  The performance of codes defined from graphs depends on the expansion
property of the underlying graph in a crucial way. Graph products, such as the
zig-zag product and replacement product provide new infinite families of
constant degree expander graphs. The paper investigates the use of zig-zag and
replacement product graphs for the construction of codes on graphs. A
modification of the zig-zag product is also introduced, which can operate on
two unbalanced biregular bipartite graphs.
