  This letter investigates the role of index coding in the capacity of AWGN
broadcast channels with receiver message side information. We first show that
index coding is unnecessary where there are two receivers; multiplexing coding
and superposition coding are sufficient to achieve the capacity region. We next
show that, for more than two receivers, multiplexing coding and superposition
coding alone can be suboptimal. We give an example where these two coding
schemes alone cannot achieve the capacity region, but adding index coding can.
This demonstrates that, in contrast to the two-receiver case, multiplexing
coding cannot fulfill the function of index coding where there are three or
more receivers.
