  Interference limits performance in wireless networks, and cooperation among
receivers or transmitters can help mitigate interference by forming distributed
MIMO systems. Earlier work shows how limited receiver cooperation helps
mitigate interference. The scenario with transmitter cooperation, however, is
more difficult to tackle. In this paper we study the two-user Gaussian
interference channel with conferencing transmitters to make progress towards
this direction. We characterize the capacity region to within 6.5 bits/s/Hz,
regardless of channel parameters. Based on the constant-to-optimality result,
we show that there is an interesting reciprocity between the scenario with
conferencing transmitters and the scenario with conferencing receivers, and
their capacity regions are within a constant gap to each other. Hence in the
interference-limited regime, the behavior of the benefit brought by transmitter
cooperation is the same as that by receiver cooperation.
