Channel Capacity with Side Information - A Unified View

We identify the common underlying form of the capacity expression that is applicable to both cases where causal or non-causal side information is made available to the transmitter. Using this common form we find that the capacity with causal and non-causal side information are identical when all the transmitter side information is also made available to the receiver. A genie-aided outerbound is developed that states that when a genie provides one bit of side information to the receiver the resulting capacity improvement can not be more than a bit. Combining these two results we are able to bound the relative capacity advantage of non-causal side information over causal side information for both single user as well as multiple user communication scenarios. Applications of these capacity bounds are demonstrated through examples of random access channels. It is also shown that even one bit of side information at the transmitter can result in unbounded capacity improvement.