Use of active phased arrays for multiple-beam cellular communications systems

Abstract

Active phased array antennas offer an efficient and more versatile alternative to reflector antennas for use in communications systems that require multiple beams. The use of such antennas is evident in newly developed satellite communications systems and in base stations for wireless systems. In satellite systems, global, regional and zone coverage on the earth surface are traditionally accomplished using multibeam reflector antennas that use a number of feed elements or shaped reflectors that use a single feed per beam. Recent demands for large number of spot beams with high beam-to-beam isolation and the need for flexibility in steering the beams imposed new requirements on the communications system that are easier met with active phased arrays. The advances in Monolithic Microwave Integrated Circuits (MMIC) and digital technologies played a major role in realizing these arrays for satellite applications. However, the added requirements and active array characteristics introduce a set of transmission impairments in the communications link that did not necessarily exist in the conventional systems. This paper reviews phased array developments for multiple-beam cellular communications applications. The paper describes hardware designs to meet coverage requirements. These include the modular MMIC beam-forming matrices that generate a number of simultaneous and independent beams that are controlled digitally. The communications link and system aspects of the active phased array are assessed through measurements and analysis. Three link parameters are investigated and their effects on the overall carrier-to-interference ratio are quantified. The first parameter is the intermodulation components that are generated at the non-linear amplifier outputs and contribute to well-formed interference in the far field radiation of the array. The second is the bit-error ratio (BER) degradation due to the multi-carrier operation of the active array. Measurement results are shown to demonstrate this effect. The third link parameter is the co-channel interference caused by frequency re-use in multiple beam systems. Antenna performance for both user terminals and satellite payload (or cellular system base station) as well as coverage requirements and user traffic within each beam contribute to this interference effect.Introduction: