  Degraded broadcast channels (DBC) are a typical multiuser communication
scenario, Semantic communications over DBC still lack in-depth research. In
this paper, we design a semantic communications approach based on multi-user
semantic fusion for wireless image transmission over DBC. In the proposed
method, the transmitter extracts semantic features for two users separately. It
then effectively fuses these semantic features for broadcasting by leveraging
semantic similarity. Unlike traditional allocation of time, power, or
bandwidth, the semantic fusion scheme can dynamically control the weight of the
semantic features of the two users to balance the performance between the two
users. Considering the different channel state information (CSI) of both users
over DBC, a DBC-Aware method is developed that embeds the CSI of both users
into the joint source-channel coding encoder and fusion module to adapt to the
channel. Experimental results show that the proposed system outperforms the
traditional broadcasting schemes.
