Radio 102: Noise in an information carrying system

Each letter in the word is represented by different signal amplitude using thirty two scale order. If we do so information flow speed gets greatly increased as per Hartley law. Since now each letter is represented by one symbol instead of five. But in such a case noise will cause numerous error making the system useless extremely large transmitting power is used. This fact may be established on comparing the power requirements for the binary coding system and any N-level system under the same noise conditions. For a given transmission and coding system there results are threshold noise levels below which practically no errors occur due to noise. Using binary code noise has to compete with the full power of the transmitter to cause any serious error. It is found that in a practical channel, signal to noise ratio of thirty decibel ensures almost error free reception. This thirty decibel ratio implies that noise power be one by thousand of signal power or root mean square noise voltage be one by thirty one of root mean square signal voltage.

The transmitted power is required to rise tremendously if a desired high signal to noise power ratio is to be maintained on increasing signaling speed that is on increasing the number of coding levels. Shannon-Hartley theorem gives the maximum signaling speed in a channel in which the noise is purely random. This theorem may be used as a very good approximation for the ultimate channel capacity of most of the transmission channels in spite of the fact that the noise present in most channels is never perfectly random. It is found that the limiting channel speed for a typical telephone channel is about thirty three kilo bits per second. How ever speeds used in practice over such channels do not normally exceed eleven kilo bits per second. Doubling the speed the bandwidth of a noise limited channel will double its capacity would amount to misinterpretation. Actually the capacity gets increased by only eighty percent depending upon signal to noise ratio. Thus we see that there exists possibility of trading bandwidth for signal to noise ratio. It may also be noted that a low channel capacity does not mean that the desired amount of information can not be seen over a given channel. It simply means sending this amount of information takes longer time. Lastly it may be seen that the Shannon-Hartley theorem represents a fundamental limitation. Any attempt to exceed the Shannon limit would result in unacceptable error rate. In good quality transmission system maximum acceptable error ratio is one in 1000000. All the messages sent though the noise limited channel are unpredictable or random.

Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect XM Satellite Radio system for their needs.