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Understanding Signal Power and S/N Ratio
Dec 10,2006 00:00
by
admin
One of the principle requirements for wireless communication is that the transmitted EM wave must reach the receiver with ample power to allow the receiver to distinguish the wave from the background noise. An analogy can be made to human hearing: when someone is talking to you, they must talk loud enough for your ears to pick up the sound, and it has to be loud enough and clear enough for your brain to be able to recognize and translate the sounds into individual words. Just as your ears and brain require a minimum volume and clarity level to be able to discern what is being said, a radio receiver also requires a minimum power level of received signal in order to discern and recreate the transmitted modulating wave. Signal strength for EM waves is usually measured in watts, or more specifically, a logarithmic ratio of the signal strength divided by 1 milliwatt. This logarithmic ratio is called decibels above 1 milliwatt (dBm). Another common property used to describe signal strength is the S/N ratio. The S/N ratio does not describe the absolute power in the signal, but instead describes the power of the signal in comparison to the power of the background noise.The higher the S/N ratio, the better or more powerful the signal. Looking back at the hearing analogy: someone talking to you in a quiet room would be able to whisper and you would still be able to hear; however, if there was a lot of background noise, say at a rock concert, that person would have to yell in order for you to hear.The same concept applies to RF wireless communication. Since the S/N ratio accounts for the level of background noise, it is a very valuable and widely used indicator of signal strength. Different modulation and encoding technologies require different minimum S/N ratios to function. Most digital modulation schemes require a lower S/N ratio than analog modulation schemes, because the receiver of a digitally modulated carrier wave only has to distinguish between certain levels that represent a logical 1 and a logical 0. Even in the presence of a lot of noise, the receiver is able to distinguish between the predefined threshold levels and then regenerate the digital square wave. In contrast, the receiver of an analog-modulated signal has an infinite number of levels that must be distinguished and maintained. It cannot assume that the received signal was supposed to be a 1 or a 0 and regenerate the signal—it must receive the signal, demodulate it, and pass the resulting representation of the original signal to the next processing device, such as an amplifier.Therefore, any noise added to an analog signal during propagation will alter the original signal. When the power of the modulated RF signal is several times greater than the power of the background noise, the added noise will not be noticed or can be reduced by filtering. On the other hand, when the noise is nearly as powerful as the signal, the resulting demodulated signal will be noticeably different than the original modulating signal.This is commonly called static. The S/N ratio is an important aspect of network design.There are engineering rules established by vendors, specific to their equipment, to provide a set of guidelines for your design. Depending on the geographic span of your design, the S/N ratio may warrant devices to amplify or regenerate the transmitted signal. Attenuation, discussed in the following section, is another important consideration in wireless networking.This will dictate the acceptable span between antennas.The engineering rules mentioned earlier also include the attenuation parameters acceptable by specific equipment. |