An Introduction to 5G

6Multiple‐antenna Techniques

Multiple antennas are an important part of the 5G air interface, which uses them in two distinct ways. At high radio frequencies, for example millimetre waves, analogue beam selection compensates for the loss of coverage that would otherwise result from the radio propagation issues that we introduced in Chapter 4. At low radio frequencies, for example the 700 MHz band, digital beamforming is the basis for a technique known as multiple input multiple output (MIMO) antennas, which increases the capacity of the air interface in situations where the bandwidth is low. At intermediate radio frequencies, for example the 3500 MHz band, hybrid beamforming systems employ a mixture of the two. We will examine all of those techniques in this chapter, starting with the simplest and gradually progressing to the more complex ones.

Multiple antennas are a rather technical subject, and some previous knowledge of matrices would help the reader to understand the more mathematical parts of this chapter. There are some valuable introductions to matrix algebra in the Bibliography, as well as some more detailed treatments of multiple‐antenna techniques in general. Once again, however, we will keep the maths as lightweight as we reasonably can, and we will introduce the important concepts and terminology as we go along.

6.1 Analogue Beam Selection

6.1.1 Spatial Filtering

To begin our discussion, Figure 6.1 shows a base station that is equipped with four transmit antennas. ...