Approximate Ergodic Capacity of a Class of Fading 2x2x2 Networks

We study a 2-user 2-hop network with 2 relays in which channel coefficients are independently drawn from continuous distributions and vary over time. For a broad class of channel distributions, we characterize the ergodic sum capacity within a constant number of bits/sec/Hz, independent of signal- to-noise ratio. Specifically, we characterize the ergodic sum capacity within 4 bits/sec/Hz for independent and identically distributed (i.i.d.) uniform phase fading and approximately 4.7 bits/sec/Hz for i.i.d. Rayleigh fading. For achievability, we propose ergodic interference neutralization in which the relays amplify and forward their received signals with appropriate delays such that interference can be neutralized at each destination.