Tuan Anh Le

Green radio: radio techniques to enable energy-efficient wireless networks

Recent analysis by manufacturers and network operators has shown that current wireless networks are not very energy efficient, particularly the base stations by which terminals access services from the network. In response to this observation the Mobile Virtual Centre of Excellence (VCE) Green Radio project was established in 2009 to establish how significant energy savings may be obtained in future wireless systems. This article discusses the technical background to the project and discusses models of current energy consumption in base station devices. It also describes some of the most promising research directions in reducing the energy consumption of future base stations.

Power-efficient downlink transmission in multicell networks with limited wireless backhaul

This article shows that division of a cell into tiers of smaller cells reduces power consumption. However, using the same frequency-time resources within multiple divided cells causes strong intercell interference. Given this circumstance, three beamforming techniques for multicell networks are presented to tackle the resultant challenging intercell interference environment. The schemes minimize the total transmit power across the coordinating base stations while simultaneously considering the quality of service of each user so that the latter is not unduly affected. Since the beamforming approaches require the circulation of information, an energy-efficient backhaul protocol is demonstrated.

Downlink Optimization with Interference Pricing and Statistical CSI

In this paper, we propose a downlink transmission strategy based on intercell interference pricing and a distributed algorithm that enables each base station (BS) to design locally its own beamforming vectors without relying on downlink channel state information of links from other BSs to the users. This algorithm is the solution to an optimization problem that minimizes a linear combination of data transmission power and the resulting weighted intercell interference with pricing factors at each BS and maintains the required signal-to-interference-plus-noise ratios (SINR) at user terminals. We provide a convergence analysis for the proposed distributed algorithm and derive conditions for its existence. We characterize the impact of the pricing factors in expanding the operational range of SINR targets at user terminals in a power-efficient manner. Simulation results confirm that the proposed algorithm converges to a network-wide equilibrium point by balancing and stabilizing the intercell interference levels and assigning power optimal beamforming vectors to the BSs. The results also show the effectiveness of the proposed algorithm in closely following the performance limits of its centralized coordinated beamforming counterpart.

An Iterative Algorithm for Downlink Multi-Cell Beam-Forming

In this paper, we address the problem of severe performance degradation of cell-edge users in conventional cellular networks via multicell processing with improved spectral efficiency. Using uplink-downlink duality, we propose an iterative multicell beamforming algorithm that minimises the total transmit power within a virtual cell of 3 adjacent sectors of 3 neighbouring BSs. We derive and employ a channel model that includes the angular spread caused by random local scatterers in the vicinity of a user. Simulation results confirm that the proposed algorithm converges fast to the optimal solution.

Coordinated beamforming using semidefinite programming

In this paper, we study a coordinated beamforming (CBF) scheme for a multi-cell scenario where precoding/beamforming vectors for all coordinating base stations (BSs) are jointly designed in a manner that each BS transmits to its local users. We minimise the total transmit power across coordinating BSs while keeping the signal-to-interference-plus-noise ratio (SINR) at each user above the required level. We formulate the optimisation problem for CBF in a standard semidefinite programming form using instantaneous channel state information. Then, taking into consideration the uncertainty in the estimation of channel covariance matrix, we formulate the CBF problem based on the second order statistical properties of channel so that the robustness of the designed system parameters is guaranteed within a tolerable channel estimation error. We show that, although this robustness comes at the expense of increased overall power, a significant reduction in signaling overhead can be attained with a minor increase in total transmit power at moderately low SINRs.

Robust Chance-Constrained Optimization for Power-Efficient and Secure SWIPT Systems

In this paper, we propose beamforming schemes to simultaneously transmit data securely to multiple information receivers while transferring power wirelessly to multiple energy-harvesting receivers. Taking into account the imperfection of the instantaneous channel state information (CSI), we introduce a chance-constrained optimization problem to minimize the total transmit power while guaranteeing data transmission reliability, data transmission security, and power transfer reliability. As the proposed optimization problem is non-convex due to the chance constraints, we propose two robust reformulations of the original problem based on safe-convex-approximation techniques. Subsequently, applying semidefinite programming relaxation (SDR), the derived robust reformulations can be effectively solved by standard convex optimization packages. We show that the adopted SDR is tight and thus the globally optimal solutions of the reformulated problems can be recovered. Simulation results confirm the superiority of the proposed methods in guaranteeing transmission security compared to a baseline scheme. Furthermore, the performance of proposed methods can closely follow that of a benchmark scheme where perfect CSI is available for resource allocation.

