January 6, 2016
In our previous post, we described areas of concern for passive intermodulation (PIM) in passive Distributed Antenna Systems (DAS). Today, we will continue that discussion with a focus on PIM concerns in active DAS installations.
Figure 1 shows a typical active DAS, where RF signals from multiple sources are conditioned, combined and converted to light for distribution over fiber optic cables to radio units distributed throughout the building. Radio units convert the optical inputs back to RF, amplify the signals and re-broadcast them through either integrated or external antennas to users inside the building. In the uplink direction, the process is reversed, with the radio units converting received signals to light and transporting them back to the head-end equipment over fiber. In the head-end unit, the uplink signals are converted back to RF, separated by frequency and delivered to each operator’s base station equipment. Active electronics containing gain elements in both the uplink and downlink directions are used in this configuration, hence the name “active DAS.”
We’ve previously presented that PIM interference is highest when:
- low order IM products exist
- RF power is high
- highly non-liner objects exits
Now, let’s take these factors into account and describe areas of PIM concern in active DAS construction.
Figure 1
Signal Sources
The level of PIM concern in an active DAS near the signal source is the same as in a passive DAS. Since only a single operator’s frequencies are present in this section it is often IM9 or higher that will impact the operator’s uplink band. Because this is also the highest power section of the DAS, PIM is still a concern. The area between each signal source and its associated point of interface (POI) tray has an “orange” or “medium” level of PIM concern. Use of low PIM materials and low PIM construction techniques is advised.
After The POI
In an active DAS, the POI typically performs two functions. First, it separates uplink signals from downlink signals using an internal duplexer. Second, the POI applies attenuation to the downlink path to reduce the RF power level to approximately 1mW (0 dBm). Small diameter, highly flexible RF cables with SMA or QMA connectors are typically used between the POI and the DAS head-end equipment.
PIM is not a concern in this section of the DAS. To begin with, the RF power level is extremely low on both the uplink and downlink cables. Plus, any PIM generated on the uplink path would be attenuated by the isolation of the duplexer (typically >60 dB) before appearing in the downlink path. As a result, PIM in this DAS section is well below the receiver noise floor and therefore can’t impact performance.
Optical Distribution
PIM is not a concern in the optical cables connecting the head-end equipment to the distributed radio units. While there are known mechanisms for intermodulation in optical fibers (Four Wave Mixing), this is only an issue in tightly spaced, wave division multiplex (WDM) systems. The optical link in a typical DAS system uses one fiber for uplink and a separate fiber for downlink, eliminating any possibility of crosstalk.
Antenna Level
The radio unit converts the optical signal received from the DAS head-end equipment back to RF. In some applications, such as outdoor DAS (oDAS) or stadium application, high-power radio units may be deployed. In this case, the level of PIM concern would be somewhere between “orange” and “red,” depending on power level and number of frequency bands combined. In an indoor application where each radio unit feeds an individual antenna, the RF power is much lower, typically on the order of 100mW (20 dBm). As with a passive DAS, PIM is still a concern near these antennas due to the highly non-liner objects often found in the indoor environment. Overall, the level of PIM concern at or beyond the antenna in this configuration is “yellow” or “low,” for the same reasons explained with passive DAS.
For more information on distributed antenna system (DAS) construction, download the free white paper Understanding In-building Wireless Solutions. In addition to DAS construction, this paper also discusses the pros & cons of Distributed Radio Systems (DRS) and Distributed Small Cells (DSC.)
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