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Wikipedia / Relay channel

In information theory, a relay channel is a probability model of the communication between a sender and a receiver aided by one or more intermediate relay nodes.

General discrete-time memoryless relay channel

, and a conditional probability on these sets. The probability distribution.

There exist three main relaying schemes: Decode-and-Forward, Compress-and-Forward and Amplify-and-Forward. The first two schemes were first proposed in the pioneer article by Cover and El-Gamal.

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  • Amplify-and-Forward (AF): In this relaying scheme, the relay sends an amplified version of the received signal in the last time-slot. Comparing with DF and CF, AF requires much less delay as the relay node operates time-slot by time-slot. Also, AF requires much less computing power as no decoding or quantizing operation is performed at the relay side.

Cut-set upper bound

The first upper bound on the capacity where C is the capacity of the relay channel. The first term and second term in the minimization above are called broadcast bound and multi-access bound, respectively.

Degraded relay channel

A relay channel is said to be degraded if y. In the article by Cover and El-Gamal it is shown that the capacity of the degraded relay channel can be achieved using Decode-and-Forward scheme. It turns out that the capacity in this case is equal to the Cut-set upper bound.

Reversely degraded relay channel

. Cover and El-Gamal proved that the Direct Transmission Lower Bound (wherein relay is not used) is tight when the relay channel is reversely degraded.

Feedback relay channel

Relay without delay channel

In a relay-without-delay channel (RWD), each transmitted relay symbol can depend on relay's past as well as present received symbols. Relay Without Delay was shown to achieve rates that are outside the Cut-set upper bound. Recently, it was also shown that instantaneous relays (a special case of relay-without-delay) are capable of improving not only the capacity, but also Degrees of Freedom (DoF) of the 2-user interference channel.

See also

References