An Adaptive Array Antenna Steered by If Local Signal Phase Shifters for K-band Broadband Wireless Access Base Station

1. Introduction Adaptive arrays for broadband wireless access base stations are currently being considered as a promising means to enhance subscriber capacity [1]-[6]. The transmission speed for broadband wireless access systems are significantly higher than personal communication systems (PCS) to which the adaptive arrays are partly applied [7]. Digital beam forming is being eagerly explored for application to PCS on account of its flexibility, but it will be difficult to apply to the high-speed systems, because the digital signal processing (DSP) and analog-to-digital converters (ADCs) require huge resources and power consumption. A pattern updating method with input vectors that are sparsely acquired to reduce the signal processing load [3] is proposed for wireless access systems with low mobility. But, very fast and power-consuming ADCs, which sample the modulated signals with a sufficiently higher rate than the signal baud rate, as well as a DSP circuit which weights and combines the outputs sampled by the ADCs are still required for real-time reception. Conventional phased arrays, on the other hand, utilize phase shifters through RF or IF signal lines. The phase shifters are usually both bulky and expensive. Digital phase shifters are preferred for computer-based control, but the signal attenuation becomes larger and the noise figure is degraded as for the conventional type, if the number of bits increases to achieve precise control. The authors have proposed an adaptive array steered by IF local signal phase shifters [2]. Figure 1 shows a typical configuration based on the proposal. An adaptive array as a radio receiver usually requires two or more frequency conversion stages to achieve the sensitivity performance. The phase shift and the frequency shift of the signal from each antenna element can be simultaneously accomplished at the down conversion stage by the phase-controlled local signal. The configuration can avoid the additional loss or noise with the received signal due to the phase shifter itself, and the ordinary commercial quadrature modulator IC for mobile communication can be utilized for the CW local phase shifter. The adaptive array does not require the high-speed digital signal processing sections. Also, it utilizes only one fast ADC in the digital demodulator for the composed output signal, not for all input vectors. A concern may be that LMS (Least Mean Square) or other adaptive algorithms, which require input vectors, cannot be used straightforwardly with this configuration. However, some algorithms to be used with the configuration have also …