Dealing with interference in a crowded RF spectrum
'Interference' is in fact a catch-all term for a variety of phenomena that disrupt or even disable transmission and reception of wide-area wireless communications. It's therefore not in itself a useful term: it does not help network engineers and wireless equipment developers to troubleshoot and repair a specific problem that is compromising a system, or to design equipment that is immune to the effects of this problem.
This article sets out to pick apart the concept of interference as it applies to cellular networks and broadcast television transmissions, and to show how the correct representation and diagnosis of the common causes of interference make it easier to fix.
Figure 1: Co-channel interference occurs when transmissions radiate further than expected.
Self-disturbance in cellular networks
Interference problems in European cellular networks have in part been the unwitting result of government restrictions. Environmental legislation has had the effect of limiting the availability of new base station sites. To increase network capacity, service providers have therefore had to increase the density of antennas on existing cell towers. Of course, this in itself increases the potential for one network to interfere with another.
Often, however, the interference is the result of a cellular network disturbing itself. One example is co-channel interference (CCI), which can occur in GSM and FDD-LTE networks. Normally, network operators allocate different frequency bands (250kHz for GSM, up to 20MHz for LTE) to neighbouring cells. The goal of the network planning process is to ensure that signals from two cells using the same frequency band do not share the same air space. In their planning, service providers take account of the geography of the cell location. For instance, transmissions from a base station at the top of a hill can radiate further than those from a base station in a valley.
CCI arises when a base station radiates further than was expected by the network planner. This can sometimes happen in particular weather conditions: for example, humidity helps electromagnetic waves to travel further. This means that fog extends a base station's coverage, and can thus cause transmissions to reach the coverage area of another, distant site with the same frequency allocation (figure 1).
CCI as a cause of network problems can be discovered by correlating the pattern of network disturbance events with weather patterns, and by the use of a direction-finding measurement instrument. For instance, the MA2700A Interference Hunter from Anritsu includes a GPS location device and electronic compass. Connected to an Anritsu spectrum analyser and a directional antenna, it enables a network engineer to find the location of any interferer by triangulation.
Rust as a source of interference
A different cause of self-disturbance is Passive Intermodulation (PIM). It is caused when two or more strong RF signals combine in a non-linear device, such as a transistor or diode. In fact, the crystals found in corrosion or rust on an antenna or cable connector can cause PIM. Even corrosion outside the intended radio signal chain can cause interference: a rusty fence, rusty bolts, corroded rooftop air conditioners or even a rusty barn roof in proximity to the base station are a hazard. Of course, it's also possible that loose connectors in an antenna feed line or poorly configured transmitters can cause PIM.
PIM frequencies are predictable. If you have two signals at frequencies F1 and F2, the third-, fifth- and seventh-order intermodulation products will be found equally spaced above and below the two signals (figure 2).
Figure 2: The incidence of intermodulation products of two carrier signals.
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