PIM is the generation of interfering signals caused by non-linearity in the mechanical components of a wireless system. Two signals mix together (amplitude modulation) to produce sum and difference signals and products within the same band, causing interference. Passive Intermodulation has become new benchmark in determining the health of a cell site. Today’s mobile handset users expect consistent high throughput from their devices and, consequently, push current networks to their limit.

The upcoming next generations networks feature an increased mobile data rate of 100 Mb/s and this higher transmission rate will expose PIM vulnerabilities in today’s networks like never before.

4G Has Different Needs

Fourth generation FDD networks require superior network transmission fidelity, higher than previous ones.

MNO are now facing the challenge to maintain customer loyalty in an unforgiving competitive arena. As such, good network PIM performance is now imperative! Although important, these PIM sources can easily be resolved with a regular cell site transmission line maintenance and, of course, by giving the right training to site Engineers, about high quality installation.

Descriptively, Passive Inter-Modulation is an undesired, nonlinear, signal energy generated as a bi-product of two or more carriers sharing the same downlink path in wireless networks.

Due to network hardware configurations, this multi carrier interaction can cause significant interferences in the uplink receive band, which can lead to reduced receiver sensitivity.

To the mobile phone user, this is often translated to a loss in audio fidelity in conversations, decreased data speeds, or in extreme circumstances, dropped calls or an inability to make or receive calls or utilize data services.

Since there is a mathematical correlation between the known carrier frequencies and the resultant interference signal in the receive band, accurate measurements of PIM signals can be achieved consistently. For practical PIM testing applications, we will only concern ourselves with those PIM signals which interfere directly with our network’s receive band. Typically these PIM signals are:

3rd order PIM, = 2xf1 – f2

5th order PIM, = 3xf1 – 2xf2

Since Passive Intermods cannot be mathematically modeled and cannot be simulated using today’s engineering design tools, using a PIM analyzer is the only way to quantify it.

What causes PIM?

Ferromagnetic materials, when in the current path, exhibit a non-linear voltage to current ratio. This non-linear effect is accentuated at higher power levels because of increased current density.

Looking at Ohm’s law from the perspective of “Power” helps clarify the fact that the squaring effect of current results in a higher magnetic flux, which makes metals with high bulk resistivity, such as, iron, steel and nickel exhibit a magnet like memory effect.

This effect is better known as magnetic hysteresis

Metals that exhibit this asymmetrical magnetic flux are often the main contributor of PIM energy.

Poor metal to metal contact junctions can create additional nonlinearities resulting in PIM. Such nonlinearities can come from under-torqued male to female DIN 7/16 mates, as well as irregular contact surfaces such as poorly manufactured connectors and surface metal oxidation. Oxidation (corrosion)

Example:

CCI’s PIM-Pro 850 analyzer has a default setup with two transmit frequencies at 869 and 894 MHz, producing a 3rd order IM at 844MHz and a 5th order IM at 819 MHz.

In this example, the focus would be on the 3rd order IM at 844 MHz since it falls within the receiver range of 824 to 849 MHz. The 5th order IM at 819 MHz is outside of the receiver range and, as such, can be ignored for the purposes of PIM testing.

It is important to observe that the actual IM frequency is determined by the two transmit frequencies and the spacing between them. A 25 MHz frequency spacing between the transmitters also results in a 25 MHz spacing between the IM signals.

Typically, the 3rd and 5th order PIM signals are the most likely to fall within the receive band with enough PIM energy to cause disturbances, while 7th and 9th order PIM signals are usually very low in power.  [/one_half][one_half_last animation=”Fade In From Right” delay=””]CCI’s PimPro Passive Intermod Analyzer allows you to select which order PIM you want to measure and highlights the ones that fall in the receive band for simplicity.

It should be noted that PIM signals exist as a result of the combined transmission of multiple carrier frequencies within a transmission line path. The objective is to ensure that these levels, by design and in practice, should occur at an amplitude which is below the Base Stations receiver sensitivity.

The amplitude of these undesired signals is directly influenced by the fidelity of the transmission line path, including all components and junctions that can introduce a non-linear effect to the signals passing through them. laroccasolutions is proud to offer his specialized Engineers for PIM Site Optimization Service using CCI PIMPro Tester.

