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The problem with pim, how can pimforensics help?

How do networks test for pim today?

rf pim tester

•Good for site install test
•Not possible to test 4/5G multi band PIM issues.
•Expensive tower climb, out of service test •Measures CW PIM, cannot identify impact to modulated carriers
•Intrusive test , can introduce PIM!

rf pim wand

•Useful for identifying exact location of PIM on site.
•Labour intensive, manual process
•You need to be close to the PIM source to find it!

remote uplink monitor

•Low cost, remote test
•Indicates you MIGHT have PIM
•Requires site visit to diagnose/rectify

All of these methods are tried and tested ways to find SOME PIM Current PIM test methods are either limited in capability or costly to use

Operators and how they view pim

Operators for whom PIM is not the focus
•Operators who check KPIs and network metrics and state they have no PIM in their networks.
•Operators where the cost of investigating PIM is prohibitively expensive, live with the problem rather than fix it.
•Operators for whom the option of PIM Cancellation is seen as a solution BUT PIM Cancellation can only be used where you understand what PIM you need to fix!
•Operators who cannot test PIM due to regulatory restrictions on swept CW transmissions

Operators who are investing in addressing pim
•Operator procedures to check for PIM on installation and tools to check for PIM in live networks.
•Active work to allocate spectrum and optimize network design
•Investment in network equipment that can in specific cases mitigate PIM (RRH with active cancellation)  

All networks have PIM to a greater or lesser extent. PIM is dynamic and constantly changing.
The real issue is that today it is very difficult to quantify the financial impact of PIM at a site. If you cannot quantify the impact, why try to fix it? How do you know any fix offers an ROI?

Aceaxis, Requirements for a PIM test solution

  • Provide a dB measure of impact to traffic on the cell site to support PIM ROI analysis strategy  
  • Measure PIM in non intrusive, zero downtime way*
  • Provide tools to accurately define where the PIM is and the type of PIM (multi/cross-band, IM3+5)
  • Provide one box that can be used for all 4G and many 5G RF bands**  

*a very brief outage (few mins) to install an optical tap, if applicable.
**Supports all 3GPP bands, except millimetric bands.

PIMForensics

We need to survey the site to determine:
  • What type of PIM is it (IM3+5)
  • Where the PIM is
  • What is the cost of the PIM at the cell site (dB impact on carriers)

As there is no service disruption with PIMForensics, surveying the cell site can now be done either when investigation is required or during normal periodic maintenance

From this information strategies to tackle PIM can be identified namely :
  • No action taken, cost of PIM at the cell site is negligible
    Level of PIM is low and/or PIM occurs at specific times of the day with little impact.
  • Fix PIM at cell site, cost of PIM is minor but cost of fix is low
    PIM is identified in feeder cables or bracket,  technicians at site can fix the problem whilst at site
  • Fix PIM at the cell site, cost of PIM is significant and can be fixed with return visit
    PIM is identified in antenna or adjacent to site
  • Fix PIM with PIM mitigation solution, PIM is significantly affecting site
    PIM is significant and cannot be fixed without using PIM mitigation,level of PIM impact and root cause drive ROI analysis to fit PIM mitigation at cell site.

Surveying the site to understand the cost of PIM, drives cost effective solutions to PIM

Pim forensics, how does it work?

Optical tap
Inserted into the CPRI link between BBU and RRU. The optical tap may be pre-installed in some networks or can be supplied  with the sensor
During installation of the tap into the optical link, there will be a short interruption on the CPRI link to service, following which the network will continue to operate, supporting live traffic, as before.
The required optical signals are fed from tap to sensor.

IQ sensor
The sensor collects base band IQ data from the downlink band(s) which are causing the PIM and the uplink band suffering from  PIM.
Baseband IQ data preserves all the information in the RF link, just in digitised form, so any measurement that could be made at an RF connection can also be done by processing baseband IQ data.  
It will simultaneously capture all MIMO antennas.
The captured IQ data is processed on a laptop.

GUI
The set up, measurement control, display of results and report generation are under control of the GUI running on the laptop.

PIM Algorithm (patented)
The algorithm, which identifies and measures the PIM, runs on the laptop under control of the GUI.
It measures the level of PIM, even in the presence of UL traffic, thermal noise and any other interference.

PIM Algorithm

The algorithm uses all the downlink LTE signals that contribute to the PIM to adaptively build a model of the PIM.

