On the stability of adaptive antenna arrays with weights at an intermediate frequency

The author focuses on the stability problem that exists in adaptive antenna arrays when the weighting functions operate at a nonzero frequency. The application of the least-mean-square algorithm when the weights are written in terms of an in-phase and a quadrature component is considered. The behavior of the signal processing algorithm is then described by a set of coupled differential equations derived under zero-delay conditions. When time delays in the feedback loops are taken into account, the stability and therefore the performance of the array are greatly affected. It is shown how stability can be achieved by a simple time-compensation procedure. Simulation results show that interference rejection comparable to the zero-delay case is experienced