Cross-Band PIM Generation in Co-located Cellular Base Stations

PIM testing in cellular base stations has traditionally focused on intra-band PIM problems. This refers to situations in which down-link signals from a base transceiver station (BTS) generate PIM in the up-link band of the same system.A less well-known but increasingly more common type of PIM is cross-band PIM. This occurs when down-link signals from two or more BTS's operating in different frequency bands mix together to produce PIM in the up-link band of one or more of those systems. This problem is set to intensify as spectrum is re-framed for use in the 600 MHz, 700 MHz, 2500 MHz and 3500MHz bands, thereby creating opportunities for mixing between entirely new combinations of down link signals.An insidious feature of cross-band PIM faults is that they do not reveal themselves when only one BTS is operating. This means that, depending on its traffic configuration, a BTS can be PIM-free when tested in isolation but still experience a large PIM-induced noise- floor rise when all co-located BTSs are transmitting simultaneously.