We address the problem of joint downlink beamforming in a power-controlled network, where independent data streams are to be transmitted from a multiantenna base station to several decentralized single-antenna terminals. The total transmit power is limited and channel information (possibly statistical) is available at the transmitter. The design goal: jointly adjust the beamformers and transmission powers according to individual SINR requirements. In this context, there are two closely related optimization problems. P1: maximize the jointly achievable SINR margin under a total power constraint. P2: minimize the total transmission power while satisfying a set of SINR constraints. In this paper, both problems are solved within a unified analytical framework. Problem P1 is solved by minimizing the maximal eigenvalue of an extended crosstalk matrix. The solution provides a necessary and sufficient condition for the feasibility of the SINR requirements. Problem P2 is a variation of problem P1. An important step in our analysis is to show that the global optimum of the downlink beamforming problem is equivalently obtained from solving a dual uplink problem, which has an easier-to-handle analytical structure. Then, we make use of the special structure of the extended crosstalk matrix to develop a rapidly converging iterative algorithm. The optimality and global convergence of the algorithm is proven and stopping criteria are given.

Solution of the multiuser downlink beamforming problem with individual SINR constraints

The demand for wireless mobile communications services is growing at an explosive rate, with the anticipation that communication to a mobile device anywhere on the globe at all times will be available in the near future. An array of antennas mounted on vehicles, ships, aircraft, satellites, and base stations is expected to play an important role in fulfilling the increased demand of channel requirement for these services, as well as for the realization of the dream that a portable communications device the size of a wristwatch be available at an affordable cost for such services. This paper is the first of a two-part study. It provides a comprehensive treatment, at a level appropriate to nonspecialists, of the use of an antenna array to enhance the efficiency of mobile communications systems. It presents an overview of mobile communications as well as details of how an array may be used in various mobile communications systems, including land-mobile, indoor-radio, and satellite-based systems. It discusses advantages of an array of antennas in a mobile communications system, highlights improvements that are possible by using multiple antennas compared to a single antenna in a system, and provides details on the feasibility of antenna arrays for mobile communications applications.

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Applications of antenna arrays to mobile communications. I. Performance improvement, feasibility, and system considerations

In this article, an overview of advanced convex optimization approaches to multisensor beamforming is presented, and connections are drawn between different types of optimization-based beamformers that apply to a broad class of receive, transmit, and network beamformer design problems. It is demonstrated that convex optimization provides an indispensable set of tools for beamforming, enabling rigorous formulation and effective solution of both long-standing and emerging design problems.

Convex Optimization-Based Beamforming

The use of multiple antennas at base stations is a key component in the design of cellular communication systems that can meet high-capacity demands in the downlink. Under ideal conditions, the gai ...

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Optimal Resource Allocation in Coordinated Multi-Cell Systems

A method and apparatus for allocating subcarriers in an orthogonal frequency division multiple access (OFDMA) system is described. In one embodiment, the method comprises allocating at least one diversity cluster of subcarriers to a first subscriber and allocating at least one coherence cluster to a second subscriber.

Ofdma with adaptive subcarrier-cluster configuration and selective loading

On the downlink of an SDMA mobile radio system, different users who share the same frequency band and time slot have to be spatially separated by beamforming. In this paper we analyze an approach to jointly calculate weights in such a way that all users receive their signal with a given signal-to-noise-and-interference ratio. The computations are based on spatial covariance matrices which describe the user-specific channels averaged over the fast fading. The presented algorithm succeeds in minimizing the array transmit power while spatially separating the users, no matter which rank their spatial covariance matrices have. But since it needs to solve a complex nonlinear constrained optimization problem, several approximations of this algorithm are presented and compared to each other on the basis of array power, complexity, and convergence.

Spatial covariance based downlink beamforming in an SDMA mobile radio system

On the downlink of an SDMA mobile radio system, co-channel interference has to be kept down by beamforming. In this paper two DOA-based beamforming approaches are presented minimizing array signal power while maintaining given signal-to-noise-and-interference ratios for all users. The linear approach is computationally cheap, so that it is suited for channel allocation as well, quickly evaluating the spatial separability of a specific mobile radio scenario. The nonlinear algorithm yields optimum results but only converges quickly, if provided with a good starting point. Therefore, its basic application is burst-to-burst updating of the beamformer.

Channel allocation and downlink beamforming in an SDMA mobile radio system

In SDMA mobile radio systems where only base stations, but no mobiles are equipped with antenna arrays, co-channel interference on the downlink has to be kept down by beamforming. The performance of the beamformer is determined by the directions of arrival (DOAs) of the users operating in the same frequency/time/code slot. Thus the full capacity of an SDMA cell can only be made use of by an effective channel allocation scheme. In this paper, a real time concept for channel allocation is presented. Furthermore, several criteria to quickly evaluate the spatial separability of subsets of users, were defined and then compared to each other by means of simulations.

A real time downlink channel allocation scheme for an SDMA mobile radio system

An approach is proposed to introduce a space division multiple access (SDMA) component into a frequency/time division multiple access (FTDMA) mobile radio system like the European GSM system providing higher frequency reuse and spectral efficiency. Except from signalling aspects the mobiles remain uneffected by the presented SDMA method without any need for antenna diversity or more sophisticated equalizers. Additional hard- and software-demands due to the new SDMA features are only restricted to base stations as they have to be equipped with an adaptive antenna array. This array is used to separate wavefronts in coherent multipath environments by means of a new algorithm. The number of antenna elements needed is significantly lower than the number of wavefronts impinging on the array.

Application of space division multiple access to mobile radio

In a cellular mobile radio system an SDMA component can be implemented for the reuse of radio channels physically incorporated by time, frequency or code slots. In this paper we will present simulation results on an SDMA downlink channel allocation scheme operating in a typical urban mobile radio cell. Based on the resulting blocking probabilities we will give estimations on the capacity increase which can be expected after adding an SDMA component to a conventional mobile radio system.

C. Farsakh

Papers

Spatial covariance based downlink beamforming in an SDMA mobile radio system

Channel allocation and downlink beamforming in an SDMA mobile radio system

A real time downlink channel allocation scheme for an SDMA mobile radio system

Application of space division multiple access to mobile radio

On the mobile radio capacity increase through SDMA