Interference in a crowded RF spectrum: why it occurs, and what to do about it
‘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.
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 (250 kHz for GSM, up to 20 MHz 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 (see figure 1).
Figure 1: co-channel interference occurs when transmissions radiate further than expected.
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 (see figure 2).
Figure 2: the incidence of intermodulation products of two carrier signals. Click to enlarge.
So for instance, if a base station transmits two signals at 925MHz and 960MHz (the extremities of the EGSM band), intermodulation products will be found at 890MHz, 855MHz and 820MHz. The problem here is that the 890MHz intermodulation signal is in the middle of the EGSM receiving band (880-915MHz), so the intermodulation product will therefore interfere with valid incoming signals that the base station is trying to receive.
For LTE, most European operators have allocated a single, wideband channel (of up to 20 MHz). Even here, though, PIM – which requires two strong signals – can occur. This is because the channel includes sub-carriers in the OFDM modulation scheme that LTE uses. So if the infrastructure is liable to cause PIM, all the sub-carriers will intermodulate with each other and so disturb the receiver (see figure 3).
Figure 3: sub-carriers in LTE transmissions can generate intermodulation products. Click to enlarge.
Interference is not only caused internally: external interference also affects cellular networks. The biggest such source is jammers installed for the specific purpose of blocking mobile phone transmissions. These devices, which can be readily bought online for less than €300, are used by places of worship and cinemas to prevent mobile phones from causing disturbance, and by universities to stop students using mobile phones to find information during examinations.
In fact, usage of jammers is strictly forbidden in Europe: this is mainly to ensure that emergency services (available by dialing 112 in the European Union) can be reached at any time from any place. But a secondary problem is that users of jammers cannot control their coverage, and this means they frequently block mobile phone reception close to, as well as inside, the buildings that host them.
Interference in broadcast networks
Cellular network operation is also responsible for instances of interference with reception of television broadcasts. The problem arises from the way that analogue TV spectrum has been released for mobile phone usage.
Initially, the plan was to allocate the freed spectrum for digital video broadcast. In fact, DVB-T broadcasting needs less spectrum than analogue TV, so a portion of the spectrum in the 800MHz band remained free. In Europe, the European Commission is set to release it for LTE transmissions.
Unfortunately, this new wideband cellular standard adjacent to digital broadcast spectrum increases the chances of interference, as DVB-T receivers and antennas are designed to receive signals in the 800MHz band, including the part now reserved for use by LTE transmissions.
This means that a strong LTE signal broadcast can interact or even overlap with a DVB-T signal, with effects that can range from barely noticeable video and/or audio errors, to freezing of the video signal, up to complete loss of service.
The interference can come from both LTE network base stations and user equipment. The more common source is user equipment, even though its transmissions are much weaker than a base station’s. This is because the base station is usually far from a DVB-T receiver (a set-top box), while a user’s mobile phone can be very close – in the worst case, the user might even put it on top of a set-top box.
This source of interference can be particularly hard to find, since a mobile handset or dongle only broadcasts intermittently, when linked with a base station during a voice call or data session.
800 MHz LTE transmissions can also interfere with digital cable TV (DVB-C) reception if the receiver, or the cables connecting to the receiver, are improperly shielded. This can impair not only TV reception, but also telephone and internet service carried on the same cable.
A number of techniques are used today to try to prevent interference with TV broadcasts, with varying levels of success. One of the most common is to install a low-pass/band-pass filter between the antenna and the receiver; if this helps DVB-T interference, however, it will not fix DVB-C problems.
Unfortunately, commercially available and affordable filters that block 800MHz LTE transmissions also attenuate the DVB-T channels that are close to the LTE bands. While this is not a problem in locations with a good DVB-T signal, it can actually disrupt the service for a location with a weak DVB-T signal.
Effective methods for dealing with interference
The first requirement in dealing with interference problems is to locate and identify the source of the interference, whether it be CCI, PIM, a jammer, or (in the case of digital video broadcast) LTE transmissions. Many types of RF test instrument can potentially be used for this. Dedicated interference detection equipment, however, will save the user a great deal of time, because of the precision with which it locates and analyses the source of interference.
For PIM problems, the Anritsu MW8219A PIM Master is ideal: it can generate two tones of up to 40W into the transmission system, and measure precisely the occurrences of passive intermodulation. Its ‘Distance to PIM’ feature lets the user exactly locate the source of the interference. The MA2700A Interference Hunter is equally helpful for other kinds of interference.
Once located, the interference can normally be eliminated easily. In the case of DVB-C broadcasts, filtering is ineffective because the technology uses overlapping frequency bands. The correct approach here is to specify the receiver, cables, modem and other components with high resistance to interferers.
As for DVB-T, it can be helpful to replace an omni-directional antenna with a uni-directional antenna (the smaller the beam width, the better). This is not a guaranteed fix: it will make no difference if the LTE base station is located in the same direction as the DVB-T transmitter.
And in general, the best cure for interference is prevention: using excellent components and installing them properly eliminates almost all risk of interference. But since no real-world operation is ever perfect, it is useful to know that precise and accurate instruments are available that will quickly find the source of an interference problem when it occurs.
About the authors:
Eder Eiras is Business Development Manager at Anritsu — www.anritsu.com
Mathias Hofer is Regional Account Manager.