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Discrete Fourier transforms (DFTs) may be used to create fixed orthogonal beams, and the receiver can then select the beam with the strongest desired signal to provide discrimination against interference. A more accurate method for locating the desired signal and its application in speeding up the adaptive convergence rate in a direct-sequence code division multiple access receiver are described.
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Introduction: Conventional base-station antennas are designed to have a fixed broad-beam coverage. As these antennas do not provide any discrimination against interference falling within its wide beamwidth, weak mobile signals received from near the edge of the cell are very vulnerable to interference. To improve the signal-to-interference ratio, a hardwired Butler beamforming network may be used to create fixed orthogonal beams. The receiver can then select the beam with the strongest desired-signal component, but a certain loss in signal strength must be accepted with a maximum loss of > 3 dB at the crossover of two adjacent beams.
Instead of using hardwired networks, software-based digital beamforming may be adopted. With the advent of high-speed digital signal processors (DSPs), software techniques have been developed which can provide a viable alternative, as they are more flexible, and, as shown below, can be used to further process the signal to improve the discrimination against interference. Briefly, with digital beamforming, the antenna signals are first downconverted to a low frequency or baseband, and then digitised before being sent to the DSP module for further processing [1].
Method for determining desired-signal direction-of-arrival: With digital beamforming schemes, the task of the above-mentioned hardwired Butler transformer can be realised by carrying out a spatial DFT of the signals received by elements of the array. Without compromising the generality of the theory, in the following discussion we will assume a linear array of omnidirectional antennas, with inter-element spacing equal to l /2, where l is the operating wavelength. The expression of the resulting orthogonal beams at the output of the DFT is as follows: