On the Capacity of the Two-user Gaussian Causal Cognitive Interference Channel

TL;DR

This paper analyzes the capacity of the two-user Gaussian Causal Cognitive Interference Channel, providing approximate capacity results and insights into when causal cooperation offers significant benefits over non-cognitive methods.

Contribution

It characterizes the approximate capacity region of the GCCIC within a constant gap, extending symmetric results to general cases and identifying regimes where causal cooperation is most beneficial.

Findings

Sum-capacity of symmetric GCCIC determined within a constant gap.

Capacity region approximated within 2 bits for various channel configurations.

Identifies regimes where causal cooperation matches noncooperative or non-causal cognitive channels.

Abstract

This paper considers the two-user Gaussian Causal Cognitive Interference Channel (GCCIC), which consists of two source-destination pairs that share the same channel and where one full-duplex cognitive source can causally learn the message of the primary source through a noisy link. The GCCIC is an interference channel with unilateral source cooperation that better models practical cognitive radio networks than the commonly used model which assumes that one source has perfect non-causal knowledge of the other source's message. First the sum-capacity of the symmetric GCCIC is determined to within a constant gap. Then, the insights gained from the derivation of the symmetric sum-capacity are extended to characterize the whole capacity region to within a constant gap for more general cases. In particular, the capacity is determined (a) to within 2 bits for the fully connected GCCIC when, roughly speaking, the interference is not weak at both receivers, (b) to within 2 bits for the Z-channel, i.e., when there is no interference from the primary user, and (c) to within 2 bits for the S-channel, i.e., when there is no interference from the secondary user. The parameter regimes where the GCCIC is equivalent, in terms of generalized degrees-of-freedom, to the noncooperative interference channel (i.e., unilateral causal cooperation is not useful), to the non-causal cognitive interference channel (i.e., causal cooperation attains the ultimate limit of cognitive radio technology), and to bilateral source cooperation are identified. These comparisons shed lights into the parameter regimes and network topologies that in practice might provide an unbounded throughput gain compared to currently available (non cognitive) technologies.