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EETE APR 2014

Interference in a crowded RF spectrum: why it occurs, and what to do about it By Eder Eiras and Mathias Hofer As the number of wireless services has grown in recent years, so interference, once uncommon, has become a regular headache for wireless and broadcast service providers. ‘Interference’ is in fact a catch-all term for a variety of phenomena that disrupt or even disable transmission and reception of widearea 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, PCIM_2014_ANZ_E_210x140 14.10.13 11:25 Seite 1 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. 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 mea- This is your powerful connection! The marketplace for developers and innovators. Future starts here! International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management Nuremberg, 20 – 22 May 2014 More information at +49 711 61946-0 pcim@mesago.com or pcim-europe.com


EETE APR 2014
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