Radio 102: Noise In An Information Carrying System

Each letter in the word is represented bynoise is purely random. This theorem may be
different signal amplitude using thirty two scaleused as a very good approximation for the
order. If we do so information flow speed getsultimate channel capacity of most of the
greatly increased as per Hartley law. Since nowtransmission channels in spite of the fact that the
each letter is represented by one symbol insteadnoise present in most channels is never perfectly
of five. But in such a case noise will causerandom. It is found that the limiting channel speed
numerous error making the system uselessfor a typical telephone channel is about thirty
extremely large transmitting power is used. Thisthree kilo bits per second. How ever speeds used
fact may be established on comparing the powerin practice over such channels do not normally
requirements for the binary coding system andexceed eleven kilo bits per second. Doubling the
any N-level system under the same noisespeed the bandwidth of a noise limited channel will
conditions. For a given transmission and codingdouble its capacity would amount to
system there results are threshold noise levelsmisinterpretation. Actually the capacity gets
below which practically no errors occur due toincreased by only eighty percent depending upon
noise. Using binary code noise has to competesignal to noise ratio. Thus we see that there
with the full power of the transmitter to causeexists possibility of trading bandwidth for signal to
any serious error. It is found that in a practicalnoise ratio. It may also be noted that a low
channel, signal to noise ratio of thirty decibelchannel capacity does not mean that the desired
ensures almost error free reception. This thirtyamount of information can not be seen over a
decibel ratio implies that noise power be one bygiven channel. It simply means sending this
thousand of signal power or root mean squareamount of information takes longer time. Lastly it
noise voltage be one by thirty one of root meanmay be seen that the Shannon-Hartley theorem
square signal voltage.represents a fundamental limitation. Any attempt
The transmitted power is required to riseto exceed the Shannon limit would result in
tremendously if a desired high signal to noiseunacceptable error rate. In good quality
power ratio is to be maintained on increasingtransmission system maximum acceptable error
signaling speed that is on increasing the number ofratio is one in 1000000. All the messages sent
coding levels. Shannon-Hartley theorem gives thethough the noise limited channel are unpredictable
maximum signaling speed in a channel in which theor random.