This paper proposes a novel hybrid residual least mean square (HRLMS) algorithm for adaptive filtering followed by an antenna beamformer using 16-element linear array. The hybridization process involves a switching between the residual-LMS (RLMS) and the conventional-LMS (CLMS) algorithms after the eighth iteration, if the square errors for four consecutive iterations are less than a threshold. The novelty of HRLMS lies in estimating best step size factor through residuals for speedy convergence followed by the CLMS switching for minimum steady state error (SSE). The novelty also includes in realizing a real-time antenna beamformer with significant sidelobe level (SLL) reduction and improved interference nulling by integration of HRLMS and space selective digital filter (SSDF). The adaptive filter and smart beamformer, based on HRLMS and HRLMS-SSDF have been implemented on TMS320VC5416 digital signal processor. The comparative performance evaluation of HRLMS has been done for convergence speed, SSE, interference nulling and SLL reduction with the existing variable step size LMS (VSSLMS) algorithms. The iteration count for convergence has been reduced by about 50% with paltry additional computational burden over the other VSSLMS algorithms. The HRLMS-SSDF provide attenuations of about 76, 33, and >50 dB, respectively for interfering signals, first SLL and higher order SLLs of beamformer. K E Y W O R D Sadaptive antenna beamformer, convergence speed, HRLMS algorithm, interference nulling, residuals, SLL
The mismatch between the actual direction of arrival (DOA) of signal of interest (SOI) and the estimated DOA drastically deteriorates the performance of the antenna beamformer. Diagonal loading (DL) is a popular method to improve the robustness of the antenna beamformer against such mismatch.However, the evaluation of optimum diagonal load level (DLL) involves much complexity. This paper proposes a simple and user parameter free novel data dependent algorithm for the development of minimum variance distortion less response (MVDR) based robust antenna beamformer which provides uniform output SINR over the entire range of mismatch. The behavior of the beamformer for varying DLL has been studied and observed the fact that, under optimum DLL condition, the main lobe of beam pattern is focused towards the estimated DOA of SOI within a mismatch of 4 0 . The novelty includes the logical exploitation of the analogy between the frequency and space domains Fourier Transforms followed by the frequency shift property and convolutions with necessary modifications. Next, the proposed algorithm has been extended to minimize the side lobe level (SLL) below À35 dB, in addition to achieving the desired robustness. Another advantage of this algorithm is the requirement of minimum additional computational burden of M complex multiplications and one convolution. The implementation of the robust antenna beamformer has also been done through the Digital Signal Processor TMS320C5416 supported with Code Composer Studio (CCS) for real time operations. Extensive comparative simulations have been included to justify the superiority of the proposed algorithm over the existing algorithms.
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