EMC Awareness
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Radiocommunications Agency EMC Awareness |
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| Co-channel rejection |  |
Introduction
The problem of multiple signals on the same frequency is almost as old as radio itself. In order to use the spectrum efficiently, radio authorities can allocate the same frequency to several different users. Interference between them is expected to be avoided by several means, for instance:
separation distance
transmitted power level
transmission at different times
coding techniques
frequency agility
If any of these are relied on but are inadequately policed or maintained, then co-channel interference can result. This is not really an EMC problem, rather one of spectrum management, but an overview of the issues is given here.
How these techniques are used
Separation distance
In the far field, radiated fields decay at a rate inversely proportional to the distance from the emitting source. In the near field, the rate can approach an inverse square or even inverse cube law. This provides the simplest mechanism for dealing with interference: separation. Channels can be re-allocated on a geographical basis if the transmitters are fixed and the extent of their coverage, due to these propagation laws, is well controlled. At the local level, any radiated interference problem can be reduced simply by moving the victim away from the source, and even a small distance may be sufficient if a square or cube law is involved.
Power level and directivity
Licensed radio transmitters have limits placed on their transmitted power. The field strength E volts per metre at a distance d metres, in free space conditions, is directly related to this power (P watts radiated):
Limits on the interfering field strength in a given channel therefore can be achieved by controlling the power of potentially interfering transmitters. It is normally possible to tailor an antenna’s directional properties, so that the main power is only transmitted towards a particular area of coverage and the power transmitted in other directions is less, often much less.
Time segregation
Typical land mobile service users have brief conversations, interrupted by long periods of silence. Efficient spectrum utilisation demands that a channel is not idle during silent periods, and therefore systems for sharing the same channel are popular. Determining channel capacity then becomes an exercise in statistical analysis of the usage patterns.
Interference may occur if different users transmit at the same time, and therefore some mechanism is needed for arbitrating between them if this cannot be prevented by, say, synchronisation.
Coding
One extension of the technique of time separation is to slice the transmitted information into discrete timeslots by encoding the data, and allocate different transmitters different slots within a given frame. This is known as Time Division Multiplexing, and is part of the principle of operation of the cellular mobile phone and TETRA systems, in which each time slot is of the order of a few milliseconds.
An alternative sharing mechanism is known as Code Division Multiplexing, in which each digital signal is deliberately transmitted on the same channel at the same time but with different encoding “keys”. A third party listening on the channel would hear nothing but apparently random noise, but a receiver with the correct key would be able to decode the required data despite it being jumbled up with several other uncorrelated transmissions. The success of this technique depends on the various transmissions being carefully controlled to have roughly equal amplitudes.
Frequency agility
An alternative method for a radio system to avoid the effects of interference while maintaining a continuous link is to spread its coverage over several channels. This is known variously as “frequency hopping” or “spread spectrum” transmission depending on the methods employed to select the channel in use at any instant. A particular frequency might be used for just a few milliseconds, and the hopping may be dynamically arranged to avoid channels which are suffering interference. Naturally the receiver and transmitter must be synchronised in their hopping patterns. Such systems are attractive for secure communications, but also because of their adaptive ability they can be extremely efficient spectrum users and, as long as their allocated channels are not completely obliterated, very tolerant to interference.
Key issues in employing these techniques
While all of the above methods for dealing with co-channel interference are in use and successful, none are 100% optimum for all situations. The more complex systems may be unsuitable for widespread commercial use on cost grounds. The simple methods of spatial separation, time separation and constraints on power level are also imperfect:
propagation characteristics do not always follow a simple and predictable distance law; for instance at VHF, certain weather conditions can result in the occasional formation of atmospheric “waveguides” that carry signals many times further than happens in normal conditions. Local topography can result in large attenuation changes over quite short distances. And at HF, ionospheric conditions cause regular changes in the propagation patterns, on both a diurnal and yearly cycle. For certain types of interfering source, for instance broadband data, it may not be possible to increase separation distance because the source is all-pervasive.
relying on low duty cycles to prevent interference runs into difficulties when channel capacity limits are approached, or when unusual circumstances result in much greater use of a channel than is normal.
