By Nicholas Cannon
In this current exciting cellular industry, Cellular system performance requirements
are at an all time high. This is an extremely important part of the day-to-day operation
of a modern telecommunications network and directly reflects on the overall perceived
quality levels as experienced by each individual subscriber. We have all experienced
dropped calls and poor quality connections while using our cellular devices and these frustrating performance problems directly contribute to subscriber churn, sometimes
to a level exceeding 30% of the overall reasons a subscriber decides to change
service providers. This has a massive impact for the service provider, as it is a key
indicator reflecting the sentiment of the customer. It is not surprising to learn that
staff bonus plans take this indicator of performance into account heavily.
The modern communications system is required to perform at a very efficient level.
This quality standard has changed greatly during the past few years and will
continue to change as current 3G and 4G systems compete for market share and
increase infrastructure and coverage. Line sweep testing is more important than
ever and emerging Passive Intermodulation (PIM) testing requirements have lifted the bar even higher. This ultimately means the Contractors, Cell Technicians and
Performance Engineers must work closely together to achieve the required quality
of service. In this white paper, we will examine the basics of both Line Sweeping
and PIM testing, how they work together and why we need to perform these tests to a high standard.
2.0 Line sweep basics
Cellular system performance starts with correct line sweep measurements. This acceptance
test is usually broken down into two basic tests: Return loss and insertion loss. Both are very
frequency dependant and can vary greatly within a specified band.
Return loss measures the power transfer efficiency of the antenna system. It is essential that
minimal power is reflected back towards the transmitter. Any reflected power can distort the
transmitted signal and when powerful enough, cause damage to the transmitter. A return loss
figure of 20 dB indicates that 1% of the transmitted signal is being reflected back to the transmitter
and 99% is reaching the antenna, this is generally considered good performance. A return loss
of 10 dB indicates that 10% of the signal is being reflected and should be considered poor. If the
return loss measured 0dB, 100% of the power would be reflected, this would likely be an open
or short circuit. Every connection erodes the possible return loss and thus minimal connections
are recommended. Return loss can be masked by high insertion loss and this is why DTF plots
are sometimes requested.
Insertion loss measures the amount of signal loss through a coaxial system. Ideally the insertion
loss will be very low. Again, every connection adds to the system loss. If a cable has 3 dB of
insertion loss, 50% of the power is being lost within the cable system. This effectively turns a
16 watt transmitter into an 8 watt transmitter. High loss affects the site’s ability to not only get
signals out to the subscribers, but heavily affect the handsets ability to transmit back to the
BTS receivers. Both problems can seem very slight but do drastically affect the subscribers’
user experience in reduced coverage area and short handset battery life.
Cell site insertion loss is commonly measured with a short circuit attached to the top end of the
cable and site master measuring return loss at the bottom end. The signal will travel up the cable.
When it reaches the short circuit 100% will reflect back down to the instrument. In this test
scenario, the signal experiences the cable loss twice (up and down) so the final figure should be
divided by 2. A measurement of 3dB will equate to an insertion loss of 1.5dB. As requirements
vary greatly between operators, specific operator test procedures must be used in all cases to
ensure compliance of test results.
While system designers take this information into account when designing the site specifications,
it is important to be aware of the effects that insertion loss and cable return loss can have on the
overall system return loss. A very good system return loss can mask possible component / antenna
problems and therefore may not be a good thing; it could easily be due to a cable that exhibits
high insertion loss and a faulty antenna. As a result, a smaller than expected signal level would
reach the antenna and a large percentage would be reflected, only to be dissipated within the high loss cable. This results in a transmitted signal level much lower than expected and the
coverage area would be restricted greatly. Comprehensive testing is required.
3.0 PIM test characteristics
PIM testing is a measure of linearity within a cellular transmission system. It is a very comprehensive
test of mechanical integrity. Two high power test signals are injected into the antenna line. If any
corroded, dirty or mechanically compromised connections or components exist, the two test
signals will mix and produce intermodulation. When performing PIM tests, the test system usually
monitors the 3rd order product as it is the strongest of all intermodulation orders. By minimizing
the 3rd order products, all other intermodulation is also minimized. Some site configurations
cannot generate PIM, particularly if only one Transmitter is used. There is still a compelling
argument to perform PIM testing on these smaller sites to ensure that further expansion will
yield excellent performance, as the RF plumbing will be in great operating condition and
suitable for multi carrier operation. A methodical approach must be followed when PIM testing
as continual breaking of connections will cause premature failure, particularly when testing
antennas and TTLNA’s.
