Page 34

EETE APR 2014

TEST & MEASUREMENT surement 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 Fig. 1: the incidence of intermodulation products of two carrier signals above and below the two signals (see figure 1). 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 2). 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. Fig. 2: sub-carriers in LTE transmissions can generate intermodulation products 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. 800MHz 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. Eder Eiras is Business Development Manager at Anritsu - www.anritsu.com Mathias Hofer is Regional Account Manager. 34 Electronic Engineering Times Europe April 2014 www.electronics-eetimes.com


EETE APR 2014
To see the actual publication please follow the link above