PIM Requirements Must Increase to Support

26 Sep

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

few years

1.

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

[1]. 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

highest levels.

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:

Power splitter

Hybrid coupler

Directional coupler

Tapper

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

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