  The interference channel with common information (IC-CI) consists of two
transmit-receive pairs that communicate over a common noisy medium. Each
transmitter has an individual message for its paired receiver, and
additionally, both transmitters have a common message to deliver to both
receivers. In this paper, through explicit inner and outer bounds on the
capacity region, we establish the capacity region of the Gaussian IC-CI to
within a bounded gap of one bit, independently of the values of all channel
parameters. Using this constant-gap characterization, the generalized degrees
of freedom (GDoF) region is determined. It is shown that the introduction of
the common message leads to an increase in the GDoF over that achievable over
the Gaussian interference channel without a common message, and hence to an
unbounded improvement in the achievable rate. A surprising feature of the
capacity-within-one-bit result is that most of the available benefit (i.e., to
within one bit of capacity) due to the common message is achieved through a
simple and explicit coding scheme that involves independent signaling at the
two transmitters so that, in effect, this scheme forgoes the opportunity for
transmitter cooperation that is inherently available due to shared knowledge of
the common message at both transmitters.
