A mixed wireline/wireless network and its graph is shown in Figure 3.6. Note that the graphs for the networks of Figures 3.4 and 3.6 are the same, but they are interpreted differently. In general, we will say that every node u has one channel input Xu and one channel output Yu. We draw a directed edge from node u to node v if Xu contributes to Yv. For example, the wireline network of Figure 3.4 has X1 being a vector [X12 X13], and similarly for X2 and Y3. The X12 and X13 here represent symbols sent on non-interfering (or parallel) channels. The same is true for the mixed network of Figure 3.6. However, instead of as in Figure 3.4, we might now have
Fig. 3.6 A mixed wireline/wireless network and its graph.
where X13 and X23 are complex random variables. Hence, the signal X13 interferes with X23. Furthermore, if there are "port" constraints in the network of Figure 3.6 then one might wish to add (3.2a) and (3.2b) to the model.