The capacity region of the symmetric Gaussian interference channel with common information to within a constant gap

The interference channel with common information (IC-CI) is studied. In an IC-CI, each transmitter has an individual message for its paired receiver, and additionally, both transmitters collaboratively must deliver a common message to both the receivers. For the symmetric Gaussian IC-CI, the capacity region is characterized to within a gap of 3 bits independently of the values of the channel parameters. Further, using this constant-gap characterization, the generalized degrees of freedom region is also determined. Insight is provided on how the common message helps in markedly improving the achievable rates over that in an IC without a common message.