August 11, 2016
Field engineers and technicians have become all-too-familiar with In-building Wireless systems. The proliferation of these network extensions to accommodate the overwhelming use of mobile services indoors has placed a great deal of attention on proper installation and maintenance of Distributed Antenna Systems (DAS), Distributed Radio Systems (DRS) and Distributed Small Cells (DSC).
All three systems have similar obstacles when it comes to Passive Intermodulation (PIM) that go above and beyond interference issues associated with conventional base stations. There are some simple ways, however, to prevent PIM in these In-building Wireless systems to maintain optimum performance.
It’s important to understand the system architecture differences between the three options that deliver mobile service in malls, office buildings, arenas and other public buildings. One aspect to be aware of – especially when it comes to eliminating PIM – is where the network equipment manufacturers place the baseband processing and radio functions. Each architecture is slightly different, as show in Figure 1:
DAS – The radio functions and baseband processing are centralized and the signals are distributed to the antennas in the respective building/venue.
DRS – Only the baseband processing is centralized; all radio functions are distributed with the antennas.
DSC – All three elements, the baseband processing, radio functions and antennas, are distributed throughout the system.
Figure 1
As it relates to PIM, DRS and DSC have some commonality. A DRS is typically a single-operator system and doesn’t have multiple frequencies deployed at the antenna. So, there are usually IM7 or IM9 products, rather than low-order products such as IM3, meaning PIM is not a common issue. Most DSC systems are also for a single operator, though there are some occasions when multi-operator DSC systems are deployed. PIM can become a problem, however, when multiple bands operating at lower frequencies are used in a DRS or DSC.
PIM can be prevented by following any of these rules:
- Avoid frequency combinations that create low order IM products in the uplink
- Use low PIM construction techniques and materials
- Avoid non-linear objects near the antenna
Field engineers and technicians can only control one of these rules – low PIM construction techniques and materials. Here are four ways you can adhere to this rule to help mitigate external PIM:
- Antenna Placement – The adverse effects of PIM can be significantly reduced by moving the antenna so the interfering source falls into the far-field rather than near-field. It doesn’t have to be a large adjustment to have a dramatic improvement. We’ve seen cases where altering the antenna location by as little as one meter can produce a significant PIM benefit.
- Move PIM Source – Often times, cables, rods, lighting and other external causes of PIM common in buildings are installed after the In-building Wireless system. This can create an unacceptable drop in network performance. Figure 2 (right) shows an electrical conduit that was installed after a DAS went live at a sports stadium. As you can see, it is much too close to the antenna and caused PIM. Once the season was over, the cable was re-routed and network performance returned to specification.
- Add Attenuation Between Antenna and PIM Source – In this scenario, simple solutions can be very effective. At one site, the PIM source was covered by aluminum duct tape and the customer saw a PIM reduction of >40 dB!
- Radiate Less Power at the PIM Source – Most in-building wireless systems use omni-directional antennas. A notch can be added in the horizontal pattern of the antenna to intentionally reduce the gain. You can rotate this “notch-beam antenna” until you steer the null in the pattern towards the PIM source. An approach such as this can reduce PIM by 30 dB.
Figure 2
To learn more about these simple tips to help reduce PIM that may interfere with you in-building wireless system, watch this webinar entitled PIM Concerns in In-building Wireless Systems.
