Telecom Lead Asia: Livingston India director Shailendra Kalra says PIM testing is vital for mobile QoS.
Livingston India — in this byline article — says passive intermodulation (PIM) was identified as a source of interference in the field of wireless communication when the first mobile infrastructure started to be put in place several decades ago.
Until now, it has been thought of as little more than a minor hindrance, but it appears that things are about to change. As the importance of the quality of service (QoS) becomes greater and networks need to support a combination of voice and data services, the influence on overall system performance will become more noticeable.
This means that PIM is no longer something that can be ignored – whether it is in western economies now beginning to migrate to advanced mobile technology or in emerging economies that are deploying it from scratch.
PIM occurs when multiple wireless signals from one or more base stations interact with some form of nonlinearity and thus generate new unwanted frequencies that cause interference.
It thereby raising base station noise characteristics and impinges on overall performance.
Generally the nonlinearities causing some form of PIM distortion will be of a mechanical nature – such as wireless signals coming into contact with inconsistent metal-to-metal contacts in areas where the current density is at a high level. As PIM raises the noise floor, it becomes more difficult to distinguish the base station’s transmit/receive signal from the background interference. This will impact on the QoS that a mobile operator’s network can provide – potentially leading to dropped calls, poor call performance, reduced data rates and a shrinking of cell coverage. If PIM issues are to be contended with PIM signals need to be kept well below the receiver sensitivity of the base station in question. Though PIM is simple enough to grasp as a concept, there may be difficulties when attempting to find exactly where the faults responsible for it occur.
Origins of PIM
There are a multitude of different components that make up a modern base station. Among these are various cable assemblies, RF components, connection interfaces, filters and antennas. Though each of these component parts may function correctly and have passed all the necessary compliancy tests, if any of them are poorly installed or have been damaged in some way after installation, the effect they can have in terms of generating PIM can be profound. The problem will be further compounded as components within a system begin to reach the end of their operational lifespan.
The reason for the growing concern about PIM, which is now far greater than it had been previously, is that mobile networks now need to support much higher data capacities. The increased noise floor caused by PIM is having a far greater effect as a result. Though many
people believe that this only going to be cause real trouble as on networks as they starting to utilise 4G mobile technologies, such as LTE, HSPA+ and LTE-Advanced, in fact it also has serious implications for 2.5 and 3G mobile technologies, such as GSM and W-CDMA.
Through comprehensive PIM testing, the build quality of a newly deployed base station can be evaluated. Likewise, the upkeep of existing base stations can be scrutinised and potential complications dealt with, such as poor connector terminations, etc. PIM will be particularly acute if base station hardware is poorly installed.
Mobile network operators who, in an attempt to save money, do not invest in contract installers who do a good job, may be punished with low subscriber retention levels due to PIM issues impinging on the QoS of their networks, or by having to allocate considerable engineering resource to resolving the problem afterwards. In the long run, the financial bearing is likely to be much greater.
Dynamic PIM testing, where test occurs while the component or RF connection in question is under some form of mechanical stress, allows collation of test data that would not be possible within a lab environment and thus has far greater value. If any component generates a level of PIM that is unacceptable when subjected such relatively small stresses then replacement or repair will be required. PIM sources may also be present which do not produce large spikes
when dynamic test procedures are done. Pinpointing these static PIM sources in order to eliminate them can be very difficult as well as incredibly time consuming.
PIM Testing Techniques
Conventional PIM testing equipment transmits two high power signals at fixed frequencies into the line or device under test. If these signals encounter a non-linearity, mixing takes pace with PIM frequencies thereby arising. The magnitude of the generated PIM signal is then measures by the test engineer. Any sizeable spikes that are encountered will signify the existence of a fault. This fault can then be dealt with, as soon it is clear where it is situated.
The difficult part is that engineers using this technique will struggle to get the precise position of the fault.
Leading equipment manufacturers, such as Kaelus, have now developed test solutions that are able to more accurately locate where faults lie. This can be done through the translation of frequency data into time domain plots using complex mathematical functions and
enhancement algorithms. Kaelus’ proprietary Range to Fault (RTF) technology transmits two test frequencies, as with conventional PIM tests, however here one of these frequencies is fixed, while the other sweeps across a range of frequencies in order to produce intermodulation events in the receive band of the system being tested.
The ability to combat the effect of PIM will be of prime importance to mobile operators as the look to improve QoS levels on their networks, so that they have competitive advantage over the rivals. Undertaking PIM testing will ensure that base station hardware has the robustness and operational performance to deal with the exacting pressures of the outdoor environmental in which they are located. Action can then be taken so that so that loose cabling is refitted or defective components are replaced – thereby eradicating the sources of better processes must now be put in place to ensure the assembly of base station hardware is carried out correctly.
In addition to this, operators need to be able to perform accurate PIM tests in the field, so that they can uncover where hardware has been poorly installed or if it has been damaged by exposure to the elements, as well as pinpointing component parts that are not performing to spec. The industry will need to employ more sophisticated test procedures and have access to highly advanced equipment capable of analysing a variety of different parameters. In particular there will be ever greater demand for high quality, portable equipment that can be used to tackle the problem in-situ.
The costs involved in sourcing such items can, unfortunately be prohibitive in some cases. Through partnership with a highly experienced test rental firm which has an extensive inventory of testing tools, companies within the mobile telecom sector will be better placed to deal with the challenges of PIM and various other phenomena. The rental approach has a number of clear advantages over direct purchasing, enabling contractors charged with carrying out base station deployment and subsequently testing of performance levels to complete the job while still coping with stringent budget constraints.
It is clear that PIM now has the potential to seriously disrupt mobile communication systems. The issues arising from it could have a highly detrimental effect on network performance. In response to this, PIM testing is set to become an essential part of ensuring compliance of newly deployed base stations.
Shailendra Kalra, director, Livingston India