Tobias J. Oechtering

In a three-node network bidirectional communication between two nodes can be enabled by a half-duplex relay node with a decode-and-forward protocol. In the first phase, the messages of two nodes are transmitted to the relay node. In the second phase a re-encoded composition is broadcasted by the relay node. In this work the capacity region of the broadcast phase in terms of the maximal probability of error is determined. It is characterized by the mutual informations of the separate channels coupled by the common input.

We consider the problem of relay selection in a network with N relay nodes. A half-duplex relay node enables bidirectional communication between two nodes with a spectrally efficient two-phase protocol. In the first phase both nodes transmit their messages to a relay node, which decodes the messages and broadcasts a composition using superposition encoding in the succeeding phase. The probability that the achievable rate region of one relay node contains all other rate regions decreases with the number of relay nodes N. Therefore, we propose a relay selection criterion that decides according to the weighted rate sum for any bidirectional rate pair on the boundary of the achievable rate region individually. If we allow time-sharing between the usage of different relay nodes, we can enlarge the achievable rate region. In an iid Rayleigh fading scenario relay selection realizes multi-user diversity so that the sum-rate of any rate pair on the boundary of the ergodic rate region asymptotically grows with Theta(log(log(/V))).

We propose a new resource efficient transmission scheme to achieve cooperative diversity of a distributed smart antenna system consisting of an information source, destination, and two alternating supportive relay stations. Each station is equipped with one antenna element. Furthermore we assume perfect synchronization and channel knowledge at the receivers. The proposed linear encoding functions with interference cancellation at one relay station result in an equivalent space-time delay coded system, but with additionally colored noise from the relay stations. Based on the maximum likelihood sequence estimator we derive criteria to evaluate the performance compared to the direct S1SO transmission. Finally the numerical simulation results of an suboptimal sequence estimator show significant performance gains if the power gain from the hopping is gainful exploited. If not, the additional noise from the relay stations will degrade the performance at reasonable transmit powers.

The main result of the paper is a new achievable rate region for the two-phase two-way relay channel with half duplex nodes. The new region is obtained using a compress- and-forward like protocol. Even though all information passes the relay, there are channels where decoding at the relay is suboptimal. The proposed protocol establishes a virtual two-way channel by forwarding a good enough version of the MAC output at the relay to both the receivers. In effect, the side information at the receivers can be used for the decoding of the transmission in the first phase.

We consider the broadcast phase of a three-node network, where a relay node establishes a bidirectional communication between two nodes using a spectrally efficient two-phase decode-and-forward protocol. In the first phase the two nodes transmit their messages to the relay node. Then the relay node decodes the messages and broadcasts a re-encoded composition of them in the second phase. We consider Gaussian MIMO channels and determine the capacity region for the second phase which we call the Gaussian MIMO bidirectional broadcast channel.