Читать книгу Advanced Antenna Array Engineering for 6G and Beyond Wireless Communications - Richard W. Ziolkowski - Страница 27

Before we end this chapter, we would like to clarify the concept of multi‐beam antennas. A conventional antenna has only one input port for one single beam with a specified polarization. We call such an antenna a single input and single output (SISO) antenna. There are two options to create multiple beams. The first option is that a single signal is fed into one port and then is split or distributed to sets of radiating elements and, hence, into a number of beams. We call this type of antenna a single input and multiple output (SIMO) multi‐beam antenna; it is illustrated in Figure 1.12a. The second option is that multiple signals are fed into multiple ports, separately, and then each input signal is delivered to a specific set of radiating elements to produce one dedicated beam. We call this type of antenna a multiple input and multiple output (MIMO) multi‐beam antenna; it is illustrated in Figure 1.12b. SIMO multi‐beam antennas are useful for data distribution and targeted broadcasting services. On the other hand, MIMO multi‐beam antennas are useful for multiuser communications in which each user is at a different location and communicates different information. The multiple beams created by SIMO and MIMO antennas can be fixed or steerable. The latter is much harder to achieve and would serve as a major research direction for the future. This topic is addressed further in Chapters 5 and 6.

Figure 1.12 Illustration of (a) SIMO and (b) MIMO multi‐beam antennas.

Notice that the SIMO and MIMO multi‐beam antenna concepts presented above are substantially different from the concepts of SIMO and MIMO in wireless communication systems. All of the inputs and outputs in the former reside in one transmitter or receiver system, typically in the base stations. The multi‐beams produced by the antennas are distinct beam patterns. In contrast, all of the inputs and outputs of the latter reside separately in the transmitter, typically at the base station, and the receivers, typically in the user terminals. The transmitted RF signal may not have distinct conventional beam patterns; certain types of multiuser detection or spatial–temporal decoding algorithms are employed at the receivers with no regard to specific beam patterns.