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 dependent 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.
HINLAR Cell Site Services utilizes Anritsu Site Masters operated by certified operators to perform all line sweeps.
Passive Intermodulation (PIM) Testing
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 this category.
HINLAR Cell Site Services utilizes Kaelus PIM Testers operated by certified operators to perform all PIM testing.
Fiber-to-the-Antenna (FTTA) Testing
Dirty fiber end-faces are the #1 cause of fiber link failure, but are the easiest to prevent. Damaged end-faces, in the form of scratches, pits, cracks, or chips can also bring a fiber network down, and are often the result of poor terminations or mated contamination. Finding and eliminating any area of reflectance will help high bandwidth performance.
HINLAR Cell Site Services utilizes JDSU and ODM testers operated by certified operators to perform all fiber testing.