Why worry about PIM for indoor DAS deployment nowadays?
Writer: Kwan Fui Hiew
As far as I could remember, 4-5 years ago when cellular network is still dominated by 2G and 3G technology carried on a few typical frequency bands such as AWS, 1900MHz, 850MHz etc, PIM (Passive intermodulation) isn’t a concern at all to many mobile operators. In fact, mobile operator doesn’t care much about PIM until recently.
The main reason behind this mindset change is that overwhelming data usage especially indoor area has been constantly pushing operators to acquire more spectrum for LTE deployment. As a result, more and more frequency band carriers are combined into indoor DAS nowadays from each operator. PIM issue started surface out to result in uplink noise rise on certain frequency band after introducing additional LTE carriers into the DAS. For instance, introducing LTE 700MHz mixing with existing 850MHz carrier may generate 4th order PIM product falling on 1900MHz (band 2) uplink band. Secondly, to stay competitive, mobile operators care a lot about the user experience in term of throughput, latency performance etc. Serious PIM issue will negatively affect user experience which is critical for user churn rate, considering similar data subscription plan offered by the operators.
A couple of times when meeting up with mobile operator, they often raise a question “What kind of PIM specification is required for those passive product like combiner, antenna etc?” The answer really depends on which stage the passive component is located in the DAS and also the input power level into the component. Typically, -160dBc is recommended for first stage of passive DAS, which is a combiner or POI or the first passive component (splitter, coupler) immediately after base station. This stage is particularly critical due to the high base station output power. For other passive components far away or at least 10dB DAS loss away, -150dBc is generally good specs to handle the situation.
However, to achieve high linearity in the whole system or cell site, good workmanship is required on top of the low PIM product as discussed above. Cell site linearity will drastically decline if there is any PIM source found in your DAS network. What is a PIM source? PIM source is nothing but can be an untightened connection, bending on RF cable, external metal object in front of antenna, metal flakes in connector, rusty mounting bracket, and poor connector termination. Strong PIM product will be generated by non-linearity in the cell site as mentioned above.
In summary, there are three key takeaways:
1. Low PIM product is recommended for DAS location where there is high input power (or it is close to base station)
2. Based on the spectrum allocation, operator must work out the potential PIM victim (which frequency band) under multi-carrier and multi-operator combined DAS
3. Operator requires to monitor the cell site statistics closely to determine whether there is any PIM issue. The only way to solve the PIM issue is to remove the PIM source. Hence, good workmanship for building a DAS network is very important to ensure high linearity in the cell site.