Simulation Study of Wideband Interference Rejection using Adaptive Array Antenna

Array antenna systems are often used to enhance the received signal to interference and noise ratio when the signal operates in heavily jammed environment. Proper modeling of the received data at different antenna elements is important when evaluating the performance of this system, especially when both the signal and interference have wide frequency bands. The received data has two dimensions: space, because different antenna elements are separated on the antenna face and time, because sampled data from each element is collected over time. Therefore received data processing can be regarded as space-time processing. The time sequence data is referred to as the received waveform of a given antenna element. The antenna output is the linear combination of data from all the antenna elements. In conventional narrowband beamforming, time sequences at different antenna elements are related by some fixed phase shift. The phase shift is determined by the waveform´s direction of arrival (DOA). For a wideband waveform, time sequences at different antenna elements are delayed in time and not simply related by a phase shift. If a wideband waveform arrives at an off boresight angle, the time delays may not be integer multiples of the sampling time (fractional delay). Thus, to properly process the received data at each antenna element, interpolation is necessary to derive the correct time samples. Fractional delay was first employed in a digital array phase shifter (Crochiere et al., 1975), and later in null-steering of antenna arrays (Ko and Lim, 1988) and also finds wide applications in other fields, such as digital modems (Zovojnovic and Meyr, 1988), speech coding (Kroon and Atal, 1991) and so on. The interpolation function used in this simulation study is sin(x)/x, where the power spectral density for the signal is assumed to be uniform. The received waveform is approximated by using a finite number of time samples in the interpolation formula, which is equivalent to approxim- ation in a least square error sense (Laakso et al, 1996). If the power spectral density function were not uniform, then we would have to use a different interpolation function. The array antenna in this simulation study is assumed to have seven omni-directional elements placed on a plane arranged as a honeycomb. To avoid grating lobes, the spacing between the elements is assumed to half a wavelength of the center frequency. Simulation results based on adaptive processing algorithm of conventional adaptive array antenna and space time adaptive processor (STAP) are compared with the theoretical calculations. Convergence rates and interference rejection for the two approaches are discussed below. An important conclusion is that the wideband processing required by STAP significantly improves performance against wideband interference