The present limits on complexity are:
•One or two carriers which can be from the same or different bands.
•Up to 4 transmit MIMO antennas
•IM3 or IM5 and Harmonics

The model adapts to:

•Internal/ external PIM sources
•Differing round trip delays through different radio infrastructure
•Multiple PIM sources
•Time varying PIM sources
It correlates the modelled PIM with the uplink signal from each of the MIMO antennas.

Only the PIM within the uplink signal will correlate with the modelled PIM.


•The measurement therefore differentiates PIM from uplink traffic, thermal noise and other forms of interference (a significant source of false positive PIM results with remote PIM test)

Detailed analysis and PIM source diagnosis

Building a PIM model allows a level of analysis far beyond a simple examination of the uplink spectrum.
•The PIM can be monitored through time as the downlink and uplink traffic levels vary.
•Low level PIM can be identified, regardless of the level of uplink traffic.
•The contribution to the PIM from individual MIMO transmit antennas can be identified aiding the diagnosis of PIM faults.

PIM source diagnosis.This is a two step process:

1.Heat Map: Uses the individual antenna contributions to indicate the likely source and position of those dominant PIM sources that are internal to the RF network or in the immediate vicinity of the antennas

2. Distance Measurement: Uses a power delay profile, similar to that used by an RF PIM tester,to report the distance between antenna calibration point and dominant PIM source.

How to use pimforensics

System components and connectivity

Configuring the System

Configuration is in two stages:

•Stage 1 – Configure the instrument for the environment it will work in(site info)  

•Stage 2 – Configure the instrument for the test (test roster)

•To remove complexity AceAxis offers the option to load scripts to support test or to manually enter the information to support the test.

•Beta version of auto-discovery of CPRI + RF data is in development.

Site information and Test roster

Site Information

Contains site specific information, fully editable within the GUI:-
•Site ID / name
•Lat, long, antenna beam width& azimuth (for PIM location)
•Carrier details – UL / DL center frequency, LTE bandwidth
•CPRI mapping – No. of antennas, AxC start number (typically 1,5 (2 ant)), max rate (supports all rates 1-8)

Test Roster

•For each site, the test roster will define the set of measurements to be made and in what order. The test roster is editable within the GUI
•For each sector within the site,one (or more) possible PIM scenarios are identified as likely to cause a problem and for each problem, the following are defined:

The specific band combination.
e.g. DL1=Band 17, DL2 = Band 14, UL1 = Band 17
The IM caused.
e.g. IM3 upper/lower

Using the GUI- Configuration

Once site info & test roster are configured and loaded. The tap and sensor can be connected, using the on-screen, colour-coded fibre diagram.

A configuration check is run to ensure correct fibre connectivity. The TSSI (DL) and RSSI (UL) levels will be displayed along with pass/fail.

Low traffic levels will require OCNS to be activated – this will be represented with low TSSI values (resulting in fail configuration check).

Uplink & Downlink spectrums can be viewed on the ‘analyser’ tab.

Using the GUI- PIM Measurements

Once tap & sensor are configured PIM measurements for each UL antenna port can be performed.Six tabs display various useful information, each set of results can be saved to a report.

Displays (wanted) UL signal and PIM signal – changeable units, dBm & dBlsb (dB Full Scale, dB Least Significant Bit)

Displays PIM over time, useful for long-term monitoring, saves an ‘event log’ detailingtime/level when PIM exceeds pre-defined threshold.

CDF detailing percentage of time PIM exceeds threshold.

Bar chart displaying PIM counts exceeding threshold over a 24 hour period.

Intuitive display to help identify the potential source / location of PIM.

Distance of PIM source from a reference point (typically the antennaface).

How does PIMForensics solve your problem?

New Insights+ Lower Costs + Zero Downtime = PIMForensics

Field test of PIM

Capex & Opex
1 – Passive test for PIM, no cell site down time, ROI measure of PIM
2 – One PIM over CPRI test box covers all RF bands  (future proof)
3 –No certification testing needed, product is designed to be simple to use
4 – No need to calibrate the unit

Physical
1 –Tower base/ shelter test for PIM
2 –No need to power down site and disconnect RF cabling
3 –Equipment can be left at site for long term monitoring
4 –Testing can be performed at any time of the day
5 –Small sensor size and weight (2lb weight and 1.3L volume).

Measurement/ Measurement Accuracy
1 –Measures with live traffic providing the most accurate assessment of PIM impact to network
2 –Measures dBm or dB full scale to represent PIM magnitude directly related to the RF carriers under analysis.
3 –Measures PIM to below the noise floor.
4 –Measures single band PIM, multi band PIM and cross sector PIM, on site/off site PIM, PIM location
5 – Measures RF over CPRI