Multiple Access Channels With Combined Cooperation and Partial Cribbing

Abstract

In this paper, the multiple access channel (MAC) with combined cooperation and partial cribbing is studied, and its capacity region is characterized. Cooperation means that each of the two encoders sends a message to the other via a rate-limited link prior to transmission, while partial cribbing means that each of the two encoders obtains a deterministic function of the other encoder’s output with or without delay. Prior work in this field dealt separately with cooperation and partial cribbing, but by combining these two methods, we can achieve significantly higher rates. Surprisingly, the capacity region of the MAC with combined cooperation and partial cribbing can be expressed using only one auxiliary random variable (RV) similar to the capacity regions of the MAC with cooperation and with partially cribbing encoders. The reason is that in an optimal coding scheme, the encoders use both cooperation and partial cribbing to generate a common message between the encoders. Furthermore, the Gaussian MAC with combined one-sided cooperation and quantized cribbing is studied. For this model, an achievability scheme is given. This scheme shows how many cooperation or quantization bits are required to practically achieve the capacity region of the Gaussian MAC with full message cooperation or perfect cribbing. To ratify the main results, two additional models are studied. In both models, only one auxiliary RV is needed. The first is a rate distortion dual setting for the MAC with degraded message set and combined cooperation and cribbing. The second is a state-dependent MAC with cooperation, where the state is known at a partially cribbing encoder and at the decoder. However, there are cases where more than one auxiliary RV is needed, e.g., when the cooperation and the cribbing are not used for the same purposes. The MAC with an action-dependent state is presented, where the action is based on the cooperation but not on the cribbing. Therefore, in this case, more than one auxiliary RV is needed. As a result, when the common information shared by the two encoders is used unevenly by the users in the channel, more than one auxiliary RV is needed to express the capacity region.