Wideband Smart Antenna Avoiding Tapped-Delay Lines and Filters

Introduction

In the past two decades, radio systems (also known as wireless systems) have grown with an unprecedented speed from early radio paging, cordless telephone, and cellular telephony to today’s personal communication and mobile computing. This rapid expansion of radio systems has a profound impact on today’s business world and people’s daily lives. One undesired outcome is a heavy utilization of the available frequency spectrum. Because of this situation, a considerable interest has been shown in methods and techniques to overcome the limited frequency spectrum. One technique that is capable to increase the wireless system capacity without additional frequency spectrum is the smart antenna technique. Smart antennas are multiple element antennas accompanied by suitable signal processing algorithms either at the transmitter or receiver sides of a communication link. By pointing their beam towards a desired user and nulls or low side lobes towards interfering sources they are capable to considerably improve the quality of signal transmission in a multi-user environment. A significant value of smart antenna techniques in the efficient use of wireless spectrum has been addressed in (Kang, 2002; Jiang 1997). These multiple element antenna systems can also offer other advantages. These include the capability of minimizing the cost of establishing new wireless networks (Shao, 2003; Lee, 2005; Kawitkar, 2003), a better service quality (Hettak, 2000), and transparent operation across multi−technology wireless networks (Alexiou, 2004). It has to be noted that the benefits of smart antennas have been largely demonstrated for the case of narrowband communication systems. As the rapid growth of wireless technologies demands high bit rate data transmission, there is an interest in smart antennas which would operate over an increased frequency band. The design of such wideband intelligent antenna systems creates a challenge in terms of processing techniques and associated costs.

This book chapter gives a brief overview of wideband beamforming techniques. In particular, their advantages and disadvantages are discussed. As a result of these considerations the focus is on a wideband smart antenna system that relies on a fully spatial wideband beamforming technique. Full theoretical and experimental investigations into this wideband smart antenna system are presented.

The chapter is organized as follows. Firstly, shortfalls of narrowband smart antenna/beamforming techniques with respect to wideband signals are demonstrated via a suitable example. A number of wideband signal processing techniques are introduced and discussed to overcome this impairment. They are classified into three categories: space-time, space-frequency and fully spatial techniques. A brief comparison of these three wideband beamforming techniques is presented. As a result of this comparison, the fully spatial beamforming technique is selected for further considerations. Because of this choice, the main part of the chapter is devoted to a wideband spatial beamformer and its practical realization. The considerations commence with the introduction of configuration and the basic principles of operation of a wideband spatial beamformer that is created around a rectangular array of wideband antenna elements. The original beamforming algorithm, as reported in literature, is introduced and its shortfalls with respect to a small size array are pointed out. A suitable rectification of the original beamforming algorithm is proposed so it is valid for an arbitrary size array. The remaining considerations focus on small size arrays, which are easy to realize in practice. A 4×4 element wideband beamformer prototype is developed and tested over a specified frequency band. Various radiation patterns are realized by applying suitably devised signal weighting coefficients. The chapter is finalized with conclusions and remarks concerning future plans for wideband spatial beamformers.