PIMPro Tower series

• Available LTE 700, Cellular 850, GSM 900, GSM 1800/UMTS 2100, PCS 1900/AWS 2100 & 2600 MHz models
• Real Time PIM, Return Loss measurements and distance to PIM
• 40W PIM sensitivity -135 dBm
• Lightest full power unit less than 19 lbs in a durable backpack enclosure with over three hours of battery life
• Simultaneous Real Time PIM & Return Loss measurements
• Smart phone app Wi-Fi remote
• Automatic GPS site location
• Integrated DAS test feature
• New Distance to Fault feature allows for simultaneous view of PiMPoint and Distance to Fault impedance reflections on the same graph
• New PiMPoint feature (integrated) allows distance approximation to largest PIM source, in 50 Ω path and outside the antenna

PIM Non-Linearity Analysis

PIM non-linearity increases, in theory, at a ratio of 3:1 (PIM to signal). A 1 dB increase in carrier power correlates to a theoretical increase of 3 dB in PIM signal power.

In practice, the actual effect is closer to 2.3 dB as the thermal noise constant -174 dBm/ Hz becomes an error contributor.

This thermal noise floor gets closer to -140dBm as PIMs are measured in a narrow IF filter which allows the noise level to increase at a theoretical 10 dB/decade.

This -140 dBm floor is considered a PIM analyzer’s residual IM level

40 Watt Vs 20 Watt

In order to better represent real traffic network conditions, PIM measurements should be performed at the BTS radio power level or slightly higher.

In the last several years, a handful of 2 Watt PIM analyzers have entered the market place touting their benefits as being, smaller, more portable, and conveniently battery operated.

Although these features are obvious, these units offer limited value since 2 Watt PIM testing is not representative of typical BTS power levels of 20 Watts or higher, where PIMs are likely to be generated. PIM testing, when measured in dBc, is a measurement of relative nonlinearity.

Network operators wants to be confident about their network while under the real traffic stress

Networks engineers want a confidence buffer in their power range where PIM begins to show non-linearity. Although most of today’s BTS units output 20 Watts, the new RRU technology (roof top or tower top radios) is now at 30 or 40 Watts and in some cases even higher power levels.

Network operators need to question whether testing at 20 watts (43 dBm) is satisfactory, as it may not expose marginal network PIM conditions. This is the main reason why CCI engineers designed the PIM-Pro PIM analyzer family with 40 Watts of output power.

Above graph displays actual PIM measurement results of a load. Note the slope of the red (PIM dBm) and green (PIM dBc) compared the 2 tone signal power. Also note that there is hardly any measurable non-linearity in the 2-10 W power range, due to lack of PIM generating power.Measurement recommendations

Due to their low power levels, less than -80dBm, PIM measurements are difficult to make with good accuracy in the best of lab conditions, let alone the harsh conditions that can be experienced at actual cell sites. A valid and repeatable PIM measurement requires an analyzer with stable linear amplifiers, exceptionally low PIM duplexer and related components, and a well-designed receiver with a very low receiver noise floor. The CCI PIM-Pro with a residual IM level of <-140dBm is well suited to perform PIM measurements in this regard.

Since Passive Intermodulations cannot be mathematically modeled and cannot be simulated using today’s engineering design tools, using a PIM analyzer is the only way to quantify them.

Recommended measurement practice includes:

• Visually inspect and clean all connectors before mating them.
• Torque all connections to a minimum of 16, and maximum of 18 ft-lbs (23-24 Nm).
• Allow measurements to thermally stabilize, especially in cold weather. Use PIM vs Time mode at highest available power (40W on PIM-Pro) to establish confirmation of a stable measurement using a low PIM load on the test port.
• In order to maintain measurement confidence, regularly verify measurement accuracy using a quality load and a PIM Standard – a calibrated device which looks like a connector, and is designed to deliberately generate a known PIM (usually -100dBm), which can be used to check that the PIM analyzer is measuring correctly. Using a quality low PIM load will confirm faulty components.
• Due to the non-linearity of the PIM response it is wise to test at higher power levels than necessary to ensure an acceptable measurement error margin.
• Use higher power to confirm marginal measurements as 2 x 20W-tone PIM testing is often not enough power to uncover a marginal PIM situation. Higher power, 2 x40W testing provides additional field diagnostic capability.

The DIN 7-16 connector is rated for 500 mates, although the connector can probably survive up to 1,000 mates, it is important to be cognizant of the constant wear and tear on cables and the PIM tester’s output connector. In the world of RF measurements, problems often start in the components used to perform the measurement at hand. Test cables are typical culprits.