On smart base-station antennas for mobile communications

Conventional base-station sector antennas for mobile communications are relatively inexpensive, but do not give an effective protection against interference. An alternative approach is to use directional adaptive phased arrays, which could additionally provide an increased protection against multipath fading and an ability to match antenna radiation to local conditions. For a narrow-band mobile system, we believe that a DSP-based phased array would provide a great deal of flexibility. As the signal from each element of the array is down converted to a low IF frequency and digitised before being processed, the phased array may be treated as a bank of digital spatial filters, each of which will pass signals coming from a given direction. The principle is fairly simple, because even though the signal received from each element of the array may contain information from many sources coming from many different directions, this signal may be stored in a common memory, and then read out for subsequent processing by all the ´filters´ of the bank. This paper discusses a concept-demonstration model which we have built and tested. The experimental model was designed for the ´uplink´, with the base-station antenna receiving a speech-modulated carrier transmitted by a moving terminal. For this purpose, the array is analysed in its receive mode, and the output of this digital spatial filter in response to a desired source in the presence of a set of interference is described.