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    PIM Requirements for Low Power Wireless Components and Subsystems

    Microlab

    Posted March 14, 2014

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    Murat Eron, Ph.D., Vice President of Engineering, Wireless Telecom Group

    Passive intermodulation, PIM, continues to be a concern for high speed Wireless networks. A good understanding of how it impacts the network operation and how to detect and avoid it has been rapidly developing. This has reflected in PIM testing procedures and limits for high power RF components and subsystems. As these specifications were mostly geared towards high power, +40 W applications and components, there is a need to address the low power PIM requirements as such systems are playing a bigger role in new roll outs. We attempt to define here those reasonable limits.

    The best method of assuring good PIM performance is to obviously start with low-PIM guaranteed components in the first place. Second, try to specify PIM as close to working conditions and power as possible to avoid needlessly expensive solutions. The closest we have to a PIM standard is the IEC 62037-1 and -4 that specify the conditions for testing PIM. These documents "recommend" the use of 2 x 20 W CW tones leaving the choice of frequencies and separation to the user per the needs of system in use. 

    Meanwhile, Wireless industry has developed ad hoc specifications for PIM levels mostly based on field tests and trial and error. A maximum PIM level of -110 dBm in the receive band is found to be sufficient in many cases, especially when BTS interfaces are involved. Many component vendors aim for -115 dBm or better depending on the type of component. Given the standard test level of 43 dBm per tone (CW), this -110 dBm of absolute IM power level corresponds to a -153 dBc of relative PIM level. Note that it may be hard to relate this analytically to an ideal state of the art base station receive sensitivity of around -120 dBm range since these are defined for specific bandwidths and for CDMA/OFDM type signals that resemble anything but CW. 

    One of the drawbacks of fixed power testing per IEC is it its unsuitability to lower power applications. Designing and manufacturing a passive component or assembly for a 2 x 20 W test when let us say the application is only 2 W, results in a costly and heavier solution than required.  In the absence of a standard and accepted guideline, some users are inclined to stick with the 2 x 20 W test requirement which results in significant cost, size and weight increase for the product. Even when the test requirement is pulled back to 2 x 2 W for example, then the false expectation is for the PIM performance to be now 30 dB better, around -140 dBm, based on traditional IP3 analysis as shown in Fig. 1. This level is about -173 dBc from the test tone, beyond the reliable measurement limit of most of the state of the art PIM test instruments. It gets even worse if the PIM of specific interest is 5th or 7th order. In Fig. 2 reducing power down to 1W per tone for example, the actual IM5 measured may be about 20 dB higher than what a simple 5:1 extrapolation would predict. It is possible to lower PIM for a component to such low levels by careful design, numerous iterations and very well controlled manufacturing process, but very unlikely to obtain reasonable yield for commercial applications, in addition to being unnecessary for proper system operation.  So there is an obvious need to re-think the PIM specifications as the base stations get smaller and power outputs get lower.

    PIM Requirements for Low Power Wireless Components and Subsystems
    PIM Requirements for Low Power Wireless Components and Subsystems
    Fig. 1.  IM3 vs power for two different devices compared with ideal at 1900 MHz. These are two different passive devices with rather high PIM. None follow the 3:1 slope expected of IM3 products. Typical PIM IM3 rises at a rate 1.5-2.5:1. Noise floor is near -140 dBm.
    Fig. 2. IM5 for PIM plotted as a function test tone power for two different DUTs at 700 MHz. IM5s do not follow 5:1 slope either. IM3 for one of the devices also included for reference.

    Some of the confusion can directly be traced to the convention of specifying PIM as a relative measure in dBc rather than an absolute power level. In reality, a radio receiver responds to power not dBc. Sensitivity of the radio is not affected by PIM or other low-level interference if the absolute levels are much below the sensitivity of the receiver. Since power levels involved may be substantially lower than 40 W for many new wireless infrastructure applications, one could then in theory adjust the required dBc value (based on 2 x 20 W) and establish a new requirement in dBc appropriate for the actual power levels. 

    Unfortunately this would be straightforward only if PIM behaved according to a known power law. It does not, as shown in Figs. 1 and 2. So while higher power level components and systems are tested with a standard level of 20 W tones, which has proven to be a good figure of merit, lower power system specs still remain open to interpretation and requires better knowledge of specific operating conditions, receiver sensitivity requirements and PIM test limits. Specifically, for a low power system or component that cannot be characterized for PIM by using 2 x 20 W tones for physical reasons, then one may need to find out what exactly are the corresponding specifications for lower power, say 2 x 2 W tones.

    One cannot simply extrapolate -153 dBc to -173 dBc based on a theoretical 3rd order nonlinearity behavior.  Third order IMD generated by 2 x 20 W when PIM is -153 dBc down, turns out to be -110 dBm level. In reality this is the accepted PIM performance for standard 2 x 20 W test. Question is, what is the dBc requirement for low power levels that would generate the same -110 dBm distortion level. For a 2 W test this turns out to be -143 dBc. 

    On the other hand, if we expect an improvement in intermodulation distortion of 30 dB, corresponding to the 10 dB drop in tone power as one would expect in standard 3rd order behavior, than we would be looking for -140 dBm (-173 dBc) PIM, which happens to be the noise floor of most of the respectable PIM testers available today. In fact, absolute PIM measurements much less than -130 dBm or so tend to be highly unrepeatable in general. 

    Test Tones  PIM -153 dBc  PIM -143 dBc  IMD for 1:3 Slope  IMD for 1:2 Slope  PIM -150 dBc 
    2 x 20 W  -110 dBm IMD  -100 dBm IMD  -110 dBm (ref.)  -110 dBm (ref.)  -107 dBm 
    2 x 2 W  -120 dBm IMD  -110 dBm IMD  -140 dBm  -130 dBm  -117 dBm 
    Noise Floor  -140 dBm         
    Table: Maximum PIM values expected in dBm for various PIM test power levels and dBc specs.

    In addition, PIM very rarely follows 3:1 slope vs. power for 3rd order components as shown before. Considering a typical slope range of 1.5-2.5:1, one should then expect a PIM level of -125 dBm to -135 dBm. This is a far cry from -140 dBm estimated from basic theory. It seems for a 2 W test, -117 dBm (-150 dBc) seems to be a very reasonable benchmark and where -110 dBm PIM is found to be transparent to system operation, then surely a 7 dB improvement should not be a cause for any concern. In practice, there will be many situations where the small cell or remote used may have power levels higher than 2+2 W, or there may be power combining. Even considering the worst case scenario of IMD increase with power, -150 dBc guarantees low PIM up to 8 W of total power.

    One word of caution is that in many applications high power macros and small cells are co-located and used together; as such there is always a risk of low-power elements being exposed to high power levels by accident. It would be a good design practice to have such low-power systems at least be able to tolerate high power levels of 30-60 W even with poor PIM performance but without damage.