September 1, 2016
In LTE networks – like all mobile communications systems – the BTS serves as both a transmitter and a receiver. As downlink signals pass though non-linear devices in the feed system, passive intermodulation (PIM) caused by something as simple as a loose connecter can be generated and travel back toward the BTS receiver, causing interference. The magnitude of PIM signals arriving at the BTS receiver can be altered as the insertion loss between the non-linear device and the BTS changes.
We have demonstrated this fact on numerous occasions to customers. For example, we can insert a non-liner device very close to the BTS which might generate an IM3 signal of –70 dBm. In this case, PIM entering the BTS receiver would be approximately –70 dBm. If we disconnected the system and added 10 dB of cable loss between the non-linear device and the BTS, the power arriving at the non-linear device would be reduced by 10 dB. As a result, the PIM produced by the non-liner device would be reduced by 30 dB to –100 dBm (10 dB power loss x 3 dB/dB = –30 dB).
That’s not the entire story, though! The PIM signal would now have to travel back through the 10 dB of cable loss to reach the BTS receiver, further attenuating the PIM signal by an additional 10 dB. By adding 10 dB of cable loss between the non-linear device and the BTS receiver, the PIM level entering the BTS receiver is reduced by 40 dB. This can be verified by replacing the BTS with a PIM analyzer, such as the PIM Master™ MW82119B (figure 1).
The new PIM value (NPV) measured at the BTS when insertion loss is added can be expressed as:
NPV = PV – ( PS * IL ) – ( 1 * IL )
PV = PIM value (measured with no insertion loss)
PS = PIM slope (change in PIM level for every 1 dB change in signal power) in dB/dB
IL = Insertion loss added between the PIM source and the BTS in dB
Of course, PIM occurs in the field, where PIM sources don’t follow the theoretical 3.0 dB/dB PIM slope for IM3. Rather, the IM3 level of a PIM source typically varies 2.2 to 2.8 dB/dB with changing power. Substituting an average PIM slope value of 2.5 dB/dB, the above equation becomes:
NPV = PV – ( 3.5 * IL )
Note that this simplified equation is an approximation that is only appropriate for evaluating changes in IM3. A different PIM slope is required to evaluate IM orders IM2, IM5, IM7, and so forth.
DAS Branch Example
In an in-building system such as a Distributed Antenna System (DAS), the power from the BTS is split and distributed to multiple antennas using coaxial cables. In the example shown in figure 2, the outputs from four different frequency band BTS are combined together using hybrid combiners and fed into a passive distribution network. Tapper “S1” couples energy from the riser to feed the first floor of the building, which is served by three antennas (A1, A2 and A3).
To evaluate the magnitude of PIM seen by a BTS receiver when a non-liner device is placed at points A, B and C, the insertion loss between the DAS input and these three points of interest must be calculated. To do this, the cable and splitter losses between the input and each point of interest must be added together as shown:
Loss to point A = H2 + H1 = 3.1 + 3.1 = 6.2 dB
Loss to point B = H2 + H1 + C1+ S1coupled + C2 + S2through
= 3.1 + 3.1 + 2.8 + 6.1 + 1.4 + 1.3 = 17.8 dB
Loss to point C = H2 + H1 + C1+ S1coupled + C2 + S2through + C3 + S3through + C4
= 3.1 + 3.1 + 2.8 + 6.1 + 1.4 + 1.3 + 2.1 + 1.8 + 2.8 = 24.5 dB
For the sake of discussion, let’s assume the non-liner device generates IM3 at –50 dBm (–93 dBc) when subjected to 2x 43 dBm CW test signals. A typical system pass/fail level for IM3 is –97 dBm (–140 dBc). The PIM signal produced by this non-linear device would need to be 47 dB lower before being considered acceptable. Another way to view it is that the PIM magnitude would need to be reduced by a factor of 50,000 before it would be acceptable.
Let’s further assume that this “bad” PIM source can be inserted at the locations A, B and C in the DAS design shown in figure 2. Keeping the test power constant, the new PIM value (NPV) measured at the DAS input can be estimated for each location.
If the operator’s IM3 pass/fail level had been –97 dBm (–140 dBc) when tested at the DAS input using 2x 43 dBm test tones, the site would only have failed when this “bad” PIM source was located close to the DAS input. When the “bad” PIM source was placed far away from the DAS input, the insertion loss of the DAS reduced the PIM signal to a level that would no longer impact site performance.
To learn more about the effects of insertion loss on measured PIM levels, you can download a white paper from Anritsu.