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Information Capacity and Communication Systems

Information Capacity and Communication Systems. By : Mr. Gaurav Verma Asst. Prof. ECE Dept. NIEC. SHANNON’S LAW. Shannon's law is any statement defining the theoretical maximum rate at which error free digits can be transmitted over a bandwidth

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Information Capacity and Communication Systems

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  1. Information Capacity and Communication Systems By : Mr. Gaurav Verma Asst. Prof. ECE Dept. NIEC

  2. SHANNON’S LAW Shannon's law is any statement defining the theoretical maximum rate at which error free digits can be transmitted over a bandwidth limited channel  in the presence of noise

  3. Shannon’s Theorem(Shannon’s Limit for Information Capacity) • Claude Shannon at Bell Labs figured out how much information a channel could theoretically carry: I = B log2 (1 + S/N) • Where I is Information Capacity in bits per second (bps) • B is the channel bandwidth in Hz • S/N is Signal-to-Noise ratio (SNR: unitless…don’t make into decibel: dB) Note that the log is base 2!

  4. Signal-to-Noise Ratio • S/N is normally measured in dB (decibel). It is a relationship between the signal we want versus the noise that we do not want, which is in the medium. • It can be thought of as a fractional relationship (that is, before we take the logarithm): • 1000W of signal power versus 20W of noise power is either: • 1000/20=50 (unitless!) • or: about 17 dB ==> 10 log10 1000/20 = 16.9897 dB

  5. Example If the SNR is 20 dB, and the bandwidth available is 4 kHz, which is appropriate for telephone communications, then C = 4 log2(1 + 100) = 4 log2 (101) = 26.63 kbit/s. Note that the value of 100 is appropriate for an SNR of 20 dB.

  6. Example If it is required to transmit at 50 kbit/s, and a bandwidth of 1 MHz is used, then the minimum SNR required is given by 50 = 1000 log2(1+S/N) so S/N = 2C/W -1 = 0.035 corresponding to an SNR of -14.5 dB. This shows that it is possible to transmit using signals which are actually much weaker than the background noise level.

  7. Communication systems • The block diagram on the top shows the blocks common to all communication systems Digital Analog

  8. We recall the components of a communication system: • Input transducer: The device that converts a physical signal from source to an electrical, mechanical or electromagnetic signal more suitable for communicating • Transmitter: The device that sends the transduced signal • Transmission channel: The physical medium on which the signal is carried • Receiver: The device that recovers the transmitted signal from the channel • Output transducer: The device that converts the received signal back into a useful quantity

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