Chapter 1
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This course examines communications by electrical signals
Noise is one of the basic factors that set limits on the rate of communication
The signal-to-noise ratio (SNR) is the ratio of signal power to noise power
With analog signals, SNR is continuously decreasing along the length of the
communications channel
Digital communications systems can better withstand limited amounts of noise
and distortion than analog communications systems
Because of the advent of optical fiber and dramatic cost reductions made possible,
almost all new communications systems being installed are digital
When a waveform is sampled and quantized, the information is said to be
digitized
Virtually all digital communications today is binary, due to simplicity of use and
ease of detection
Digital signal regeneration makes it possible to transmit the digital message
virtually error-free
When analog messages are digitized and transmitted digitally, the only error in
the received signal is that caused by quantization
The price to pay for the benefits of digital communications is an increase of
bandwidth of transmission
The transistor made Pulse Code Modulation (PCM) practical
The fundamental parameters controlling rate and quality of information
transmission are channel bandwidth and signal power
The number of pulses per second being transmitted over a channel is directly
proportional to the channel bandwidth
Channel bandwidth and signal power are exchangable
One usually increases bandwidth to reduce power, not increases power to reduce
bandwidth
Shannon’s equation C = B log2(1 + SNR) bits/sec describes the maximum limit on
the amount of information that may be transmitted over a communication channel
having bandwidth B and signal-to-noise ratio SNR
For a communication channel with zero noise, there is no theoretical maximum
limit on the amount of information that may be transmitted over this channel in a
fixed time interval
Practical systems operate at rates below the Shannon rate
Modulation is used to facilitate transmission by easing the requirements for
antenna design, allowing the channel to be shared by many users without
interfering with one another, and effecting the exchange of SNR with bandwidth
Randomness is the essence of communication
The information of a message with probability P is proportional to log(1/P)
Message redundancy and coding may be used to detect or even correct
transmission errors