Network Coding and Wireless Physical-layer ... - Jacobs University

33
The emergence of scalable video coding standard means that some parts of transmitted
data should be better protected than others. This concept is called unequal error/erasure
protection (UEP). We will show in Chapter 4 that network coding inherently supports
this concept. However, this will inevitably leads to conflicts among receiving nodes in a
multicast session. We provide an economic analysis of the conflicts as well as an auction
algorithm to resolve them.
Rateless codes, such as LT-codes and Rapter codes, are originally designed to guarantee
that erasures in the network do not affect data recovery. This seems, at first thought,
to imply that they do not support UEP, since the recovery of every data priority seems
to be guaranteed. However, a subsequent work by Rahnavard, vellambi, and Fekri, shows
that UEP can still be implemented using a similar concept of unequal recovery time
(URT) [47]. Their technique ensures that, for an unlimited time, data of all priorities is
recovered, but for a specific time range, higher-priority data will be recovered before the
lower-priority one. This allows the receiving nodes to decide how long they want to spend
time receiving data, according to the number of priorities that they need.
However, when rateless codes are used in a network with network coding, another
problem arises. We call it the degree distribution distortion problem. This will make the
recovery time of every data priority longer than the normal case without network coding.
In Chapter 5, we investigate this problem in LT-codes with the simplest case of a butterfly
network and gives a solution.