Several years ago passive intermodulation (PIM) was a virtually unknown performance
metric in distributed antenna systems (DAS). Today it is recognized as one of the most critical
requirements for optimum system performance. Hypersensitive antennas and radios, multiple
frequency overlays, and more components in the RF path create an environment in which the
margin for error regarding PIM continues to shrink
1. Given the high susceptibility of current DAS
systems, even small levels of PIM distortion can significantly impact network performance, as
measured by upload speed.
Outdoor macro sites were the first deployment scenarios where the PIM issues had to be
tackled. High power levels from the base transceiver station (BTS) ports and a more complex
RF path to the antennas—including jumpers, filters and tower mounted amplifiers (TMAs)—
contribute to generating PIM that can be very detrimental to the quality of wireless service.
Due to the limited uplink (UL) transmit power of mobile terminals, the uplink receive sensitivity
is a critical parameter to optimize in outdoor scenarios to allow a balanced downlink/uplink
maximum path-loss. Best practices for macro site deployments have been defined over the past
High and reliable data throughput values are even more important in DAS environments, such as
stadiums, where there are many components in the RF path that can contribute to PIM generation.
The minimum PIM specification for each and every component is improving continually. PIM
specifications for RF components (splitters, couplers, etc.) and antennas have transitioned from
–140 dBc to –150 dBc and now are moving to –153 dBc and –160 dBc
. With the passive
components—such as splitters, hybrid couplers, and directional couplers—being placed closer to
the signal sources in these systems, it is critical that the PIM specification for these devices is at the
It should be noted that, at the DAS point of interface (POI), the PIM requirements are actually
less stringent than at DAS remote unit ports coupled to a passive network. This is because
DAS POIs typically feature filters that limit the frequency range of the generated PIM products.
Moreover, BTS output ports are typically band-specific, so multiband carriers cannot mix
together and generate PIM products falling in multiple UL bands. In this case, a –153 dBc PIM
specification for POIs is typically sufficient to handle the input signals from macro BTS ports.
On the other hand, passive components used in RF signal distribution networks have wideband
frequency support. Therefore, multiband and multicarrier signals from DAS remote unit output
ports can mix together at every passive stage and generate a large variety of detrimental
PIM products falling in multiple uplink bands. As such, PIM requirements for these passive
components must be more stringent.
CommScope has introduced –160 dBc (i.e., –117 dBm IM power) passive components
in the product portfolio to provide a solution for demanding DAS applications where PIM
performance is critical. The following CommScope passive device families are offered with a
PIM specification of –160 dBc:
More detailed infomation – Source: http://img.en25.com/Web/ArdenMediaCompanyLLC/%7B4a2bcc16-f160-46f7-904b-903e12d2294d%7D_PIM_Requirements_Must_Increase_to_Support_Evolving_DAS_Systems_WP-108243___.pdf