Non-Orthogonal Amplify-And-Forward Relaying with Partial Channel State Information.
ABSTRACT Wireless amplify-and-forward relay networks in which the source communicates with the relays and destination in the first phase and the relays simultaneously forward signals to the destination in the second phase over uncorrelated Rayleigh fading channels are considered. We examine the scenario in which each relay only knows the perfect information of its source-relay channel while the destination knows the exact information of the relay-destination channels and the statistics of the source-relay channels. Based on a combiner developed at the destination, we propose an efficient beamforming scheme at the relays and develop its quantized version using Lloyd's algorithm to work with a limited-rate feedback channel. Simulation results show that the non-orthogonal relaying with the proposed beamforming scheme outperforms the orthogonal relaying with power allocation in terms of the ergodic capacity. In terms of the signal-to-noise ratio, the non-orthogonal scheme also becomes superior to the orthogonal scheme when the number of quantization regions increases.
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ABSTRACT: This paper deals with beamforming in wireless relay networks with perfect channel information at the relays, receiver, and transmitter if there is a direct link between the transmitter and receiver. It is assumed that every node in the network has its own power constraint. A two-step amplify-and-forward protocol is used, in which the transmitter and relays not only use match filters to form a beam at the receiver but also adaptively adjust their transmit powers according to the channel strength information. For networks with no direct link, an algorithm is proposed to analytically find the exact solution with linear (in network size) complexity. It is shown that the transmitter should always use its maximal power while the optimal power of a relay ca.n take any value between zero and its maxima. Also, this value depends on the quality of all other channels in addition to the relay's own. Despite this coupling fact, distributive strategies are proposed in which, with the aid of a low-rate receiver broadcast, a relay needs only its own channel information to implement the optimal power control. Then, beamforming in networks with a direct link is considered. When the direct link exists during the first step only, the optimal power control is the same as that of networks with no direct link. For networks with a direct link during the second step only and both steps, recursive numerical algorithms are proposed. Simulation shows that network beamforming achieves the maximal diversity order and outperforms other existing schemes.IEEE Transactions on Information Theory 07/2009; · 2.62 Impact Factor
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ABSTRACT: Cooperative diversity systems rely on using relay nodes to relay copies of transmitted information to the destination such that each copy experiences different channel fading, hence increasing the diversity of the system. However, without proper processing of the message at the relays, the performance of the cooperative system may not necessarily perform better than direct transmission systems. In this paper, we proposed a distributed beamforming and power allocation algorithm which substantially improves the diversity of the system with only very limited feedback from the destination node. We also derive outage probability as well as study the outage behavior of this scheme.IEEE Transactions on Wireless Communications 06/2008; · 2.42 Impact Factor
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ABSTRACT: In wireless networks, power allocation is an effective technique for prolonging network lifetime, achieving better quality-of-service (QoS), and reducing network interference. However, these benefits depend on knowledge of the channel state information (CSI), which is hardly perfect. Therefore, robust algorithms that take into account such CSI uncertainties play an important role in the design of practical systems. In this paper, we develop relay power allocation algorithms for noncoherent and coherent amplify-and-forward (AF) relay networks. The goal is to minimize the total relay transmission power under individual relay power constraints, while satisfying a QoS requirement. To make our algorithms practical and attractive, our power update rate is designed to follow large-scale fading, i.e., in the order of seconds. We show that, in the presence of perfect global CSI, our power optimization problems for noncoherent and coherent AF relay networks can be formulated as a linear program and a second-order cone program (SOCP), respectively. We then introduce robust optimization methodology that accounts for uncertainties in the global CSI. In the presence of ellipsoidal uncertainty sets, the robust counterparts of our optimization problems for noncoherent and coherent AF relay networks are shown to be an SOCP and a semi-definite program, respectively. Our results reveal that ignoring uncertainties associated with global CSI often leads to poor performance. We verify that our proposed algorithms can provide significant power savings over a naive scheme that employs maximum transmission power at each relay node. This work highlights the importance of robust algorithms with practical power update rates in realistic wireless networks.IEEE Journal of Selected Topics in Signal Processing 01/2008; · 3.30 Impact Factor