channel capacity

channel capacity

A variety of impairments can distort or corrupt a signal. A common impairment is

noise, which is any unwanted signal that combines with and hence distorts the signal

intended for transmission and reception. Noise and other impairments are discussed

in Chapter 5. For the purposes of this section, we simply need to know that noise is

something that degrades signal quality. For digital data, the question that then arises

is to what extent these impairments limit the data rate that can be achieved. The

maximum rate at which data can be transmitted over a given communication path,

or channel, under given conditions is referred to as the channel capacity.

There are four concepts here that we are trying to relate to one another:

.. Data rate: This is the rate, in bits per second (bps), at which data can be

communicated.

'" Bandwidth: This is the bandwidth of the transmitted signal as constrained by

the transmitter and the nature of the transmission medium, expressed in cycles

per second, or Hertz.

1& Noise: For this discussion, we are concerned with the average level of noise

over the communications path.

.. Error rate: This is the rate at which errors occur, where an error is the reception

of a 1 when a 0 was transmitted or the reception of a 0 when a 1 was transmitted.

The problem we are addressing is this: Communications facilities are expensive

and, in general, the greater the bandwidth of a facility, the greater the cost. Furthermore,

all transmission channels of any practical interest are of limited bandwidth. The

limitations arise from the physical properties of the transmission medium or from

deliberate limitations at the transmitter on the bandwidth to prevent interference

from other sources. Accordingly, we would like to make as efficient use as possible

of a given bandwidth. For digital data, this means that we would like to get as high a

data rate as possible at a particular limit of error rate for a given bandwidth.The main

constraint on achieving this efficiency is noise.