5.4. Interference Cancellation Suppression and Signal Enhancement

The available antenna elements at either the transmitter or receiver can be used to suppress undesired signals and/or enhance the power of the desired signal. In a multiantenna system, the channel is multidimensional and so the dimensions of the channel can be applied to null interference in a certain direction, while amplifying signals in another direction. Put another way, rather than increasing the statistical diversity of the total signal as in the preceding section, the energy can instead be steered. Perhaps most intuitively, the energy can be steered physically, resulting in actual electromagnetic wave patterns with certain properties. We discuss this approach first. Alternatively, the dimensions can be used to create linear transmitters and receivers that project desired signals into strong dimensions while attenuating the dimensions that interferers appear in. We consider two such approaches in Sections 5.4.2 and 5.4.3, depending on the amount of information available about the interfering channels.

5.4.1. DOA-Based Beamsteering

Electromagnetic waves can be physically steered to create beam patterns at either the transmitter or the receiver. At the transmitter, this causes energy to be sent predominantly in a desired direction, while only a small amount of residual energy is leaked in other directions. The more antennas are used, the more control over the beam pattern. The most common and simple form of this is static pattern-gain beamsteering, which is known as “sectoring” and was discussed in Section 2.3.3 in the context of cellular systems. In sectoring, static patterns are created. For example, in a threesector cell, a strong beam is projected over approximately 120 degrees, while very little energy is projected over the remaining 240 degrees. At a sectored receiver, the idea is similar: energy is accepted from the desired direction while suppressed from other directions.