band at a cell site interfere with operation in another frequency band.

The most significant effect of multi-band PIM is that a specific PIM-impacted frequency band often has to be operated at considerably below optimum transmit power – by as much as 10 – 15dB – simply to avoid compromising performance in other bands.

Multi-band PIM behaviour has a significant financial impact on network operators, since it affects the viability of purchasing and utilising spectrum.

“PIM is a huge source of concern for radio engineers,” said Simon Gale, head of research at AceAxis. “Normally it is verified during installation, and further checks are only made if performance degradation becomes apparent. By that time subscriber KPIs may already have shown significant deterioration, and user experience will have suffered.”

A demonstration platform for PIM detection and cancellation is available from AceAxis, and is suitable for trials with CPRI-based Radio Access Network (RAN) equipment.

The unit can be inserted into the link between a radio baseband unit and a remote radio head (RRH), or alternatively can accept two RF inputs for use with legacy base stations without an accessible CPRI interface.

AceAxis is now seeking partners to license the technology and work together with them to integrate the technology into LTE base stations.

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