This chapter presents a concern regarding the nature of wireless communications using multiple antennas. Multi-antenna systems are mainly developed using array processing methodology mostly derived for a scalar rather than a vector problem. However, as wireless communication systems operate in microwave frequency region, the vector nature of electromagnetic waves cannot be neglected in any system design levels. Failure in doing so will lead to an erroneous interpretation of a system performance. The goal of this chapter is to show that when the vector nature of electromagnetic wave is taken into account, an expected system performance may not be realized. Therefore, the electromagnetic effects must be integrated into a system design process in order to achieve the best system design. Many researches are underway regarding this important issue.
An array of antennas has proved its significant role in both radar and communication systems. It has been shown that with multiple antennas at a receiver, the improvement in the signal-to-noise ratio (SNR) of the designed signal is feasible while any other interference is suppressed from the receiver. Similarly, by reciprocity property, multiple antennas can be used at the transmitter to concentrate energy to a particular receiver and to reduce its interference with other receivers. In an adaptive array processing context, it is customary to assume a receiver being an ideal point source situated in free space. This is generally valid for sonar and acoustic signals. However, when applied to electromagnetic waves, this assumption has never been met since there is always mutual coupling between antennas themselves. Mutual coupling destroys the linear wavefront assumption for the received signal, which is the first assumption in array processing. Moreover, when an antenna is mounted on a tower or any kind of platform, the coupling between the antenna and platform is also important as pointed out in (Sarkar, 2006).
In this section, the effect of mutual coupling on adaptive processing is demonstrated. A method for compensation of the effect of mutual coupling will be also discussed. The mutual coupling compensation, in general, is too tedious to be derived analytically. A computer code may be used for this purpose. However, to provide a physical picture to the readers, the work of (Adve & Sarkar, 2000) will be discussed here to show how the mutual coupling affects the array performance and how the mutual coupling can be integrated into the adaptive processing.
Let us consider an array of thin wire dipole antennas. The dipoles are assumed to be z-directed of length and radius and are placed along the , separated by a distance . The port of each antenna is center loaded with an impedance of Ohms primarily to make them resonant and thereby increase its efficiency. Figure 1 shows the model of the antenna arrays. To simplify the problem, the following assumptions are made (Djordjevic, 1995; Strait 1973):