Figure 2
From: Opportunistic Adaptive Transmission for Network Coding Using Nonbinary LDPC Codes
Network group formation.
4 is going to access the channel. Node 
4 knows which packets are stored in its neighbors' buffers. Based on this knowledge it must choose which packets to XOR together in order to maximize the number of packets decoded in the transmission slot. A possible choice is, for example, 
which allows nodes 
1 and 
2 to decode, but not 
. A better choice is to encode 
, so that 3 packets can be decoded in a single transmission. The difference in SNR for the three sinks (
,
, and 
) can lead to high packet loss probability on some of the links if a single channel rate is used for all the sinks. 
is the vector of SNRs.