Various interference management techniques have been proposed over the past few decades. However, these techniques are usually based on availability of instantaneous channel state information (CSI) at the transmitters. Such an assumption is perhaps not very realistic in practical systems, at least when dealing with fast fading links. Surprisingly, it is shown by Maddah-Ali and Tse that even delayed CSI is helpful to improve the achievable rate of wireless network with multiple flows, even if the channel realizations are independent over time. Availability of delayed CSI has been examined for various networks by different research groups, and have shown to be quite useful. A general study of effect of delayed CSIT on the performance of a wireless network is however still missing.
On the other hand, it is well-known that output feedback does not increase the capacity of point-to-point discrete memoryless channels. However, output feedback is beneficial in improving the capacity regions of more complex networks. This because in a source distributed network, output feedback allow each transmitter to (partially and causally) learn the signal sent by the other transmitter.
We have studied various multi-user networks configurations (including K-user SISO Interference and X channels, as well as 2-user MIMO Interference and X channels). In each configuration, characterizing the degrees of freedom for the network under different models of availability of delayed CSI and/or delayed feedback. It is shown that for certain network configurations, combination of delayed CSI and output feedback is as strong as having instantaneous CSI at the transmitters. Many other interesting results are reported for various networks.