  This paper deals with superposition coding for cooperative broadcasting in
the case of two coordinated source nodes, as introduced in the seminal work of
Bergmans and Cover in 1974. A scheme is introduced for two classes of
destination (or relay) nodes: Close nodes and far nodes, as ranked by their
spatial distances to the pair of transmitting nodes. Two linear codes are
combined using the (u,u+v)-construction devised by Plotkin to construct
two-level linear unequal error protection (LUEP) codes. However, instead of
binary addition of subcode codewords in the source encoder, here modulated
subcode sequences are combined at the destination (or relay) nodes antennae.
Bergmans and Cover referred to this as over-the-air mixing. In the case of
Rayleigh fading, additional diversity order as well as robustness to channel
estimation errors are obtained when source nodes transmit pairs of coded
sequences in accordance to Alamouti's transmit diversity scheme. We refer to
this combination as a Plotkin-Alamouti scheme and study its performance over
AWGN and Rayleigh fading channels with a properly partitioned QPSK
constellation.
