In previous issues of RCR, we discussed Near-Far and its “Effect on In-Building DAS Performance” and “Uplink Near-Far: Causing Degraded DAS Performance”. Near-Far is a term describing performance reduction when a mobile device is operating within a DAS coverage area, but is being serviced by a distant macro cell tower.
Near-Far affects people on the DAS (participants) as well as those in the DAS coverage area who are not on the DAS (non-participants). When people on the DAS are affected, it is known as uplink Near-Far. Conversely, when those not on the DAS are affected, it is known as downlink Near-Far. For today’s reading, we will focus on downlink Near-Far.
Downlink Near-Far DAS Overview
In the last post we saw how Near-Far can affect DAS participants when a non-participant is in the coverage area transmitting at high power to connect to a distant macro site. To a similar extent, the non-participants can see degraded performance caused by the DAS when they are in the DAS coverage area. As previously noted, DASs are usually designed to provide downlink signal levels strong enough to maintain dominance over signals penetrating the building from macro sites in the frequency band(s) of interest. To get the downlink signals to the desired level, there are power amplifiers (PA’s) in the DAS remotes to boost the signals.
The amplifiers used in DAS remotes are broadband to support the full band of interest, i.e. PCS 1900, Cellular 850, LTE 700, etc. Along with the amplifier, most of the other components in a DAS are typically broadband including the duplexers in the Point of Interface (POI), the lasers and detectors, and filters that combine the signals to a common antenna port. Because the components are broadband, the amplifiers in the remote receive broadband noise along with the wanted signals. This includes the noise generated by the lasers in the DASs optical network. With the PA’s being broadband, they amplify the broadband noise along with the signal in the desired band of interest.
In a perfect world, the filters on the output of the remote would only pass the wanted signals and reject the unwanted signals. Because DASs are designed to work over broad frequency ranges, the filters on the output of the DAS pass the full band. Therefore the broadband noise is transmitted into the DAS coverage area along with the desired signals. Although the broadband noise is not at levels high enough to violate FCC transmission requirements, they can be higher than the noise floor the non-participants device would have otherwise experienced. When the power level of the noise from the DAS is higher than the receive sensitivity of the non-participants device, the DAS sets the noise floor for the non-participant. This in-turn decreases the Signal to Noise level received by the participant which could impact their level of service.
The Bottom Line Impact on Non-participants in a DAS Environment
Some examples of degraded performance include: slow data rates, poor voice quality, and/or loss of service. When the non-participant experiences slower performance, they require more interaction with the macro site. Depending on the level of degradation, their slow connection with the macro could degrade the performance of other users on the macro site that are not in a DAS coverage area. As non-participants move closer to a DAS antenna, the broadband noise received by the non-participant is stronger, resulting in greater noise floor rise and higher impact on non-participants’ device performance. Depending on the non-participants’ signal strength relative to the noise floor, the non-participants’ service could degrade until they lose total service.
Public Safety and Downlink Near-Far DAS
The effect of downlink Near-Far is not limited to commercial bands. If a public safety band is within the passband of the filter in the DAS remote, it could be affected by the noise generated by the DAS. If the noise level is too great, public safety phones may not work in the DAS coverage footprint.
For a quick informational overview on Near-Far, please check out the Westell video.
Near-Far DAS Comparison Tool
Check out a new web-based tool which compares Near-Far performance under defined conditions. The Near-Far performance tool, presented by Westell, will be available in a future edition of RCR Wireless.
