Answer:

Multi-antenna techniques can be seen as a joint name for a set of techniques with the common theme that they rely on the use of multiple antennas at the receiver and/or the transmitter, in combination with more or less advanced signal processing. Multi-antenna techniques can be used to achieve improved system performance. including improved system capacity (more users per cell) and improved coverage (possibility for larger cells). as well as improved service provisioning—for example. higher per-user data rates.

Multi-antenna configurations

An important characteristic of any multi-antenna configuration is the distance between the different antenna elements, to a large extent due to the relation between the antenna distance and the mutual correlation between the radio-channel fading experienced by the signals at the different antennas.

The antennas in a multi-antenna configuration can be located relatively far from each other, typically implying a relatively low mutual correlation. Alternatively, the antennas can be located relatively close to each other, typically implying a high mutual fading correlation—in essence, the different antennas experience the same, or at least very similar, instantaneous fading. Whether high or low correlation is desirable depends on what is to be achieved with the multi-antenna configuration (diversity, beam-forming, or spatial multi-plexing). What actual antenna distance is needed for low (alternatively high) fading correlation depends on the wavelength or, equivalently, the carrier frequency used for the radio communication. However, it also depends on the deployment scenario.

In the case of base-station antennas in typical macro-cell environments (relatively large cells, relatively high base-station antenna positions, etc.), an antenna distance of the order of 10 wavelengths is typically needed to ensure a low mutual fading correlation. At the same time, for a terminal in the same kind of environment, an antenna distance of the order of only half a wavelength is often sufficient to achieve relatively low mutual correlation. The reason for the difference between the base station and the terminal in this respect is that, in the macro-cell scenario. the multi-path reflections that cause the fading mainly occur in the near-zone around the terminal. Thus, as seen from the terminal, the different paths will typically arrive from a wide angle, implying a low fading correlation already with a relatively small antenna distance. At the same time, as seen from the (macro-cell) base station the different paths will typically arrive within a much smaller angle, implying the need for significantly larger antenna distance to achieve low fading correlation.

On the other hand, in other deployment scenarios, such as micro-cell deployments with base-station antennas below rooftop level and indoor deployments, the environment as seen from the base station is more similar to the environment as seen from the terminal. in such scenarios, a smaller base-station antenna distance is typically sufficient to ensure relatively low mutual correlation between the fading experienced by the different antennas.

Benefits of multi-antenna techniques

The availability of multiple antennas at the transmitter and/or the receiver can be utilized in different ways to achieve different aims:

• Multiple antennas at the transmitter and/or the receiver can be used to provide additional diversity against fading on the radio channel. In this case, the channels experienced by the different antennas should have low mutual correlation, implying the need for a sufficiently large inter-antenna distance (spatial diversity), or the use of different antenna polarization directions (polarization diversity).
• Multiple antennas at the transmitter and/or the receiver can be used to "shape" the overall antenna beam (transmit beam and receive beam respectively) in a certain way—for example, to maximize the overall antenna gain in the direction of the target receiver/ transmitter or to suppress specific dominant interfering signals. Such beam-forming can be based either on high or low fading correlation between the antennas.
• The simultaneous availability of multiple antennas at the transmitter and the receiver can be used to create what can be seen as multiple parallel communication "channels" over the radio interface. This provides the possibility for very high bandwidth utilization without a corresponding reduction in power efficiency or, in other words, the possibility for very high data rates within a limited bandwidth without a disproportionately large degradation in terms of coverage. Herein we will refer to this as spatial multiplexing.