Secure Information Transmission and Power Transfer in Cellular Networks

This letter studies simultaneous data transmission and power transfer for multiple information receivers (IRs) and energy-harvesting receivers (ERs) in cellular networks. We formulate an optimization problem to minimize the total transmit power across a network subject to the following three sets of constraints: 1) data reliability by maintaining the required level of signal-to-interference-plus-noise ratio (SINR) for all IRs; 2) information security by keeping all the SINR levels of the intended IRs measured at each ER below a predefined value, which helps prevent possible eavesdroppers, i.e., ERs, from detecting the information aimed for the IRs; and 3) energy harvesting by guaranteeing the required level of received power at each ER. Using a semidefinite relaxation technique, the proposed problem is then transformed into a convex form that is proved to always yield a rank-one optimal solution.

Downlink Beamforming in Underlay Cognitive Cellular Networks

We propose a novel scheme for downlink beamforming design in an underlay cognitive cellular system. The beamforming design is formulated as an optimization problem with the objective of keeping the cognitive base station transmit power as well as the induced interference on the primary users, below predefined system thresholds. This is subject to providing a certain level of signal-to-interference-plus-noise ratio (SINR) to the secondary users. We then derive the corresponding semidefinite programming form for the formulated optimization problem and propose an iterative algorithm to obtain the beamforming vectors as the optimal solutions. We further analytically show the convergence of the proposed iterative algorithm. Extensive simulations verify that the proposed algorithm quickly converges to the optimal solution. We then compare the proposed scheme with a benchmarking system defined based on the previous methods proposed in the related literature. Comparisons show that the proposed algorithm outperforms the benchmarking system and induces lower interference at the primary service receivers. It is also observed that the proposed algorithm offers a higher sum rate in comparison to the benchmarking system. Simulation results further reveal that the proposed approach effectively works at a relatively high SINR level required by secondary users and strict interference threshold set by the primary system while the benchmarking system fails to do so.

A decentralized downlink beamforming algorithm for multicell processing

In this paper, we tackle the problem of intercell-interference in the absence of cooperation amongst base stations (BSs). The problem originates from the simultaneous transmissions of several BSs to their local users within their corresponding cells using the same frequency band. We propose a decentralized downlink beamforming scheme in cellular interference channels so that individual BSs can independently design their own beamforming vectors based on the locally attainable second order statistical channel state information at each BS. In this scheme, while each BS minimizes a combination of its total transmit power and the resulting total interference on the other vulnerable users of the adjacent cells, it assures that certain targeted signal-to-interference-plus-noise ratio (SINR) levels are achieved at its local users. Using the reciprocity of wireless network and the uplink-downlink duality, we develop a fast-converging iterative algorithm for downlink transmissions by distributed BSs. Finally, we derive a feasibility condition for the existence of a unique solution to the proposed optimization problem. This condition can be used to develop a scheduling algorithm for choosing a set of feasible users within each cell. Simulation results confirm the effectiveness of the proposed decentralized algorithm in closely following the performance curve of its centralized counterpart.

Throughput analysis of network coding enabled wireless backhauls

This study derives upper bounds on the throughput of the wireless backhaul link within a cluster of inter-connected three base stations (BSs) or a cluster of a controlling BS and three fixed relay stations. The authors propose and analyse the Ring protocol for the former and the Star protocol for the latter using network coding concept. These protocols can be used individually or in an overlaid fashion in a coordinated multi-cell system or in a cellular-distributed-antenna system to exchange information in the backhaul. To avoid the weakest link acting as a bottleneck and determining the backhaul throughput, as a result, the authors derive throughput optimising time sharing factors to compensate for the resulting bit imbalances in the backhaul.