An arbitrarily varying channel (AVC) is a communication channel model used in coding theory, and was first introduced by Blackwell, Breiman, and Thomasian. This particular channel has unknown parameters that can change over time and these changes may not have a uniform pattern during the transmission of a codeword. uses of this channel can be described using a stochastic matrix, where is the input alphabet, is the output alphabet, and is the probability over a given set of states , that the transmitted input leads to the received output . The state in set can vary arbitrarily at each time unit . This channel was developed as an alternative to Shannon'sBinary Symmetric Channel (BSC), where the entire nature of the channel is known, to be more realistic to actual network channel situations.
An AVC's capacity can vary depending on the certain parameters.
is an achievable rate for a deterministic AVC code if it is larger than , and if for every positive and , and very large , length-block codes exist that satisfy the following equations: and , where is the highest value in and where is the average probability of error for a state sequence . The largest rate represents the capacity of the AVC, denoted by .
As you can see, the only useful situations are when the capacity of the AVC is greater than , because then the channel can transmit a guaranteed amount of data without errors. So we start out with a theorem that shows when is positive in an AVC and the theorems discussed afterward will narrow down the range of for different circumstances.
Before stating Theorem 1, a few definitions need to be addressed:
An AVC is symmetric if for every , where , , and is a channel function .
, where the minimum is over all random variables such that , , and are distributed in the form of .
Theorem 1: if and only if the AVC is not symmetric. If , then .
Proof of 1st part for symmetry: If we can prove that is positive when the AVC is not symmetric, and then prove that , we will be able to prove Theorem 1. Assume were equal to . From the definition of , this would make and independentrandom variables, for some , because this would mean that neither random variable's entropy would rely on the other random variable's value. By using equation , (and remembering ,) we can get,
So now we have a probability distribution on that is independent of . So now the definition of a symmetric AVC can be rewritten as follows: since and are both functions based on , they have been replaced with functions based on and only. As you can see, both sides are now equal to the we calculated earlier, so the AVC is indeed symmetric when is equal to . Therefore, can only be positive if the AVC is not symmetric.
Proof of second part for capacity: See the paper "The capacity of the arbitrarily varying channel revisited: positivity, constraints," referenced below for full proof.
Capacity of AVCs with input and state constraints
The next theorem will deal with the capacity for AVCs with input and/or state constraints. These constraints help to decrease the very large range of possibilities for transmission and error on an AVC, making it a bit easier to see how the AVC behaves.
Before we go on to Theorem 2, we need to define a few definitions and lemmas:
For such AVCs, there exists:
- An input constraint based on the equation , where and .
- A state constraint , based on the equation , where and .
- is very similar to equation mentioned previously, , but now any state or in the equation must follow the state restriction.
Assume is a given non-negative-valued function on and is a given non-negative-valued function on and that the minimum values for both is . In the literature I have read on this subject, the exact definitions of both and (for one variable ,) is never described formally. The usefulness of the input constraint and the state constraint will be based on these equations.
For AVCs with input and/or state constraints, the rate is now limited to codewords of format that satisfy , and now the state is limited to all states that satisfy . The largest rate is still considered the capacity of the AVC, and is now denoted as .
Lemma 1: Any codes where is greater than cannot be considered "good" codes, because those kinds of codes have a maximum average probability of error greater than or equal to , where is the maximum value of . This isn't a good maximum average error probability because it is fairly large, is close to , and the other part of the equation will be very small since the value is squared, and is set to be larger than . Therefore, it would be very unlikely to receive a codeword without error. This is why the condition is present in Theorem 2.
Theorem 2: Given a positive and arbitrarily small , , , for any block length and for any type with conditions and , and where , there exists a code with codewords, each of type , that satisfy the following equations: , , and where positive and depend only on , , , and the given AVC.
Proof of Theorem 2: See the paper "The capacity of the arbitrarily varying channel revisited: positivity, constraints," referenced below for full proof.
The next theorem will be for AVCs with randomizedcode. For such AVCs the code is a random variable with values from a family of length-n block codes, and these codes are not allowed to depend/rely on the actual value of the codeword. These codes have the same maximum and average error probability value for any channel because of its random nature. These types of codes also help to make certain properties of the AVC more clear.
Before we go on to Theorem 3, we need to define a couple important terms first:
is very similar to the equation mentioned previously, , but now the pmf is added to the equation, making the minimum of based a new form of , where replaces .