**1. Introduction**

Antenna systems can be classified as omnidirectional, directional, phased array, or adaptive array. An omnidirectional antenna (also called isotropic antenna) has equal gain in all directions. On the other hand, directional antennas have more gain in certain directions and less in others. A phased array antenna uses an array of antenna elements and combines the signals received on these elements with appropriate phase shifts to form the output of the array. The direction of the maximum gain (main beam) can be controlled by adjusting the phase between the elements of the array. For the case of narrow-band signals, which is considered here, the term adaptive antenna is used when the weights (magnitudes/gains and phases) of the signals induced on the array elements are regularly updated before combining, in order to control the radiation pattern of the array dynamically according to the requirements of the system [1–3].

In an optimal adaptive antenna array system, the gain and phase of each antenna element are adjusted to achieve the optimal performance of the array in some sense. For example, one basis for adjusting the gain and phase of each element is to obtain maximum output signal-to-interference-plus-noise ratio (*SINR*) by canceling undesired interferences while receiving the desired signal. Such an arrangement is referred to as optimal combining in the mobile communication literature [2, 4].

Desired signal tracking with an adaptive array can be accomplished in various ways. One can either supply a reference signal in the feedback loop of the array and use an adaptive algorithm such as the least mean squares algorithm (LMS); or one can inject steering weights into the feedback loops based on the prior knowledge of the arrival angle of the desired signal and use optimal combining to adjust the weights of the array; or as a third approach, blind adaptation can be used which is similar to LMS but it does not require a reference signal. The second approach is vastly simpler than the first one since no reference signal is required at all. The only difficulty with this approach is that the designer must know the arrival angle of the desired signal accurately [5]. In many situations, however, the angle of arrival of the desired signal is known to some degree of accuracy. The disadvantages of the third approach lie in the fact that the blind adaptation is too slow and/or complex in the mobile environment, because fading and movement may introduce sudden large variations in levels of desired signals and interferers, which complicate the blind adaptation process.

This chapter focuses on the second approach, and in particular optimal combining steered beam adaptive arrays. Such arrays typically aim to maximize the *SINR*. The achieved *SINR* depends largely on the "pointing error" in steering the main beam of the array toward the direction of the desired signal [5–7]. Therefore, the effect of pointing error will be discussed extensively, and it will be considered as one of the assessment criteria of the array performance.

Here are some basic concepts and considerations related to beam formation using antenna arrays mounted on base stations:


It is assumed that the elements of the adaptive array are short dipoles (such that their individual radiation patterns are very close to isotropic antennas) and arranged in vertical positions with their beams that are formed in a broadside direction. The array elements are spatially arranged in such a way to provide sufficient coverage over the expected range of directions of the desired signal. The

## *Steered Beam Adaptive Antenna Arrays DOI: http://dx.doi.org/10.5772/intechopen.100168*

type and the arrangement of the elements set significant limitations on the maximum capability of the array system, in terms of its capability to detect the desired signal and to null the interference signals with acceptable *SINR* [13, 14].

The individual output of each antenna element is fed to the pattern-forming network shown in **Figure 1**, where outputs are weighted by complex weights (amplifiers/attenuators and phase shifters), and then added to generate the overall output of the array. The values of the weights (which are determined by the adaptive signal processor based on the available information and the physical arrangement of the individual isotropic sensor elements) determine the overall array beam sensitivity pattern.

The steered beam adaptive arrays are based on the concept of maximizing the desired-to-undesired (interference plus thermal white noise) signal ratio at the output of the array [1–3, 5, 9]. Conceivably, this leads to placing nulls in the directions of the interference and noise while placing a maximum gain in the direction of the desired signal.
