This chapter presents direction of arrival (DoA) estimation with a compact array antenna using methods based on reactance switching. The compact array is the single-port electronically steerable parasitic array radiator (Espar) antenna. The antenna beam pattern is controlled though parasitic elements loaded with reactances. DoA estimation using an Espar antenna is proposed with the power pattern cross correlation (PPCC), reactance-domain (RD) multiple signal classification (MUSIC), and, RD estimation of signal parameters via rotational invariance techniques (ESPRIT) algorithms. The three methods exploit the reactance diversity provided by an Espar antenna to correlate different antenna output signals measured at different times and for different reactance values. The authors hope that this chapter allows the researchers to appreciate the issues that may be encountered in the implementation of direction-finding application with a single-port compact array like the Espar antenna.
The efficient use of direction-of-arrival (DoA) estimation techniques with smart antennas is an important research topic in wireless systems like ad hoc networks (Alexiou, 2004) or for the design of a DoA finder.
As a kind of smart antenna, switched parasitic antennas were proposed for cellular communications (Vaughan, 1999; Scott, 1999; Almhdie, 2000; Svantesson, 2002; Thiel, 2002). Compared to a conventional array antenna, which needs as many active radio receivers as antenna elements, a switched parasitic array antenna needs only a single active radio receiver. Therefore, the use of a switched parasitic antenna at the user side is an interesting alternative, since it can provide a small size, low cost and low power consumption for the receiver part. The switched parasitic antenna forms beams by using passive antenna elements that serve as reflectors when shorted to ground. Thus, a fixed number of directional patterns can be achieved by switching the short-circuits of the passive elements using p.i.n. diodes (Thiel, 2002). Switched parasitic antennas are known to improve the communication capacity in wireless communication systems (Almhdie, 2000), to perform high-resolution DoA estimation (Svantesson, 2002), such as that for personal locating services, and to provide antenna diversity (Vaughan, 1999; Scott, 1999) for adaptive communication systems.
The electronically steerable parasitic array radiator (Espar antenna), a kind of reactively controlled antenna (Harrington, 1978), was first proposed for low-cost user terminal applications (Ohira, 2000). Compared with simple switched parasitic antennas, the Espar antenna exhibits greater steerability control by means of its electronically controllable reactances and a more complex system for controlling the reactances (Thiel, 2004). Indeed, the parasitic element is connected to the ground by means of a reactance made with a reverse-bias varactor diode that can be controlled through loaded voltage. Thus, as a function of the reactance value, the parasitic element can variably act as a reflector or a radiator (Ohira, 2004, pp. 184-204). The continuous variability of the loaded reactance of the Espar antenna makes it more flexible than switched parasitic antennas because the number of possible directional patterns becomes greater (Thiel, 2004). Such a feature could be successfully employed, for example, in adaptive control processes involving beam and null forming, where the radiation pattern of an Espar antenna with a beampointing direction in the desired signal direction and nulls in the interference directions is obtained by optimizing the loaded reactances (Sun, 2004).