Sites that have multiple transmitters will show performance degradation from internally generated
PIM when poor connections exist. This may become apparent with Rx noise imbalance alarms
and dropped calls/early handoff. The BTS supervisory system should inform the operator of
these conditions. In cases like these, repairing the connections/antennas/linearity problems with
a PIM tester is the only real solution. Unfortunately, a large percentage of old sites fall into
4.0 The difference between Line Sweeping and PIM testing
Line Sweep testing and PIM are very different tests. Both are very important and accurate measures
of a cell site’s ability to provide service and perform optimally. Line sweeping measures the signal
losses and reflections of the transmission system. PIM testing is a measure of construction
quality and poor quality will result in self-interference.
PIM testing performance measurements are not relevant unless accompanied by comprehensive
line sweep tests. It is important to recognize that PIM test measurements performed on a
transmission system that has poor microwave performance are irrelevant indicators of the
performance of the transmission system. Unfortunately, lack of understanding of the way these
tests relate to performance has not only resulted in compromised test data, it also causes the
need to re-test the systems repeatedly. In turn, connections and components are becoming
increasingly overworked and greatly contributes to line sweep and PIM problems.
PIM requires both low system loss and good return loss (VSWR) to perform to an acceptable
standard. If PIM testing is performed prior to line sweep testing the operator may not be aware
of the impedance characteristics of the transmission line. High insertion loss attenuates the
PIM test signals preventing the high power characteristics from reaching the very components
which require this stringent testing. Poor Return loss reflects a percentage of the PIM test signals
back into the test set causing some signal cancellation that can report a false positive.
It is becoming increasingly common to take advantage of poor line sweep performance to pass a PIM test.
By performing the line sweep test prior to PIM testing, the operator can be confident the
insertion loss and return loss data at acceptable levels. This data in turn ensures that the PIM
test signals actually reach the components at the highest possible signal level, offering the most
accurate indicator of true PIM performance. By constructing a system using modern low PIM
practices, the need to break the transmission system back open will be minimized. If the lines
are disassembled again to repair or clean a connector, the line sweep data will need to be
5.0 Site performance indicators
BTS alarm conditions are designed to provide insight into the current performance issues. Many
network operators are assuming that unfixable performance faults are due to poor PIM levels.
It needs to be carefully considered if PIM can actually be a possible problem within the cell site.
Unless the site has two or more transmitters on one feed line, self-generated PIM is unlikely.
The BTS will offer indications of PIM performance and this is usually reported as a “Rx/Main
diversity noise imbalance” alarm. This data indicates that the noise floor between the main line
and the diversity line are different in level. All major BTS system manufacturers offer this alarm
condition but different terms are are sometimes used.
If the feed line that carries the transmit signals has the raised noise floor, then PIM is highly
suspected. External interference will be present on both the main and diversity feed lines. This
interference or raised noise floor could be externally generated PIM and can be quickly isolated
by wilting (Shutting down) one or more transmitters on the affected sector. It is worth noting
that a system that has a calculatable 5th or 7th order relationship usually needs three or four
transmitters on air to generate enough power within the PIM products to elevate the Rx noise
floor and disrupt service.
A variance in download speed problems are often blamed on PIM and the operators are shocked
to learn that the issue is two or three different site sectors are actually competing for dominance
where the coverage overlaps. A field strength measurement system that incorporates a site ID
decoding capability is an invaluable tool for adjusting transmitter power levels where a number
of sites overlap in an area that covers a high density of subscribers. The real issue in this case
is unhappy subscribers due to a signal dominance battle between a high capacity BTS sector and
a lower capacity site where the coverage overlaps. A subscriber does not receive consistent
data performance due to the cellular device having a number of connection choices, all with
different data speed performance. The performance engineer can simply instruct the operations
center to vary the transmit power on one site while the download speed, pilot levels and site ID
are monitored live. This can take an hour or less to rectify without even the need for a tower crew.
A cellular site that reports elevated Rx noise floor on both Rx lines (Duplex/Simplex) would
indicate that the source is external. This is often a cable TV amplifier that is leaking RF power.
The BTS receiver is very sensitive and can easily suffer from external noise that happens to cross
into the sites receive band. Another good example of this type of fault is problems caused by
cellular jammers. Church groups and Schools often employ this technology without realizing the
possible problems these items can cause.
Today’s cellular systems are pushing site performance to a level never before seen within the
industry. It is essential to approach performance problems with a multi disciplinary methodical
system for troubleshooting and not relying on just one or two measurement platforms. By arming
Contractors, Cell Tech’s and Performance Engineers with the correct testing tools and high level
training coupled with strong industry experience so the exact performance parameter can be
isolated and measured, a whole new level of cellular system performance can be realized. Line
Sweep and PIM testing are key tools to find possible BTS issues. To complete the toolkit, over
the air testing and interference measurement solutions are also required, as is a thorough testing
capability and understanding of backhaul capacity and performance. 4G/ LTE communications
platforms force network operators to approach performance optimization using both conventional
and modern troubleshooting methods.