## Wednesday, August 22, 2007

### Another Deletion Code Open Problem

In a binary symmetric error channel, n bits are sent, and the channel flips each bit independently with probability p. So, for example, the message sent might be 00110011 and the received message could be 01100011 if the 2nd and 4th bits were flipped. Now suppose the same message is sent through k independent channels, and the receiver sees all of results. (Here k should be thought of as a small constant.) The capacity of this channel can be computed; essentially, in this channel, each bit gets maps to a number in the range [0,k], corresponding to the number of 1's in the appropriate position. (Since all errors are independent, exactly which channels flip a specific bit doesn't matter, just the number of flips matter.) As a specific example, when k = 2, we can think in the following nice way -- if we see two 1's (resp 0's) in bit position i, we think the original bit was 1 (resp 0), and now we have an error with probability p^2. With probability 2p(1-p), we see a 1 and a 0 in the ith position -- this corresponds to an "erasure", since the bit is now equally likely to be a 1 and a 0. So we have a channel that gives errors with probability p^2 and erasures with probability 2p(1-p); we can find the capacity (and codes for) such a channel.

In a binary deletion channel, n bits are sent, and the channel deletes each bit independently with probability p. So, for example, the message sent might be 00110011 and the received message could be 010011 if the 2nd and 4th bits were deleted. Now suppose the same message is sent through k independent binary deletion channels, and the receiver sees all of results. Can we say anything useful here? The problem is automatically more challenging since we only have bounds and don't even know the capacity of the standard deletion channel (when k is 1). This is yet another simply stated question from the theory of coding for deletion channels in need of an idea.

#### 1 comment:

Anonymous said...

This description (and the one about the Poisson channel) are both pretty nice... thanks for the glimpse of the coding world!