Amplify-and-forward relaying in wireless communication networks

Wireless networks are growing rapidly as demand for reliable, high data rate, and efficient communication technologies has intensified during recent years. To address the requested increase in capacity and to improve wireless link performance, co-operative wireless relay based networks have been adopted. Among the proposed relay strategies, amplify-and-forward (AF) has been more widely adopted due its simplicity.In this thesis, first, we propose a protocol for a single antenna AF relay network with power constraint at the destination. The power constraint is defined such that the received signal power is bounded between two values in order to control interference to neighboring cells using the same spectrum. The proposed scheme simultaneously maximizes SNR and minimizes MMSE. It is shown that the proposed scheme achieves full diversity order. Furthermore, the effect of channel uncertainties on the system performance is investigated and a modified solution for the proposed scheme is provided.Next, we consider a multiple input multiple output (MIMO) AF relay network. We investigate maximum SNR solutions subject to zero-forcing (ZF) criteria, as well as approximate MMSE equalizers with specified target SNR without power constraint. First, without considering any predetermined MIMO equalizer, we design the optimal relay matrix such that it maximizes the output SNR without post-equalization. This is achieved under a zero-forcing (ZF) criterion with a specified target output SNR. Second, we design the relay matrix and equalizer pair under a MMSE criterion, considering two distinct cases: (i) where relay matrix and equalizer are designed in two independent steps (MMSE in two steps) (ii) where relay matrix and equalizer are designed jointly. Then, we extend our previous results to include a power constraint at the receiver. We derive the optimal relay matrix that minimizes the overall MMSE subject to the power constraint at the destination. We, also maximize SNR subject to ZF at the output of equalizer.Also, a closed form solution is provided for a single antenna AF relay network by minimizing MMSE subject to a global power constraint at the output of relays where each node in the network is equipped with one antenna.Finally, we propose a training based channel estimation for single antenna AF relay networks. We consider LMMSE channel estimator and derive a lower bound for the capacity considering the effect of channel estimation error.