New beamforming and DOA estimation techniques in wireless communications
Date
2008-04-10T06:04:08Z
Authors
Wang, Nanyan.
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Abstract
The development and performance evaluation of new techniques for direction of arrival (DOA) estimation, single-user beamforming (SUB), and multiuser beamforming (MUB) to be used in wireless communications are investigated. Two of the most commonly used beamformer configurations in direct-sequence code-division multiple access (DS-CDMA) systems, the chip-based (CB) and the symbol-based (SB) configurations for the base station (BS) receiver, are studied and their performance is evaluated. It is shown that using the CB configuration, different interfering components are rejected based on the spatial distribution of their power. In the SB configuration, spatial diversity is exploited after despreading and different interfering components are rejected based on their interfering strength which depends on both their power and code correlation with the signal of interest. For the SB configuration, more effort is applied to rejecting the interfering components with higher interfering strength and thus a more selective and efficient system is achieved. Detailed performance analysis and simulations show that in the presence of multipleaccess interference, the SB configuration can lead to a significant improvement in the signal-to-interference-plus-noise ratio relative to that achieved with the CB configuration for both asynchronous and synchronous DS-CDMA systems. A new technique for DOA estimation is proposed. The new technique, called subarray beamforming-based DOA (SBDOA) estimation, uses two virtual subarrays to form a signal whose phase relative to the reference signal is a function of the DOA. The DOA is then estimated based on the computation of the phase shift between the reference signal and the phaseshifted target signal. Since the phase-shifted target signal is obtained after interference rejection through beamforrning, the effect of co-channel interference on the estimation is significantly reduced. The proposed technique is computationally simple and the number of signal sources detectable is iii not bounded by the number of antenna elements used. Performance analysis and extensive simulations show that the proposed technique offers significantly improved estimation resolution, capacity, accuracy, and tracking capability relative to existing techniques. A new SUB algorithm is proposed for the downlink in wireless communication systems. The beam pattern at the BS is determined using a new optimality criterion which takes into consideration the fact that signals from the BS to different mobile stations (MSs) have different power and thus have different resistance to co-channel interference. In this way, the power of co-channel interference in the direction of an MS whose downlink signal has low resistance to co-channel interference can be significantly reduced. Simulation results show that the new algorithm leads to better performance than conventional algorithms in terms of system outage probability. A new MUB algorithm is proposed for joint beamforming and power control for the downlink in wireless communication systems. The optimization problem of optimal MUB is reformulated by modifying the constraints so that the weight vectors at the BS for different MSs are optimized in a feasible region which is a subset of the one of the original MUB problem. The downlink beamforming weight vectors of different MSs are then jointly optimized in a subspace instead of searching in the entire parameter space. Simulation results show that the modified optimization problem leads to solutions that satisfy the signal-to-noise-plus-interference ratio specification at each MS and, at the same time, the total power transmitted from the BS is very close to the optimal one. The solution of the modified optimization problem requires significantly less computation than that of the optimal MUB problem.