8/6/2019 Mobile Interference

1/3

Co-channel interference or CCI is crosstalk from two different radio transmitters using thesame frequency. There can be several causes of co-channelradio interference; four examples arelisted here.

y Cellular Mobile Networks: In cellular mobile communication (GSM & LTE Systems,for instance), frequency spectrum is a precious resource which is divided into non-overlapping spectrum bands which are assigned to different cells (In cellularcommunications, a cell refers to the hexagonal/circular area around the base stationantenna). However, after certain geographical distance, the frequency bands are re-used,

i.e. the same spectrum bands are re-assigned to other distant cells. The co-channelinterference arises in the cellular mobile networks owing to this phenomenon ofFrequency reuse. Thus, besides the intended signal from with in the cell, signals at thesame frequencies (co-channel signals) arrive at the receiver from the undesiredtransmitters located (far away) in some other cells and lead to deterioration in receiverperformance.

y Adverse weather conditions: During periods of abnormally high-pressureweather, VHFsignals which would normally exit through the atmosphere can instead be reflected by thetroposphere. This tropospheric ducting will cause the signal to travel much further thanintended; often causing interference to local transmitters in the areas affected by the

increased range of the distant transmitter.

y Poor frequency planning: Poor planning of frequencies by broadcasters can cause CCI,although this is rare. A very localised example is Listowel in the south-west ofIreland.The RTNL UHF television transmitter systems in Listowel and Knockmoyle (nearTralee) are on the same frequencies but with opposite polarisation. However in someoutskirts of Listowel town, both transmitters can be picked up causing heavy CCI. This problem forces residents in these areas to use alternative transmitters to receive RTprogramming.

y Overly-crowded radio spectrum: In many populated areas, there just isn't much room inthe radio spectrum. Stations will be jam-packed in, sometimes to the point that one canhear loud and clear two, three, or more stations on the same frequency, at once. In theUSA, the FCC propagation models used to space stations on the same frequency are notalways accurate in prediction of signals and interference. An example of this situation isin some parts of Fayetteville, Arkansas the local 99.5 FM KAKS is displaced by KXBL99.5 FM in Tulsa, particularly on the west side of significant hills. Another examplewould be ofCleveland's WKKY 104.7 having interference from Toledo's WIOT 104.7FM on the Ontario shore of Lake Erie, as well as Woodstock's CIHR-FM (on rareoccasions), which is also on 104.7 FM, due to the signals travelling very far across LakeErie.

Radio resource management (RRM) is the system level control ofco-channel interference andother radio transmission characteristics in wireless communication systems, for example

cellular networks, wireless networks and broadcasting systems. RRM involves strategies and

algorithms for controlling parameters such as transmit power, channel allocation, data rates,

handover criteria, modulation scheme, error coding scheme, etc. The objective is to utilize the

limited radio spectrum resources and radio network infrastructure as efficiently as possible.

8/6/2019 Mobile Interference

2/3

Adjacent-channel interference

orACI is interference caused by extraneous power from a signal in an adjacent channel. ACImay be caused by inadequate filtering, such as incomplete filtering of unwanted modulationproducts in frequency modulation (FM) systems, impropertuning, or poor frequency control, ineither the reference channel or the interfering channel, or both.

ACI is distinguished from crosstalk.

Broadcast regulators frequently manage the broadcast spectrum in order to minimize adjacent-channel interference. For example, in North America FM radio stations in a single region cannotbe licensed on adjacent frequencies that is, if a station is licensed on 99.5 MHz in a city, thefrequencies of 99.3 MHz and 99.7 MHz cannot be used anywhere within a certain distance ofthat station's transmitter, and the second-adjacent frequencies of 99.1 MHz and 99.9 MHz arerestricted to specialized usages such as low-power stations. Similar restrictions formerly appliedto third-adjacent frequencies as well i.e. 98.9 MHz and 100.1 MHz in the example above but these are no longer observed.

The Adjacent Channel Interference that a receiver A experiences from a transmitter B is the sumof power that B emits into A's channel ( which is called the unwanted emission and representedby the ACLR (Adjacent Channel Leakage Ratio) and the power that A picks up from B's channelwhich is represented by the ACS (Adjacent Channel Selectivity). B emitting power into A'schannel is called Adjacent Channel Leakage, or unwanted emissions. It occurs because all RFfilters require a roll off, and never eliminate a signal completely. Therefore B emits some powerin the adjacent channel which is picked up by A. A receives some emissions from B's channeldue to the roll off of the selectivity filters. Selectivity filters are designed to 'select' a channel.

Intermodulation orintermodulation distortion (IMD) is the unwanted amplitude modulationofsignals containing two or more different frequencies in a system with nonlinearities. Theintermodulation between each frequency component will form additional signals at frequenciesthat are not just at harmonic frequencies (integermultiples) of either, but also at the sum anddifference frequencies of the original frequencies and at multiples of those sum and differencefrequencies.

Intermodulation is caused by non-linearbehaviour of the signal processing being used. Thetheoretical outcome of these non-linearities can be calculated by generating a Volterra series ofthe characteristic, while the usual approximation of those non-linearities is obtained bygenerating a Taylor series.

Intermodulation is rarely desirable in radio or audio processing, as it creates unwanted spuriousemissions, often in the form ofsidebands. For radio transmissions this increases the occupiedbandwidth, leading to adjacent channel interference, which can reduce audio clarity or increasespectrum usage. It should not be confused with harmonic distortion (which does have widespreaduse in audio effects processing), nor with intentional modulation (such as a frequency mixerinsuperheterodyne receivers) where signals to be modulated are presented to an intentional

8/6/2019 Mobile Interference

3/3

nonlinear element (multiplied) (see non-linearmixers such as mixerdiodes and even single-transistoroscillator-mixer circuits). In audio, the intermodulation products are nonharmonicallyrelated to the input frequencies and therefore "off-key" with respect to the common Westernmusical scale.

In electronics, crosstalk (XT) is any phenomenon by which a signal transmitted on one circuit or channel

of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually

caused by undesired capacitive, inductive, or conductive coupling from one circuit, part of a circuit, orchannel, to another

In structured cabling, crosstalk can refer to electromagnetic interference from one unshieldedtwisted pairto another twisted pair, normally running in parallel.

Near End Crosstalk (NEXT)Interference between two pairs in a cable measured at the same end of the cable as thetransmitter.

Far end crosstalk (FEXT)Interference between two pairs of a cable measured at the other end of the cable from the

transmitter.Alien crosstalk (AXT)Interference caused by other cables routed close to the cable of interest