In electrical engineering, computer science and information theory, channel capacity is the tightest upper bound on the rate of information that can be reliably transmitted over a communications channel. By the noisy-channel coding theorem, the channel capacity of a given channel is the limiting information rate (in units of information per unit time) that can be achieved with arbitrarily small error probability. 
Information theory, developed by Claude E. Shannon during World War II, defines the notion of channel capacity and provides a mathematical model by which one can compute it. The key result states that the capacity of the channel, as defined above, is given by the maximum of the mutual information between the input and output of the channel, where the maximization is with respect to the input distribution.
Formal definition 
Let and be the random variables representing the input and output of the channel, respectively. Let be the conditional distribution function of given , which is an inherent fixed property of the communications channel. Then the choice of the marginal distribution completely determines the joint distribution due to the identity
which, in turn, induces a mutual information . The channel capacity is defined as
where the supremum is taken over all possible choices of .
Noisy-channel coding theorem 
The noisy-channel coding theorem states that for any ε > 0 and for any rate R less than the channel capacity C, there is an encoding and decoding scheme that can be used to ensure that the probability of decoding error is less than ε for a sufficiently large block length. Also, for any rate greater than the channel capacity, the probability of error at the receiver goes to one as the block length goes to infinity.
Example application 
C is measured in bits per second if the logarithm is taken in base 2, or nats per second if the natural logarithm is used, assuming B is in hertz; the signal and noise powers S and N are measured in watts or volts2, so the signal-to-noise ratio here is expressed as a power ratio, not in decibels (dB); since figures are often cited in dB, a conversion may be needed. For example, 30 dB is a power ratio of .
Channel capacity in wireless communications 
AWGN channel 
If the average received power is [W] and the noise power spectral density is [W/Hz], the AWGN channel capacity is
where is the received signal-to-noise ratio (SNR).
When the SNR is large (SNR >> 0 dB), the capacity is logarithmic in power and approximately linear in bandwidth. This is called the bandwidth-limited regime.
When the SNR is small (SNR << 0 dB), the capacity is linear in power but insensitive to bandwidth. This is called the power-limited regime.
The bandwidth-limited regime and power-limited regime are illustrated in the figure.
Frequency-selective channel 
where and is the gain of subchannel , with chosen to meet the power constraint.
Slow-fading channel 
In a slow-fading channel, where the coherence time is greater than the latency requirement, there is no definite capacity as the maximum rate of reliable communications supported by the channel, , depends on the random channel gain , which is unknown to the transmitter. If the transmitter encodes data at rate [bits/s/Hz], there is a non-zero probability that the decoding error probability cannot be made arbitrarily small,
in which case the system is said to be in outage. With a non-zero probability that the channel is in deep fade, the capacity of the slow-fading channel in strict sense is zero. However, it is possible to determine the largest value of such that the outage probability is less than . This value is known as the -outage capacity.
Fast-fading channel 
In a fast-fading channel, where the latency requirement is greater than the coherence time and the codeword length spans many coherence periods, one can average over many independent channel fades by coding over a large number of coherence time intervals. Thus, it is possible to achieve a reliable rate of communication of [bits/s/Hz] and it is meaningful to speak of this value as the capacity of the fast-fading channel.
See also 
- Bandwidth (computing)
- Bandwidth (signal processing)
- Bit rate
- Code rate
- Error exponent
- Nyquist rate
- Sender, Encoder, Decoder, Receiver
- Shannon–Hartley theorem
- Spectral efficiency
Advanced Communication Topics 
- Hazewinkel, Michiel, ed. (2001), "Transmission rate of a channel", Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
- Saleem Bhatti. "Channel capacity". Lecture notes for M.Sc. Data Communication Networks and Distributed Systems D51 -- Basic Communications and Networks.
- Jim Lesurf. "Signals look like noise!". Information and Measurement, 2nd ed.
- Thomas M. Cover, Joy A. Thomas (2006). Elements of Information Theory. John Wiley & Sons, New York.
- David Tse, Pramod Viswanath (2005), Fundamentals of Wireless Communication, Cambridge University Press, UK
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