Hardware limitations of interference suppressing beamforming

[摘要] ENGLISH ABSTRACT: This thesis proposes two blind beamforming techniques for use in global positioningsystem (GPS) applications as well as a dual band multi-layered micro strip antenna for use at L1 and L2 GPS frequencies. Various direction of arrival (DOA) algorithms were tested to investigate their failure points.The antenna was built and measured. It was found that S11 is below -10 dB across the L1 and L2 bands from 1.22 GHz - 1.57 GHz. The gain is 1.7 dB below the simulation value in the L1 band however the L2 bands gain matchesthe simulation value of 7 dB. The results show that the antenna will function for GPS applications.The algorithms were to be tested, in practice, using the transient array radio telescope (TART) system which is a 24 element radio telescope designed for testing of imaging algorithms and surveying transient events. The systemmakes use of the MAX2769B universal GPS receiver which produces binary samples at a sample frequency of 16.368 MHz and a centre frequency of 4.092 MHz.Algorithm 1 uses an orthogonal subspace beamforming method to null interference and place antenna gain in the directions of the signals of interest (SOI).Algorithm 2 uses linearly constraint minimum variance (LCMV) beamforming and a beam search method in order to find available GPS satellites and null interference. Both algorithms were tested thoroughly by varying parameters such as signal to noise ratio (SNR), signal to interference ratio (SIR), integration time and the number of analogue to digital converter (ADC) bits to find their failure points and determine if they are viable techniques on the TART system.It was found that, when using a peak search method to determine successrates, and sweeping SNR, SIR, integration time (number of samples) and the number of ADC bits, the Bartlett DOA algorithm performs the best, possibly due to the number of signals present on the simulated array.It is shown that the success rates of algorithms 1 and 2 are similar, for the parameters that were swept. According to simulations, both methods are viable beamformers, for GPS applications, on the TART system and are not limitedby its hardware. Algorithm 1 is more sensitive to variations in SIR, SNR and integration time when using 1 bit shifted data. The complex bit shifted data is created using a technique that allows one to create quadrature (Q) data byshifting the in-phase (I) data 1 bit, and is only possible due to the intermediate frequency (IF) being one quarter that of the sample frequency. Algorithm 1 also requires a priori information about the number of SOI's which is notrequired for algorithm 2. Algorithm 2 is more computationally expensive compared to algorithm 1 for GPS applications.Preliminary practical results were collected using the TART system, and analysed.The measurements taken show the potential for algorithm 1 to be implemented, as a beamformer, in the TART system, however thorough practical testing was not possible due to the TART system becoming unusable.

[发布机构] Stellenbosch University