Spectrally Efficient Bidirectional Decode-and-Forward Relaying for Wireless Networks

Abstract

Relaying concepts will play a central role in future wireless networks. In this thesis we consider a three-node network where two nodes communicate with each other by the support of a relay node. We study a two-phase decode-and-forward bidirectional relaying protocol without feedback. Bidirectional relaying has the ability to compensate the spectral loss due to the half-duplex constraint of nodes in wireless communications. In the first part we study cross-layer design aspects of bidirectional relaying using superposition encoding at the relay node. For the two phases we consider the fixed and optimal time division case. For single-antenna nodes an intensive study of the combinatorial structure of the achievable rate region allows us to characterize the rate pairs which maximize the weighted rate sum for the equal and fixed time division case in closed form. These are used for the design of a throughput optimal resource allocation policy based on the backpressure strategy and to derive a relay selection criterion for routing in network with N relay nodes. It shows that it is beneficial to allow time-sharing between the usage of relay nodes. We see that the sum of any rate pair on the boundary of the ergodic rate region for independent and identical distributed Rayleigh fading channels grows asymptotically with log(log(N)). Then we add a relay multicast to the bidirectional relay communication. The joint resource allocation of two routing tasks improves the overall efficiency and enables new rate tradeoffs. It shows that it is always optimal to decode the relay message first. Furthermore, we characterize and discuss the total sum-rate maximum of both routing tasks. After that we study the achievable rate region of bidirectional relaying between nodes equipped with multiple antennas. Therefore, we specify the optimal transmit strategy and show that the achievable rate region scales linearly with respect to the spatial degrees of the vector channels and time division. In the second part we find an optimal channel coding strategy for the bidirectional broadcast channel considering finite size alphabets. Thereby, we consider achievable rates with respect to the maximal probability of error. For the coding theorem we follow the philosophy of network coding and regard information flows not as “fluids”. In the final conclusion we give an outlook on future research work and show how the bidirectional relaying protocol can be integrated in wireless networks.